5: DataLink Layer5-1 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing.

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5 DataLink Layer 5-1

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-2

Link Layer IntroductionSome terminology hosts and routers are nodes communication channels

that connect adjacent nodes along communication path are linkso wired links wireless linkso Point-to-point links multi-

access (or broadcast) links layer-2 packet is a frame

encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5 DataLink Layer 5-3

Link layer context

datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on

intermediate links 80211 on last link

each link protocol provides different serviceso eg may or may not provide rdt over link

Introduction 1-4

source

application

transportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

5 DataLink Layer 5-5

Link Layer Services framing

encapsulate datagram into frame adding header trailer

link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted

pair)o wireless links high error ratesbull Q why both link-level and end-end reliability

5 DataLink Layer 5-6

Link Layer Services (more)

flow control o pacing between adjacent sending and receiving

nodes

error detection o errors caused by signal attenuation noise o receiver detects presence of errors

bull signals sender for retransmission or drops frame

error correction o receiver identifies and corrects bit error(s) without

resorting to retransmission

5 DataLink Layer 5-7

Where is the link layer implemented

in each and every host link layer implemented in

ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

80211 cardo implements link physical

layer

attaches into hostrsquos system buses

combination of hardware software firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

5 DataLink Layer 5-8

Adaptors Communicating

sending sideo encapsulates datagram

in frameo adds error checking bits

rdt flow control etc

receiving sideo looks for errors rdt flow

control etco extracts datagram passes

to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

5 DataLink Layer 5-9

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-10

Parity Checking

Single Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

5 DataLink Layer 5-11

Internet checksum (review)

Sender treat segment contents

as sequence of 16-bit integers

checksum addition (1rsquos complement sum) of segment contents

sender puts checksum value into UDP checksum field

Receiver compute checksum of

received segment check if computed

checksum equals checksum field valueo NO - error detectedo YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

5 DataLink Layer 5-12

Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

remainder error detectedo can detect all burst errors less than r+1 bits

widely used in practice (Ethernet 80211 WiFi ATM)

Cyclic Redundancy Check Modulo 2 arithmetic

o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

A polynomial G(x) of degree r is known to both sender and receiver

Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

If not there are transmission errors in the frame

7 4 3( ) 1D x x x x x

Common polynomials for G(x)

CRC

CRC-8

CRC-10

CRC-12

CRC-16

CRC-CCITT

CRC-32

C(x)

x8+x2+x1+1

x10+x9+x5+x4+x1+1

x12+x11+x3+x2+x1+1

x16+x15+x2+1

x16+x12+x5+1

x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

5 DataLink Layer 5-15

CRC Example

31001 represents 1x

5 DataLink Layer 5-16

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-17

Multiple Access Links and Protocols

Two types of ldquolinksrdquo point-to-point

o PPP for dial-up accesso point-to-point link between Ethernet switch and host

broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Introduction 1-18

Cable Network Architecture Overview

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5000 homes

5 DataLink Layer 5-19

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference o collision if node receives two or more signals at the same

time

multiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself o no out-of-band channel for coordination

5 DataLink Layer 5-20

Ideal Multiple Access Protocol

Broadcast channel of rate R bps1 when one node wants to transmit it can send

at the full rate say R2 when M nodes want to transmit each can

send at average rate RM3 fully decentralized

o no special node to coordinate transmissionso no synchronization of clocks slots

4 simple

5 DataLink Layer 5-21

MAC Protocols a taxonomy

Three broad classes Channel Partitioning

o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

o allocate piece to node for exclusive use

Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

take longer turns

5 DataLink Layer 5-22

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

5 DataLink Layer 5-23

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

idle example 6-station LAN 134 have pkt

frequency bands 256 idle fr

equ

ency

bands time

FDM cable

5 DataLink Layer 5-24

Random Access Protocols

When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

retransmissions)

Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

5 DataLink Layer 5-25

ALOHA

When a node has a frame to send send immediately

Set a timer for a random amount of time If an ACK arrives before the timer expires

fine otherwise resend the frame

(Works like stop-and-wait with random timeout interval)

5 DataLink Layer 5-26

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmit

If channel sensed idle transmit entire frame

If channel sensed busy defer transmission

human analogy donrsquot interrupt others

5 DataLink Layer 5-27

CSMA collisions

collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

5 DataLink Layer 5-28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

o collisions detected within short timeo colliding transmissions aborted reducing

channel wastage collision detection

o easy in wired LANs measure signal strengths compare transmitted amp received signals

o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

human analogy the polite conversationalist

5 DataLink Layer 5-29

CSMACD collision detection

5 DataLink Layer 5-30

ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

o share channel efficiently and fairly at high load

o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolso efficient at low load single node can fully

utilize channelo high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

5 DataLink Layer 5-31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

typically used with ldquodumbrdquo slave devices

concernso polling overhead o latencyo single point of

failure (master)

master

slaves

poll

data

data

5 DataLink Layer 5-32

ldquoTaking Turnsrdquo MAC protocolsToken passing control token

passed from one node to next sequentially

token message concerns

o token overhead o latencyo single point of failure

(token)

T

data

(nothingto send)

T

5 DataLink Layer 5-33

Summary of MAC protocols

channel partitioning by time frequency or codeo Time Division Frequency Division

random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

5 DataLink Layer 5-34

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-Layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-35

MAC Addresses and ARP

32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

another physically-connected interface (in same network)

o 48 bit MAC addressbull burned in NIC ROM

5 DataLink Layer 5-36

LAN Addresses and ARPEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

5 DataLink Layer 5-37

LAN Address (more)

MAC address allocation administered by IEEE manufacturer buys portion of MAC address

space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

o can move LAN card from one LAN to another

IP hierarchical address NOT portableo address depends on IP subnet to which node is

attached

5 DataLink Layer 5-38

ARP Address Resolution Protocol

Each IP node (host router) on LAN has ARP table

ARP table IPMAC address mappings for some LAN nodes

lt IP address MAC address TTLgto TTL (Time To Live)

time after which address mapping will be forgotten (typically 20 min)

Q given a nodersquos IP address how to determine its MAC address

1A-2F-BB-76-09-AD

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137196723

137196778

137196714

137196788

5 DataLink Layer 5-39

ARP protocol Same LAN (network) A wants to send datagram

to B and Brsquos MAC address not in Arsquos ARP table

A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

FF-FF-FF-FF-FFo all machines on LAN

receive ARP query B receives ARP packet

replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

ARP is ldquoplug-and-playrdquoo nodes create their

ARP tables without intervention from net administrator

5 DataLink Layer 5-40

Addressing routing to another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

walkthrough send datagram from A to B via R assume A knows Brsquos IP address

two ARP tables in router R one for each IP network (LAN)

LAN LAN

5 DataLink Layer 5-41

A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

R

1A-23-F9-CD-06-9B

222222222220

111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

5 DataLink Layer 5-42

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-Layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-43

Ethernet

ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

5 DataLink Layer 5-44

Star topology bus topology popular through mid 90s

o all nodes in same collision domain (can collide with each other)

today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

5 DataLink Layer 5-45

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver amp sender clock

rates

5 DataLink Layer 5-46

Ethernet Frame Structure (more) Addresses 6 bytes

o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

o otherwise adapter discards frame

Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

CRC checked at receiver if error is detected frame is dropped

5 DataLink Layer 5-47

Ethernet Unreliable connectionless connectionless No handshaking between sending

and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

send acks or nacks to sending NICo stream of datagrams passed to network layer can have

gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

Ethernetrsquos MAC protocol CSMACD

5 DataLink Layer 5-48

Ethernet CSMACD algorithm

1 NIC receives datagram from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects collision while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff

after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

(1 slot = 512 bit-times)

5 DataLink Layer 5-49

Ethernetrsquos CSMACD (more)

Jam Signal make sure all other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current loado heavy load random

wait will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

5 DataLink Layer 5-50

8023 Ethernet Standards Link amp Physical Layers

many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpso different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

5 DataLink Layer 5-51

Manchester encoding

used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

synchronize to each othero no need for a centralized global clock among nodes

Hey this is physical-layer stuff

5 DataLink Layer 5-52

Link Layer

51 Introduction and services

52 Error detection and correction

53 Multiple access protocols

54 Link-layer Addressing

55 Ethernet

56 Link-layer switches LANs

59 A day in the life of a web request

5 DataLink Layer 5-53

Hubsphysical-layer (ldquodumbrdquo) repeaters

o bits coming in one link go out all other links at same rate

o all nodes connected to hub can collide with one another

o no frame bufferingo no CSMACD at hub host NICs detect collisions

twisted pair

hub

5 DataLink Layer 5-54

Switch link-layer device smarter than hubs take active

roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

transparento hosts are unaware of presence of switches

plug-and-play self-learningo switches do not need to be configured

5 DataLink Layer 5-55

Switch allows multiple simultaneous transmissions

hosts have dedicated direct connection to switch

switches buffer packets Ethernet protocol used on

each incoming link but no collisions full duplexo each link is its own collision

domain switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions o not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

5 DataLink Layer 5-56

Switch Table

Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

A each switch has a switch table each entryo (MAC address of host interface

to reach host time stamp)

looks like a routing table Q how are entries created

maintained in switch table o something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

5 DataLink Layer 5-57

Switch self-learning

switch learns which hosts can be reached through which interfaceso when frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

o records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

5 DataLink Layer 5-58

Switch frame filteringforwardingWhen frame received

1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

5 DataLink Layer 5-59

Self-learning forwarding example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

frame destination unknownflood

Arsquo A

destination A location known

Arsquo 4 60

selective send

5 DataLink Layer 5-60

Interconnecting switches

switches can be connected together

A

B

Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

A self learning (works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

5 DataLink Layer 5-61

Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

Q show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2 3

34

5 DataLink Layer 5-62

Institutional network

to externalnetwork

router

IP subnet

mail server

web server

5 DataLink Layer 5-63

Switches vs Routers both store-and-forward devices

o routers network layer devices (examine network layer headers)o switches are link layer devices

routers maintain routing tables implement routing algorithms

switches maintain switch tables implement filtering learning algorithms

Switch

5 DataLink Layer 5-64

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-Layer Addressing

55 Ethernet

56 Link-layer switches 59 A day in the life of

a web request

5 DataLink Layer 5-65

Synthesis a day in the life of a web request

journey down protocol stack completeo application transport network link

putting-it-all-together synthesiso goal identify review understand protocols

(at all layers) involved in seemingly simple scenario requesting www page

o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

5 DataLink Layer 5-66

A day in the life scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

5 DataLink Layer 5-67

A day in the lifehellip connecting to the Internet

connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

5 DataLink Layer 5-68

A day in the lifehellip connecting to the Internet

DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

DHCP client receives DHCP ACK reply

5 DataLink Layer 5-69

A day in the lifehellip ARP (before DNS before HTTP)

before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

5 DataLink Layer 5-70

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

demuxrsquoed to DNS server DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS server

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

5 DataLink Layer 5-71

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

to send HTTP request client first opens TCP socket to web server

TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

TCP connection established

64233169105

web server

SYN

SYN

SYN

SYN

TCPIP

EthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

web server responds with TCP SYNACK (step 2 in 3-way handshake)

5 DataLink Layer 5-72

A day in the lifehellip HTTP requestreply

HTTPTCPIP

EthPhy

HTTP

HTTP request sent into TCP socket

IP datagram containing HTTP request routed to wwwgooglecom

IP datgram containing HTTP reply routed back to client

64233169105

web server

HTTPTCPIP

EthPhy

web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

web page finally () displayed

5 DataLink Layer 5-73

Chapter 5 letrsquos take a breath journey down protocol stack complete

(except PHY) solid understanding of networking

principles practice hellip could stop here hellip but lots of

interesting topicso Internetworking (CSE 678 TCPIP socket

programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

  • Link Layer
  • Link Layer Introduction
  • Link layer context
  • Encapsulation
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Slide 9
  • Parity Checking
  • Internet checksum (review)
  • Checksumming Cyclic Redundancy Check
  • Cyclic Redundancy Check
  • Slide 14
  • CRC Example
  • Slide 16
  • Multiple Access Links and Protocols
  • Cable Network Architecture Overview
  • Multiple Access protocols
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • ALOHA
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Slide 34
  • MAC Addresses and ARP
  • LAN Addresses and ARP
  • LAN Address (more)
  • ARP Address Resolution Protocol
  • ARP protocol Same LAN (network)
  • Addressing routing to another LAN
  • Slide 41
  • Slide 42
  • Ethernet
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • 8023 Ethernet Standards Link amp Physical Layers
  • Manchester encoding
  • Slide 52
  • Hubs
  • Switch
  • Switch allows multiple simultaneous transmissions
  • Switch Table
  • Switch self-learning
  • Switch frame filteringforwarding
  • Self-learning forwarding example
  • Interconnecting switches
  • Self-learning multi-switch example
  • Institutional network
  • Switches vs Routers
  • Slide 64
  • Synthesis a day in the life of a web request
  • A day in the life scenario
  • A day in the lifehellip connecting to the Internet
  • Slide 68
  • A day in the lifehellip ARP (before DNS before HTTP)
  • A day in the lifehellip using DNS
  • A day in the lifehellip TCP connection carrying HTTP
  • A day in the lifehellip HTTP requestreply
  • Chapter 5 letrsquos take a breath

    5 DataLink Layer 5-2

    Link Layer IntroductionSome terminology hosts and routers are nodes communication channels

    that connect adjacent nodes along communication path are linkso wired links wireless linkso Point-to-point links multi-

    access (or broadcast) links layer-2 packet is a frame

    encapsulates datagram

    data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

    5 DataLink Layer 5-3

    Link layer context

    datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on

    intermediate links 80211 on last link

    each link protocol provides different serviceso eg may or may not provide rdt over link

    Introduction 1-4

    source

    application

    transportnetwork

    linkphysical

    HtHn M

    segment Ht

    datagram

    destination

    application

    transportnetwork

    linkphysical

    HtHnHl M

    HtHn M

    Ht M

    M

    networklink

    physical

    linkphysical

    HtHnHl M

    HtHn M

    HtHn M

    HtHnHl M

    router

    switch

    Encapsulationmessage M

    Ht M

    Hn

    frame

    5 DataLink Layer 5-5

    Link Layer Services framing

    encapsulate datagram into frame adding header trailer

    link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify

    source dest bull different from IP address

    reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted

    pair)o wireless links high error ratesbull Q why both link-level and end-end reliability

    5 DataLink Layer 5-6

    Link Layer Services (more)

    flow control o pacing between adjacent sending and receiving

    nodes

    error detection o errors caused by signal attenuation noise o receiver detects presence of errors

    bull signals sender for retransmission or drops frame

    error correction o receiver identifies and corrects bit error(s) without

    resorting to retransmission

    5 DataLink Layer 5-7

    Where is the link layer implemented

    in each and every host link layer implemented in

    ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

    80211 cardo implements link physical

    layer

    attaches into hostrsquos system buses

    combination of hardware software firmware

    controller

    physicaltransmission

    cpu memory

    host bus (eg PCI)

    network adaptercard

    host schematic

    applicationtransportnetwork

    link

    linkphysical

    5 DataLink Layer 5-8

    Adaptors Communicating

    sending sideo encapsulates datagram

    in frameo adds error checking bits

    rdt flow control etc

    receiving sideo looks for errors rdt flow

    control etco extracts datagram passes

    to upper layer at receiving side

    controller controller

    sending host receiving host

    datagram datagram

    datagram

    frame

    5 DataLink Layer 5-9

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53Multiple access protocols

    54 Link-layer Addressing

    55 Ethernet

    56 Link-layer switches 59 A day in the life of

    a web request

    5 DataLink Layer 5-10

    Parity Checking

    Single Bit ParityDetect single bit errors

    Two Dimensional Bit ParityDetect and correct single bit errors

    0 0

    5 DataLink Layer 5-11

    Internet checksum (review)

    Sender treat segment contents

    as sequence of 16-bit integers

    checksum addition (1rsquos complement sum) of segment contents

    sender puts checksum value into UDP checksum field

    Receiver compute checksum of

    received segment check if computed

    checksum equals checksum field valueo NO - error detectedo YES - no error detected

    But maybe errors nonetheless

    Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

    5 DataLink Layer 5-12

    Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

    polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

    o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

    remainder error detectedo can detect all burst errors less than r+1 bits

    widely used in practice (Ethernet 80211 WiFi ATM)

    Cyclic Redundancy Check Modulo 2 arithmetic

    o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

    A polynomial G(x) of degree r is known to both sender and receiver

    Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

    Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

    If not there are transmission errors in the frame

    7 4 3( ) 1D x x x x x

    Common polynomials for G(x)

    CRC

    CRC-8

    CRC-10

    CRC-12

    CRC-16

    CRC-CCITT

    CRC-32

    C(x)

    x8+x2+x1+1

    x10+x9+x5+x4+x1+1

    x12+x11+x3+x2+x1+1

    x16+x15+x2+1

    x16+x12+x5+1

    x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

    5 DataLink Layer 5-15

    CRC Example

    31001 represents 1x

    5 DataLink Layer 5-16

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53Multiple access protocols

    54 Link-layer Addressing

    55 Ethernet

    56 Link-layer switches 59 A day in the life of

    a web request

    5 DataLink Layer 5-17

    Multiple Access Links and Protocols

    Two types of ldquolinksrdquo point-to-point

    o PPP for dial-up accesso point-to-point link between Ethernet switch and host

    broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

    shared wire (eg cabled Ethernet)

    shared RF (eg 80211 WiFi)

    shared RF(satellite)

    humans at acocktail party

    (shared air acoustical)

    Introduction 1-18

    Cable Network Architecture Overview

    home

    cable headend

    cable distributionnetwork (simplified)

    Typically 500 to 5000 homes

    5 DataLink Layer 5-19

    Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

    interference o collision if node receives two or more signals at the same

    time

    multiple access protocol distributed algorithm that determines how nodes

    share channel ie determine when node can transmit

    communication about channel sharing must use channel itself o no out-of-band channel for coordination

    5 DataLink Layer 5-20

    Ideal Multiple Access Protocol

    Broadcast channel of rate R bps1 when one node wants to transmit it can send

    at the full rate say R2 when M nodes want to transmit each can

    send at average rate RM3 fully decentralized

    o no special node to coordinate transmissionso no synchronization of clocks slots

    4 simple

    5 DataLink Layer 5-21

    MAC Protocols a taxonomy

    Three broad classes Channel Partitioning

    o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

    o allocate piece to node for exclusive use

    Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

    ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

    take longer turns

    5 DataLink Layer 5-22

    Channel Partitioning MAC protocols TDMA

    TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

    pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

    256 idle

    1 3 4 1 3 4

    6-slotframe

    5 DataLink Layer 5-23

    Channel Partitioning MAC protocols FDMA

    FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

    idle example 6-station LAN 134 have pkt

    frequency bands 256 idle fr

    equ

    ency

    bands time

    FDM cable

    5 DataLink Layer 5-24

    Random Access Protocols

    When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

    two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

    o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

    retransmissions)

    Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

    5 DataLink Layer 5-25

    ALOHA

    When a node has a frame to send send immediately

    Set a timer for a random amount of time If an ACK arrives before the timer expires

    fine otherwise resend the frame

    (Works like stop-and-wait with random timeout interval)

    5 DataLink Layer 5-26

    CSMA (Carrier Sense Multiple Access)

    CSMA listen before transmit

    If channel sensed idle transmit entire frame

    If channel sensed busy defer transmission

    human analogy donrsquot interrupt others

    5 DataLink Layer 5-27

    CSMA collisions

    collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

    spatial layout of nodes

    5 DataLink Layer 5-28

    CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

    o collisions detected within short timeo colliding transmissions aborted reducing

    channel wastage collision detection

    o easy in wired LANs measure signal strengths compare transmitted amp received signals

    o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

    human analogy the polite conversationalist

    5 DataLink Layer 5-29

    CSMACD collision detection

    5 DataLink Layer 5-30

    ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

    o share channel efficiently and fairly at high load

    o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

    Random access MAC protocolso efficient at low load single node can fully

    utilize channelo high load collision overhead

    ldquotaking turnsrdquo protocolslook for best of both worlds

    5 DataLink Layer 5-31

    ldquoTaking Turnsrdquo MAC protocolsPolling master node

    ldquoinvitesrdquo slave nodes to transmit in turn

    typically used with ldquodumbrdquo slave devices

    concernso polling overhead o latencyo single point of

    failure (master)

    master

    slaves

    poll

    data

    data

    5 DataLink Layer 5-32

    ldquoTaking Turnsrdquo MAC protocolsToken passing control token

    passed from one node to next sequentially

    token message concerns

    o token overhead o latencyo single point of failure

    (token)

    T

    data

    (nothingto send)

    T

    5 DataLink Layer 5-33

    Summary of MAC protocols

    channel partitioning by time frequency or codeo Time Division Frequency Division

    random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

    hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

    taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

    5 DataLink Layer 5-34

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53Multiple access protocols

    54 Link-Layer Addressing

    55 Ethernet

    56 Link-layer switches 59 A day in the life of

    a web request

    5 DataLink Layer 5-35

    MAC Addresses and ARP

    32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

    MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

    another physically-connected interface (in same network)

    o 48 bit MAC addressbull burned in NIC ROM

    5 DataLink Layer 5-36

    LAN Addresses and ARPEach adapter on LAN has unique LAN address

    Broadcast address =FF-FF-FF-FF-FF-FF

    = adapter

    1A-2F-BB-76-09-AD

    58-23-D7-FA-20-B0

    0C-C4-11-6F-E3-98

    71-65-F7-2B-08-53

    LAN(wired orwireless)

    5 DataLink Layer 5-37

    LAN Address (more)

    MAC address allocation administered by IEEE manufacturer buys portion of MAC address

    space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

    o can move LAN card from one LAN to another

    IP hierarchical address NOT portableo address depends on IP subnet to which node is

    attached

    5 DataLink Layer 5-38

    ARP Address Resolution Protocol

    Each IP node (host router) on LAN has ARP table

    ARP table IPMAC address mappings for some LAN nodes

    lt IP address MAC address TTLgto TTL (Time To Live)

    time after which address mapping will be forgotten (typically 20 min)

    Q given a nodersquos IP address how to determine its MAC address

    1A-2F-BB-76-09-AD

    0C-C4-11-6F-E3-98

    71-65-F7-2B-08-53

    LAN

    137196723

    137196778

    137196714

    137196788

    5 DataLink Layer 5-39

    ARP protocol Same LAN (network) A wants to send datagram

    to B and Brsquos MAC address not in Arsquos ARP table

    A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

    FF-FF-FF-FF-FFo all machines on LAN

    receive ARP query B receives ARP packet

    replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

    address (unicast)

    A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

    ARP is ldquoplug-and-playrdquoo nodes create their

    ARP tables without intervention from net administrator

    5 DataLink Layer 5-40

    Addressing routing to another LAN

    R

    1A-23-F9-CD-06-9B

    222222222220111111111110

    E6-E9-00-17-BB-4B

    CC-49-DE-D0-AB-7D

    111111111112

    111111111111

    A74-29-9C-E8-FF-55

    222222222221

    88-B2-2F-54-1A-0F

    B222222222222

    49-BD-D2-C7-56-2A

    walkthrough send datagram from A to B via R assume A knows Brsquos IP address

    two ARP tables in router R one for each IP network (LAN)

    LAN LAN

    5 DataLink Layer 5-41

    A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

    frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

    destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

    R

    1A-23-F9-CD-06-9B

    222222222220

    111111111110

    E6-E9-00-17-BB-4B

    CC-49-DE-D0-AB-7D

    111111111112

    111111111111

    A74-29-9C-E8-FF-55

    222222222221

    88-B2-2F-54-1A-0F

    B222222222222

    49-BD-D2-C7-56-2A

    This is a really importantexample ndash make sure youunderstand

    5 DataLink Layer 5-42

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53Multiple access protocols

    54 Link-Layer Addressing

    55 Ethernet

    56 Link-layer switches 59 A day in the life of

    a web request

    5 DataLink Layer 5-43

    Ethernet

    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

    Metcalfersquos Ethernetsketch

    5 DataLink Layer 5-44

    Star topology bus topology popular through mid 90s

    o all nodes in same collision domain (can collide with each other)

    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

    (nodes do not collide with each other)

    switch

    bus coaxial cable star

    5 DataLink Layer 5-45

    Ethernet Frame Structure

    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

    Preamble 7 bytes with pattern 10101010 followed by one

    byte with pattern 10101011 used to synchronize receiver amp sender clock

    rates

    5 DataLink Layer 5-46

    Ethernet Frame Structure (more) Addresses 6 bytes

    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

    o otherwise adapter discards frame

    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

    CRC checked at receiver if error is detected frame is dropped

    5 DataLink Layer 5-47

    Ethernet Unreliable connectionless connectionless No handshaking between sending

    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

    Ethernetrsquos MAC protocol CSMACD

    5 DataLink Layer 5-48

    Ethernet CSMACD algorithm

    1 NIC receives datagram from network layer creates frame

    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

    4 If NIC detects collision while transmitting aborts and sends jam signal

    5 After aborting NIC enters exponential backoff

    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

    (1 slot = 512 bit-times)

    5 DataLink Layer 5-49

    Ethernetrsquos CSMACD (more)

    Jam Signal make sure all other transmitters are aware of collision 48 bits

    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

    Exponential Backoff Goal adapt retransmission

    attempts to estimated current loado heavy load random

    wait will be longer first collision choose K

    from 01 delay is K 512 bit transmission times

    after second collision choose K from 0123hellip

    after ten collisions choose K from 01234hellip1023

    5 DataLink Layer 5-50

    8023 Ethernet Standards Link amp Physical Layers

    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

    Mbps 1Gbps 10G bpso different physical layer media fiber cable

    applicationtransportnetwork

    linkphysical

    MAC protocoland frame format

    100BASE-TX

    100BASE-T4

    100BASE-FX100BASE-T2

    100BASE-SX 100BASE-BX

    fiber physical layercopper (twisterpair) physical layer

    5 DataLink Layer 5-51

    Manchester encoding

    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

    synchronize to each othero no need for a centralized global clock among nodes

    Hey this is physical-layer stuff

    5 DataLink Layer 5-52

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53 Multiple access protocols

    54 Link-layer Addressing

    55 Ethernet

    56 Link-layer switches LANs

    59 A day in the life of a web request

    5 DataLink Layer 5-53

    Hubsphysical-layer (ldquodumbrdquo) repeaters

    o bits coming in one link go out all other links at same rate

    o all nodes connected to hub can collide with one another

    o no frame bufferingo no CSMACD at hub host NICs detect collisions

    twisted pair

    hub

    5 DataLink Layer 5-54

    Switch link-layer device smarter than hubs take active

    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

    transparento hosts are unaware of presence of switches

    plug-and-play self-learningo switches do not need to be configured

    5 DataLink Layer 5-55

    Switch allows multiple simultaneous transmissions

    hosts have dedicated direct connection to switch

    switches buffer packets Ethernet protocol used on

    each incoming link but no collisions full duplexo each link is its own collision

    domain switching A-to-Arsquo and B-

    to-Brsquo simultaneously without collisions o not possible with dumb hub

    A

    Arsquo

    B

    Brsquo

    C

    Crsquo

    switch with six interfaces(123456)

    1 23

    45

    6

    5 DataLink Layer 5-56

    Switch Table

    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

    A each switch has a switch table each entryo (MAC address of host interface

    to reach host time stamp)

    looks like a routing table Q how are entries created

    maintained in switch table o something like a routing

    protocol

    A

    Arsquo

    B

    Brsquo

    C

    Crsquo

    switch with six interfaces(123456)

    1 23

    45

    6

    5 DataLink Layer 5-57

    Switch self-learning

    switch learns which hosts can be reached through which interfaceso when frame received

    switch ldquolearnsrdquo location of sender incoming LAN segment

    o records senderlocation pair in switch table

    A

    Arsquo

    B

    Brsquo

    C

    Crsquo

    1 23

    45

    6

    A Arsquo

    Source ADest Arsquo

    MAC addr interface TTL

    Switch table (initially empty)

    A 1 60

    5 DataLink Layer 5-58

    Switch frame filteringforwardingWhen frame received

    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

    then if dest on segment from which frame arrived

    then drop the frame else forward the frame on interface indicated else flood

    forward on all but the interface on which the frame arrived

    5 DataLink Layer 5-59

    Self-learning forwarding example

    A

    Arsquo

    B

    Brsquo

    C

    Crsquo

    1 23

    45

    6

    A Arsquo

    Source ADest Arsquo

    MAC addr interface TTL

    Switch table (initially empty)

    A 1 60

    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

    frame destination unknownflood

    Arsquo A

    destination A location known

    Arsquo 4 60

    selective send

    5 DataLink Layer 5-60

    Interconnecting switches

    switches can be connected together

    A

    B

    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

    A self learning (works exactly the same as in single-switch case)

    S1

    C D

    E

    FS2

    S4

    S3

    H

    I

    G

    5 DataLink Layer 5-61

    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

    Q show switch tables and packet forwarding in S1 S2 S3 S4

    A

    B

    S1

    C D

    E

    FS2

    S4

    S3

    H

    I

    G

    1

    2 3

    34

    5 DataLink Layer 5-62

    Institutional network

    to externalnetwork

    router

    IP subnet

    mail server

    web server

    5 DataLink Layer 5-63

    Switches vs Routers both store-and-forward devices

    o routers network layer devices (examine network layer headers)o switches are link layer devices

    routers maintain routing tables implement routing algorithms

    switches maintain switch tables implement filtering learning algorithms

    Switch

    5 DataLink Layer 5-64

    Link Layer

    51 Introduction and services

    52 Error detection and correction

    53Multiple access protocols

    54 Link-Layer Addressing

    55 Ethernet

    56 Link-layer switches 59 A day in the life of

    a web request

    5 DataLink Layer 5-65

    Synthesis a day in the life of a web request

    journey down protocol stack completeo application transport network link

    putting-it-all-together synthesiso goal identify review understand protocols

    (at all layers) involved in seemingly simple scenario requesting www page

    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

    5 DataLink Layer 5-66

    A day in the life scenario

    Comcast network 68800013

    Googlersquos network 64233160019 64233169105

    web server

    DNS server

    school network 68802024

    browser

    web page

    5 DataLink Layer 5-67

    A day in the lifehellip connecting to the Internet

    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

    router(runs DHCP)

    DHCPUDP

    IPEthPhy

    DHCP

    DHCP

    DHCP

    DHCP

    DHCP

    DHCPUDP

    IPEthPhy

    DHCP

    DHCP

    DHCP

    DHCPDHCP

    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

    5 DataLink Layer 5-68

    A day in the lifehellip connecting to the Internet

    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

    router(runs DHCP)

    DHCPUDP

    IPEthPhy

    DHCP

    DHCP

    DHCP

    DHCP

    DHCPUDP

    IPEthPhy

    DHCP

    DHCP

    DHCP

    DHCP

    DHCP

    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

    DHCP client receives DHCP ACK reply

    5 DataLink Layer 5-69

    A day in the lifehellip ARP (before DNS before HTTP)

    before sending HTTP request need IP address of wwwgooglecom DNS

    DNSUDP

    IPEthPhy

    DNS

    DNS

    DNS

    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

    ARP query

    EthPhy

    ARP

    ARP

    ARP reply

    5 DataLink Layer 5-70

    A day in the lifehellip using DNS

    DNSUDP

    IPEthPhy

    DNS

    DNS

    DNS

    DNS

    DNS

    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

    demuxrsquoed to DNS server DNS server replies to

    client with IP address of wwwgooglecom

    Comcast network 68800013

    DNS server

    DNSUDP

    IPEthPhy

    DNS

    DNS

    DNS

    DNS

    5 DataLink Layer 5-71

    A day in the lifehellip TCP connection carrying HTTP

    HTTPTCPIP

    EthPhy

    HTTP

    to send HTTP request client first opens TCP socket to web server

    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

    TCP connection established

    64233169105

    web server

    SYN

    SYN

    SYN

    SYN

    TCPIP

    EthPhy

    SYN

    SYN

    SYN

    SYNACK

    SYNACK

    SYNACK

    SYNACK

    SYNACK

    SYNACK

    SYNACK

    web server responds with TCP SYNACK (step 2 in 3-way handshake)

    5 DataLink Layer 5-72

    A day in the lifehellip HTTP requestreply

    HTTPTCPIP

    EthPhy

    HTTP

    HTTP request sent into TCP socket

    IP datagram containing HTTP request routed to wwwgooglecom

    IP datgram containing HTTP reply routed back to client

    64233169105

    web server

    HTTPTCPIP

    EthPhy

    web server responds with HTTP reply (containing web page)

    HTTP

    HTTP

    HTTPHTTP

    HTTP

    HTTP

    HTTP

    HTTP

    HTTP

    HTTP

    HTTP

    HTTP

    HTTP

    web page finally () displayed

    5 DataLink Layer 5-73

    Chapter 5 letrsquos take a breath journey down protocol stack complete

    (except PHY) solid understanding of networking

    principles practice hellip could stop here hellip but lots of

    interesting topicso Internetworking (CSE 678 TCPIP socket

    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

    • Link Layer
    • Link Layer Introduction
    • Link layer context
    • Encapsulation
    • Link Layer Services
    • Link Layer Services (more)
    • Where is the link layer implemented
    • Adaptors Communicating
    • Slide 9
    • Parity Checking
    • Internet checksum (review)
    • Checksumming Cyclic Redundancy Check
    • Cyclic Redundancy Check
    • Slide 14
    • CRC Example
    • Slide 16
    • Multiple Access Links and Protocols
    • Cable Network Architecture Overview
    • Multiple Access protocols
    • Ideal Multiple Access Protocol
    • MAC Protocols a taxonomy
    • Channel Partitioning MAC protocols TDMA
    • Channel Partitioning MAC protocols FDMA
    • Random Access Protocols
    • ALOHA
    • CSMA (Carrier Sense Multiple Access)
    • CSMA collisions
    • CSMACD (Collision Detection)
    • CSMACD collision detection
    • ldquoTaking Turnsrdquo MAC protocols
    • Slide 31
    • Slide 32
    • Summary of MAC protocols
    • Slide 34
    • MAC Addresses and ARP
    • LAN Addresses and ARP
    • LAN Address (more)
    • ARP Address Resolution Protocol
    • ARP protocol Same LAN (network)
    • Addressing routing to another LAN
    • Slide 41
    • Slide 42
    • Ethernet
    • Star topology
    • Ethernet Frame Structure
    • Ethernet Frame Structure (more)
    • Ethernet Unreliable connectionless
    • Ethernet CSMACD algorithm
    • Ethernetrsquos CSMACD (more)
    • 8023 Ethernet Standards Link amp Physical Layers
    • Manchester encoding
    • Slide 52
    • Hubs
    • Switch
    • Switch allows multiple simultaneous transmissions
    • Switch Table
    • Switch self-learning
    • Switch frame filteringforwarding
    • Self-learning forwarding example
    • Interconnecting switches
    • Self-learning multi-switch example
    • Institutional network
    • Switches vs Routers
    • Slide 64
    • Synthesis a day in the life of a web request
    • A day in the life scenario
    • A day in the lifehellip connecting to the Internet
    • Slide 68
    • A day in the lifehellip ARP (before DNS before HTTP)
    • A day in the lifehellip using DNS
    • A day in the lifehellip TCP connection carrying HTTP
    • A day in the lifehellip HTTP requestreply
    • Chapter 5 letrsquos take a breath

      5 DataLink Layer 5-3

      Link layer context

      datagram transferred by different link protocols over different linkso eg Ethernet on first link frame relay on

      intermediate links 80211 on last link

      each link protocol provides different serviceso eg may or may not provide rdt over link

      Introduction 1-4

      source

      application

      transportnetwork

      linkphysical

      HtHn M

      segment Ht

      datagram

      destination

      application

      transportnetwork

      linkphysical

      HtHnHl M

      HtHn M

      Ht M

      M

      networklink

      physical

      linkphysical

      HtHnHl M

      HtHn M

      HtHn M

      HtHnHl M

      router

      switch

      Encapsulationmessage M

      Ht M

      Hn

      frame

      5 DataLink Layer 5-5

      Link Layer Services framing

      encapsulate datagram into frame adding header trailer

      link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify

      source dest bull different from IP address

      reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted

      pair)o wireless links high error ratesbull Q why both link-level and end-end reliability

      5 DataLink Layer 5-6

      Link Layer Services (more)

      flow control o pacing between adjacent sending and receiving

      nodes

      error detection o errors caused by signal attenuation noise o receiver detects presence of errors

      bull signals sender for retransmission or drops frame

      error correction o receiver identifies and corrects bit error(s) without

      resorting to retransmission

      5 DataLink Layer 5-7

      Where is the link layer implemented

      in each and every host link layer implemented in

      ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

      80211 cardo implements link physical

      layer

      attaches into hostrsquos system buses

      combination of hardware software firmware

      controller

      physicaltransmission

      cpu memory

      host bus (eg PCI)

      network adaptercard

      host schematic

      applicationtransportnetwork

      link

      linkphysical

      5 DataLink Layer 5-8

      Adaptors Communicating

      sending sideo encapsulates datagram

      in frameo adds error checking bits

      rdt flow control etc

      receiving sideo looks for errors rdt flow

      control etco extracts datagram passes

      to upper layer at receiving side

      controller controller

      sending host receiving host

      datagram datagram

      datagram

      frame

      5 DataLink Layer 5-9

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53Multiple access protocols

      54 Link-layer Addressing

      55 Ethernet

      56 Link-layer switches 59 A day in the life of

      a web request

      5 DataLink Layer 5-10

      Parity Checking

      Single Bit ParityDetect single bit errors

      Two Dimensional Bit ParityDetect and correct single bit errors

      0 0

      5 DataLink Layer 5-11

      Internet checksum (review)

      Sender treat segment contents

      as sequence of 16-bit integers

      checksum addition (1rsquos complement sum) of segment contents

      sender puts checksum value into UDP checksum field

      Receiver compute checksum of

      received segment check if computed

      checksum equals checksum field valueo NO - error detectedo YES - no error detected

      But maybe errors nonetheless

      Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

      5 DataLink Layer 5-12

      Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

      polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

      o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

      remainder error detectedo can detect all burst errors less than r+1 bits

      widely used in practice (Ethernet 80211 WiFi ATM)

      Cyclic Redundancy Check Modulo 2 arithmetic

      o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

      A polynomial G(x) of degree r is known to both sender and receiver

      Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

      Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

      If not there are transmission errors in the frame

      7 4 3( ) 1D x x x x x

      Common polynomials for G(x)

      CRC

      CRC-8

      CRC-10

      CRC-12

      CRC-16

      CRC-CCITT

      CRC-32

      C(x)

      x8+x2+x1+1

      x10+x9+x5+x4+x1+1

      x12+x11+x3+x2+x1+1

      x16+x15+x2+1

      x16+x12+x5+1

      x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

      5 DataLink Layer 5-15

      CRC Example

      31001 represents 1x

      5 DataLink Layer 5-16

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53Multiple access protocols

      54 Link-layer Addressing

      55 Ethernet

      56 Link-layer switches 59 A day in the life of

      a web request

      5 DataLink Layer 5-17

      Multiple Access Links and Protocols

      Two types of ldquolinksrdquo point-to-point

      o PPP for dial-up accesso point-to-point link between Ethernet switch and host

      broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

      shared wire (eg cabled Ethernet)

      shared RF (eg 80211 WiFi)

      shared RF(satellite)

      humans at acocktail party

      (shared air acoustical)

      Introduction 1-18

      Cable Network Architecture Overview

      home

      cable headend

      cable distributionnetwork (simplified)

      Typically 500 to 5000 homes

      5 DataLink Layer 5-19

      Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

      interference o collision if node receives two or more signals at the same

      time

      multiple access protocol distributed algorithm that determines how nodes

      share channel ie determine when node can transmit

      communication about channel sharing must use channel itself o no out-of-band channel for coordination

      5 DataLink Layer 5-20

      Ideal Multiple Access Protocol

      Broadcast channel of rate R bps1 when one node wants to transmit it can send

      at the full rate say R2 when M nodes want to transmit each can

      send at average rate RM3 fully decentralized

      o no special node to coordinate transmissionso no synchronization of clocks slots

      4 simple

      5 DataLink Layer 5-21

      MAC Protocols a taxonomy

      Three broad classes Channel Partitioning

      o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

      o allocate piece to node for exclusive use

      Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

      ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

      take longer turns

      5 DataLink Layer 5-22

      Channel Partitioning MAC protocols TDMA

      TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

      pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

      256 idle

      1 3 4 1 3 4

      6-slotframe

      5 DataLink Layer 5-23

      Channel Partitioning MAC protocols FDMA

      FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

      idle example 6-station LAN 134 have pkt

      frequency bands 256 idle fr

      equ

      ency

      bands time

      FDM cable

      5 DataLink Layer 5-24

      Random Access Protocols

      When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

      two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

      o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

      retransmissions)

      Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

      5 DataLink Layer 5-25

      ALOHA

      When a node has a frame to send send immediately

      Set a timer for a random amount of time If an ACK arrives before the timer expires

      fine otherwise resend the frame

      (Works like stop-and-wait with random timeout interval)

      5 DataLink Layer 5-26

      CSMA (Carrier Sense Multiple Access)

      CSMA listen before transmit

      If channel sensed idle transmit entire frame

      If channel sensed busy defer transmission

      human analogy donrsquot interrupt others

      5 DataLink Layer 5-27

      CSMA collisions

      collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

      spatial layout of nodes

      5 DataLink Layer 5-28

      CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

      o collisions detected within short timeo colliding transmissions aborted reducing

      channel wastage collision detection

      o easy in wired LANs measure signal strengths compare transmitted amp received signals

      o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

      human analogy the polite conversationalist

      5 DataLink Layer 5-29

      CSMACD collision detection

      5 DataLink Layer 5-30

      ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

      o share channel efficiently and fairly at high load

      o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

      Random access MAC protocolso efficient at low load single node can fully

      utilize channelo high load collision overhead

      ldquotaking turnsrdquo protocolslook for best of both worlds

      5 DataLink Layer 5-31

      ldquoTaking Turnsrdquo MAC protocolsPolling master node

      ldquoinvitesrdquo slave nodes to transmit in turn

      typically used with ldquodumbrdquo slave devices

      concernso polling overhead o latencyo single point of

      failure (master)

      master

      slaves

      poll

      data

      data

      5 DataLink Layer 5-32

      ldquoTaking Turnsrdquo MAC protocolsToken passing control token

      passed from one node to next sequentially

      token message concerns

      o token overhead o latencyo single point of failure

      (token)

      T

      data

      (nothingto send)

      T

      5 DataLink Layer 5-33

      Summary of MAC protocols

      channel partitioning by time frequency or codeo Time Division Frequency Division

      random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

      hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

      taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

      5 DataLink Layer 5-34

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53Multiple access protocols

      54 Link-Layer Addressing

      55 Ethernet

      56 Link-layer switches 59 A day in the life of

      a web request

      5 DataLink Layer 5-35

      MAC Addresses and ARP

      32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

      MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

      another physically-connected interface (in same network)

      o 48 bit MAC addressbull burned in NIC ROM

      5 DataLink Layer 5-36

      LAN Addresses and ARPEach adapter on LAN has unique LAN address

      Broadcast address =FF-FF-FF-FF-FF-FF

      = adapter

      1A-2F-BB-76-09-AD

      58-23-D7-FA-20-B0

      0C-C4-11-6F-E3-98

      71-65-F7-2B-08-53

      LAN(wired orwireless)

      5 DataLink Layer 5-37

      LAN Address (more)

      MAC address allocation administered by IEEE manufacturer buys portion of MAC address

      space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

      o can move LAN card from one LAN to another

      IP hierarchical address NOT portableo address depends on IP subnet to which node is

      attached

      5 DataLink Layer 5-38

      ARP Address Resolution Protocol

      Each IP node (host router) on LAN has ARP table

      ARP table IPMAC address mappings for some LAN nodes

      lt IP address MAC address TTLgto TTL (Time To Live)

      time after which address mapping will be forgotten (typically 20 min)

      Q given a nodersquos IP address how to determine its MAC address

      1A-2F-BB-76-09-AD

      0C-C4-11-6F-E3-98

      71-65-F7-2B-08-53

      LAN

      137196723

      137196778

      137196714

      137196788

      5 DataLink Layer 5-39

      ARP protocol Same LAN (network) A wants to send datagram

      to B and Brsquos MAC address not in Arsquos ARP table

      A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

      FF-FF-FF-FF-FFo all machines on LAN

      receive ARP query B receives ARP packet

      replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

      address (unicast)

      A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

      ARP is ldquoplug-and-playrdquoo nodes create their

      ARP tables without intervention from net administrator

      5 DataLink Layer 5-40

      Addressing routing to another LAN

      R

      1A-23-F9-CD-06-9B

      222222222220111111111110

      E6-E9-00-17-BB-4B

      CC-49-DE-D0-AB-7D

      111111111112

      111111111111

      A74-29-9C-E8-FF-55

      222222222221

      88-B2-2F-54-1A-0F

      B222222222222

      49-BD-D2-C7-56-2A

      walkthrough send datagram from A to B via R assume A knows Brsquos IP address

      two ARP tables in router R one for each IP network (LAN)

      LAN LAN

      5 DataLink Layer 5-41

      A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

      frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

      destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

      R

      1A-23-F9-CD-06-9B

      222222222220

      111111111110

      E6-E9-00-17-BB-4B

      CC-49-DE-D0-AB-7D

      111111111112

      111111111111

      A74-29-9C-E8-FF-55

      222222222221

      88-B2-2F-54-1A-0F

      B222222222222

      49-BD-D2-C7-56-2A

      This is a really importantexample ndash make sure youunderstand

      5 DataLink Layer 5-42

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53Multiple access protocols

      54 Link-Layer Addressing

      55 Ethernet

      56 Link-layer switches 59 A day in the life of

      a web request

      5 DataLink Layer 5-43

      Ethernet

      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

      Metcalfersquos Ethernetsketch

      5 DataLink Layer 5-44

      Star topology bus topology popular through mid 90s

      o all nodes in same collision domain (can collide with each other)

      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

      (nodes do not collide with each other)

      switch

      bus coaxial cable star

      5 DataLink Layer 5-45

      Ethernet Frame Structure

      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

      Preamble 7 bytes with pattern 10101010 followed by one

      byte with pattern 10101011 used to synchronize receiver amp sender clock

      rates

      5 DataLink Layer 5-46

      Ethernet Frame Structure (more) Addresses 6 bytes

      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

      o otherwise adapter discards frame

      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

      CRC checked at receiver if error is detected frame is dropped

      5 DataLink Layer 5-47

      Ethernet Unreliable connectionless connectionless No handshaking between sending

      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

      Ethernetrsquos MAC protocol CSMACD

      5 DataLink Layer 5-48

      Ethernet CSMACD algorithm

      1 NIC receives datagram from network layer creates frame

      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

      4 If NIC detects collision while transmitting aborts and sends jam signal

      5 After aborting NIC enters exponential backoff

      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

      (1 slot = 512 bit-times)

      5 DataLink Layer 5-49

      Ethernetrsquos CSMACD (more)

      Jam Signal make sure all other transmitters are aware of collision 48 bits

      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

      Exponential Backoff Goal adapt retransmission

      attempts to estimated current loado heavy load random

      wait will be longer first collision choose K

      from 01 delay is K 512 bit transmission times

      after second collision choose K from 0123hellip

      after ten collisions choose K from 01234hellip1023

      5 DataLink Layer 5-50

      8023 Ethernet Standards Link amp Physical Layers

      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

      Mbps 1Gbps 10G bpso different physical layer media fiber cable

      applicationtransportnetwork

      linkphysical

      MAC protocoland frame format

      100BASE-TX

      100BASE-T4

      100BASE-FX100BASE-T2

      100BASE-SX 100BASE-BX

      fiber physical layercopper (twisterpair) physical layer

      5 DataLink Layer 5-51

      Manchester encoding

      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

      synchronize to each othero no need for a centralized global clock among nodes

      Hey this is physical-layer stuff

      5 DataLink Layer 5-52

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53 Multiple access protocols

      54 Link-layer Addressing

      55 Ethernet

      56 Link-layer switches LANs

      59 A day in the life of a web request

      5 DataLink Layer 5-53

      Hubsphysical-layer (ldquodumbrdquo) repeaters

      o bits coming in one link go out all other links at same rate

      o all nodes connected to hub can collide with one another

      o no frame bufferingo no CSMACD at hub host NICs detect collisions

      twisted pair

      hub

      5 DataLink Layer 5-54

      Switch link-layer device smarter than hubs take active

      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

      transparento hosts are unaware of presence of switches

      plug-and-play self-learningo switches do not need to be configured

      5 DataLink Layer 5-55

      Switch allows multiple simultaneous transmissions

      hosts have dedicated direct connection to switch

      switches buffer packets Ethernet protocol used on

      each incoming link but no collisions full duplexo each link is its own collision

      domain switching A-to-Arsquo and B-

      to-Brsquo simultaneously without collisions o not possible with dumb hub

      A

      Arsquo

      B

      Brsquo

      C

      Crsquo

      switch with six interfaces(123456)

      1 23

      45

      6

      5 DataLink Layer 5-56

      Switch Table

      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

      A each switch has a switch table each entryo (MAC address of host interface

      to reach host time stamp)

      looks like a routing table Q how are entries created

      maintained in switch table o something like a routing

      protocol

      A

      Arsquo

      B

      Brsquo

      C

      Crsquo

      switch with six interfaces(123456)

      1 23

      45

      6

      5 DataLink Layer 5-57

      Switch self-learning

      switch learns which hosts can be reached through which interfaceso when frame received

      switch ldquolearnsrdquo location of sender incoming LAN segment

      o records senderlocation pair in switch table

      A

      Arsquo

      B

      Brsquo

      C

      Crsquo

      1 23

      45

      6

      A Arsquo

      Source ADest Arsquo

      MAC addr interface TTL

      Switch table (initially empty)

      A 1 60

      5 DataLink Layer 5-58

      Switch frame filteringforwardingWhen frame received

      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

      then if dest on segment from which frame arrived

      then drop the frame else forward the frame on interface indicated else flood

      forward on all but the interface on which the frame arrived

      5 DataLink Layer 5-59

      Self-learning forwarding example

      A

      Arsquo

      B

      Brsquo

      C

      Crsquo

      1 23

      45

      6

      A Arsquo

      Source ADest Arsquo

      MAC addr interface TTL

      Switch table (initially empty)

      A 1 60

      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

      frame destination unknownflood

      Arsquo A

      destination A location known

      Arsquo 4 60

      selective send

      5 DataLink Layer 5-60

      Interconnecting switches

      switches can be connected together

      A

      B

      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

      A self learning (works exactly the same as in single-switch case)

      S1

      C D

      E

      FS2

      S4

      S3

      H

      I

      G

      5 DataLink Layer 5-61

      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

      Q show switch tables and packet forwarding in S1 S2 S3 S4

      A

      B

      S1

      C D

      E

      FS2

      S4

      S3

      H

      I

      G

      1

      2 3

      34

      5 DataLink Layer 5-62

      Institutional network

      to externalnetwork

      router

      IP subnet

      mail server

      web server

      5 DataLink Layer 5-63

      Switches vs Routers both store-and-forward devices

      o routers network layer devices (examine network layer headers)o switches are link layer devices

      routers maintain routing tables implement routing algorithms

      switches maintain switch tables implement filtering learning algorithms

      Switch

      5 DataLink Layer 5-64

      Link Layer

      51 Introduction and services

      52 Error detection and correction

      53Multiple access protocols

      54 Link-Layer Addressing

      55 Ethernet

      56 Link-layer switches 59 A day in the life of

      a web request

      5 DataLink Layer 5-65

      Synthesis a day in the life of a web request

      journey down protocol stack completeo application transport network link

      putting-it-all-together synthesiso goal identify review understand protocols

      (at all layers) involved in seemingly simple scenario requesting www page

      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

      5 DataLink Layer 5-66

      A day in the life scenario

      Comcast network 68800013

      Googlersquos network 64233160019 64233169105

      web server

      DNS server

      school network 68802024

      browser

      web page

      5 DataLink Layer 5-67

      A day in the lifehellip connecting to the Internet

      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

      router(runs DHCP)

      DHCPUDP

      IPEthPhy

      DHCP

      DHCP

      DHCP

      DHCP

      DHCP

      DHCPUDP

      IPEthPhy

      DHCP

      DHCP

      DHCP

      DHCPDHCP

      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

      5 DataLink Layer 5-68

      A day in the lifehellip connecting to the Internet

      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

      router(runs DHCP)

      DHCPUDP

      IPEthPhy

      DHCP

      DHCP

      DHCP

      DHCP

      DHCPUDP

      IPEthPhy

      DHCP

      DHCP

      DHCP

      DHCP

      DHCP

      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

      DHCP client receives DHCP ACK reply

      5 DataLink Layer 5-69

      A day in the lifehellip ARP (before DNS before HTTP)

      before sending HTTP request need IP address of wwwgooglecom DNS

      DNSUDP

      IPEthPhy

      DNS

      DNS

      DNS

      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

      ARP query

      EthPhy

      ARP

      ARP

      ARP reply

      5 DataLink Layer 5-70

      A day in the lifehellip using DNS

      DNSUDP

      IPEthPhy

      DNS

      DNS

      DNS

      DNS

      DNS

      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

      demuxrsquoed to DNS server DNS server replies to

      client with IP address of wwwgooglecom

      Comcast network 68800013

      DNS server

      DNSUDP

      IPEthPhy

      DNS

      DNS

      DNS

      DNS

      5 DataLink Layer 5-71

      A day in the lifehellip TCP connection carrying HTTP

      HTTPTCPIP

      EthPhy

      HTTP

      to send HTTP request client first opens TCP socket to web server

      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

      TCP connection established

      64233169105

      web server

      SYN

      SYN

      SYN

      SYN

      TCPIP

      EthPhy

      SYN

      SYN

      SYN

      SYNACK

      SYNACK

      SYNACK

      SYNACK

      SYNACK

      SYNACK

      SYNACK

      web server responds with TCP SYNACK (step 2 in 3-way handshake)

      5 DataLink Layer 5-72

      A day in the lifehellip HTTP requestreply

      HTTPTCPIP

      EthPhy

      HTTP

      HTTP request sent into TCP socket

      IP datagram containing HTTP request routed to wwwgooglecom

      IP datgram containing HTTP reply routed back to client

      64233169105

      web server

      HTTPTCPIP

      EthPhy

      web server responds with HTTP reply (containing web page)

      HTTP

      HTTP

      HTTPHTTP

      HTTP

      HTTP

      HTTP

      HTTP

      HTTP

      HTTP

      HTTP

      HTTP

      HTTP

      web page finally () displayed

      5 DataLink Layer 5-73

      Chapter 5 letrsquos take a breath journey down protocol stack complete

      (except PHY) solid understanding of networking

      principles practice hellip could stop here hellip but lots of

      interesting topicso Internetworking (CSE 678 TCPIP socket

      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

      • Link Layer
      • Link Layer Introduction
      • Link layer context
      • Encapsulation
      • Link Layer Services
      • Link Layer Services (more)
      • Where is the link layer implemented
      • Adaptors Communicating
      • Slide 9
      • Parity Checking
      • Internet checksum (review)
      • Checksumming Cyclic Redundancy Check
      • Cyclic Redundancy Check
      • Slide 14
      • CRC Example
      • Slide 16
      • Multiple Access Links and Protocols
      • Cable Network Architecture Overview
      • Multiple Access protocols
      • Ideal Multiple Access Protocol
      • MAC Protocols a taxonomy
      • Channel Partitioning MAC protocols TDMA
      • Channel Partitioning MAC protocols FDMA
      • Random Access Protocols
      • ALOHA
      • CSMA (Carrier Sense Multiple Access)
      • CSMA collisions
      • CSMACD (Collision Detection)
      • CSMACD collision detection
      • ldquoTaking Turnsrdquo MAC protocols
      • Slide 31
      • Slide 32
      • Summary of MAC protocols
      • Slide 34
      • MAC Addresses and ARP
      • LAN Addresses and ARP
      • LAN Address (more)
      • ARP Address Resolution Protocol
      • ARP protocol Same LAN (network)
      • Addressing routing to another LAN
      • Slide 41
      • Slide 42
      • Ethernet
      • Star topology
      • Ethernet Frame Structure
      • Ethernet Frame Structure (more)
      • Ethernet Unreliable connectionless
      • Ethernet CSMACD algorithm
      • Ethernetrsquos CSMACD (more)
      • 8023 Ethernet Standards Link amp Physical Layers
      • Manchester encoding
      • Slide 52
      • Hubs
      • Switch
      • Switch allows multiple simultaneous transmissions
      • Switch Table
      • Switch self-learning
      • Switch frame filteringforwarding
      • Self-learning forwarding example
      • Interconnecting switches
      • Self-learning multi-switch example
      • Institutional network
      • Switches vs Routers
      • Slide 64
      • Synthesis a day in the life of a web request
      • A day in the life scenario
      • A day in the lifehellip connecting to the Internet
      • Slide 68
      • A day in the lifehellip ARP (before DNS before HTTP)
      • A day in the lifehellip using DNS
      • A day in the lifehellip TCP connection carrying HTTP
      • A day in the lifehellip HTTP requestreply
      • Chapter 5 letrsquos take a breath

        Introduction 1-4

        source

        application

        transportnetwork

        linkphysical

        HtHn M

        segment Ht

        datagram

        destination

        application

        transportnetwork

        linkphysical

        HtHnHl M

        HtHn M

        Ht M

        M

        networklink

        physical

        linkphysical

        HtHnHl M

        HtHn M

        HtHn M

        HtHnHl M

        router

        switch

        Encapsulationmessage M

        Ht M

        Hn

        frame

        5 DataLink Layer 5-5

        Link Layer Services framing

        encapsulate datagram into frame adding header trailer

        link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify

        source dest bull different from IP address

        reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted

        pair)o wireless links high error ratesbull Q why both link-level and end-end reliability

        5 DataLink Layer 5-6

        Link Layer Services (more)

        flow control o pacing between adjacent sending and receiving

        nodes

        error detection o errors caused by signal attenuation noise o receiver detects presence of errors

        bull signals sender for retransmission or drops frame

        error correction o receiver identifies and corrects bit error(s) without

        resorting to retransmission

        5 DataLink Layer 5-7

        Where is the link layer implemented

        in each and every host link layer implemented in

        ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

        80211 cardo implements link physical

        layer

        attaches into hostrsquos system buses

        combination of hardware software firmware

        controller

        physicaltransmission

        cpu memory

        host bus (eg PCI)

        network adaptercard

        host schematic

        applicationtransportnetwork

        link

        linkphysical

        5 DataLink Layer 5-8

        Adaptors Communicating

        sending sideo encapsulates datagram

        in frameo adds error checking bits

        rdt flow control etc

        receiving sideo looks for errors rdt flow

        control etco extracts datagram passes

        to upper layer at receiving side

        controller controller

        sending host receiving host

        datagram datagram

        datagram

        frame

        5 DataLink Layer 5-9

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53Multiple access protocols

        54 Link-layer Addressing

        55 Ethernet

        56 Link-layer switches 59 A day in the life of

        a web request

        5 DataLink Layer 5-10

        Parity Checking

        Single Bit ParityDetect single bit errors

        Two Dimensional Bit ParityDetect and correct single bit errors

        0 0

        5 DataLink Layer 5-11

        Internet checksum (review)

        Sender treat segment contents

        as sequence of 16-bit integers

        checksum addition (1rsquos complement sum) of segment contents

        sender puts checksum value into UDP checksum field

        Receiver compute checksum of

        received segment check if computed

        checksum equals checksum field valueo NO - error detectedo YES - no error detected

        But maybe errors nonetheless

        Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

        5 DataLink Layer 5-12

        Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

        polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

        o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

        remainder error detectedo can detect all burst errors less than r+1 bits

        widely used in practice (Ethernet 80211 WiFi ATM)

        Cyclic Redundancy Check Modulo 2 arithmetic

        o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

        A polynomial G(x) of degree r is known to both sender and receiver

        Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

        Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

        If not there are transmission errors in the frame

        7 4 3( ) 1D x x x x x

        Common polynomials for G(x)

        CRC

        CRC-8

        CRC-10

        CRC-12

        CRC-16

        CRC-CCITT

        CRC-32

        C(x)

        x8+x2+x1+1

        x10+x9+x5+x4+x1+1

        x12+x11+x3+x2+x1+1

        x16+x15+x2+1

        x16+x12+x5+1

        x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

        5 DataLink Layer 5-15

        CRC Example

        31001 represents 1x

        5 DataLink Layer 5-16

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53Multiple access protocols

        54 Link-layer Addressing

        55 Ethernet

        56 Link-layer switches 59 A day in the life of

        a web request

        5 DataLink Layer 5-17

        Multiple Access Links and Protocols

        Two types of ldquolinksrdquo point-to-point

        o PPP for dial-up accesso point-to-point link between Ethernet switch and host

        broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

        shared wire (eg cabled Ethernet)

        shared RF (eg 80211 WiFi)

        shared RF(satellite)

        humans at acocktail party

        (shared air acoustical)

        Introduction 1-18

        Cable Network Architecture Overview

        home

        cable headend

        cable distributionnetwork (simplified)

        Typically 500 to 5000 homes

        5 DataLink Layer 5-19

        Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

        interference o collision if node receives two or more signals at the same

        time

        multiple access protocol distributed algorithm that determines how nodes

        share channel ie determine when node can transmit

        communication about channel sharing must use channel itself o no out-of-band channel for coordination

        5 DataLink Layer 5-20

        Ideal Multiple Access Protocol

        Broadcast channel of rate R bps1 when one node wants to transmit it can send

        at the full rate say R2 when M nodes want to transmit each can

        send at average rate RM3 fully decentralized

        o no special node to coordinate transmissionso no synchronization of clocks slots

        4 simple

        5 DataLink Layer 5-21

        MAC Protocols a taxonomy

        Three broad classes Channel Partitioning

        o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

        o allocate piece to node for exclusive use

        Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

        ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

        take longer turns

        5 DataLink Layer 5-22

        Channel Partitioning MAC protocols TDMA

        TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

        pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

        256 idle

        1 3 4 1 3 4

        6-slotframe

        5 DataLink Layer 5-23

        Channel Partitioning MAC protocols FDMA

        FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

        idle example 6-station LAN 134 have pkt

        frequency bands 256 idle fr

        equ

        ency

        bands time

        FDM cable

        5 DataLink Layer 5-24

        Random Access Protocols

        When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

        two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

        o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

        retransmissions)

        Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

        5 DataLink Layer 5-25

        ALOHA

        When a node has a frame to send send immediately

        Set a timer for a random amount of time If an ACK arrives before the timer expires

        fine otherwise resend the frame

        (Works like stop-and-wait with random timeout interval)

        5 DataLink Layer 5-26

        CSMA (Carrier Sense Multiple Access)

        CSMA listen before transmit

        If channel sensed idle transmit entire frame

        If channel sensed busy defer transmission

        human analogy donrsquot interrupt others

        5 DataLink Layer 5-27

        CSMA collisions

        collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

        spatial layout of nodes

        5 DataLink Layer 5-28

        CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

        o collisions detected within short timeo colliding transmissions aborted reducing

        channel wastage collision detection

        o easy in wired LANs measure signal strengths compare transmitted amp received signals

        o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

        human analogy the polite conversationalist

        5 DataLink Layer 5-29

        CSMACD collision detection

        5 DataLink Layer 5-30

        ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

        o share channel efficiently and fairly at high load

        o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

        Random access MAC protocolso efficient at low load single node can fully

        utilize channelo high load collision overhead

        ldquotaking turnsrdquo protocolslook for best of both worlds

        5 DataLink Layer 5-31

        ldquoTaking Turnsrdquo MAC protocolsPolling master node

        ldquoinvitesrdquo slave nodes to transmit in turn

        typically used with ldquodumbrdquo slave devices

        concernso polling overhead o latencyo single point of

        failure (master)

        master

        slaves

        poll

        data

        data

        5 DataLink Layer 5-32

        ldquoTaking Turnsrdquo MAC protocolsToken passing control token

        passed from one node to next sequentially

        token message concerns

        o token overhead o latencyo single point of failure

        (token)

        T

        data

        (nothingto send)

        T

        5 DataLink Layer 5-33

        Summary of MAC protocols

        channel partitioning by time frequency or codeo Time Division Frequency Division

        random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

        hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

        taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

        5 DataLink Layer 5-34

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53Multiple access protocols

        54 Link-Layer Addressing

        55 Ethernet

        56 Link-layer switches 59 A day in the life of

        a web request

        5 DataLink Layer 5-35

        MAC Addresses and ARP

        32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

        MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

        another physically-connected interface (in same network)

        o 48 bit MAC addressbull burned in NIC ROM

        5 DataLink Layer 5-36

        LAN Addresses and ARPEach adapter on LAN has unique LAN address

        Broadcast address =FF-FF-FF-FF-FF-FF

        = adapter

        1A-2F-BB-76-09-AD

        58-23-D7-FA-20-B0

        0C-C4-11-6F-E3-98

        71-65-F7-2B-08-53

        LAN(wired orwireless)

        5 DataLink Layer 5-37

        LAN Address (more)

        MAC address allocation administered by IEEE manufacturer buys portion of MAC address

        space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

        o can move LAN card from one LAN to another

        IP hierarchical address NOT portableo address depends on IP subnet to which node is

        attached

        5 DataLink Layer 5-38

        ARP Address Resolution Protocol

        Each IP node (host router) on LAN has ARP table

        ARP table IPMAC address mappings for some LAN nodes

        lt IP address MAC address TTLgto TTL (Time To Live)

        time after which address mapping will be forgotten (typically 20 min)

        Q given a nodersquos IP address how to determine its MAC address

        1A-2F-BB-76-09-AD

        0C-C4-11-6F-E3-98

        71-65-F7-2B-08-53

        LAN

        137196723

        137196778

        137196714

        137196788

        5 DataLink Layer 5-39

        ARP protocol Same LAN (network) A wants to send datagram

        to B and Brsquos MAC address not in Arsquos ARP table

        A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

        FF-FF-FF-FF-FFo all machines on LAN

        receive ARP query B receives ARP packet

        replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

        address (unicast)

        A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

        ARP is ldquoplug-and-playrdquoo nodes create their

        ARP tables without intervention from net administrator

        5 DataLink Layer 5-40

        Addressing routing to another LAN

        R

        1A-23-F9-CD-06-9B

        222222222220111111111110

        E6-E9-00-17-BB-4B

        CC-49-DE-D0-AB-7D

        111111111112

        111111111111

        A74-29-9C-E8-FF-55

        222222222221

        88-B2-2F-54-1A-0F

        B222222222222

        49-BD-D2-C7-56-2A

        walkthrough send datagram from A to B via R assume A knows Brsquos IP address

        two ARP tables in router R one for each IP network (LAN)

        LAN LAN

        5 DataLink Layer 5-41

        A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

        frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

        destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

        R

        1A-23-F9-CD-06-9B

        222222222220

        111111111110

        E6-E9-00-17-BB-4B

        CC-49-DE-D0-AB-7D

        111111111112

        111111111111

        A74-29-9C-E8-FF-55

        222222222221

        88-B2-2F-54-1A-0F

        B222222222222

        49-BD-D2-C7-56-2A

        This is a really importantexample ndash make sure youunderstand

        5 DataLink Layer 5-42

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53Multiple access protocols

        54 Link-Layer Addressing

        55 Ethernet

        56 Link-layer switches 59 A day in the life of

        a web request

        5 DataLink Layer 5-43

        Ethernet

        ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

        Metcalfersquos Ethernetsketch

        5 DataLink Layer 5-44

        Star topology bus topology popular through mid 90s

        o all nodes in same collision domain (can collide with each other)

        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

        (nodes do not collide with each other)

        switch

        bus coaxial cable star

        5 DataLink Layer 5-45

        Ethernet Frame Structure

        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

        Preamble 7 bytes with pattern 10101010 followed by one

        byte with pattern 10101011 used to synchronize receiver amp sender clock

        rates

        5 DataLink Layer 5-46

        Ethernet Frame Structure (more) Addresses 6 bytes

        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

        o otherwise adapter discards frame

        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

        CRC checked at receiver if error is detected frame is dropped

        5 DataLink Layer 5-47

        Ethernet Unreliable connectionless connectionless No handshaking between sending

        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

        Ethernetrsquos MAC protocol CSMACD

        5 DataLink Layer 5-48

        Ethernet CSMACD algorithm

        1 NIC receives datagram from network layer creates frame

        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

        4 If NIC detects collision while transmitting aborts and sends jam signal

        5 After aborting NIC enters exponential backoff

        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

        (1 slot = 512 bit-times)

        5 DataLink Layer 5-49

        Ethernetrsquos CSMACD (more)

        Jam Signal make sure all other transmitters are aware of collision 48 bits

        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

        Exponential Backoff Goal adapt retransmission

        attempts to estimated current loado heavy load random

        wait will be longer first collision choose K

        from 01 delay is K 512 bit transmission times

        after second collision choose K from 0123hellip

        after ten collisions choose K from 01234hellip1023

        5 DataLink Layer 5-50

        8023 Ethernet Standards Link amp Physical Layers

        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

        Mbps 1Gbps 10G bpso different physical layer media fiber cable

        applicationtransportnetwork

        linkphysical

        MAC protocoland frame format

        100BASE-TX

        100BASE-T4

        100BASE-FX100BASE-T2

        100BASE-SX 100BASE-BX

        fiber physical layercopper (twisterpair) physical layer

        5 DataLink Layer 5-51

        Manchester encoding

        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

        synchronize to each othero no need for a centralized global clock among nodes

        Hey this is physical-layer stuff

        5 DataLink Layer 5-52

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53 Multiple access protocols

        54 Link-layer Addressing

        55 Ethernet

        56 Link-layer switches LANs

        59 A day in the life of a web request

        5 DataLink Layer 5-53

        Hubsphysical-layer (ldquodumbrdquo) repeaters

        o bits coming in one link go out all other links at same rate

        o all nodes connected to hub can collide with one another

        o no frame bufferingo no CSMACD at hub host NICs detect collisions

        twisted pair

        hub

        5 DataLink Layer 5-54

        Switch link-layer device smarter than hubs take active

        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

        transparento hosts are unaware of presence of switches

        plug-and-play self-learningo switches do not need to be configured

        5 DataLink Layer 5-55

        Switch allows multiple simultaneous transmissions

        hosts have dedicated direct connection to switch

        switches buffer packets Ethernet protocol used on

        each incoming link but no collisions full duplexo each link is its own collision

        domain switching A-to-Arsquo and B-

        to-Brsquo simultaneously without collisions o not possible with dumb hub

        A

        Arsquo

        B

        Brsquo

        C

        Crsquo

        switch with six interfaces(123456)

        1 23

        45

        6

        5 DataLink Layer 5-56

        Switch Table

        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

        A each switch has a switch table each entryo (MAC address of host interface

        to reach host time stamp)

        looks like a routing table Q how are entries created

        maintained in switch table o something like a routing

        protocol

        A

        Arsquo

        B

        Brsquo

        C

        Crsquo

        switch with six interfaces(123456)

        1 23

        45

        6

        5 DataLink Layer 5-57

        Switch self-learning

        switch learns which hosts can be reached through which interfaceso when frame received

        switch ldquolearnsrdquo location of sender incoming LAN segment

        o records senderlocation pair in switch table

        A

        Arsquo

        B

        Brsquo

        C

        Crsquo

        1 23

        45

        6

        A Arsquo

        Source ADest Arsquo

        MAC addr interface TTL

        Switch table (initially empty)

        A 1 60

        5 DataLink Layer 5-58

        Switch frame filteringforwardingWhen frame received

        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

        then if dest on segment from which frame arrived

        then drop the frame else forward the frame on interface indicated else flood

        forward on all but the interface on which the frame arrived

        5 DataLink Layer 5-59

        Self-learning forwarding example

        A

        Arsquo

        B

        Brsquo

        C

        Crsquo

        1 23

        45

        6

        A Arsquo

        Source ADest Arsquo

        MAC addr interface TTL

        Switch table (initially empty)

        A 1 60

        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

        frame destination unknownflood

        Arsquo A

        destination A location known

        Arsquo 4 60

        selective send

        5 DataLink Layer 5-60

        Interconnecting switches

        switches can be connected together

        A

        B

        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

        A self learning (works exactly the same as in single-switch case)

        S1

        C D

        E

        FS2

        S4

        S3

        H

        I

        G

        5 DataLink Layer 5-61

        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

        Q show switch tables and packet forwarding in S1 S2 S3 S4

        A

        B

        S1

        C D

        E

        FS2

        S4

        S3

        H

        I

        G

        1

        2 3

        34

        5 DataLink Layer 5-62

        Institutional network

        to externalnetwork

        router

        IP subnet

        mail server

        web server

        5 DataLink Layer 5-63

        Switches vs Routers both store-and-forward devices

        o routers network layer devices (examine network layer headers)o switches are link layer devices

        routers maintain routing tables implement routing algorithms

        switches maintain switch tables implement filtering learning algorithms

        Switch

        5 DataLink Layer 5-64

        Link Layer

        51 Introduction and services

        52 Error detection and correction

        53Multiple access protocols

        54 Link-Layer Addressing

        55 Ethernet

        56 Link-layer switches 59 A day in the life of

        a web request

        5 DataLink Layer 5-65

        Synthesis a day in the life of a web request

        journey down protocol stack completeo application transport network link

        putting-it-all-together synthesiso goal identify review understand protocols

        (at all layers) involved in seemingly simple scenario requesting www page

        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

        5 DataLink Layer 5-66

        A day in the life scenario

        Comcast network 68800013

        Googlersquos network 64233160019 64233169105

        web server

        DNS server

        school network 68802024

        browser

        web page

        5 DataLink Layer 5-67

        A day in the lifehellip connecting to the Internet

        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

        router(runs DHCP)

        DHCPUDP

        IPEthPhy

        DHCP

        DHCP

        DHCP

        DHCP

        DHCP

        DHCPUDP

        IPEthPhy

        DHCP

        DHCP

        DHCP

        DHCPDHCP

        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

        5 DataLink Layer 5-68

        A day in the lifehellip connecting to the Internet

        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

        router(runs DHCP)

        DHCPUDP

        IPEthPhy

        DHCP

        DHCP

        DHCP

        DHCP

        DHCPUDP

        IPEthPhy

        DHCP

        DHCP

        DHCP

        DHCP

        DHCP

        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

        DHCP client receives DHCP ACK reply

        5 DataLink Layer 5-69

        A day in the lifehellip ARP (before DNS before HTTP)

        before sending HTTP request need IP address of wwwgooglecom DNS

        DNSUDP

        IPEthPhy

        DNS

        DNS

        DNS

        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

        ARP query

        EthPhy

        ARP

        ARP

        ARP reply

        5 DataLink Layer 5-70

        A day in the lifehellip using DNS

        DNSUDP

        IPEthPhy

        DNS

        DNS

        DNS

        DNS

        DNS

        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

        demuxrsquoed to DNS server DNS server replies to

        client with IP address of wwwgooglecom

        Comcast network 68800013

        DNS server

        DNSUDP

        IPEthPhy

        DNS

        DNS

        DNS

        DNS

        5 DataLink Layer 5-71

        A day in the lifehellip TCP connection carrying HTTP

        HTTPTCPIP

        EthPhy

        HTTP

        to send HTTP request client first opens TCP socket to web server

        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

        TCP connection established

        64233169105

        web server

        SYN

        SYN

        SYN

        SYN

        TCPIP

        EthPhy

        SYN

        SYN

        SYN

        SYNACK

        SYNACK

        SYNACK

        SYNACK

        SYNACK

        SYNACK

        SYNACK

        web server responds with TCP SYNACK (step 2 in 3-way handshake)

        5 DataLink Layer 5-72

        A day in the lifehellip HTTP requestreply

        HTTPTCPIP

        EthPhy

        HTTP

        HTTP request sent into TCP socket

        IP datagram containing HTTP request routed to wwwgooglecom

        IP datgram containing HTTP reply routed back to client

        64233169105

        web server

        HTTPTCPIP

        EthPhy

        web server responds with HTTP reply (containing web page)

        HTTP

        HTTP

        HTTPHTTP

        HTTP

        HTTP

        HTTP

        HTTP

        HTTP

        HTTP

        HTTP

        HTTP

        HTTP

        web page finally () displayed

        5 DataLink Layer 5-73

        Chapter 5 letrsquos take a breath journey down protocol stack complete

        (except PHY) solid understanding of networking

        principles practice hellip could stop here hellip but lots of

        interesting topicso Internetworking (CSE 678 TCPIP socket

        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

        • Link Layer
        • Link Layer Introduction
        • Link layer context
        • Encapsulation
        • Link Layer Services
        • Link Layer Services (more)
        • Where is the link layer implemented
        • Adaptors Communicating
        • Slide 9
        • Parity Checking
        • Internet checksum (review)
        • Checksumming Cyclic Redundancy Check
        • Cyclic Redundancy Check
        • Slide 14
        • CRC Example
        • Slide 16
        • Multiple Access Links and Protocols
        • Cable Network Architecture Overview
        • Multiple Access protocols
        • Ideal Multiple Access Protocol
        • MAC Protocols a taxonomy
        • Channel Partitioning MAC protocols TDMA
        • Channel Partitioning MAC protocols FDMA
        • Random Access Protocols
        • ALOHA
        • CSMA (Carrier Sense Multiple Access)
        • CSMA collisions
        • CSMACD (Collision Detection)
        • CSMACD collision detection
        • ldquoTaking Turnsrdquo MAC protocols
        • Slide 31
        • Slide 32
        • Summary of MAC protocols
        • Slide 34
        • MAC Addresses and ARP
        • LAN Addresses and ARP
        • LAN Address (more)
        • ARP Address Resolution Protocol
        • ARP protocol Same LAN (network)
        • Addressing routing to another LAN
        • Slide 41
        • Slide 42
        • Ethernet
        • Star topology
        • Ethernet Frame Structure
        • Ethernet Frame Structure (more)
        • Ethernet Unreliable connectionless
        • Ethernet CSMACD algorithm
        • Ethernetrsquos CSMACD (more)
        • 8023 Ethernet Standards Link amp Physical Layers
        • Manchester encoding
        • Slide 52
        • Hubs
        • Switch
        • Switch allows multiple simultaneous transmissions
        • Switch Table
        • Switch self-learning
        • Switch frame filteringforwarding
        • Self-learning forwarding example
        • Interconnecting switches
        • Self-learning multi-switch example
        • Institutional network
        • Switches vs Routers
        • Slide 64
        • Synthesis a day in the life of a web request
        • A day in the life scenario
        • A day in the lifehellip connecting to the Internet
        • Slide 68
        • A day in the lifehellip ARP (before DNS before HTTP)
        • A day in the lifehellip using DNS
        • A day in the lifehellip TCP connection carrying HTTP
        • A day in the lifehellip HTTP requestreply
        • Chapter 5 letrsquos take a breath

          5 DataLink Layer 5-5

          Link Layer Services framing

          encapsulate datagram into frame adding header trailer

          link access o channel access if shared mediumo ldquoMACrdquo addresses used in frame headers to identify

          source dest bull different from IP address

          reliable delivery between adjacent nodeso we learned how to do this already (chapter 3)o seldom used on low bit-error link (fiber some twisted

          pair)o wireless links high error ratesbull Q why both link-level and end-end reliability

          5 DataLink Layer 5-6

          Link Layer Services (more)

          flow control o pacing between adjacent sending and receiving

          nodes

          error detection o errors caused by signal attenuation noise o receiver detects presence of errors

          bull signals sender for retransmission or drops frame

          error correction o receiver identifies and corrects bit error(s) without

          resorting to retransmission

          5 DataLink Layer 5-7

          Where is the link layer implemented

          in each and every host link layer implemented in

          ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

          80211 cardo implements link physical

          layer

          attaches into hostrsquos system buses

          combination of hardware software firmware

          controller

          physicaltransmission

          cpu memory

          host bus (eg PCI)

          network adaptercard

          host schematic

          applicationtransportnetwork

          link

          linkphysical

          5 DataLink Layer 5-8

          Adaptors Communicating

          sending sideo encapsulates datagram

          in frameo adds error checking bits

          rdt flow control etc

          receiving sideo looks for errors rdt flow

          control etco extracts datagram passes

          to upper layer at receiving side

          controller controller

          sending host receiving host

          datagram datagram

          datagram

          frame

          5 DataLink Layer 5-9

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53Multiple access protocols

          54 Link-layer Addressing

          55 Ethernet

          56 Link-layer switches 59 A day in the life of

          a web request

          5 DataLink Layer 5-10

          Parity Checking

          Single Bit ParityDetect single bit errors

          Two Dimensional Bit ParityDetect and correct single bit errors

          0 0

          5 DataLink Layer 5-11

          Internet checksum (review)

          Sender treat segment contents

          as sequence of 16-bit integers

          checksum addition (1rsquos complement sum) of segment contents

          sender puts checksum value into UDP checksum field

          Receiver compute checksum of

          received segment check if computed

          checksum equals checksum field valueo NO - error detectedo YES - no error detected

          But maybe errors nonetheless

          Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

          5 DataLink Layer 5-12

          Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

          polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

          o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

          remainder error detectedo can detect all burst errors less than r+1 bits

          widely used in practice (Ethernet 80211 WiFi ATM)

          Cyclic Redundancy Check Modulo 2 arithmetic

          o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

          A polynomial G(x) of degree r is known to both sender and receiver

          Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

          Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

          If not there are transmission errors in the frame

          7 4 3( ) 1D x x x x x

          Common polynomials for G(x)

          CRC

          CRC-8

          CRC-10

          CRC-12

          CRC-16

          CRC-CCITT

          CRC-32

          C(x)

          x8+x2+x1+1

          x10+x9+x5+x4+x1+1

          x12+x11+x3+x2+x1+1

          x16+x15+x2+1

          x16+x12+x5+1

          x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

          5 DataLink Layer 5-15

          CRC Example

          31001 represents 1x

          5 DataLink Layer 5-16

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53Multiple access protocols

          54 Link-layer Addressing

          55 Ethernet

          56 Link-layer switches 59 A day in the life of

          a web request

          5 DataLink Layer 5-17

          Multiple Access Links and Protocols

          Two types of ldquolinksrdquo point-to-point

          o PPP for dial-up accesso point-to-point link between Ethernet switch and host

          broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

          shared wire (eg cabled Ethernet)

          shared RF (eg 80211 WiFi)

          shared RF(satellite)

          humans at acocktail party

          (shared air acoustical)

          Introduction 1-18

          Cable Network Architecture Overview

          home

          cable headend

          cable distributionnetwork (simplified)

          Typically 500 to 5000 homes

          5 DataLink Layer 5-19

          Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

          interference o collision if node receives two or more signals at the same

          time

          multiple access protocol distributed algorithm that determines how nodes

          share channel ie determine when node can transmit

          communication about channel sharing must use channel itself o no out-of-band channel for coordination

          5 DataLink Layer 5-20

          Ideal Multiple Access Protocol

          Broadcast channel of rate R bps1 when one node wants to transmit it can send

          at the full rate say R2 when M nodes want to transmit each can

          send at average rate RM3 fully decentralized

          o no special node to coordinate transmissionso no synchronization of clocks slots

          4 simple

          5 DataLink Layer 5-21

          MAC Protocols a taxonomy

          Three broad classes Channel Partitioning

          o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

          o allocate piece to node for exclusive use

          Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

          ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

          take longer turns

          5 DataLink Layer 5-22

          Channel Partitioning MAC protocols TDMA

          TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

          pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

          256 idle

          1 3 4 1 3 4

          6-slotframe

          5 DataLink Layer 5-23

          Channel Partitioning MAC protocols FDMA

          FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

          idle example 6-station LAN 134 have pkt

          frequency bands 256 idle fr

          equ

          ency

          bands time

          FDM cable

          5 DataLink Layer 5-24

          Random Access Protocols

          When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

          two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

          o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

          retransmissions)

          Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

          5 DataLink Layer 5-25

          ALOHA

          When a node has a frame to send send immediately

          Set a timer for a random amount of time If an ACK arrives before the timer expires

          fine otherwise resend the frame

          (Works like stop-and-wait with random timeout interval)

          5 DataLink Layer 5-26

          CSMA (Carrier Sense Multiple Access)

          CSMA listen before transmit

          If channel sensed idle transmit entire frame

          If channel sensed busy defer transmission

          human analogy donrsquot interrupt others

          5 DataLink Layer 5-27

          CSMA collisions

          collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

          spatial layout of nodes

          5 DataLink Layer 5-28

          CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

          o collisions detected within short timeo colliding transmissions aborted reducing

          channel wastage collision detection

          o easy in wired LANs measure signal strengths compare transmitted amp received signals

          o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

          human analogy the polite conversationalist

          5 DataLink Layer 5-29

          CSMACD collision detection

          5 DataLink Layer 5-30

          ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

          o share channel efficiently and fairly at high load

          o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

          Random access MAC protocolso efficient at low load single node can fully

          utilize channelo high load collision overhead

          ldquotaking turnsrdquo protocolslook for best of both worlds

          5 DataLink Layer 5-31

          ldquoTaking Turnsrdquo MAC protocolsPolling master node

          ldquoinvitesrdquo slave nodes to transmit in turn

          typically used with ldquodumbrdquo slave devices

          concernso polling overhead o latencyo single point of

          failure (master)

          master

          slaves

          poll

          data

          data

          5 DataLink Layer 5-32

          ldquoTaking Turnsrdquo MAC protocolsToken passing control token

          passed from one node to next sequentially

          token message concerns

          o token overhead o latencyo single point of failure

          (token)

          T

          data

          (nothingto send)

          T

          5 DataLink Layer 5-33

          Summary of MAC protocols

          channel partitioning by time frequency or codeo Time Division Frequency Division

          random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

          hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

          taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

          5 DataLink Layer 5-34

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53Multiple access protocols

          54 Link-Layer Addressing

          55 Ethernet

          56 Link-layer switches 59 A day in the life of

          a web request

          5 DataLink Layer 5-35

          MAC Addresses and ARP

          32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

          MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

          another physically-connected interface (in same network)

          o 48 bit MAC addressbull burned in NIC ROM

          5 DataLink Layer 5-36

          LAN Addresses and ARPEach adapter on LAN has unique LAN address

          Broadcast address =FF-FF-FF-FF-FF-FF

          = adapter

          1A-2F-BB-76-09-AD

          58-23-D7-FA-20-B0

          0C-C4-11-6F-E3-98

          71-65-F7-2B-08-53

          LAN(wired orwireless)

          5 DataLink Layer 5-37

          LAN Address (more)

          MAC address allocation administered by IEEE manufacturer buys portion of MAC address

          space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

          o can move LAN card from one LAN to another

          IP hierarchical address NOT portableo address depends on IP subnet to which node is

          attached

          5 DataLink Layer 5-38

          ARP Address Resolution Protocol

          Each IP node (host router) on LAN has ARP table

          ARP table IPMAC address mappings for some LAN nodes

          lt IP address MAC address TTLgto TTL (Time To Live)

          time after which address mapping will be forgotten (typically 20 min)

          Q given a nodersquos IP address how to determine its MAC address

          1A-2F-BB-76-09-AD

          0C-C4-11-6F-E3-98

          71-65-F7-2B-08-53

          LAN

          137196723

          137196778

          137196714

          137196788

          5 DataLink Layer 5-39

          ARP protocol Same LAN (network) A wants to send datagram

          to B and Brsquos MAC address not in Arsquos ARP table

          A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

          FF-FF-FF-FF-FFo all machines on LAN

          receive ARP query B receives ARP packet

          replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

          address (unicast)

          A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

          ARP is ldquoplug-and-playrdquoo nodes create their

          ARP tables without intervention from net administrator

          5 DataLink Layer 5-40

          Addressing routing to another LAN

          R

          1A-23-F9-CD-06-9B

          222222222220111111111110

          E6-E9-00-17-BB-4B

          CC-49-DE-D0-AB-7D

          111111111112

          111111111111

          A74-29-9C-E8-FF-55

          222222222221

          88-B2-2F-54-1A-0F

          B222222222222

          49-BD-D2-C7-56-2A

          walkthrough send datagram from A to B via R assume A knows Brsquos IP address

          two ARP tables in router R one for each IP network (LAN)

          LAN LAN

          5 DataLink Layer 5-41

          A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

          frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

          destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

          R

          1A-23-F9-CD-06-9B

          222222222220

          111111111110

          E6-E9-00-17-BB-4B

          CC-49-DE-D0-AB-7D

          111111111112

          111111111111

          A74-29-9C-E8-FF-55

          222222222221

          88-B2-2F-54-1A-0F

          B222222222222

          49-BD-D2-C7-56-2A

          This is a really importantexample ndash make sure youunderstand

          5 DataLink Layer 5-42

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53Multiple access protocols

          54 Link-Layer Addressing

          55 Ethernet

          56 Link-layer switches 59 A day in the life of

          a web request

          5 DataLink Layer 5-43

          Ethernet

          ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

          Metcalfersquos Ethernetsketch

          5 DataLink Layer 5-44

          Star topology bus topology popular through mid 90s

          o all nodes in same collision domain (can collide with each other)

          today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

          (nodes do not collide with each other)

          switch

          bus coaxial cable star

          5 DataLink Layer 5-45

          Ethernet Frame Structure

          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

          Preamble 7 bytes with pattern 10101010 followed by one

          byte with pattern 10101011 used to synchronize receiver amp sender clock

          rates

          5 DataLink Layer 5-46

          Ethernet Frame Structure (more) Addresses 6 bytes

          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

          o otherwise adapter discards frame

          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

          CRC checked at receiver if error is detected frame is dropped

          5 DataLink Layer 5-47

          Ethernet Unreliable connectionless connectionless No handshaking between sending

          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

          Ethernetrsquos MAC protocol CSMACD

          5 DataLink Layer 5-48

          Ethernet CSMACD algorithm

          1 NIC receives datagram from network layer creates frame

          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

          4 If NIC detects collision while transmitting aborts and sends jam signal

          5 After aborting NIC enters exponential backoff

          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

          (1 slot = 512 bit-times)

          5 DataLink Layer 5-49

          Ethernetrsquos CSMACD (more)

          Jam Signal make sure all other transmitters are aware of collision 48 bits

          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

          Exponential Backoff Goal adapt retransmission

          attempts to estimated current loado heavy load random

          wait will be longer first collision choose K

          from 01 delay is K 512 bit transmission times

          after second collision choose K from 0123hellip

          after ten collisions choose K from 01234hellip1023

          5 DataLink Layer 5-50

          8023 Ethernet Standards Link amp Physical Layers

          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

          Mbps 1Gbps 10G bpso different physical layer media fiber cable

          applicationtransportnetwork

          linkphysical

          MAC protocoland frame format

          100BASE-TX

          100BASE-T4

          100BASE-FX100BASE-T2

          100BASE-SX 100BASE-BX

          fiber physical layercopper (twisterpair) physical layer

          5 DataLink Layer 5-51

          Manchester encoding

          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

          synchronize to each othero no need for a centralized global clock among nodes

          Hey this is physical-layer stuff

          5 DataLink Layer 5-52

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53 Multiple access protocols

          54 Link-layer Addressing

          55 Ethernet

          56 Link-layer switches LANs

          59 A day in the life of a web request

          5 DataLink Layer 5-53

          Hubsphysical-layer (ldquodumbrdquo) repeaters

          o bits coming in one link go out all other links at same rate

          o all nodes connected to hub can collide with one another

          o no frame bufferingo no CSMACD at hub host NICs detect collisions

          twisted pair

          hub

          5 DataLink Layer 5-54

          Switch link-layer device smarter than hubs take active

          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

          transparento hosts are unaware of presence of switches

          plug-and-play self-learningo switches do not need to be configured

          5 DataLink Layer 5-55

          Switch allows multiple simultaneous transmissions

          hosts have dedicated direct connection to switch

          switches buffer packets Ethernet protocol used on

          each incoming link but no collisions full duplexo each link is its own collision

          domain switching A-to-Arsquo and B-

          to-Brsquo simultaneously without collisions o not possible with dumb hub

          A

          Arsquo

          B

          Brsquo

          C

          Crsquo

          switch with six interfaces(123456)

          1 23

          45

          6

          5 DataLink Layer 5-56

          Switch Table

          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

          A each switch has a switch table each entryo (MAC address of host interface

          to reach host time stamp)

          looks like a routing table Q how are entries created

          maintained in switch table o something like a routing

          protocol

          A

          Arsquo

          B

          Brsquo

          C

          Crsquo

          switch with six interfaces(123456)

          1 23

          45

          6

          5 DataLink Layer 5-57

          Switch self-learning

          switch learns which hosts can be reached through which interfaceso when frame received

          switch ldquolearnsrdquo location of sender incoming LAN segment

          o records senderlocation pair in switch table

          A

          Arsquo

          B

          Brsquo

          C

          Crsquo

          1 23

          45

          6

          A Arsquo

          Source ADest Arsquo

          MAC addr interface TTL

          Switch table (initially empty)

          A 1 60

          5 DataLink Layer 5-58

          Switch frame filteringforwardingWhen frame received

          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

          then if dest on segment from which frame arrived

          then drop the frame else forward the frame on interface indicated else flood

          forward on all but the interface on which the frame arrived

          5 DataLink Layer 5-59

          Self-learning forwarding example

          A

          Arsquo

          B

          Brsquo

          C

          Crsquo

          1 23

          45

          6

          A Arsquo

          Source ADest Arsquo

          MAC addr interface TTL

          Switch table (initially empty)

          A 1 60

          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

          frame destination unknownflood

          Arsquo A

          destination A location known

          Arsquo 4 60

          selective send

          5 DataLink Layer 5-60

          Interconnecting switches

          switches can be connected together

          A

          B

          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

          A self learning (works exactly the same as in single-switch case)

          S1

          C D

          E

          FS2

          S4

          S3

          H

          I

          G

          5 DataLink Layer 5-61

          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

          Q show switch tables and packet forwarding in S1 S2 S3 S4

          A

          B

          S1

          C D

          E

          FS2

          S4

          S3

          H

          I

          G

          1

          2 3

          34

          5 DataLink Layer 5-62

          Institutional network

          to externalnetwork

          router

          IP subnet

          mail server

          web server

          5 DataLink Layer 5-63

          Switches vs Routers both store-and-forward devices

          o routers network layer devices (examine network layer headers)o switches are link layer devices

          routers maintain routing tables implement routing algorithms

          switches maintain switch tables implement filtering learning algorithms

          Switch

          5 DataLink Layer 5-64

          Link Layer

          51 Introduction and services

          52 Error detection and correction

          53Multiple access protocols

          54 Link-Layer Addressing

          55 Ethernet

          56 Link-layer switches 59 A day in the life of

          a web request

          5 DataLink Layer 5-65

          Synthesis a day in the life of a web request

          journey down protocol stack completeo application transport network link

          putting-it-all-together synthesiso goal identify review understand protocols

          (at all layers) involved in seemingly simple scenario requesting www page

          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

          5 DataLink Layer 5-66

          A day in the life scenario

          Comcast network 68800013

          Googlersquos network 64233160019 64233169105

          web server

          DNS server

          school network 68802024

          browser

          web page

          5 DataLink Layer 5-67

          A day in the lifehellip connecting to the Internet

          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

          router(runs DHCP)

          DHCPUDP

          IPEthPhy

          DHCP

          DHCP

          DHCP

          DHCP

          DHCP

          DHCPUDP

          IPEthPhy

          DHCP

          DHCP

          DHCP

          DHCPDHCP

          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

          5 DataLink Layer 5-68

          A day in the lifehellip connecting to the Internet

          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

          router(runs DHCP)

          DHCPUDP

          IPEthPhy

          DHCP

          DHCP

          DHCP

          DHCP

          DHCPUDP

          IPEthPhy

          DHCP

          DHCP

          DHCP

          DHCP

          DHCP

          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

          DHCP client receives DHCP ACK reply

          5 DataLink Layer 5-69

          A day in the lifehellip ARP (before DNS before HTTP)

          before sending HTTP request need IP address of wwwgooglecom DNS

          DNSUDP

          IPEthPhy

          DNS

          DNS

          DNS

          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

          ARP query

          EthPhy

          ARP

          ARP

          ARP reply

          5 DataLink Layer 5-70

          A day in the lifehellip using DNS

          DNSUDP

          IPEthPhy

          DNS

          DNS

          DNS

          DNS

          DNS

          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

          demuxrsquoed to DNS server DNS server replies to

          client with IP address of wwwgooglecom

          Comcast network 68800013

          DNS server

          DNSUDP

          IPEthPhy

          DNS

          DNS

          DNS

          DNS

          5 DataLink Layer 5-71

          A day in the lifehellip TCP connection carrying HTTP

          HTTPTCPIP

          EthPhy

          HTTP

          to send HTTP request client first opens TCP socket to web server

          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

          TCP connection established

          64233169105

          web server

          SYN

          SYN

          SYN

          SYN

          TCPIP

          EthPhy

          SYN

          SYN

          SYN

          SYNACK

          SYNACK

          SYNACK

          SYNACK

          SYNACK

          SYNACK

          SYNACK

          web server responds with TCP SYNACK (step 2 in 3-way handshake)

          5 DataLink Layer 5-72

          A day in the lifehellip HTTP requestreply

          HTTPTCPIP

          EthPhy

          HTTP

          HTTP request sent into TCP socket

          IP datagram containing HTTP request routed to wwwgooglecom

          IP datgram containing HTTP reply routed back to client

          64233169105

          web server

          HTTPTCPIP

          EthPhy

          web server responds with HTTP reply (containing web page)

          HTTP

          HTTP

          HTTPHTTP

          HTTP

          HTTP

          HTTP

          HTTP

          HTTP

          HTTP

          HTTP

          HTTP

          HTTP

          web page finally () displayed

          5 DataLink Layer 5-73

          Chapter 5 letrsquos take a breath journey down protocol stack complete

          (except PHY) solid understanding of networking

          principles practice hellip could stop here hellip but lots of

          interesting topicso Internetworking (CSE 678 TCPIP socket

          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

          • Link Layer
          • Link Layer Introduction
          • Link layer context
          • Encapsulation
          • Link Layer Services
          • Link Layer Services (more)
          • Where is the link layer implemented
          • Adaptors Communicating
          • Slide 9
          • Parity Checking
          • Internet checksum (review)
          • Checksumming Cyclic Redundancy Check
          • Cyclic Redundancy Check
          • Slide 14
          • CRC Example
          • Slide 16
          • Multiple Access Links and Protocols
          • Cable Network Architecture Overview
          • Multiple Access protocols
          • Ideal Multiple Access Protocol
          • MAC Protocols a taxonomy
          • Channel Partitioning MAC protocols TDMA
          • Channel Partitioning MAC protocols FDMA
          • Random Access Protocols
          • ALOHA
          • CSMA (Carrier Sense Multiple Access)
          • CSMA collisions
          • CSMACD (Collision Detection)
          • CSMACD collision detection
          • ldquoTaking Turnsrdquo MAC protocols
          • Slide 31
          • Slide 32
          • Summary of MAC protocols
          • Slide 34
          • MAC Addresses and ARP
          • LAN Addresses and ARP
          • LAN Address (more)
          • ARP Address Resolution Protocol
          • ARP protocol Same LAN (network)
          • Addressing routing to another LAN
          • Slide 41
          • Slide 42
          • Ethernet
          • Star topology
          • Ethernet Frame Structure
          • Ethernet Frame Structure (more)
          • Ethernet Unreliable connectionless
          • Ethernet CSMACD algorithm
          • Ethernetrsquos CSMACD (more)
          • 8023 Ethernet Standards Link amp Physical Layers
          • Manchester encoding
          • Slide 52
          • Hubs
          • Switch
          • Switch allows multiple simultaneous transmissions
          • Switch Table
          • Switch self-learning
          • Switch frame filteringforwarding
          • Self-learning forwarding example
          • Interconnecting switches
          • Self-learning multi-switch example
          • Institutional network
          • Switches vs Routers
          • Slide 64
          • Synthesis a day in the life of a web request
          • A day in the life scenario
          • A day in the lifehellip connecting to the Internet
          • Slide 68
          • A day in the lifehellip ARP (before DNS before HTTP)
          • A day in the lifehellip using DNS
          • A day in the lifehellip TCP connection carrying HTTP
          • A day in the lifehellip HTTP requestreply
          • Chapter 5 letrsquos take a breath

            5 DataLink Layer 5-6

            Link Layer Services (more)

            flow control o pacing between adjacent sending and receiving

            nodes

            error detection o errors caused by signal attenuation noise o receiver detects presence of errors

            bull signals sender for retransmission or drops frame

            error correction o receiver identifies and corrects bit error(s) without

            resorting to retransmission

            5 DataLink Layer 5-7

            Where is the link layer implemented

            in each and every host link layer implemented in

            ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

            80211 cardo implements link physical

            layer

            attaches into hostrsquos system buses

            combination of hardware software firmware

            controller

            physicaltransmission

            cpu memory

            host bus (eg PCI)

            network adaptercard

            host schematic

            applicationtransportnetwork

            link

            linkphysical

            5 DataLink Layer 5-8

            Adaptors Communicating

            sending sideo encapsulates datagram

            in frameo adds error checking bits

            rdt flow control etc

            receiving sideo looks for errors rdt flow

            control etco extracts datagram passes

            to upper layer at receiving side

            controller controller

            sending host receiving host

            datagram datagram

            datagram

            frame

            5 DataLink Layer 5-9

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53Multiple access protocols

            54 Link-layer Addressing

            55 Ethernet

            56 Link-layer switches 59 A day in the life of

            a web request

            5 DataLink Layer 5-10

            Parity Checking

            Single Bit ParityDetect single bit errors

            Two Dimensional Bit ParityDetect and correct single bit errors

            0 0

            5 DataLink Layer 5-11

            Internet checksum (review)

            Sender treat segment contents

            as sequence of 16-bit integers

            checksum addition (1rsquos complement sum) of segment contents

            sender puts checksum value into UDP checksum field

            Receiver compute checksum of

            received segment check if computed

            checksum equals checksum field valueo NO - error detectedo YES - no error detected

            But maybe errors nonetheless

            Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

            5 DataLink Layer 5-12

            Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

            polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

            o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

            remainder error detectedo can detect all burst errors less than r+1 bits

            widely used in practice (Ethernet 80211 WiFi ATM)

            Cyclic Redundancy Check Modulo 2 arithmetic

            o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

            A polynomial G(x) of degree r is known to both sender and receiver

            Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

            Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

            If not there are transmission errors in the frame

            7 4 3( ) 1D x x x x x

            Common polynomials for G(x)

            CRC

            CRC-8

            CRC-10

            CRC-12

            CRC-16

            CRC-CCITT

            CRC-32

            C(x)

            x8+x2+x1+1

            x10+x9+x5+x4+x1+1

            x12+x11+x3+x2+x1+1

            x16+x15+x2+1

            x16+x12+x5+1

            x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

            5 DataLink Layer 5-15

            CRC Example

            31001 represents 1x

            5 DataLink Layer 5-16

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53Multiple access protocols

            54 Link-layer Addressing

            55 Ethernet

            56 Link-layer switches 59 A day in the life of

            a web request

            5 DataLink Layer 5-17

            Multiple Access Links and Protocols

            Two types of ldquolinksrdquo point-to-point

            o PPP for dial-up accesso point-to-point link between Ethernet switch and host

            broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

            shared wire (eg cabled Ethernet)

            shared RF (eg 80211 WiFi)

            shared RF(satellite)

            humans at acocktail party

            (shared air acoustical)

            Introduction 1-18

            Cable Network Architecture Overview

            home

            cable headend

            cable distributionnetwork (simplified)

            Typically 500 to 5000 homes

            5 DataLink Layer 5-19

            Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

            interference o collision if node receives two or more signals at the same

            time

            multiple access protocol distributed algorithm that determines how nodes

            share channel ie determine when node can transmit

            communication about channel sharing must use channel itself o no out-of-band channel for coordination

            5 DataLink Layer 5-20

            Ideal Multiple Access Protocol

            Broadcast channel of rate R bps1 when one node wants to transmit it can send

            at the full rate say R2 when M nodes want to transmit each can

            send at average rate RM3 fully decentralized

            o no special node to coordinate transmissionso no synchronization of clocks slots

            4 simple

            5 DataLink Layer 5-21

            MAC Protocols a taxonomy

            Three broad classes Channel Partitioning

            o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

            o allocate piece to node for exclusive use

            Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

            ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

            take longer turns

            5 DataLink Layer 5-22

            Channel Partitioning MAC protocols TDMA

            TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

            pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

            256 idle

            1 3 4 1 3 4

            6-slotframe

            5 DataLink Layer 5-23

            Channel Partitioning MAC protocols FDMA

            FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

            idle example 6-station LAN 134 have pkt

            frequency bands 256 idle fr

            equ

            ency

            bands time

            FDM cable

            5 DataLink Layer 5-24

            Random Access Protocols

            When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

            two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

            o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

            retransmissions)

            Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

            5 DataLink Layer 5-25

            ALOHA

            When a node has a frame to send send immediately

            Set a timer for a random amount of time If an ACK arrives before the timer expires

            fine otherwise resend the frame

            (Works like stop-and-wait with random timeout interval)

            5 DataLink Layer 5-26

            CSMA (Carrier Sense Multiple Access)

            CSMA listen before transmit

            If channel sensed idle transmit entire frame

            If channel sensed busy defer transmission

            human analogy donrsquot interrupt others

            5 DataLink Layer 5-27

            CSMA collisions

            collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

            spatial layout of nodes

            5 DataLink Layer 5-28

            CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

            o collisions detected within short timeo colliding transmissions aborted reducing

            channel wastage collision detection

            o easy in wired LANs measure signal strengths compare transmitted amp received signals

            o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

            human analogy the polite conversationalist

            5 DataLink Layer 5-29

            CSMACD collision detection

            5 DataLink Layer 5-30

            ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

            o share channel efficiently and fairly at high load

            o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

            Random access MAC protocolso efficient at low load single node can fully

            utilize channelo high load collision overhead

            ldquotaking turnsrdquo protocolslook for best of both worlds

            5 DataLink Layer 5-31

            ldquoTaking Turnsrdquo MAC protocolsPolling master node

            ldquoinvitesrdquo slave nodes to transmit in turn

            typically used with ldquodumbrdquo slave devices

            concernso polling overhead o latencyo single point of

            failure (master)

            master

            slaves

            poll

            data

            data

            5 DataLink Layer 5-32

            ldquoTaking Turnsrdquo MAC protocolsToken passing control token

            passed from one node to next sequentially

            token message concerns

            o token overhead o latencyo single point of failure

            (token)

            T

            data

            (nothingto send)

            T

            5 DataLink Layer 5-33

            Summary of MAC protocols

            channel partitioning by time frequency or codeo Time Division Frequency Division

            random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

            hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

            taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

            5 DataLink Layer 5-34

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53Multiple access protocols

            54 Link-Layer Addressing

            55 Ethernet

            56 Link-layer switches 59 A day in the life of

            a web request

            5 DataLink Layer 5-35

            MAC Addresses and ARP

            32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

            MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

            another physically-connected interface (in same network)

            o 48 bit MAC addressbull burned in NIC ROM

            5 DataLink Layer 5-36

            LAN Addresses and ARPEach adapter on LAN has unique LAN address

            Broadcast address =FF-FF-FF-FF-FF-FF

            = adapter

            1A-2F-BB-76-09-AD

            58-23-D7-FA-20-B0

            0C-C4-11-6F-E3-98

            71-65-F7-2B-08-53

            LAN(wired orwireless)

            5 DataLink Layer 5-37

            LAN Address (more)

            MAC address allocation administered by IEEE manufacturer buys portion of MAC address

            space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

            o can move LAN card from one LAN to another

            IP hierarchical address NOT portableo address depends on IP subnet to which node is

            attached

            5 DataLink Layer 5-38

            ARP Address Resolution Protocol

            Each IP node (host router) on LAN has ARP table

            ARP table IPMAC address mappings for some LAN nodes

            lt IP address MAC address TTLgto TTL (Time To Live)

            time after which address mapping will be forgotten (typically 20 min)

            Q given a nodersquos IP address how to determine its MAC address

            1A-2F-BB-76-09-AD

            0C-C4-11-6F-E3-98

            71-65-F7-2B-08-53

            LAN

            137196723

            137196778

            137196714

            137196788

            5 DataLink Layer 5-39

            ARP protocol Same LAN (network) A wants to send datagram

            to B and Brsquos MAC address not in Arsquos ARP table

            A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

            FF-FF-FF-FF-FFo all machines on LAN

            receive ARP query B receives ARP packet

            replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

            address (unicast)

            A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

            ARP is ldquoplug-and-playrdquoo nodes create their

            ARP tables without intervention from net administrator

            5 DataLink Layer 5-40

            Addressing routing to another LAN

            R

            1A-23-F9-CD-06-9B

            222222222220111111111110

            E6-E9-00-17-BB-4B

            CC-49-DE-D0-AB-7D

            111111111112

            111111111111

            A74-29-9C-E8-FF-55

            222222222221

            88-B2-2F-54-1A-0F

            B222222222222

            49-BD-D2-C7-56-2A

            walkthrough send datagram from A to B via R assume A knows Brsquos IP address

            two ARP tables in router R one for each IP network (LAN)

            LAN LAN

            5 DataLink Layer 5-41

            A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

            frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

            destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

            R

            1A-23-F9-CD-06-9B

            222222222220

            111111111110

            E6-E9-00-17-BB-4B

            CC-49-DE-D0-AB-7D

            111111111112

            111111111111

            A74-29-9C-E8-FF-55

            222222222221

            88-B2-2F-54-1A-0F

            B222222222222

            49-BD-D2-C7-56-2A

            This is a really importantexample ndash make sure youunderstand

            5 DataLink Layer 5-42

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53Multiple access protocols

            54 Link-Layer Addressing

            55 Ethernet

            56 Link-layer switches 59 A day in the life of

            a web request

            5 DataLink Layer 5-43

            Ethernet

            ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

            Metcalfersquos Ethernetsketch

            5 DataLink Layer 5-44

            Star topology bus topology popular through mid 90s

            o all nodes in same collision domain (can collide with each other)

            today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

            (nodes do not collide with each other)

            switch

            bus coaxial cable star

            5 DataLink Layer 5-45

            Ethernet Frame Structure

            Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

            Preamble 7 bytes with pattern 10101010 followed by one

            byte with pattern 10101011 used to synchronize receiver amp sender clock

            rates

            5 DataLink Layer 5-46

            Ethernet Frame Structure (more) Addresses 6 bytes

            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

            o otherwise adapter discards frame

            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

            CRC checked at receiver if error is detected frame is dropped

            5 DataLink Layer 5-47

            Ethernet Unreliable connectionless connectionless No handshaking between sending

            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

            Ethernetrsquos MAC protocol CSMACD

            5 DataLink Layer 5-48

            Ethernet CSMACD algorithm

            1 NIC receives datagram from network layer creates frame

            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

            4 If NIC detects collision while transmitting aborts and sends jam signal

            5 After aborting NIC enters exponential backoff

            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

            (1 slot = 512 bit-times)

            5 DataLink Layer 5-49

            Ethernetrsquos CSMACD (more)

            Jam Signal make sure all other transmitters are aware of collision 48 bits

            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

            Exponential Backoff Goal adapt retransmission

            attempts to estimated current loado heavy load random

            wait will be longer first collision choose K

            from 01 delay is K 512 bit transmission times

            after second collision choose K from 0123hellip

            after ten collisions choose K from 01234hellip1023

            5 DataLink Layer 5-50

            8023 Ethernet Standards Link amp Physical Layers

            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

            Mbps 1Gbps 10G bpso different physical layer media fiber cable

            applicationtransportnetwork

            linkphysical

            MAC protocoland frame format

            100BASE-TX

            100BASE-T4

            100BASE-FX100BASE-T2

            100BASE-SX 100BASE-BX

            fiber physical layercopper (twisterpair) physical layer

            5 DataLink Layer 5-51

            Manchester encoding

            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

            synchronize to each othero no need for a centralized global clock among nodes

            Hey this is physical-layer stuff

            5 DataLink Layer 5-52

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53 Multiple access protocols

            54 Link-layer Addressing

            55 Ethernet

            56 Link-layer switches LANs

            59 A day in the life of a web request

            5 DataLink Layer 5-53

            Hubsphysical-layer (ldquodumbrdquo) repeaters

            o bits coming in one link go out all other links at same rate

            o all nodes connected to hub can collide with one another

            o no frame bufferingo no CSMACD at hub host NICs detect collisions

            twisted pair

            hub

            5 DataLink Layer 5-54

            Switch link-layer device smarter than hubs take active

            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

            transparento hosts are unaware of presence of switches

            plug-and-play self-learningo switches do not need to be configured

            5 DataLink Layer 5-55

            Switch allows multiple simultaneous transmissions

            hosts have dedicated direct connection to switch

            switches buffer packets Ethernet protocol used on

            each incoming link but no collisions full duplexo each link is its own collision

            domain switching A-to-Arsquo and B-

            to-Brsquo simultaneously without collisions o not possible with dumb hub

            A

            Arsquo

            B

            Brsquo

            C

            Crsquo

            switch with six interfaces(123456)

            1 23

            45

            6

            5 DataLink Layer 5-56

            Switch Table

            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

            A each switch has a switch table each entryo (MAC address of host interface

            to reach host time stamp)

            looks like a routing table Q how are entries created

            maintained in switch table o something like a routing

            protocol

            A

            Arsquo

            B

            Brsquo

            C

            Crsquo

            switch with six interfaces(123456)

            1 23

            45

            6

            5 DataLink Layer 5-57

            Switch self-learning

            switch learns which hosts can be reached through which interfaceso when frame received

            switch ldquolearnsrdquo location of sender incoming LAN segment

            o records senderlocation pair in switch table

            A

            Arsquo

            B

            Brsquo

            C

            Crsquo

            1 23

            45

            6

            A Arsquo

            Source ADest Arsquo

            MAC addr interface TTL

            Switch table (initially empty)

            A 1 60

            5 DataLink Layer 5-58

            Switch frame filteringforwardingWhen frame received

            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

            then if dest on segment from which frame arrived

            then drop the frame else forward the frame on interface indicated else flood

            forward on all but the interface on which the frame arrived

            5 DataLink Layer 5-59

            Self-learning forwarding example

            A

            Arsquo

            B

            Brsquo

            C

            Crsquo

            1 23

            45

            6

            A Arsquo

            Source ADest Arsquo

            MAC addr interface TTL

            Switch table (initially empty)

            A 1 60

            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

            frame destination unknownflood

            Arsquo A

            destination A location known

            Arsquo 4 60

            selective send

            5 DataLink Layer 5-60

            Interconnecting switches

            switches can be connected together

            A

            B

            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

            A self learning (works exactly the same as in single-switch case)

            S1

            C D

            E

            FS2

            S4

            S3

            H

            I

            G

            5 DataLink Layer 5-61

            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

            Q show switch tables and packet forwarding in S1 S2 S3 S4

            A

            B

            S1

            C D

            E

            FS2

            S4

            S3

            H

            I

            G

            1

            2 3

            34

            5 DataLink Layer 5-62

            Institutional network

            to externalnetwork

            router

            IP subnet

            mail server

            web server

            5 DataLink Layer 5-63

            Switches vs Routers both store-and-forward devices

            o routers network layer devices (examine network layer headers)o switches are link layer devices

            routers maintain routing tables implement routing algorithms

            switches maintain switch tables implement filtering learning algorithms

            Switch

            5 DataLink Layer 5-64

            Link Layer

            51 Introduction and services

            52 Error detection and correction

            53Multiple access protocols

            54 Link-Layer Addressing

            55 Ethernet

            56 Link-layer switches 59 A day in the life of

            a web request

            5 DataLink Layer 5-65

            Synthesis a day in the life of a web request

            journey down protocol stack completeo application transport network link

            putting-it-all-together synthesiso goal identify review understand protocols

            (at all layers) involved in seemingly simple scenario requesting www page

            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

            5 DataLink Layer 5-66

            A day in the life scenario

            Comcast network 68800013

            Googlersquos network 64233160019 64233169105

            web server

            DNS server

            school network 68802024

            browser

            web page

            5 DataLink Layer 5-67

            A day in the lifehellip connecting to the Internet

            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

            router(runs DHCP)

            DHCPUDP

            IPEthPhy

            DHCP

            DHCP

            DHCP

            DHCP

            DHCP

            DHCPUDP

            IPEthPhy

            DHCP

            DHCP

            DHCP

            DHCPDHCP

            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

            5 DataLink Layer 5-68

            A day in the lifehellip connecting to the Internet

            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

            router(runs DHCP)

            DHCPUDP

            IPEthPhy

            DHCP

            DHCP

            DHCP

            DHCP

            DHCPUDP

            IPEthPhy

            DHCP

            DHCP

            DHCP

            DHCP

            DHCP

            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

            DHCP client receives DHCP ACK reply

            5 DataLink Layer 5-69

            A day in the lifehellip ARP (before DNS before HTTP)

            before sending HTTP request need IP address of wwwgooglecom DNS

            DNSUDP

            IPEthPhy

            DNS

            DNS

            DNS

            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

            ARP query

            EthPhy

            ARP

            ARP

            ARP reply

            5 DataLink Layer 5-70

            A day in the lifehellip using DNS

            DNSUDP

            IPEthPhy

            DNS

            DNS

            DNS

            DNS

            DNS

            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

            demuxrsquoed to DNS server DNS server replies to

            client with IP address of wwwgooglecom

            Comcast network 68800013

            DNS server

            DNSUDP

            IPEthPhy

            DNS

            DNS

            DNS

            DNS

            5 DataLink Layer 5-71

            A day in the lifehellip TCP connection carrying HTTP

            HTTPTCPIP

            EthPhy

            HTTP

            to send HTTP request client first opens TCP socket to web server

            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

            TCP connection established

            64233169105

            web server

            SYN

            SYN

            SYN

            SYN

            TCPIP

            EthPhy

            SYN

            SYN

            SYN

            SYNACK

            SYNACK

            SYNACK

            SYNACK

            SYNACK

            SYNACK

            SYNACK

            web server responds with TCP SYNACK (step 2 in 3-way handshake)

            5 DataLink Layer 5-72

            A day in the lifehellip HTTP requestreply

            HTTPTCPIP

            EthPhy

            HTTP

            HTTP request sent into TCP socket

            IP datagram containing HTTP request routed to wwwgooglecom

            IP datgram containing HTTP reply routed back to client

            64233169105

            web server

            HTTPTCPIP

            EthPhy

            web server responds with HTTP reply (containing web page)

            HTTP

            HTTP

            HTTPHTTP

            HTTP

            HTTP

            HTTP

            HTTP

            HTTP

            HTTP

            HTTP

            HTTP

            HTTP

            web page finally () displayed

            5 DataLink Layer 5-73

            Chapter 5 letrsquos take a breath journey down protocol stack complete

            (except PHY) solid understanding of networking

            principles practice hellip could stop here hellip but lots of

            interesting topicso Internetworking (CSE 678 TCPIP socket

            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

            • Link Layer
            • Link Layer Introduction
            • Link layer context
            • Encapsulation
            • Link Layer Services
            • Link Layer Services (more)
            • Where is the link layer implemented
            • Adaptors Communicating
            • Slide 9
            • Parity Checking
            • Internet checksum (review)
            • Checksumming Cyclic Redundancy Check
            • Cyclic Redundancy Check
            • Slide 14
            • CRC Example
            • Slide 16
            • Multiple Access Links and Protocols
            • Cable Network Architecture Overview
            • Multiple Access protocols
            • Ideal Multiple Access Protocol
            • MAC Protocols a taxonomy
            • Channel Partitioning MAC protocols TDMA
            • Channel Partitioning MAC protocols FDMA
            • Random Access Protocols
            • ALOHA
            • CSMA (Carrier Sense Multiple Access)
            • CSMA collisions
            • CSMACD (Collision Detection)
            • CSMACD collision detection
            • ldquoTaking Turnsrdquo MAC protocols
            • Slide 31
            • Slide 32
            • Summary of MAC protocols
            • Slide 34
            • MAC Addresses and ARP
            • LAN Addresses and ARP
            • LAN Address (more)
            • ARP Address Resolution Protocol
            • ARP protocol Same LAN (network)
            • Addressing routing to another LAN
            • Slide 41
            • Slide 42
            • Ethernet
            • Star topology
            • Ethernet Frame Structure
            • Ethernet Frame Structure (more)
            • Ethernet Unreliable connectionless
            • Ethernet CSMACD algorithm
            • Ethernetrsquos CSMACD (more)
            • 8023 Ethernet Standards Link amp Physical Layers
            • Manchester encoding
            • Slide 52
            • Hubs
            • Switch
            • Switch allows multiple simultaneous transmissions
            • Switch Table
            • Switch self-learning
            • Switch frame filteringforwarding
            • Self-learning forwarding example
            • Interconnecting switches
            • Self-learning multi-switch example
            • Institutional network
            • Switches vs Routers
            • Slide 64
            • Synthesis a day in the life of a web request
            • A day in the life scenario
            • A day in the lifehellip connecting to the Internet
            • Slide 68
            • A day in the lifehellip ARP (before DNS before HTTP)
            • A day in the lifehellip using DNS
            • A day in the lifehellip TCP connection carrying HTTP
            • A day in the lifehellip HTTP requestreply
            • Chapter 5 letrsquos take a breath

              5 DataLink Layer 5-7

              Where is the link layer implemented

              in each and every host link layer implemented in

              ldquoadaptorrdquo (aka network interface card NIC)o Ethernet card PCMCI card

              80211 cardo implements link physical

              layer

              attaches into hostrsquos system buses

              combination of hardware software firmware

              controller

              physicaltransmission

              cpu memory

              host bus (eg PCI)

              network adaptercard

              host schematic

              applicationtransportnetwork

              link

              linkphysical

              5 DataLink Layer 5-8

              Adaptors Communicating

              sending sideo encapsulates datagram

              in frameo adds error checking bits

              rdt flow control etc

              receiving sideo looks for errors rdt flow

              control etco extracts datagram passes

              to upper layer at receiving side

              controller controller

              sending host receiving host

              datagram datagram

              datagram

              frame

              5 DataLink Layer 5-9

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53Multiple access protocols

              54 Link-layer Addressing

              55 Ethernet

              56 Link-layer switches 59 A day in the life of

              a web request

              5 DataLink Layer 5-10

              Parity Checking

              Single Bit ParityDetect single bit errors

              Two Dimensional Bit ParityDetect and correct single bit errors

              0 0

              5 DataLink Layer 5-11

              Internet checksum (review)

              Sender treat segment contents

              as sequence of 16-bit integers

              checksum addition (1rsquos complement sum) of segment contents

              sender puts checksum value into UDP checksum field

              Receiver compute checksum of

              received segment check if computed

              checksum equals checksum field valueo NO - error detectedo YES - no error detected

              But maybe errors nonetheless

              Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

              5 DataLink Layer 5-12

              Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

              polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

              o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

              remainder error detectedo can detect all burst errors less than r+1 bits

              widely used in practice (Ethernet 80211 WiFi ATM)

              Cyclic Redundancy Check Modulo 2 arithmetic

              o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

              A polynomial G(x) of degree r is known to both sender and receiver

              Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

              Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

              If not there are transmission errors in the frame

              7 4 3( ) 1D x x x x x

              Common polynomials for G(x)

              CRC

              CRC-8

              CRC-10

              CRC-12

              CRC-16

              CRC-CCITT

              CRC-32

              C(x)

              x8+x2+x1+1

              x10+x9+x5+x4+x1+1

              x12+x11+x3+x2+x1+1

              x16+x15+x2+1

              x16+x12+x5+1

              x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

              5 DataLink Layer 5-15

              CRC Example

              31001 represents 1x

              5 DataLink Layer 5-16

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53Multiple access protocols

              54 Link-layer Addressing

              55 Ethernet

              56 Link-layer switches 59 A day in the life of

              a web request

              5 DataLink Layer 5-17

              Multiple Access Links and Protocols

              Two types of ldquolinksrdquo point-to-point

              o PPP for dial-up accesso point-to-point link between Ethernet switch and host

              broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

              shared wire (eg cabled Ethernet)

              shared RF (eg 80211 WiFi)

              shared RF(satellite)

              humans at acocktail party

              (shared air acoustical)

              Introduction 1-18

              Cable Network Architecture Overview

              home

              cable headend

              cable distributionnetwork (simplified)

              Typically 500 to 5000 homes

              5 DataLink Layer 5-19

              Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

              interference o collision if node receives two or more signals at the same

              time

              multiple access protocol distributed algorithm that determines how nodes

              share channel ie determine when node can transmit

              communication about channel sharing must use channel itself o no out-of-band channel for coordination

              5 DataLink Layer 5-20

              Ideal Multiple Access Protocol

              Broadcast channel of rate R bps1 when one node wants to transmit it can send

              at the full rate say R2 when M nodes want to transmit each can

              send at average rate RM3 fully decentralized

              o no special node to coordinate transmissionso no synchronization of clocks slots

              4 simple

              5 DataLink Layer 5-21

              MAC Protocols a taxonomy

              Three broad classes Channel Partitioning

              o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

              o allocate piece to node for exclusive use

              Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

              ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

              take longer turns

              5 DataLink Layer 5-22

              Channel Partitioning MAC protocols TDMA

              TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

              pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

              256 idle

              1 3 4 1 3 4

              6-slotframe

              5 DataLink Layer 5-23

              Channel Partitioning MAC protocols FDMA

              FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

              idle example 6-station LAN 134 have pkt

              frequency bands 256 idle fr

              equ

              ency

              bands time

              FDM cable

              5 DataLink Layer 5-24

              Random Access Protocols

              When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

              two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

              o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

              retransmissions)

              Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

              5 DataLink Layer 5-25

              ALOHA

              When a node has a frame to send send immediately

              Set a timer for a random amount of time If an ACK arrives before the timer expires

              fine otherwise resend the frame

              (Works like stop-and-wait with random timeout interval)

              5 DataLink Layer 5-26

              CSMA (Carrier Sense Multiple Access)

              CSMA listen before transmit

              If channel sensed idle transmit entire frame

              If channel sensed busy defer transmission

              human analogy donrsquot interrupt others

              5 DataLink Layer 5-27

              CSMA collisions

              collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

              spatial layout of nodes

              5 DataLink Layer 5-28

              CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

              o collisions detected within short timeo colliding transmissions aborted reducing

              channel wastage collision detection

              o easy in wired LANs measure signal strengths compare transmitted amp received signals

              o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

              human analogy the polite conversationalist

              5 DataLink Layer 5-29

              CSMACD collision detection

              5 DataLink Layer 5-30

              ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

              o share channel efficiently and fairly at high load

              o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

              Random access MAC protocolso efficient at low load single node can fully

              utilize channelo high load collision overhead

              ldquotaking turnsrdquo protocolslook for best of both worlds

              5 DataLink Layer 5-31

              ldquoTaking Turnsrdquo MAC protocolsPolling master node

              ldquoinvitesrdquo slave nodes to transmit in turn

              typically used with ldquodumbrdquo slave devices

              concernso polling overhead o latencyo single point of

              failure (master)

              master

              slaves

              poll

              data

              data

              5 DataLink Layer 5-32

              ldquoTaking Turnsrdquo MAC protocolsToken passing control token

              passed from one node to next sequentially

              token message concerns

              o token overhead o latencyo single point of failure

              (token)

              T

              data

              (nothingto send)

              T

              5 DataLink Layer 5-33

              Summary of MAC protocols

              channel partitioning by time frequency or codeo Time Division Frequency Division

              random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

              hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

              taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

              5 DataLink Layer 5-34

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53Multiple access protocols

              54 Link-Layer Addressing

              55 Ethernet

              56 Link-layer switches 59 A day in the life of

              a web request

              5 DataLink Layer 5-35

              MAC Addresses and ARP

              32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

              MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

              another physically-connected interface (in same network)

              o 48 bit MAC addressbull burned in NIC ROM

              5 DataLink Layer 5-36

              LAN Addresses and ARPEach adapter on LAN has unique LAN address

              Broadcast address =FF-FF-FF-FF-FF-FF

              = adapter

              1A-2F-BB-76-09-AD

              58-23-D7-FA-20-B0

              0C-C4-11-6F-E3-98

              71-65-F7-2B-08-53

              LAN(wired orwireless)

              5 DataLink Layer 5-37

              LAN Address (more)

              MAC address allocation administered by IEEE manufacturer buys portion of MAC address

              space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

              o can move LAN card from one LAN to another

              IP hierarchical address NOT portableo address depends on IP subnet to which node is

              attached

              5 DataLink Layer 5-38

              ARP Address Resolution Protocol

              Each IP node (host router) on LAN has ARP table

              ARP table IPMAC address mappings for some LAN nodes

              lt IP address MAC address TTLgto TTL (Time To Live)

              time after which address mapping will be forgotten (typically 20 min)

              Q given a nodersquos IP address how to determine its MAC address

              1A-2F-BB-76-09-AD

              0C-C4-11-6F-E3-98

              71-65-F7-2B-08-53

              LAN

              137196723

              137196778

              137196714

              137196788

              5 DataLink Layer 5-39

              ARP protocol Same LAN (network) A wants to send datagram

              to B and Brsquos MAC address not in Arsquos ARP table

              A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

              FF-FF-FF-FF-FFo all machines on LAN

              receive ARP query B receives ARP packet

              replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

              address (unicast)

              A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

              ARP is ldquoplug-and-playrdquoo nodes create their

              ARP tables without intervention from net administrator

              5 DataLink Layer 5-40

              Addressing routing to another LAN

              R

              1A-23-F9-CD-06-9B

              222222222220111111111110

              E6-E9-00-17-BB-4B

              CC-49-DE-D0-AB-7D

              111111111112

              111111111111

              A74-29-9C-E8-FF-55

              222222222221

              88-B2-2F-54-1A-0F

              B222222222222

              49-BD-D2-C7-56-2A

              walkthrough send datagram from A to B via R assume A knows Brsquos IP address

              two ARP tables in router R one for each IP network (LAN)

              LAN LAN

              5 DataLink Layer 5-41

              A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

              frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

              destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

              R

              1A-23-F9-CD-06-9B

              222222222220

              111111111110

              E6-E9-00-17-BB-4B

              CC-49-DE-D0-AB-7D

              111111111112

              111111111111

              A74-29-9C-E8-FF-55

              222222222221

              88-B2-2F-54-1A-0F

              B222222222222

              49-BD-D2-C7-56-2A

              This is a really importantexample ndash make sure youunderstand

              5 DataLink Layer 5-42

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53Multiple access protocols

              54 Link-Layer Addressing

              55 Ethernet

              56 Link-layer switches 59 A day in the life of

              a web request

              5 DataLink Layer 5-43

              Ethernet

              ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

              Metcalfersquos Ethernetsketch

              5 DataLink Layer 5-44

              Star topology bus topology popular through mid 90s

              o all nodes in same collision domain (can collide with each other)

              today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

              (nodes do not collide with each other)

              switch

              bus coaxial cable star

              5 DataLink Layer 5-45

              Ethernet Frame Structure

              Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

              Preamble 7 bytes with pattern 10101010 followed by one

              byte with pattern 10101011 used to synchronize receiver amp sender clock

              rates

              5 DataLink Layer 5-46

              Ethernet Frame Structure (more) Addresses 6 bytes

              o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

              o otherwise adapter discards frame

              Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

              CRC checked at receiver if error is detected frame is dropped

              5 DataLink Layer 5-47

              Ethernet Unreliable connectionless connectionless No handshaking between sending

              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

              Ethernetrsquos MAC protocol CSMACD

              5 DataLink Layer 5-48

              Ethernet CSMACD algorithm

              1 NIC receives datagram from network layer creates frame

              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

              4 If NIC detects collision while transmitting aborts and sends jam signal

              5 After aborting NIC enters exponential backoff

              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

              (1 slot = 512 bit-times)

              5 DataLink Layer 5-49

              Ethernetrsquos CSMACD (more)

              Jam Signal make sure all other transmitters are aware of collision 48 bits

              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

              Exponential Backoff Goal adapt retransmission

              attempts to estimated current loado heavy load random

              wait will be longer first collision choose K

              from 01 delay is K 512 bit transmission times

              after second collision choose K from 0123hellip

              after ten collisions choose K from 01234hellip1023

              5 DataLink Layer 5-50

              8023 Ethernet Standards Link amp Physical Layers

              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

              Mbps 1Gbps 10G bpso different physical layer media fiber cable

              applicationtransportnetwork

              linkphysical

              MAC protocoland frame format

              100BASE-TX

              100BASE-T4

              100BASE-FX100BASE-T2

              100BASE-SX 100BASE-BX

              fiber physical layercopper (twisterpair) physical layer

              5 DataLink Layer 5-51

              Manchester encoding

              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

              synchronize to each othero no need for a centralized global clock among nodes

              Hey this is physical-layer stuff

              5 DataLink Layer 5-52

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53 Multiple access protocols

              54 Link-layer Addressing

              55 Ethernet

              56 Link-layer switches LANs

              59 A day in the life of a web request

              5 DataLink Layer 5-53

              Hubsphysical-layer (ldquodumbrdquo) repeaters

              o bits coming in one link go out all other links at same rate

              o all nodes connected to hub can collide with one another

              o no frame bufferingo no CSMACD at hub host NICs detect collisions

              twisted pair

              hub

              5 DataLink Layer 5-54

              Switch link-layer device smarter than hubs take active

              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

              transparento hosts are unaware of presence of switches

              plug-and-play self-learningo switches do not need to be configured

              5 DataLink Layer 5-55

              Switch allows multiple simultaneous transmissions

              hosts have dedicated direct connection to switch

              switches buffer packets Ethernet protocol used on

              each incoming link but no collisions full duplexo each link is its own collision

              domain switching A-to-Arsquo and B-

              to-Brsquo simultaneously without collisions o not possible with dumb hub

              A

              Arsquo

              B

              Brsquo

              C

              Crsquo

              switch with six interfaces(123456)

              1 23

              45

              6

              5 DataLink Layer 5-56

              Switch Table

              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

              A each switch has a switch table each entryo (MAC address of host interface

              to reach host time stamp)

              looks like a routing table Q how are entries created

              maintained in switch table o something like a routing

              protocol

              A

              Arsquo

              B

              Brsquo

              C

              Crsquo

              switch with six interfaces(123456)

              1 23

              45

              6

              5 DataLink Layer 5-57

              Switch self-learning

              switch learns which hosts can be reached through which interfaceso when frame received

              switch ldquolearnsrdquo location of sender incoming LAN segment

              o records senderlocation pair in switch table

              A

              Arsquo

              B

              Brsquo

              C

              Crsquo

              1 23

              45

              6

              A Arsquo

              Source ADest Arsquo

              MAC addr interface TTL

              Switch table (initially empty)

              A 1 60

              5 DataLink Layer 5-58

              Switch frame filteringforwardingWhen frame received

              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

              then if dest on segment from which frame arrived

              then drop the frame else forward the frame on interface indicated else flood

              forward on all but the interface on which the frame arrived

              5 DataLink Layer 5-59

              Self-learning forwarding example

              A

              Arsquo

              B

              Brsquo

              C

              Crsquo

              1 23

              45

              6

              A Arsquo

              Source ADest Arsquo

              MAC addr interface TTL

              Switch table (initially empty)

              A 1 60

              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

              frame destination unknownflood

              Arsquo A

              destination A location known

              Arsquo 4 60

              selective send

              5 DataLink Layer 5-60

              Interconnecting switches

              switches can be connected together

              A

              B

              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

              A self learning (works exactly the same as in single-switch case)

              S1

              C D

              E

              FS2

              S4

              S3

              H

              I

              G

              5 DataLink Layer 5-61

              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

              Q show switch tables and packet forwarding in S1 S2 S3 S4

              A

              B

              S1

              C D

              E

              FS2

              S4

              S3

              H

              I

              G

              1

              2 3

              34

              5 DataLink Layer 5-62

              Institutional network

              to externalnetwork

              router

              IP subnet

              mail server

              web server

              5 DataLink Layer 5-63

              Switches vs Routers both store-and-forward devices

              o routers network layer devices (examine network layer headers)o switches are link layer devices

              routers maintain routing tables implement routing algorithms

              switches maintain switch tables implement filtering learning algorithms

              Switch

              5 DataLink Layer 5-64

              Link Layer

              51 Introduction and services

              52 Error detection and correction

              53Multiple access protocols

              54 Link-Layer Addressing

              55 Ethernet

              56 Link-layer switches 59 A day in the life of

              a web request

              5 DataLink Layer 5-65

              Synthesis a day in the life of a web request

              journey down protocol stack completeo application transport network link

              putting-it-all-together synthesiso goal identify review understand protocols

              (at all layers) involved in seemingly simple scenario requesting www page

              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

              5 DataLink Layer 5-66

              A day in the life scenario

              Comcast network 68800013

              Googlersquos network 64233160019 64233169105

              web server

              DNS server

              school network 68802024

              browser

              web page

              5 DataLink Layer 5-67

              A day in the lifehellip connecting to the Internet

              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

              router(runs DHCP)

              DHCPUDP

              IPEthPhy

              DHCP

              DHCP

              DHCP

              DHCP

              DHCP

              DHCPUDP

              IPEthPhy

              DHCP

              DHCP

              DHCP

              DHCPDHCP

              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

              5 DataLink Layer 5-68

              A day in the lifehellip connecting to the Internet

              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

              router(runs DHCP)

              DHCPUDP

              IPEthPhy

              DHCP

              DHCP

              DHCP

              DHCP

              DHCPUDP

              IPEthPhy

              DHCP

              DHCP

              DHCP

              DHCP

              DHCP

              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

              DHCP client receives DHCP ACK reply

              5 DataLink Layer 5-69

              A day in the lifehellip ARP (before DNS before HTTP)

              before sending HTTP request need IP address of wwwgooglecom DNS

              DNSUDP

              IPEthPhy

              DNS

              DNS

              DNS

              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

              ARP query

              EthPhy

              ARP

              ARP

              ARP reply

              5 DataLink Layer 5-70

              A day in the lifehellip using DNS

              DNSUDP

              IPEthPhy

              DNS

              DNS

              DNS

              DNS

              DNS

              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

              demuxrsquoed to DNS server DNS server replies to

              client with IP address of wwwgooglecom

              Comcast network 68800013

              DNS server

              DNSUDP

              IPEthPhy

              DNS

              DNS

              DNS

              DNS

              5 DataLink Layer 5-71

              A day in the lifehellip TCP connection carrying HTTP

              HTTPTCPIP

              EthPhy

              HTTP

              to send HTTP request client first opens TCP socket to web server

              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

              TCP connection established

              64233169105

              web server

              SYN

              SYN

              SYN

              SYN

              TCPIP

              EthPhy

              SYN

              SYN

              SYN

              SYNACK

              SYNACK

              SYNACK

              SYNACK

              SYNACK

              SYNACK

              SYNACK

              web server responds with TCP SYNACK (step 2 in 3-way handshake)

              5 DataLink Layer 5-72

              A day in the lifehellip HTTP requestreply

              HTTPTCPIP

              EthPhy

              HTTP

              HTTP request sent into TCP socket

              IP datagram containing HTTP request routed to wwwgooglecom

              IP datgram containing HTTP reply routed back to client

              64233169105

              web server

              HTTPTCPIP

              EthPhy

              web server responds with HTTP reply (containing web page)

              HTTP

              HTTP

              HTTPHTTP

              HTTP

              HTTP

              HTTP

              HTTP

              HTTP

              HTTP

              HTTP

              HTTP

              HTTP

              web page finally () displayed

              5 DataLink Layer 5-73

              Chapter 5 letrsquos take a breath journey down protocol stack complete

              (except PHY) solid understanding of networking

              principles practice hellip could stop here hellip but lots of

              interesting topicso Internetworking (CSE 678 TCPIP socket

              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

              • Link Layer
              • Link Layer Introduction
              • Link layer context
              • Encapsulation
              • Link Layer Services
              • Link Layer Services (more)
              • Where is the link layer implemented
              • Adaptors Communicating
              • Slide 9
              • Parity Checking
              • Internet checksum (review)
              • Checksumming Cyclic Redundancy Check
              • Cyclic Redundancy Check
              • Slide 14
              • CRC Example
              • Slide 16
              • Multiple Access Links and Protocols
              • Cable Network Architecture Overview
              • Multiple Access protocols
              • Ideal Multiple Access Protocol
              • MAC Protocols a taxonomy
              • Channel Partitioning MAC protocols TDMA
              • Channel Partitioning MAC protocols FDMA
              • Random Access Protocols
              • ALOHA
              • CSMA (Carrier Sense Multiple Access)
              • CSMA collisions
              • CSMACD (Collision Detection)
              • CSMACD collision detection
              • ldquoTaking Turnsrdquo MAC protocols
              • Slide 31
              • Slide 32
              • Summary of MAC protocols
              • Slide 34
              • MAC Addresses and ARP
              • LAN Addresses and ARP
              • LAN Address (more)
              • ARP Address Resolution Protocol
              • ARP protocol Same LAN (network)
              • Addressing routing to another LAN
              • Slide 41
              • Slide 42
              • Ethernet
              • Star topology
              • Ethernet Frame Structure
              • Ethernet Frame Structure (more)
              • Ethernet Unreliable connectionless
              • Ethernet CSMACD algorithm
              • Ethernetrsquos CSMACD (more)
              • 8023 Ethernet Standards Link amp Physical Layers
              • Manchester encoding
              • Slide 52
              • Hubs
              • Switch
              • Switch allows multiple simultaneous transmissions
              • Switch Table
              • Switch self-learning
              • Switch frame filteringforwarding
              • Self-learning forwarding example
              • Interconnecting switches
              • Self-learning multi-switch example
              • Institutional network
              • Switches vs Routers
              • Slide 64
              • Synthesis a day in the life of a web request
              • A day in the life scenario
              • A day in the lifehellip connecting to the Internet
              • Slide 68
              • A day in the lifehellip ARP (before DNS before HTTP)
              • A day in the lifehellip using DNS
              • A day in the lifehellip TCP connection carrying HTTP
              • A day in the lifehellip HTTP requestreply
              • Chapter 5 letrsquos take a breath

                5 DataLink Layer 5-8

                Adaptors Communicating

                sending sideo encapsulates datagram

                in frameo adds error checking bits

                rdt flow control etc

                receiving sideo looks for errors rdt flow

                control etco extracts datagram passes

                to upper layer at receiving side

                controller controller

                sending host receiving host

                datagram datagram

                datagram

                frame

                5 DataLink Layer 5-9

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53Multiple access protocols

                54 Link-layer Addressing

                55 Ethernet

                56 Link-layer switches 59 A day in the life of

                a web request

                5 DataLink Layer 5-10

                Parity Checking

                Single Bit ParityDetect single bit errors

                Two Dimensional Bit ParityDetect and correct single bit errors

                0 0

                5 DataLink Layer 5-11

                Internet checksum (review)

                Sender treat segment contents

                as sequence of 16-bit integers

                checksum addition (1rsquos complement sum) of segment contents

                sender puts checksum value into UDP checksum field

                Receiver compute checksum of

                received segment check if computed

                checksum equals checksum field valueo NO - error detectedo YES - no error detected

                But maybe errors nonetheless

                Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

                5 DataLink Layer 5-12

                Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

                polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

                o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

                remainder error detectedo can detect all burst errors less than r+1 bits

                widely used in practice (Ethernet 80211 WiFi ATM)

                Cyclic Redundancy Check Modulo 2 arithmetic

                o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                A polynomial G(x) of degree r is known to both sender and receiver

                Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                If not there are transmission errors in the frame

                7 4 3( ) 1D x x x x x

                Common polynomials for G(x)

                CRC

                CRC-8

                CRC-10

                CRC-12

                CRC-16

                CRC-CCITT

                CRC-32

                C(x)

                x8+x2+x1+1

                x10+x9+x5+x4+x1+1

                x12+x11+x3+x2+x1+1

                x16+x15+x2+1

                x16+x12+x5+1

                x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                5 DataLink Layer 5-15

                CRC Example

                31001 represents 1x

                5 DataLink Layer 5-16

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53Multiple access protocols

                54 Link-layer Addressing

                55 Ethernet

                56 Link-layer switches 59 A day in the life of

                a web request

                5 DataLink Layer 5-17

                Multiple Access Links and Protocols

                Two types of ldquolinksrdquo point-to-point

                o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                shared wire (eg cabled Ethernet)

                shared RF (eg 80211 WiFi)

                shared RF(satellite)

                humans at acocktail party

                (shared air acoustical)

                Introduction 1-18

                Cable Network Architecture Overview

                home

                cable headend

                cable distributionnetwork (simplified)

                Typically 500 to 5000 homes

                5 DataLink Layer 5-19

                Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                interference o collision if node receives two or more signals at the same

                time

                multiple access protocol distributed algorithm that determines how nodes

                share channel ie determine when node can transmit

                communication about channel sharing must use channel itself o no out-of-band channel for coordination

                5 DataLink Layer 5-20

                Ideal Multiple Access Protocol

                Broadcast channel of rate R bps1 when one node wants to transmit it can send

                at the full rate say R2 when M nodes want to transmit each can

                send at average rate RM3 fully decentralized

                o no special node to coordinate transmissionso no synchronization of clocks slots

                4 simple

                5 DataLink Layer 5-21

                MAC Protocols a taxonomy

                Three broad classes Channel Partitioning

                o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                o allocate piece to node for exclusive use

                Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                take longer turns

                5 DataLink Layer 5-22

                Channel Partitioning MAC protocols TDMA

                TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                256 idle

                1 3 4 1 3 4

                6-slotframe

                5 DataLink Layer 5-23

                Channel Partitioning MAC protocols FDMA

                FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                idle example 6-station LAN 134 have pkt

                frequency bands 256 idle fr

                equ

                ency

                bands time

                FDM cable

                5 DataLink Layer 5-24

                Random Access Protocols

                When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                retransmissions)

                Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                5 DataLink Layer 5-25

                ALOHA

                When a node has a frame to send send immediately

                Set a timer for a random amount of time If an ACK arrives before the timer expires

                fine otherwise resend the frame

                (Works like stop-and-wait with random timeout interval)

                5 DataLink Layer 5-26

                CSMA (Carrier Sense Multiple Access)

                CSMA listen before transmit

                If channel sensed idle transmit entire frame

                If channel sensed busy defer transmission

                human analogy donrsquot interrupt others

                5 DataLink Layer 5-27

                CSMA collisions

                collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                spatial layout of nodes

                5 DataLink Layer 5-28

                CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                o collisions detected within short timeo colliding transmissions aborted reducing

                channel wastage collision detection

                o easy in wired LANs measure signal strengths compare transmitted amp received signals

                o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                human analogy the polite conversationalist

                5 DataLink Layer 5-29

                CSMACD collision detection

                5 DataLink Layer 5-30

                ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                o share channel efficiently and fairly at high load

                o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                Random access MAC protocolso efficient at low load single node can fully

                utilize channelo high load collision overhead

                ldquotaking turnsrdquo protocolslook for best of both worlds

                5 DataLink Layer 5-31

                ldquoTaking Turnsrdquo MAC protocolsPolling master node

                ldquoinvitesrdquo slave nodes to transmit in turn

                typically used with ldquodumbrdquo slave devices

                concernso polling overhead o latencyo single point of

                failure (master)

                master

                slaves

                poll

                data

                data

                5 DataLink Layer 5-32

                ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                passed from one node to next sequentially

                token message concerns

                o token overhead o latencyo single point of failure

                (token)

                T

                data

                (nothingto send)

                T

                5 DataLink Layer 5-33

                Summary of MAC protocols

                channel partitioning by time frequency or codeo Time Division Frequency Division

                random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                5 DataLink Layer 5-34

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53Multiple access protocols

                54 Link-Layer Addressing

                55 Ethernet

                56 Link-layer switches 59 A day in the life of

                a web request

                5 DataLink Layer 5-35

                MAC Addresses and ARP

                32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                another physically-connected interface (in same network)

                o 48 bit MAC addressbull burned in NIC ROM

                5 DataLink Layer 5-36

                LAN Addresses and ARPEach adapter on LAN has unique LAN address

                Broadcast address =FF-FF-FF-FF-FF-FF

                = adapter

                1A-2F-BB-76-09-AD

                58-23-D7-FA-20-B0

                0C-C4-11-6F-E3-98

                71-65-F7-2B-08-53

                LAN(wired orwireless)

                5 DataLink Layer 5-37

                LAN Address (more)

                MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                o can move LAN card from one LAN to another

                IP hierarchical address NOT portableo address depends on IP subnet to which node is

                attached

                5 DataLink Layer 5-38

                ARP Address Resolution Protocol

                Each IP node (host router) on LAN has ARP table

                ARP table IPMAC address mappings for some LAN nodes

                lt IP address MAC address TTLgto TTL (Time To Live)

                time after which address mapping will be forgotten (typically 20 min)

                Q given a nodersquos IP address how to determine its MAC address

                1A-2F-BB-76-09-AD

                0C-C4-11-6F-E3-98

                71-65-F7-2B-08-53

                LAN

                137196723

                137196778

                137196714

                137196788

                5 DataLink Layer 5-39

                ARP protocol Same LAN (network) A wants to send datagram

                to B and Brsquos MAC address not in Arsquos ARP table

                A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                FF-FF-FF-FF-FFo all machines on LAN

                receive ARP query B receives ARP packet

                replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                address (unicast)

                A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                ARP is ldquoplug-and-playrdquoo nodes create their

                ARP tables without intervention from net administrator

                5 DataLink Layer 5-40

                Addressing routing to another LAN

                R

                1A-23-F9-CD-06-9B

                222222222220111111111110

                E6-E9-00-17-BB-4B

                CC-49-DE-D0-AB-7D

                111111111112

                111111111111

                A74-29-9C-E8-FF-55

                222222222221

                88-B2-2F-54-1A-0F

                B222222222222

                49-BD-D2-C7-56-2A

                walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                two ARP tables in router R one for each IP network (LAN)

                LAN LAN

                5 DataLink Layer 5-41

                A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                R

                1A-23-F9-CD-06-9B

                222222222220

                111111111110

                E6-E9-00-17-BB-4B

                CC-49-DE-D0-AB-7D

                111111111112

                111111111111

                A74-29-9C-E8-FF-55

                222222222221

                88-B2-2F-54-1A-0F

                B222222222222

                49-BD-D2-C7-56-2A

                This is a really importantexample ndash make sure youunderstand

                5 DataLink Layer 5-42

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53Multiple access protocols

                54 Link-Layer Addressing

                55 Ethernet

                56 Link-layer switches 59 A day in the life of

                a web request

                5 DataLink Layer 5-43

                Ethernet

                ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                Metcalfersquos Ethernetsketch

                5 DataLink Layer 5-44

                Star topology bus topology popular through mid 90s

                o all nodes in same collision domain (can collide with each other)

                today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                (nodes do not collide with each other)

                switch

                bus coaxial cable star

                5 DataLink Layer 5-45

                Ethernet Frame Structure

                Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                Preamble 7 bytes with pattern 10101010 followed by one

                byte with pattern 10101011 used to synchronize receiver amp sender clock

                rates

                5 DataLink Layer 5-46

                Ethernet Frame Structure (more) Addresses 6 bytes

                o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                o otherwise adapter discards frame

                Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                CRC checked at receiver if error is detected frame is dropped

                5 DataLink Layer 5-47

                Ethernet Unreliable connectionless connectionless No handshaking between sending

                and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                Ethernetrsquos MAC protocol CSMACD

                5 DataLink Layer 5-48

                Ethernet CSMACD algorithm

                1 NIC receives datagram from network layer creates frame

                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                4 If NIC detects collision while transmitting aborts and sends jam signal

                5 After aborting NIC enters exponential backoff

                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                (1 slot = 512 bit-times)

                5 DataLink Layer 5-49

                Ethernetrsquos CSMACD (more)

                Jam Signal make sure all other transmitters are aware of collision 48 bits

                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                Exponential Backoff Goal adapt retransmission

                attempts to estimated current loado heavy load random

                wait will be longer first collision choose K

                from 01 delay is K 512 bit transmission times

                after second collision choose K from 0123hellip

                after ten collisions choose K from 01234hellip1023

                5 DataLink Layer 5-50

                8023 Ethernet Standards Link amp Physical Layers

                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                applicationtransportnetwork

                linkphysical

                MAC protocoland frame format

                100BASE-TX

                100BASE-T4

                100BASE-FX100BASE-T2

                100BASE-SX 100BASE-BX

                fiber physical layercopper (twisterpair) physical layer

                5 DataLink Layer 5-51

                Manchester encoding

                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                synchronize to each othero no need for a centralized global clock among nodes

                Hey this is physical-layer stuff

                5 DataLink Layer 5-52

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53 Multiple access protocols

                54 Link-layer Addressing

                55 Ethernet

                56 Link-layer switches LANs

                59 A day in the life of a web request

                5 DataLink Layer 5-53

                Hubsphysical-layer (ldquodumbrdquo) repeaters

                o bits coming in one link go out all other links at same rate

                o all nodes connected to hub can collide with one another

                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                twisted pair

                hub

                5 DataLink Layer 5-54

                Switch link-layer device smarter than hubs take active

                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                transparento hosts are unaware of presence of switches

                plug-and-play self-learningo switches do not need to be configured

                5 DataLink Layer 5-55

                Switch allows multiple simultaneous transmissions

                hosts have dedicated direct connection to switch

                switches buffer packets Ethernet protocol used on

                each incoming link but no collisions full duplexo each link is its own collision

                domain switching A-to-Arsquo and B-

                to-Brsquo simultaneously without collisions o not possible with dumb hub

                A

                Arsquo

                B

                Brsquo

                C

                Crsquo

                switch with six interfaces(123456)

                1 23

                45

                6

                5 DataLink Layer 5-56

                Switch Table

                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                A each switch has a switch table each entryo (MAC address of host interface

                to reach host time stamp)

                looks like a routing table Q how are entries created

                maintained in switch table o something like a routing

                protocol

                A

                Arsquo

                B

                Brsquo

                C

                Crsquo

                switch with six interfaces(123456)

                1 23

                45

                6

                5 DataLink Layer 5-57

                Switch self-learning

                switch learns which hosts can be reached through which interfaceso when frame received

                switch ldquolearnsrdquo location of sender incoming LAN segment

                o records senderlocation pair in switch table

                A

                Arsquo

                B

                Brsquo

                C

                Crsquo

                1 23

                45

                6

                A Arsquo

                Source ADest Arsquo

                MAC addr interface TTL

                Switch table (initially empty)

                A 1 60

                5 DataLink Layer 5-58

                Switch frame filteringforwardingWhen frame received

                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                then if dest on segment from which frame arrived

                then drop the frame else forward the frame on interface indicated else flood

                forward on all but the interface on which the frame arrived

                5 DataLink Layer 5-59

                Self-learning forwarding example

                A

                Arsquo

                B

                Brsquo

                C

                Crsquo

                1 23

                45

                6

                A Arsquo

                Source ADest Arsquo

                MAC addr interface TTL

                Switch table (initially empty)

                A 1 60

                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                frame destination unknownflood

                Arsquo A

                destination A location known

                Arsquo 4 60

                selective send

                5 DataLink Layer 5-60

                Interconnecting switches

                switches can be connected together

                A

                B

                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                A self learning (works exactly the same as in single-switch case)

                S1

                C D

                E

                FS2

                S4

                S3

                H

                I

                G

                5 DataLink Layer 5-61

                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                Q show switch tables and packet forwarding in S1 S2 S3 S4

                A

                B

                S1

                C D

                E

                FS2

                S4

                S3

                H

                I

                G

                1

                2 3

                34

                5 DataLink Layer 5-62

                Institutional network

                to externalnetwork

                router

                IP subnet

                mail server

                web server

                5 DataLink Layer 5-63

                Switches vs Routers both store-and-forward devices

                o routers network layer devices (examine network layer headers)o switches are link layer devices

                routers maintain routing tables implement routing algorithms

                switches maintain switch tables implement filtering learning algorithms

                Switch

                5 DataLink Layer 5-64

                Link Layer

                51 Introduction and services

                52 Error detection and correction

                53Multiple access protocols

                54 Link-Layer Addressing

                55 Ethernet

                56 Link-layer switches 59 A day in the life of

                a web request

                5 DataLink Layer 5-65

                Synthesis a day in the life of a web request

                journey down protocol stack completeo application transport network link

                putting-it-all-together synthesiso goal identify review understand protocols

                (at all layers) involved in seemingly simple scenario requesting www page

                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                5 DataLink Layer 5-66

                A day in the life scenario

                Comcast network 68800013

                Googlersquos network 64233160019 64233169105

                web server

                DNS server

                school network 68802024

                browser

                web page

                5 DataLink Layer 5-67

                A day in the lifehellip connecting to the Internet

                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                router(runs DHCP)

                DHCPUDP

                IPEthPhy

                DHCP

                DHCP

                DHCP

                DHCP

                DHCP

                DHCPUDP

                IPEthPhy

                DHCP

                DHCP

                DHCP

                DHCPDHCP

                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                5 DataLink Layer 5-68

                A day in the lifehellip connecting to the Internet

                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                router(runs DHCP)

                DHCPUDP

                IPEthPhy

                DHCP

                DHCP

                DHCP

                DHCP

                DHCPUDP

                IPEthPhy

                DHCP

                DHCP

                DHCP

                DHCP

                DHCP

                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                DHCP client receives DHCP ACK reply

                5 DataLink Layer 5-69

                A day in the lifehellip ARP (before DNS before HTTP)

                before sending HTTP request need IP address of wwwgooglecom DNS

                DNSUDP

                IPEthPhy

                DNS

                DNS

                DNS

                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                ARP query

                EthPhy

                ARP

                ARP

                ARP reply

                5 DataLink Layer 5-70

                A day in the lifehellip using DNS

                DNSUDP

                IPEthPhy

                DNS

                DNS

                DNS

                DNS

                DNS

                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                demuxrsquoed to DNS server DNS server replies to

                client with IP address of wwwgooglecom

                Comcast network 68800013

                DNS server

                DNSUDP

                IPEthPhy

                DNS

                DNS

                DNS

                DNS

                5 DataLink Layer 5-71

                A day in the lifehellip TCP connection carrying HTTP

                HTTPTCPIP

                EthPhy

                HTTP

                to send HTTP request client first opens TCP socket to web server

                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                TCP connection established

                64233169105

                web server

                SYN

                SYN

                SYN

                SYN

                TCPIP

                EthPhy

                SYN

                SYN

                SYN

                SYNACK

                SYNACK

                SYNACK

                SYNACK

                SYNACK

                SYNACK

                SYNACK

                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                5 DataLink Layer 5-72

                A day in the lifehellip HTTP requestreply

                HTTPTCPIP

                EthPhy

                HTTP

                HTTP request sent into TCP socket

                IP datagram containing HTTP request routed to wwwgooglecom

                IP datgram containing HTTP reply routed back to client

                64233169105

                web server

                HTTPTCPIP

                EthPhy

                web server responds with HTTP reply (containing web page)

                HTTP

                HTTP

                HTTPHTTP

                HTTP

                HTTP

                HTTP

                HTTP

                HTTP

                HTTP

                HTTP

                HTTP

                HTTP

                web page finally () displayed

                5 DataLink Layer 5-73

                Chapter 5 letrsquos take a breath journey down protocol stack complete

                (except PHY) solid understanding of networking

                principles practice hellip could stop here hellip but lots of

                interesting topicso Internetworking (CSE 678 TCPIP socket

                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                • Link Layer
                • Link Layer Introduction
                • Link layer context
                • Encapsulation
                • Link Layer Services
                • Link Layer Services (more)
                • Where is the link layer implemented
                • Adaptors Communicating
                • Slide 9
                • Parity Checking
                • Internet checksum (review)
                • Checksumming Cyclic Redundancy Check
                • Cyclic Redundancy Check
                • Slide 14
                • CRC Example
                • Slide 16
                • Multiple Access Links and Protocols
                • Cable Network Architecture Overview
                • Multiple Access protocols
                • Ideal Multiple Access Protocol
                • MAC Protocols a taxonomy
                • Channel Partitioning MAC protocols TDMA
                • Channel Partitioning MAC protocols FDMA
                • Random Access Protocols
                • ALOHA
                • CSMA (Carrier Sense Multiple Access)
                • CSMA collisions
                • CSMACD (Collision Detection)
                • CSMACD collision detection
                • ldquoTaking Turnsrdquo MAC protocols
                • Slide 31
                • Slide 32
                • Summary of MAC protocols
                • Slide 34
                • MAC Addresses and ARP
                • LAN Addresses and ARP
                • LAN Address (more)
                • ARP Address Resolution Protocol
                • ARP protocol Same LAN (network)
                • Addressing routing to another LAN
                • Slide 41
                • Slide 42
                • Ethernet
                • Star topology
                • Ethernet Frame Structure
                • Ethernet Frame Structure (more)
                • Ethernet Unreliable connectionless
                • Ethernet CSMACD algorithm
                • Ethernetrsquos CSMACD (more)
                • 8023 Ethernet Standards Link amp Physical Layers
                • Manchester encoding
                • Slide 52
                • Hubs
                • Switch
                • Switch allows multiple simultaneous transmissions
                • Switch Table
                • Switch self-learning
                • Switch frame filteringforwarding
                • Self-learning forwarding example
                • Interconnecting switches
                • Self-learning multi-switch example
                • Institutional network
                • Switches vs Routers
                • Slide 64
                • Synthesis a day in the life of a web request
                • A day in the life scenario
                • A day in the lifehellip connecting to the Internet
                • Slide 68
                • A day in the lifehellip ARP (before DNS before HTTP)
                • A day in the lifehellip using DNS
                • A day in the lifehellip TCP connection carrying HTTP
                • A day in the lifehellip HTTP requestreply
                • Chapter 5 letrsquos take a breath

                  5 DataLink Layer 5-9

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53Multiple access protocols

                  54 Link-layer Addressing

                  55 Ethernet

                  56 Link-layer switches 59 A day in the life of

                  a web request

                  5 DataLink Layer 5-10

                  Parity Checking

                  Single Bit ParityDetect single bit errors

                  Two Dimensional Bit ParityDetect and correct single bit errors

                  0 0

                  5 DataLink Layer 5-11

                  Internet checksum (review)

                  Sender treat segment contents

                  as sequence of 16-bit integers

                  checksum addition (1rsquos complement sum) of segment contents

                  sender puts checksum value into UDP checksum field

                  Receiver compute checksum of

                  received segment check if computed

                  checksum equals checksum field valueo NO - error detectedo YES - no error detected

                  But maybe errors nonetheless

                  Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

                  5 DataLink Layer 5-12

                  Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

                  polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

                  o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

                  remainder error detectedo can detect all burst errors less than r+1 bits

                  widely used in practice (Ethernet 80211 WiFi ATM)

                  Cyclic Redundancy Check Modulo 2 arithmetic

                  o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                  A polynomial G(x) of degree r is known to both sender and receiver

                  Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                  Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                  If not there are transmission errors in the frame

                  7 4 3( ) 1D x x x x x

                  Common polynomials for G(x)

                  CRC

                  CRC-8

                  CRC-10

                  CRC-12

                  CRC-16

                  CRC-CCITT

                  CRC-32

                  C(x)

                  x8+x2+x1+1

                  x10+x9+x5+x4+x1+1

                  x12+x11+x3+x2+x1+1

                  x16+x15+x2+1

                  x16+x12+x5+1

                  x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                  5 DataLink Layer 5-15

                  CRC Example

                  31001 represents 1x

                  5 DataLink Layer 5-16

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53Multiple access protocols

                  54 Link-layer Addressing

                  55 Ethernet

                  56 Link-layer switches 59 A day in the life of

                  a web request

                  5 DataLink Layer 5-17

                  Multiple Access Links and Protocols

                  Two types of ldquolinksrdquo point-to-point

                  o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                  broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                  shared wire (eg cabled Ethernet)

                  shared RF (eg 80211 WiFi)

                  shared RF(satellite)

                  humans at acocktail party

                  (shared air acoustical)

                  Introduction 1-18

                  Cable Network Architecture Overview

                  home

                  cable headend

                  cable distributionnetwork (simplified)

                  Typically 500 to 5000 homes

                  5 DataLink Layer 5-19

                  Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                  interference o collision if node receives two or more signals at the same

                  time

                  multiple access protocol distributed algorithm that determines how nodes

                  share channel ie determine when node can transmit

                  communication about channel sharing must use channel itself o no out-of-band channel for coordination

                  5 DataLink Layer 5-20

                  Ideal Multiple Access Protocol

                  Broadcast channel of rate R bps1 when one node wants to transmit it can send

                  at the full rate say R2 when M nodes want to transmit each can

                  send at average rate RM3 fully decentralized

                  o no special node to coordinate transmissionso no synchronization of clocks slots

                  4 simple

                  5 DataLink Layer 5-21

                  MAC Protocols a taxonomy

                  Three broad classes Channel Partitioning

                  o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                  o allocate piece to node for exclusive use

                  Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                  ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                  take longer turns

                  5 DataLink Layer 5-22

                  Channel Partitioning MAC protocols TDMA

                  TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                  pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                  256 idle

                  1 3 4 1 3 4

                  6-slotframe

                  5 DataLink Layer 5-23

                  Channel Partitioning MAC protocols FDMA

                  FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                  idle example 6-station LAN 134 have pkt

                  frequency bands 256 idle fr

                  equ

                  ency

                  bands time

                  FDM cable

                  5 DataLink Layer 5-24

                  Random Access Protocols

                  When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                  two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                  o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                  retransmissions)

                  Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                  5 DataLink Layer 5-25

                  ALOHA

                  When a node has a frame to send send immediately

                  Set a timer for a random amount of time If an ACK arrives before the timer expires

                  fine otherwise resend the frame

                  (Works like stop-and-wait with random timeout interval)

                  5 DataLink Layer 5-26

                  CSMA (Carrier Sense Multiple Access)

                  CSMA listen before transmit

                  If channel sensed idle transmit entire frame

                  If channel sensed busy defer transmission

                  human analogy donrsquot interrupt others

                  5 DataLink Layer 5-27

                  CSMA collisions

                  collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                  spatial layout of nodes

                  5 DataLink Layer 5-28

                  CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                  o collisions detected within short timeo colliding transmissions aborted reducing

                  channel wastage collision detection

                  o easy in wired LANs measure signal strengths compare transmitted amp received signals

                  o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                  human analogy the polite conversationalist

                  5 DataLink Layer 5-29

                  CSMACD collision detection

                  5 DataLink Layer 5-30

                  ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                  o share channel efficiently and fairly at high load

                  o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                  Random access MAC protocolso efficient at low load single node can fully

                  utilize channelo high load collision overhead

                  ldquotaking turnsrdquo protocolslook for best of both worlds

                  5 DataLink Layer 5-31

                  ldquoTaking Turnsrdquo MAC protocolsPolling master node

                  ldquoinvitesrdquo slave nodes to transmit in turn

                  typically used with ldquodumbrdquo slave devices

                  concernso polling overhead o latencyo single point of

                  failure (master)

                  master

                  slaves

                  poll

                  data

                  data

                  5 DataLink Layer 5-32

                  ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                  passed from one node to next sequentially

                  token message concerns

                  o token overhead o latencyo single point of failure

                  (token)

                  T

                  data

                  (nothingto send)

                  T

                  5 DataLink Layer 5-33

                  Summary of MAC protocols

                  channel partitioning by time frequency or codeo Time Division Frequency Division

                  random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                  hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                  taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                  5 DataLink Layer 5-34

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53Multiple access protocols

                  54 Link-Layer Addressing

                  55 Ethernet

                  56 Link-layer switches 59 A day in the life of

                  a web request

                  5 DataLink Layer 5-35

                  MAC Addresses and ARP

                  32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                  MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                  another physically-connected interface (in same network)

                  o 48 bit MAC addressbull burned in NIC ROM

                  5 DataLink Layer 5-36

                  LAN Addresses and ARPEach adapter on LAN has unique LAN address

                  Broadcast address =FF-FF-FF-FF-FF-FF

                  = adapter

                  1A-2F-BB-76-09-AD

                  58-23-D7-FA-20-B0

                  0C-C4-11-6F-E3-98

                  71-65-F7-2B-08-53

                  LAN(wired orwireless)

                  5 DataLink Layer 5-37

                  LAN Address (more)

                  MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                  space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                  o can move LAN card from one LAN to another

                  IP hierarchical address NOT portableo address depends on IP subnet to which node is

                  attached

                  5 DataLink Layer 5-38

                  ARP Address Resolution Protocol

                  Each IP node (host router) on LAN has ARP table

                  ARP table IPMAC address mappings for some LAN nodes

                  lt IP address MAC address TTLgto TTL (Time To Live)

                  time after which address mapping will be forgotten (typically 20 min)

                  Q given a nodersquos IP address how to determine its MAC address

                  1A-2F-BB-76-09-AD

                  0C-C4-11-6F-E3-98

                  71-65-F7-2B-08-53

                  LAN

                  137196723

                  137196778

                  137196714

                  137196788

                  5 DataLink Layer 5-39

                  ARP protocol Same LAN (network) A wants to send datagram

                  to B and Brsquos MAC address not in Arsquos ARP table

                  A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                  FF-FF-FF-FF-FFo all machines on LAN

                  receive ARP query B receives ARP packet

                  replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                  address (unicast)

                  A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                  ARP is ldquoplug-and-playrdquoo nodes create their

                  ARP tables without intervention from net administrator

                  5 DataLink Layer 5-40

                  Addressing routing to another LAN

                  R

                  1A-23-F9-CD-06-9B

                  222222222220111111111110

                  E6-E9-00-17-BB-4B

                  CC-49-DE-D0-AB-7D

                  111111111112

                  111111111111

                  A74-29-9C-E8-FF-55

                  222222222221

                  88-B2-2F-54-1A-0F

                  B222222222222

                  49-BD-D2-C7-56-2A

                  walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                  two ARP tables in router R one for each IP network (LAN)

                  LAN LAN

                  5 DataLink Layer 5-41

                  A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                  frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                  destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                  R

                  1A-23-F9-CD-06-9B

                  222222222220

                  111111111110

                  E6-E9-00-17-BB-4B

                  CC-49-DE-D0-AB-7D

                  111111111112

                  111111111111

                  A74-29-9C-E8-FF-55

                  222222222221

                  88-B2-2F-54-1A-0F

                  B222222222222

                  49-BD-D2-C7-56-2A

                  This is a really importantexample ndash make sure youunderstand

                  5 DataLink Layer 5-42

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53Multiple access protocols

                  54 Link-Layer Addressing

                  55 Ethernet

                  56 Link-layer switches 59 A day in the life of

                  a web request

                  5 DataLink Layer 5-43

                  Ethernet

                  ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                  Metcalfersquos Ethernetsketch

                  5 DataLink Layer 5-44

                  Star topology bus topology popular through mid 90s

                  o all nodes in same collision domain (can collide with each other)

                  today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                  (nodes do not collide with each other)

                  switch

                  bus coaxial cable star

                  5 DataLink Layer 5-45

                  Ethernet Frame Structure

                  Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                  Preamble 7 bytes with pattern 10101010 followed by one

                  byte with pattern 10101011 used to synchronize receiver amp sender clock

                  rates

                  5 DataLink Layer 5-46

                  Ethernet Frame Structure (more) Addresses 6 bytes

                  o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                  o otherwise adapter discards frame

                  Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                  CRC checked at receiver if error is detected frame is dropped

                  5 DataLink Layer 5-47

                  Ethernet Unreliable connectionless connectionless No handshaking between sending

                  and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                  send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                  gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                  Ethernetrsquos MAC protocol CSMACD

                  5 DataLink Layer 5-48

                  Ethernet CSMACD algorithm

                  1 NIC receives datagram from network layer creates frame

                  2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                  3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                  4 If NIC detects collision while transmitting aborts and sends jam signal

                  5 After aborting NIC enters exponential backoff

                  after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                  (1 slot = 512 bit-times)

                  5 DataLink Layer 5-49

                  Ethernetrsquos CSMACD (more)

                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                  Exponential Backoff Goal adapt retransmission

                  attempts to estimated current loado heavy load random

                  wait will be longer first collision choose K

                  from 01 delay is K 512 bit transmission times

                  after second collision choose K from 0123hellip

                  after ten collisions choose K from 01234hellip1023

                  5 DataLink Layer 5-50

                  8023 Ethernet Standards Link amp Physical Layers

                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                  applicationtransportnetwork

                  linkphysical

                  MAC protocoland frame format

                  100BASE-TX

                  100BASE-T4

                  100BASE-FX100BASE-T2

                  100BASE-SX 100BASE-BX

                  fiber physical layercopper (twisterpair) physical layer

                  5 DataLink Layer 5-51

                  Manchester encoding

                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                  synchronize to each othero no need for a centralized global clock among nodes

                  Hey this is physical-layer stuff

                  5 DataLink Layer 5-52

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53 Multiple access protocols

                  54 Link-layer Addressing

                  55 Ethernet

                  56 Link-layer switches LANs

                  59 A day in the life of a web request

                  5 DataLink Layer 5-53

                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                  o bits coming in one link go out all other links at same rate

                  o all nodes connected to hub can collide with one another

                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                  twisted pair

                  hub

                  5 DataLink Layer 5-54

                  Switch link-layer device smarter than hubs take active

                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                  transparento hosts are unaware of presence of switches

                  plug-and-play self-learningo switches do not need to be configured

                  5 DataLink Layer 5-55

                  Switch allows multiple simultaneous transmissions

                  hosts have dedicated direct connection to switch

                  switches buffer packets Ethernet protocol used on

                  each incoming link but no collisions full duplexo each link is its own collision

                  domain switching A-to-Arsquo and B-

                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                  A

                  Arsquo

                  B

                  Brsquo

                  C

                  Crsquo

                  switch with six interfaces(123456)

                  1 23

                  45

                  6

                  5 DataLink Layer 5-56

                  Switch Table

                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                  A each switch has a switch table each entryo (MAC address of host interface

                  to reach host time stamp)

                  looks like a routing table Q how are entries created

                  maintained in switch table o something like a routing

                  protocol

                  A

                  Arsquo

                  B

                  Brsquo

                  C

                  Crsquo

                  switch with six interfaces(123456)

                  1 23

                  45

                  6

                  5 DataLink Layer 5-57

                  Switch self-learning

                  switch learns which hosts can be reached through which interfaceso when frame received

                  switch ldquolearnsrdquo location of sender incoming LAN segment

                  o records senderlocation pair in switch table

                  A

                  Arsquo

                  B

                  Brsquo

                  C

                  Crsquo

                  1 23

                  45

                  6

                  A Arsquo

                  Source ADest Arsquo

                  MAC addr interface TTL

                  Switch table (initially empty)

                  A 1 60

                  5 DataLink Layer 5-58

                  Switch frame filteringforwardingWhen frame received

                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                  then if dest on segment from which frame arrived

                  then drop the frame else forward the frame on interface indicated else flood

                  forward on all but the interface on which the frame arrived

                  5 DataLink Layer 5-59

                  Self-learning forwarding example

                  A

                  Arsquo

                  B

                  Brsquo

                  C

                  Crsquo

                  1 23

                  45

                  6

                  A Arsquo

                  Source ADest Arsquo

                  MAC addr interface TTL

                  Switch table (initially empty)

                  A 1 60

                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                  frame destination unknownflood

                  Arsquo A

                  destination A location known

                  Arsquo 4 60

                  selective send

                  5 DataLink Layer 5-60

                  Interconnecting switches

                  switches can be connected together

                  A

                  B

                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                  A self learning (works exactly the same as in single-switch case)

                  S1

                  C D

                  E

                  FS2

                  S4

                  S3

                  H

                  I

                  G

                  5 DataLink Layer 5-61

                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                  A

                  B

                  S1

                  C D

                  E

                  FS2

                  S4

                  S3

                  H

                  I

                  G

                  1

                  2 3

                  34

                  5 DataLink Layer 5-62

                  Institutional network

                  to externalnetwork

                  router

                  IP subnet

                  mail server

                  web server

                  5 DataLink Layer 5-63

                  Switches vs Routers both store-and-forward devices

                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                  routers maintain routing tables implement routing algorithms

                  switches maintain switch tables implement filtering learning algorithms

                  Switch

                  5 DataLink Layer 5-64

                  Link Layer

                  51 Introduction and services

                  52 Error detection and correction

                  53Multiple access protocols

                  54 Link-Layer Addressing

                  55 Ethernet

                  56 Link-layer switches 59 A day in the life of

                  a web request

                  5 DataLink Layer 5-65

                  Synthesis a day in the life of a web request

                  journey down protocol stack completeo application transport network link

                  putting-it-all-together synthesiso goal identify review understand protocols

                  (at all layers) involved in seemingly simple scenario requesting www page

                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                  5 DataLink Layer 5-66

                  A day in the life scenario

                  Comcast network 68800013

                  Googlersquos network 64233160019 64233169105

                  web server

                  DNS server

                  school network 68802024

                  browser

                  web page

                  5 DataLink Layer 5-67

                  A day in the lifehellip connecting to the Internet

                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                  router(runs DHCP)

                  DHCPUDP

                  IPEthPhy

                  DHCP

                  DHCP

                  DHCP

                  DHCP

                  DHCP

                  DHCPUDP

                  IPEthPhy

                  DHCP

                  DHCP

                  DHCP

                  DHCPDHCP

                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                  5 DataLink Layer 5-68

                  A day in the lifehellip connecting to the Internet

                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                  router(runs DHCP)

                  DHCPUDP

                  IPEthPhy

                  DHCP

                  DHCP

                  DHCP

                  DHCP

                  DHCPUDP

                  IPEthPhy

                  DHCP

                  DHCP

                  DHCP

                  DHCP

                  DHCP

                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                  DHCP client receives DHCP ACK reply

                  5 DataLink Layer 5-69

                  A day in the lifehellip ARP (before DNS before HTTP)

                  before sending HTTP request need IP address of wwwgooglecom DNS

                  DNSUDP

                  IPEthPhy

                  DNS

                  DNS

                  DNS

                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                  ARP query

                  EthPhy

                  ARP

                  ARP

                  ARP reply

                  5 DataLink Layer 5-70

                  A day in the lifehellip using DNS

                  DNSUDP

                  IPEthPhy

                  DNS

                  DNS

                  DNS

                  DNS

                  DNS

                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                  demuxrsquoed to DNS server DNS server replies to

                  client with IP address of wwwgooglecom

                  Comcast network 68800013

                  DNS server

                  DNSUDP

                  IPEthPhy

                  DNS

                  DNS

                  DNS

                  DNS

                  5 DataLink Layer 5-71

                  A day in the lifehellip TCP connection carrying HTTP

                  HTTPTCPIP

                  EthPhy

                  HTTP

                  to send HTTP request client first opens TCP socket to web server

                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                  TCP connection established

                  64233169105

                  web server

                  SYN

                  SYN

                  SYN

                  SYN

                  TCPIP

                  EthPhy

                  SYN

                  SYN

                  SYN

                  SYNACK

                  SYNACK

                  SYNACK

                  SYNACK

                  SYNACK

                  SYNACK

                  SYNACK

                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                  5 DataLink Layer 5-72

                  A day in the lifehellip HTTP requestreply

                  HTTPTCPIP

                  EthPhy

                  HTTP

                  HTTP request sent into TCP socket

                  IP datagram containing HTTP request routed to wwwgooglecom

                  IP datgram containing HTTP reply routed back to client

                  64233169105

                  web server

                  HTTPTCPIP

                  EthPhy

                  web server responds with HTTP reply (containing web page)

                  HTTP

                  HTTP

                  HTTPHTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  HTTP

                  web page finally () displayed

                  5 DataLink Layer 5-73

                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                  (except PHY) solid understanding of networking

                  principles practice hellip could stop here hellip but lots of

                  interesting topicso Internetworking (CSE 678 TCPIP socket

                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                  • Link Layer
                  • Link Layer Introduction
                  • Link layer context
                  • Encapsulation
                  • Link Layer Services
                  • Link Layer Services (more)
                  • Where is the link layer implemented
                  • Adaptors Communicating
                  • Slide 9
                  • Parity Checking
                  • Internet checksum (review)
                  • Checksumming Cyclic Redundancy Check
                  • Cyclic Redundancy Check
                  • Slide 14
                  • CRC Example
                  • Slide 16
                  • Multiple Access Links and Protocols
                  • Cable Network Architecture Overview
                  • Multiple Access protocols
                  • Ideal Multiple Access Protocol
                  • MAC Protocols a taxonomy
                  • Channel Partitioning MAC protocols TDMA
                  • Channel Partitioning MAC protocols FDMA
                  • Random Access Protocols
                  • ALOHA
                  • CSMA (Carrier Sense Multiple Access)
                  • CSMA collisions
                  • CSMACD (Collision Detection)
                  • CSMACD collision detection
                  • ldquoTaking Turnsrdquo MAC protocols
                  • Slide 31
                  • Slide 32
                  • Summary of MAC protocols
                  • Slide 34
                  • MAC Addresses and ARP
                  • LAN Addresses and ARP
                  • LAN Address (more)
                  • ARP Address Resolution Protocol
                  • ARP protocol Same LAN (network)
                  • Addressing routing to another LAN
                  • Slide 41
                  • Slide 42
                  • Ethernet
                  • Star topology
                  • Ethernet Frame Structure
                  • Ethernet Frame Structure (more)
                  • Ethernet Unreliable connectionless
                  • Ethernet CSMACD algorithm
                  • Ethernetrsquos CSMACD (more)
                  • 8023 Ethernet Standards Link amp Physical Layers
                  • Manchester encoding
                  • Slide 52
                  • Hubs
                  • Switch
                  • Switch allows multiple simultaneous transmissions
                  • Switch Table
                  • Switch self-learning
                  • Switch frame filteringforwarding
                  • Self-learning forwarding example
                  • Interconnecting switches
                  • Self-learning multi-switch example
                  • Institutional network
                  • Switches vs Routers
                  • Slide 64
                  • Synthesis a day in the life of a web request
                  • A day in the life scenario
                  • A day in the lifehellip connecting to the Internet
                  • Slide 68
                  • A day in the lifehellip ARP (before DNS before HTTP)
                  • A day in the lifehellip using DNS
                  • A day in the lifehellip TCP connection carrying HTTP
                  • A day in the lifehellip HTTP requestreply
                  • Chapter 5 letrsquos take a breath

                    5 DataLink Layer 5-10

                    Parity Checking

                    Single Bit ParityDetect single bit errors

                    Two Dimensional Bit ParityDetect and correct single bit errors

                    0 0

                    5 DataLink Layer 5-11

                    Internet checksum (review)

                    Sender treat segment contents

                    as sequence of 16-bit integers

                    checksum addition (1rsquos complement sum) of segment contents

                    sender puts checksum value into UDP checksum field

                    Receiver compute checksum of

                    received segment check if computed

                    checksum equals checksum field valueo NO - error detectedo YES - no error detected

                    But maybe errors nonetheless

                    Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

                    5 DataLink Layer 5-12

                    Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

                    polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

                    o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

                    remainder error detectedo can detect all burst errors less than r+1 bits

                    widely used in practice (Ethernet 80211 WiFi ATM)

                    Cyclic Redundancy Check Modulo 2 arithmetic

                    o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                    A polynomial G(x) of degree r is known to both sender and receiver

                    Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                    Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                    If not there are transmission errors in the frame

                    7 4 3( ) 1D x x x x x

                    Common polynomials for G(x)

                    CRC

                    CRC-8

                    CRC-10

                    CRC-12

                    CRC-16

                    CRC-CCITT

                    CRC-32

                    C(x)

                    x8+x2+x1+1

                    x10+x9+x5+x4+x1+1

                    x12+x11+x3+x2+x1+1

                    x16+x15+x2+1

                    x16+x12+x5+1

                    x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                    5 DataLink Layer 5-15

                    CRC Example

                    31001 represents 1x

                    5 DataLink Layer 5-16

                    Link Layer

                    51 Introduction and services

                    52 Error detection and correction

                    53Multiple access protocols

                    54 Link-layer Addressing

                    55 Ethernet

                    56 Link-layer switches 59 A day in the life of

                    a web request

                    5 DataLink Layer 5-17

                    Multiple Access Links and Protocols

                    Two types of ldquolinksrdquo point-to-point

                    o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                    broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                    shared wire (eg cabled Ethernet)

                    shared RF (eg 80211 WiFi)

                    shared RF(satellite)

                    humans at acocktail party

                    (shared air acoustical)

                    Introduction 1-18

                    Cable Network Architecture Overview

                    home

                    cable headend

                    cable distributionnetwork (simplified)

                    Typically 500 to 5000 homes

                    5 DataLink Layer 5-19

                    Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                    interference o collision if node receives two or more signals at the same

                    time

                    multiple access protocol distributed algorithm that determines how nodes

                    share channel ie determine when node can transmit

                    communication about channel sharing must use channel itself o no out-of-band channel for coordination

                    5 DataLink Layer 5-20

                    Ideal Multiple Access Protocol

                    Broadcast channel of rate R bps1 when one node wants to transmit it can send

                    at the full rate say R2 when M nodes want to transmit each can

                    send at average rate RM3 fully decentralized

                    o no special node to coordinate transmissionso no synchronization of clocks slots

                    4 simple

                    5 DataLink Layer 5-21

                    MAC Protocols a taxonomy

                    Three broad classes Channel Partitioning

                    o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                    o allocate piece to node for exclusive use

                    Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                    ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                    take longer turns

                    5 DataLink Layer 5-22

                    Channel Partitioning MAC protocols TDMA

                    TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                    pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                    256 idle

                    1 3 4 1 3 4

                    6-slotframe

                    5 DataLink Layer 5-23

                    Channel Partitioning MAC protocols FDMA

                    FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                    idle example 6-station LAN 134 have pkt

                    frequency bands 256 idle fr

                    equ

                    ency

                    bands time

                    FDM cable

                    5 DataLink Layer 5-24

                    Random Access Protocols

                    When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                    two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                    o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                    retransmissions)

                    Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                    5 DataLink Layer 5-25

                    ALOHA

                    When a node has a frame to send send immediately

                    Set a timer for a random amount of time If an ACK arrives before the timer expires

                    fine otherwise resend the frame

                    (Works like stop-and-wait with random timeout interval)

                    5 DataLink Layer 5-26

                    CSMA (Carrier Sense Multiple Access)

                    CSMA listen before transmit

                    If channel sensed idle transmit entire frame

                    If channel sensed busy defer transmission

                    human analogy donrsquot interrupt others

                    5 DataLink Layer 5-27

                    CSMA collisions

                    collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                    spatial layout of nodes

                    5 DataLink Layer 5-28

                    CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                    o collisions detected within short timeo colliding transmissions aborted reducing

                    channel wastage collision detection

                    o easy in wired LANs measure signal strengths compare transmitted amp received signals

                    o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                    human analogy the polite conversationalist

                    5 DataLink Layer 5-29

                    CSMACD collision detection

                    5 DataLink Layer 5-30

                    ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                    o share channel efficiently and fairly at high load

                    o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                    Random access MAC protocolso efficient at low load single node can fully

                    utilize channelo high load collision overhead

                    ldquotaking turnsrdquo protocolslook for best of both worlds

                    5 DataLink Layer 5-31

                    ldquoTaking Turnsrdquo MAC protocolsPolling master node

                    ldquoinvitesrdquo slave nodes to transmit in turn

                    typically used with ldquodumbrdquo slave devices

                    concernso polling overhead o latencyo single point of

                    failure (master)

                    master

                    slaves

                    poll

                    data

                    data

                    5 DataLink Layer 5-32

                    ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                    passed from one node to next sequentially

                    token message concerns

                    o token overhead o latencyo single point of failure

                    (token)

                    T

                    data

                    (nothingto send)

                    T

                    5 DataLink Layer 5-33

                    Summary of MAC protocols

                    channel partitioning by time frequency or codeo Time Division Frequency Division

                    random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                    hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                    taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                    5 DataLink Layer 5-34

                    Link Layer

                    51 Introduction and services

                    52 Error detection and correction

                    53Multiple access protocols

                    54 Link-Layer Addressing

                    55 Ethernet

                    56 Link-layer switches 59 A day in the life of

                    a web request

                    5 DataLink Layer 5-35

                    MAC Addresses and ARP

                    32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                    MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                    another physically-connected interface (in same network)

                    o 48 bit MAC addressbull burned in NIC ROM

                    5 DataLink Layer 5-36

                    LAN Addresses and ARPEach adapter on LAN has unique LAN address

                    Broadcast address =FF-FF-FF-FF-FF-FF

                    = adapter

                    1A-2F-BB-76-09-AD

                    58-23-D7-FA-20-B0

                    0C-C4-11-6F-E3-98

                    71-65-F7-2B-08-53

                    LAN(wired orwireless)

                    5 DataLink Layer 5-37

                    LAN Address (more)

                    MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                    space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                    o can move LAN card from one LAN to another

                    IP hierarchical address NOT portableo address depends on IP subnet to which node is

                    attached

                    5 DataLink Layer 5-38

                    ARP Address Resolution Protocol

                    Each IP node (host router) on LAN has ARP table

                    ARP table IPMAC address mappings for some LAN nodes

                    lt IP address MAC address TTLgto TTL (Time To Live)

                    time after which address mapping will be forgotten (typically 20 min)

                    Q given a nodersquos IP address how to determine its MAC address

                    1A-2F-BB-76-09-AD

                    0C-C4-11-6F-E3-98

                    71-65-F7-2B-08-53

                    LAN

                    137196723

                    137196778

                    137196714

                    137196788

                    5 DataLink Layer 5-39

                    ARP protocol Same LAN (network) A wants to send datagram

                    to B and Brsquos MAC address not in Arsquos ARP table

                    A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                    FF-FF-FF-FF-FFo all machines on LAN

                    receive ARP query B receives ARP packet

                    replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                    address (unicast)

                    A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                    ARP is ldquoplug-and-playrdquoo nodes create their

                    ARP tables without intervention from net administrator

                    5 DataLink Layer 5-40

                    Addressing routing to another LAN

                    R

                    1A-23-F9-CD-06-9B

                    222222222220111111111110

                    E6-E9-00-17-BB-4B

                    CC-49-DE-D0-AB-7D

                    111111111112

                    111111111111

                    A74-29-9C-E8-FF-55

                    222222222221

                    88-B2-2F-54-1A-0F

                    B222222222222

                    49-BD-D2-C7-56-2A

                    walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                    two ARP tables in router R one for each IP network (LAN)

                    LAN LAN

                    5 DataLink Layer 5-41

                    A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                    frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                    destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                    R

                    1A-23-F9-CD-06-9B

                    222222222220

                    111111111110

                    E6-E9-00-17-BB-4B

                    CC-49-DE-D0-AB-7D

                    111111111112

                    111111111111

                    A74-29-9C-E8-FF-55

                    222222222221

                    88-B2-2F-54-1A-0F

                    B222222222222

                    49-BD-D2-C7-56-2A

                    This is a really importantexample ndash make sure youunderstand

                    5 DataLink Layer 5-42

                    Link Layer

                    51 Introduction and services

                    52 Error detection and correction

                    53Multiple access protocols

                    54 Link-Layer Addressing

                    55 Ethernet

                    56 Link-layer switches 59 A day in the life of

                    a web request

                    5 DataLink Layer 5-43

                    Ethernet

                    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                    Metcalfersquos Ethernetsketch

                    5 DataLink Layer 5-44

                    Star topology bus topology popular through mid 90s

                    o all nodes in same collision domain (can collide with each other)

                    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                    (nodes do not collide with each other)

                    switch

                    bus coaxial cable star

                    5 DataLink Layer 5-45

                    Ethernet Frame Structure

                    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                    Preamble 7 bytes with pattern 10101010 followed by one

                    byte with pattern 10101011 used to synchronize receiver amp sender clock

                    rates

                    5 DataLink Layer 5-46

                    Ethernet Frame Structure (more) Addresses 6 bytes

                    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                    o otherwise adapter discards frame

                    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                    CRC checked at receiver if error is detected frame is dropped

                    5 DataLink Layer 5-47

                    Ethernet Unreliable connectionless connectionless No handshaking between sending

                    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                    Ethernetrsquos MAC protocol CSMACD

                    5 DataLink Layer 5-48

                    Ethernet CSMACD algorithm

                    1 NIC receives datagram from network layer creates frame

                    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                    4 If NIC detects collision while transmitting aborts and sends jam signal

                    5 After aborting NIC enters exponential backoff

                    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                    (1 slot = 512 bit-times)

                    5 DataLink Layer 5-49

                    Ethernetrsquos CSMACD (more)

                    Jam Signal make sure all other transmitters are aware of collision 48 bits

                    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                    Exponential Backoff Goal adapt retransmission

                    attempts to estimated current loado heavy load random

                    wait will be longer first collision choose K

                    from 01 delay is K 512 bit transmission times

                    after second collision choose K from 0123hellip

                    after ten collisions choose K from 01234hellip1023

                    5 DataLink Layer 5-50

                    8023 Ethernet Standards Link amp Physical Layers

                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                    applicationtransportnetwork

                    linkphysical

                    MAC protocoland frame format

                    100BASE-TX

                    100BASE-T4

                    100BASE-FX100BASE-T2

                    100BASE-SX 100BASE-BX

                    fiber physical layercopper (twisterpair) physical layer

                    5 DataLink Layer 5-51

                    Manchester encoding

                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                    synchronize to each othero no need for a centralized global clock among nodes

                    Hey this is physical-layer stuff

                    5 DataLink Layer 5-52

                    Link Layer

                    51 Introduction and services

                    52 Error detection and correction

                    53 Multiple access protocols

                    54 Link-layer Addressing

                    55 Ethernet

                    56 Link-layer switches LANs

                    59 A day in the life of a web request

                    5 DataLink Layer 5-53

                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                    o bits coming in one link go out all other links at same rate

                    o all nodes connected to hub can collide with one another

                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                    twisted pair

                    hub

                    5 DataLink Layer 5-54

                    Switch link-layer device smarter than hubs take active

                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                    transparento hosts are unaware of presence of switches

                    plug-and-play self-learningo switches do not need to be configured

                    5 DataLink Layer 5-55

                    Switch allows multiple simultaneous transmissions

                    hosts have dedicated direct connection to switch

                    switches buffer packets Ethernet protocol used on

                    each incoming link but no collisions full duplexo each link is its own collision

                    domain switching A-to-Arsquo and B-

                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                    A

                    Arsquo

                    B

                    Brsquo

                    C

                    Crsquo

                    switch with six interfaces(123456)

                    1 23

                    45

                    6

                    5 DataLink Layer 5-56

                    Switch Table

                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                    A each switch has a switch table each entryo (MAC address of host interface

                    to reach host time stamp)

                    looks like a routing table Q how are entries created

                    maintained in switch table o something like a routing

                    protocol

                    A

                    Arsquo

                    B

                    Brsquo

                    C

                    Crsquo

                    switch with six interfaces(123456)

                    1 23

                    45

                    6

                    5 DataLink Layer 5-57

                    Switch self-learning

                    switch learns which hosts can be reached through which interfaceso when frame received

                    switch ldquolearnsrdquo location of sender incoming LAN segment

                    o records senderlocation pair in switch table

                    A

                    Arsquo

                    B

                    Brsquo

                    C

                    Crsquo

                    1 23

                    45

                    6

                    A Arsquo

                    Source ADest Arsquo

                    MAC addr interface TTL

                    Switch table (initially empty)

                    A 1 60

                    5 DataLink Layer 5-58

                    Switch frame filteringforwardingWhen frame received

                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                    then if dest on segment from which frame arrived

                    then drop the frame else forward the frame on interface indicated else flood

                    forward on all but the interface on which the frame arrived

                    5 DataLink Layer 5-59

                    Self-learning forwarding example

                    A

                    Arsquo

                    B

                    Brsquo

                    C

                    Crsquo

                    1 23

                    45

                    6

                    A Arsquo

                    Source ADest Arsquo

                    MAC addr interface TTL

                    Switch table (initially empty)

                    A 1 60

                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                    frame destination unknownflood

                    Arsquo A

                    destination A location known

                    Arsquo 4 60

                    selective send

                    5 DataLink Layer 5-60

                    Interconnecting switches

                    switches can be connected together

                    A

                    B

                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                    A self learning (works exactly the same as in single-switch case)

                    S1

                    C D

                    E

                    FS2

                    S4

                    S3

                    H

                    I

                    G

                    5 DataLink Layer 5-61

                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                    A

                    B

                    S1

                    C D

                    E

                    FS2

                    S4

                    S3

                    H

                    I

                    G

                    1

                    2 3

                    34

                    5 DataLink Layer 5-62

                    Institutional network

                    to externalnetwork

                    router

                    IP subnet

                    mail server

                    web server

                    5 DataLink Layer 5-63

                    Switches vs Routers both store-and-forward devices

                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                    routers maintain routing tables implement routing algorithms

                    switches maintain switch tables implement filtering learning algorithms

                    Switch

                    5 DataLink Layer 5-64

                    Link Layer

                    51 Introduction and services

                    52 Error detection and correction

                    53Multiple access protocols

                    54 Link-Layer Addressing

                    55 Ethernet

                    56 Link-layer switches 59 A day in the life of

                    a web request

                    5 DataLink Layer 5-65

                    Synthesis a day in the life of a web request

                    journey down protocol stack completeo application transport network link

                    putting-it-all-together synthesiso goal identify review understand protocols

                    (at all layers) involved in seemingly simple scenario requesting www page

                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                    5 DataLink Layer 5-66

                    A day in the life scenario

                    Comcast network 68800013

                    Googlersquos network 64233160019 64233169105

                    web server

                    DNS server

                    school network 68802024

                    browser

                    web page

                    5 DataLink Layer 5-67

                    A day in the lifehellip connecting to the Internet

                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                    router(runs DHCP)

                    DHCPUDP

                    IPEthPhy

                    DHCP

                    DHCP

                    DHCP

                    DHCP

                    DHCP

                    DHCPUDP

                    IPEthPhy

                    DHCP

                    DHCP

                    DHCP

                    DHCPDHCP

                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                    5 DataLink Layer 5-68

                    A day in the lifehellip connecting to the Internet

                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                    router(runs DHCP)

                    DHCPUDP

                    IPEthPhy

                    DHCP

                    DHCP

                    DHCP

                    DHCP

                    DHCPUDP

                    IPEthPhy

                    DHCP

                    DHCP

                    DHCP

                    DHCP

                    DHCP

                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                    DHCP client receives DHCP ACK reply

                    5 DataLink Layer 5-69

                    A day in the lifehellip ARP (before DNS before HTTP)

                    before sending HTTP request need IP address of wwwgooglecom DNS

                    DNSUDP

                    IPEthPhy

                    DNS

                    DNS

                    DNS

                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                    ARP query

                    EthPhy

                    ARP

                    ARP

                    ARP reply

                    5 DataLink Layer 5-70

                    A day in the lifehellip using DNS

                    DNSUDP

                    IPEthPhy

                    DNS

                    DNS

                    DNS

                    DNS

                    DNS

                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                    demuxrsquoed to DNS server DNS server replies to

                    client with IP address of wwwgooglecom

                    Comcast network 68800013

                    DNS server

                    DNSUDP

                    IPEthPhy

                    DNS

                    DNS

                    DNS

                    DNS

                    5 DataLink Layer 5-71

                    A day in the lifehellip TCP connection carrying HTTP

                    HTTPTCPIP

                    EthPhy

                    HTTP

                    to send HTTP request client first opens TCP socket to web server

                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                    TCP connection established

                    64233169105

                    web server

                    SYN

                    SYN

                    SYN

                    SYN

                    TCPIP

                    EthPhy

                    SYN

                    SYN

                    SYN

                    SYNACK

                    SYNACK

                    SYNACK

                    SYNACK

                    SYNACK

                    SYNACK

                    SYNACK

                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                    5 DataLink Layer 5-72

                    A day in the lifehellip HTTP requestreply

                    HTTPTCPIP

                    EthPhy

                    HTTP

                    HTTP request sent into TCP socket

                    IP datagram containing HTTP request routed to wwwgooglecom

                    IP datgram containing HTTP reply routed back to client

                    64233169105

                    web server

                    HTTPTCPIP

                    EthPhy

                    web server responds with HTTP reply (containing web page)

                    HTTP

                    HTTP

                    HTTPHTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    HTTP

                    web page finally () displayed

                    5 DataLink Layer 5-73

                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                    (except PHY) solid understanding of networking

                    principles practice hellip could stop here hellip but lots of

                    interesting topicso Internetworking (CSE 678 TCPIP socket

                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                    • Link Layer
                    • Link Layer Introduction
                    • Link layer context
                    • Encapsulation
                    • Link Layer Services
                    • Link Layer Services (more)
                    • Where is the link layer implemented
                    • Adaptors Communicating
                    • Slide 9
                    • Parity Checking
                    • Internet checksum (review)
                    • Checksumming Cyclic Redundancy Check
                    • Cyclic Redundancy Check
                    • Slide 14
                    • CRC Example
                    • Slide 16
                    • Multiple Access Links and Protocols
                    • Cable Network Architecture Overview
                    • Multiple Access protocols
                    • Ideal Multiple Access Protocol
                    • MAC Protocols a taxonomy
                    • Channel Partitioning MAC protocols TDMA
                    • Channel Partitioning MAC protocols FDMA
                    • Random Access Protocols
                    • ALOHA
                    • CSMA (Carrier Sense Multiple Access)
                    • CSMA collisions
                    • CSMACD (Collision Detection)
                    • CSMACD collision detection
                    • ldquoTaking Turnsrdquo MAC protocols
                    • Slide 31
                    • Slide 32
                    • Summary of MAC protocols
                    • Slide 34
                    • MAC Addresses and ARP
                    • LAN Addresses and ARP
                    • LAN Address (more)
                    • ARP Address Resolution Protocol
                    • ARP protocol Same LAN (network)
                    • Addressing routing to another LAN
                    • Slide 41
                    • Slide 42
                    • Ethernet
                    • Star topology
                    • Ethernet Frame Structure
                    • Ethernet Frame Structure (more)
                    • Ethernet Unreliable connectionless
                    • Ethernet CSMACD algorithm
                    • Ethernetrsquos CSMACD (more)
                    • 8023 Ethernet Standards Link amp Physical Layers
                    • Manchester encoding
                    • Slide 52
                    • Hubs
                    • Switch
                    • Switch allows multiple simultaneous transmissions
                    • Switch Table
                    • Switch self-learning
                    • Switch frame filteringforwarding
                    • Self-learning forwarding example
                    • Interconnecting switches
                    • Self-learning multi-switch example
                    • Institutional network
                    • Switches vs Routers
                    • Slide 64
                    • Synthesis a day in the life of a web request
                    • A day in the life scenario
                    • A day in the lifehellip connecting to the Internet
                    • Slide 68
                    • A day in the lifehellip ARP (before DNS before HTTP)
                    • A day in the lifehellip using DNS
                    • A day in the lifehellip TCP connection carrying HTTP
                    • A day in the lifehellip HTTP requestreply
                    • Chapter 5 letrsquos take a breath

                      5 DataLink Layer 5-11

                      Internet checksum (review)

                      Sender treat segment contents

                      as sequence of 16-bit integers

                      checksum addition (1rsquos complement sum) of segment contents

                      sender puts checksum value into UDP checksum field

                      Receiver compute checksum of

                      received segment check if computed

                      checksum equals checksum field valueo NO - error detectedo YES - no error detected

                      But maybe errors nonetheless

                      Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)

                      5 DataLink Layer 5-12

                      Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

                      polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

                      o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

                      remainder error detectedo can detect all burst errors less than r+1 bits

                      widely used in practice (Ethernet 80211 WiFi ATM)

                      Cyclic Redundancy Check Modulo 2 arithmetic

                      o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                      A polynomial G(x) of degree r is known to both sender and receiver

                      Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                      Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                      If not there are transmission errors in the frame

                      7 4 3( ) 1D x x x x x

                      Common polynomials for G(x)

                      CRC

                      CRC-8

                      CRC-10

                      CRC-12

                      CRC-16

                      CRC-CCITT

                      CRC-32

                      C(x)

                      x8+x2+x1+1

                      x10+x9+x5+x4+x1+1

                      x12+x11+x3+x2+x1+1

                      x16+x15+x2+1

                      x16+x12+x5+1

                      x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                      5 DataLink Layer 5-15

                      CRC Example

                      31001 represents 1x

                      5 DataLink Layer 5-16

                      Link Layer

                      51 Introduction and services

                      52 Error detection and correction

                      53Multiple access protocols

                      54 Link-layer Addressing

                      55 Ethernet

                      56 Link-layer switches 59 A day in the life of

                      a web request

                      5 DataLink Layer 5-17

                      Multiple Access Links and Protocols

                      Two types of ldquolinksrdquo point-to-point

                      o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                      broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                      shared wire (eg cabled Ethernet)

                      shared RF (eg 80211 WiFi)

                      shared RF(satellite)

                      humans at acocktail party

                      (shared air acoustical)

                      Introduction 1-18

                      Cable Network Architecture Overview

                      home

                      cable headend

                      cable distributionnetwork (simplified)

                      Typically 500 to 5000 homes

                      5 DataLink Layer 5-19

                      Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                      interference o collision if node receives two or more signals at the same

                      time

                      multiple access protocol distributed algorithm that determines how nodes

                      share channel ie determine when node can transmit

                      communication about channel sharing must use channel itself o no out-of-band channel for coordination

                      5 DataLink Layer 5-20

                      Ideal Multiple Access Protocol

                      Broadcast channel of rate R bps1 when one node wants to transmit it can send

                      at the full rate say R2 when M nodes want to transmit each can

                      send at average rate RM3 fully decentralized

                      o no special node to coordinate transmissionso no synchronization of clocks slots

                      4 simple

                      5 DataLink Layer 5-21

                      MAC Protocols a taxonomy

                      Three broad classes Channel Partitioning

                      o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                      o allocate piece to node for exclusive use

                      Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                      ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                      take longer turns

                      5 DataLink Layer 5-22

                      Channel Partitioning MAC protocols TDMA

                      TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                      pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                      256 idle

                      1 3 4 1 3 4

                      6-slotframe

                      5 DataLink Layer 5-23

                      Channel Partitioning MAC protocols FDMA

                      FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                      idle example 6-station LAN 134 have pkt

                      frequency bands 256 idle fr

                      equ

                      ency

                      bands time

                      FDM cable

                      5 DataLink Layer 5-24

                      Random Access Protocols

                      When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                      two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                      o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                      retransmissions)

                      Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                      5 DataLink Layer 5-25

                      ALOHA

                      When a node has a frame to send send immediately

                      Set a timer for a random amount of time If an ACK arrives before the timer expires

                      fine otherwise resend the frame

                      (Works like stop-and-wait with random timeout interval)

                      5 DataLink Layer 5-26

                      CSMA (Carrier Sense Multiple Access)

                      CSMA listen before transmit

                      If channel sensed idle transmit entire frame

                      If channel sensed busy defer transmission

                      human analogy donrsquot interrupt others

                      5 DataLink Layer 5-27

                      CSMA collisions

                      collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                      spatial layout of nodes

                      5 DataLink Layer 5-28

                      CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                      o collisions detected within short timeo colliding transmissions aborted reducing

                      channel wastage collision detection

                      o easy in wired LANs measure signal strengths compare transmitted amp received signals

                      o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                      human analogy the polite conversationalist

                      5 DataLink Layer 5-29

                      CSMACD collision detection

                      5 DataLink Layer 5-30

                      ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                      o share channel efficiently and fairly at high load

                      o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                      Random access MAC protocolso efficient at low load single node can fully

                      utilize channelo high load collision overhead

                      ldquotaking turnsrdquo protocolslook for best of both worlds

                      5 DataLink Layer 5-31

                      ldquoTaking Turnsrdquo MAC protocolsPolling master node

                      ldquoinvitesrdquo slave nodes to transmit in turn

                      typically used with ldquodumbrdquo slave devices

                      concernso polling overhead o latencyo single point of

                      failure (master)

                      master

                      slaves

                      poll

                      data

                      data

                      5 DataLink Layer 5-32

                      ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                      passed from one node to next sequentially

                      token message concerns

                      o token overhead o latencyo single point of failure

                      (token)

                      T

                      data

                      (nothingto send)

                      T

                      5 DataLink Layer 5-33

                      Summary of MAC protocols

                      channel partitioning by time frequency or codeo Time Division Frequency Division

                      random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                      hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                      taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                      5 DataLink Layer 5-34

                      Link Layer

                      51 Introduction and services

                      52 Error detection and correction

                      53Multiple access protocols

                      54 Link-Layer Addressing

                      55 Ethernet

                      56 Link-layer switches 59 A day in the life of

                      a web request

                      5 DataLink Layer 5-35

                      MAC Addresses and ARP

                      32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                      MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                      another physically-connected interface (in same network)

                      o 48 bit MAC addressbull burned in NIC ROM

                      5 DataLink Layer 5-36

                      LAN Addresses and ARPEach adapter on LAN has unique LAN address

                      Broadcast address =FF-FF-FF-FF-FF-FF

                      = adapter

                      1A-2F-BB-76-09-AD

                      58-23-D7-FA-20-B0

                      0C-C4-11-6F-E3-98

                      71-65-F7-2B-08-53

                      LAN(wired orwireless)

                      5 DataLink Layer 5-37

                      LAN Address (more)

                      MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                      space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                      o can move LAN card from one LAN to another

                      IP hierarchical address NOT portableo address depends on IP subnet to which node is

                      attached

                      5 DataLink Layer 5-38

                      ARP Address Resolution Protocol

                      Each IP node (host router) on LAN has ARP table

                      ARP table IPMAC address mappings for some LAN nodes

                      lt IP address MAC address TTLgto TTL (Time To Live)

                      time after which address mapping will be forgotten (typically 20 min)

                      Q given a nodersquos IP address how to determine its MAC address

                      1A-2F-BB-76-09-AD

                      0C-C4-11-6F-E3-98

                      71-65-F7-2B-08-53

                      LAN

                      137196723

                      137196778

                      137196714

                      137196788

                      5 DataLink Layer 5-39

                      ARP protocol Same LAN (network) A wants to send datagram

                      to B and Brsquos MAC address not in Arsquos ARP table

                      A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                      FF-FF-FF-FF-FFo all machines on LAN

                      receive ARP query B receives ARP packet

                      replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                      address (unicast)

                      A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                      ARP is ldquoplug-and-playrdquoo nodes create their

                      ARP tables without intervention from net administrator

                      5 DataLink Layer 5-40

                      Addressing routing to another LAN

                      R

                      1A-23-F9-CD-06-9B

                      222222222220111111111110

                      E6-E9-00-17-BB-4B

                      CC-49-DE-D0-AB-7D

                      111111111112

                      111111111111

                      A74-29-9C-E8-FF-55

                      222222222221

                      88-B2-2F-54-1A-0F

                      B222222222222

                      49-BD-D2-C7-56-2A

                      walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                      two ARP tables in router R one for each IP network (LAN)

                      LAN LAN

                      5 DataLink Layer 5-41

                      A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                      frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                      destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                      R

                      1A-23-F9-CD-06-9B

                      222222222220

                      111111111110

                      E6-E9-00-17-BB-4B

                      CC-49-DE-D0-AB-7D

                      111111111112

                      111111111111

                      A74-29-9C-E8-FF-55

                      222222222221

                      88-B2-2F-54-1A-0F

                      B222222222222

                      49-BD-D2-C7-56-2A

                      This is a really importantexample ndash make sure youunderstand

                      5 DataLink Layer 5-42

                      Link Layer

                      51 Introduction and services

                      52 Error detection and correction

                      53Multiple access protocols

                      54 Link-Layer Addressing

                      55 Ethernet

                      56 Link-layer switches 59 A day in the life of

                      a web request

                      5 DataLink Layer 5-43

                      Ethernet

                      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                      Metcalfersquos Ethernetsketch

                      5 DataLink Layer 5-44

                      Star topology bus topology popular through mid 90s

                      o all nodes in same collision domain (can collide with each other)

                      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                      (nodes do not collide with each other)

                      switch

                      bus coaxial cable star

                      5 DataLink Layer 5-45

                      Ethernet Frame Structure

                      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                      Preamble 7 bytes with pattern 10101010 followed by one

                      byte with pattern 10101011 used to synchronize receiver amp sender clock

                      rates

                      5 DataLink Layer 5-46

                      Ethernet Frame Structure (more) Addresses 6 bytes

                      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                      o otherwise adapter discards frame

                      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                      CRC checked at receiver if error is detected frame is dropped

                      5 DataLink Layer 5-47

                      Ethernet Unreliable connectionless connectionless No handshaking between sending

                      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                      Ethernetrsquos MAC protocol CSMACD

                      5 DataLink Layer 5-48

                      Ethernet CSMACD algorithm

                      1 NIC receives datagram from network layer creates frame

                      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                      4 If NIC detects collision while transmitting aborts and sends jam signal

                      5 After aborting NIC enters exponential backoff

                      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                      (1 slot = 512 bit-times)

                      5 DataLink Layer 5-49

                      Ethernetrsquos CSMACD (more)

                      Jam Signal make sure all other transmitters are aware of collision 48 bits

                      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                      Exponential Backoff Goal adapt retransmission

                      attempts to estimated current loado heavy load random

                      wait will be longer first collision choose K

                      from 01 delay is K 512 bit transmission times

                      after second collision choose K from 0123hellip

                      after ten collisions choose K from 01234hellip1023

                      5 DataLink Layer 5-50

                      8023 Ethernet Standards Link amp Physical Layers

                      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                      Mbps 1Gbps 10G bpso different physical layer media fiber cable

                      applicationtransportnetwork

                      linkphysical

                      MAC protocoland frame format

                      100BASE-TX

                      100BASE-T4

                      100BASE-FX100BASE-T2

                      100BASE-SX 100BASE-BX

                      fiber physical layercopper (twisterpair) physical layer

                      5 DataLink Layer 5-51

                      Manchester encoding

                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                      synchronize to each othero no need for a centralized global clock among nodes

                      Hey this is physical-layer stuff

                      5 DataLink Layer 5-52

                      Link Layer

                      51 Introduction and services

                      52 Error detection and correction

                      53 Multiple access protocols

                      54 Link-layer Addressing

                      55 Ethernet

                      56 Link-layer switches LANs

                      59 A day in the life of a web request

                      5 DataLink Layer 5-53

                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                      o bits coming in one link go out all other links at same rate

                      o all nodes connected to hub can collide with one another

                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                      twisted pair

                      hub

                      5 DataLink Layer 5-54

                      Switch link-layer device smarter than hubs take active

                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                      transparento hosts are unaware of presence of switches

                      plug-and-play self-learningo switches do not need to be configured

                      5 DataLink Layer 5-55

                      Switch allows multiple simultaneous transmissions

                      hosts have dedicated direct connection to switch

                      switches buffer packets Ethernet protocol used on

                      each incoming link but no collisions full duplexo each link is its own collision

                      domain switching A-to-Arsquo and B-

                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                      A

                      Arsquo

                      B

                      Brsquo

                      C

                      Crsquo

                      switch with six interfaces(123456)

                      1 23

                      45

                      6

                      5 DataLink Layer 5-56

                      Switch Table

                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                      A each switch has a switch table each entryo (MAC address of host interface

                      to reach host time stamp)

                      looks like a routing table Q how are entries created

                      maintained in switch table o something like a routing

                      protocol

                      A

                      Arsquo

                      B

                      Brsquo

                      C

                      Crsquo

                      switch with six interfaces(123456)

                      1 23

                      45

                      6

                      5 DataLink Layer 5-57

                      Switch self-learning

                      switch learns which hosts can be reached through which interfaceso when frame received

                      switch ldquolearnsrdquo location of sender incoming LAN segment

                      o records senderlocation pair in switch table

                      A

                      Arsquo

                      B

                      Brsquo

                      C

                      Crsquo

                      1 23

                      45

                      6

                      A Arsquo

                      Source ADest Arsquo

                      MAC addr interface TTL

                      Switch table (initially empty)

                      A 1 60

                      5 DataLink Layer 5-58

                      Switch frame filteringforwardingWhen frame received

                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                      then if dest on segment from which frame arrived

                      then drop the frame else forward the frame on interface indicated else flood

                      forward on all but the interface on which the frame arrived

                      5 DataLink Layer 5-59

                      Self-learning forwarding example

                      A

                      Arsquo

                      B

                      Brsquo

                      C

                      Crsquo

                      1 23

                      45

                      6

                      A Arsquo

                      Source ADest Arsquo

                      MAC addr interface TTL

                      Switch table (initially empty)

                      A 1 60

                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                      frame destination unknownflood

                      Arsquo A

                      destination A location known

                      Arsquo 4 60

                      selective send

                      5 DataLink Layer 5-60

                      Interconnecting switches

                      switches can be connected together

                      A

                      B

                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                      A self learning (works exactly the same as in single-switch case)

                      S1

                      C D

                      E

                      FS2

                      S4

                      S3

                      H

                      I

                      G

                      5 DataLink Layer 5-61

                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                      A

                      B

                      S1

                      C D

                      E

                      FS2

                      S4

                      S3

                      H

                      I

                      G

                      1

                      2 3

                      34

                      5 DataLink Layer 5-62

                      Institutional network

                      to externalnetwork

                      router

                      IP subnet

                      mail server

                      web server

                      5 DataLink Layer 5-63

                      Switches vs Routers both store-and-forward devices

                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                      routers maintain routing tables implement routing algorithms

                      switches maintain switch tables implement filtering learning algorithms

                      Switch

                      5 DataLink Layer 5-64

                      Link Layer

                      51 Introduction and services

                      52 Error detection and correction

                      53Multiple access protocols

                      54 Link-Layer Addressing

                      55 Ethernet

                      56 Link-layer switches 59 A day in the life of

                      a web request

                      5 DataLink Layer 5-65

                      Synthesis a day in the life of a web request

                      journey down protocol stack completeo application transport network link

                      putting-it-all-together synthesiso goal identify review understand protocols

                      (at all layers) involved in seemingly simple scenario requesting www page

                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                      5 DataLink Layer 5-66

                      A day in the life scenario

                      Comcast network 68800013

                      Googlersquos network 64233160019 64233169105

                      web server

                      DNS server

                      school network 68802024

                      browser

                      web page

                      5 DataLink Layer 5-67

                      A day in the lifehellip connecting to the Internet

                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                      router(runs DHCP)

                      DHCPUDP

                      IPEthPhy

                      DHCP

                      DHCP

                      DHCP

                      DHCP

                      DHCP

                      DHCPUDP

                      IPEthPhy

                      DHCP

                      DHCP

                      DHCP

                      DHCPDHCP

                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                      5 DataLink Layer 5-68

                      A day in the lifehellip connecting to the Internet

                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                      router(runs DHCP)

                      DHCPUDP

                      IPEthPhy

                      DHCP

                      DHCP

                      DHCP

                      DHCP

                      DHCPUDP

                      IPEthPhy

                      DHCP

                      DHCP

                      DHCP

                      DHCP

                      DHCP

                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                      DHCP client receives DHCP ACK reply

                      5 DataLink Layer 5-69

                      A day in the lifehellip ARP (before DNS before HTTP)

                      before sending HTTP request need IP address of wwwgooglecom DNS

                      DNSUDP

                      IPEthPhy

                      DNS

                      DNS

                      DNS

                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                      ARP query

                      EthPhy

                      ARP

                      ARP

                      ARP reply

                      5 DataLink Layer 5-70

                      A day in the lifehellip using DNS

                      DNSUDP

                      IPEthPhy

                      DNS

                      DNS

                      DNS

                      DNS

                      DNS

                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                      demuxrsquoed to DNS server DNS server replies to

                      client with IP address of wwwgooglecom

                      Comcast network 68800013

                      DNS server

                      DNSUDP

                      IPEthPhy

                      DNS

                      DNS

                      DNS

                      DNS

                      5 DataLink Layer 5-71

                      A day in the lifehellip TCP connection carrying HTTP

                      HTTPTCPIP

                      EthPhy

                      HTTP

                      to send HTTP request client first opens TCP socket to web server

                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                      TCP connection established

                      64233169105

                      web server

                      SYN

                      SYN

                      SYN

                      SYN

                      TCPIP

                      EthPhy

                      SYN

                      SYN

                      SYN

                      SYNACK

                      SYNACK

                      SYNACK

                      SYNACK

                      SYNACK

                      SYNACK

                      SYNACK

                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                      5 DataLink Layer 5-72

                      A day in the lifehellip HTTP requestreply

                      HTTPTCPIP

                      EthPhy

                      HTTP

                      HTTP request sent into TCP socket

                      IP datagram containing HTTP request routed to wwwgooglecom

                      IP datgram containing HTTP reply routed back to client

                      64233169105

                      web server

                      HTTPTCPIP

                      EthPhy

                      web server responds with HTTP reply (containing web page)

                      HTTP

                      HTTP

                      HTTPHTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      HTTP

                      web page finally () displayed

                      5 DataLink Layer 5-73

                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                      (except PHY) solid understanding of networking

                      principles practice hellip could stop here hellip but lots of

                      interesting topicso Internetworking (CSE 678 TCPIP socket

                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                      • Link Layer
                      • Link Layer Introduction
                      • Link layer context
                      • Encapsulation
                      • Link Layer Services
                      • Link Layer Services (more)
                      • Where is the link layer implemented
                      • Adaptors Communicating
                      • Slide 9
                      • Parity Checking
                      • Internet checksum (review)
                      • Checksumming Cyclic Redundancy Check
                      • Cyclic Redundancy Check
                      • Slide 14
                      • CRC Example
                      • Slide 16
                      • Multiple Access Links and Protocols
                      • Cable Network Architecture Overview
                      • Multiple Access protocols
                      • Ideal Multiple Access Protocol
                      • MAC Protocols a taxonomy
                      • Channel Partitioning MAC protocols TDMA
                      • Channel Partitioning MAC protocols FDMA
                      • Random Access Protocols
                      • ALOHA
                      • CSMA (Carrier Sense Multiple Access)
                      • CSMA collisions
                      • CSMACD (Collision Detection)
                      • CSMACD collision detection
                      • ldquoTaking Turnsrdquo MAC protocols
                      • Slide 31
                      • Slide 32
                      • Summary of MAC protocols
                      • Slide 34
                      • MAC Addresses and ARP
                      • LAN Addresses and ARP
                      • LAN Address (more)
                      • ARP Address Resolution Protocol
                      • ARP protocol Same LAN (network)
                      • Addressing routing to another LAN
                      • Slide 41
                      • Slide 42
                      • Ethernet
                      • Star topology
                      • Ethernet Frame Structure
                      • Ethernet Frame Structure (more)
                      • Ethernet Unreliable connectionless
                      • Ethernet CSMACD algorithm
                      • Ethernetrsquos CSMACD (more)
                      • 8023 Ethernet Standards Link amp Physical Layers
                      • Manchester encoding
                      • Slide 52
                      • Hubs
                      • Switch
                      • Switch allows multiple simultaneous transmissions
                      • Switch Table
                      • Switch self-learning
                      • Switch frame filteringforwarding
                      • Self-learning forwarding example
                      • Interconnecting switches
                      • Self-learning multi-switch example
                      • Institutional network
                      • Switches vs Routers
                      • Slide 64
                      • Synthesis a day in the life of a web request
                      • A day in the life scenario
                      • A day in the lifehellip connecting to the Internet
                      • Slide 68
                      • A day in the lifehellip ARP (before DNS before HTTP)
                      • A day in the lifehellip using DNS
                      • A day in the lifehellip TCP connection carrying HTTP
                      • A day in the lifehellip HTTP requestreply
                      • Chapter 5 letrsquos take a breath

                        5 DataLink Layer 5-12

                        Checksumming Cyclic Redundancy Check view data bits D as a binary number (actually a

                        polynomial with binary coefficients) choose r+1 bit pattern (generator) G goal choose r CRC bits R such that

                        o ltDRgt exactly divisible by G (modulo 2) o receiver knows G divides ltDRgt by G If non-zero

                        remainder error detectedo can detect all burst errors less than r+1 bits

                        widely used in practice (Ethernet 80211 WiFi ATM)

                        Cyclic Redundancy Check Modulo 2 arithmetic

                        o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                        A polynomial G(x) of degree r is known to both sender and receiver

                        Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                        Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                        If not there are transmission errors in the frame

                        7 4 3( ) 1D x x x x x

                        Common polynomials for G(x)

                        CRC

                        CRC-8

                        CRC-10

                        CRC-12

                        CRC-16

                        CRC-CCITT

                        CRC-32

                        C(x)

                        x8+x2+x1+1

                        x10+x9+x5+x4+x1+1

                        x12+x11+x3+x2+x1+1

                        x16+x15+x2+1

                        x16+x12+x5+1

                        x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                        5 DataLink Layer 5-15

                        CRC Example

                        31001 represents 1x

                        5 DataLink Layer 5-16

                        Link Layer

                        51 Introduction and services

                        52 Error detection and correction

                        53Multiple access protocols

                        54 Link-layer Addressing

                        55 Ethernet

                        56 Link-layer switches 59 A day in the life of

                        a web request

                        5 DataLink Layer 5-17

                        Multiple Access Links and Protocols

                        Two types of ldquolinksrdquo point-to-point

                        o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                        broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                        shared wire (eg cabled Ethernet)

                        shared RF (eg 80211 WiFi)

                        shared RF(satellite)

                        humans at acocktail party

                        (shared air acoustical)

                        Introduction 1-18

                        Cable Network Architecture Overview

                        home

                        cable headend

                        cable distributionnetwork (simplified)

                        Typically 500 to 5000 homes

                        5 DataLink Layer 5-19

                        Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                        interference o collision if node receives two or more signals at the same

                        time

                        multiple access protocol distributed algorithm that determines how nodes

                        share channel ie determine when node can transmit

                        communication about channel sharing must use channel itself o no out-of-band channel for coordination

                        5 DataLink Layer 5-20

                        Ideal Multiple Access Protocol

                        Broadcast channel of rate R bps1 when one node wants to transmit it can send

                        at the full rate say R2 when M nodes want to transmit each can

                        send at average rate RM3 fully decentralized

                        o no special node to coordinate transmissionso no synchronization of clocks slots

                        4 simple

                        5 DataLink Layer 5-21

                        MAC Protocols a taxonomy

                        Three broad classes Channel Partitioning

                        o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                        o allocate piece to node for exclusive use

                        Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                        ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                        take longer turns

                        5 DataLink Layer 5-22

                        Channel Partitioning MAC protocols TDMA

                        TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                        pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                        256 idle

                        1 3 4 1 3 4

                        6-slotframe

                        5 DataLink Layer 5-23

                        Channel Partitioning MAC protocols FDMA

                        FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                        idle example 6-station LAN 134 have pkt

                        frequency bands 256 idle fr

                        equ

                        ency

                        bands time

                        FDM cable

                        5 DataLink Layer 5-24

                        Random Access Protocols

                        When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                        two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                        o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                        retransmissions)

                        Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                        5 DataLink Layer 5-25

                        ALOHA

                        When a node has a frame to send send immediately

                        Set a timer for a random amount of time If an ACK arrives before the timer expires

                        fine otherwise resend the frame

                        (Works like stop-and-wait with random timeout interval)

                        5 DataLink Layer 5-26

                        CSMA (Carrier Sense Multiple Access)

                        CSMA listen before transmit

                        If channel sensed idle transmit entire frame

                        If channel sensed busy defer transmission

                        human analogy donrsquot interrupt others

                        5 DataLink Layer 5-27

                        CSMA collisions

                        collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                        spatial layout of nodes

                        5 DataLink Layer 5-28

                        CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                        o collisions detected within short timeo colliding transmissions aborted reducing

                        channel wastage collision detection

                        o easy in wired LANs measure signal strengths compare transmitted amp received signals

                        o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                        human analogy the polite conversationalist

                        5 DataLink Layer 5-29

                        CSMACD collision detection

                        5 DataLink Layer 5-30

                        ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                        o share channel efficiently and fairly at high load

                        o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                        Random access MAC protocolso efficient at low load single node can fully

                        utilize channelo high load collision overhead

                        ldquotaking turnsrdquo protocolslook for best of both worlds

                        5 DataLink Layer 5-31

                        ldquoTaking Turnsrdquo MAC protocolsPolling master node

                        ldquoinvitesrdquo slave nodes to transmit in turn

                        typically used with ldquodumbrdquo slave devices

                        concernso polling overhead o latencyo single point of

                        failure (master)

                        master

                        slaves

                        poll

                        data

                        data

                        5 DataLink Layer 5-32

                        ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                        passed from one node to next sequentially

                        token message concerns

                        o token overhead o latencyo single point of failure

                        (token)

                        T

                        data

                        (nothingto send)

                        T

                        5 DataLink Layer 5-33

                        Summary of MAC protocols

                        channel partitioning by time frequency or codeo Time Division Frequency Division

                        random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                        hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                        taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                        5 DataLink Layer 5-34

                        Link Layer

                        51 Introduction and services

                        52 Error detection and correction

                        53Multiple access protocols

                        54 Link-Layer Addressing

                        55 Ethernet

                        56 Link-layer switches 59 A day in the life of

                        a web request

                        5 DataLink Layer 5-35

                        MAC Addresses and ARP

                        32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                        MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                        another physically-connected interface (in same network)

                        o 48 bit MAC addressbull burned in NIC ROM

                        5 DataLink Layer 5-36

                        LAN Addresses and ARPEach adapter on LAN has unique LAN address

                        Broadcast address =FF-FF-FF-FF-FF-FF

                        = adapter

                        1A-2F-BB-76-09-AD

                        58-23-D7-FA-20-B0

                        0C-C4-11-6F-E3-98

                        71-65-F7-2B-08-53

                        LAN(wired orwireless)

                        5 DataLink Layer 5-37

                        LAN Address (more)

                        MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                        space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                        o can move LAN card from one LAN to another

                        IP hierarchical address NOT portableo address depends on IP subnet to which node is

                        attached

                        5 DataLink Layer 5-38

                        ARP Address Resolution Protocol

                        Each IP node (host router) on LAN has ARP table

                        ARP table IPMAC address mappings for some LAN nodes

                        lt IP address MAC address TTLgto TTL (Time To Live)

                        time after which address mapping will be forgotten (typically 20 min)

                        Q given a nodersquos IP address how to determine its MAC address

                        1A-2F-BB-76-09-AD

                        0C-C4-11-6F-E3-98

                        71-65-F7-2B-08-53

                        LAN

                        137196723

                        137196778

                        137196714

                        137196788

                        5 DataLink Layer 5-39

                        ARP protocol Same LAN (network) A wants to send datagram

                        to B and Brsquos MAC address not in Arsquos ARP table

                        A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                        FF-FF-FF-FF-FFo all machines on LAN

                        receive ARP query B receives ARP packet

                        replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                        address (unicast)

                        A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                        ARP is ldquoplug-and-playrdquoo nodes create their

                        ARP tables without intervention from net administrator

                        5 DataLink Layer 5-40

                        Addressing routing to another LAN

                        R

                        1A-23-F9-CD-06-9B

                        222222222220111111111110

                        E6-E9-00-17-BB-4B

                        CC-49-DE-D0-AB-7D

                        111111111112

                        111111111111

                        A74-29-9C-E8-FF-55

                        222222222221

                        88-B2-2F-54-1A-0F

                        B222222222222

                        49-BD-D2-C7-56-2A

                        walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                        two ARP tables in router R one for each IP network (LAN)

                        LAN LAN

                        5 DataLink Layer 5-41

                        A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                        frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                        destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                        R

                        1A-23-F9-CD-06-9B

                        222222222220

                        111111111110

                        E6-E9-00-17-BB-4B

                        CC-49-DE-D0-AB-7D

                        111111111112

                        111111111111

                        A74-29-9C-E8-FF-55

                        222222222221

                        88-B2-2F-54-1A-0F

                        B222222222222

                        49-BD-D2-C7-56-2A

                        This is a really importantexample ndash make sure youunderstand

                        5 DataLink Layer 5-42

                        Link Layer

                        51 Introduction and services

                        52 Error detection and correction

                        53Multiple access protocols

                        54 Link-Layer Addressing

                        55 Ethernet

                        56 Link-layer switches 59 A day in the life of

                        a web request

                        5 DataLink Layer 5-43

                        Ethernet

                        ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                        Metcalfersquos Ethernetsketch

                        5 DataLink Layer 5-44

                        Star topology bus topology popular through mid 90s

                        o all nodes in same collision domain (can collide with each other)

                        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                        (nodes do not collide with each other)

                        switch

                        bus coaxial cable star

                        5 DataLink Layer 5-45

                        Ethernet Frame Structure

                        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                        Preamble 7 bytes with pattern 10101010 followed by one

                        byte with pattern 10101011 used to synchronize receiver amp sender clock

                        rates

                        5 DataLink Layer 5-46

                        Ethernet Frame Structure (more) Addresses 6 bytes

                        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                        o otherwise adapter discards frame

                        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                        CRC checked at receiver if error is detected frame is dropped

                        5 DataLink Layer 5-47

                        Ethernet Unreliable connectionless connectionless No handshaking between sending

                        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                        Ethernetrsquos MAC protocol CSMACD

                        5 DataLink Layer 5-48

                        Ethernet CSMACD algorithm

                        1 NIC receives datagram from network layer creates frame

                        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                        4 If NIC detects collision while transmitting aborts and sends jam signal

                        5 After aborting NIC enters exponential backoff

                        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                        (1 slot = 512 bit-times)

                        5 DataLink Layer 5-49

                        Ethernetrsquos CSMACD (more)

                        Jam Signal make sure all other transmitters are aware of collision 48 bits

                        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                        Exponential Backoff Goal adapt retransmission

                        attempts to estimated current loado heavy load random

                        wait will be longer first collision choose K

                        from 01 delay is K 512 bit transmission times

                        after second collision choose K from 0123hellip

                        after ten collisions choose K from 01234hellip1023

                        5 DataLink Layer 5-50

                        8023 Ethernet Standards Link amp Physical Layers

                        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                        Mbps 1Gbps 10G bpso different physical layer media fiber cable

                        applicationtransportnetwork

                        linkphysical

                        MAC protocoland frame format

                        100BASE-TX

                        100BASE-T4

                        100BASE-FX100BASE-T2

                        100BASE-SX 100BASE-BX

                        fiber physical layercopper (twisterpair) physical layer

                        5 DataLink Layer 5-51

                        Manchester encoding

                        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                        synchronize to each othero no need for a centralized global clock among nodes

                        Hey this is physical-layer stuff

                        5 DataLink Layer 5-52

                        Link Layer

                        51 Introduction and services

                        52 Error detection and correction

                        53 Multiple access protocols

                        54 Link-layer Addressing

                        55 Ethernet

                        56 Link-layer switches LANs

                        59 A day in the life of a web request

                        5 DataLink Layer 5-53

                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                        o bits coming in one link go out all other links at same rate

                        o all nodes connected to hub can collide with one another

                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                        twisted pair

                        hub

                        5 DataLink Layer 5-54

                        Switch link-layer device smarter than hubs take active

                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                        transparento hosts are unaware of presence of switches

                        plug-and-play self-learningo switches do not need to be configured

                        5 DataLink Layer 5-55

                        Switch allows multiple simultaneous transmissions

                        hosts have dedicated direct connection to switch

                        switches buffer packets Ethernet protocol used on

                        each incoming link but no collisions full duplexo each link is its own collision

                        domain switching A-to-Arsquo and B-

                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                        A

                        Arsquo

                        B

                        Brsquo

                        C

                        Crsquo

                        switch with six interfaces(123456)

                        1 23

                        45

                        6

                        5 DataLink Layer 5-56

                        Switch Table

                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                        A each switch has a switch table each entryo (MAC address of host interface

                        to reach host time stamp)

                        looks like a routing table Q how are entries created

                        maintained in switch table o something like a routing

                        protocol

                        A

                        Arsquo

                        B

                        Brsquo

                        C

                        Crsquo

                        switch with six interfaces(123456)

                        1 23

                        45

                        6

                        5 DataLink Layer 5-57

                        Switch self-learning

                        switch learns which hosts can be reached through which interfaceso when frame received

                        switch ldquolearnsrdquo location of sender incoming LAN segment

                        o records senderlocation pair in switch table

                        A

                        Arsquo

                        B

                        Brsquo

                        C

                        Crsquo

                        1 23

                        45

                        6

                        A Arsquo

                        Source ADest Arsquo

                        MAC addr interface TTL

                        Switch table (initially empty)

                        A 1 60

                        5 DataLink Layer 5-58

                        Switch frame filteringforwardingWhen frame received

                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                        then if dest on segment from which frame arrived

                        then drop the frame else forward the frame on interface indicated else flood

                        forward on all but the interface on which the frame arrived

                        5 DataLink Layer 5-59

                        Self-learning forwarding example

                        A

                        Arsquo

                        B

                        Brsquo

                        C

                        Crsquo

                        1 23

                        45

                        6

                        A Arsquo

                        Source ADest Arsquo

                        MAC addr interface TTL

                        Switch table (initially empty)

                        A 1 60

                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                        frame destination unknownflood

                        Arsquo A

                        destination A location known

                        Arsquo 4 60

                        selective send

                        5 DataLink Layer 5-60

                        Interconnecting switches

                        switches can be connected together

                        A

                        B

                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                        A self learning (works exactly the same as in single-switch case)

                        S1

                        C D

                        E

                        FS2

                        S4

                        S3

                        H

                        I

                        G

                        5 DataLink Layer 5-61

                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                        A

                        B

                        S1

                        C D

                        E

                        FS2

                        S4

                        S3

                        H

                        I

                        G

                        1

                        2 3

                        34

                        5 DataLink Layer 5-62

                        Institutional network

                        to externalnetwork

                        router

                        IP subnet

                        mail server

                        web server

                        5 DataLink Layer 5-63

                        Switches vs Routers both store-and-forward devices

                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                        routers maintain routing tables implement routing algorithms

                        switches maintain switch tables implement filtering learning algorithms

                        Switch

                        5 DataLink Layer 5-64

                        Link Layer

                        51 Introduction and services

                        52 Error detection and correction

                        53Multiple access protocols

                        54 Link-Layer Addressing

                        55 Ethernet

                        56 Link-layer switches 59 A day in the life of

                        a web request

                        5 DataLink Layer 5-65

                        Synthesis a day in the life of a web request

                        journey down protocol stack completeo application transport network link

                        putting-it-all-together synthesiso goal identify review understand protocols

                        (at all layers) involved in seemingly simple scenario requesting www page

                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                        5 DataLink Layer 5-66

                        A day in the life scenario

                        Comcast network 68800013

                        Googlersquos network 64233160019 64233169105

                        web server

                        DNS server

                        school network 68802024

                        browser

                        web page

                        5 DataLink Layer 5-67

                        A day in the lifehellip connecting to the Internet

                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                        router(runs DHCP)

                        DHCPUDP

                        IPEthPhy

                        DHCP

                        DHCP

                        DHCP

                        DHCP

                        DHCP

                        DHCPUDP

                        IPEthPhy

                        DHCP

                        DHCP

                        DHCP

                        DHCPDHCP

                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                        5 DataLink Layer 5-68

                        A day in the lifehellip connecting to the Internet

                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                        router(runs DHCP)

                        DHCPUDP

                        IPEthPhy

                        DHCP

                        DHCP

                        DHCP

                        DHCP

                        DHCPUDP

                        IPEthPhy

                        DHCP

                        DHCP

                        DHCP

                        DHCP

                        DHCP

                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                        DHCP client receives DHCP ACK reply

                        5 DataLink Layer 5-69

                        A day in the lifehellip ARP (before DNS before HTTP)

                        before sending HTTP request need IP address of wwwgooglecom DNS

                        DNSUDP

                        IPEthPhy

                        DNS

                        DNS

                        DNS

                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                        ARP query

                        EthPhy

                        ARP

                        ARP

                        ARP reply

                        5 DataLink Layer 5-70

                        A day in the lifehellip using DNS

                        DNSUDP

                        IPEthPhy

                        DNS

                        DNS

                        DNS

                        DNS

                        DNS

                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                        demuxrsquoed to DNS server DNS server replies to

                        client with IP address of wwwgooglecom

                        Comcast network 68800013

                        DNS server

                        DNSUDP

                        IPEthPhy

                        DNS

                        DNS

                        DNS

                        DNS

                        5 DataLink Layer 5-71

                        A day in the lifehellip TCP connection carrying HTTP

                        HTTPTCPIP

                        EthPhy

                        HTTP

                        to send HTTP request client first opens TCP socket to web server

                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                        TCP connection established

                        64233169105

                        web server

                        SYN

                        SYN

                        SYN

                        SYN

                        TCPIP

                        EthPhy

                        SYN

                        SYN

                        SYN

                        SYNACK

                        SYNACK

                        SYNACK

                        SYNACK

                        SYNACK

                        SYNACK

                        SYNACK

                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                        5 DataLink Layer 5-72

                        A day in the lifehellip HTTP requestreply

                        HTTPTCPIP

                        EthPhy

                        HTTP

                        HTTP request sent into TCP socket

                        IP datagram containing HTTP request routed to wwwgooglecom

                        IP datgram containing HTTP reply routed back to client

                        64233169105

                        web server

                        HTTPTCPIP

                        EthPhy

                        web server responds with HTTP reply (containing web page)

                        HTTP

                        HTTP

                        HTTPHTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        HTTP

                        web page finally () displayed

                        5 DataLink Layer 5-73

                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                        (except PHY) solid understanding of networking

                        principles practice hellip could stop here hellip but lots of

                        interesting topicso Internetworking (CSE 678 TCPIP socket

                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                        • Link Layer
                        • Link Layer Introduction
                        • Link layer context
                        • Encapsulation
                        • Link Layer Services
                        • Link Layer Services (more)
                        • Where is the link layer implemented
                        • Adaptors Communicating
                        • Slide 9
                        • Parity Checking
                        • Internet checksum (review)
                        • Checksumming Cyclic Redundancy Check
                        • Cyclic Redundancy Check
                        • Slide 14
                        • CRC Example
                        • Slide 16
                        • Multiple Access Links and Protocols
                        • Cable Network Architecture Overview
                        • Multiple Access protocols
                        • Ideal Multiple Access Protocol
                        • MAC Protocols a taxonomy
                        • Channel Partitioning MAC protocols TDMA
                        • Channel Partitioning MAC protocols FDMA
                        • Random Access Protocols
                        • ALOHA
                        • CSMA (Carrier Sense Multiple Access)
                        • CSMA collisions
                        • CSMACD (Collision Detection)
                        • CSMACD collision detection
                        • ldquoTaking Turnsrdquo MAC protocols
                        • Slide 31
                        • Slide 32
                        • Summary of MAC protocols
                        • Slide 34
                        • MAC Addresses and ARP
                        • LAN Addresses and ARP
                        • LAN Address (more)
                        • ARP Address Resolution Protocol
                        • ARP protocol Same LAN (network)
                        • Addressing routing to another LAN
                        • Slide 41
                        • Slide 42
                        • Ethernet
                        • Star topology
                        • Ethernet Frame Structure
                        • Ethernet Frame Structure (more)
                        • Ethernet Unreliable connectionless
                        • Ethernet CSMACD algorithm
                        • Ethernetrsquos CSMACD (more)
                        • 8023 Ethernet Standards Link amp Physical Layers
                        • Manchester encoding
                        • Slide 52
                        • Hubs
                        • Switch
                        • Switch allows multiple simultaneous transmissions
                        • Switch Table
                        • Switch self-learning
                        • Switch frame filteringforwarding
                        • Self-learning forwarding example
                        • Interconnecting switches
                        • Self-learning multi-switch example
                        • Institutional network
                        • Switches vs Routers
                        • Slide 64
                        • Synthesis a day in the life of a web request
                        • A day in the life scenario
                        • A day in the lifehellip connecting to the Internet
                        • Slide 68
                        • A day in the lifehellip ARP (before DNS before HTTP)
                        • A day in the lifehellip using DNS
                        • A day in the lifehellip TCP connection carrying HTTP
                        • A day in the lifehellip HTTP requestreply
                        • Chapter 5 letrsquos take a breath

                          Cyclic Redundancy Check Modulo 2 arithmetic

                          o addition = subtraction = XOR Each bit string represents a polynomial Example 10011011 corresponds to

                          A polynomial G(x) of degree r is known to both sender and receiver

                          Sender appends r bits (called CRC code) to the message so that the resulting polynomial can be divided evenly by G(x)

                          Receiver checks if the received frame (message together with CRC) is still divisible by G(x)

                          If not there are transmission errors in the frame

                          7 4 3( ) 1D x x x x x

                          Common polynomials for G(x)

                          CRC

                          CRC-8

                          CRC-10

                          CRC-12

                          CRC-16

                          CRC-CCITT

                          CRC-32

                          C(x)

                          x8+x2+x1+1

                          x10+x9+x5+x4+x1+1

                          x12+x11+x3+x2+x1+1

                          x16+x15+x2+1

                          x16+x12+x5+1

                          x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                          5 DataLink Layer 5-15

                          CRC Example

                          31001 represents 1x

                          5 DataLink Layer 5-16

                          Link Layer

                          51 Introduction and services

                          52 Error detection and correction

                          53Multiple access protocols

                          54 Link-layer Addressing

                          55 Ethernet

                          56 Link-layer switches 59 A day in the life of

                          a web request

                          5 DataLink Layer 5-17

                          Multiple Access Links and Protocols

                          Two types of ldquolinksrdquo point-to-point

                          o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                          broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                          shared wire (eg cabled Ethernet)

                          shared RF (eg 80211 WiFi)

                          shared RF(satellite)

                          humans at acocktail party

                          (shared air acoustical)

                          Introduction 1-18

                          Cable Network Architecture Overview

                          home

                          cable headend

                          cable distributionnetwork (simplified)

                          Typically 500 to 5000 homes

                          5 DataLink Layer 5-19

                          Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                          interference o collision if node receives two or more signals at the same

                          time

                          multiple access protocol distributed algorithm that determines how nodes

                          share channel ie determine when node can transmit

                          communication about channel sharing must use channel itself o no out-of-band channel for coordination

                          5 DataLink Layer 5-20

                          Ideal Multiple Access Protocol

                          Broadcast channel of rate R bps1 when one node wants to transmit it can send

                          at the full rate say R2 when M nodes want to transmit each can

                          send at average rate RM3 fully decentralized

                          o no special node to coordinate transmissionso no synchronization of clocks slots

                          4 simple

                          5 DataLink Layer 5-21

                          MAC Protocols a taxonomy

                          Three broad classes Channel Partitioning

                          o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                          o allocate piece to node for exclusive use

                          Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                          ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                          take longer turns

                          5 DataLink Layer 5-22

                          Channel Partitioning MAC protocols TDMA

                          TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                          pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                          256 idle

                          1 3 4 1 3 4

                          6-slotframe

                          5 DataLink Layer 5-23

                          Channel Partitioning MAC protocols FDMA

                          FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                          idle example 6-station LAN 134 have pkt

                          frequency bands 256 idle fr

                          equ

                          ency

                          bands time

                          FDM cable

                          5 DataLink Layer 5-24

                          Random Access Protocols

                          When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                          two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                          o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                          retransmissions)

                          Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                          5 DataLink Layer 5-25

                          ALOHA

                          When a node has a frame to send send immediately

                          Set a timer for a random amount of time If an ACK arrives before the timer expires

                          fine otherwise resend the frame

                          (Works like stop-and-wait with random timeout interval)

                          5 DataLink Layer 5-26

                          CSMA (Carrier Sense Multiple Access)

                          CSMA listen before transmit

                          If channel sensed idle transmit entire frame

                          If channel sensed busy defer transmission

                          human analogy donrsquot interrupt others

                          5 DataLink Layer 5-27

                          CSMA collisions

                          collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                          spatial layout of nodes

                          5 DataLink Layer 5-28

                          CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                          o collisions detected within short timeo colliding transmissions aborted reducing

                          channel wastage collision detection

                          o easy in wired LANs measure signal strengths compare transmitted amp received signals

                          o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                          human analogy the polite conversationalist

                          5 DataLink Layer 5-29

                          CSMACD collision detection

                          5 DataLink Layer 5-30

                          ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                          o share channel efficiently and fairly at high load

                          o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                          Random access MAC protocolso efficient at low load single node can fully

                          utilize channelo high load collision overhead

                          ldquotaking turnsrdquo protocolslook for best of both worlds

                          5 DataLink Layer 5-31

                          ldquoTaking Turnsrdquo MAC protocolsPolling master node

                          ldquoinvitesrdquo slave nodes to transmit in turn

                          typically used with ldquodumbrdquo slave devices

                          concernso polling overhead o latencyo single point of

                          failure (master)

                          master

                          slaves

                          poll

                          data

                          data

                          5 DataLink Layer 5-32

                          ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                          passed from one node to next sequentially

                          token message concerns

                          o token overhead o latencyo single point of failure

                          (token)

                          T

                          data

                          (nothingto send)

                          T

                          5 DataLink Layer 5-33

                          Summary of MAC protocols

                          channel partitioning by time frequency or codeo Time Division Frequency Division

                          random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                          hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                          taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                          5 DataLink Layer 5-34

                          Link Layer

                          51 Introduction and services

                          52 Error detection and correction

                          53Multiple access protocols

                          54 Link-Layer Addressing

                          55 Ethernet

                          56 Link-layer switches 59 A day in the life of

                          a web request

                          5 DataLink Layer 5-35

                          MAC Addresses and ARP

                          32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                          MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                          another physically-connected interface (in same network)

                          o 48 bit MAC addressbull burned in NIC ROM

                          5 DataLink Layer 5-36

                          LAN Addresses and ARPEach adapter on LAN has unique LAN address

                          Broadcast address =FF-FF-FF-FF-FF-FF

                          = adapter

                          1A-2F-BB-76-09-AD

                          58-23-D7-FA-20-B0

                          0C-C4-11-6F-E3-98

                          71-65-F7-2B-08-53

                          LAN(wired orwireless)

                          5 DataLink Layer 5-37

                          LAN Address (more)

                          MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                          space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                          o can move LAN card from one LAN to another

                          IP hierarchical address NOT portableo address depends on IP subnet to which node is

                          attached

                          5 DataLink Layer 5-38

                          ARP Address Resolution Protocol

                          Each IP node (host router) on LAN has ARP table

                          ARP table IPMAC address mappings for some LAN nodes

                          lt IP address MAC address TTLgto TTL (Time To Live)

                          time after which address mapping will be forgotten (typically 20 min)

                          Q given a nodersquos IP address how to determine its MAC address

                          1A-2F-BB-76-09-AD

                          0C-C4-11-6F-E3-98

                          71-65-F7-2B-08-53

                          LAN

                          137196723

                          137196778

                          137196714

                          137196788

                          5 DataLink Layer 5-39

                          ARP protocol Same LAN (network) A wants to send datagram

                          to B and Brsquos MAC address not in Arsquos ARP table

                          A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                          FF-FF-FF-FF-FFo all machines on LAN

                          receive ARP query B receives ARP packet

                          replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                          address (unicast)

                          A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                          ARP is ldquoplug-and-playrdquoo nodes create their

                          ARP tables without intervention from net administrator

                          5 DataLink Layer 5-40

                          Addressing routing to another LAN

                          R

                          1A-23-F9-CD-06-9B

                          222222222220111111111110

                          E6-E9-00-17-BB-4B

                          CC-49-DE-D0-AB-7D

                          111111111112

                          111111111111

                          A74-29-9C-E8-FF-55

                          222222222221

                          88-B2-2F-54-1A-0F

                          B222222222222

                          49-BD-D2-C7-56-2A

                          walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                          two ARP tables in router R one for each IP network (LAN)

                          LAN LAN

                          5 DataLink Layer 5-41

                          A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                          frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                          destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                          R

                          1A-23-F9-CD-06-9B

                          222222222220

                          111111111110

                          E6-E9-00-17-BB-4B

                          CC-49-DE-D0-AB-7D

                          111111111112

                          111111111111

                          A74-29-9C-E8-FF-55

                          222222222221

                          88-B2-2F-54-1A-0F

                          B222222222222

                          49-BD-D2-C7-56-2A

                          This is a really importantexample ndash make sure youunderstand

                          5 DataLink Layer 5-42

                          Link Layer

                          51 Introduction and services

                          52 Error detection and correction

                          53Multiple access protocols

                          54 Link-Layer Addressing

                          55 Ethernet

                          56 Link-layer switches 59 A day in the life of

                          a web request

                          5 DataLink Layer 5-43

                          Ethernet

                          ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                          Metcalfersquos Ethernetsketch

                          5 DataLink Layer 5-44

                          Star topology bus topology popular through mid 90s

                          o all nodes in same collision domain (can collide with each other)

                          today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                          (nodes do not collide with each other)

                          switch

                          bus coaxial cable star

                          5 DataLink Layer 5-45

                          Ethernet Frame Structure

                          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                          Preamble 7 bytes with pattern 10101010 followed by one

                          byte with pattern 10101011 used to synchronize receiver amp sender clock

                          rates

                          5 DataLink Layer 5-46

                          Ethernet Frame Structure (more) Addresses 6 bytes

                          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                          o otherwise adapter discards frame

                          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                          CRC checked at receiver if error is detected frame is dropped

                          5 DataLink Layer 5-47

                          Ethernet Unreliable connectionless connectionless No handshaking between sending

                          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                          Ethernetrsquos MAC protocol CSMACD

                          5 DataLink Layer 5-48

                          Ethernet CSMACD algorithm

                          1 NIC receives datagram from network layer creates frame

                          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                          4 If NIC detects collision while transmitting aborts and sends jam signal

                          5 After aborting NIC enters exponential backoff

                          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                          (1 slot = 512 bit-times)

                          5 DataLink Layer 5-49

                          Ethernetrsquos CSMACD (more)

                          Jam Signal make sure all other transmitters are aware of collision 48 bits

                          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                          Exponential Backoff Goal adapt retransmission

                          attempts to estimated current loado heavy load random

                          wait will be longer first collision choose K

                          from 01 delay is K 512 bit transmission times

                          after second collision choose K from 0123hellip

                          after ten collisions choose K from 01234hellip1023

                          5 DataLink Layer 5-50

                          8023 Ethernet Standards Link amp Physical Layers

                          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                          Mbps 1Gbps 10G bpso different physical layer media fiber cable

                          applicationtransportnetwork

                          linkphysical

                          MAC protocoland frame format

                          100BASE-TX

                          100BASE-T4

                          100BASE-FX100BASE-T2

                          100BASE-SX 100BASE-BX

                          fiber physical layercopper (twisterpair) physical layer

                          5 DataLink Layer 5-51

                          Manchester encoding

                          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                          synchronize to each othero no need for a centralized global clock among nodes

                          Hey this is physical-layer stuff

                          5 DataLink Layer 5-52

                          Link Layer

                          51 Introduction and services

                          52 Error detection and correction

                          53 Multiple access protocols

                          54 Link-layer Addressing

                          55 Ethernet

                          56 Link-layer switches LANs

                          59 A day in the life of a web request

                          5 DataLink Layer 5-53

                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                          o bits coming in one link go out all other links at same rate

                          o all nodes connected to hub can collide with one another

                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                          twisted pair

                          hub

                          5 DataLink Layer 5-54

                          Switch link-layer device smarter than hubs take active

                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                          transparento hosts are unaware of presence of switches

                          plug-and-play self-learningo switches do not need to be configured

                          5 DataLink Layer 5-55

                          Switch allows multiple simultaneous transmissions

                          hosts have dedicated direct connection to switch

                          switches buffer packets Ethernet protocol used on

                          each incoming link but no collisions full duplexo each link is its own collision

                          domain switching A-to-Arsquo and B-

                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                          A

                          Arsquo

                          B

                          Brsquo

                          C

                          Crsquo

                          switch with six interfaces(123456)

                          1 23

                          45

                          6

                          5 DataLink Layer 5-56

                          Switch Table

                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                          A each switch has a switch table each entryo (MAC address of host interface

                          to reach host time stamp)

                          looks like a routing table Q how are entries created

                          maintained in switch table o something like a routing

                          protocol

                          A

                          Arsquo

                          B

                          Brsquo

                          C

                          Crsquo

                          switch with six interfaces(123456)

                          1 23

                          45

                          6

                          5 DataLink Layer 5-57

                          Switch self-learning

                          switch learns which hosts can be reached through which interfaceso when frame received

                          switch ldquolearnsrdquo location of sender incoming LAN segment

                          o records senderlocation pair in switch table

                          A

                          Arsquo

                          B

                          Brsquo

                          C

                          Crsquo

                          1 23

                          45

                          6

                          A Arsquo

                          Source ADest Arsquo

                          MAC addr interface TTL

                          Switch table (initially empty)

                          A 1 60

                          5 DataLink Layer 5-58

                          Switch frame filteringforwardingWhen frame received

                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                          then if dest on segment from which frame arrived

                          then drop the frame else forward the frame on interface indicated else flood

                          forward on all but the interface on which the frame arrived

                          5 DataLink Layer 5-59

                          Self-learning forwarding example

                          A

                          Arsquo

                          B

                          Brsquo

                          C

                          Crsquo

                          1 23

                          45

                          6

                          A Arsquo

                          Source ADest Arsquo

                          MAC addr interface TTL

                          Switch table (initially empty)

                          A 1 60

                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                          frame destination unknownflood

                          Arsquo A

                          destination A location known

                          Arsquo 4 60

                          selective send

                          5 DataLink Layer 5-60

                          Interconnecting switches

                          switches can be connected together

                          A

                          B

                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                          A self learning (works exactly the same as in single-switch case)

                          S1

                          C D

                          E

                          FS2

                          S4

                          S3

                          H

                          I

                          G

                          5 DataLink Layer 5-61

                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                          A

                          B

                          S1

                          C D

                          E

                          FS2

                          S4

                          S3

                          H

                          I

                          G

                          1

                          2 3

                          34

                          5 DataLink Layer 5-62

                          Institutional network

                          to externalnetwork

                          router

                          IP subnet

                          mail server

                          web server

                          5 DataLink Layer 5-63

                          Switches vs Routers both store-and-forward devices

                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                          routers maintain routing tables implement routing algorithms

                          switches maintain switch tables implement filtering learning algorithms

                          Switch

                          5 DataLink Layer 5-64

                          Link Layer

                          51 Introduction and services

                          52 Error detection and correction

                          53Multiple access protocols

                          54 Link-Layer Addressing

                          55 Ethernet

                          56 Link-layer switches 59 A day in the life of

                          a web request

                          5 DataLink Layer 5-65

                          Synthesis a day in the life of a web request

                          journey down protocol stack completeo application transport network link

                          putting-it-all-together synthesiso goal identify review understand protocols

                          (at all layers) involved in seemingly simple scenario requesting www page

                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                          5 DataLink Layer 5-66

                          A day in the life scenario

                          Comcast network 68800013

                          Googlersquos network 64233160019 64233169105

                          web server

                          DNS server

                          school network 68802024

                          browser

                          web page

                          5 DataLink Layer 5-67

                          A day in the lifehellip connecting to the Internet

                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                          router(runs DHCP)

                          DHCPUDP

                          IPEthPhy

                          DHCP

                          DHCP

                          DHCP

                          DHCP

                          DHCP

                          DHCPUDP

                          IPEthPhy

                          DHCP

                          DHCP

                          DHCP

                          DHCPDHCP

                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                          5 DataLink Layer 5-68

                          A day in the lifehellip connecting to the Internet

                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                          router(runs DHCP)

                          DHCPUDP

                          IPEthPhy

                          DHCP

                          DHCP

                          DHCP

                          DHCP

                          DHCPUDP

                          IPEthPhy

                          DHCP

                          DHCP

                          DHCP

                          DHCP

                          DHCP

                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                          DHCP client receives DHCP ACK reply

                          5 DataLink Layer 5-69

                          A day in the lifehellip ARP (before DNS before HTTP)

                          before sending HTTP request need IP address of wwwgooglecom DNS

                          DNSUDP

                          IPEthPhy

                          DNS

                          DNS

                          DNS

                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                          ARP query

                          EthPhy

                          ARP

                          ARP

                          ARP reply

                          5 DataLink Layer 5-70

                          A day in the lifehellip using DNS

                          DNSUDP

                          IPEthPhy

                          DNS

                          DNS

                          DNS

                          DNS

                          DNS

                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                          demuxrsquoed to DNS server DNS server replies to

                          client with IP address of wwwgooglecom

                          Comcast network 68800013

                          DNS server

                          DNSUDP

                          IPEthPhy

                          DNS

                          DNS

                          DNS

                          DNS

                          5 DataLink Layer 5-71

                          A day in the lifehellip TCP connection carrying HTTP

                          HTTPTCPIP

                          EthPhy

                          HTTP

                          to send HTTP request client first opens TCP socket to web server

                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                          TCP connection established

                          64233169105

                          web server

                          SYN

                          SYN

                          SYN

                          SYN

                          TCPIP

                          EthPhy

                          SYN

                          SYN

                          SYN

                          SYNACK

                          SYNACK

                          SYNACK

                          SYNACK

                          SYNACK

                          SYNACK

                          SYNACK

                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                          5 DataLink Layer 5-72

                          A day in the lifehellip HTTP requestreply

                          HTTPTCPIP

                          EthPhy

                          HTTP

                          HTTP request sent into TCP socket

                          IP datagram containing HTTP request routed to wwwgooglecom

                          IP datgram containing HTTP reply routed back to client

                          64233169105

                          web server

                          HTTPTCPIP

                          EthPhy

                          web server responds with HTTP reply (containing web page)

                          HTTP

                          HTTP

                          HTTPHTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          HTTP

                          web page finally () displayed

                          5 DataLink Layer 5-73

                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                          (except PHY) solid understanding of networking

                          principles practice hellip could stop here hellip but lots of

                          interesting topicso Internetworking (CSE 678 TCPIP socket

                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                          • Link Layer
                          • Link Layer Introduction
                          • Link layer context
                          • Encapsulation
                          • Link Layer Services
                          • Link Layer Services (more)
                          • Where is the link layer implemented
                          • Adaptors Communicating
                          • Slide 9
                          • Parity Checking
                          • Internet checksum (review)
                          • Checksumming Cyclic Redundancy Check
                          • Cyclic Redundancy Check
                          • Slide 14
                          • CRC Example
                          • Slide 16
                          • Multiple Access Links and Protocols
                          • Cable Network Architecture Overview
                          • Multiple Access protocols
                          • Ideal Multiple Access Protocol
                          • MAC Protocols a taxonomy
                          • Channel Partitioning MAC protocols TDMA
                          • Channel Partitioning MAC protocols FDMA
                          • Random Access Protocols
                          • ALOHA
                          • CSMA (Carrier Sense Multiple Access)
                          • CSMA collisions
                          • CSMACD (Collision Detection)
                          • CSMACD collision detection
                          • ldquoTaking Turnsrdquo MAC protocols
                          • Slide 31
                          • Slide 32
                          • Summary of MAC protocols
                          • Slide 34
                          • MAC Addresses and ARP
                          • LAN Addresses and ARP
                          • LAN Address (more)
                          • ARP Address Resolution Protocol
                          • ARP protocol Same LAN (network)
                          • Addressing routing to another LAN
                          • Slide 41
                          • Slide 42
                          • Ethernet
                          • Star topology
                          • Ethernet Frame Structure
                          • Ethernet Frame Structure (more)
                          • Ethernet Unreliable connectionless
                          • Ethernet CSMACD algorithm
                          • Ethernetrsquos CSMACD (more)
                          • 8023 Ethernet Standards Link amp Physical Layers
                          • Manchester encoding
                          • Slide 52
                          • Hubs
                          • Switch
                          • Switch allows multiple simultaneous transmissions
                          • Switch Table
                          • Switch self-learning
                          • Switch frame filteringforwarding
                          • Self-learning forwarding example
                          • Interconnecting switches
                          • Self-learning multi-switch example
                          • Institutional network
                          • Switches vs Routers
                          • Slide 64
                          • Synthesis a day in the life of a web request
                          • A day in the life scenario
                          • A day in the lifehellip connecting to the Internet
                          • Slide 68
                          • A day in the lifehellip ARP (before DNS before HTTP)
                          • A day in the lifehellip using DNS
                          • A day in the lifehellip TCP connection carrying HTTP
                          • A day in the lifehellip HTTP requestreply
                          • Chapter 5 letrsquos take a breath

                            Common polynomials for G(x)

                            CRC

                            CRC-8

                            CRC-10

                            CRC-12

                            CRC-16

                            CRC-CCITT

                            CRC-32

                            C(x)

                            x8+x2+x1+1

                            x10+x9+x5+x4+x1+1

                            x12+x11+x3+x2+x1+1

                            x16+x15+x2+1

                            x16+x12+x5+1

                            x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

                            5 DataLink Layer 5-15

                            CRC Example

                            31001 represents 1x

                            5 DataLink Layer 5-16

                            Link Layer

                            51 Introduction and services

                            52 Error detection and correction

                            53Multiple access protocols

                            54 Link-layer Addressing

                            55 Ethernet

                            56 Link-layer switches 59 A day in the life of

                            a web request

                            5 DataLink Layer 5-17

                            Multiple Access Links and Protocols

                            Two types of ldquolinksrdquo point-to-point

                            o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                            broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                            shared wire (eg cabled Ethernet)

                            shared RF (eg 80211 WiFi)

                            shared RF(satellite)

                            humans at acocktail party

                            (shared air acoustical)

                            Introduction 1-18

                            Cable Network Architecture Overview

                            home

                            cable headend

                            cable distributionnetwork (simplified)

                            Typically 500 to 5000 homes

                            5 DataLink Layer 5-19

                            Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                            interference o collision if node receives two or more signals at the same

                            time

                            multiple access protocol distributed algorithm that determines how nodes

                            share channel ie determine when node can transmit

                            communication about channel sharing must use channel itself o no out-of-band channel for coordination

                            5 DataLink Layer 5-20

                            Ideal Multiple Access Protocol

                            Broadcast channel of rate R bps1 when one node wants to transmit it can send

                            at the full rate say R2 when M nodes want to transmit each can

                            send at average rate RM3 fully decentralized

                            o no special node to coordinate transmissionso no synchronization of clocks slots

                            4 simple

                            5 DataLink Layer 5-21

                            MAC Protocols a taxonomy

                            Three broad classes Channel Partitioning

                            o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                            o allocate piece to node for exclusive use

                            Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                            ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                            take longer turns

                            5 DataLink Layer 5-22

                            Channel Partitioning MAC protocols TDMA

                            TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                            pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                            256 idle

                            1 3 4 1 3 4

                            6-slotframe

                            5 DataLink Layer 5-23

                            Channel Partitioning MAC protocols FDMA

                            FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                            idle example 6-station LAN 134 have pkt

                            frequency bands 256 idle fr

                            equ

                            ency

                            bands time

                            FDM cable

                            5 DataLink Layer 5-24

                            Random Access Protocols

                            When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                            two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                            o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                            retransmissions)

                            Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                            5 DataLink Layer 5-25

                            ALOHA

                            When a node has a frame to send send immediately

                            Set a timer for a random amount of time If an ACK arrives before the timer expires

                            fine otherwise resend the frame

                            (Works like stop-and-wait with random timeout interval)

                            5 DataLink Layer 5-26

                            CSMA (Carrier Sense Multiple Access)

                            CSMA listen before transmit

                            If channel sensed idle transmit entire frame

                            If channel sensed busy defer transmission

                            human analogy donrsquot interrupt others

                            5 DataLink Layer 5-27

                            CSMA collisions

                            collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                            spatial layout of nodes

                            5 DataLink Layer 5-28

                            CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                            o collisions detected within short timeo colliding transmissions aborted reducing

                            channel wastage collision detection

                            o easy in wired LANs measure signal strengths compare transmitted amp received signals

                            o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                            human analogy the polite conversationalist

                            5 DataLink Layer 5-29

                            CSMACD collision detection

                            5 DataLink Layer 5-30

                            ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                            o share channel efficiently and fairly at high load

                            o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                            Random access MAC protocolso efficient at low load single node can fully

                            utilize channelo high load collision overhead

                            ldquotaking turnsrdquo protocolslook for best of both worlds

                            5 DataLink Layer 5-31

                            ldquoTaking Turnsrdquo MAC protocolsPolling master node

                            ldquoinvitesrdquo slave nodes to transmit in turn

                            typically used with ldquodumbrdquo slave devices

                            concernso polling overhead o latencyo single point of

                            failure (master)

                            master

                            slaves

                            poll

                            data

                            data

                            5 DataLink Layer 5-32

                            ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                            passed from one node to next sequentially

                            token message concerns

                            o token overhead o latencyo single point of failure

                            (token)

                            T

                            data

                            (nothingto send)

                            T

                            5 DataLink Layer 5-33

                            Summary of MAC protocols

                            channel partitioning by time frequency or codeo Time Division Frequency Division

                            random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                            hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                            taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                            5 DataLink Layer 5-34

                            Link Layer

                            51 Introduction and services

                            52 Error detection and correction

                            53Multiple access protocols

                            54 Link-Layer Addressing

                            55 Ethernet

                            56 Link-layer switches 59 A day in the life of

                            a web request

                            5 DataLink Layer 5-35

                            MAC Addresses and ARP

                            32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                            MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                            another physically-connected interface (in same network)

                            o 48 bit MAC addressbull burned in NIC ROM

                            5 DataLink Layer 5-36

                            LAN Addresses and ARPEach adapter on LAN has unique LAN address

                            Broadcast address =FF-FF-FF-FF-FF-FF

                            = adapter

                            1A-2F-BB-76-09-AD

                            58-23-D7-FA-20-B0

                            0C-C4-11-6F-E3-98

                            71-65-F7-2B-08-53

                            LAN(wired orwireless)

                            5 DataLink Layer 5-37

                            LAN Address (more)

                            MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                            space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                            o can move LAN card from one LAN to another

                            IP hierarchical address NOT portableo address depends on IP subnet to which node is

                            attached

                            5 DataLink Layer 5-38

                            ARP Address Resolution Protocol

                            Each IP node (host router) on LAN has ARP table

                            ARP table IPMAC address mappings for some LAN nodes

                            lt IP address MAC address TTLgto TTL (Time To Live)

                            time after which address mapping will be forgotten (typically 20 min)

                            Q given a nodersquos IP address how to determine its MAC address

                            1A-2F-BB-76-09-AD

                            0C-C4-11-6F-E3-98

                            71-65-F7-2B-08-53

                            LAN

                            137196723

                            137196778

                            137196714

                            137196788

                            5 DataLink Layer 5-39

                            ARP protocol Same LAN (network) A wants to send datagram

                            to B and Brsquos MAC address not in Arsquos ARP table

                            A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                            FF-FF-FF-FF-FFo all machines on LAN

                            receive ARP query B receives ARP packet

                            replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                            address (unicast)

                            A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                            ARP is ldquoplug-and-playrdquoo nodes create their

                            ARP tables without intervention from net administrator

                            5 DataLink Layer 5-40

                            Addressing routing to another LAN

                            R

                            1A-23-F9-CD-06-9B

                            222222222220111111111110

                            E6-E9-00-17-BB-4B

                            CC-49-DE-D0-AB-7D

                            111111111112

                            111111111111

                            A74-29-9C-E8-FF-55

                            222222222221

                            88-B2-2F-54-1A-0F

                            B222222222222

                            49-BD-D2-C7-56-2A

                            walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                            two ARP tables in router R one for each IP network (LAN)

                            LAN LAN

                            5 DataLink Layer 5-41

                            A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                            frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                            destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                            R

                            1A-23-F9-CD-06-9B

                            222222222220

                            111111111110

                            E6-E9-00-17-BB-4B

                            CC-49-DE-D0-AB-7D

                            111111111112

                            111111111111

                            A74-29-9C-E8-FF-55

                            222222222221

                            88-B2-2F-54-1A-0F

                            B222222222222

                            49-BD-D2-C7-56-2A

                            This is a really importantexample ndash make sure youunderstand

                            5 DataLink Layer 5-42

                            Link Layer

                            51 Introduction and services

                            52 Error detection and correction

                            53Multiple access protocols

                            54 Link-Layer Addressing

                            55 Ethernet

                            56 Link-layer switches 59 A day in the life of

                            a web request

                            5 DataLink Layer 5-43

                            Ethernet

                            ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                            Metcalfersquos Ethernetsketch

                            5 DataLink Layer 5-44

                            Star topology bus topology popular through mid 90s

                            o all nodes in same collision domain (can collide with each other)

                            today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                            (nodes do not collide with each other)

                            switch

                            bus coaxial cable star

                            5 DataLink Layer 5-45

                            Ethernet Frame Structure

                            Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                            Preamble 7 bytes with pattern 10101010 followed by one

                            byte with pattern 10101011 used to synchronize receiver amp sender clock

                            rates

                            5 DataLink Layer 5-46

                            Ethernet Frame Structure (more) Addresses 6 bytes

                            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                            o otherwise adapter discards frame

                            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                            CRC checked at receiver if error is detected frame is dropped

                            5 DataLink Layer 5-47

                            Ethernet Unreliable connectionless connectionless No handshaking between sending

                            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                            Ethernetrsquos MAC protocol CSMACD

                            5 DataLink Layer 5-48

                            Ethernet CSMACD algorithm

                            1 NIC receives datagram from network layer creates frame

                            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                            4 If NIC detects collision while transmitting aborts and sends jam signal

                            5 After aborting NIC enters exponential backoff

                            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                            (1 slot = 512 bit-times)

                            5 DataLink Layer 5-49

                            Ethernetrsquos CSMACD (more)

                            Jam Signal make sure all other transmitters are aware of collision 48 bits

                            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                            Exponential Backoff Goal adapt retransmission

                            attempts to estimated current loado heavy load random

                            wait will be longer first collision choose K

                            from 01 delay is K 512 bit transmission times

                            after second collision choose K from 0123hellip

                            after ten collisions choose K from 01234hellip1023

                            5 DataLink Layer 5-50

                            8023 Ethernet Standards Link amp Physical Layers

                            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                            Mbps 1Gbps 10G bpso different physical layer media fiber cable

                            applicationtransportnetwork

                            linkphysical

                            MAC protocoland frame format

                            100BASE-TX

                            100BASE-T4

                            100BASE-FX100BASE-T2

                            100BASE-SX 100BASE-BX

                            fiber physical layercopper (twisterpair) physical layer

                            5 DataLink Layer 5-51

                            Manchester encoding

                            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                            synchronize to each othero no need for a centralized global clock among nodes

                            Hey this is physical-layer stuff

                            5 DataLink Layer 5-52

                            Link Layer

                            51 Introduction and services

                            52 Error detection and correction

                            53 Multiple access protocols

                            54 Link-layer Addressing

                            55 Ethernet

                            56 Link-layer switches LANs

                            59 A day in the life of a web request

                            5 DataLink Layer 5-53

                            Hubsphysical-layer (ldquodumbrdquo) repeaters

                            o bits coming in one link go out all other links at same rate

                            o all nodes connected to hub can collide with one another

                            o no frame bufferingo no CSMACD at hub host NICs detect collisions

                            twisted pair

                            hub

                            5 DataLink Layer 5-54

                            Switch link-layer device smarter than hubs take active

                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                            transparento hosts are unaware of presence of switches

                            plug-and-play self-learningo switches do not need to be configured

                            5 DataLink Layer 5-55

                            Switch allows multiple simultaneous transmissions

                            hosts have dedicated direct connection to switch

                            switches buffer packets Ethernet protocol used on

                            each incoming link but no collisions full duplexo each link is its own collision

                            domain switching A-to-Arsquo and B-

                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                            A

                            Arsquo

                            B

                            Brsquo

                            C

                            Crsquo

                            switch with six interfaces(123456)

                            1 23

                            45

                            6

                            5 DataLink Layer 5-56

                            Switch Table

                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                            A each switch has a switch table each entryo (MAC address of host interface

                            to reach host time stamp)

                            looks like a routing table Q how are entries created

                            maintained in switch table o something like a routing

                            protocol

                            A

                            Arsquo

                            B

                            Brsquo

                            C

                            Crsquo

                            switch with six interfaces(123456)

                            1 23

                            45

                            6

                            5 DataLink Layer 5-57

                            Switch self-learning

                            switch learns which hosts can be reached through which interfaceso when frame received

                            switch ldquolearnsrdquo location of sender incoming LAN segment

                            o records senderlocation pair in switch table

                            A

                            Arsquo

                            B

                            Brsquo

                            C

                            Crsquo

                            1 23

                            45

                            6

                            A Arsquo

                            Source ADest Arsquo

                            MAC addr interface TTL

                            Switch table (initially empty)

                            A 1 60

                            5 DataLink Layer 5-58

                            Switch frame filteringforwardingWhen frame received

                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                            then if dest on segment from which frame arrived

                            then drop the frame else forward the frame on interface indicated else flood

                            forward on all but the interface on which the frame arrived

                            5 DataLink Layer 5-59

                            Self-learning forwarding example

                            A

                            Arsquo

                            B

                            Brsquo

                            C

                            Crsquo

                            1 23

                            45

                            6

                            A Arsquo

                            Source ADest Arsquo

                            MAC addr interface TTL

                            Switch table (initially empty)

                            A 1 60

                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                            frame destination unknownflood

                            Arsquo A

                            destination A location known

                            Arsquo 4 60

                            selective send

                            5 DataLink Layer 5-60

                            Interconnecting switches

                            switches can be connected together

                            A

                            B

                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                            A self learning (works exactly the same as in single-switch case)

                            S1

                            C D

                            E

                            FS2

                            S4

                            S3

                            H

                            I

                            G

                            5 DataLink Layer 5-61

                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                            A

                            B

                            S1

                            C D

                            E

                            FS2

                            S4

                            S3

                            H

                            I

                            G

                            1

                            2 3

                            34

                            5 DataLink Layer 5-62

                            Institutional network

                            to externalnetwork

                            router

                            IP subnet

                            mail server

                            web server

                            5 DataLink Layer 5-63

                            Switches vs Routers both store-and-forward devices

                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                            routers maintain routing tables implement routing algorithms

                            switches maintain switch tables implement filtering learning algorithms

                            Switch

                            5 DataLink Layer 5-64

                            Link Layer

                            51 Introduction and services

                            52 Error detection and correction

                            53Multiple access protocols

                            54 Link-Layer Addressing

                            55 Ethernet

                            56 Link-layer switches 59 A day in the life of

                            a web request

                            5 DataLink Layer 5-65

                            Synthesis a day in the life of a web request

                            journey down protocol stack completeo application transport network link

                            putting-it-all-together synthesiso goal identify review understand protocols

                            (at all layers) involved in seemingly simple scenario requesting www page

                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                            5 DataLink Layer 5-66

                            A day in the life scenario

                            Comcast network 68800013

                            Googlersquos network 64233160019 64233169105

                            web server

                            DNS server

                            school network 68802024

                            browser

                            web page

                            5 DataLink Layer 5-67

                            A day in the lifehellip connecting to the Internet

                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                            router(runs DHCP)

                            DHCPUDP

                            IPEthPhy

                            DHCP

                            DHCP

                            DHCP

                            DHCP

                            DHCP

                            DHCPUDP

                            IPEthPhy

                            DHCP

                            DHCP

                            DHCP

                            DHCPDHCP

                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                            5 DataLink Layer 5-68

                            A day in the lifehellip connecting to the Internet

                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                            router(runs DHCP)

                            DHCPUDP

                            IPEthPhy

                            DHCP

                            DHCP

                            DHCP

                            DHCP

                            DHCPUDP

                            IPEthPhy

                            DHCP

                            DHCP

                            DHCP

                            DHCP

                            DHCP

                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                            DHCP client receives DHCP ACK reply

                            5 DataLink Layer 5-69

                            A day in the lifehellip ARP (before DNS before HTTP)

                            before sending HTTP request need IP address of wwwgooglecom DNS

                            DNSUDP

                            IPEthPhy

                            DNS

                            DNS

                            DNS

                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                            ARP query

                            EthPhy

                            ARP

                            ARP

                            ARP reply

                            5 DataLink Layer 5-70

                            A day in the lifehellip using DNS

                            DNSUDP

                            IPEthPhy

                            DNS

                            DNS

                            DNS

                            DNS

                            DNS

                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                            demuxrsquoed to DNS server DNS server replies to

                            client with IP address of wwwgooglecom

                            Comcast network 68800013

                            DNS server

                            DNSUDP

                            IPEthPhy

                            DNS

                            DNS

                            DNS

                            DNS

                            5 DataLink Layer 5-71

                            A day in the lifehellip TCP connection carrying HTTP

                            HTTPTCPIP

                            EthPhy

                            HTTP

                            to send HTTP request client first opens TCP socket to web server

                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                            TCP connection established

                            64233169105

                            web server

                            SYN

                            SYN

                            SYN

                            SYN

                            TCPIP

                            EthPhy

                            SYN

                            SYN

                            SYN

                            SYNACK

                            SYNACK

                            SYNACK

                            SYNACK

                            SYNACK

                            SYNACK

                            SYNACK

                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                            5 DataLink Layer 5-72

                            A day in the lifehellip HTTP requestreply

                            HTTPTCPIP

                            EthPhy

                            HTTP

                            HTTP request sent into TCP socket

                            IP datagram containing HTTP request routed to wwwgooglecom

                            IP datgram containing HTTP reply routed back to client

                            64233169105

                            web server

                            HTTPTCPIP

                            EthPhy

                            web server responds with HTTP reply (containing web page)

                            HTTP

                            HTTP

                            HTTPHTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            HTTP

                            web page finally () displayed

                            5 DataLink Layer 5-73

                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                            (except PHY) solid understanding of networking

                            principles practice hellip could stop here hellip but lots of

                            interesting topicso Internetworking (CSE 678 TCPIP socket

                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                            • Link Layer
                            • Link Layer Introduction
                            • Link layer context
                            • Encapsulation
                            • Link Layer Services
                            • Link Layer Services (more)
                            • Where is the link layer implemented
                            • Adaptors Communicating
                            • Slide 9
                            • Parity Checking
                            • Internet checksum (review)
                            • Checksumming Cyclic Redundancy Check
                            • Cyclic Redundancy Check
                            • Slide 14
                            • CRC Example
                            • Slide 16
                            • Multiple Access Links and Protocols
                            • Cable Network Architecture Overview
                            • Multiple Access protocols
                            • Ideal Multiple Access Protocol
                            • MAC Protocols a taxonomy
                            • Channel Partitioning MAC protocols TDMA
                            • Channel Partitioning MAC protocols FDMA
                            • Random Access Protocols
                            • ALOHA
                            • CSMA (Carrier Sense Multiple Access)
                            • CSMA collisions
                            • CSMACD (Collision Detection)
                            • CSMACD collision detection
                            • ldquoTaking Turnsrdquo MAC protocols
                            • Slide 31
                            • Slide 32
                            • Summary of MAC protocols
                            • Slide 34
                            • MAC Addresses and ARP
                            • LAN Addresses and ARP
                            • LAN Address (more)
                            • ARP Address Resolution Protocol
                            • ARP protocol Same LAN (network)
                            • Addressing routing to another LAN
                            • Slide 41
                            • Slide 42
                            • Ethernet
                            • Star topology
                            • Ethernet Frame Structure
                            • Ethernet Frame Structure (more)
                            • Ethernet Unreliable connectionless
                            • Ethernet CSMACD algorithm
                            • Ethernetrsquos CSMACD (more)
                            • 8023 Ethernet Standards Link amp Physical Layers
                            • Manchester encoding
                            • Slide 52
                            • Hubs
                            • Switch
                            • Switch allows multiple simultaneous transmissions
                            • Switch Table
                            • Switch self-learning
                            • Switch frame filteringforwarding
                            • Self-learning forwarding example
                            • Interconnecting switches
                            • Self-learning multi-switch example
                            • Institutional network
                            • Switches vs Routers
                            • Slide 64
                            • Synthesis a day in the life of a web request
                            • A day in the life scenario
                            • A day in the lifehellip connecting to the Internet
                            • Slide 68
                            • A day in the lifehellip ARP (before DNS before HTTP)
                            • A day in the lifehellip using DNS
                            • A day in the lifehellip TCP connection carrying HTTP
                            • A day in the lifehellip HTTP requestreply
                            • Chapter 5 letrsquos take a breath

                              5 DataLink Layer 5-15

                              CRC Example

                              31001 represents 1x

                              5 DataLink Layer 5-16

                              Link Layer

                              51 Introduction and services

                              52 Error detection and correction

                              53Multiple access protocols

                              54 Link-layer Addressing

                              55 Ethernet

                              56 Link-layer switches 59 A day in the life of

                              a web request

                              5 DataLink Layer 5-17

                              Multiple Access Links and Protocols

                              Two types of ldquolinksrdquo point-to-point

                              o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                              broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                              shared wire (eg cabled Ethernet)

                              shared RF (eg 80211 WiFi)

                              shared RF(satellite)

                              humans at acocktail party

                              (shared air acoustical)

                              Introduction 1-18

                              Cable Network Architecture Overview

                              home

                              cable headend

                              cable distributionnetwork (simplified)

                              Typically 500 to 5000 homes

                              5 DataLink Layer 5-19

                              Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                              interference o collision if node receives two or more signals at the same

                              time

                              multiple access protocol distributed algorithm that determines how nodes

                              share channel ie determine when node can transmit

                              communication about channel sharing must use channel itself o no out-of-band channel for coordination

                              5 DataLink Layer 5-20

                              Ideal Multiple Access Protocol

                              Broadcast channel of rate R bps1 when one node wants to transmit it can send

                              at the full rate say R2 when M nodes want to transmit each can

                              send at average rate RM3 fully decentralized

                              o no special node to coordinate transmissionso no synchronization of clocks slots

                              4 simple

                              5 DataLink Layer 5-21

                              MAC Protocols a taxonomy

                              Three broad classes Channel Partitioning

                              o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                              o allocate piece to node for exclusive use

                              Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                              ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                              take longer turns

                              5 DataLink Layer 5-22

                              Channel Partitioning MAC protocols TDMA

                              TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                              pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                              256 idle

                              1 3 4 1 3 4

                              6-slotframe

                              5 DataLink Layer 5-23

                              Channel Partitioning MAC protocols FDMA

                              FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                              idle example 6-station LAN 134 have pkt

                              frequency bands 256 idle fr

                              equ

                              ency

                              bands time

                              FDM cable

                              5 DataLink Layer 5-24

                              Random Access Protocols

                              When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                              two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                              o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                              retransmissions)

                              Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                              5 DataLink Layer 5-25

                              ALOHA

                              When a node has a frame to send send immediately

                              Set a timer for a random amount of time If an ACK arrives before the timer expires

                              fine otherwise resend the frame

                              (Works like stop-and-wait with random timeout interval)

                              5 DataLink Layer 5-26

                              CSMA (Carrier Sense Multiple Access)

                              CSMA listen before transmit

                              If channel sensed idle transmit entire frame

                              If channel sensed busy defer transmission

                              human analogy donrsquot interrupt others

                              5 DataLink Layer 5-27

                              CSMA collisions

                              collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                              spatial layout of nodes

                              5 DataLink Layer 5-28

                              CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                              o collisions detected within short timeo colliding transmissions aborted reducing

                              channel wastage collision detection

                              o easy in wired LANs measure signal strengths compare transmitted amp received signals

                              o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                              human analogy the polite conversationalist

                              5 DataLink Layer 5-29

                              CSMACD collision detection

                              5 DataLink Layer 5-30

                              ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                              o share channel efficiently and fairly at high load

                              o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                              Random access MAC protocolso efficient at low load single node can fully

                              utilize channelo high load collision overhead

                              ldquotaking turnsrdquo protocolslook for best of both worlds

                              5 DataLink Layer 5-31

                              ldquoTaking Turnsrdquo MAC protocolsPolling master node

                              ldquoinvitesrdquo slave nodes to transmit in turn

                              typically used with ldquodumbrdquo slave devices

                              concernso polling overhead o latencyo single point of

                              failure (master)

                              master

                              slaves

                              poll

                              data

                              data

                              5 DataLink Layer 5-32

                              ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                              passed from one node to next sequentially

                              token message concerns

                              o token overhead o latencyo single point of failure

                              (token)

                              T

                              data

                              (nothingto send)

                              T

                              5 DataLink Layer 5-33

                              Summary of MAC protocols

                              channel partitioning by time frequency or codeo Time Division Frequency Division

                              random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                              hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                              taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                              5 DataLink Layer 5-34

                              Link Layer

                              51 Introduction and services

                              52 Error detection and correction

                              53Multiple access protocols

                              54 Link-Layer Addressing

                              55 Ethernet

                              56 Link-layer switches 59 A day in the life of

                              a web request

                              5 DataLink Layer 5-35

                              MAC Addresses and ARP

                              32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                              MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                              another physically-connected interface (in same network)

                              o 48 bit MAC addressbull burned in NIC ROM

                              5 DataLink Layer 5-36

                              LAN Addresses and ARPEach adapter on LAN has unique LAN address

                              Broadcast address =FF-FF-FF-FF-FF-FF

                              = adapter

                              1A-2F-BB-76-09-AD

                              58-23-D7-FA-20-B0

                              0C-C4-11-6F-E3-98

                              71-65-F7-2B-08-53

                              LAN(wired orwireless)

                              5 DataLink Layer 5-37

                              LAN Address (more)

                              MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                              space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                              o can move LAN card from one LAN to another

                              IP hierarchical address NOT portableo address depends on IP subnet to which node is

                              attached

                              5 DataLink Layer 5-38

                              ARP Address Resolution Protocol

                              Each IP node (host router) on LAN has ARP table

                              ARP table IPMAC address mappings for some LAN nodes

                              lt IP address MAC address TTLgto TTL (Time To Live)

                              time after which address mapping will be forgotten (typically 20 min)

                              Q given a nodersquos IP address how to determine its MAC address

                              1A-2F-BB-76-09-AD

                              0C-C4-11-6F-E3-98

                              71-65-F7-2B-08-53

                              LAN

                              137196723

                              137196778

                              137196714

                              137196788

                              5 DataLink Layer 5-39

                              ARP protocol Same LAN (network) A wants to send datagram

                              to B and Brsquos MAC address not in Arsquos ARP table

                              A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                              FF-FF-FF-FF-FFo all machines on LAN

                              receive ARP query B receives ARP packet

                              replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                              address (unicast)

                              A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                              ARP is ldquoplug-and-playrdquoo nodes create their

                              ARP tables without intervention from net administrator

                              5 DataLink Layer 5-40

                              Addressing routing to another LAN

                              R

                              1A-23-F9-CD-06-9B

                              222222222220111111111110

                              E6-E9-00-17-BB-4B

                              CC-49-DE-D0-AB-7D

                              111111111112

                              111111111111

                              A74-29-9C-E8-FF-55

                              222222222221

                              88-B2-2F-54-1A-0F

                              B222222222222

                              49-BD-D2-C7-56-2A

                              walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                              two ARP tables in router R one for each IP network (LAN)

                              LAN LAN

                              5 DataLink Layer 5-41

                              A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                              frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                              destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                              R

                              1A-23-F9-CD-06-9B

                              222222222220

                              111111111110

                              E6-E9-00-17-BB-4B

                              CC-49-DE-D0-AB-7D

                              111111111112

                              111111111111

                              A74-29-9C-E8-FF-55

                              222222222221

                              88-B2-2F-54-1A-0F

                              B222222222222

                              49-BD-D2-C7-56-2A

                              This is a really importantexample ndash make sure youunderstand

                              5 DataLink Layer 5-42

                              Link Layer

                              51 Introduction and services

                              52 Error detection and correction

                              53Multiple access protocols

                              54 Link-Layer Addressing

                              55 Ethernet

                              56 Link-layer switches 59 A day in the life of

                              a web request

                              5 DataLink Layer 5-43

                              Ethernet

                              ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                              Metcalfersquos Ethernetsketch

                              5 DataLink Layer 5-44

                              Star topology bus topology popular through mid 90s

                              o all nodes in same collision domain (can collide with each other)

                              today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                              (nodes do not collide with each other)

                              switch

                              bus coaxial cable star

                              5 DataLink Layer 5-45

                              Ethernet Frame Structure

                              Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                              Preamble 7 bytes with pattern 10101010 followed by one

                              byte with pattern 10101011 used to synchronize receiver amp sender clock

                              rates

                              5 DataLink Layer 5-46

                              Ethernet Frame Structure (more) Addresses 6 bytes

                              o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                              o otherwise adapter discards frame

                              Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                              CRC checked at receiver if error is detected frame is dropped

                              5 DataLink Layer 5-47

                              Ethernet Unreliable connectionless connectionless No handshaking between sending

                              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                              Ethernetrsquos MAC protocol CSMACD

                              5 DataLink Layer 5-48

                              Ethernet CSMACD algorithm

                              1 NIC receives datagram from network layer creates frame

                              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                              4 If NIC detects collision while transmitting aborts and sends jam signal

                              5 After aborting NIC enters exponential backoff

                              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                              (1 slot = 512 bit-times)

                              5 DataLink Layer 5-49

                              Ethernetrsquos CSMACD (more)

                              Jam Signal make sure all other transmitters are aware of collision 48 bits

                              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                              Exponential Backoff Goal adapt retransmission

                              attempts to estimated current loado heavy load random

                              wait will be longer first collision choose K

                              from 01 delay is K 512 bit transmission times

                              after second collision choose K from 0123hellip

                              after ten collisions choose K from 01234hellip1023

                              5 DataLink Layer 5-50

                              8023 Ethernet Standards Link amp Physical Layers

                              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                              Mbps 1Gbps 10G bpso different physical layer media fiber cable

                              applicationtransportnetwork

                              linkphysical

                              MAC protocoland frame format

                              100BASE-TX

                              100BASE-T4

                              100BASE-FX100BASE-T2

                              100BASE-SX 100BASE-BX

                              fiber physical layercopper (twisterpair) physical layer

                              5 DataLink Layer 5-51

                              Manchester encoding

                              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                              synchronize to each othero no need for a centralized global clock among nodes

                              Hey this is physical-layer stuff

                              5 DataLink Layer 5-52

                              Link Layer

                              51 Introduction and services

                              52 Error detection and correction

                              53 Multiple access protocols

                              54 Link-layer Addressing

                              55 Ethernet

                              56 Link-layer switches LANs

                              59 A day in the life of a web request

                              5 DataLink Layer 5-53

                              Hubsphysical-layer (ldquodumbrdquo) repeaters

                              o bits coming in one link go out all other links at same rate

                              o all nodes connected to hub can collide with one another

                              o no frame bufferingo no CSMACD at hub host NICs detect collisions

                              twisted pair

                              hub

                              5 DataLink Layer 5-54

                              Switch link-layer device smarter than hubs take active

                              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                              transparento hosts are unaware of presence of switches

                              plug-and-play self-learningo switches do not need to be configured

                              5 DataLink Layer 5-55

                              Switch allows multiple simultaneous transmissions

                              hosts have dedicated direct connection to switch

                              switches buffer packets Ethernet protocol used on

                              each incoming link but no collisions full duplexo each link is its own collision

                              domain switching A-to-Arsquo and B-

                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                              A

                              Arsquo

                              B

                              Brsquo

                              C

                              Crsquo

                              switch with six interfaces(123456)

                              1 23

                              45

                              6

                              5 DataLink Layer 5-56

                              Switch Table

                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                              A each switch has a switch table each entryo (MAC address of host interface

                              to reach host time stamp)

                              looks like a routing table Q how are entries created

                              maintained in switch table o something like a routing

                              protocol

                              A

                              Arsquo

                              B

                              Brsquo

                              C

                              Crsquo

                              switch with six interfaces(123456)

                              1 23

                              45

                              6

                              5 DataLink Layer 5-57

                              Switch self-learning

                              switch learns which hosts can be reached through which interfaceso when frame received

                              switch ldquolearnsrdquo location of sender incoming LAN segment

                              o records senderlocation pair in switch table

                              A

                              Arsquo

                              B

                              Brsquo

                              C

                              Crsquo

                              1 23

                              45

                              6

                              A Arsquo

                              Source ADest Arsquo

                              MAC addr interface TTL

                              Switch table (initially empty)

                              A 1 60

                              5 DataLink Layer 5-58

                              Switch frame filteringforwardingWhen frame received

                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                              then if dest on segment from which frame arrived

                              then drop the frame else forward the frame on interface indicated else flood

                              forward on all but the interface on which the frame arrived

                              5 DataLink Layer 5-59

                              Self-learning forwarding example

                              A

                              Arsquo

                              B

                              Brsquo

                              C

                              Crsquo

                              1 23

                              45

                              6

                              A Arsquo

                              Source ADest Arsquo

                              MAC addr interface TTL

                              Switch table (initially empty)

                              A 1 60

                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                              frame destination unknownflood

                              Arsquo A

                              destination A location known

                              Arsquo 4 60

                              selective send

                              5 DataLink Layer 5-60

                              Interconnecting switches

                              switches can be connected together

                              A

                              B

                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                              A self learning (works exactly the same as in single-switch case)

                              S1

                              C D

                              E

                              FS2

                              S4

                              S3

                              H

                              I

                              G

                              5 DataLink Layer 5-61

                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                              A

                              B

                              S1

                              C D

                              E

                              FS2

                              S4

                              S3

                              H

                              I

                              G

                              1

                              2 3

                              34

                              5 DataLink Layer 5-62

                              Institutional network

                              to externalnetwork

                              router

                              IP subnet

                              mail server

                              web server

                              5 DataLink Layer 5-63

                              Switches vs Routers both store-and-forward devices

                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                              routers maintain routing tables implement routing algorithms

                              switches maintain switch tables implement filtering learning algorithms

                              Switch

                              5 DataLink Layer 5-64

                              Link Layer

                              51 Introduction and services

                              52 Error detection and correction

                              53Multiple access protocols

                              54 Link-Layer Addressing

                              55 Ethernet

                              56 Link-layer switches 59 A day in the life of

                              a web request

                              5 DataLink Layer 5-65

                              Synthesis a day in the life of a web request

                              journey down protocol stack completeo application transport network link

                              putting-it-all-together synthesiso goal identify review understand protocols

                              (at all layers) involved in seemingly simple scenario requesting www page

                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                              5 DataLink Layer 5-66

                              A day in the life scenario

                              Comcast network 68800013

                              Googlersquos network 64233160019 64233169105

                              web server

                              DNS server

                              school network 68802024

                              browser

                              web page

                              5 DataLink Layer 5-67

                              A day in the lifehellip connecting to the Internet

                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                              router(runs DHCP)

                              DHCPUDP

                              IPEthPhy

                              DHCP

                              DHCP

                              DHCP

                              DHCP

                              DHCP

                              DHCPUDP

                              IPEthPhy

                              DHCP

                              DHCP

                              DHCP

                              DHCPDHCP

                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                              5 DataLink Layer 5-68

                              A day in the lifehellip connecting to the Internet

                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                              router(runs DHCP)

                              DHCPUDP

                              IPEthPhy

                              DHCP

                              DHCP

                              DHCP

                              DHCP

                              DHCPUDP

                              IPEthPhy

                              DHCP

                              DHCP

                              DHCP

                              DHCP

                              DHCP

                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                              DHCP client receives DHCP ACK reply

                              5 DataLink Layer 5-69

                              A day in the lifehellip ARP (before DNS before HTTP)

                              before sending HTTP request need IP address of wwwgooglecom DNS

                              DNSUDP

                              IPEthPhy

                              DNS

                              DNS

                              DNS

                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                              ARP query

                              EthPhy

                              ARP

                              ARP

                              ARP reply

                              5 DataLink Layer 5-70

                              A day in the lifehellip using DNS

                              DNSUDP

                              IPEthPhy

                              DNS

                              DNS

                              DNS

                              DNS

                              DNS

                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                              demuxrsquoed to DNS server DNS server replies to

                              client with IP address of wwwgooglecom

                              Comcast network 68800013

                              DNS server

                              DNSUDP

                              IPEthPhy

                              DNS

                              DNS

                              DNS

                              DNS

                              5 DataLink Layer 5-71

                              A day in the lifehellip TCP connection carrying HTTP

                              HTTPTCPIP

                              EthPhy

                              HTTP

                              to send HTTP request client first opens TCP socket to web server

                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                              TCP connection established

                              64233169105

                              web server

                              SYN

                              SYN

                              SYN

                              SYN

                              TCPIP

                              EthPhy

                              SYN

                              SYN

                              SYN

                              SYNACK

                              SYNACK

                              SYNACK

                              SYNACK

                              SYNACK

                              SYNACK

                              SYNACK

                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                              5 DataLink Layer 5-72

                              A day in the lifehellip HTTP requestreply

                              HTTPTCPIP

                              EthPhy

                              HTTP

                              HTTP request sent into TCP socket

                              IP datagram containing HTTP request routed to wwwgooglecom

                              IP datgram containing HTTP reply routed back to client

                              64233169105

                              web server

                              HTTPTCPIP

                              EthPhy

                              web server responds with HTTP reply (containing web page)

                              HTTP

                              HTTP

                              HTTPHTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              HTTP

                              web page finally () displayed

                              5 DataLink Layer 5-73

                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                              (except PHY) solid understanding of networking

                              principles practice hellip could stop here hellip but lots of

                              interesting topicso Internetworking (CSE 678 TCPIP socket

                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                              • Link Layer
                              • Link Layer Introduction
                              • Link layer context
                              • Encapsulation
                              • Link Layer Services
                              • Link Layer Services (more)
                              • Where is the link layer implemented
                              • Adaptors Communicating
                              • Slide 9
                              • Parity Checking
                              • Internet checksum (review)
                              • Checksumming Cyclic Redundancy Check
                              • Cyclic Redundancy Check
                              • Slide 14
                              • CRC Example
                              • Slide 16
                              • Multiple Access Links and Protocols
                              • Cable Network Architecture Overview
                              • Multiple Access protocols
                              • Ideal Multiple Access Protocol
                              • MAC Protocols a taxonomy
                              • Channel Partitioning MAC protocols TDMA
                              • Channel Partitioning MAC protocols FDMA
                              • Random Access Protocols
                              • ALOHA
                              • CSMA (Carrier Sense Multiple Access)
                              • CSMA collisions
                              • CSMACD (Collision Detection)
                              • CSMACD collision detection
                              • ldquoTaking Turnsrdquo MAC protocols
                              • Slide 31
                              • Slide 32
                              • Summary of MAC protocols
                              • Slide 34
                              • MAC Addresses and ARP
                              • LAN Addresses and ARP
                              • LAN Address (more)
                              • ARP Address Resolution Protocol
                              • ARP protocol Same LAN (network)
                              • Addressing routing to another LAN
                              • Slide 41
                              • Slide 42
                              • Ethernet
                              • Star topology
                              • Ethernet Frame Structure
                              • Ethernet Frame Structure (more)
                              • Ethernet Unreliable connectionless
                              • Ethernet CSMACD algorithm
                              • Ethernetrsquos CSMACD (more)
                              • 8023 Ethernet Standards Link amp Physical Layers
                              • Manchester encoding
                              • Slide 52
                              • Hubs
                              • Switch
                              • Switch allows multiple simultaneous transmissions
                              • Switch Table
                              • Switch self-learning
                              • Switch frame filteringforwarding
                              • Self-learning forwarding example
                              • Interconnecting switches
                              • Self-learning multi-switch example
                              • Institutional network
                              • Switches vs Routers
                              • Slide 64
                              • Synthesis a day in the life of a web request
                              • A day in the life scenario
                              • A day in the lifehellip connecting to the Internet
                              • Slide 68
                              • A day in the lifehellip ARP (before DNS before HTTP)
                              • A day in the lifehellip using DNS
                              • A day in the lifehellip TCP connection carrying HTTP
                              • A day in the lifehellip HTTP requestreply
                              • Chapter 5 letrsquos take a breath

                                5 DataLink Layer 5-16

                                Link Layer

                                51 Introduction and services

                                52 Error detection and correction

                                53Multiple access protocols

                                54 Link-layer Addressing

                                55 Ethernet

                                56 Link-layer switches 59 A day in the life of

                                a web request

                                5 DataLink Layer 5-17

                                Multiple Access Links and Protocols

                                Two types of ldquolinksrdquo point-to-point

                                o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                                broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                                shared wire (eg cabled Ethernet)

                                shared RF (eg 80211 WiFi)

                                shared RF(satellite)

                                humans at acocktail party

                                (shared air acoustical)

                                Introduction 1-18

                                Cable Network Architecture Overview

                                home

                                cable headend

                                cable distributionnetwork (simplified)

                                Typically 500 to 5000 homes

                                5 DataLink Layer 5-19

                                Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                                interference o collision if node receives two or more signals at the same

                                time

                                multiple access protocol distributed algorithm that determines how nodes

                                share channel ie determine when node can transmit

                                communication about channel sharing must use channel itself o no out-of-band channel for coordination

                                5 DataLink Layer 5-20

                                Ideal Multiple Access Protocol

                                Broadcast channel of rate R bps1 when one node wants to transmit it can send

                                at the full rate say R2 when M nodes want to transmit each can

                                send at average rate RM3 fully decentralized

                                o no special node to coordinate transmissionso no synchronization of clocks slots

                                4 simple

                                5 DataLink Layer 5-21

                                MAC Protocols a taxonomy

                                Three broad classes Channel Partitioning

                                o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                o allocate piece to node for exclusive use

                                Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                take longer turns

                                5 DataLink Layer 5-22

                                Channel Partitioning MAC protocols TDMA

                                TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                256 idle

                                1 3 4 1 3 4

                                6-slotframe

                                5 DataLink Layer 5-23

                                Channel Partitioning MAC protocols FDMA

                                FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                idle example 6-station LAN 134 have pkt

                                frequency bands 256 idle fr

                                equ

                                ency

                                bands time

                                FDM cable

                                5 DataLink Layer 5-24

                                Random Access Protocols

                                When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                retransmissions)

                                Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                5 DataLink Layer 5-25

                                ALOHA

                                When a node has a frame to send send immediately

                                Set a timer for a random amount of time If an ACK arrives before the timer expires

                                fine otherwise resend the frame

                                (Works like stop-and-wait with random timeout interval)

                                5 DataLink Layer 5-26

                                CSMA (Carrier Sense Multiple Access)

                                CSMA listen before transmit

                                If channel sensed idle transmit entire frame

                                If channel sensed busy defer transmission

                                human analogy donrsquot interrupt others

                                5 DataLink Layer 5-27

                                CSMA collisions

                                collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                spatial layout of nodes

                                5 DataLink Layer 5-28

                                CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                o collisions detected within short timeo colliding transmissions aborted reducing

                                channel wastage collision detection

                                o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                human analogy the polite conversationalist

                                5 DataLink Layer 5-29

                                CSMACD collision detection

                                5 DataLink Layer 5-30

                                ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                o share channel efficiently and fairly at high load

                                o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                Random access MAC protocolso efficient at low load single node can fully

                                utilize channelo high load collision overhead

                                ldquotaking turnsrdquo protocolslook for best of both worlds

                                5 DataLink Layer 5-31

                                ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                ldquoinvitesrdquo slave nodes to transmit in turn

                                typically used with ldquodumbrdquo slave devices

                                concernso polling overhead o latencyo single point of

                                failure (master)

                                master

                                slaves

                                poll

                                data

                                data

                                5 DataLink Layer 5-32

                                ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                passed from one node to next sequentially

                                token message concerns

                                o token overhead o latencyo single point of failure

                                (token)

                                T

                                data

                                (nothingto send)

                                T

                                5 DataLink Layer 5-33

                                Summary of MAC protocols

                                channel partitioning by time frequency or codeo Time Division Frequency Division

                                random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                5 DataLink Layer 5-34

                                Link Layer

                                51 Introduction and services

                                52 Error detection and correction

                                53Multiple access protocols

                                54 Link-Layer Addressing

                                55 Ethernet

                                56 Link-layer switches 59 A day in the life of

                                a web request

                                5 DataLink Layer 5-35

                                MAC Addresses and ARP

                                32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                another physically-connected interface (in same network)

                                o 48 bit MAC addressbull burned in NIC ROM

                                5 DataLink Layer 5-36

                                LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                Broadcast address =FF-FF-FF-FF-FF-FF

                                = adapter

                                1A-2F-BB-76-09-AD

                                58-23-D7-FA-20-B0

                                0C-C4-11-6F-E3-98

                                71-65-F7-2B-08-53

                                LAN(wired orwireless)

                                5 DataLink Layer 5-37

                                LAN Address (more)

                                MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                o can move LAN card from one LAN to another

                                IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                attached

                                5 DataLink Layer 5-38

                                ARP Address Resolution Protocol

                                Each IP node (host router) on LAN has ARP table

                                ARP table IPMAC address mappings for some LAN nodes

                                lt IP address MAC address TTLgto TTL (Time To Live)

                                time after which address mapping will be forgotten (typically 20 min)

                                Q given a nodersquos IP address how to determine its MAC address

                                1A-2F-BB-76-09-AD

                                0C-C4-11-6F-E3-98

                                71-65-F7-2B-08-53

                                LAN

                                137196723

                                137196778

                                137196714

                                137196788

                                5 DataLink Layer 5-39

                                ARP protocol Same LAN (network) A wants to send datagram

                                to B and Brsquos MAC address not in Arsquos ARP table

                                A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                FF-FF-FF-FF-FFo all machines on LAN

                                receive ARP query B receives ARP packet

                                replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                address (unicast)

                                A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                ARP is ldquoplug-and-playrdquoo nodes create their

                                ARP tables without intervention from net administrator

                                5 DataLink Layer 5-40

                                Addressing routing to another LAN

                                R

                                1A-23-F9-CD-06-9B

                                222222222220111111111110

                                E6-E9-00-17-BB-4B

                                CC-49-DE-D0-AB-7D

                                111111111112

                                111111111111

                                A74-29-9C-E8-FF-55

                                222222222221

                                88-B2-2F-54-1A-0F

                                B222222222222

                                49-BD-D2-C7-56-2A

                                walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                two ARP tables in router R one for each IP network (LAN)

                                LAN LAN

                                5 DataLink Layer 5-41

                                A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                R

                                1A-23-F9-CD-06-9B

                                222222222220

                                111111111110

                                E6-E9-00-17-BB-4B

                                CC-49-DE-D0-AB-7D

                                111111111112

                                111111111111

                                A74-29-9C-E8-FF-55

                                222222222221

                                88-B2-2F-54-1A-0F

                                B222222222222

                                49-BD-D2-C7-56-2A

                                This is a really importantexample ndash make sure youunderstand

                                5 DataLink Layer 5-42

                                Link Layer

                                51 Introduction and services

                                52 Error detection and correction

                                53Multiple access protocols

                                54 Link-Layer Addressing

                                55 Ethernet

                                56 Link-layer switches 59 A day in the life of

                                a web request

                                5 DataLink Layer 5-43

                                Ethernet

                                ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                Metcalfersquos Ethernetsketch

                                5 DataLink Layer 5-44

                                Star topology bus topology popular through mid 90s

                                o all nodes in same collision domain (can collide with each other)

                                today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                (nodes do not collide with each other)

                                switch

                                bus coaxial cable star

                                5 DataLink Layer 5-45

                                Ethernet Frame Structure

                                Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                Preamble 7 bytes with pattern 10101010 followed by one

                                byte with pattern 10101011 used to synchronize receiver amp sender clock

                                rates

                                5 DataLink Layer 5-46

                                Ethernet Frame Structure (more) Addresses 6 bytes

                                o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                o otherwise adapter discards frame

                                Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                CRC checked at receiver if error is detected frame is dropped

                                5 DataLink Layer 5-47

                                Ethernet Unreliable connectionless connectionless No handshaking between sending

                                and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                Ethernetrsquos MAC protocol CSMACD

                                5 DataLink Layer 5-48

                                Ethernet CSMACD algorithm

                                1 NIC receives datagram from network layer creates frame

                                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                4 If NIC detects collision while transmitting aborts and sends jam signal

                                5 After aborting NIC enters exponential backoff

                                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                (1 slot = 512 bit-times)

                                5 DataLink Layer 5-49

                                Ethernetrsquos CSMACD (more)

                                Jam Signal make sure all other transmitters are aware of collision 48 bits

                                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                Exponential Backoff Goal adapt retransmission

                                attempts to estimated current loado heavy load random

                                wait will be longer first collision choose K

                                from 01 delay is K 512 bit transmission times

                                after second collision choose K from 0123hellip

                                after ten collisions choose K from 01234hellip1023

                                5 DataLink Layer 5-50

                                8023 Ethernet Standards Link amp Physical Layers

                                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                applicationtransportnetwork

                                linkphysical

                                MAC protocoland frame format

                                100BASE-TX

                                100BASE-T4

                                100BASE-FX100BASE-T2

                                100BASE-SX 100BASE-BX

                                fiber physical layercopper (twisterpair) physical layer

                                5 DataLink Layer 5-51

                                Manchester encoding

                                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                synchronize to each othero no need for a centralized global clock among nodes

                                Hey this is physical-layer stuff

                                5 DataLink Layer 5-52

                                Link Layer

                                51 Introduction and services

                                52 Error detection and correction

                                53 Multiple access protocols

                                54 Link-layer Addressing

                                55 Ethernet

                                56 Link-layer switches LANs

                                59 A day in the life of a web request

                                5 DataLink Layer 5-53

                                Hubsphysical-layer (ldquodumbrdquo) repeaters

                                o bits coming in one link go out all other links at same rate

                                o all nodes connected to hub can collide with one another

                                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                twisted pair

                                hub

                                5 DataLink Layer 5-54

                                Switch link-layer device smarter than hubs take active

                                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                transparento hosts are unaware of presence of switches

                                plug-and-play self-learningo switches do not need to be configured

                                5 DataLink Layer 5-55

                                Switch allows multiple simultaneous transmissions

                                hosts have dedicated direct connection to switch

                                switches buffer packets Ethernet protocol used on

                                each incoming link but no collisions full duplexo each link is its own collision

                                domain switching A-to-Arsquo and B-

                                to-Brsquo simultaneously without collisions o not possible with dumb hub

                                A

                                Arsquo

                                B

                                Brsquo

                                C

                                Crsquo

                                switch with six interfaces(123456)

                                1 23

                                45

                                6

                                5 DataLink Layer 5-56

                                Switch Table

                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                A each switch has a switch table each entryo (MAC address of host interface

                                to reach host time stamp)

                                looks like a routing table Q how are entries created

                                maintained in switch table o something like a routing

                                protocol

                                A

                                Arsquo

                                B

                                Brsquo

                                C

                                Crsquo

                                switch with six interfaces(123456)

                                1 23

                                45

                                6

                                5 DataLink Layer 5-57

                                Switch self-learning

                                switch learns which hosts can be reached through which interfaceso when frame received

                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                o records senderlocation pair in switch table

                                A

                                Arsquo

                                B

                                Brsquo

                                C

                                Crsquo

                                1 23

                                45

                                6

                                A Arsquo

                                Source ADest Arsquo

                                MAC addr interface TTL

                                Switch table (initially empty)

                                A 1 60

                                5 DataLink Layer 5-58

                                Switch frame filteringforwardingWhen frame received

                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                then if dest on segment from which frame arrived

                                then drop the frame else forward the frame on interface indicated else flood

                                forward on all but the interface on which the frame arrived

                                5 DataLink Layer 5-59

                                Self-learning forwarding example

                                A

                                Arsquo

                                B

                                Brsquo

                                C

                                Crsquo

                                1 23

                                45

                                6

                                A Arsquo

                                Source ADest Arsquo

                                MAC addr interface TTL

                                Switch table (initially empty)

                                A 1 60

                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                frame destination unknownflood

                                Arsquo A

                                destination A location known

                                Arsquo 4 60

                                selective send

                                5 DataLink Layer 5-60

                                Interconnecting switches

                                switches can be connected together

                                A

                                B

                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                A self learning (works exactly the same as in single-switch case)

                                S1

                                C D

                                E

                                FS2

                                S4

                                S3

                                H

                                I

                                G

                                5 DataLink Layer 5-61

                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                A

                                B

                                S1

                                C D

                                E

                                FS2

                                S4

                                S3

                                H

                                I

                                G

                                1

                                2 3

                                34

                                5 DataLink Layer 5-62

                                Institutional network

                                to externalnetwork

                                router

                                IP subnet

                                mail server

                                web server

                                5 DataLink Layer 5-63

                                Switches vs Routers both store-and-forward devices

                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                routers maintain routing tables implement routing algorithms

                                switches maintain switch tables implement filtering learning algorithms

                                Switch

                                5 DataLink Layer 5-64

                                Link Layer

                                51 Introduction and services

                                52 Error detection and correction

                                53Multiple access protocols

                                54 Link-Layer Addressing

                                55 Ethernet

                                56 Link-layer switches 59 A day in the life of

                                a web request

                                5 DataLink Layer 5-65

                                Synthesis a day in the life of a web request

                                journey down protocol stack completeo application transport network link

                                putting-it-all-together synthesiso goal identify review understand protocols

                                (at all layers) involved in seemingly simple scenario requesting www page

                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                5 DataLink Layer 5-66

                                A day in the life scenario

                                Comcast network 68800013

                                Googlersquos network 64233160019 64233169105

                                web server

                                DNS server

                                school network 68802024

                                browser

                                web page

                                5 DataLink Layer 5-67

                                A day in the lifehellip connecting to the Internet

                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                router(runs DHCP)

                                DHCPUDP

                                IPEthPhy

                                DHCP

                                DHCP

                                DHCP

                                DHCP

                                DHCP

                                DHCPUDP

                                IPEthPhy

                                DHCP

                                DHCP

                                DHCP

                                DHCPDHCP

                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                5 DataLink Layer 5-68

                                A day in the lifehellip connecting to the Internet

                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                router(runs DHCP)

                                DHCPUDP

                                IPEthPhy

                                DHCP

                                DHCP

                                DHCP

                                DHCP

                                DHCPUDP

                                IPEthPhy

                                DHCP

                                DHCP

                                DHCP

                                DHCP

                                DHCP

                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                DHCP client receives DHCP ACK reply

                                5 DataLink Layer 5-69

                                A day in the lifehellip ARP (before DNS before HTTP)

                                before sending HTTP request need IP address of wwwgooglecom DNS

                                DNSUDP

                                IPEthPhy

                                DNS

                                DNS

                                DNS

                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                ARP query

                                EthPhy

                                ARP

                                ARP

                                ARP reply

                                5 DataLink Layer 5-70

                                A day in the lifehellip using DNS

                                DNSUDP

                                IPEthPhy

                                DNS

                                DNS

                                DNS

                                DNS

                                DNS

                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                demuxrsquoed to DNS server DNS server replies to

                                client with IP address of wwwgooglecom

                                Comcast network 68800013

                                DNS server

                                DNSUDP

                                IPEthPhy

                                DNS

                                DNS

                                DNS

                                DNS

                                5 DataLink Layer 5-71

                                A day in the lifehellip TCP connection carrying HTTP

                                HTTPTCPIP

                                EthPhy

                                HTTP

                                to send HTTP request client first opens TCP socket to web server

                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                TCP connection established

                                64233169105

                                web server

                                SYN

                                SYN

                                SYN

                                SYN

                                TCPIP

                                EthPhy

                                SYN

                                SYN

                                SYN

                                SYNACK

                                SYNACK

                                SYNACK

                                SYNACK

                                SYNACK

                                SYNACK

                                SYNACK

                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                5 DataLink Layer 5-72

                                A day in the lifehellip HTTP requestreply

                                HTTPTCPIP

                                EthPhy

                                HTTP

                                HTTP request sent into TCP socket

                                IP datagram containing HTTP request routed to wwwgooglecom

                                IP datgram containing HTTP reply routed back to client

                                64233169105

                                web server

                                HTTPTCPIP

                                EthPhy

                                web server responds with HTTP reply (containing web page)

                                HTTP

                                HTTP

                                HTTPHTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                HTTP

                                web page finally () displayed

                                5 DataLink Layer 5-73

                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                (except PHY) solid understanding of networking

                                principles practice hellip could stop here hellip but lots of

                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                • Link Layer
                                • Link Layer Introduction
                                • Link layer context
                                • Encapsulation
                                • Link Layer Services
                                • Link Layer Services (more)
                                • Where is the link layer implemented
                                • Adaptors Communicating
                                • Slide 9
                                • Parity Checking
                                • Internet checksum (review)
                                • Checksumming Cyclic Redundancy Check
                                • Cyclic Redundancy Check
                                • Slide 14
                                • CRC Example
                                • Slide 16
                                • Multiple Access Links and Protocols
                                • Cable Network Architecture Overview
                                • Multiple Access protocols
                                • Ideal Multiple Access Protocol
                                • MAC Protocols a taxonomy
                                • Channel Partitioning MAC protocols TDMA
                                • Channel Partitioning MAC protocols FDMA
                                • Random Access Protocols
                                • ALOHA
                                • CSMA (Carrier Sense Multiple Access)
                                • CSMA collisions
                                • CSMACD (Collision Detection)
                                • CSMACD collision detection
                                • ldquoTaking Turnsrdquo MAC protocols
                                • Slide 31
                                • Slide 32
                                • Summary of MAC protocols
                                • Slide 34
                                • MAC Addresses and ARP
                                • LAN Addresses and ARP
                                • LAN Address (more)
                                • ARP Address Resolution Protocol
                                • ARP protocol Same LAN (network)
                                • Addressing routing to another LAN
                                • Slide 41
                                • Slide 42
                                • Ethernet
                                • Star topology
                                • Ethernet Frame Structure
                                • Ethernet Frame Structure (more)
                                • Ethernet Unreliable connectionless
                                • Ethernet CSMACD algorithm
                                • Ethernetrsquos CSMACD (more)
                                • 8023 Ethernet Standards Link amp Physical Layers
                                • Manchester encoding
                                • Slide 52
                                • Hubs
                                • Switch
                                • Switch allows multiple simultaneous transmissions
                                • Switch Table
                                • Switch self-learning
                                • Switch frame filteringforwarding
                                • Self-learning forwarding example
                                • Interconnecting switches
                                • Self-learning multi-switch example
                                • Institutional network
                                • Switches vs Routers
                                • Slide 64
                                • Synthesis a day in the life of a web request
                                • A day in the life scenario
                                • A day in the lifehellip connecting to the Internet
                                • Slide 68
                                • A day in the lifehellip ARP (before DNS before HTTP)
                                • A day in the lifehellip using DNS
                                • A day in the lifehellip TCP connection carrying HTTP
                                • A day in the lifehellip HTTP requestreply
                                • Chapter 5 letrsquos take a breath

                                  5 DataLink Layer 5-17

                                  Multiple Access Links and Protocols

                                  Two types of ldquolinksrdquo point-to-point

                                  o PPP for dial-up accesso point-to-point link between Ethernet switch and host

                                  broadcast (shared wire or medium)o old-fashioned Etherneto upstream HFC (cable network)o 80211 wireless LAN

                                  shared wire (eg cabled Ethernet)

                                  shared RF (eg 80211 WiFi)

                                  shared RF(satellite)

                                  humans at acocktail party

                                  (shared air acoustical)

                                  Introduction 1-18

                                  Cable Network Architecture Overview

                                  home

                                  cable headend

                                  cable distributionnetwork (simplified)

                                  Typically 500 to 5000 homes

                                  5 DataLink Layer 5-19

                                  Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                                  interference o collision if node receives two or more signals at the same

                                  time

                                  multiple access protocol distributed algorithm that determines how nodes

                                  share channel ie determine when node can transmit

                                  communication about channel sharing must use channel itself o no out-of-band channel for coordination

                                  5 DataLink Layer 5-20

                                  Ideal Multiple Access Protocol

                                  Broadcast channel of rate R bps1 when one node wants to transmit it can send

                                  at the full rate say R2 when M nodes want to transmit each can

                                  send at average rate RM3 fully decentralized

                                  o no special node to coordinate transmissionso no synchronization of clocks slots

                                  4 simple

                                  5 DataLink Layer 5-21

                                  MAC Protocols a taxonomy

                                  Three broad classes Channel Partitioning

                                  o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                  o allocate piece to node for exclusive use

                                  Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                  ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                  take longer turns

                                  5 DataLink Layer 5-22

                                  Channel Partitioning MAC protocols TDMA

                                  TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                  pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                  256 idle

                                  1 3 4 1 3 4

                                  6-slotframe

                                  5 DataLink Layer 5-23

                                  Channel Partitioning MAC protocols FDMA

                                  FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                  idle example 6-station LAN 134 have pkt

                                  frequency bands 256 idle fr

                                  equ

                                  ency

                                  bands time

                                  FDM cable

                                  5 DataLink Layer 5-24

                                  Random Access Protocols

                                  When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                  two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                  o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                  retransmissions)

                                  Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                  5 DataLink Layer 5-25

                                  ALOHA

                                  When a node has a frame to send send immediately

                                  Set a timer for a random amount of time If an ACK arrives before the timer expires

                                  fine otherwise resend the frame

                                  (Works like stop-and-wait with random timeout interval)

                                  5 DataLink Layer 5-26

                                  CSMA (Carrier Sense Multiple Access)

                                  CSMA listen before transmit

                                  If channel sensed idle transmit entire frame

                                  If channel sensed busy defer transmission

                                  human analogy donrsquot interrupt others

                                  5 DataLink Layer 5-27

                                  CSMA collisions

                                  collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                  spatial layout of nodes

                                  5 DataLink Layer 5-28

                                  CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                  o collisions detected within short timeo colliding transmissions aborted reducing

                                  channel wastage collision detection

                                  o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                  o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                  human analogy the polite conversationalist

                                  5 DataLink Layer 5-29

                                  CSMACD collision detection

                                  5 DataLink Layer 5-30

                                  ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                  o share channel efficiently and fairly at high load

                                  o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                  Random access MAC protocolso efficient at low load single node can fully

                                  utilize channelo high load collision overhead

                                  ldquotaking turnsrdquo protocolslook for best of both worlds

                                  5 DataLink Layer 5-31

                                  ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                  ldquoinvitesrdquo slave nodes to transmit in turn

                                  typically used with ldquodumbrdquo slave devices

                                  concernso polling overhead o latencyo single point of

                                  failure (master)

                                  master

                                  slaves

                                  poll

                                  data

                                  data

                                  5 DataLink Layer 5-32

                                  ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                  passed from one node to next sequentially

                                  token message concerns

                                  o token overhead o latencyo single point of failure

                                  (token)

                                  T

                                  data

                                  (nothingto send)

                                  T

                                  5 DataLink Layer 5-33

                                  Summary of MAC protocols

                                  channel partitioning by time frequency or codeo Time Division Frequency Division

                                  random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                  hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                  taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                  5 DataLink Layer 5-34

                                  Link Layer

                                  51 Introduction and services

                                  52 Error detection and correction

                                  53Multiple access protocols

                                  54 Link-Layer Addressing

                                  55 Ethernet

                                  56 Link-layer switches 59 A day in the life of

                                  a web request

                                  5 DataLink Layer 5-35

                                  MAC Addresses and ARP

                                  32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                  MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                  another physically-connected interface (in same network)

                                  o 48 bit MAC addressbull burned in NIC ROM

                                  5 DataLink Layer 5-36

                                  LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                  Broadcast address =FF-FF-FF-FF-FF-FF

                                  = adapter

                                  1A-2F-BB-76-09-AD

                                  58-23-D7-FA-20-B0

                                  0C-C4-11-6F-E3-98

                                  71-65-F7-2B-08-53

                                  LAN(wired orwireless)

                                  5 DataLink Layer 5-37

                                  LAN Address (more)

                                  MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                  space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                  o can move LAN card from one LAN to another

                                  IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                  attached

                                  5 DataLink Layer 5-38

                                  ARP Address Resolution Protocol

                                  Each IP node (host router) on LAN has ARP table

                                  ARP table IPMAC address mappings for some LAN nodes

                                  lt IP address MAC address TTLgto TTL (Time To Live)

                                  time after which address mapping will be forgotten (typically 20 min)

                                  Q given a nodersquos IP address how to determine its MAC address

                                  1A-2F-BB-76-09-AD

                                  0C-C4-11-6F-E3-98

                                  71-65-F7-2B-08-53

                                  LAN

                                  137196723

                                  137196778

                                  137196714

                                  137196788

                                  5 DataLink Layer 5-39

                                  ARP protocol Same LAN (network) A wants to send datagram

                                  to B and Brsquos MAC address not in Arsquos ARP table

                                  A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                  FF-FF-FF-FF-FFo all machines on LAN

                                  receive ARP query B receives ARP packet

                                  replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                  address (unicast)

                                  A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                  ARP is ldquoplug-and-playrdquoo nodes create their

                                  ARP tables without intervention from net administrator

                                  5 DataLink Layer 5-40

                                  Addressing routing to another LAN

                                  R

                                  1A-23-F9-CD-06-9B

                                  222222222220111111111110

                                  E6-E9-00-17-BB-4B

                                  CC-49-DE-D0-AB-7D

                                  111111111112

                                  111111111111

                                  A74-29-9C-E8-FF-55

                                  222222222221

                                  88-B2-2F-54-1A-0F

                                  B222222222222

                                  49-BD-D2-C7-56-2A

                                  walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                  two ARP tables in router R one for each IP network (LAN)

                                  LAN LAN

                                  5 DataLink Layer 5-41

                                  A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                  frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                  destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                  R

                                  1A-23-F9-CD-06-9B

                                  222222222220

                                  111111111110

                                  E6-E9-00-17-BB-4B

                                  CC-49-DE-D0-AB-7D

                                  111111111112

                                  111111111111

                                  A74-29-9C-E8-FF-55

                                  222222222221

                                  88-B2-2F-54-1A-0F

                                  B222222222222

                                  49-BD-D2-C7-56-2A

                                  This is a really importantexample ndash make sure youunderstand

                                  5 DataLink Layer 5-42

                                  Link Layer

                                  51 Introduction and services

                                  52 Error detection and correction

                                  53Multiple access protocols

                                  54 Link-Layer Addressing

                                  55 Ethernet

                                  56 Link-layer switches 59 A day in the life of

                                  a web request

                                  5 DataLink Layer 5-43

                                  Ethernet

                                  ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                  Metcalfersquos Ethernetsketch

                                  5 DataLink Layer 5-44

                                  Star topology bus topology popular through mid 90s

                                  o all nodes in same collision domain (can collide with each other)

                                  today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                  (nodes do not collide with each other)

                                  switch

                                  bus coaxial cable star

                                  5 DataLink Layer 5-45

                                  Ethernet Frame Structure

                                  Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                  Preamble 7 bytes with pattern 10101010 followed by one

                                  byte with pattern 10101011 used to synchronize receiver amp sender clock

                                  rates

                                  5 DataLink Layer 5-46

                                  Ethernet Frame Structure (more) Addresses 6 bytes

                                  o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                  o otherwise adapter discards frame

                                  Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                  CRC checked at receiver if error is detected frame is dropped

                                  5 DataLink Layer 5-47

                                  Ethernet Unreliable connectionless connectionless No handshaking between sending

                                  and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                  send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                  gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                  Ethernetrsquos MAC protocol CSMACD

                                  5 DataLink Layer 5-48

                                  Ethernet CSMACD algorithm

                                  1 NIC receives datagram from network layer creates frame

                                  2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                  3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                  4 If NIC detects collision while transmitting aborts and sends jam signal

                                  5 After aborting NIC enters exponential backoff

                                  after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                  (1 slot = 512 bit-times)

                                  5 DataLink Layer 5-49

                                  Ethernetrsquos CSMACD (more)

                                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                  Exponential Backoff Goal adapt retransmission

                                  attempts to estimated current loado heavy load random

                                  wait will be longer first collision choose K

                                  from 01 delay is K 512 bit transmission times

                                  after second collision choose K from 0123hellip

                                  after ten collisions choose K from 01234hellip1023

                                  5 DataLink Layer 5-50

                                  8023 Ethernet Standards Link amp Physical Layers

                                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                  applicationtransportnetwork

                                  linkphysical

                                  MAC protocoland frame format

                                  100BASE-TX

                                  100BASE-T4

                                  100BASE-FX100BASE-T2

                                  100BASE-SX 100BASE-BX

                                  fiber physical layercopper (twisterpair) physical layer

                                  5 DataLink Layer 5-51

                                  Manchester encoding

                                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                  synchronize to each othero no need for a centralized global clock among nodes

                                  Hey this is physical-layer stuff

                                  5 DataLink Layer 5-52

                                  Link Layer

                                  51 Introduction and services

                                  52 Error detection and correction

                                  53 Multiple access protocols

                                  54 Link-layer Addressing

                                  55 Ethernet

                                  56 Link-layer switches LANs

                                  59 A day in the life of a web request

                                  5 DataLink Layer 5-53

                                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                                  o bits coming in one link go out all other links at same rate

                                  o all nodes connected to hub can collide with one another

                                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                  twisted pair

                                  hub

                                  5 DataLink Layer 5-54

                                  Switch link-layer device smarter than hubs take active

                                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                  transparento hosts are unaware of presence of switches

                                  plug-and-play self-learningo switches do not need to be configured

                                  5 DataLink Layer 5-55

                                  Switch allows multiple simultaneous transmissions

                                  hosts have dedicated direct connection to switch

                                  switches buffer packets Ethernet protocol used on

                                  each incoming link but no collisions full duplexo each link is its own collision

                                  domain switching A-to-Arsquo and B-

                                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                                  A

                                  Arsquo

                                  B

                                  Brsquo

                                  C

                                  Crsquo

                                  switch with six interfaces(123456)

                                  1 23

                                  45

                                  6

                                  5 DataLink Layer 5-56

                                  Switch Table

                                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                  A each switch has a switch table each entryo (MAC address of host interface

                                  to reach host time stamp)

                                  looks like a routing table Q how are entries created

                                  maintained in switch table o something like a routing

                                  protocol

                                  A

                                  Arsquo

                                  B

                                  Brsquo

                                  C

                                  Crsquo

                                  switch with six interfaces(123456)

                                  1 23

                                  45

                                  6

                                  5 DataLink Layer 5-57

                                  Switch self-learning

                                  switch learns which hosts can be reached through which interfaceso when frame received

                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                  o records senderlocation pair in switch table

                                  A

                                  Arsquo

                                  B

                                  Brsquo

                                  C

                                  Crsquo

                                  1 23

                                  45

                                  6

                                  A Arsquo

                                  Source ADest Arsquo

                                  MAC addr interface TTL

                                  Switch table (initially empty)

                                  A 1 60

                                  5 DataLink Layer 5-58

                                  Switch frame filteringforwardingWhen frame received

                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                  then if dest on segment from which frame arrived

                                  then drop the frame else forward the frame on interface indicated else flood

                                  forward on all but the interface on which the frame arrived

                                  5 DataLink Layer 5-59

                                  Self-learning forwarding example

                                  A

                                  Arsquo

                                  B

                                  Brsquo

                                  C

                                  Crsquo

                                  1 23

                                  45

                                  6

                                  A Arsquo

                                  Source ADest Arsquo

                                  MAC addr interface TTL

                                  Switch table (initially empty)

                                  A 1 60

                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                  frame destination unknownflood

                                  Arsquo A

                                  destination A location known

                                  Arsquo 4 60

                                  selective send

                                  5 DataLink Layer 5-60

                                  Interconnecting switches

                                  switches can be connected together

                                  A

                                  B

                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                  A self learning (works exactly the same as in single-switch case)

                                  S1

                                  C D

                                  E

                                  FS2

                                  S4

                                  S3

                                  H

                                  I

                                  G

                                  5 DataLink Layer 5-61

                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                  A

                                  B

                                  S1

                                  C D

                                  E

                                  FS2

                                  S4

                                  S3

                                  H

                                  I

                                  G

                                  1

                                  2 3

                                  34

                                  5 DataLink Layer 5-62

                                  Institutional network

                                  to externalnetwork

                                  router

                                  IP subnet

                                  mail server

                                  web server

                                  5 DataLink Layer 5-63

                                  Switches vs Routers both store-and-forward devices

                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                  routers maintain routing tables implement routing algorithms

                                  switches maintain switch tables implement filtering learning algorithms

                                  Switch

                                  5 DataLink Layer 5-64

                                  Link Layer

                                  51 Introduction and services

                                  52 Error detection and correction

                                  53Multiple access protocols

                                  54 Link-Layer Addressing

                                  55 Ethernet

                                  56 Link-layer switches 59 A day in the life of

                                  a web request

                                  5 DataLink Layer 5-65

                                  Synthesis a day in the life of a web request

                                  journey down protocol stack completeo application transport network link

                                  putting-it-all-together synthesiso goal identify review understand protocols

                                  (at all layers) involved in seemingly simple scenario requesting www page

                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                  5 DataLink Layer 5-66

                                  A day in the life scenario

                                  Comcast network 68800013

                                  Googlersquos network 64233160019 64233169105

                                  web server

                                  DNS server

                                  school network 68802024

                                  browser

                                  web page

                                  5 DataLink Layer 5-67

                                  A day in the lifehellip connecting to the Internet

                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                  router(runs DHCP)

                                  DHCPUDP

                                  IPEthPhy

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCPUDP

                                  IPEthPhy

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCPDHCP

                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                  5 DataLink Layer 5-68

                                  A day in the lifehellip connecting to the Internet

                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                  router(runs DHCP)

                                  DHCPUDP

                                  IPEthPhy

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCPUDP

                                  IPEthPhy

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCP

                                  DHCP

                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                  DHCP client receives DHCP ACK reply

                                  5 DataLink Layer 5-69

                                  A day in the lifehellip ARP (before DNS before HTTP)

                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                  DNSUDP

                                  IPEthPhy

                                  DNS

                                  DNS

                                  DNS

                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                  ARP query

                                  EthPhy

                                  ARP

                                  ARP

                                  ARP reply

                                  5 DataLink Layer 5-70

                                  A day in the lifehellip using DNS

                                  DNSUDP

                                  IPEthPhy

                                  DNS

                                  DNS

                                  DNS

                                  DNS

                                  DNS

                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                  demuxrsquoed to DNS server DNS server replies to

                                  client with IP address of wwwgooglecom

                                  Comcast network 68800013

                                  DNS server

                                  DNSUDP

                                  IPEthPhy

                                  DNS

                                  DNS

                                  DNS

                                  DNS

                                  5 DataLink Layer 5-71

                                  A day in the lifehellip TCP connection carrying HTTP

                                  HTTPTCPIP

                                  EthPhy

                                  HTTP

                                  to send HTTP request client first opens TCP socket to web server

                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                  TCP connection established

                                  64233169105

                                  web server

                                  SYN

                                  SYN

                                  SYN

                                  SYN

                                  TCPIP

                                  EthPhy

                                  SYN

                                  SYN

                                  SYN

                                  SYNACK

                                  SYNACK

                                  SYNACK

                                  SYNACK

                                  SYNACK

                                  SYNACK

                                  SYNACK

                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                  5 DataLink Layer 5-72

                                  A day in the lifehellip HTTP requestreply

                                  HTTPTCPIP

                                  EthPhy

                                  HTTP

                                  HTTP request sent into TCP socket

                                  IP datagram containing HTTP request routed to wwwgooglecom

                                  IP datgram containing HTTP reply routed back to client

                                  64233169105

                                  web server

                                  HTTPTCPIP

                                  EthPhy

                                  web server responds with HTTP reply (containing web page)

                                  HTTP

                                  HTTP

                                  HTTPHTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  HTTP

                                  web page finally () displayed

                                  5 DataLink Layer 5-73

                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                  (except PHY) solid understanding of networking

                                  principles practice hellip could stop here hellip but lots of

                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                  • Link Layer
                                  • Link Layer Introduction
                                  • Link layer context
                                  • Encapsulation
                                  • Link Layer Services
                                  • Link Layer Services (more)
                                  • Where is the link layer implemented
                                  • Adaptors Communicating
                                  • Slide 9
                                  • Parity Checking
                                  • Internet checksum (review)
                                  • Checksumming Cyclic Redundancy Check
                                  • Cyclic Redundancy Check
                                  • Slide 14
                                  • CRC Example
                                  • Slide 16
                                  • Multiple Access Links and Protocols
                                  • Cable Network Architecture Overview
                                  • Multiple Access protocols
                                  • Ideal Multiple Access Protocol
                                  • MAC Protocols a taxonomy
                                  • Channel Partitioning MAC protocols TDMA
                                  • Channel Partitioning MAC protocols FDMA
                                  • Random Access Protocols
                                  • ALOHA
                                  • CSMA (Carrier Sense Multiple Access)
                                  • CSMA collisions
                                  • CSMACD (Collision Detection)
                                  • CSMACD collision detection
                                  • ldquoTaking Turnsrdquo MAC protocols
                                  • Slide 31
                                  • Slide 32
                                  • Summary of MAC protocols
                                  • Slide 34
                                  • MAC Addresses and ARP
                                  • LAN Addresses and ARP
                                  • LAN Address (more)
                                  • ARP Address Resolution Protocol
                                  • ARP protocol Same LAN (network)
                                  • Addressing routing to another LAN
                                  • Slide 41
                                  • Slide 42
                                  • Ethernet
                                  • Star topology
                                  • Ethernet Frame Structure
                                  • Ethernet Frame Structure (more)
                                  • Ethernet Unreliable connectionless
                                  • Ethernet CSMACD algorithm
                                  • Ethernetrsquos CSMACD (more)
                                  • 8023 Ethernet Standards Link amp Physical Layers
                                  • Manchester encoding
                                  • Slide 52
                                  • Hubs
                                  • Switch
                                  • Switch allows multiple simultaneous transmissions
                                  • Switch Table
                                  • Switch self-learning
                                  • Switch frame filteringforwarding
                                  • Self-learning forwarding example
                                  • Interconnecting switches
                                  • Self-learning multi-switch example
                                  • Institutional network
                                  • Switches vs Routers
                                  • Slide 64
                                  • Synthesis a day in the life of a web request
                                  • A day in the life scenario
                                  • A day in the lifehellip connecting to the Internet
                                  • Slide 68
                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                  • A day in the lifehellip using DNS
                                  • A day in the lifehellip TCP connection carrying HTTP
                                  • A day in the lifehellip HTTP requestreply
                                  • Chapter 5 letrsquos take a breath

                                    Introduction 1-18

                                    Cable Network Architecture Overview

                                    home

                                    cable headend

                                    cable distributionnetwork (simplified)

                                    Typically 500 to 5000 homes

                                    5 DataLink Layer 5-19

                                    Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                                    interference o collision if node receives two or more signals at the same

                                    time

                                    multiple access protocol distributed algorithm that determines how nodes

                                    share channel ie determine when node can transmit

                                    communication about channel sharing must use channel itself o no out-of-band channel for coordination

                                    5 DataLink Layer 5-20

                                    Ideal Multiple Access Protocol

                                    Broadcast channel of rate R bps1 when one node wants to transmit it can send

                                    at the full rate say R2 when M nodes want to transmit each can

                                    send at average rate RM3 fully decentralized

                                    o no special node to coordinate transmissionso no synchronization of clocks slots

                                    4 simple

                                    5 DataLink Layer 5-21

                                    MAC Protocols a taxonomy

                                    Three broad classes Channel Partitioning

                                    o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                    o allocate piece to node for exclusive use

                                    Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                    ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                    take longer turns

                                    5 DataLink Layer 5-22

                                    Channel Partitioning MAC protocols TDMA

                                    TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                    pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                    256 idle

                                    1 3 4 1 3 4

                                    6-slotframe

                                    5 DataLink Layer 5-23

                                    Channel Partitioning MAC protocols FDMA

                                    FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                    idle example 6-station LAN 134 have pkt

                                    frequency bands 256 idle fr

                                    equ

                                    ency

                                    bands time

                                    FDM cable

                                    5 DataLink Layer 5-24

                                    Random Access Protocols

                                    When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                    two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                    o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                    retransmissions)

                                    Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                    5 DataLink Layer 5-25

                                    ALOHA

                                    When a node has a frame to send send immediately

                                    Set a timer for a random amount of time If an ACK arrives before the timer expires

                                    fine otherwise resend the frame

                                    (Works like stop-and-wait with random timeout interval)

                                    5 DataLink Layer 5-26

                                    CSMA (Carrier Sense Multiple Access)

                                    CSMA listen before transmit

                                    If channel sensed idle transmit entire frame

                                    If channel sensed busy defer transmission

                                    human analogy donrsquot interrupt others

                                    5 DataLink Layer 5-27

                                    CSMA collisions

                                    collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                    spatial layout of nodes

                                    5 DataLink Layer 5-28

                                    CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                    o collisions detected within short timeo colliding transmissions aborted reducing

                                    channel wastage collision detection

                                    o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                    o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                    human analogy the polite conversationalist

                                    5 DataLink Layer 5-29

                                    CSMACD collision detection

                                    5 DataLink Layer 5-30

                                    ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                    o share channel efficiently and fairly at high load

                                    o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                    Random access MAC protocolso efficient at low load single node can fully

                                    utilize channelo high load collision overhead

                                    ldquotaking turnsrdquo protocolslook for best of both worlds

                                    5 DataLink Layer 5-31

                                    ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                    ldquoinvitesrdquo slave nodes to transmit in turn

                                    typically used with ldquodumbrdquo slave devices

                                    concernso polling overhead o latencyo single point of

                                    failure (master)

                                    master

                                    slaves

                                    poll

                                    data

                                    data

                                    5 DataLink Layer 5-32

                                    ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                    passed from one node to next sequentially

                                    token message concerns

                                    o token overhead o latencyo single point of failure

                                    (token)

                                    T

                                    data

                                    (nothingto send)

                                    T

                                    5 DataLink Layer 5-33

                                    Summary of MAC protocols

                                    channel partitioning by time frequency or codeo Time Division Frequency Division

                                    random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                    hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                    taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                    5 DataLink Layer 5-34

                                    Link Layer

                                    51 Introduction and services

                                    52 Error detection and correction

                                    53Multiple access protocols

                                    54 Link-Layer Addressing

                                    55 Ethernet

                                    56 Link-layer switches 59 A day in the life of

                                    a web request

                                    5 DataLink Layer 5-35

                                    MAC Addresses and ARP

                                    32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                    MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                    another physically-connected interface (in same network)

                                    o 48 bit MAC addressbull burned in NIC ROM

                                    5 DataLink Layer 5-36

                                    LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                    Broadcast address =FF-FF-FF-FF-FF-FF

                                    = adapter

                                    1A-2F-BB-76-09-AD

                                    58-23-D7-FA-20-B0

                                    0C-C4-11-6F-E3-98

                                    71-65-F7-2B-08-53

                                    LAN(wired orwireless)

                                    5 DataLink Layer 5-37

                                    LAN Address (more)

                                    MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                    space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                    o can move LAN card from one LAN to another

                                    IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                    attached

                                    5 DataLink Layer 5-38

                                    ARP Address Resolution Protocol

                                    Each IP node (host router) on LAN has ARP table

                                    ARP table IPMAC address mappings for some LAN nodes

                                    lt IP address MAC address TTLgto TTL (Time To Live)

                                    time after which address mapping will be forgotten (typically 20 min)

                                    Q given a nodersquos IP address how to determine its MAC address

                                    1A-2F-BB-76-09-AD

                                    0C-C4-11-6F-E3-98

                                    71-65-F7-2B-08-53

                                    LAN

                                    137196723

                                    137196778

                                    137196714

                                    137196788

                                    5 DataLink Layer 5-39

                                    ARP protocol Same LAN (network) A wants to send datagram

                                    to B and Brsquos MAC address not in Arsquos ARP table

                                    A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                    FF-FF-FF-FF-FFo all machines on LAN

                                    receive ARP query B receives ARP packet

                                    replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                    address (unicast)

                                    A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                    ARP is ldquoplug-and-playrdquoo nodes create their

                                    ARP tables without intervention from net administrator

                                    5 DataLink Layer 5-40

                                    Addressing routing to another LAN

                                    R

                                    1A-23-F9-CD-06-9B

                                    222222222220111111111110

                                    E6-E9-00-17-BB-4B

                                    CC-49-DE-D0-AB-7D

                                    111111111112

                                    111111111111

                                    A74-29-9C-E8-FF-55

                                    222222222221

                                    88-B2-2F-54-1A-0F

                                    B222222222222

                                    49-BD-D2-C7-56-2A

                                    walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                    two ARP tables in router R one for each IP network (LAN)

                                    LAN LAN

                                    5 DataLink Layer 5-41

                                    A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                    frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                    destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                    R

                                    1A-23-F9-CD-06-9B

                                    222222222220

                                    111111111110

                                    E6-E9-00-17-BB-4B

                                    CC-49-DE-D0-AB-7D

                                    111111111112

                                    111111111111

                                    A74-29-9C-E8-FF-55

                                    222222222221

                                    88-B2-2F-54-1A-0F

                                    B222222222222

                                    49-BD-D2-C7-56-2A

                                    This is a really importantexample ndash make sure youunderstand

                                    5 DataLink Layer 5-42

                                    Link Layer

                                    51 Introduction and services

                                    52 Error detection and correction

                                    53Multiple access protocols

                                    54 Link-Layer Addressing

                                    55 Ethernet

                                    56 Link-layer switches 59 A day in the life of

                                    a web request

                                    5 DataLink Layer 5-43

                                    Ethernet

                                    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                    Metcalfersquos Ethernetsketch

                                    5 DataLink Layer 5-44

                                    Star topology bus topology popular through mid 90s

                                    o all nodes in same collision domain (can collide with each other)

                                    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                    (nodes do not collide with each other)

                                    switch

                                    bus coaxial cable star

                                    5 DataLink Layer 5-45

                                    Ethernet Frame Structure

                                    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                    Preamble 7 bytes with pattern 10101010 followed by one

                                    byte with pattern 10101011 used to synchronize receiver amp sender clock

                                    rates

                                    5 DataLink Layer 5-46

                                    Ethernet Frame Structure (more) Addresses 6 bytes

                                    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                    o otherwise adapter discards frame

                                    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                    CRC checked at receiver if error is detected frame is dropped

                                    5 DataLink Layer 5-47

                                    Ethernet Unreliable connectionless connectionless No handshaking between sending

                                    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                    Ethernetrsquos MAC protocol CSMACD

                                    5 DataLink Layer 5-48

                                    Ethernet CSMACD algorithm

                                    1 NIC receives datagram from network layer creates frame

                                    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                    4 If NIC detects collision while transmitting aborts and sends jam signal

                                    5 After aborting NIC enters exponential backoff

                                    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                    (1 slot = 512 bit-times)

                                    5 DataLink Layer 5-49

                                    Ethernetrsquos CSMACD (more)

                                    Jam Signal make sure all other transmitters are aware of collision 48 bits

                                    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                    Exponential Backoff Goal adapt retransmission

                                    attempts to estimated current loado heavy load random

                                    wait will be longer first collision choose K

                                    from 01 delay is K 512 bit transmission times

                                    after second collision choose K from 0123hellip

                                    after ten collisions choose K from 01234hellip1023

                                    5 DataLink Layer 5-50

                                    8023 Ethernet Standards Link amp Physical Layers

                                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                    applicationtransportnetwork

                                    linkphysical

                                    MAC protocoland frame format

                                    100BASE-TX

                                    100BASE-T4

                                    100BASE-FX100BASE-T2

                                    100BASE-SX 100BASE-BX

                                    fiber physical layercopper (twisterpair) physical layer

                                    5 DataLink Layer 5-51

                                    Manchester encoding

                                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                    synchronize to each othero no need for a centralized global clock among nodes

                                    Hey this is physical-layer stuff

                                    5 DataLink Layer 5-52

                                    Link Layer

                                    51 Introduction and services

                                    52 Error detection and correction

                                    53 Multiple access protocols

                                    54 Link-layer Addressing

                                    55 Ethernet

                                    56 Link-layer switches LANs

                                    59 A day in the life of a web request

                                    5 DataLink Layer 5-53

                                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                                    o bits coming in one link go out all other links at same rate

                                    o all nodes connected to hub can collide with one another

                                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                    twisted pair

                                    hub

                                    5 DataLink Layer 5-54

                                    Switch link-layer device smarter than hubs take active

                                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                    transparento hosts are unaware of presence of switches

                                    plug-and-play self-learningo switches do not need to be configured

                                    5 DataLink Layer 5-55

                                    Switch allows multiple simultaneous transmissions

                                    hosts have dedicated direct connection to switch

                                    switches buffer packets Ethernet protocol used on

                                    each incoming link but no collisions full duplexo each link is its own collision

                                    domain switching A-to-Arsquo and B-

                                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                                    A

                                    Arsquo

                                    B

                                    Brsquo

                                    C

                                    Crsquo

                                    switch with six interfaces(123456)

                                    1 23

                                    45

                                    6

                                    5 DataLink Layer 5-56

                                    Switch Table

                                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                    A each switch has a switch table each entryo (MAC address of host interface

                                    to reach host time stamp)

                                    looks like a routing table Q how are entries created

                                    maintained in switch table o something like a routing

                                    protocol

                                    A

                                    Arsquo

                                    B

                                    Brsquo

                                    C

                                    Crsquo

                                    switch with six interfaces(123456)

                                    1 23

                                    45

                                    6

                                    5 DataLink Layer 5-57

                                    Switch self-learning

                                    switch learns which hosts can be reached through which interfaceso when frame received

                                    switch ldquolearnsrdquo location of sender incoming LAN segment

                                    o records senderlocation pair in switch table

                                    A

                                    Arsquo

                                    B

                                    Brsquo

                                    C

                                    Crsquo

                                    1 23

                                    45

                                    6

                                    A Arsquo

                                    Source ADest Arsquo

                                    MAC addr interface TTL

                                    Switch table (initially empty)

                                    A 1 60

                                    5 DataLink Layer 5-58

                                    Switch frame filteringforwardingWhen frame received

                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                    then if dest on segment from which frame arrived

                                    then drop the frame else forward the frame on interface indicated else flood

                                    forward on all but the interface on which the frame arrived

                                    5 DataLink Layer 5-59

                                    Self-learning forwarding example

                                    A

                                    Arsquo

                                    B

                                    Brsquo

                                    C

                                    Crsquo

                                    1 23

                                    45

                                    6

                                    A Arsquo

                                    Source ADest Arsquo

                                    MAC addr interface TTL

                                    Switch table (initially empty)

                                    A 1 60

                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                    frame destination unknownflood

                                    Arsquo A

                                    destination A location known

                                    Arsquo 4 60

                                    selective send

                                    5 DataLink Layer 5-60

                                    Interconnecting switches

                                    switches can be connected together

                                    A

                                    B

                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                    A self learning (works exactly the same as in single-switch case)

                                    S1

                                    C D

                                    E

                                    FS2

                                    S4

                                    S3

                                    H

                                    I

                                    G

                                    5 DataLink Layer 5-61

                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                    A

                                    B

                                    S1

                                    C D

                                    E

                                    FS2

                                    S4

                                    S3

                                    H

                                    I

                                    G

                                    1

                                    2 3

                                    34

                                    5 DataLink Layer 5-62

                                    Institutional network

                                    to externalnetwork

                                    router

                                    IP subnet

                                    mail server

                                    web server

                                    5 DataLink Layer 5-63

                                    Switches vs Routers both store-and-forward devices

                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                    routers maintain routing tables implement routing algorithms

                                    switches maintain switch tables implement filtering learning algorithms

                                    Switch

                                    5 DataLink Layer 5-64

                                    Link Layer

                                    51 Introduction and services

                                    52 Error detection and correction

                                    53Multiple access protocols

                                    54 Link-Layer Addressing

                                    55 Ethernet

                                    56 Link-layer switches 59 A day in the life of

                                    a web request

                                    5 DataLink Layer 5-65

                                    Synthesis a day in the life of a web request

                                    journey down protocol stack completeo application transport network link

                                    putting-it-all-together synthesiso goal identify review understand protocols

                                    (at all layers) involved in seemingly simple scenario requesting www page

                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                    5 DataLink Layer 5-66

                                    A day in the life scenario

                                    Comcast network 68800013

                                    Googlersquos network 64233160019 64233169105

                                    web server

                                    DNS server

                                    school network 68802024

                                    browser

                                    web page

                                    5 DataLink Layer 5-67

                                    A day in the lifehellip connecting to the Internet

                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                    router(runs DHCP)

                                    DHCPUDP

                                    IPEthPhy

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCPUDP

                                    IPEthPhy

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCPDHCP

                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                    5 DataLink Layer 5-68

                                    A day in the lifehellip connecting to the Internet

                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                    router(runs DHCP)

                                    DHCPUDP

                                    IPEthPhy

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCPUDP

                                    IPEthPhy

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCP

                                    DHCP

                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                    DHCP client receives DHCP ACK reply

                                    5 DataLink Layer 5-69

                                    A day in the lifehellip ARP (before DNS before HTTP)

                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                    DNSUDP

                                    IPEthPhy

                                    DNS

                                    DNS

                                    DNS

                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                    ARP query

                                    EthPhy

                                    ARP

                                    ARP

                                    ARP reply

                                    5 DataLink Layer 5-70

                                    A day in the lifehellip using DNS

                                    DNSUDP

                                    IPEthPhy

                                    DNS

                                    DNS

                                    DNS

                                    DNS

                                    DNS

                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                    demuxrsquoed to DNS server DNS server replies to

                                    client with IP address of wwwgooglecom

                                    Comcast network 68800013

                                    DNS server

                                    DNSUDP

                                    IPEthPhy

                                    DNS

                                    DNS

                                    DNS

                                    DNS

                                    5 DataLink Layer 5-71

                                    A day in the lifehellip TCP connection carrying HTTP

                                    HTTPTCPIP

                                    EthPhy

                                    HTTP

                                    to send HTTP request client first opens TCP socket to web server

                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                    TCP connection established

                                    64233169105

                                    web server

                                    SYN

                                    SYN

                                    SYN

                                    SYN

                                    TCPIP

                                    EthPhy

                                    SYN

                                    SYN

                                    SYN

                                    SYNACK

                                    SYNACK

                                    SYNACK

                                    SYNACK

                                    SYNACK

                                    SYNACK

                                    SYNACK

                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                    5 DataLink Layer 5-72

                                    A day in the lifehellip HTTP requestreply

                                    HTTPTCPIP

                                    EthPhy

                                    HTTP

                                    HTTP request sent into TCP socket

                                    IP datagram containing HTTP request routed to wwwgooglecom

                                    IP datgram containing HTTP reply routed back to client

                                    64233169105

                                    web server

                                    HTTPTCPIP

                                    EthPhy

                                    web server responds with HTTP reply (containing web page)

                                    HTTP

                                    HTTP

                                    HTTPHTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    HTTP

                                    web page finally () displayed

                                    5 DataLink Layer 5-73

                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                    (except PHY) solid understanding of networking

                                    principles practice hellip could stop here hellip but lots of

                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                    • Link Layer
                                    • Link Layer Introduction
                                    • Link layer context
                                    • Encapsulation
                                    • Link Layer Services
                                    • Link Layer Services (more)
                                    • Where is the link layer implemented
                                    • Adaptors Communicating
                                    • Slide 9
                                    • Parity Checking
                                    • Internet checksum (review)
                                    • Checksumming Cyclic Redundancy Check
                                    • Cyclic Redundancy Check
                                    • Slide 14
                                    • CRC Example
                                    • Slide 16
                                    • Multiple Access Links and Protocols
                                    • Cable Network Architecture Overview
                                    • Multiple Access protocols
                                    • Ideal Multiple Access Protocol
                                    • MAC Protocols a taxonomy
                                    • Channel Partitioning MAC protocols TDMA
                                    • Channel Partitioning MAC protocols FDMA
                                    • Random Access Protocols
                                    • ALOHA
                                    • CSMA (Carrier Sense Multiple Access)
                                    • CSMA collisions
                                    • CSMACD (Collision Detection)
                                    • CSMACD collision detection
                                    • ldquoTaking Turnsrdquo MAC protocols
                                    • Slide 31
                                    • Slide 32
                                    • Summary of MAC protocols
                                    • Slide 34
                                    • MAC Addresses and ARP
                                    • LAN Addresses and ARP
                                    • LAN Address (more)
                                    • ARP Address Resolution Protocol
                                    • ARP protocol Same LAN (network)
                                    • Addressing routing to another LAN
                                    • Slide 41
                                    • Slide 42
                                    • Ethernet
                                    • Star topology
                                    • Ethernet Frame Structure
                                    • Ethernet Frame Structure (more)
                                    • Ethernet Unreliable connectionless
                                    • Ethernet CSMACD algorithm
                                    • Ethernetrsquos CSMACD (more)
                                    • 8023 Ethernet Standards Link amp Physical Layers
                                    • Manchester encoding
                                    • Slide 52
                                    • Hubs
                                    • Switch
                                    • Switch allows multiple simultaneous transmissions
                                    • Switch Table
                                    • Switch self-learning
                                    • Switch frame filteringforwarding
                                    • Self-learning forwarding example
                                    • Interconnecting switches
                                    • Self-learning multi-switch example
                                    • Institutional network
                                    • Switches vs Routers
                                    • Slide 64
                                    • Synthesis a day in the life of a web request
                                    • A day in the life scenario
                                    • A day in the lifehellip connecting to the Internet
                                    • Slide 68
                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                    • A day in the lifehellip using DNS
                                    • A day in the lifehellip TCP connection carrying HTTP
                                    • A day in the lifehellip HTTP requestreply
                                    • Chapter 5 letrsquos take a breath

                                      5 DataLink Layer 5-19

                                      Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

                                      interference o collision if node receives two or more signals at the same

                                      time

                                      multiple access protocol distributed algorithm that determines how nodes

                                      share channel ie determine when node can transmit

                                      communication about channel sharing must use channel itself o no out-of-band channel for coordination

                                      5 DataLink Layer 5-20

                                      Ideal Multiple Access Protocol

                                      Broadcast channel of rate R bps1 when one node wants to transmit it can send

                                      at the full rate say R2 when M nodes want to transmit each can

                                      send at average rate RM3 fully decentralized

                                      o no special node to coordinate transmissionso no synchronization of clocks slots

                                      4 simple

                                      5 DataLink Layer 5-21

                                      MAC Protocols a taxonomy

                                      Three broad classes Channel Partitioning

                                      o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                      o allocate piece to node for exclusive use

                                      Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                      ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                      take longer turns

                                      5 DataLink Layer 5-22

                                      Channel Partitioning MAC protocols TDMA

                                      TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                      pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                      256 idle

                                      1 3 4 1 3 4

                                      6-slotframe

                                      5 DataLink Layer 5-23

                                      Channel Partitioning MAC protocols FDMA

                                      FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                      idle example 6-station LAN 134 have pkt

                                      frequency bands 256 idle fr

                                      equ

                                      ency

                                      bands time

                                      FDM cable

                                      5 DataLink Layer 5-24

                                      Random Access Protocols

                                      When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                      two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                      o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                      retransmissions)

                                      Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                      5 DataLink Layer 5-25

                                      ALOHA

                                      When a node has a frame to send send immediately

                                      Set a timer for a random amount of time If an ACK arrives before the timer expires

                                      fine otherwise resend the frame

                                      (Works like stop-and-wait with random timeout interval)

                                      5 DataLink Layer 5-26

                                      CSMA (Carrier Sense Multiple Access)

                                      CSMA listen before transmit

                                      If channel sensed idle transmit entire frame

                                      If channel sensed busy defer transmission

                                      human analogy donrsquot interrupt others

                                      5 DataLink Layer 5-27

                                      CSMA collisions

                                      collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                      spatial layout of nodes

                                      5 DataLink Layer 5-28

                                      CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                      o collisions detected within short timeo colliding transmissions aborted reducing

                                      channel wastage collision detection

                                      o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                      o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                      human analogy the polite conversationalist

                                      5 DataLink Layer 5-29

                                      CSMACD collision detection

                                      5 DataLink Layer 5-30

                                      ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                      o share channel efficiently and fairly at high load

                                      o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                      Random access MAC protocolso efficient at low load single node can fully

                                      utilize channelo high load collision overhead

                                      ldquotaking turnsrdquo protocolslook for best of both worlds

                                      5 DataLink Layer 5-31

                                      ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                      ldquoinvitesrdquo slave nodes to transmit in turn

                                      typically used with ldquodumbrdquo slave devices

                                      concernso polling overhead o latencyo single point of

                                      failure (master)

                                      master

                                      slaves

                                      poll

                                      data

                                      data

                                      5 DataLink Layer 5-32

                                      ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                      passed from one node to next sequentially

                                      token message concerns

                                      o token overhead o latencyo single point of failure

                                      (token)

                                      T

                                      data

                                      (nothingto send)

                                      T

                                      5 DataLink Layer 5-33

                                      Summary of MAC protocols

                                      channel partitioning by time frequency or codeo Time Division Frequency Division

                                      random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                      hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                      taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                      5 DataLink Layer 5-34

                                      Link Layer

                                      51 Introduction and services

                                      52 Error detection and correction

                                      53Multiple access protocols

                                      54 Link-Layer Addressing

                                      55 Ethernet

                                      56 Link-layer switches 59 A day in the life of

                                      a web request

                                      5 DataLink Layer 5-35

                                      MAC Addresses and ARP

                                      32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                      MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                      another physically-connected interface (in same network)

                                      o 48 bit MAC addressbull burned in NIC ROM

                                      5 DataLink Layer 5-36

                                      LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                      Broadcast address =FF-FF-FF-FF-FF-FF

                                      = adapter

                                      1A-2F-BB-76-09-AD

                                      58-23-D7-FA-20-B0

                                      0C-C4-11-6F-E3-98

                                      71-65-F7-2B-08-53

                                      LAN(wired orwireless)

                                      5 DataLink Layer 5-37

                                      LAN Address (more)

                                      MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                      space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                      o can move LAN card from one LAN to another

                                      IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                      attached

                                      5 DataLink Layer 5-38

                                      ARP Address Resolution Protocol

                                      Each IP node (host router) on LAN has ARP table

                                      ARP table IPMAC address mappings for some LAN nodes

                                      lt IP address MAC address TTLgto TTL (Time To Live)

                                      time after which address mapping will be forgotten (typically 20 min)

                                      Q given a nodersquos IP address how to determine its MAC address

                                      1A-2F-BB-76-09-AD

                                      0C-C4-11-6F-E3-98

                                      71-65-F7-2B-08-53

                                      LAN

                                      137196723

                                      137196778

                                      137196714

                                      137196788

                                      5 DataLink Layer 5-39

                                      ARP protocol Same LAN (network) A wants to send datagram

                                      to B and Brsquos MAC address not in Arsquos ARP table

                                      A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                      FF-FF-FF-FF-FFo all machines on LAN

                                      receive ARP query B receives ARP packet

                                      replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                      address (unicast)

                                      A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                      ARP is ldquoplug-and-playrdquoo nodes create their

                                      ARP tables without intervention from net administrator

                                      5 DataLink Layer 5-40

                                      Addressing routing to another LAN

                                      R

                                      1A-23-F9-CD-06-9B

                                      222222222220111111111110

                                      E6-E9-00-17-BB-4B

                                      CC-49-DE-D0-AB-7D

                                      111111111112

                                      111111111111

                                      A74-29-9C-E8-FF-55

                                      222222222221

                                      88-B2-2F-54-1A-0F

                                      B222222222222

                                      49-BD-D2-C7-56-2A

                                      walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                      two ARP tables in router R one for each IP network (LAN)

                                      LAN LAN

                                      5 DataLink Layer 5-41

                                      A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                      frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                      destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                      R

                                      1A-23-F9-CD-06-9B

                                      222222222220

                                      111111111110

                                      E6-E9-00-17-BB-4B

                                      CC-49-DE-D0-AB-7D

                                      111111111112

                                      111111111111

                                      A74-29-9C-E8-FF-55

                                      222222222221

                                      88-B2-2F-54-1A-0F

                                      B222222222222

                                      49-BD-D2-C7-56-2A

                                      This is a really importantexample ndash make sure youunderstand

                                      5 DataLink Layer 5-42

                                      Link Layer

                                      51 Introduction and services

                                      52 Error detection and correction

                                      53Multiple access protocols

                                      54 Link-Layer Addressing

                                      55 Ethernet

                                      56 Link-layer switches 59 A day in the life of

                                      a web request

                                      5 DataLink Layer 5-43

                                      Ethernet

                                      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                      Metcalfersquos Ethernetsketch

                                      5 DataLink Layer 5-44

                                      Star topology bus topology popular through mid 90s

                                      o all nodes in same collision domain (can collide with each other)

                                      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                      (nodes do not collide with each other)

                                      switch

                                      bus coaxial cable star

                                      5 DataLink Layer 5-45

                                      Ethernet Frame Structure

                                      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                      Preamble 7 bytes with pattern 10101010 followed by one

                                      byte with pattern 10101011 used to synchronize receiver amp sender clock

                                      rates

                                      5 DataLink Layer 5-46

                                      Ethernet Frame Structure (more) Addresses 6 bytes

                                      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                      o otherwise adapter discards frame

                                      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                      CRC checked at receiver if error is detected frame is dropped

                                      5 DataLink Layer 5-47

                                      Ethernet Unreliable connectionless connectionless No handshaking between sending

                                      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                      Ethernetrsquos MAC protocol CSMACD

                                      5 DataLink Layer 5-48

                                      Ethernet CSMACD algorithm

                                      1 NIC receives datagram from network layer creates frame

                                      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                      4 If NIC detects collision while transmitting aborts and sends jam signal

                                      5 After aborting NIC enters exponential backoff

                                      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                      (1 slot = 512 bit-times)

                                      5 DataLink Layer 5-49

                                      Ethernetrsquos CSMACD (more)

                                      Jam Signal make sure all other transmitters are aware of collision 48 bits

                                      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                      Exponential Backoff Goal adapt retransmission

                                      attempts to estimated current loado heavy load random

                                      wait will be longer first collision choose K

                                      from 01 delay is K 512 bit transmission times

                                      after second collision choose K from 0123hellip

                                      after ten collisions choose K from 01234hellip1023

                                      5 DataLink Layer 5-50

                                      8023 Ethernet Standards Link amp Physical Layers

                                      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                      Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                      applicationtransportnetwork

                                      linkphysical

                                      MAC protocoland frame format

                                      100BASE-TX

                                      100BASE-T4

                                      100BASE-FX100BASE-T2

                                      100BASE-SX 100BASE-BX

                                      fiber physical layercopper (twisterpair) physical layer

                                      5 DataLink Layer 5-51

                                      Manchester encoding

                                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                      synchronize to each othero no need for a centralized global clock among nodes

                                      Hey this is physical-layer stuff

                                      5 DataLink Layer 5-52

                                      Link Layer

                                      51 Introduction and services

                                      52 Error detection and correction

                                      53 Multiple access protocols

                                      54 Link-layer Addressing

                                      55 Ethernet

                                      56 Link-layer switches LANs

                                      59 A day in the life of a web request

                                      5 DataLink Layer 5-53

                                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                                      o bits coming in one link go out all other links at same rate

                                      o all nodes connected to hub can collide with one another

                                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                      twisted pair

                                      hub

                                      5 DataLink Layer 5-54

                                      Switch link-layer device smarter than hubs take active

                                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                      transparento hosts are unaware of presence of switches

                                      plug-and-play self-learningo switches do not need to be configured

                                      5 DataLink Layer 5-55

                                      Switch allows multiple simultaneous transmissions

                                      hosts have dedicated direct connection to switch

                                      switches buffer packets Ethernet protocol used on

                                      each incoming link but no collisions full duplexo each link is its own collision

                                      domain switching A-to-Arsquo and B-

                                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                                      A

                                      Arsquo

                                      B

                                      Brsquo

                                      C

                                      Crsquo

                                      switch with six interfaces(123456)

                                      1 23

                                      45

                                      6

                                      5 DataLink Layer 5-56

                                      Switch Table

                                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                      A each switch has a switch table each entryo (MAC address of host interface

                                      to reach host time stamp)

                                      looks like a routing table Q how are entries created

                                      maintained in switch table o something like a routing

                                      protocol

                                      A

                                      Arsquo

                                      B

                                      Brsquo

                                      C

                                      Crsquo

                                      switch with six interfaces(123456)

                                      1 23

                                      45

                                      6

                                      5 DataLink Layer 5-57

                                      Switch self-learning

                                      switch learns which hosts can be reached through which interfaceso when frame received

                                      switch ldquolearnsrdquo location of sender incoming LAN segment

                                      o records senderlocation pair in switch table

                                      A

                                      Arsquo

                                      B

                                      Brsquo

                                      C

                                      Crsquo

                                      1 23

                                      45

                                      6

                                      A Arsquo

                                      Source ADest Arsquo

                                      MAC addr interface TTL

                                      Switch table (initially empty)

                                      A 1 60

                                      5 DataLink Layer 5-58

                                      Switch frame filteringforwardingWhen frame received

                                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                      then if dest on segment from which frame arrived

                                      then drop the frame else forward the frame on interface indicated else flood

                                      forward on all but the interface on which the frame arrived

                                      5 DataLink Layer 5-59

                                      Self-learning forwarding example

                                      A

                                      Arsquo

                                      B

                                      Brsquo

                                      C

                                      Crsquo

                                      1 23

                                      45

                                      6

                                      A Arsquo

                                      Source ADest Arsquo

                                      MAC addr interface TTL

                                      Switch table (initially empty)

                                      A 1 60

                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                      frame destination unknownflood

                                      Arsquo A

                                      destination A location known

                                      Arsquo 4 60

                                      selective send

                                      5 DataLink Layer 5-60

                                      Interconnecting switches

                                      switches can be connected together

                                      A

                                      B

                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                      A self learning (works exactly the same as in single-switch case)

                                      S1

                                      C D

                                      E

                                      FS2

                                      S4

                                      S3

                                      H

                                      I

                                      G

                                      5 DataLink Layer 5-61

                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                      A

                                      B

                                      S1

                                      C D

                                      E

                                      FS2

                                      S4

                                      S3

                                      H

                                      I

                                      G

                                      1

                                      2 3

                                      34

                                      5 DataLink Layer 5-62

                                      Institutional network

                                      to externalnetwork

                                      router

                                      IP subnet

                                      mail server

                                      web server

                                      5 DataLink Layer 5-63

                                      Switches vs Routers both store-and-forward devices

                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                      routers maintain routing tables implement routing algorithms

                                      switches maintain switch tables implement filtering learning algorithms

                                      Switch

                                      5 DataLink Layer 5-64

                                      Link Layer

                                      51 Introduction and services

                                      52 Error detection and correction

                                      53Multiple access protocols

                                      54 Link-Layer Addressing

                                      55 Ethernet

                                      56 Link-layer switches 59 A day in the life of

                                      a web request

                                      5 DataLink Layer 5-65

                                      Synthesis a day in the life of a web request

                                      journey down protocol stack completeo application transport network link

                                      putting-it-all-together synthesiso goal identify review understand protocols

                                      (at all layers) involved in seemingly simple scenario requesting www page

                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                      5 DataLink Layer 5-66

                                      A day in the life scenario

                                      Comcast network 68800013

                                      Googlersquos network 64233160019 64233169105

                                      web server

                                      DNS server

                                      school network 68802024

                                      browser

                                      web page

                                      5 DataLink Layer 5-67

                                      A day in the lifehellip connecting to the Internet

                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                      router(runs DHCP)

                                      DHCPUDP

                                      IPEthPhy

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCPUDP

                                      IPEthPhy

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCPDHCP

                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                      5 DataLink Layer 5-68

                                      A day in the lifehellip connecting to the Internet

                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                      router(runs DHCP)

                                      DHCPUDP

                                      IPEthPhy

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCPUDP

                                      IPEthPhy

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCP

                                      DHCP

                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                      DHCP client receives DHCP ACK reply

                                      5 DataLink Layer 5-69

                                      A day in the lifehellip ARP (before DNS before HTTP)

                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                      DNSUDP

                                      IPEthPhy

                                      DNS

                                      DNS

                                      DNS

                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                      ARP query

                                      EthPhy

                                      ARP

                                      ARP

                                      ARP reply

                                      5 DataLink Layer 5-70

                                      A day in the lifehellip using DNS

                                      DNSUDP

                                      IPEthPhy

                                      DNS

                                      DNS

                                      DNS

                                      DNS

                                      DNS

                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                      demuxrsquoed to DNS server DNS server replies to

                                      client with IP address of wwwgooglecom

                                      Comcast network 68800013

                                      DNS server

                                      DNSUDP

                                      IPEthPhy

                                      DNS

                                      DNS

                                      DNS

                                      DNS

                                      5 DataLink Layer 5-71

                                      A day in the lifehellip TCP connection carrying HTTP

                                      HTTPTCPIP

                                      EthPhy

                                      HTTP

                                      to send HTTP request client first opens TCP socket to web server

                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                      TCP connection established

                                      64233169105

                                      web server

                                      SYN

                                      SYN

                                      SYN

                                      SYN

                                      TCPIP

                                      EthPhy

                                      SYN

                                      SYN

                                      SYN

                                      SYNACK

                                      SYNACK

                                      SYNACK

                                      SYNACK

                                      SYNACK

                                      SYNACK

                                      SYNACK

                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                      5 DataLink Layer 5-72

                                      A day in the lifehellip HTTP requestreply

                                      HTTPTCPIP

                                      EthPhy

                                      HTTP

                                      HTTP request sent into TCP socket

                                      IP datagram containing HTTP request routed to wwwgooglecom

                                      IP datgram containing HTTP reply routed back to client

                                      64233169105

                                      web server

                                      HTTPTCPIP

                                      EthPhy

                                      web server responds with HTTP reply (containing web page)

                                      HTTP

                                      HTTP

                                      HTTPHTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      HTTP

                                      web page finally () displayed

                                      5 DataLink Layer 5-73

                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                      (except PHY) solid understanding of networking

                                      principles practice hellip could stop here hellip but lots of

                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                      • Link Layer
                                      • Link Layer Introduction
                                      • Link layer context
                                      • Encapsulation
                                      • Link Layer Services
                                      • Link Layer Services (more)
                                      • Where is the link layer implemented
                                      • Adaptors Communicating
                                      • Slide 9
                                      • Parity Checking
                                      • Internet checksum (review)
                                      • Checksumming Cyclic Redundancy Check
                                      • Cyclic Redundancy Check
                                      • Slide 14
                                      • CRC Example
                                      • Slide 16
                                      • Multiple Access Links and Protocols
                                      • Cable Network Architecture Overview
                                      • Multiple Access protocols
                                      • Ideal Multiple Access Protocol
                                      • MAC Protocols a taxonomy
                                      • Channel Partitioning MAC protocols TDMA
                                      • Channel Partitioning MAC protocols FDMA
                                      • Random Access Protocols
                                      • ALOHA
                                      • CSMA (Carrier Sense Multiple Access)
                                      • CSMA collisions
                                      • CSMACD (Collision Detection)
                                      • CSMACD collision detection
                                      • ldquoTaking Turnsrdquo MAC protocols
                                      • Slide 31
                                      • Slide 32
                                      • Summary of MAC protocols
                                      • Slide 34
                                      • MAC Addresses and ARP
                                      • LAN Addresses and ARP
                                      • LAN Address (more)
                                      • ARP Address Resolution Protocol
                                      • ARP protocol Same LAN (network)
                                      • Addressing routing to another LAN
                                      • Slide 41
                                      • Slide 42
                                      • Ethernet
                                      • Star topology
                                      • Ethernet Frame Structure
                                      • Ethernet Frame Structure (more)
                                      • Ethernet Unreliable connectionless
                                      • Ethernet CSMACD algorithm
                                      • Ethernetrsquos CSMACD (more)
                                      • 8023 Ethernet Standards Link amp Physical Layers
                                      • Manchester encoding
                                      • Slide 52
                                      • Hubs
                                      • Switch
                                      • Switch allows multiple simultaneous transmissions
                                      • Switch Table
                                      • Switch self-learning
                                      • Switch frame filteringforwarding
                                      • Self-learning forwarding example
                                      • Interconnecting switches
                                      • Self-learning multi-switch example
                                      • Institutional network
                                      • Switches vs Routers
                                      • Slide 64
                                      • Synthesis a day in the life of a web request
                                      • A day in the life scenario
                                      • A day in the lifehellip connecting to the Internet
                                      • Slide 68
                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                      • A day in the lifehellip using DNS
                                      • A day in the lifehellip TCP connection carrying HTTP
                                      • A day in the lifehellip HTTP requestreply
                                      • Chapter 5 letrsquos take a breath

                                        5 DataLink Layer 5-20

                                        Ideal Multiple Access Protocol

                                        Broadcast channel of rate R bps1 when one node wants to transmit it can send

                                        at the full rate say R2 when M nodes want to transmit each can

                                        send at average rate RM3 fully decentralized

                                        o no special node to coordinate transmissionso no synchronization of clocks slots

                                        4 simple

                                        5 DataLink Layer 5-21

                                        MAC Protocols a taxonomy

                                        Three broad classes Channel Partitioning

                                        o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                        o allocate piece to node for exclusive use

                                        Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                        ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                        take longer turns

                                        5 DataLink Layer 5-22

                                        Channel Partitioning MAC protocols TDMA

                                        TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                        pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                        256 idle

                                        1 3 4 1 3 4

                                        6-slotframe

                                        5 DataLink Layer 5-23

                                        Channel Partitioning MAC protocols FDMA

                                        FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                        idle example 6-station LAN 134 have pkt

                                        frequency bands 256 idle fr

                                        equ

                                        ency

                                        bands time

                                        FDM cable

                                        5 DataLink Layer 5-24

                                        Random Access Protocols

                                        When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                        two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                        o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                        retransmissions)

                                        Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                        5 DataLink Layer 5-25

                                        ALOHA

                                        When a node has a frame to send send immediately

                                        Set a timer for a random amount of time If an ACK arrives before the timer expires

                                        fine otherwise resend the frame

                                        (Works like stop-and-wait with random timeout interval)

                                        5 DataLink Layer 5-26

                                        CSMA (Carrier Sense Multiple Access)

                                        CSMA listen before transmit

                                        If channel sensed idle transmit entire frame

                                        If channel sensed busy defer transmission

                                        human analogy donrsquot interrupt others

                                        5 DataLink Layer 5-27

                                        CSMA collisions

                                        collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                        spatial layout of nodes

                                        5 DataLink Layer 5-28

                                        CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                        o collisions detected within short timeo colliding transmissions aborted reducing

                                        channel wastage collision detection

                                        o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                        o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                        human analogy the polite conversationalist

                                        5 DataLink Layer 5-29

                                        CSMACD collision detection

                                        5 DataLink Layer 5-30

                                        ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                        o share channel efficiently and fairly at high load

                                        o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                        Random access MAC protocolso efficient at low load single node can fully

                                        utilize channelo high load collision overhead

                                        ldquotaking turnsrdquo protocolslook for best of both worlds

                                        5 DataLink Layer 5-31

                                        ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                        ldquoinvitesrdquo slave nodes to transmit in turn

                                        typically used with ldquodumbrdquo slave devices

                                        concernso polling overhead o latencyo single point of

                                        failure (master)

                                        master

                                        slaves

                                        poll

                                        data

                                        data

                                        5 DataLink Layer 5-32

                                        ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                        passed from one node to next sequentially

                                        token message concerns

                                        o token overhead o latencyo single point of failure

                                        (token)

                                        T

                                        data

                                        (nothingto send)

                                        T

                                        5 DataLink Layer 5-33

                                        Summary of MAC protocols

                                        channel partitioning by time frequency or codeo Time Division Frequency Division

                                        random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                        hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                        taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                        5 DataLink Layer 5-34

                                        Link Layer

                                        51 Introduction and services

                                        52 Error detection and correction

                                        53Multiple access protocols

                                        54 Link-Layer Addressing

                                        55 Ethernet

                                        56 Link-layer switches 59 A day in the life of

                                        a web request

                                        5 DataLink Layer 5-35

                                        MAC Addresses and ARP

                                        32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                        MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                        another physically-connected interface (in same network)

                                        o 48 bit MAC addressbull burned in NIC ROM

                                        5 DataLink Layer 5-36

                                        LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                        Broadcast address =FF-FF-FF-FF-FF-FF

                                        = adapter

                                        1A-2F-BB-76-09-AD

                                        58-23-D7-FA-20-B0

                                        0C-C4-11-6F-E3-98

                                        71-65-F7-2B-08-53

                                        LAN(wired orwireless)

                                        5 DataLink Layer 5-37

                                        LAN Address (more)

                                        MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                        space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                        o can move LAN card from one LAN to another

                                        IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                        attached

                                        5 DataLink Layer 5-38

                                        ARP Address Resolution Protocol

                                        Each IP node (host router) on LAN has ARP table

                                        ARP table IPMAC address mappings for some LAN nodes

                                        lt IP address MAC address TTLgto TTL (Time To Live)

                                        time after which address mapping will be forgotten (typically 20 min)

                                        Q given a nodersquos IP address how to determine its MAC address

                                        1A-2F-BB-76-09-AD

                                        0C-C4-11-6F-E3-98

                                        71-65-F7-2B-08-53

                                        LAN

                                        137196723

                                        137196778

                                        137196714

                                        137196788

                                        5 DataLink Layer 5-39

                                        ARP protocol Same LAN (network) A wants to send datagram

                                        to B and Brsquos MAC address not in Arsquos ARP table

                                        A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                        FF-FF-FF-FF-FFo all machines on LAN

                                        receive ARP query B receives ARP packet

                                        replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                        address (unicast)

                                        A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                        ARP is ldquoplug-and-playrdquoo nodes create their

                                        ARP tables without intervention from net administrator

                                        5 DataLink Layer 5-40

                                        Addressing routing to another LAN

                                        R

                                        1A-23-F9-CD-06-9B

                                        222222222220111111111110

                                        E6-E9-00-17-BB-4B

                                        CC-49-DE-D0-AB-7D

                                        111111111112

                                        111111111111

                                        A74-29-9C-E8-FF-55

                                        222222222221

                                        88-B2-2F-54-1A-0F

                                        B222222222222

                                        49-BD-D2-C7-56-2A

                                        walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                        two ARP tables in router R one for each IP network (LAN)

                                        LAN LAN

                                        5 DataLink Layer 5-41

                                        A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                        frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                        destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                        R

                                        1A-23-F9-CD-06-9B

                                        222222222220

                                        111111111110

                                        E6-E9-00-17-BB-4B

                                        CC-49-DE-D0-AB-7D

                                        111111111112

                                        111111111111

                                        A74-29-9C-E8-FF-55

                                        222222222221

                                        88-B2-2F-54-1A-0F

                                        B222222222222

                                        49-BD-D2-C7-56-2A

                                        This is a really importantexample ndash make sure youunderstand

                                        5 DataLink Layer 5-42

                                        Link Layer

                                        51 Introduction and services

                                        52 Error detection and correction

                                        53Multiple access protocols

                                        54 Link-Layer Addressing

                                        55 Ethernet

                                        56 Link-layer switches 59 A day in the life of

                                        a web request

                                        5 DataLink Layer 5-43

                                        Ethernet

                                        ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                        Metcalfersquos Ethernetsketch

                                        5 DataLink Layer 5-44

                                        Star topology bus topology popular through mid 90s

                                        o all nodes in same collision domain (can collide with each other)

                                        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                        (nodes do not collide with each other)

                                        switch

                                        bus coaxial cable star

                                        5 DataLink Layer 5-45

                                        Ethernet Frame Structure

                                        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                        Preamble 7 bytes with pattern 10101010 followed by one

                                        byte with pattern 10101011 used to synchronize receiver amp sender clock

                                        rates

                                        5 DataLink Layer 5-46

                                        Ethernet Frame Structure (more) Addresses 6 bytes

                                        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                        o otherwise adapter discards frame

                                        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                        CRC checked at receiver if error is detected frame is dropped

                                        5 DataLink Layer 5-47

                                        Ethernet Unreliable connectionless connectionless No handshaking between sending

                                        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                        Ethernetrsquos MAC protocol CSMACD

                                        5 DataLink Layer 5-48

                                        Ethernet CSMACD algorithm

                                        1 NIC receives datagram from network layer creates frame

                                        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                        4 If NIC detects collision while transmitting aborts and sends jam signal

                                        5 After aborting NIC enters exponential backoff

                                        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                        (1 slot = 512 bit-times)

                                        5 DataLink Layer 5-49

                                        Ethernetrsquos CSMACD (more)

                                        Jam Signal make sure all other transmitters are aware of collision 48 bits

                                        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                        Exponential Backoff Goal adapt retransmission

                                        attempts to estimated current loado heavy load random

                                        wait will be longer first collision choose K

                                        from 01 delay is K 512 bit transmission times

                                        after second collision choose K from 0123hellip

                                        after ten collisions choose K from 01234hellip1023

                                        5 DataLink Layer 5-50

                                        8023 Ethernet Standards Link amp Physical Layers

                                        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                        Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                        applicationtransportnetwork

                                        linkphysical

                                        MAC protocoland frame format

                                        100BASE-TX

                                        100BASE-T4

                                        100BASE-FX100BASE-T2

                                        100BASE-SX 100BASE-BX

                                        fiber physical layercopper (twisterpair) physical layer

                                        5 DataLink Layer 5-51

                                        Manchester encoding

                                        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                        synchronize to each othero no need for a centralized global clock among nodes

                                        Hey this is physical-layer stuff

                                        5 DataLink Layer 5-52

                                        Link Layer

                                        51 Introduction and services

                                        52 Error detection and correction

                                        53 Multiple access protocols

                                        54 Link-layer Addressing

                                        55 Ethernet

                                        56 Link-layer switches LANs

                                        59 A day in the life of a web request

                                        5 DataLink Layer 5-53

                                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                                        o bits coming in one link go out all other links at same rate

                                        o all nodes connected to hub can collide with one another

                                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                        twisted pair

                                        hub

                                        5 DataLink Layer 5-54

                                        Switch link-layer device smarter than hubs take active

                                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                        transparento hosts are unaware of presence of switches

                                        plug-and-play self-learningo switches do not need to be configured

                                        5 DataLink Layer 5-55

                                        Switch allows multiple simultaneous transmissions

                                        hosts have dedicated direct connection to switch

                                        switches buffer packets Ethernet protocol used on

                                        each incoming link but no collisions full duplexo each link is its own collision

                                        domain switching A-to-Arsquo and B-

                                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                                        A

                                        Arsquo

                                        B

                                        Brsquo

                                        C

                                        Crsquo

                                        switch with six interfaces(123456)

                                        1 23

                                        45

                                        6

                                        5 DataLink Layer 5-56

                                        Switch Table

                                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                        A each switch has a switch table each entryo (MAC address of host interface

                                        to reach host time stamp)

                                        looks like a routing table Q how are entries created

                                        maintained in switch table o something like a routing

                                        protocol

                                        A

                                        Arsquo

                                        B

                                        Brsquo

                                        C

                                        Crsquo

                                        switch with six interfaces(123456)

                                        1 23

                                        45

                                        6

                                        5 DataLink Layer 5-57

                                        Switch self-learning

                                        switch learns which hosts can be reached through which interfaceso when frame received

                                        switch ldquolearnsrdquo location of sender incoming LAN segment

                                        o records senderlocation pair in switch table

                                        A

                                        Arsquo

                                        B

                                        Brsquo

                                        C

                                        Crsquo

                                        1 23

                                        45

                                        6

                                        A Arsquo

                                        Source ADest Arsquo

                                        MAC addr interface TTL

                                        Switch table (initially empty)

                                        A 1 60

                                        5 DataLink Layer 5-58

                                        Switch frame filteringforwardingWhen frame received

                                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                        then if dest on segment from which frame arrived

                                        then drop the frame else forward the frame on interface indicated else flood

                                        forward on all but the interface on which the frame arrived

                                        5 DataLink Layer 5-59

                                        Self-learning forwarding example

                                        A

                                        Arsquo

                                        B

                                        Brsquo

                                        C

                                        Crsquo

                                        1 23

                                        45

                                        6

                                        A Arsquo

                                        Source ADest Arsquo

                                        MAC addr interface TTL

                                        Switch table (initially empty)

                                        A 1 60

                                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                        frame destination unknownflood

                                        Arsquo A

                                        destination A location known

                                        Arsquo 4 60

                                        selective send

                                        5 DataLink Layer 5-60

                                        Interconnecting switches

                                        switches can be connected together

                                        A

                                        B

                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                        A self learning (works exactly the same as in single-switch case)

                                        S1

                                        C D

                                        E

                                        FS2

                                        S4

                                        S3

                                        H

                                        I

                                        G

                                        5 DataLink Layer 5-61

                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                        A

                                        B

                                        S1

                                        C D

                                        E

                                        FS2

                                        S4

                                        S3

                                        H

                                        I

                                        G

                                        1

                                        2 3

                                        34

                                        5 DataLink Layer 5-62

                                        Institutional network

                                        to externalnetwork

                                        router

                                        IP subnet

                                        mail server

                                        web server

                                        5 DataLink Layer 5-63

                                        Switches vs Routers both store-and-forward devices

                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                        routers maintain routing tables implement routing algorithms

                                        switches maintain switch tables implement filtering learning algorithms

                                        Switch

                                        5 DataLink Layer 5-64

                                        Link Layer

                                        51 Introduction and services

                                        52 Error detection and correction

                                        53Multiple access protocols

                                        54 Link-Layer Addressing

                                        55 Ethernet

                                        56 Link-layer switches 59 A day in the life of

                                        a web request

                                        5 DataLink Layer 5-65

                                        Synthesis a day in the life of a web request

                                        journey down protocol stack completeo application transport network link

                                        putting-it-all-together synthesiso goal identify review understand protocols

                                        (at all layers) involved in seemingly simple scenario requesting www page

                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                        5 DataLink Layer 5-66

                                        A day in the life scenario

                                        Comcast network 68800013

                                        Googlersquos network 64233160019 64233169105

                                        web server

                                        DNS server

                                        school network 68802024

                                        browser

                                        web page

                                        5 DataLink Layer 5-67

                                        A day in the lifehellip connecting to the Internet

                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                        router(runs DHCP)

                                        DHCPUDP

                                        IPEthPhy

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCPUDP

                                        IPEthPhy

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCPDHCP

                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                        5 DataLink Layer 5-68

                                        A day in the lifehellip connecting to the Internet

                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                        router(runs DHCP)

                                        DHCPUDP

                                        IPEthPhy

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCPUDP

                                        IPEthPhy

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCP

                                        DHCP

                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                        DHCP client receives DHCP ACK reply

                                        5 DataLink Layer 5-69

                                        A day in the lifehellip ARP (before DNS before HTTP)

                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                        DNSUDP

                                        IPEthPhy

                                        DNS

                                        DNS

                                        DNS

                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                        ARP query

                                        EthPhy

                                        ARP

                                        ARP

                                        ARP reply

                                        5 DataLink Layer 5-70

                                        A day in the lifehellip using DNS

                                        DNSUDP

                                        IPEthPhy

                                        DNS

                                        DNS

                                        DNS

                                        DNS

                                        DNS

                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                        demuxrsquoed to DNS server DNS server replies to

                                        client with IP address of wwwgooglecom

                                        Comcast network 68800013

                                        DNS server

                                        DNSUDP

                                        IPEthPhy

                                        DNS

                                        DNS

                                        DNS

                                        DNS

                                        5 DataLink Layer 5-71

                                        A day in the lifehellip TCP connection carrying HTTP

                                        HTTPTCPIP

                                        EthPhy

                                        HTTP

                                        to send HTTP request client first opens TCP socket to web server

                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                        TCP connection established

                                        64233169105

                                        web server

                                        SYN

                                        SYN

                                        SYN

                                        SYN

                                        TCPIP

                                        EthPhy

                                        SYN

                                        SYN

                                        SYN

                                        SYNACK

                                        SYNACK

                                        SYNACK

                                        SYNACK

                                        SYNACK

                                        SYNACK

                                        SYNACK

                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                        5 DataLink Layer 5-72

                                        A day in the lifehellip HTTP requestreply

                                        HTTPTCPIP

                                        EthPhy

                                        HTTP

                                        HTTP request sent into TCP socket

                                        IP datagram containing HTTP request routed to wwwgooglecom

                                        IP datgram containing HTTP reply routed back to client

                                        64233169105

                                        web server

                                        HTTPTCPIP

                                        EthPhy

                                        web server responds with HTTP reply (containing web page)

                                        HTTP

                                        HTTP

                                        HTTPHTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        HTTP

                                        web page finally () displayed

                                        5 DataLink Layer 5-73

                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                        (except PHY) solid understanding of networking

                                        principles practice hellip could stop here hellip but lots of

                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                        • Link Layer
                                        • Link Layer Introduction
                                        • Link layer context
                                        • Encapsulation
                                        • Link Layer Services
                                        • Link Layer Services (more)
                                        • Where is the link layer implemented
                                        • Adaptors Communicating
                                        • Slide 9
                                        • Parity Checking
                                        • Internet checksum (review)
                                        • Checksumming Cyclic Redundancy Check
                                        • Cyclic Redundancy Check
                                        • Slide 14
                                        • CRC Example
                                        • Slide 16
                                        • Multiple Access Links and Protocols
                                        • Cable Network Architecture Overview
                                        • Multiple Access protocols
                                        • Ideal Multiple Access Protocol
                                        • MAC Protocols a taxonomy
                                        • Channel Partitioning MAC protocols TDMA
                                        • Channel Partitioning MAC protocols FDMA
                                        • Random Access Protocols
                                        • ALOHA
                                        • CSMA (Carrier Sense Multiple Access)
                                        • CSMA collisions
                                        • CSMACD (Collision Detection)
                                        • CSMACD collision detection
                                        • ldquoTaking Turnsrdquo MAC protocols
                                        • Slide 31
                                        • Slide 32
                                        • Summary of MAC protocols
                                        • Slide 34
                                        • MAC Addresses and ARP
                                        • LAN Addresses and ARP
                                        • LAN Address (more)
                                        • ARP Address Resolution Protocol
                                        • ARP protocol Same LAN (network)
                                        • Addressing routing to another LAN
                                        • Slide 41
                                        • Slide 42
                                        • Ethernet
                                        • Star topology
                                        • Ethernet Frame Structure
                                        • Ethernet Frame Structure (more)
                                        • Ethernet Unreliable connectionless
                                        • Ethernet CSMACD algorithm
                                        • Ethernetrsquos CSMACD (more)
                                        • 8023 Ethernet Standards Link amp Physical Layers
                                        • Manchester encoding
                                        • Slide 52
                                        • Hubs
                                        • Switch
                                        • Switch allows multiple simultaneous transmissions
                                        • Switch Table
                                        • Switch self-learning
                                        • Switch frame filteringforwarding
                                        • Self-learning forwarding example
                                        • Interconnecting switches
                                        • Self-learning multi-switch example
                                        • Institutional network
                                        • Switches vs Routers
                                        • Slide 64
                                        • Synthesis a day in the life of a web request
                                        • A day in the life scenario
                                        • A day in the lifehellip connecting to the Internet
                                        • Slide 68
                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                        • A day in the lifehellip using DNS
                                        • A day in the lifehellip TCP connection carrying HTTP
                                        • A day in the lifehellip HTTP requestreply
                                        • Chapter 5 letrsquos take a breath

                                          5 DataLink Layer 5-21

                                          MAC Protocols a taxonomy

                                          Three broad classes Channel Partitioning

                                          o divide channel into smaller ldquopiecesrdquo (time slots frequency code)

                                          o allocate piece to node for exclusive use

                                          Random Accesso channel not divided allow collisionso ldquorecoverrdquo from collisions

                                          ldquoTaking turnsrdquoo nodes take turns but nodes with more to send can

                                          take longer turns

                                          5 DataLink Layer 5-22

                                          Channel Partitioning MAC protocols TDMA

                                          TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                          pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                          256 idle

                                          1 3 4 1 3 4

                                          6-slotframe

                                          5 DataLink Layer 5-23

                                          Channel Partitioning MAC protocols FDMA

                                          FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                          idle example 6-station LAN 134 have pkt

                                          frequency bands 256 idle fr

                                          equ

                                          ency

                                          bands time

                                          FDM cable

                                          5 DataLink Layer 5-24

                                          Random Access Protocols

                                          When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                          two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                          o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                          retransmissions)

                                          Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                          5 DataLink Layer 5-25

                                          ALOHA

                                          When a node has a frame to send send immediately

                                          Set a timer for a random amount of time If an ACK arrives before the timer expires

                                          fine otherwise resend the frame

                                          (Works like stop-and-wait with random timeout interval)

                                          5 DataLink Layer 5-26

                                          CSMA (Carrier Sense Multiple Access)

                                          CSMA listen before transmit

                                          If channel sensed idle transmit entire frame

                                          If channel sensed busy defer transmission

                                          human analogy donrsquot interrupt others

                                          5 DataLink Layer 5-27

                                          CSMA collisions

                                          collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                          spatial layout of nodes

                                          5 DataLink Layer 5-28

                                          CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                          o collisions detected within short timeo colliding transmissions aborted reducing

                                          channel wastage collision detection

                                          o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                          o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                          human analogy the polite conversationalist

                                          5 DataLink Layer 5-29

                                          CSMACD collision detection

                                          5 DataLink Layer 5-30

                                          ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                          o share channel efficiently and fairly at high load

                                          o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                          Random access MAC protocolso efficient at low load single node can fully

                                          utilize channelo high load collision overhead

                                          ldquotaking turnsrdquo protocolslook for best of both worlds

                                          5 DataLink Layer 5-31

                                          ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                          ldquoinvitesrdquo slave nodes to transmit in turn

                                          typically used with ldquodumbrdquo slave devices

                                          concernso polling overhead o latencyo single point of

                                          failure (master)

                                          master

                                          slaves

                                          poll

                                          data

                                          data

                                          5 DataLink Layer 5-32

                                          ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                          passed from one node to next sequentially

                                          token message concerns

                                          o token overhead o latencyo single point of failure

                                          (token)

                                          T

                                          data

                                          (nothingto send)

                                          T

                                          5 DataLink Layer 5-33

                                          Summary of MAC protocols

                                          channel partitioning by time frequency or codeo Time Division Frequency Division

                                          random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                          hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                          taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                          5 DataLink Layer 5-34

                                          Link Layer

                                          51 Introduction and services

                                          52 Error detection and correction

                                          53Multiple access protocols

                                          54 Link-Layer Addressing

                                          55 Ethernet

                                          56 Link-layer switches 59 A day in the life of

                                          a web request

                                          5 DataLink Layer 5-35

                                          MAC Addresses and ARP

                                          32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                          MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                          another physically-connected interface (in same network)

                                          o 48 bit MAC addressbull burned in NIC ROM

                                          5 DataLink Layer 5-36

                                          LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                          Broadcast address =FF-FF-FF-FF-FF-FF

                                          = adapter

                                          1A-2F-BB-76-09-AD

                                          58-23-D7-FA-20-B0

                                          0C-C4-11-6F-E3-98

                                          71-65-F7-2B-08-53

                                          LAN(wired orwireless)

                                          5 DataLink Layer 5-37

                                          LAN Address (more)

                                          MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                          space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                          o can move LAN card from one LAN to another

                                          IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                          attached

                                          5 DataLink Layer 5-38

                                          ARP Address Resolution Protocol

                                          Each IP node (host router) on LAN has ARP table

                                          ARP table IPMAC address mappings for some LAN nodes

                                          lt IP address MAC address TTLgto TTL (Time To Live)

                                          time after which address mapping will be forgotten (typically 20 min)

                                          Q given a nodersquos IP address how to determine its MAC address

                                          1A-2F-BB-76-09-AD

                                          0C-C4-11-6F-E3-98

                                          71-65-F7-2B-08-53

                                          LAN

                                          137196723

                                          137196778

                                          137196714

                                          137196788

                                          5 DataLink Layer 5-39

                                          ARP protocol Same LAN (network) A wants to send datagram

                                          to B and Brsquos MAC address not in Arsquos ARP table

                                          A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                          FF-FF-FF-FF-FFo all machines on LAN

                                          receive ARP query B receives ARP packet

                                          replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                          address (unicast)

                                          A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                          ARP is ldquoplug-and-playrdquoo nodes create their

                                          ARP tables without intervention from net administrator

                                          5 DataLink Layer 5-40

                                          Addressing routing to another LAN

                                          R

                                          1A-23-F9-CD-06-9B

                                          222222222220111111111110

                                          E6-E9-00-17-BB-4B

                                          CC-49-DE-D0-AB-7D

                                          111111111112

                                          111111111111

                                          A74-29-9C-E8-FF-55

                                          222222222221

                                          88-B2-2F-54-1A-0F

                                          B222222222222

                                          49-BD-D2-C7-56-2A

                                          walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                          two ARP tables in router R one for each IP network (LAN)

                                          LAN LAN

                                          5 DataLink Layer 5-41

                                          A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                          frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                          destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                          R

                                          1A-23-F9-CD-06-9B

                                          222222222220

                                          111111111110

                                          E6-E9-00-17-BB-4B

                                          CC-49-DE-D0-AB-7D

                                          111111111112

                                          111111111111

                                          A74-29-9C-E8-FF-55

                                          222222222221

                                          88-B2-2F-54-1A-0F

                                          B222222222222

                                          49-BD-D2-C7-56-2A

                                          This is a really importantexample ndash make sure youunderstand

                                          5 DataLink Layer 5-42

                                          Link Layer

                                          51 Introduction and services

                                          52 Error detection and correction

                                          53Multiple access protocols

                                          54 Link-Layer Addressing

                                          55 Ethernet

                                          56 Link-layer switches 59 A day in the life of

                                          a web request

                                          5 DataLink Layer 5-43

                                          Ethernet

                                          ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                          Metcalfersquos Ethernetsketch

                                          5 DataLink Layer 5-44

                                          Star topology bus topology popular through mid 90s

                                          o all nodes in same collision domain (can collide with each other)

                                          today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                          (nodes do not collide with each other)

                                          switch

                                          bus coaxial cable star

                                          5 DataLink Layer 5-45

                                          Ethernet Frame Structure

                                          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                          Preamble 7 bytes with pattern 10101010 followed by one

                                          byte with pattern 10101011 used to synchronize receiver amp sender clock

                                          rates

                                          5 DataLink Layer 5-46

                                          Ethernet Frame Structure (more) Addresses 6 bytes

                                          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                          o otherwise adapter discards frame

                                          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                          CRC checked at receiver if error is detected frame is dropped

                                          5 DataLink Layer 5-47

                                          Ethernet Unreliable connectionless connectionless No handshaking between sending

                                          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                          Ethernetrsquos MAC protocol CSMACD

                                          5 DataLink Layer 5-48

                                          Ethernet CSMACD algorithm

                                          1 NIC receives datagram from network layer creates frame

                                          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                          4 If NIC detects collision while transmitting aborts and sends jam signal

                                          5 After aborting NIC enters exponential backoff

                                          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                          (1 slot = 512 bit-times)

                                          5 DataLink Layer 5-49

                                          Ethernetrsquos CSMACD (more)

                                          Jam Signal make sure all other transmitters are aware of collision 48 bits

                                          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                          Exponential Backoff Goal adapt retransmission

                                          attempts to estimated current loado heavy load random

                                          wait will be longer first collision choose K

                                          from 01 delay is K 512 bit transmission times

                                          after second collision choose K from 0123hellip

                                          after ten collisions choose K from 01234hellip1023

                                          5 DataLink Layer 5-50

                                          8023 Ethernet Standards Link amp Physical Layers

                                          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                          Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                          applicationtransportnetwork

                                          linkphysical

                                          MAC protocoland frame format

                                          100BASE-TX

                                          100BASE-T4

                                          100BASE-FX100BASE-T2

                                          100BASE-SX 100BASE-BX

                                          fiber physical layercopper (twisterpair) physical layer

                                          5 DataLink Layer 5-51

                                          Manchester encoding

                                          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                          synchronize to each othero no need for a centralized global clock among nodes

                                          Hey this is physical-layer stuff

                                          5 DataLink Layer 5-52

                                          Link Layer

                                          51 Introduction and services

                                          52 Error detection and correction

                                          53 Multiple access protocols

                                          54 Link-layer Addressing

                                          55 Ethernet

                                          56 Link-layer switches LANs

                                          59 A day in the life of a web request

                                          5 DataLink Layer 5-53

                                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                                          o bits coming in one link go out all other links at same rate

                                          o all nodes connected to hub can collide with one another

                                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                          twisted pair

                                          hub

                                          5 DataLink Layer 5-54

                                          Switch link-layer device smarter than hubs take active

                                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                          transparento hosts are unaware of presence of switches

                                          plug-and-play self-learningo switches do not need to be configured

                                          5 DataLink Layer 5-55

                                          Switch allows multiple simultaneous transmissions

                                          hosts have dedicated direct connection to switch

                                          switches buffer packets Ethernet protocol used on

                                          each incoming link but no collisions full duplexo each link is its own collision

                                          domain switching A-to-Arsquo and B-

                                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                                          A

                                          Arsquo

                                          B

                                          Brsquo

                                          C

                                          Crsquo

                                          switch with six interfaces(123456)

                                          1 23

                                          45

                                          6

                                          5 DataLink Layer 5-56

                                          Switch Table

                                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                          A each switch has a switch table each entryo (MAC address of host interface

                                          to reach host time stamp)

                                          looks like a routing table Q how are entries created

                                          maintained in switch table o something like a routing

                                          protocol

                                          A

                                          Arsquo

                                          B

                                          Brsquo

                                          C

                                          Crsquo

                                          switch with six interfaces(123456)

                                          1 23

                                          45

                                          6

                                          5 DataLink Layer 5-57

                                          Switch self-learning

                                          switch learns which hosts can be reached through which interfaceso when frame received

                                          switch ldquolearnsrdquo location of sender incoming LAN segment

                                          o records senderlocation pair in switch table

                                          A

                                          Arsquo

                                          B

                                          Brsquo

                                          C

                                          Crsquo

                                          1 23

                                          45

                                          6

                                          A Arsquo

                                          Source ADest Arsquo

                                          MAC addr interface TTL

                                          Switch table (initially empty)

                                          A 1 60

                                          5 DataLink Layer 5-58

                                          Switch frame filteringforwardingWhen frame received

                                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                          then if dest on segment from which frame arrived

                                          then drop the frame else forward the frame on interface indicated else flood

                                          forward on all but the interface on which the frame arrived

                                          5 DataLink Layer 5-59

                                          Self-learning forwarding example

                                          A

                                          Arsquo

                                          B

                                          Brsquo

                                          C

                                          Crsquo

                                          1 23

                                          45

                                          6

                                          A Arsquo

                                          Source ADest Arsquo

                                          MAC addr interface TTL

                                          Switch table (initially empty)

                                          A 1 60

                                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                          frame destination unknownflood

                                          Arsquo A

                                          destination A location known

                                          Arsquo 4 60

                                          selective send

                                          5 DataLink Layer 5-60

                                          Interconnecting switches

                                          switches can be connected together

                                          A

                                          B

                                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                          A self learning (works exactly the same as in single-switch case)

                                          S1

                                          C D

                                          E

                                          FS2

                                          S4

                                          S3

                                          H

                                          I

                                          G

                                          5 DataLink Layer 5-61

                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                          A

                                          B

                                          S1

                                          C D

                                          E

                                          FS2

                                          S4

                                          S3

                                          H

                                          I

                                          G

                                          1

                                          2 3

                                          34

                                          5 DataLink Layer 5-62

                                          Institutional network

                                          to externalnetwork

                                          router

                                          IP subnet

                                          mail server

                                          web server

                                          5 DataLink Layer 5-63

                                          Switches vs Routers both store-and-forward devices

                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                          routers maintain routing tables implement routing algorithms

                                          switches maintain switch tables implement filtering learning algorithms

                                          Switch

                                          5 DataLink Layer 5-64

                                          Link Layer

                                          51 Introduction and services

                                          52 Error detection and correction

                                          53Multiple access protocols

                                          54 Link-Layer Addressing

                                          55 Ethernet

                                          56 Link-layer switches 59 A day in the life of

                                          a web request

                                          5 DataLink Layer 5-65

                                          Synthesis a day in the life of a web request

                                          journey down protocol stack completeo application transport network link

                                          putting-it-all-together synthesiso goal identify review understand protocols

                                          (at all layers) involved in seemingly simple scenario requesting www page

                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                          5 DataLink Layer 5-66

                                          A day in the life scenario

                                          Comcast network 68800013

                                          Googlersquos network 64233160019 64233169105

                                          web server

                                          DNS server

                                          school network 68802024

                                          browser

                                          web page

                                          5 DataLink Layer 5-67

                                          A day in the lifehellip connecting to the Internet

                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                          router(runs DHCP)

                                          DHCPUDP

                                          IPEthPhy

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCPUDP

                                          IPEthPhy

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCPDHCP

                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                          5 DataLink Layer 5-68

                                          A day in the lifehellip connecting to the Internet

                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                          router(runs DHCP)

                                          DHCPUDP

                                          IPEthPhy

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCPUDP

                                          IPEthPhy

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCP

                                          DHCP

                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                          DHCP client receives DHCP ACK reply

                                          5 DataLink Layer 5-69

                                          A day in the lifehellip ARP (before DNS before HTTP)

                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                          DNSUDP

                                          IPEthPhy

                                          DNS

                                          DNS

                                          DNS

                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                          ARP query

                                          EthPhy

                                          ARP

                                          ARP

                                          ARP reply

                                          5 DataLink Layer 5-70

                                          A day in the lifehellip using DNS

                                          DNSUDP

                                          IPEthPhy

                                          DNS

                                          DNS

                                          DNS

                                          DNS

                                          DNS

                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                          demuxrsquoed to DNS server DNS server replies to

                                          client with IP address of wwwgooglecom

                                          Comcast network 68800013

                                          DNS server

                                          DNSUDP

                                          IPEthPhy

                                          DNS

                                          DNS

                                          DNS

                                          DNS

                                          5 DataLink Layer 5-71

                                          A day in the lifehellip TCP connection carrying HTTP

                                          HTTPTCPIP

                                          EthPhy

                                          HTTP

                                          to send HTTP request client first opens TCP socket to web server

                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                          TCP connection established

                                          64233169105

                                          web server

                                          SYN

                                          SYN

                                          SYN

                                          SYN

                                          TCPIP

                                          EthPhy

                                          SYN

                                          SYN

                                          SYN

                                          SYNACK

                                          SYNACK

                                          SYNACK

                                          SYNACK

                                          SYNACK

                                          SYNACK

                                          SYNACK

                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                          5 DataLink Layer 5-72

                                          A day in the lifehellip HTTP requestreply

                                          HTTPTCPIP

                                          EthPhy

                                          HTTP

                                          HTTP request sent into TCP socket

                                          IP datagram containing HTTP request routed to wwwgooglecom

                                          IP datgram containing HTTP reply routed back to client

                                          64233169105

                                          web server

                                          HTTPTCPIP

                                          EthPhy

                                          web server responds with HTTP reply (containing web page)

                                          HTTP

                                          HTTP

                                          HTTPHTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          HTTP

                                          web page finally () displayed

                                          5 DataLink Layer 5-73

                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                          (except PHY) solid understanding of networking

                                          principles practice hellip could stop here hellip but lots of

                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                          • Link Layer
                                          • Link Layer Introduction
                                          • Link layer context
                                          • Encapsulation
                                          • Link Layer Services
                                          • Link Layer Services (more)
                                          • Where is the link layer implemented
                                          • Adaptors Communicating
                                          • Slide 9
                                          • Parity Checking
                                          • Internet checksum (review)
                                          • Checksumming Cyclic Redundancy Check
                                          • Cyclic Redundancy Check
                                          • Slide 14
                                          • CRC Example
                                          • Slide 16
                                          • Multiple Access Links and Protocols
                                          • Cable Network Architecture Overview
                                          • Multiple Access protocols
                                          • Ideal Multiple Access Protocol
                                          • MAC Protocols a taxonomy
                                          • Channel Partitioning MAC protocols TDMA
                                          • Channel Partitioning MAC protocols FDMA
                                          • Random Access Protocols
                                          • ALOHA
                                          • CSMA (Carrier Sense Multiple Access)
                                          • CSMA collisions
                                          • CSMACD (Collision Detection)
                                          • CSMACD collision detection
                                          • ldquoTaking Turnsrdquo MAC protocols
                                          • Slide 31
                                          • Slide 32
                                          • Summary of MAC protocols
                                          • Slide 34
                                          • MAC Addresses and ARP
                                          • LAN Addresses and ARP
                                          • LAN Address (more)
                                          • ARP Address Resolution Protocol
                                          • ARP protocol Same LAN (network)
                                          • Addressing routing to another LAN
                                          • Slide 41
                                          • Slide 42
                                          • Ethernet
                                          • Star topology
                                          • Ethernet Frame Structure
                                          • Ethernet Frame Structure (more)
                                          • Ethernet Unreliable connectionless
                                          • Ethernet CSMACD algorithm
                                          • Ethernetrsquos CSMACD (more)
                                          • 8023 Ethernet Standards Link amp Physical Layers
                                          • Manchester encoding
                                          • Slide 52
                                          • Hubs
                                          • Switch
                                          • Switch allows multiple simultaneous transmissions
                                          • Switch Table
                                          • Switch self-learning
                                          • Switch frame filteringforwarding
                                          • Self-learning forwarding example
                                          • Interconnecting switches
                                          • Self-learning multi-switch example
                                          • Institutional network
                                          • Switches vs Routers
                                          • Slide 64
                                          • Synthesis a day in the life of a web request
                                          • A day in the life scenario
                                          • A day in the lifehellip connecting to the Internet
                                          • Slide 68
                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                          • A day in the lifehellip using DNS
                                          • A day in the lifehellip TCP connection carrying HTTP
                                          • A day in the lifehellip HTTP requestreply
                                          • Chapter 5 letrsquos take a breath

                                            5 DataLink Layer 5-22

                                            Channel Partitioning MAC protocols TDMA

                                            TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =

                                            pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots

                                            256 idle

                                            1 3 4 1 3 4

                                            6-slotframe

                                            5 DataLink Layer 5-23

                                            Channel Partitioning MAC protocols FDMA

                                            FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                            idle example 6-station LAN 134 have pkt

                                            frequency bands 256 idle fr

                                            equ

                                            ency

                                            bands time

                                            FDM cable

                                            5 DataLink Layer 5-24

                                            Random Access Protocols

                                            When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                            two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                            o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                            retransmissions)

                                            Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                            5 DataLink Layer 5-25

                                            ALOHA

                                            When a node has a frame to send send immediately

                                            Set a timer for a random amount of time If an ACK arrives before the timer expires

                                            fine otherwise resend the frame

                                            (Works like stop-and-wait with random timeout interval)

                                            5 DataLink Layer 5-26

                                            CSMA (Carrier Sense Multiple Access)

                                            CSMA listen before transmit

                                            If channel sensed idle transmit entire frame

                                            If channel sensed busy defer transmission

                                            human analogy donrsquot interrupt others

                                            5 DataLink Layer 5-27

                                            CSMA collisions

                                            collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                            spatial layout of nodes

                                            5 DataLink Layer 5-28

                                            CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                            o collisions detected within short timeo colliding transmissions aborted reducing

                                            channel wastage collision detection

                                            o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                            o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                            human analogy the polite conversationalist

                                            5 DataLink Layer 5-29

                                            CSMACD collision detection

                                            5 DataLink Layer 5-30

                                            ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                            o share channel efficiently and fairly at high load

                                            o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                            Random access MAC protocolso efficient at low load single node can fully

                                            utilize channelo high load collision overhead

                                            ldquotaking turnsrdquo protocolslook for best of both worlds

                                            5 DataLink Layer 5-31

                                            ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                            ldquoinvitesrdquo slave nodes to transmit in turn

                                            typically used with ldquodumbrdquo slave devices

                                            concernso polling overhead o latencyo single point of

                                            failure (master)

                                            master

                                            slaves

                                            poll

                                            data

                                            data

                                            5 DataLink Layer 5-32

                                            ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                            passed from one node to next sequentially

                                            token message concerns

                                            o token overhead o latencyo single point of failure

                                            (token)

                                            T

                                            data

                                            (nothingto send)

                                            T

                                            5 DataLink Layer 5-33

                                            Summary of MAC protocols

                                            channel partitioning by time frequency or codeo Time Division Frequency Division

                                            random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                            hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                            taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                            5 DataLink Layer 5-34

                                            Link Layer

                                            51 Introduction and services

                                            52 Error detection and correction

                                            53Multiple access protocols

                                            54 Link-Layer Addressing

                                            55 Ethernet

                                            56 Link-layer switches 59 A day in the life of

                                            a web request

                                            5 DataLink Layer 5-35

                                            MAC Addresses and ARP

                                            32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                            MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                            another physically-connected interface (in same network)

                                            o 48 bit MAC addressbull burned in NIC ROM

                                            5 DataLink Layer 5-36

                                            LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                            Broadcast address =FF-FF-FF-FF-FF-FF

                                            = adapter

                                            1A-2F-BB-76-09-AD

                                            58-23-D7-FA-20-B0

                                            0C-C4-11-6F-E3-98

                                            71-65-F7-2B-08-53

                                            LAN(wired orwireless)

                                            5 DataLink Layer 5-37

                                            LAN Address (more)

                                            MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                            space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                            o can move LAN card from one LAN to another

                                            IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                            attached

                                            5 DataLink Layer 5-38

                                            ARP Address Resolution Protocol

                                            Each IP node (host router) on LAN has ARP table

                                            ARP table IPMAC address mappings for some LAN nodes

                                            lt IP address MAC address TTLgto TTL (Time To Live)

                                            time after which address mapping will be forgotten (typically 20 min)

                                            Q given a nodersquos IP address how to determine its MAC address

                                            1A-2F-BB-76-09-AD

                                            0C-C4-11-6F-E3-98

                                            71-65-F7-2B-08-53

                                            LAN

                                            137196723

                                            137196778

                                            137196714

                                            137196788

                                            5 DataLink Layer 5-39

                                            ARP protocol Same LAN (network) A wants to send datagram

                                            to B and Brsquos MAC address not in Arsquos ARP table

                                            A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                            FF-FF-FF-FF-FFo all machines on LAN

                                            receive ARP query B receives ARP packet

                                            replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                            address (unicast)

                                            A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                            ARP is ldquoplug-and-playrdquoo nodes create their

                                            ARP tables without intervention from net administrator

                                            5 DataLink Layer 5-40

                                            Addressing routing to another LAN

                                            R

                                            1A-23-F9-CD-06-9B

                                            222222222220111111111110

                                            E6-E9-00-17-BB-4B

                                            CC-49-DE-D0-AB-7D

                                            111111111112

                                            111111111111

                                            A74-29-9C-E8-FF-55

                                            222222222221

                                            88-B2-2F-54-1A-0F

                                            B222222222222

                                            49-BD-D2-C7-56-2A

                                            walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                            two ARP tables in router R one for each IP network (LAN)

                                            LAN LAN

                                            5 DataLink Layer 5-41

                                            A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                            frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                            destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                            R

                                            1A-23-F9-CD-06-9B

                                            222222222220

                                            111111111110

                                            E6-E9-00-17-BB-4B

                                            CC-49-DE-D0-AB-7D

                                            111111111112

                                            111111111111

                                            A74-29-9C-E8-FF-55

                                            222222222221

                                            88-B2-2F-54-1A-0F

                                            B222222222222

                                            49-BD-D2-C7-56-2A

                                            This is a really importantexample ndash make sure youunderstand

                                            5 DataLink Layer 5-42

                                            Link Layer

                                            51 Introduction and services

                                            52 Error detection and correction

                                            53Multiple access protocols

                                            54 Link-Layer Addressing

                                            55 Ethernet

                                            56 Link-layer switches 59 A day in the life of

                                            a web request

                                            5 DataLink Layer 5-43

                                            Ethernet

                                            ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                            Metcalfersquos Ethernetsketch

                                            5 DataLink Layer 5-44

                                            Star topology bus topology popular through mid 90s

                                            o all nodes in same collision domain (can collide with each other)

                                            today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                            (nodes do not collide with each other)

                                            switch

                                            bus coaxial cable star

                                            5 DataLink Layer 5-45

                                            Ethernet Frame Structure

                                            Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                            Preamble 7 bytes with pattern 10101010 followed by one

                                            byte with pattern 10101011 used to synchronize receiver amp sender clock

                                            rates

                                            5 DataLink Layer 5-46

                                            Ethernet Frame Structure (more) Addresses 6 bytes

                                            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                            o otherwise adapter discards frame

                                            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                            CRC checked at receiver if error is detected frame is dropped

                                            5 DataLink Layer 5-47

                                            Ethernet Unreliable connectionless connectionless No handshaking between sending

                                            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                            Ethernetrsquos MAC protocol CSMACD

                                            5 DataLink Layer 5-48

                                            Ethernet CSMACD algorithm

                                            1 NIC receives datagram from network layer creates frame

                                            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                            4 If NIC detects collision while transmitting aborts and sends jam signal

                                            5 After aborting NIC enters exponential backoff

                                            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                            (1 slot = 512 bit-times)

                                            5 DataLink Layer 5-49

                                            Ethernetrsquos CSMACD (more)

                                            Jam Signal make sure all other transmitters are aware of collision 48 bits

                                            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                            Exponential Backoff Goal adapt retransmission

                                            attempts to estimated current loado heavy load random

                                            wait will be longer first collision choose K

                                            from 01 delay is K 512 bit transmission times

                                            after second collision choose K from 0123hellip

                                            after ten collisions choose K from 01234hellip1023

                                            5 DataLink Layer 5-50

                                            8023 Ethernet Standards Link amp Physical Layers

                                            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                            Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                            applicationtransportnetwork

                                            linkphysical

                                            MAC protocoland frame format

                                            100BASE-TX

                                            100BASE-T4

                                            100BASE-FX100BASE-T2

                                            100BASE-SX 100BASE-BX

                                            fiber physical layercopper (twisterpair) physical layer

                                            5 DataLink Layer 5-51

                                            Manchester encoding

                                            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                            synchronize to each othero no need for a centralized global clock among nodes

                                            Hey this is physical-layer stuff

                                            5 DataLink Layer 5-52

                                            Link Layer

                                            51 Introduction and services

                                            52 Error detection and correction

                                            53 Multiple access protocols

                                            54 Link-layer Addressing

                                            55 Ethernet

                                            56 Link-layer switches LANs

                                            59 A day in the life of a web request

                                            5 DataLink Layer 5-53

                                            Hubsphysical-layer (ldquodumbrdquo) repeaters

                                            o bits coming in one link go out all other links at same rate

                                            o all nodes connected to hub can collide with one another

                                            o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                            twisted pair

                                            hub

                                            5 DataLink Layer 5-54

                                            Switch link-layer device smarter than hubs take active

                                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                            transparento hosts are unaware of presence of switches

                                            plug-and-play self-learningo switches do not need to be configured

                                            5 DataLink Layer 5-55

                                            Switch allows multiple simultaneous transmissions

                                            hosts have dedicated direct connection to switch

                                            switches buffer packets Ethernet protocol used on

                                            each incoming link but no collisions full duplexo each link is its own collision

                                            domain switching A-to-Arsquo and B-

                                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                                            A

                                            Arsquo

                                            B

                                            Brsquo

                                            C

                                            Crsquo

                                            switch with six interfaces(123456)

                                            1 23

                                            45

                                            6

                                            5 DataLink Layer 5-56

                                            Switch Table

                                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                            A each switch has a switch table each entryo (MAC address of host interface

                                            to reach host time stamp)

                                            looks like a routing table Q how are entries created

                                            maintained in switch table o something like a routing

                                            protocol

                                            A

                                            Arsquo

                                            B

                                            Brsquo

                                            C

                                            Crsquo

                                            switch with six interfaces(123456)

                                            1 23

                                            45

                                            6

                                            5 DataLink Layer 5-57

                                            Switch self-learning

                                            switch learns which hosts can be reached through which interfaceso when frame received

                                            switch ldquolearnsrdquo location of sender incoming LAN segment

                                            o records senderlocation pair in switch table

                                            A

                                            Arsquo

                                            B

                                            Brsquo

                                            C

                                            Crsquo

                                            1 23

                                            45

                                            6

                                            A Arsquo

                                            Source ADest Arsquo

                                            MAC addr interface TTL

                                            Switch table (initially empty)

                                            A 1 60

                                            5 DataLink Layer 5-58

                                            Switch frame filteringforwardingWhen frame received

                                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                            then if dest on segment from which frame arrived

                                            then drop the frame else forward the frame on interface indicated else flood

                                            forward on all but the interface on which the frame arrived

                                            5 DataLink Layer 5-59

                                            Self-learning forwarding example

                                            A

                                            Arsquo

                                            B

                                            Brsquo

                                            C

                                            Crsquo

                                            1 23

                                            45

                                            6

                                            A Arsquo

                                            Source ADest Arsquo

                                            MAC addr interface TTL

                                            Switch table (initially empty)

                                            A 1 60

                                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                            frame destination unknownflood

                                            Arsquo A

                                            destination A location known

                                            Arsquo 4 60

                                            selective send

                                            5 DataLink Layer 5-60

                                            Interconnecting switches

                                            switches can be connected together

                                            A

                                            B

                                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                            A self learning (works exactly the same as in single-switch case)

                                            S1

                                            C D

                                            E

                                            FS2

                                            S4

                                            S3

                                            H

                                            I

                                            G

                                            5 DataLink Layer 5-61

                                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                                            A

                                            B

                                            S1

                                            C D

                                            E

                                            FS2

                                            S4

                                            S3

                                            H

                                            I

                                            G

                                            1

                                            2 3

                                            34

                                            5 DataLink Layer 5-62

                                            Institutional network

                                            to externalnetwork

                                            router

                                            IP subnet

                                            mail server

                                            web server

                                            5 DataLink Layer 5-63

                                            Switches vs Routers both store-and-forward devices

                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                            routers maintain routing tables implement routing algorithms

                                            switches maintain switch tables implement filtering learning algorithms

                                            Switch

                                            5 DataLink Layer 5-64

                                            Link Layer

                                            51 Introduction and services

                                            52 Error detection and correction

                                            53Multiple access protocols

                                            54 Link-Layer Addressing

                                            55 Ethernet

                                            56 Link-layer switches 59 A day in the life of

                                            a web request

                                            5 DataLink Layer 5-65

                                            Synthesis a day in the life of a web request

                                            journey down protocol stack completeo application transport network link

                                            putting-it-all-together synthesiso goal identify review understand protocols

                                            (at all layers) involved in seemingly simple scenario requesting www page

                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                            5 DataLink Layer 5-66

                                            A day in the life scenario

                                            Comcast network 68800013

                                            Googlersquos network 64233160019 64233169105

                                            web server

                                            DNS server

                                            school network 68802024

                                            browser

                                            web page

                                            5 DataLink Layer 5-67

                                            A day in the lifehellip connecting to the Internet

                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                            router(runs DHCP)

                                            DHCPUDP

                                            IPEthPhy

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCPUDP

                                            IPEthPhy

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCPDHCP

                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                            5 DataLink Layer 5-68

                                            A day in the lifehellip connecting to the Internet

                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                            router(runs DHCP)

                                            DHCPUDP

                                            IPEthPhy

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCPUDP

                                            IPEthPhy

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCP

                                            DHCP

                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                            DHCP client receives DHCP ACK reply

                                            5 DataLink Layer 5-69

                                            A day in the lifehellip ARP (before DNS before HTTP)

                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                            DNSUDP

                                            IPEthPhy

                                            DNS

                                            DNS

                                            DNS

                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                            ARP query

                                            EthPhy

                                            ARP

                                            ARP

                                            ARP reply

                                            5 DataLink Layer 5-70

                                            A day in the lifehellip using DNS

                                            DNSUDP

                                            IPEthPhy

                                            DNS

                                            DNS

                                            DNS

                                            DNS

                                            DNS

                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                            demuxrsquoed to DNS server DNS server replies to

                                            client with IP address of wwwgooglecom

                                            Comcast network 68800013

                                            DNS server

                                            DNSUDP

                                            IPEthPhy

                                            DNS

                                            DNS

                                            DNS

                                            DNS

                                            5 DataLink Layer 5-71

                                            A day in the lifehellip TCP connection carrying HTTP

                                            HTTPTCPIP

                                            EthPhy

                                            HTTP

                                            to send HTTP request client first opens TCP socket to web server

                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                            TCP connection established

                                            64233169105

                                            web server

                                            SYN

                                            SYN

                                            SYN

                                            SYN

                                            TCPIP

                                            EthPhy

                                            SYN

                                            SYN

                                            SYN

                                            SYNACK

                                            SYNACK

                                            SYNACK

                                            SYNACK

                                            SYNACK

                                            SYNACK

                                            SYNACK

                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                            5 DataLink Layer 5-72

                                            A day in the lifehellip HTTP requestreply

                                            HTTPTCPIP

                                            EthPhy

                                            HTTP

                                            HTTP request sent into TCP socket

                                            IP datagram containing HTTP request routed to wwwgooglecom

                                            IP datgram containing HTTP reply routed back to client

                                            64233169105

                                            web server

                                            HTTPTCPIP

                                            EthPhy

                                            web server responds with HTTP reply (containing web page)

                                            HTTP

                                            HTTP

                                            HTTPHTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            HTTP

                                            web page finally () displayed

                                            5 DataLink Layer 5-73

                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                            (except PHY) solid understanding of networking

                                            principles practice hellip could stop here hellip but lots of

                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                            • Link Layer
                                            • Link Layer Introduction
                                            • Link layer context
                                            • Encapsulation
                                            • Link Layer Services
                                            • Link Layer Services (more)
                                            • Where is the link layer implemented
                                            • Adaptors Communicating
                                            • Slide 9
                                            • Parity Checking
                                            • Internet checksum (review)
                                            • Checksumming Cyclic Redundancy Check
                                            • Cyclic Redundancy Check
                                            • Slide 14
                                            • CRC Example
                                            • Slide 16
                                            • Multiple Access Links and Protocols
                                            • Cable Network Architecture Overview
                                            • Multiple Access protocols
                                            • Ideal Multiple Access Protocol
                                            • MAC Protocols a taxonomy
                                            • Channel Partitioning MAC protocols TDMA
                                            • Channel Partitioning MAC protocols FDMA
                                            • Random Access Protocols
                                            • ALOHA
                                            • CSMA (Carrier Sense Multiple Access)
                                            • CSMA collisions
                                            • CSMACD (Collision Detection)
                                            • CSMACD collision detection
                                            • ldquoTaking Turnsrdquo MAC protocols
                                            • Slide 31
                                            • Slide 32
                                            • Summary of MAC protocols
                                            • Slide 34
                                            • MAC Addresses and ARP
                                            • LAN Addresses and ARP
                                            • LAN Address (more)
                                            • ARP Address Resolution Protocol
                                            • ARP protocol Same LAN (network)
                                            • Addressing routing to another LAN
                                            • Slide 41
                                            • Slide 42
                                            • Ethernet
                                            • Star topology
                                            • Ethernet Frame Structure
                                            • Ethernet Frame Structure (more)
                                            • Ethernet Unreliable connectionless
                                            • Ethernet CSMACD algorithm
                                            • Ethernetrsquos CSMACD (more)
                                            • 8023 Ethernet Standards Link amp Physical Layers
                                            • Manchester encoding
                                            • Slide 52
                                            • Hubs
                                            • Switch
                                            • Switch allows multiple simultaneous transmissions
                                            • Switch Table
                                            • Switch self-learning
                                            • Switch frame filteringforwarding
                                            • Self-learning forwarding example
                                            • Interconnecting switches
                                            • Self-learning multi-switch example
                                            • Institutional network
                                            • Switches vs Routers
                                            • Slide 64
                                            • Synthesis a day in the life of a web request
                                            • A day in the life scenario
                                            • A day in the lifehellip connecting to the Internet
                                            • Slide 68
                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                            • A day in the lifehellip using DNS
                                            • A day in the lifehellip TCP connection carrying HTTP
                                            • A day in the lifehellip HTTP requestreply
                                            • Chapter 5 letrsquos take a breath

                                              5 DataLink Layer 5-23

                                              Channel Partitioning MAC protocols FDMA

                                              FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go

                                              idle example 6-station LAN 134 have pkt

                                              frequency bands 256 idle fr

                                              equ

                                              ency

                                              bands time

                                              FDM cable

                                              5 DataLink Layer 5-24

                                              Random Access Protocols

                                              When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                              two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                              o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                              retransmissions)

                                              Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                              5 DataLink Layer 5-25

                                              ALOHA

                                              When a node has a frame to send send immediately

                                              Set a timer for a random amount of time If an ACK arrives before the timer expires

                                              fine otherwise resend the frame

                                              (Works like stop-and-wait with random timeout interval)

                                              5 DataLink Layer 5-26

                                              CSMA (Carrier Sense Multiple Access)

                                              CSMA listen before transmit

                                              If channel sensed idle transmit entire frame

                                              If channel sensed busy defer transmission

                                              human analogy donrsquot interrupt others

                                              5 DataLink Layer 5-27

                                              CSMA collisions

                                              collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                              spatial layout of nodes

                                              5 DataLink Layer 5-28

                                              CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                              o collisions detected within short timeo colliding transmissions aborted reducing

                                              channel wastage collision detection

                                              o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                              o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                              human analogy the polite conversationalist

                                              5 DataLink Layer 5-29

                                              CSMACD collision detection

                                              5 DataLink Layer 5-30

                                              ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                              o share channel efficiently and fairly at high load

                                              o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                              Random access MAC protocolso efficient at low load single node can fully

                                              utilize channelo high load collision overhead

                                              ldquotaking turnsrdquo protocolslook for best of both worlds

                                              5 DataLink Layer 5-31

                                              ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                              ldquoinvitesrdquo slave nodes to transmit in turn

                                              typically used with ldquodumbrdquo slave devices

                                              concernso polling overhead o latencyo single point of

                                              failure (master)

                                              master

                                              slaves

                                              poll

                                              data

                                              data

                                              5 DataLink Layer 5-32

                                              ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                              passed from one node to next sequentially

                                              token message concerns

                                              o token overhead o latencyo single point of failure

                                              (token)

                                              T

                                              data

                                              (nothingto send)

                                              T

                                              5 DataLink Layer 5-33

                                              Summary of MAC protocols

                                              channel partitioning by time frequency or codeo Time Division Frequency Division

                                              random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                              hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                              taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                              5 DataLink Layer 5-34

                                              Link Layer

                                              51 Introduction and services

                                              52 Error detection and correction

                                              53Multiple access protocols

                                              54 Link-Layer Addressing

                                              55 Ethernet

                                              56 Link-layer switches 59 A day in the life of

                                              a web request

                                              5 DataLink Layer 5-35

                                              MAC Addresses and ARP

                                              32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                              MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                              another physically-connected interface (in same network)

                                              o 48 bit MAC addressbull burned in NIC ROM

                                              5 DataLink Layer 5-36

                                              LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                              Broadcast address =FF-FF-FF-FF-FF-FF

                                              = adapter

                                              1A-2F-BB-76-09-AD

                                              58-23-D7-FA-20-B0

                                              0C-C4-11-6F-E3-98

                                              71-65-F7-2B-08-53

                                              LAN(wired orwireless)

                                              5 DataLink Layer 5-37

                                              LAN Address (more)

                                              MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                              space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                              o can move LAN card from one LAN to another

                                              IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                              attached

                                              5 DataLink Layer 5-38

                                              ARP Address Resolution Protocol

                                              Each IP node (host router) on LAN has ARP table

                                              ARP table IPMAC address mappings for some LAN nodes

                                              lt IP address MAC address TTLgto TTL (Time To Live)

                                              time after which address mapping will be forgotten (typically 20 min)

                                              Q given a nodersquos IP address how to determine its MAC address

                                              1A-2F-BB-76-09-AD

                                              0C-C4-11-6F-E3-98

                                              71-65-F7-2B-08-53

                                              LAN

                                              137196723

                                              137196778

                                              137196714

                                              137196788

                                              5 DataLink Layer 5-39

                                              ARP protocol Same LAN (network) A wants to send datagram

                                              to B and Brsquos MAC address not in Arsquos ARP table

                                              A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                              FF-FF-FF-FF-FFo all machines on LAN

                                              receive ARP query B receives ARP packet

                                              replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                              address (unicast)

                                              A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                              ARP is ldquoplug-and-playrdquoo nodes create their

                                              ARP tables without intervention from net administrator

                                              5 DataLink Layer 5-40

                                              Addressing routing to another LAN

                                              R

                                              1A-23-F9-CD-06-9B

                                              222222222220111111111110

                                              E6-E9-00-17-BB-4B

                                              CC-49-DE-D0-AB-7D

                                              111111111112

                                              111111111111

                                              A74-29-9C-E8-FF-55

                                              222222222221

                                              88-B2-2F-54-1A-0F

                                              B222222222222

                                              49-BD-D2-C7-56-2A

                                              walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                              two ARP tables in router R one for each IP network (LAN)

                                              LAN LAN

                                              5 DataLink Layer 5-41

                                              A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                              frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                              destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                              R

                                              1A-23-F9-CD-06-9B

                                              222222222220

                                              111111111110

                                              E6-E9-00-17-BB-4B

                                              CC-49-DE-D0-AB-7D

                                              111111111112

                                              111111111111

                                              A74-29-9C-E8-FF-55

                                              222222222221

                                              88-B2-2F-54-1A-0F

                                              B222222222222

                                              49-BD-D2-C7-56-2A

                                              This is a really importantexample ndash make sure youunderstand

                                              5 DataLink Layer 5-42

                                              Link Layer

                                              51 Introduction and services

                                              52 Error detection and correction

                                              53Multiple access protocols

                                              54 Link-Layer Addressing

                                              55 Ethernet

                                              56 Link-layer switches 59 A day in the life of

                                              a web request

                                              5 DataLink Layer 5-43

                                              Ethernet

                                              ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                              Metcalfersquos Ethernetsketch

                                              5 DataLink Layer 5-44

                                              Star topology bus topology popular through mid 90s

                                              o all nodes in same collision domain (can collide with each other)

                                              today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                              (nodes do not collide with each other)

                                              switch

                                              bus coaxial cable star

                                              5 DataLink Layer 5-45

                                              Ethernet Frame Structure

                                              Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                              Preamble 7 bytes with pattern 10101010 followed by one

                                              byte with pattern 10101011 used to synchronize receiver amp sender clock

                                              rates

                                              5 DataLink Layer 5-46

                                              Ethernet Frame Structure (more) Addresses 6 bytes

                                              o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                              o otherwise adapter discards frame

                                              Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                              CRC checked at receiver if error is detected frame is dropped

                                              5 DataLink Layer 5-47

                                              Ethernet Unreliable connectionless connectionless No handshaking between sending

                                              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                              Ethernetrsquos MAC protocol CSMACD

                                              5 DataLink Layer 5-48

                                              Ethernet CSMACD algorithm

                                              1 NIC receives datagram from network layer creates frame

                                              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                              4 If NIC detects collision while transmitting aborts and sends jam signal

                                              5 After aborting NIC enters exponential backoff

                                              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                              (1 slot = 512 bit-times)

                                              5 DataLink Layer 5-49

                                              Ethernetrsquos CSMACD (more)

                                              Jam Signal make sure all other transmitters are aware of collision 48 bits

                                              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                              Exponential Backoff Goal adapt retransmission

                                              attempts to estimated current loado heavy load random

                                              wait will be longer first collision choose K

                                              from 01 delay is K 512 bit transmission times

                                              after second collision choose K from 0123hellip

                                              after ten collisions choose K from 01234hellip1023

                                              5 DataLink Layer 5-50

                                              8023 Ethernet Standards Link amp Physical Layers

                                              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                              Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                              applicationtransportnetwork

                                              linkphysical

                                              MAC protocoland frame format

                                              100BASE-TX

                                              100BASE-T4

                                              100BASE-FX100BASE-T2

                                              100BASE-SX 100BASE-BX

                                              fiber physical layercopper (twisterpair) physical layer

                                              5 DataLink Layer 5-51

                                              Manchester encoding

                                              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                              synchronize to each othero no need for a centralized global clock among nodes

                                              Hey this is physical-layer stuff

                                              5 DataLink Layer 5-52

                                              Link Layer

                                              51 Introduction and services

                                              52 Error detection and correction

                                              53 Multiple access protocols

                                              54 Link-layer Addressing

                                              55 Ethernet

                                              56 Link-layer switches LANs

                                              59 A day in the life of a web request

                                              5 DataLink Layer 5-53

                                              Hubsphysical-layer (ldquodumbrdquo) repeaters

                                              o bits coming in one link go out all other links at same rate

                                              o all nodes connected to hub can collide with one another

                                              o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                              twisted pair

                                              hub

                                              5 DataLink Layer 5-54

                                              Switch link-layer device smarter than hubs take active

                                              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                              transparento hosts are unaware of presence of switches

                                              plug-and-play self-learningo switches do not need to be configured

                                              5 DataLink Layer 5-55

                                              Switch allows multiple simultaneous transmissions

                                              hosts have dedicated direct connection to switch

                                              switches buffer packets Ethernet protocol used on

                                              each incoming link but no collisions full duplexo each link is its own collision

                                              domain switching A-to-Arsquo and B-

                                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                                              A

                                              Arsquo

                                              B

                                              Brsquo

                                              C

                                              Crsquo

                                              switch with six interfaces(123456)

                                              1 23

                                              45

                                              6

                                              5 DataLink Layer 5-56

                                              Switch Table

                                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                              A each switch has a switch table each entryo (MAC address of host interface

                                              to reach host time stamp)

                                              looks like a routing table Q how are entries created

                                              maintained in switch table o something like a routing

                                              protocol

                                              A

                                              Arsquo

                                              B

                                              Brsquo

                                              C

                                              Crsquo

                                              switch with six interfaces(123456)

                                              1 23

                                              45

                                              6

                                              5 DataLink Layer 5-57

                                              Switch self-learning

                                              switch learns which hosts can be reached through which interfaceso when frame received

                                              switch ldquolearnsrdquo location of sender incoming LAN segment

                                              o records senderlocation pair in switch table

                                              A

                                              Arsquo

                                              B

                                              Brsquo

                                              C

                                              Crsquo

                                              1 23

                                              45

                                              6

                                              A Arsquo

                                              Source ADest Arsquo

                                              MAC addr interface TTL

                                              Switch table (initially empty)

                                              A 1 60

                                              5 DataLink Layer 5-58

                                              Switch frame filteringforwardingWhen frame received

                                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                              then if dest on segment from which frame arrived

                                              then drop the frame else forward the frame on interface indicated else flood

                                              forward on all but the interface on which the frame arrived

                                              5 DataLink Layer 5-59

                                              Self-learning forwarding example

                                              A

                                              Arsquo

                                              B

                                              Brsquo

                                              C

                                              Crsquo

                                              1 23

                                              45

                                              6

                                              A Arsquo

                                              Source ADest Arsquo

                                              MAC addr interface TTL

                                              Switch table (initially empty)

                                              A 1 60

                                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                              frame destination unknownflood

                                              Arsquo A

                                              destination A location known

                                              Arsquo 4 60

                                              selective send

                                              5 DataLink Layer 5-60

                                              Interconnecting switches

                                              switches can be connected together

                                              A

                                              B

                                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                              A self learning (works exactly the same as in single-switch case)

                                              S1

                                              C D

                                              E

                                              FS2

                                              S4

                                              S3

                                              H

                                              I

                                              G

                                              5 DataLink Layer 5-61

                                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                                              A

                                              B

                                              S1

                                              C D

                                              E

                                              FS2

                                              S4

                                              S3

                                              H

                                              I

                                              G

                                              1

                                              2 3

                                              34

                                              5 DataLink Layer 5-62

                                              Institutional network

                                              to externalnetwork

                                              router

                                              IP subnet

                                              mail server

                                              web server

                                              5 DataLink Layer 5-63

                                              Switches vs Routers both store-and-forward devices

                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                              routers maintain routing tables implement routing algorithms

                                              switches maintain switch tables implement filtering learning algorithms

                                              Switch

                                              5 DataLink Layer 5-64

                                              Link Layer

                                              51 Introduction and services

                                              52 Error detection and correction

                                              53Multiple access protocols

                                              54 Link-Layer Addressing

                                              55 Ethernet

                                              56 Link-layer switches 59 A day in the life of

                                              a web request

                                              5 DataLink Layer 5-65

                                              Synthesis a day in the life of a web request

                                              journey down protocol stack completeo application transport network link

                                              putting-it-all-together synthesiso goal identify review understand protocols

                                              (at all layers) involved in seemingly simple scenario requesting www page

                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                              5 DataLink Layer 5-66

                                              A day in the life scenario

                                              Comcast network 68800013

                                              Googlersquos network 64233160019 64233169105

                                              web server

                                              DNS server

                                              school network 68802024

                                              browser

                                              web page

                                              5 DataLink Layer 5-67

                                              A day in the lifehellip connecting to the Internet

                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                              router(runs DHCP)

                                              DHCPUDP

                                              IPEthPhy

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCPUDP

                                              IPEthPhy

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCPDHCP

                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                              5 DataLink Layer 5-68

                                              A day in the lifehellip connecting to the Internet

                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                              router(runs DHCP)

                                              DHCPUDP

                                              IPEthPhy

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCPUDP

                                              IPEthPhy

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCP

                                              DHCP

                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                              DHCP client receives DHCP ACK reply

                                              5 DataLink Layer 5-69

                                              A day in the lifehellip ARP (before DNS before HTTP)

                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                              DNSUDP

                                              IPEthPhy

                                              DNS

                                              DNS

                                              DNS

                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                              ARP query

                                              EthPhy

                                              ARP

                                              ARP

                                              ARP reply

                                              5 DataLink Layer 5-70

                                              A day in the lifehellip using DNS

                                              DNSUDP

                                              IPEthPhy

                                              DNS

                                              DNS

                                              DNS

                                              DNS

                                              DNS

                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                              demuxrsquoed to DNS server DNS server replies to

                                              client with IP address of wwwgooglecom

                                              Comcast network 68800013

                                              DNS server

                                              DNSUDP

                                              IPEthPhy

                                              DNS

                                              DNS

                                              DNS

                                              DNS

                                              5 DataLink Layer 5-71

                                              A day in the lifehellip TCP connection carrying HTTP

                                              HTTPTCPIP

                                              EthPhy

                                              HTTP

                                              to send HTTP request client first opens TCP socket to web server

                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                              TCP connection established

                                              64233169105

                                              web server

                                              SYN

                                              SYN

                                              SYN

                                              SYN

                                              TCPIP

                                              EthPhy

                                              SYN

                                              SYN

                                              SYN

                                              SYNACK

                                              SYNACK

                                              SYNACK

                                              SYNACK

                                              SYNACK

                                              SYNACK

                                              SYNACK

                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                              5 DataLink Layer 5-72

                                              A day in the lifehellip HTTP requestreply

                                              HTTPTCPIP

                                              EthPhy

                                              HTTP

                                              HTTP request sent into TCP socket

                                              IP datagram containing HTTP request routed to wwwgooglecom

                                              IP datgram containing HTTP reply routed back to client

                                              64233169105

                                              web server

                                              HTTPTCPIP

                                              EthPhy

                                              web server responds with HTTP reply (containing web page)

                                              HTTP

                                              HTTP

                                              HTTPHTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              HTTP

                                              web page finally () displayed

                                              5 DataLink Layer 5-73

                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                              (except PHY) solid understanding of networking

                                              principles practice hellip could stop here hellip but lots of

                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                              • Link Layer
                                              • Link Layer Introduction
                                              • Link layer context
                                              • Encapsulation
                                              • Link Layer Services
                                              • Link Layer Services (more)
                                              • Where is the link layer implemented
                                              • Adaptors Communicating
                                              • Slide 9
                                              • Parity Checking
                                              • Internet checksum (review)
                                              • Checksumming Cyclic Redundancy Check
                                              • Cyclic Redundancy Check
                                              • Slide 14
                                              • CRC Example
                                              • Slide 16
                                              • Multiple Access Links and Protocols
                                              • Cable Network Architecture Overview
                                              • Multiple Access protocols
                                              • Ideal Multiple Access Protocol
                                              • MAC Protocols a taxonomy
                                              • Channel Partitioning MAC protocols TDMA
                                              • Channel Partitioning MAC protocols FDMA
                                              • Random Access Protocols
                                              • ALOHA
                                              • CSMA (Carrier Sense Multiple Access)
                                              • CSMA collisions
                                              • CSMACD (Collision Detection)
                                              • CSMACD collision detection
                                              • ldquoTaking Turnsrdquo MAC protocols
                                              • Slide 31
                                              • Slide 32
                                              • Summary of MAC protocols
                                              • Slide 34
                                              • MAC Addresses and ARP
                                              • LAN Addresses and ARP
                                              • LAN Address (more)
                                              • ARP Address Resolution Protocol
                                              • ARP protocol Same LAN (network)
                                              • Addressing routing to another LAN
                                              • Slide 41
                                              • Slide 42
                                              • Ethernet
                                              • Star topology
                                              • Ethernet Frame Structure
                                              • Ethernet Frame Structure (more)
                                              • Ethernet Unreliable connectionless
                                              • Ethernet CSMACD algorithm
                                              • Ethernetrsquos CSMACD (more)
                                              • 8023 Ethernet Standards Link amp Physical Layers
                                              • Manchester encoding
                                              • Slide 52
                                              • Hubs
                                              • Switch
                                              • Switch allows multiple simultaneous transmissions
                                              • Switch Table
                                              • Switch self-learning
                                              • Switch frame filteringforwarding
                                              • Self-learning forwarding example
                                              • Interconnecting switches
                                              • Self-learning multi-switch example
                                              • Institutional network
                                              • Switches vs Routers
                                              • Slide 64
                                              • Synthesis a day in the life of a web request
                                              • A day in the life scenario
                                              • A day in the lifehellip connecting to the Internet
                                              • Slide 68
                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                              • A day in the lifehellip using DNS
                                              • A day in the lifehellip TCP connection carrying HTTP
                                              • A day in the lifehellip HTTP requestreply
                                              • Chapter 5 letrsquos take a breath

                                                5 DataLink Layer 5-24

                                                Random Access Protocols

                                                When node has packet to sendo transmit at full channel data rate Ro no a priori coordination among nodes

                                                two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

                                                o when a node can send a frameo how to detect collisionso how to recover from collisions (eg via delayed

                                                retransmissions)

                                                Examples of random access MAC protocolso ALOHAo CSMA CSMACD CSMACA

                                                5 DataLink Layer 5-25

                                                ALOHA

                                                When a node has a frame to send send immediately

                                                Set a timer for a random amount of time If an ACK arrives before the timer expires

                                                fine otherwise resend the frame

                                                (Works like stop-and-wait with random timeout interval)

                                                5 DataLink Layer 5-26

                                                CSMA (Carrier Sense Multiple Access)

                                                CSMA listen before transmit

                                                If channel sensed idle transmit entire frame

                                                If channel sensed busy defer transmission

                                                human analogy donrsquot interrupt others

                                                5 DataLink Layer 5-27

                                                CSMA collisions

                                                collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                                spatial layout of nodes

                                                5 DataLink Layer 5-28

                                                CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                                o collisions detected within short timeo colliding transmissions aborted reducing

                                                channel wastage collision detection

                                                o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                                o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                                human analogy the polite conversationalist

                                                5 DataLink Layer 5-29

                                                CSMACD collision detection

                                                5 DataLink Layer 5-30

                                                ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                o share channel efficiently and fairly at high load

                                                o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                Random access MAC protocolso efficient at low load single node can fully

                                                utilize channelo high load collision overhead

                                                ldquotaking turnsrdquo protocolslook for best of both worlds

                                                5 DataLink Layer 5-31

                                                ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                ldquoinvitesrdquo slave nodes to transmit in turn

                                                typically used with ldquodumbrdquo slave devices

                                                concernso polling overhead o latencyo single point of

                                                failure (master)

                                                master

                                                slaves

                                                poll

                                                data

                                                data

                                                5 DataLink Layer 5-32

                                                ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                passed from one node to next sequentially

                                                token message concerns

                                                o token overhead o latencyo single point of failure

                                                (token)

                                                T

                                                data

                                                (nothingto send)

                                                T

                                                5 DataLink Layer 5-33

                                                Summary of MAC protocols

                                                channel partitioning by time frequency or codeo Time Division Frequency Division

                                                random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                5 DataLink Layer 5-34

                                                Link Layer

                                                51 Introduction and services

                                                52 Error detection and correction

                                                53Multiple access protocols

                                                54 Link-Layer Addressing

                                                55 Ethernet

                                                56 Link-layer switches 59 A day in the life of

                                                a web request

                                                5 DataLink Layer 5-35

                                                MAC Addresses and ARP

                                                32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                another physically-connected interface (in same network)

                                                o 48 bit MAC addressbull burned in NIC ROM

                                                5 DataLink Layer 5-36

                                                LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                Broadcast address =FF-FF-FF-FF-FF-FF

                                                = adapter

                                                1A-2F-BB-76-09-AD

                                                58-23-D7-FA-20-B0

                                                0C-C4-11-6F-E3-98

                                                71-65-F7-2B-08-53

                                                LAN(wired orwireless)

                                                5 DataLink Layer 5-37

                                                LAN Address (more)

                                                MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                o can move LAN card from one LAN to another

                                                IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                attached

                                                5 DataLink Layer 5-38

                                                ARP Address Resolution Protocol

                                                Each IP node (host router) on LAN has ARP table

                                                ARP table IPMAC address mappings for some LAN nodes

                                                lt IP address MAC address TTLgto TTL (Time To Live)

                                                time after which address mapping will be forgotten (typically 20 min)

                                                Q given a nodersquos IP address how to determine its MAC address

                                                1A-2F-BB-76-09-AD

                                                0C-C4-11-6F-E3-98

                                                71-65-F7-2B-08-53

                                                LAN

                                                137196723

                                                137196778

                                                137196714

                                                137196788

                                                5 DataLink Layer 5-39

                                                ARP protocol Same LAN (network) A wants to send datagram

                                                to B and Brsquos MAC address not in Arsquos ARP table

                                                A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                FF-FF-FF-FF-FFo all machines on LAN

                                                receive ARP query B receives ARP packet

                                                replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                address (unicast)

                                                A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                ARP is ldquoplug-and-playrdquoo nodes create their

                                                ARP tables without intervention from net administrator

                                                5 DataLink Layer 5-40

                                                Addressing routing to another LAN

                                                R

                                                1A-23-F9-CD-06-9B

                                                222222222220111111111110

                                                E6-E9-00-17-BB-4B

                                                CC-49-DE-D0-AB-7D

                                                111111111112

                                                111111111111

                                                A74-29-9C-E8-FF-55

                                                222222222221

                                                88-B2-2F-54-1A-0F

                                                B222222222222

                                                49-BD-D2-C7-56-2A

                                                walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                two ARP tables in router R one for each IP network (LAN)

                                                LAN LAN

                                                5 DataLink Layer 5-41

                                                A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                R

                                                1A-23-F9-CD-06-9B

                                                222222222220

                                                111111111110

                                                E6-E9-00-17-BB-4B

                                                CC-49-DE-D0-AB-7D

                                                111111111112

                                                111111111111

                                                A74-29-9C-E8-FF-55

                                                222222222221

                                                88-B2-2F-54-1A-0F

                                                B222222222222

                                                49-BD-D2-C7-56-2A

                                                This is a really importantexample ndash make sure youunderstand

                                                5 DataLink Layer 5-42

                                                Link Layer

                                                51 Introduction and services

                                                52 Error detection and correction

                                                53Multiple access protocols

                                                54 Link-Layer Addressing

                                                55 Ethernet

                                                56 Link-layer switches 59 A day in the life of

                                                a web request

                                                5 DataLink Layer 5-43

                                                Ethernet

                                                ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                Metcalfersquos Ethernetsketch

                                                5 DataLink Layer 5-44

                                                Star topology bus topology popular through mid 90s

                                                o all nodes in same collision domain (can collide with each other)

                                                today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                (nodes do not collide with each other)

                                                switch

                                                bus coaxial cable star

                                                5 DataLink Layer 5-45

                                                Ethernet Frame Structure

                                                Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                Preamble 7 bytes with pattern 10101010 followed by one

                                                byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                rates

                                                5 DataLink Layer 5-46

                                                Ethernet Frame Structure (more) Addresses 6 bytes

                                                o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                o otherwise adapter discards frame

                                                Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                CRC checked at receiver if error is detected frame is dropped

                                                5 DataLink Layer 5-47

                                                Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                Ethernetrsquos MAC protocol CSMACD

                                                5 DataLink Layer 5-48

                                                Ethernet CSMACD algorithm

                                                1 NIC receives datagram from network layer creates frame

                                                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                4 If NIC detects collision while transmitting aborts and sends jam signal

                                                5 After aborting NIC enters exponential backoff

                                                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                (1 slot = 512 bit-times)

                                                5 DataLink Layer 5-49

                                                Ethernetrsquos CSMACD (more)

                                                Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                Exponential Backoff Goal adapt retransmission

                                                attempts to estimated current loado heavy load random

                                                wait will be longer first collision choose K

                                                from 01 delay is K 512 bit transmission times

                                                after second collision choose K from 0123hellip

                                                after ten collisions choose K from 01234hellip1023

                                                5 DataLink Layer 5-50

                                                8023 Ethernet Standards Link amp Physical Layers

                                                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                applicationtransportnetwork

                                                linkphysical

                                                MAC protocoland frame format

                                                100BASE-TX

                                                100BASE-T4

                                                100BASE-FX100BASE-T2

                                                100BASE-SX 100BASE-BX

                                                fiber physical layercopper (twisterpair) physical layer

                                                5 DataLink Layer 5-51

                                                Manchester encoding

                                                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                synchronize to each othero no need for a centralized global clock among nodes

                                                Hey this is physical-layer stuff

                                                5 DataLink Layer 5-52

                                                Link Layer

                                                51 Introduction and services

                                                52 Error detection and correction

                                                53 Multiple access protocols

                                                54 Link-layer Addressing

                                                55 Ethernet

                                                56 Link-layer switches LANs

                                                59 A day in the life of a web request

                                                5 DataLink Layer 5-53

                                                Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                o bits coming in one link go out all other links at same rate

                                                o all nodes connected to hub can collide with one another

                                                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                twisted pair

                                                hub

                                                5 DataLink Layer 5-54

                                                Switch link-layer device smarter than hubs take active

                                                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                transparento hosts are unaware of presence of switches

                                                plug-and-play self-learningo switches do not need to be configured

                                                5 DataLink Layer 5-55

                                                Switch allows multiple simultaneous transmissions

                                                hosts have dedicated direct connection to switch

                                                switches buffer packets Ethernet protocol used on

                                                each incoming link but no collisions full duplexo each link is its own collision

                                                domain switching A-to-Arsquo and B-

                                                to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                A

                                                Arsquo

                                                B

                                                Brsquo

                                                C

                                                Crsquo

                                                switch with six interfaces(123456)

                                                1 23

                                                45

                                                6

                                                5 DataLink Layer 5-56

                                                Switch Table

                                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                A each switch has a switch table each entryo (MAC address of host interface

                                                to reach host time stamp)

                                                looks like a routing table Q how are entries created

                                                maintained in switch table o something like a routing

                                                protocol

                                                A

                                                Arsquo

                                                B

                                                Brsquo

                                                C

                                                Crsquo

                                                switch with six interfaces(123456)

                                                1 23

                                                45

                                                6

                                                5 DataLink Layer 5-57

                                                Switch self-learning

                                                switch learns which hosts can be reached through which interfaceso when frame received

                                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                                o records senderlocation pair in switch table

                                                A

                                                Arsquo

                                                B

                                                Brsquo

                                                C

                                                Crsquo

                                                1 23

                                                45

                                                6

                                                A Arsquo

                                                Source ADest Arsquo

                                                MAC addr interface TTL

                                                Switch table (initially empty)

                                                A 1 60

                                                5 DataLink Layer 5-58

                                                Switch frame filteringforwardingWhen frame received

                                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                then if dest on segment from which frame arrived

                                                then drop the frame else forward the frame on interface indicated else flood

                                                forward on all but the interface on which the frame arrived

                                                5 DataLink Layer 5-59

                                                Self-learning forwarding example

                                                A

                                                Arsquo

                                                B

                                                Brsquo

                                                C

                                                Crsquo

                                                1 23

                                                45

                                                6

                                                A Arsquo

                                                Source ADest Arsquo

                                                MAC addr interface TTL

                                                Switch table (initially empty)

                                                A 1 60

                                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                frame destination unknownflood

                                                Arsquo A

                                                destination A location known

                                                Arsquo 4 60

                                                selective send

                                                5 DataLink Layer 5-60

                                                Interconnecting switches

                                                switches can be connected together

                                                A

                                                B

                                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                A self learning (works exactly the same as in single-switch case)

                                                S1

                                                C D

                                                E

                                                FS2

                                                S4

                                                S3

                                                H

                                                I

                                                G

                                                5 DataLink Layer 5-61

                                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                A

                                                B

                                                S1

                                                C D

                                                E

                                                FS2

                                                S4

                                                S3

                                                H

                                                I

                                                G

                                                1

                                                2 3

                                                34

                                                5 DataLink Layer 5-62

                                                Institutional network

                                                to externalnetwork

                                                router

                                                IP subnet

                                                mail server

                                                web server

                                                5 DataLink Layer 5-63

                                                Switches vs Routers both store-and-forward devices

                                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                routers maintain routing tables implement routing algorithms

                                                switches maintain switch tables implement filtering learning algorithms

                                                Switch

                                                5 DataLink Layer 5-64

                                                Link Layer

                                                51 Introduction and services

                                                52 Error detection and correction

                                                53Multiple access protocols

                                                54 Link-Layer Addressing

                                                55 Ethernet

                                                56 Link-layer switches 59 A day in the life of

                                                a web request

                                                5 DataLink Layer 5-65

                                                Synthesis a day in the life of a web request

                                                journey down protocol stack completeo application transport network link

                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                5 DataLink Layer 5-66

                                                A day in the life scenario

                                                Comcast network 68800013

                                                Googlersquos network 64233160019 64233169105

                                                web server

                                                DNS server

                                                school network 68802024

                                                browser

                                                web page

                                                5 DataLink Layer 5-67

                                                A day in the lifehellip connecting to the Internet

                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                router(runs DHCP)

                                                DHCPUDP

                                                IPEthPhy

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCPUDP

                                                IPEthPhy

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCPDHCP

                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                5 DataLink Layer 5-68

                                                A day in the lifehellip connecting to the Internet

                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                router(runs DHCP)

                                                DHCPUDP

                                                IPEthPhy

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCPUDP

                                                IPEthPhy

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCP

                                                DHCP

                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                DHCP client receives DHCP ACK reply

                                                5 DataLink Layer 5-69

                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                DNSUDP

                                                IPEthPhy

                                                DNS

                                                DNS

                                                DNS

                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                ARP query

                                                EthPhy

                                                ARP

                                                ARP

                                                ARP reply

                                                5 DataLink Layer 5-70

                                                A day in the lifehellip using DNS

                                                DNSUDP

                                                IPEthPhy

                                                DNS

                                                DNS

                                                DNS

                                                DNS

                                                DNS

                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                demuxrsquoed to DNS server DNS server replies to

                                                client with IP address of wwwgooglecom

                                                Comcast network 68800013

                                                DNS server

                                                DNSUDP

                                                IPEthPhy

                                                DNS

                                                DNS

                                                DNS

                                                DNS

                                                5 DataLink Layer 5-71

                                                A day in the lifehellip TCP connection carrying HTTP

                                                HTTPTCPIP

                                                EthPhy

                                                HTTP

                                                to send HTTP request client first opens TCP socket to web server

                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                TCP connection established

                                                64233169105

                                                web server

                                                SYN

                                                SYN

                                                SYN

                                                SYN

                                                TCPIP

                                                EthPhy

                                                SYN

                                                SYN

                                                SYN

                                                SYNACK

                                                SYNACK

                                                SYNACK

                                                SYNACK

                                                SYNACK

                                                SYNACK

                                                SYNACK

                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                5 DataLink Layer 5-72

                                                A day in the lifehellip HTTP requestreply

                                                HTTPTCPIP

                                                EthPhy

                                                HTTP

                                                HTTP request sent into TCP socket

                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                IP datgram containing HTTP reply routed back to client

                                                64233169105

                                                web server

                                                HTTPTCPIP

                                                EthPhy

                                                web server responds with HTTP reply (containing web page)

                                                HTTP

                                                HTTP

                                                HTTPHTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                HTTP

                                                web page finally () displayed

                                                5 DataLink Layer 5-73

                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                (except PHY) solid understanding of networking

                                                principles practice hellip could stop here hellip but lots of

                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                • Link Layer
                                                • Link Layer Introduction
                                                • Link layer context
                                                • Encapsulation
                                                • Link Layer Services
                                                • Link Layer Services (more)
                                                • Where is the link layer implemented
                                                • Adaptors Communicating
                                                • Slide 9
                                                • Parity Checking
                                                • Internet checksum (review)
                                                • Checksumming Cyclic Redundancy Check
                                                • Cyclic Redundancy Check
                                                • Slide 14
                                                • CRC Example
                                                • Slide 16
                                                • Multiple Access Links and Protocols
                                                • Cable Network Architecture Overview
                                                • Multiple Access protocols
                                                • Ideal Multiple Access Protocol
                                                • MAC Protocols a taxonomy
                                                • Channel Partitioning MAC protocols TDMA
                                                • Channel Partitioning MAC protocols FDMA
                                                • Random Access Protocols
                                                • ALOHA
                                                • CSMA (Carrier Sense Multiple Access)
                                                • CSMA collisions
                                                • CSMACD (Collision Detection)
                                                • CSMACD collision detection
                                                • ldquoTaking Turnsrdquo MAC protocols
                                                • Slide 31
                                                • Slide 32
                                                • Summary of MAC protocols
                                                • Slide 34
                                                • MAC Addresses and ARP
                                                • LAN Addresses and ARP
                                                • LAN Address (more)
                                                • ARP Address Resolution Protocol
                                                • ARP protocol Same LAN (network)
                                                • Addressing routing to another LAN
                                                • Slide 41
                                                • Slide 42
                                                • Ethernet
                                                • Star topology
                                                • Ethernet Frame Structure
                                                • Ethernet Frame Structure (more)
                                                • Ethernet Unreliable connectionless
                                                • Ethernet CSMACD algorithm
                                                • Ethernetrsquos CSMACD (more)
                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                • Manchester encoding
                                                • Slide 52
                                                • Hubs
                                                • Switch
                                                • Switch allows multiple simultaneous transmissions
                                                • Switch Table
                                                • Switch self-learning
                                                • Switch frame filteringforwarding
                                                • Self-learning forwarding example
                                                • Interconnecting switches
                                                • Self-learning multi-switch example
                                                • Institutional network
                                                • Switches vs Routers
                                                • Slide 64
                                                • Synthesis a day in the life of a web request
                                                • A day in the life scenario
                                                • A day in the lifehellip connecting to the Internet
                                                • Slide 68
                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                • A day in the lifehellip using DNS
                                                • A day in the lifehellip TCP connection carrying HTTP
                                                • A day in the lifehellip HTTP requestreply
                                                • Chapter 5 letrsquos take a breath

                                                  5 DataLink Layer 5-25

                                                  ALOHA

                                                  When a node has a frame to send send immediately

                                                  Set a timer for a random amount of time If an ACK arrives before the timer expires

                                                  fine otherwise resend the frame

                                                  (Works like stop-and-wait with random timeout interval)

                                                  5 DataLink Layer 5-26

                                                  CSMA (Carrier Sense Multiple Access)

                                                  CSMA listen before transmit

                                                  If channel sensed idle transmit entire frame

                                                  If channel sensed busy defer transmission

                                                  human analogy donrsquot interrupt others

                                                  5 DataLink Layer 5-27

                                                  CSMA collisions

                                                  collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                                  spatial layout of nodes

                                                  5 DataLink Layer 5-28

                                                  CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                                  o collisions detected within short timeo colliding transmissions aborted reducing

                                                  channel wastage collision detection

                                                  o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                                  o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                                  human analogy the polite conversationalist

                                                  5 DataLink Layer 5-29

                                                  CSMACD collision detection

                                                  5 DataLink Layer 5-30

                                                  ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                  o share channel efficiently and fairly at high load

                                                  o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                  Random access MAC protocolso efficient at low load single node can fully

                                                  utilize channelo high load collision overhead

                                                  ldquotaking turnsrdquo protocolslook for best of both worlds

                                                  5 DataLink Layer 5-31

                                                  ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                  ldquoinvitesrdquo slave nodes to transmit in turn

                                                  typically used with ldquodumbrdquo slave devices

                                                  concernso polling overhead o latencyo single point of

                                                  failure (master)

                                                  master

                                                  slaves

                                                  poll

                                                  data

                                                  data

                                                  5 DataLink Layer 5-32

                                                  ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                  passed from one node to next sequentially

                                                  token message concerns

                                                  o token overhead o latencyo single point of failure

                                                  (token)

                                                  T

                                                  data

                                                  (nothingto send)

                                                  T

                                                  5 DataLink Layer 5-33

                                                  Summary of MAC protocols

                                                  channel partitioning by time frequency or codeo Time Division Frequency Division

                                                  random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                  hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                  taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                  5 DataLink Layer 5-34

                                                  Link Layer

                                                  51 Introduction and services

                                                  52 Error detection and correction

                                                  53Multiple access protocols

                                                  54 Link-Layer Addressing

                                                  55 Ethernet

                                                  56 Link-layer switches 59 A day in the life of

                                                  a web request

                                                  5 DataLink Layer 5-35

                                                  MAC Addresses and ARP

                                                  32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                  MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                  another physically-connected interface (in same network)

                                                  o 48 bit MAC addressbull burned in NIC ROM

                                                  5 DataLink Layer 5-36

                                                  LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                  Broadcast address =FF-FF-FF-FF-FF-FF

                                                  = adapter

                                                  1A-2F-BB-76-09-AD

                                                  58-23-D7-FA-20-B0

                                                  0C-C4-11-6F-E3-98

                                                  71-65-F7-2B-08-53

                                                  LAN(wired orwireless)

                                                  5 DataLink Layer 5-37

                                                  LAN Address (more)

                                                  MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                  space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                  o can move LAN card from one LAN to another

                                                  IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                  attached

                                                  5 DataLink Layer 5-38

                                                  ARP Address Resolution Protocol

                                                  Each IP node (host router) on LAN has ARP table

                                                  ARP table IPMAC address mappings for some LAN nodes

                                                  lt IP address MAC address TTLgto TTL (Time To Live)

                                                  time after which address mapping will be forgotten (typically 20 min)

                                                  Q given a nodersquos IP address how to determine its MAC address

                                                  1A-2F-BB-76-09-AD

                                                  0C-C4-11-6F-E3-98

                                                  71-65-F7-2B-08-53

                                                  LAN

                                                  137196723

                                                  137196778

                                                  137196714

                                                  137196788

                                                  5 DataLink Layer 5-39

                                                  ARP protocol Same LAN (network) A wants to send datagram

                                                  to B and Brsquos MAC address not in Arsquos ARP table

                                                  A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                  FF-FF-FF-FF-FFo all machines on LAN

                                                  receive ARP query B receives ARP packet

                                                  replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                  address (unicast)

                                                  A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                  ARP is ldquoplug-and-playrdquoo nodes create their

                                                  ARP tables without intervention from net administrator

                                                  5 DataLink Layer 5-40

                                                  Addressing routing to another LAN

                                                  R

                                                  1A-23-F9-CD-06-9B

                                                  222222222220111111111110

                                                  E6-E9-00-17-BB-4B

                                                  CC-49-DE-D0-AB-7D

                                                  111111111112

                                                  111111111111

                                                  A74-29-9C-E8-FF-55

                                                  222222222221

                                                  88-B2-2F-54-1A-0F

                                                  B222222222222

                                                  49-BD-D2-C7-56-2A

                                                  walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                  two ARP tables in router R one for each IP network (LAN)

                                                  LAN LAN

                                                  5 DataLink Layer 5-41

                                                  A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                  frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                  destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                  R

                                                  1A-23-F9-CD-06-9B

                                                  222222222220

                                                  111111111110

                                                  E6-E9-00-17-BB-4B

                                                  CC-49-DE-D0-AB-7D

                                                  111111111112

                                                  111111111111

                                                  A74-29-9C-E8-FF-55

                                                  222222222221

                                                  88-B2-2F-54-1A-0F

                                                  B222222222222

                                                  49-BD-D2-C7-56-2A

                                                  This is a really importantexample ndash make sure youunderstand

                                                  5 DataLink Layer 5-42

                                                  Link Layer

                                                  51 Introduction and services

                                                  52 Error detection and correction

                                                  53Multiple access protocols

                                                  54 Link-Layer Addressing

                                                  55 Ethernet

                                                  56 Link-layer switches 59 A day in the life of

                                                  a web request

                                                  5 DataLink Layer 5-43

                                                  Ethernet

                                                  ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                  Metcalfersquos Ethernetsketch

                                                  5 DataLink Layer 5-44

                                                  Star topology bus topology popular through mid 90s

                                                  o all nodes in same collision domain (can collide with each other)

                                                  today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                  (nodes do not collide with each other)

                                                  switch

                                                  bus coaxial cable star

                                                  5 DataLink Layer 5-45

                                                  Ethernet Frame Structure

                                                  Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                  Preamble 7 bytes with pattern 10101010 followed by one

                                                  byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                  rates

                                                  5 DataLink Layer 5-46

                                                  Ethernet Frame Structure (more) Addresses 6 bytes

                                                  o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                  o otherwise adapter discards frame

                                                  Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                  CRC checked at receiver if error is detected frame is dropped

                                                  5 DataLink Layer 5-47

                                                  Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                  and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                  send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                  gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                  Ethernetrsquos MAC protocol CSMACD

                                                  5 DataLink Layer 5-48

                                                  Ethernet CSMACD algorithm

                                                  1 NIC receives datagram from network layer creates frame

                                                  2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                  3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                  4 If NIC detects collision while transmitting aborts and sends jam signal

                                                  5 After aborting NIC enters exponential backoff

                                                  after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                  (1 slot = 512 bit-times)

                                                  5 DataLink Layer 5-49

                                                  Ethernetrsquos CSMACD (more)

                                                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                  Exponential Backoff Goal adapt retransmission

                                                  attempts to estimated current loado heavy load random

                                                  wait will be longer first collision choose K

                                                  from 01 delay is K 512 bit transmission times

                                                  after second collision choose K from 0123hellip

                                                  after ten collisions choose K from 01234hellip1023

                                                  5 DataLink Layer 5-50

                                                  8023 Ethernet Standards Link amp Physical Layers

                                                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                  applicationtransportnetwork

                                                  linkphysical

                                                  MAC protocoland frame format

                                                  100BASE-TX

                                                  100BASE-T4

                                                  100BASE-FX100BASE-T2

                                                  100BASE-SX 100BASE-BX

                                                  fiber physical layercopper (twisterpair) physical layer

                                                  5 DataLink Layer 5-51

                                                  Manchester encoding

                                                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                  synchronize to each othero no need for a centralized global clock among nodes

                                                  Hey this is physical-layer stuff

                                                  5 DataLink Layer 5-52

                                                  Link Layer

                                                  51 Introduction and services

                                                  52 Error detection and correction

                                                  53 Multiple access protocols

                                                  54 Link-layer Addressing

                                                  55 Ethernet

                                                  56 Link-layer switches LANs

                                                  59 A day in the life of a web request

                                                  5 DataLink Layer 5-53

                                                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                  o bits coming in one link go out all other links at same rate

                                                  o all nodes connected to hub can collide with one another

                                                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                  twisted pair

                                                  hub

                                                  5 DataLink Layer 5-54

                                                  Switch link-layer device smarter than hubs take active

                                                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                  transparento hosts are unaware of presence of switches

                                                  plug-and-play self-learningo switches do not need to be configured

                                                  5 DataLink Layer 5-55

                                                  Switch allows multiple simultaneous transmissions

                                                  hosts have dedicated direct connection to switch

                                                  switches buffer packets Ethernet protocol used on

                                                  each incoming link but no collisions full duplexo each link is its own collision

                                                  domain switching A-to-Arsquo and B-

                                                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                  A

                                                  Arsquo

                                                  B

                                                  Brsquo

                                                  C

                                                  Crsquo

                                                  switch with six interfaces(123456)

                                                  1 23

                                                  45

                                                  6

                                                  5 DataLink Layer 5-56

                                                  Switch Table

                                                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                  A each switch has a switch table each entryo (MAC address of host interface

                                                  to reach host time stamp)

                                                  looks like a routing table Q how are entries created

                                                  maintained in switch table o something like a routing

                                                  protocol

                                                  A

                                                  Arsquo

                                                  B

                                                  Brsquo

                                                  C

                                                  Crsquo

                                                  switch with six interfaces(123456)

                                                  1 23

                                                  45

                                                  6

                                                  5 DataLink Layer 5-57

                                                  Switch self-learning

                                                  switch learns which hosts can be reached through which interfaceso when frame received

                                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                                  o records senderlocation pair in switch table

                                                  A

                                                  Arsquo

                                                  B

                                                  Brsquo

                                                  C

                                                  Crsquo

                                                  1 23

                                                  45

                                                  6

                                                  A Arsquo

                                                  Source ADest Arsquo

                                                  MAC addr interface TTL

                                                  Switch table (initially empty)

                                                  A 1 60

                                                  5 DataLink Layer 5-58

                                                  Switch frame filteringforwardingWhen frame received

                                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                  then if dest on segment from which frame arrived

                                                  then drop the frame else forward the frame on interface indicated else flood

                                                  forward on all but the interface on which the frame arrived

                                                  5 DataLink Layer 5-59

                                                  Self-learning forwarding example

                                                  A

                                                  Arsquo

                                                  B

                                                  Brsquo

                                                  C

                                                  Crsquo

                                                  1 23

                                                  45

                                                  6

                                                  A Arsquo

                                                  Source ADest Arsquo

                                                  MAC addr interface TTL

                                                  Switch table (initially empty)

                                                  A 1 60

                                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                  frame destination unknownflood

                                                  Arsquo A

                                                  destination A location known

                                                  Arsquo 4 60

                                                  selective send

                                                  5 DataLink Layer 5-60

                                                  Interconnecting switches

                                                  switches can be connected together

                                                  A

                                                  B

                                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                  A self learning (works exactly the same as in single-switch case)

                                                  S1

                                                  C D

                                                  E

                                                  FS2

                                                  S4

                                                  S3

                                                  H

                                                  I

                                                  G

                                                  5 DataLink Layer 5-61

                                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                  A

                                                  B

                                                  S1

                                                  C D

                                                  E

                                                  FS2

                                                  S4

                                                  S3

                                                  H

                                                  I

                                                  G

                                                  1

                                                  2 3

                                                  34

                                                  5 DataLink Layer 5-62

                                                  Institutional network

                                                  to externalnetwork

                                                  router

                                                  IP subnet

                                                  mail server

                                                  web server

                                                  5 DataLink Layer 5-63

                                                  Switches vs Routers both store-and-forward devices

                                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                  routers maintain routing tables implement routing algorithms

                                                  switches maintain switch tables implement filtering learning algorithms

                                                  Switch

                                                  5 DataLink Layer 5-64

                                                  Link Layer

                                                  51 Introduction and services

                                                  52 Error detection and correction

                                                  53Multiple access protocols

                                                  54 Link-Layer Addressing

                                                  55 Ethernet

                                                  56 Link-layer switches 59 A day in the life of

                                                  a web request

                                                  5 DataLink Layer 5-65

                                                  Synthesis a day in the life of a web request

                                                  journey down protocol stack completeo application transport network link

                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                  5 DataLink Layer 5-66

                                                  A day in the life scenario

                                                  Comcast network 68800013

                                                  Googlersquos network 64233160019 64233169105

                                                  web server

                                                  DNS server

                                                  school network 68802024

                                                  browser

                                                  web page

                                                  5 DataLink Layer 5-67

                                                  A day in the lifehellip connecting to the Internet

                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                  router(runs DHCP)

                                                  DHCPUDP

                                                  IPEthPhy

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCPUDP

                                                  IPEthPhy

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCPDHCP

                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                  5 DataLink Layer 5-68

                                                  A day in the lifehellip connecting to the Internet

                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                  router(runs DHCP)

                                                  DHCPUDP

                                                  IPEthPhy

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCPUDP

                                                  IPEthPhy

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  DHCP

                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                  DHCP client receives DHCP ACK reply

                                                  5 DataLink Layer 5-69

                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                  DNSUDP

                                                  IPEthPhy

                                                  DNS

                                                  DNS

                                                  DNS

                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                  ARP query

                                                  EthPhy

                                                  ARP

                                                  ARP

                                                  ARP reply

                                                  5 DataLink Layer 5-70

                                                  A day in the lifehellip using DNS

                                                  DNSUDP

                                                  IPEthPhy

                                                  DNS

                                                  DNS

                                                  DNS

                                                  DNS

                                                  DNS

                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                  demuxrsquoed to DNS server DNS server replies to

                                                  client with IP address of wwwgooglecom

                                                  Comcast network 68800013

                                                  DNS server

                                                  DNSUDP

                                                  IPEthPhy

                                                  DNS

                                                  DNS

                                                  DNS

                                                  DNS

                                                  5 DataLink Layer 5-71

                                                  A day in the lifehellip TCP connection carrying HTTP

                                                  HTTPTCPIP

                                                  EthPhy

                                                  HTTP

                                                  to send HTTP request client first opens TCP socket to web server

                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                  TCP connection established

                                                  64233169105

                                                  web server

                                                  SYN

                                                  SYN

                                                  SYN

                                                  SYN

                                                  TCPIP

                                                  EthPhy

                                                  SYN

                                                  SYN

                                                  SYN

                                                  SYNACK

                                                  SYNACK

                                                  SYNACK

                                                  SYNACK

                                                  SYNACK

                                                  SYNACK

                                                  SYNACK

                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                  5 DataLink Layer 5-72

                                                  A day in the lifehellip HTTP requestreply

                                                  HTTPTCPIP

                                                  EthPhy

                                                  HTTP

                                                  HTTP request sent into TCP socket

                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                  IP datgram containing HTTP reply routed back to client

                                                  64233169105

                                                  web server

                                                  HTTPTCPIP

                                                  EthPhy

                                                  web server responds with HTTP reply (containing web page)

                                                  HTTP

                                                  HTTP

                                                  HTTPHTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  HTTP

                                                  web page finally () displayed

                                                  5 DataLink Layer 5-73

                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                  (except PHY) solid understanding of networking

                                                  principles practice hellip could stop here hellip but lots of

                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                  • Link Layer
                                                  • Link Layer Introduction
                                                  • Link layer context
                                                  • Encapsulation
                                                  • Link Layer Services
                                                  • Link Layer Services (more)
                                                  • Where is the link layer implemented
                                                  • Adaptors Communicating
                                                  • Slide 9
                                                  • Parity Checking
                                                  • Internet checksum (review)
                                                  • Checksumming Cyclic Redundancy Check
                                                  • Cyclic Redundancy Check
                                                  • Slide 14
                                                  • CRC Example
                                                  • Slide 16
                                                  • Multiple Access Links and Protocols
                                                  • Cable Network Architecture Overview
                                                  • Multiple Access protocols
                                                  • Ideal Multiple Access Protocol
                                                  • MAC Protocols a taxonomy
                                                  • Channel Partitioning MAC protocols TDMA
                                                  • Channel Partitioning MAC protocols FDMA
                                                  • Random Access Protocols
                                                  • ALOHA
                                                  • CSMA (Carrier Sense Multiple Access)
                                                  • CSMA collisions
                                                  • CSMACD (Collision Detection)
                                                  • CSMACD collision detection
                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                  • Slide 31
                                                  • Slide 32
                                                  • Summary of MAC protocols
                                                  • Slide 34
                                                  • MAC Addresses and ARP
                                                  • LAN Addresses and ARP
                                                  • LAN Address (more)
                                                  • ARP Address Resolution Protocol
                                                  • ARP protocol Same LAN (network)
                                                  • Addressing routing to another LAN
                                                  • Slide 41
                                                  • Slide 42
                                                  • Ethernet
                                                  • Star topology
                                                  • Ethernet Frame Structure
                                                  • Ethernet Frame Structure (more)
                                                  • Ethernet Unreliable connectionless
                                                  • Ethernet CSMACD algorithm
                                                  • Ethernetrsquos CSMACD (more)
                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                  • Manchester encoding
                                                  • Slide 52
                                                  • Hubs
                                                  • Switch
                                                  • Switch allows multiple simultaneous transmissions
                                                  • Switch Table
                                                  • Switch self-learning
                                                  • Switch frame filteringforwarding
                                                  • Self-learning forwarding example
                                                  • Interconnecting switches
                                                  • Self-learning multi-switch example
                                                  • Institutional network
                                                  • Switches vs Routers
                                                  • Slide 64
                                                  • Synthesis a day in the life of a web request
                                                  • A day in the life scenario
                                                  • A day in the lifehellip connecting to the Internet
                                                  • Slide 68
                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                  • A day in the lifehellip using DNS
                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                  • A day in the lifehellip HTTP requestreply
                                                  • Chapter 5 letrsquos take a breath

                                                    5 DataLink Layer 5-26

                                                    CSMA (Carrier Sense Multiple Access)

                                                    CSMA listen before transmit

                                                    If channel sensed idle transmit entire frame

                                                    If channel sensed busy defer transmission

                                                    human analogy donrsquot interrupt others

                                                    5 DataLink Layer 5-27

                                                    CSMA collisions

                                                    collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                                    spatial layout of nodes

                                                    5 DataLink Layer 5-28

                                                    CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                                    o collisions detected within short timeo colliding transmissions aborted reducing

                                                    channel wastage collision detection

                                                    o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                                    o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                                    human analogy the polite conversationalist

                                                    5 DataLink Layer 5-29

                                                    CSMACD collision detection

                                                    5 DataLink Layer 5-30

                                                    ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                    o share channel efficiently and fairly at high load

                                                    o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                    Random access MAC protocolso efficient at low load single node can fully

                                                    utilize channelo high load collision overhead

                                                    ldquotaking turnsrdquo protocolslook for best of both worlds

                                                    5 DataLink Layer 5-31

                                                    ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                    ldquoinvitesrdquo slave nodes to transmit in turn

                                                    typically used with ldquodumbrdquo slave devices

                                                    concernso polling overhead o latencyo single point of

                                                    failure (master)

                                                    master

                                                    slaves

                                                    poll

                                                    data

                                                    data

                                                    5 DataLink Layer 5-32

                                                    ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                    passed from one node to next sequentially

                                                    token message concerns

                                                    o token overhead o latencyo single point of failure

                                                    (token)

                                                    T

                                                    data

                                                    (nothingto send)

                                                    T

                                                    5 DataLink Layer 5-33

                                                    Summary of MAC protocols

                                                    channel partitioning by time frequency or codeo Time Division Frequency Division

                                                    random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                    hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                    taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                    5 DataLink Layer 5-34

                                                    Link Layer

                                                    51 Introduction and services

                                                    52 Error detection and correction

                                                    53Multiple access protocols

                                                    54 Link-Layer Addressing

                                                    55 Ethernet

                                                    56 Link-layer switches 59 A day in the life of

                                                    a web request

                                                    5 DataLink Layer 5-35

                                                    MAC Addresses and ARP

                                                    32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                    MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                    another physically-connected interface (in same network)

                                                    o 48 bit MAC addressbull burned in NIC ROM

                                                    5 DataLink Layer 5-36

                                                    LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                    Broadcast address =FF-FF-FF-FF-FF-FF

                                                    = adapter

                                                    1A-2F-BB-76-09-AD

                                                    58-23-D7-FA-20-B0

                                                    0C-C4-11-6F-E3-98

                                                    71-65-F7-2B-08-53

                                                    LAN(wired orwireless)

                                                    5 DataLink Layer 5-37

                                                    LAN Address (more)

                                                    MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                    space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                    o can move LAN card from one LAN to another

                                                    IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                    attached

                                                    5 DataLink Layer 5-38

                                                    ARP Address Resolution Protocol

                                                    Each IP node (host router) on LAN has ARP table

                                                    ARP table IPMAC address mappings for some LAN nodes

                                                    lt IP address MAC address TTLgto TTL (Time To Live)

                                                    time after which address mapping will be forgotten (typically 20 min)

                                                    Q given a nodersquos IP address how to determine its MAC address

                                                    1A-2F-BB-76-09-AD

                                                    0C-C4-11-6F-E3-98

                                                    71-65-F7-2B-08-53

                                                    LAN

                                                    137196723

                                                    137196778

                                                    137196714

                                                    137196788

                                                    5 DataLink Layer 5-39

                                                    ARP protocol Same LAN (network) A wants to send datagram

                                                    to B and Brsquos MAC address not in Arsquos ARP table

                                                    A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                    FF-FF-FF-FF-FFo all machines on LAN

                                                    receive ARP query B receives ARP packet

                                                    replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                    address (unicast)

                                                    A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                    ARP is ldquoplug-and-playrdquoo nodes create their

                                                    ARP tables without intervention from net administrator

                                                    5 DataLink Layer 5-40

                                                    Addressing routing to another LAN

                                                    R

                                                    1A-23-F9-CD-06-9B

                                                    222222222220111111111110

                                                    E6-E9-00-17-BB-4B

                                                    CC-49-DE-D0-AB-7D

                                                    111111111112

                                                    111111111111

                                                    A74-29-9C-E8-FF-55

                                                    222222222221

                                                    88-B2-2F-54-1A-0F

                                                    B222222222222

                                                    49-BD-D2-C7-56-2A

                                                    walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                    two ARP tables in router R one for each IP network (LAN)

                                                    LAN LAN

                                                    5 DataLink Layer 5-41

                                                    A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                    frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                    destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                    R

                                                    1A-23-F9-CD-06-9B

                                                    222222222220

                                                    111111111110

                                                    E6-E9-00-17-BB-4B

                                                    CC-49-DE-D0-AB-7D

                                                    111111111112

                                                    111111111111

                                                    A74-29-9C-E8-FF-55

                                                    222222222221

                                                    88-B2-2F-54-1A-0F

                                                    B222222222222

                                                    49-BD-D2-C7-56-2A

                                                    This is a really importantexample ndash make sure youunderstand

                                                    5 DataLink Layer 5-42

                                                    Link Layer

                                                    51 Introduction and services

                                                    52 Error detection and correction

                                                    53Multiple access protocols

                                                    54 Link-Layer Addressing

                                                    55 Ethernet

                                                    56 Link-layer switches 59 A day in the life of

                                                    a web request

                                                    5 DataLink Layer 5-43

                                                    Ethernet

                                                    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                    Metcalfersquos Ethernetsketch

                                                    5 DataLink Layer 5-44

                                                    Star topology bus topology popular through mid 90s

                                                    o all nodes in same collision domain (can collide with each other)

                                                    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                    (nodes do not collide with each other)

                                                    switch

                                                    bus coaxial cable star

                                                    5 DataLink Layer 5-45

                                                    Ethernet Frame Structure

                                                    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                    Preamble 7 bytes with pattern 10101010 followed by one

                                                    byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                    rates

                                                    5 DataLink Layer 5-46

                                                    Ethernet Frame Structure (more) Addresses 6 bytes

                                                    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                    o otherwise adapter discards frame

                                                    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                    CRC checked at receiver if error is detected frame is dropped

                                                    5 DataLink Layer 5-47

                                                    Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                    Ethernetrsquos MAC protocol CSMACD

                                                    5 DataLink Layer 5-48

                                                    Ethernet CSMACD algorithm

                                                    1 NIC receives datagram from network layer creates frame

                                                    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                    4 If NIC detects collision while transmitting aborts and sends jam signal

                                                    5 After aborting NIC enters exponential backoff

                                                    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                    (1 slot = 512 bit-times)

                                                    5 DataLink Layer 5-49

                                                    Ethernetrsquos CSMACD (more)

                                                    Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                    Exponential Backoff Goal adapt retransmission

                                                    attempts to estimated current loado heavy load random

                                                    wait will be longer first collision choose K

                                                    from 01 delay is K 512 bit transmission times

                                                    after second collision choose K from 0123hellip

                                                    after ten collisions choose K from 01234hellip1023

                                                    5 DataLink Layer 5-50

                                                    8023 Ethernet Standards Link amp Physical Layers

                                                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                    applicationtransportnetwork

                                                    linkphysical

                                                    MAC protocoland frame format

                                                    100BASE-TX

                                                    100BASE-T4

                                                    100BASE-FX100BASE-T2

                                                    100BASE-SX 100BASE-BX

                                                    fiber physical layercopper (twisterpair) physical layer

                                                    5 DataLink Layer 5-51

                                                    Manchester encoding

                                                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                    synchronize to each othero no need for a centralized global clock among nodes

                                                    Hey this is physical-layer stuff

                                                    5 DataLink Layer 5-52

                                                    Link Layer

                                                    51 Introduction and services

                                                    52 Error detection and correction

                                                    53 Multiple access protocols

                                                    54 Link-layer Addressing

                                                    55 Ethernet

                                                    56 Link-layer switches LANs

                                                    59 A day in the life of a web request

                                                    5 DataLink Layer 5-53

                                                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                    o bits coming in one link go out all other links at same rate

                                                    o all nodes connected to hub can collide with one another

                                                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                    twisted pair

                                                    hub

                                                    5 DataLink Layer 5-54

                                                    Switch link-layer device smarter than hubs take active

                                                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                    transparento hosts are unaware of presence of switches

                                                    plug-and-play self-learningo switches do not need to be configured

                                                    5 DataLink Layer 5-55

                                                    Switch allows multiple simultaneous transmissions

                                                    hosts have dedicated direct connection to switch

                                                    switches buffer packets Ethernet protocol used on

                                                    each incoming link but no collisions full duplexo each link is its own collision

                                                    domain switching A-to-Arsquo and B-

                                                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                    A

                                                    Arsquo

                                                    B

                                                    Brsquo

                                                    C

                                                    Crsquo

                                                    switch with six interfaces(123456)

                                                    1 23

                                                    45

                                                    6

                                                    5 DataLink Layer 5-56

                                                    Switch Table

                                                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                    A each switch has a switch table each entryo (MAC address of host interface

                                                    to reach host time stamp)

                                                    looks like a routing table Q how are entries created

                                                    maintained in switch table o something like a routing

                                                    protocol

                                                    A

                                                    Arsquo

                                                    B

                                                    Brsquo

                                                    C

                                                    Crsquo

                                                    switch with six interfaces(123456)

                                                    1 23

                                                    45

                                                    6

                                                    5 DataLink Layer 5-57

                                                    Switch self-learning

                                                    switch learns which hosts can be reached through which interfaceso when frame received

                                                    switch ldquolearnsrdquo location of sender incoming LAN segment

                                                    o records senderlocation pair in switch table

                                                    A

                                                    Arsquo

                                                    B

                                                    Brsquo

                                                    C

                                                    Crsquo

                                                    1 23

                                                    45

                                                    6

                                                    A Arsquo

                                                    Source ADest Arsquo

                                                    MAC addr interface TTL

                                                    Switch table (initially empty)

                                                    A 1 60

                                                    5 DataLink Layer 5-58

                                                    Switch frame filteringforwardingWhen frame received

                                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                    then if dest on segment from which frame arrived

                                                    then drop the frame else forward the frame on interface indicated else flood

                                                    forward on all but the interface on which the frame arrived

                                                    5 DataLink Layer 5-59

                                                    Self-learning forwarding example

                                                    A

                                                    Arsquo

                                                    B

                                                    Brsquo

                                                    C

                                                    Crsquo

                                                    1 23

                                                    45

                                                    6

                                                    A Arsquo

                                                    Source ADest Arsquo

                                                    MAC addr interface TTL

                                                    Switch table (initially empty)

                                                    A 1 60

                                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                    frame destination unknownflood

                                                    Arsquo A

                                                    destination A location known

                                                    Arsquo 4 60

                                                    selective send

                                                    5 DataLink Layer 5-60

                                                    Interconnecting switches

                                                    switches can be connected together

                                                    A

                                                    B

                                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                    A self learning (works exactly the same as in single-switch case)

                                                    S1

                                                    C D

                                                    E

                                                    FS2

                                                    S4

                                                    S3

                                                    H

                                                    I

                                                    G

                                                    5 DataLink Layer 5-61

                                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                    A

                                                    B

                                                    S1

                                                    C D

                                                    E

                                                    FS2

                                                    S4

                                                    S3

                                                    H

                                                    I

                                                    G

                                                    1

                                                    2 3

                                                    34

                                                    5 DataLink Layer 5-62

                                                    Institutional network

                                                    to externalnetwork

                                                    router

                                                    IP subnet

                                                    mail server

                                                    web server

                                                    5 DataLink Layer 5-63

                                                    Switches vs Routers both store-and-forward devices

                                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                    routers maintain routing tables implement routing algorithms

                                                    switches maintain switch tables implement filtering learning algorithms

                                                    Switch

                                                    5 DataLink Layer 5-64

                                                    Link Layer

                                                    51 Introduction and services

                                                    52 Error detection and correction

                                                    53Multiple access protocols

                                                    54 Link-Layer Addressing

                                                    55 Ethernet

                                                    56 Link-layer switches 59 A day in the life of

                                                    a web request

                                                    5 DataLink Layer 5-65

                                                    Synthesis a day in the life of a web request

                                                    journey down protocol stack completeo application transport network link

                                                    putting-it-all-together synthesiso goal identify review understand protocols

                                                    (at all layers) involved in seemingly simple scenario requesting www page

                                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                    5 DataLink Layer 5-66

                                                    A day in the life scenario

                                                    Comcast network 68800013

                                                    Googlersquos network 64233160019 64233169105

                                                    web server

                                                    DNS server

                                                    school network 68802024

                                                    browser

                                                    web page

                                                    5 DataLink Layer 5-67

                                                    A day in the lifehellip connecting to the Internet

                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                    router(runs DHCP)

                                                    DHCPUDP

                                                    IPEthPhy

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCPUDP

                                                    IPEthPhy

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCPDHCP

                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                    5 DataLink Layer 5-68

                                                    A day in the lifehellip connecting to the Internet

                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                    router(runs DHCP)

                                                    DHCPUDP

                                                    IPEthPhy

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCPUDP

                                                    IPEthPhy

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    DHCP

                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                    DHCP client receives DHCP ACK reply

                                                    5 DataLink Layer 5-69

                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                    DNSUDP

                                                    IPEthPhy

                                                    DNS

                                                    DNS

                                                    DNS

                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                    ARP query

                                                    EthPhy

                                                    ARP

                                                    ARP

                                                    ARP reply

                                                    5 DataLink Layer 5-70

                                                    A day in the lifehellip using DNS

                                                    DNSUDP

                                                    IPEthPhy

                                                    DNS

                                                    DNS

                                                    DNS

                                                    DNS

                                                    DNS

                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                    demuxrsquoed to DNS server DNS server replies to

                                                    client with IP address of wwwgooglecom

                                                    Comcast network 68800013

                                                    DNS server

                                                    DNSUDP

                                                    IPEthPhy

                                                    DNS

                                                    DNS

                                                    DNS

                                                    DNS

                                                    5 DataLink Layer 5-71

                                                    A day in the lifehellip TCP connection carrying HTTP

                                                    HTTPTCPIP

                                                    EthPhy

                                                    HTTP

                                                    to send HTTP request client first opens TCP socket to web server

                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                    TCP connection established

                                                    64233169105

                                                    web server

                                                    SYN

                                                    SYN

                                                    SYN

                                                    SYN

                                                    TCPIP

                                                    EthPhy

                                                    SYN

                                                    SYN

                                                    SYN

                                                    SYNACK

                                                    SYNACK

                                                    SYNACK

                                                    SYNACK

                                                    SYNACK

                                                    SYNACK

                                                    SYNACK

                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                    5 DataLink Layer 5-72

                                                    A day in the lifehellip HTTP requestreply

                                                    HTTPTCPIP

                                                    EthPhy

                                                    HTTP

                                                    HTTP request sent into TCP socket

                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                    IP datgram containing HTTP reply routed back to client

                                                    64233169105

                                                    web server

                                                    HTTPTCPIP

                                                    EthPhy

                                                    web server responds with HTTP reply (containing web page)

                                                    HTTP

                                                    HTTP

                                                    HTTPHTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    HTTP

                                                    web page finally () displayed

                                                    5 DataLink Layer 5-73

                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                    (except PHY) solid understanding of networking

                                                    principles practice hellip could stop here hellip but lots of

                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                    • Link Layer
                                                    • Link Layer Introduction
                                                    • Link layer context
                                                    • Encapsulation
                                                    • Link Layer Services
                                                    • Link Layer Services (more)
                                                    • Where is the link layer implemented
                                                    • Adaptors Communicating
                                                    • Slide 9
                                                    • Parity Checking
                                                    • Internet checksum (review)
                                                    • Checksumming Cyclic Redundancy Check
                                                    • Cyclic Redundancy Check
                                                    • Slide 14
                                                    • CRC Example
                                                    • Slide 16
                                                    • Multiple Access Links and Protocols
                                                    • Cable Network Architecture Overview
                                                    • Multiple Access protocols
                                                    • Ideal Multiple Access Protocol
                                                    • MAC Protocols a taxonomy
                                                    • Channel Partitioning MAC protocols TDMA
                                                    • Channel Partitioning MAC protocols FDMA
                                                    • Random Access Protocols
                                                    • ALOHA
                                                    • CSMA (Carrier Sense Multiple Access)
                                                    • CSMA collisions
                                                    • CSMACD (Collision Detection)
                                                    • CSMACD collision detection
                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                    • Slide 31
                                                    • Slide 32
                                                    • Summary of MAC protocols
                                                    • Slide 34
                                                    • MAC Addresses and ARP
                                                    • LAN Addresses and ARP
                                                    • LAN Address (more)
                                                    • ARP Address Resolution Protocol
                                                    • ARP protocol Same LAN (network)
                                                    • Addressing routing to another LAN
                                                    • Slide 41
                                                    • Slide 42
                                                    • Ethernet
                                                    • Star topology
                                                    • Ethernet Frame Structure
                                                    • Ethernet Frame Structure (more)
                                                    • Ethernet Unreliable connectionless
                                                    • Ethernet CSMACD algorithm
                                                    • Ethernetrsquos CSMACD (more)
                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                    • Manchester encoding
                                                    • Slide 52
                                                    • Hubs
                                                    • Switch
                                                    • Switch allows multiple simultaneous transmissions
                                                    • Switch Table
                                                    • Switch self-learning
                                                    • Switch frame filteringforwarding
                                                    • Self-learning forwarding example
                                                    • Interconnecting switches
                                                    • Self-learning multi-switch example
                                                    • Institutional network
                                                    • Switches vs Routers
                                                    • Slide 64
                                                    • Synthesis a day in the life of a web request
                                                    • A day in the life scenario
                                                    • A day in the lifehellip connecting to the Internet
                                                    • Slide 68
                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                    • A day in the lifehellip using DNS
                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                    • A day in the lifehellip HTTP requestreply
                                                    • Chapter 5 letrsquos take a breath

                                                      5 DataLink Layer 5-27

                                                      CSMA collisions

                                                      collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

                                                      spatial layout of nodes

                                                      5 DataLink Layer 5-28

                                                      CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                                      o collisions detected within short timeo colliding transmissions aborted reducing

                                                      channel wastage collision detection

                                                      o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                                      o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                                      human analogy the polite conversationalist

                                                      5 DataLink Layer 5-29

                                                      CSMACD collision detection

                                                      5 DataLink Layer 5-30

                                                      ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                      o share channel efficiently and fairly at high load

                                                      o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                      Random access MAC protocolso efficient at low load single node can fully

                                                      utilize channelo high load collision overhead

                                                      ldquotaking turnsrdquo protocolslook for best of both worlds

                                                      5 DataLink Layer 5-31

                                                      ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                      ldquoinvitesrdquo slave nodes to transmit in turn

                                                      typically used with ldquodumbrdquo slave devices

                                                      concernso polling overhead o latencyo single point of

                                                      failure (master)

                                                      master

                                                      slaves

                                                      poll

                                                      data

                                                      data

                                                      5 DataLink Layer 5-32

                                                      ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                      passed from one node to next sequentially

                                                      token message concerns

                                                      o token overhead o latencyo single point of failure

                                                      (token)

                                                      T

                                                      data

                                                      (nothingto send)

                                                      T

                                                      5 DataLink Layer 5-33

                                                      Summary of MAC protocols

                                                      channel partitioning by time frequency or codeo Time Division Frequency Division

                                                      random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                      hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                      taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                      5 DataLink Layer 5-34

                                                      Link Layer

                                                      51 Introduction and services

                                                      52 Error detection and correction

                                                      53Multiple access protocols

                                                      54 Link-Layer Addressing

                                                      55 Ethernet

                                                      56 Link-layer switches 59 A day in the life of

                                                      a web request

                                                      5 DataLink Layer 5-35

                                                      MAC Addresses and ARP

                                                      32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                      MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                      another physically-connected interface (in same network)

                                                      o 48 bit MAC addressbull burned in NIC ROM

                                                      5 DataLink Layer 5-36

                                                      LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                      Broadcast address =FF-FF-FF-FF-FF-FF

                                                      = adapter

                                                      1A-2F-BB-76-09-AD

                                                      58-23-D7-FA-20-B0

                                                      0C-C4-11-6F-E3-98

                                                      71-65-F7-2B-08-53

                                                      LAN(wired orwireless)

                                                      5 DataLink Layer 5-37

                                                      LAN Address (more)

                                                      MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                      space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                      o can move LAN card from one LAN to another

                                                      IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                      attached

                                                      5 DataLink Layer 5-38

                                                      ARP Address Resolution Protocol

                                                      Each IP node (host router) on LAN has ARP table

                                                      ARP table IPMAC address mappings for some LAN nodes

                                                      lt IP address MAC address TTLgto TTL (Time To Live)

                                                      time after which address mapping will be forgotten (typically 20 min)

                                                      Q given a nodersquos IP address how to determine its MAC address

                                                      1A-2F-BB-76-09-AD

                                                      0C-C4-11-6F-E3-98

                                                      71-65-F7-2B-08-53

                                                      LAN

                                                      137196723

                                                      137196778

                                                      137196714

                                                      137196788

                                                      5 DataLink Layer 5-39

                                                      ARP protocol Same LAN (network) A wants to send datagram

                                                      to B and Brsquos MAC address not in Arsquos ARP table

                                                      A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                      FF-FF-FF-FF-FFo all machines on LAN

                                                      receive ARP query B receives ARP packet

                                                      replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                      address (unicast)

                                                      A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                      ARP is ldquoplug-and-playrdquoo nodes create their

                                                      ARP tables without intervention from net administrator

                                                      5 DataLink Layer 5-40

                                                      Addressing routing to another LAN

                                                      R

                                                      1A-23-F9-CD-06-9B

                                                      222222222220111111111110

                                                      E6-E9-00-17-BB-4B

                                                      CC-49-DE-D0-AB-7D

                                                      111111111112

                                                      111111111111

                                                      A74-29-9C-E8-FF-55

                                                      222222222221

                                                      88-B2-2F-54-1A-0F

                                                      B222222222222

                                                      49-BD-D2-C7-56-2A

                                                      walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                      two ARP tables in router R one for each IP network (LAN)

                                                      LAN LAN

                                                      5 DataLink Layer 5-41

                                                      A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                      frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                      destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                      R

                                                      1A-23-F9-CD-06-9B

                                                      222222222220

                                                      111111111110

                                                      E6-E9-00-17-BB-4B

                                                      CC-49-DE-D0-AB-7D

                                                      111111111112

                                                      111111111111

                                                      A74-29-9C-E8-FF-55

                                                      222222222221

                                                      88-B2-2F-54-1A-0F

                                                      B222222222222

                                                      49-BD-D2-C7-56-2A

                                                      This is a really importantexample ndash make sure youunderstand

                                                      5 DataLink Layer 5-42

                                                      Link Layer

                                                      51 Introduction and services

                                                      52 Error detection and correction

                                                      53Multiple access protocols

                                                      54 Link-Layer Addressing

                                                      55 Ethernet

                                                      56 Link-layer switches 59 A day in the life of

                                                      a web request

                                                      5 DataLink Layer 5-43

                                                      Ethernet

                                                      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                      Metcalfersquos Ethernetsketch

                                                      5 DataLink Layer 5-44

                                                      Star topology bus topology popular through mid 90s

                                                      o all nodes in same collision domain (can collide with each other)

                                                      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                      (nodes do not collide with each other)

                                                      switch

                                                      bus coaxial cable star

                                                      5 DataLink Layer 5-45

                                                      Ethernet Frame Structure

                                                      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                      Preamble 7 bytes with pattern 10101010 followed by one

                                                      byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                      rates

                                                      5 DataLink Layer 5-46

                                                      Ethernet Frame Structure (more) Addresses 6 bytes

                                                      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                      o otherwise adapter discards frame

                                                      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                      CRC checked at receiver if error is detected frame is dropped

                                                      5 DataLink Layer 5-47

                                                      Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                      Ethernetrsquos MAC protocol CSMACD

                                                      5 DataLink Layer 5-48

                                                      Ethernet CSMACD algorithm

                                                      1 NIC receives datagram from network layer creates frame

                                                      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                      4 If NIC detects collision while transmitting aborts and sends jam signal

                                                      5 After aborting NIC enters exponential backoff

                                                      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                      (1 slot = 512 bit-times)

                                                      5 DataLink Layer 5-49

                                                      Ethernetrsquos CSMACD (more)

                                                      Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                      Exponential Backoff Goal adapt retransmission

                                                      attempts to estimated current loado heavy load random

                                                      wait will be longer first collision choose K

                                                      from 01 delay is K 512 bit transmission times

                                                      after second collision choose K from 0123hellip

                                                      after ten collisions choose K from 01234hellip1023

                                                      5 DataLink Layer 5-50

                                                      8023 Ethernet Standards Link amp Physical Layers

                                                      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                      Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                      applicationtransportnetwork

                                                      linkphysical

                                                      MAC protocoland frame format

                                                      100BASE-TX

                                                      100BASE-T4

                                                      100BASE-FX100BASE-T2

                                                      100BASE-SX 100BASE-BX

                                                      fiber physical layercopper (twisterpair) physical layer

                                                      5 DataLink Layer 5-51

                                                      Manchester encoding

                                                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                      synchronize to each othero no need for a centralized global clock among nodes

                                                      Hey this is physical-layer stuff

                                                      5 DataLink Layer 5-52

                                                      Link Layer

                                                      51 Introduction and services

                                                      52 Error detection and correction

                                                      53 Multiple access protocols

                                                      54 Link-layer Addressing

                                                      55 Ethernet

                                                      56 Link-layer switches LANs

                                                      59 A day in the life of a web request

                                                      5 DataLink Layer 5-53

                                                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                      o bits coming in one link go out all other links at same rate

                                                      o all nodes connected to hub can collide with one another

                                                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                      twisted pair

                                                      hub

                                                      5 DataLink Layer 5-54

                                                      Switch link-layer device smarter than hubs take active

                                                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                      transparento hosts are unaware of presence of switches

                                                      plug-and-play self-learningo switches do not need to be configured

                                                      5 DataLink Layer 5-55

                                                      Switch allows multiple simultaneous transmissions

                                                      hosts have dedicated direct connection to switch

                                                      switches buffer packets Ethernet protocol used on

                                                      each incoming link but no collisions full duplexo each link is its own collision

                                                      domain switching A-to-Arsquo and B-

                                                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                      A

                                                      Arsquo

                                                      B

                                                      Brsquo

                                                      C

                                                      Crsquo

                                                      switch with six interfaces(123456)

                                                      1 23

                                                      45

                                                      6

                                                      5 DataLink Layer 5-56

                                                      Switch Table

                                                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                      A each switch has a switch table each entryo (MAC address of host interface

                                                      to reach host time stamp)

                                                      looks like a routing table Q how are entries created

                                                      maintained in switch table o something like a routing

                                                      protocol

                                                      A

                                                      Arsquo

                                                      B

                                                      Brsquo

                                                      C

                                                      Crsquo

                                                      switch with six interfaces(123456)

                                                      1 23

                                                      45

                                                      6

                                                      5 DataLink Layer 5-57

                                                      Switch self-learning

                                                      switch learns which hosts can be reached through which interfaceso when frame received

                                                      switch ldquolearnsrdquo location of sender incoming LAN segment

                                                      o records senderlocation pair in switch table

                                                      A

                                                      Arsquo

                                                      B

                                                      Brsquo

                                                      C

                                                      Crsquo

                                                      1 23

                                                      45

                                                      6

                                                      A Arsquo

                                                      Source ADest Arsquo

                                                      MAC addr interface TTL

                                                      Switch table (initially empty)

                                                      A 1 60

                                                      5 DataLink Layer 5-58

                                                      Switch frame filteringforwardingWhen frame received

                                                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                      then if dest on segment from which frame arrived

                                                      then drop the frame else forward the frame on interface indicated else flood

                                                      forward on all but the interface on which the frame arrived

                                                      5 DataLink Layer 5-59

                                                      Self-learning forwarding example

                                                      A

                                                      Arsquo

                                                      B

                                                      Brsquo

                                                      C

                                                      Crsquo

                                                      1 23

                                                      45

                                                      6

                                                      A Arsquo

                                                      Source ADest Arsquo

                                                      MAC addr interface TTL

                                                      Switch table (initially empty)

                                                      A 1 60

                                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                      frame destination unknownflood

                                                      Arsquo A

                                                      destination A location known

                                                      Arsquo 4 60

                                                      selective send

                                                      5 DataLink Layer 5-60

                                                      Interconnecting switches

                                                      switches can be connected together

                                                      A

                                                      B

                                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                      A self learning (works exactly the same as in single-switch case)

                                                      S1

                                                      C D

                                                      E

                                                      FS2

                                                      S4

                                                      S3

                                                      H

                                                      I

                                                      G

                                                      5 DataLink Layer 5-61

                                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                      A

                                                      B

                                                      S1

                                                      C D

                                                      E

                                                      FS2

                                                      S4

                                                      S3

                                                      H

                                                      I

                                                      G

                                                      1

                                                      2 3

                                                      34

                                                      5 DataLink Layer 5-62

                                                      Institutional network

                                                      to externalnetwork

                                                      router

                                                      IP subnet

                                                      mail server

                                                      web server

                                                      5 DataLink Layer 5-63

                                                      Switches vs Routers both store-and-forward devices

                                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                      routers maintain routing tables implement routing algorithms

                                                      switches maintain switch tables implement filtering learning algorithms

                                                      Switch

                                                      5 DataLink Layer 5-64

                                                      Link Layer

                                                      51 Introduction and services

                                                      52 Error detection and correction

                                                      53Multiple access protocols

                                                      54 Link-Layer Addressing

                                                      55 Ethernet

                                                      56 Link-layer switches 59 A day in the life of

                                                      a web request

                                                      5 DataLink Layer 5-65

                                                      Synthesis a day in the life of a web request

                                                      journey down protocol stack completeo application transport network link

                                                      putting-it-all-together synthesiso goal identify review understand protocols

                                                      (at all layers) involved in seemingly simple scenario requesting www page

                                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                      5 DataLink Layer 5-66

                                                      A day in the life scenario

                                                      Comcast network 68800013

                                                      Googlersquos network 64233160019 64233169105

                                                      web server

                                                      DNS server

                                                      school network 68802024

                                                      browser

                                                      web page

                                                      5 DataLink Layer 5-67

                                                      A day in the lifehellip connecting to the Internet

                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                      router(runs DHCP)

                                                      DHCPUDP

                                                      IPEthPhy

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCPUDP

                                                      IPEthPhy

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCPDHCP

                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                      5 DataLink Layer 5-68

                                                      A day in the lifehellip connecting to the Internet

                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                      router(runs DHCP)

                                                      DHCPUDP

                                                      IPEthPhy

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCPUDP

                                                      IPEthPhy

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      DHCP

                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                      DHCP client receives DHCP ACK reply

                                                      5 DataLink Layer 5-69

                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                      DNSUDP

                                                      IPEthPhy

                                                      DNS

                                                      DNS

                                                      DNS

                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                      ARP query

                                                      EthPhy

                                                      ARP

                                                      ARP

                                                      ARP reply

                                                      5 DataLink Layer 5-70

                                                      A day in the lifehellip using DNS

                                                      DNSUDP

                                                      IPEthPhy

                                                      DNS

                                                      DNS

                                                      DNS

                                                      DNS

                                                      DNS

                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                      demuxrsquoed to DNS server DNS server replies to

                                                      client with IP address of wwwgooglecom

                                                      Comcast network 68800013

                                                      DNS server

                                                      DNSUDP

                                                      IPEthPhy

                                                      DNS

                                                      DNS

                                                      DNS

                                                      DNS

                                                      5 DataLink Layer 5-71

                                                      A day in the lifehellip TCP connection carrying HTTP

                                                      HTTPTCPIP

                                                      EthPhy

                                                      HTTP

                                                      to send HTTP request client first opens TCP socket to web server

                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                      TCP connection established

                                                      64233169105

                                                      web server

                                                      SYN

                                                      SYN

                                                      SYN

                                                      SYN

                                                      TCPIP

                                                      EthPhy

                                                      SYN

                                                      SYN

                                                      SYN

                                                      SYNACK

                                                      SYNACK

                                                      SYNACK

                                                      SYNACK

                                                      SYNACK

                                                      SYNACK

                                                      SYNACK

                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                      5 DataLink Layer 5-72

                                                      A day in the lifehellip HTTP requestreply

                                                      HTTPTCPIP

                                                      EthPhy

                                                      HTTP

                                                      HTTP request sent into TCP socket

                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                      IP datgram containing HTTP reply routed back to client

                                                      64233169105

                                                      web server

                                                      HTTPTCPIP

                                                      EthPhy

                                                      web server responds with HTTP reply (containing web page)

                                                      HTTP

                                                      HTTP

                                                      HTTPHTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      HTTP

                                                      web page finally () displayed

                                                      5 DataLink Layer 5-73

                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                      (except PHY) solid understanding of networking

                                                      principles practice hellip could stop here hellip but lots of

                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                      • Link Layer
                                                      • Link Layer Introduction
                                                      • Link layer context
                                                      • Encapsulation
                                                      • Link Layer Services
                                                      • Link Layer Services (more)
                                                      • Where is the link layer implemented
                                                      • Adaptors Communicating
                                                      • Slide 9
                                                      • Parity Checking
                                                      • Internet checksum (review)
                                                      • Checksumming Cyclic Redundancy Check
                                                      • Cyclic Redundancy Check
                                                      • Slide 14
                                                      • CRC Example
                                                      • Slide 16
                                                      • Multiple Access Links and Protocols
                                                      • Cable Network Architecture Overview
                                                      • Multiple Access protocols
                                                      • Ideal Multiple Access Protocol
                                                      • MAC Protocols a taxonomy
                                                      • Channel Partitioning MAC protocols TDMA
                                                      • Channel Partitioning MAC protocols FDMA
                                                      • Random Access Protocols
                                                      • ALOHA
                                                      • CSMA (Carrier Sense Multiple Access)
                                                      • CSMA collisions
                                                      • CSMACD (Collision Detection)
                                                      • CSMACD collision detection
                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                      • Slide 31
                                                      • Slide 32
                                                      • Summary of MAC protocols
                                                      • Slide 34
                                                      • MAC Addresses and ARP
                                                      • LAN Addresses and ARP
                                                      • LAN Address (more)
                                                      • ARP Address Resolution Protocol
                                                      • ARP protocol Same LAN (network)
                                                      • Addressing routing to another LAN
                                                      • Slide 41
                                                      • Slide 42
                                                      • Ethernet
                                                      • Star topology
                                                      • Ethernet Frame Structure
                                                      • Ethernet Frame Structure (more)
                                                      • Ethernet Unreliable connectionless
                                                      • Ethernet CSMACD algorithm
                                                      • Ethernetrsquos CSMACD (more)
                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                      • Manchester encoding
                                                      • Slide 52
                                                      • Hubs
                                                      • Switch
                                                      • Switch allows multiple simultaneous transmissions
                                                      • Switch Table
                                                      • Switch self-learning
                                                      • Switch frame filteringforwarding
                                                      • Self-learning forwarding example
                                                      • Interconnecting switches
                                                      • Self-learning multi-switch example
                                                      • Institutional network
                                                      • Switches vs Routers
                                                      • Slide 64
                                                      • Synthesis a day in the life of a web request
                                                      • A day in the life scenario
                                                      • A day in the lifehellip connecting to the Internet
                                                      • Slide 68
                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                      • A day in the lifehellip using DNS
                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                      • A day in the lifehellip HTTP requestreply
                                                      • Chapter 5 letrsquos take a breath

                                                        5 DataLink Layer 5-28

                                                        CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

                                                        o collisions detected within short timeo colliding transmissions aborted reducing

                                                        channel wastage collision detection

                                                        o easy in wired LANs measure signal strengths compare transmitted amp received signals

                                                        o difficult in wireless LANs received signal strength overwhelmed by local transmission strength

                                                        human analogy the polite conversationalist

                                                        5 DataLink Layer 5-29

                                                        CSMACD collision detection

                                                        5 DataLink Layer 5-30

                                                        ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                        o share channel efficiently and fairly at high load

                                                        o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                        Random access MAC protocolso efficient at low load single node can fully

                                                        utilize channelo high load collision overhead

                                                        ldquotaking turnsrdquo protocolslook for best of both worlds

                                                        5 DataLink Layer 5-31

                                                        ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                        ldquoinvitesrdquo slave nodes to transmit in turn

                                                        typically used with ldquodumbrdquo slave devices

                                                        concernso polling overhead o latencyo single point of

                                                        failure (master)

                                                        master

                                                        slaves

                                                        poll

                                                        data

                                                        data

                                                        5 DataLink Layer 5-32

                                                        ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                        passed from one node to next sequentially

                                                        token message concerns

                                                        o token overhead o latencyo single point of failure

                                                        (token)

                                                        T

                                                        data

                                                        (nothingto send)

                                                        T

                                                        5 DataLink Layer 5-33

                                                        Summary of MAC protocols

                                                        channel partitioning by time frequency or codeo Time Division Frequency Division

                                                        random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                        hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                        taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                        5 DataLink Layer 5-34

                                                        Link Layer

                                                        51 Introduction and services

                                                        52 Error detection and correction

                                                        53Multiple access protocols

                                                        54 Link-Layer Addressing

                                                        55 Ethernet

                                                        56 Link-layer switches 59 A day in the life of

                                                        a web request

                                                        5 DataLink Layer 5-35

                                                        MAC Addresses and ARP

                                                        32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                        MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                        another physically-connected interface (in same network)

                                                        o 48 bit MAC addressbull burned in NIC ROM

                                                        5 DataLink Layer 5-36

                                                        LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                        Broadcast address =FF-FF-FF-FF-FF-FF

                                                        = adapter

                                                        1A-2F-BB-76-09-AD

                                                        58-23-D7-FA-20-B0

                                                        0C-C4-11-6F-E3-98

                                                        71-65-F7-2B-08-53

                                                        LAN(wired orwireless)

                                                        5 DataLink Layer 5-37

                                                        LAN Address (more)

                                                        MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                        space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                        o can move LAN card from one LAN to another

                                                        IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                        attached

                                                        5 DataLink Layer 5-38

                                                        ARP Address Resolution Protocol

                                                        Each IP node (host router) on LAN has ARP table

                                                        ARP table IPMAC address mappings for some LAN nodes

                                                        lt IP address MAC address TTLgto TTL (Time To Live)

                                                        time after which address mapping will be forgotten (typically 20 min)

                                                        Q given a nodersquos IP address how to determine its MAC address

                                                        1A-2F-BB-76-09-AD

                                                        0C-C4-11-6F-E3-98

                                                        71-65-F7-2B-08-53

                                                        LAN

                                                        137196723

                                                        137196778

                                                        137196714

                                                        137196788

                                                        5 DataLink Layer 5-39

                                                        ARP protocol Same LAN (network) A wants to send datagram

                                                        to B and Brsquos MAC address not in Arsquos ARP table

                                                        A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                        FF-FF-FF-FF-FFo all machines on LAN

                                                        receive ARP query B receives ARP packet

                                                        replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                        address (unicast)

                                                        A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                        ARP is ldquoplug-and-playrdquoo nodes create their

                                                        ARP tables without intervention from net administrator

                                                        5 DataLink Layer 5-40

                                                        Addressing routing to another LAN

                                                        R

                                                        1A-23-F9-CD-06-9B

                                                        222222222220111111111110

                                                        E6-E9-00-17-BB-4B

                                                        CC-49-DE-D0-AB-7D

                                                        111111111112

                                                        111111111111

                                                        A74-29-9C-E8-FF-55

                                                        222222222221

                                                        88-B2-2F-54-1A-0F

                                                        B222222222222

                                                        49-BD-D2-C7-56-2A

                                                        walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                        two ARP tables in router R one for each IP network (LAN)

                                                        LAN LAN

                                                        5 DataLink Layer 5-41

                                                        A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                        frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                        destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                        R

                                                        1A-23-F9-CD-06-9B

                                                        222222222220

                                                        111111111110

                                                        E6-E9-00-17-BB-4B

                                                        CC-49-DE-D0-AB-7D

                                                        111111111112

                                                        111111111111

                                                        A74-29-9C-E8-FF-55

                                                        222222222221

                                                        88-B2-2F-54-1A-0F

                                                        B222222222222

                                                        49-BD-D2-C7-56-2A

                                                        This is a really importantexample ndash make sure youunderstand

                                                        5 DataLink Layer 5-42

                                                        Link Layer

                                                        51 Introduction and services

                                                        52 Error detection and correction

                                                        53Multiple access protocols

                                                        54 Link-Layer Addressing

                                                        55 Ethernet

                                                        56 Link-layer switches 59 A day in the life of

                                                        a web request

                                                        5 DataLink Layer 5-43

                                                        Ethernet

                                                        ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                        Metcalfersquos Ethernetsketch

                                                        5 DataLink Layer 5-44

                                                        Star topology bus topology popular through mid 90s

                                                        o all nodes in same collision domain (can collide with each other)

                                                        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                        (nodes do not collide with each other)

                                                        switch

                                                        bus coaxial cable star

                                                        5 DataLink Layer 5-45

                                                        Ethernet Frame Structure

                                                        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                        Preamble 7 bytes with pattern 10101010 followed by one

                                                        byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                        rates

                                                        5 DataLink Layer 5-46

                                                        Ethernet Frame Structure (more) Addresses 6 bytes

                                                        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                        o otherwise adapter discards frame

                                                        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                        CRC checked at receiver if error is detected frame is dropped

                                                        5 DataLink Layer 5-47

                                                        Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                        Ethernetrsquos MAC protocol CSMACD

                                                        5 DataLink Layer 5-48

                                                        Ethernet CSMACD algorithm

                                                        1 NIC receives datagram from network layer creates frame

                                                        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                        4 If NIC detects collision while transmitting aborts and sends jam signal

                                                        5 After aborting NIC enters exponential backoff

                                                        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                        (1 slot = 512 bit-times)

                                                        5 DataLink Layer 5-49

                                                        Ethernetrsquos CSMACD (more)

                                                        Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                        Exponential Backoff Goal adapt retransmission

                                                        attempts to estimated current loado heavy load random

                                                        wait will be longer first collision choose K

                                                        from 01 delay is K 512 bit transmission times

                                                        after second collision choose K from 0123hellip

                                                        after ten collisions choose K from 01234hellip1023

                                                        5 DataLink Layer 5-50

                                                        8023 Ethernet Standards Link amp Physical Layers

                                                        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                        Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                        applicationtransportnetwork

                                                        linkphysical

                                                        MAC protocoland frame format

                                                        100BASE-TX

                                                        100BASE-T4

                                                        100BASE-FX100BASE-T2

                                                        100BASE-SX 100BASE-BX

                                                        fiber physical layercopper (twisterpair) physical layer

                                                        5 DataLink Layer 5-51

                                                        Manchester encoding

                                                        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                        synchronize to each othero no need for a centralized global clock among nodes

                                                        Hey this is physical-layer stuff

                                                        5 DataLink Layer 5-52

                                                        Link Layer

                                                        51 Introduction and services

                                                        52 Error detection and correction

                                                        53 Multiple access protocols

                                                        54 Link-layer Addressing

                                                        55 Ethernet

                                                        56 Link-layer switches LANs

                                                        59 A day in the life of a web request

                                                        5 DataLink Layer 5-53

                                                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                        o bits coming in one link go out all other links at same rate

                                                        o all nodes connected to hub can collide with one another

                                                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                        twisted pair

                                                        hub

                                                        5 DataLink Layer 5-54

                                                        Switch link-layer device smarter than hubs take active

                                                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                        transparento hosts are unaware of presence of switches

                                                        plug-and-play self-learningo switches do not need to be configured

                                                        5 DataLink Layer 5-55

                                                        Switch allows multiple simultaneous transmissions

                                                        hosts have dedicated direct connection to switch

                                                        switches buffer packets Ethernet protocol used on

                                                        each incoming link but no collisions full duplexo each link is its own collision

                                                        domain switching A-to-Arsquo and B-

                                                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                        A

                                                        Arsquo

                                                        B

                                                        Brsquo

                                                        C

                                                        Crsquo

                                                        switch with six interfaces(123456)

                                                        1 23

                                                        45

                                                        6

                                                        5 DataLink Layer 5-56

                                                        Switch Table

                                                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                        A each switch has a switch table each entryo (MAC address of host interface

                                                        to reach host time stamp)

                                                        looks like a routing table Q how are entries created

                                                        maintained in switch table o something like a routing

                                                        protocol

                                                        A

                                                        Arsquo

                                                        B

                                                        Brsquo

                                                        C

                                                        Crsquo

                                                        switch with six interfaces(123456)

                                                        1 23

                                                        45

                                                        6

                                                        5 DataLink Layer 5-57

                                                        Switch self-learning

                                                        switch learns which hosts can be reached through which interfaceso when frame received

                                                        switch ldquolearnsrdquo location of sender incoming LAN segment

                                                        o records senderlocation pair in switch table

                                                        A

                                                        Arsquo

                                                        B

                                                        Brsquo

                                                        C

                                                        Crsquo

                                                        1 23

                                                        45

                                                        6

                                                        A Arsquo

                                                        Source ADest Arsquo

                                                        MAC addr interface TTL

                                                        Switch table (initially empty)

                                                        A 1 60

                                                        5 DataLink Layer 5-58

                                                        Switch frame filteringforwardingWhen frame received

                                                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                        then if dest on segment from which frame arrived

                                                        then drop the frame else forward the frame on interface indicated else flood

                                                        forward on all but the interface on which the frame arrived

                                                        5 DataLink Layer 5-59

                                                        Self-learning forwarding example

                                                        A

                                                        Arsquo

                                                        B

                                                        Brsquo

                                                        C

                                                        Crsquo

                                                        1 23

                                                        45

                                                        6

                                                        A Arsquo

                                                        Source ADest Arsquo

                                                        MAC addr interface TTL

                                                        Switch table (initially empty)

                                                        A 1 60

                                                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                        frame destination unknownflood

                                                        Arsquo A

                                                        destination A location known

                                                        Arsquo 4 60

                                                        selective send

                                                        5 DataLink Layer 5-60

                                                        Interconnecting switches

                                                        switches can be connected together

                                                        A

                                                        B

                                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                        A self learning (works exactly the same as in single-switch case)

                                                        S1

                                                        C D

                                                        E

                                                        FS2

                                                        S4

                                                        S3

                                                        H

                                                        I

                                                        G

                                                        5 DataLink Layer 5-61

                                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                        A

                                                        B

                                                        S1

                                                        C D

                                                        E

                                                        FS2

                                                        S4

                                                        S3

                                                        H

                                                        I

                                                        G

                                                        1

                                                        2 3

                                                        34

                                                        5 DataLink Layer 5-62

                                                        Institutional network

                                                        to externalnetwork

                                                        router

                                                        IP subnet

                                                        mail server

                                                        web server

                                                        5 DataLink Layer 5-63

                                                        Switches vs Routers both store-and-forward devices

                                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                        routers maintain routing tables implement routing algorithms

                                                        switches maintain switch tables implement filtering learning algorithms

                                                        Switch

                                                        5 DataLink Layer 5-64

                                                        Link Layer

                                                        51 Introduction and services

                                                        52 Error detection and correction

                                                        53Multiple access protocols

                                                        54 Link-Layer Addressing

                                                        55 Ethernet

                                                        56 Link-layer switches 59 A day in the life of

                                                        a web request

                                                        5 DataLink Layer 5-65

                                                        Synthesis a day in the life of a web request

                                                        journey down protocol stack completeo application transport network link

                                                        putting-it-all-together synthesiso goal identify review understand protocols

                                                        (at all layers) involved in seemingly simple scenario requesting www page

                                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                        5 DataLink Layer 5-66

                                                        A day in the life scenario

                                                        Comcast network 68800013

                                                        Googlersquos network 64233160019 64233169105

                                                        web server

                                                        DNS server

                                                        school network 68802024

                                                        browser

                                                        web page

                                                        5 DataLink Layer 5-67

                                                        A day in the lifehellip connecting to the Internet

                                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                        router(runs DHCP)

                                                        DHCPUDP

                                                        IPEthPhy

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCPUDP

                                                        IPEthPhy

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCPDHCP

                                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                        5 DataLink Layer 5-68

                                                        A day in the lifehellip connecting to the Internet

                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                        router(runs DHCP)

                                                        DHCPUDP

                                                        IPEthPhy

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCPUDP

                                                        IPEthPhy

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        DHCP

                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                        DHCP client receives DHCP ACK reply

                                                        5 DataLink Layer 5-69

                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                        DNSUDP

                                                        IPEthPhy

                                                        DNS

                                                        DNS

                                                        DNS

                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                        ARP query

                                                        EthPhy

                                                        ARP

                                                        ARP

                                                        ARP reply

                                                        5 DataLink Layer 5-70

                                                        A day in the lifehellip using DNS

                                                        DNSUDP

                                                        IPEthPhy

                                                        DNS

                                                        DNS

                                                        DNS

                                                        DNS

                                                        DNS

                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                        demuxrsquoed to DNS server DNS server replies to

                                                        client with IP address of wwwgooglecom

                                                        Comcast network 68800013

                                                        DNS server

                                                        DNSUDP

                                                        IPEthPhy

                                                        DNS

                                                        DNS

                                                        DNS

                                                        DNS

                                                        5 DataLink Layer 5-71

                                                        A day in the lifehellip TCP connection carrying HTTP

                                                        HTTPTCPIP

                                                        EthPhy

                                                        HTTP

                                                        to send HTTP request client first opens TCP socket to web server

                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                        TCP connection established

                                                        64233169105

                                                        web server

                                                        SYN

                                                        SYN

                                                        SYN

                                                        SYN

                                                        TCPIP

                                                        EthPhy

                                                        SYN

                                                        SYN

                                                        SYN

                                                        SYNACK

                                                        SYNACK

                                                        SYNACK

                                                        SYNACK

                                                        SYNACK

                                                        SYNACK

                                                        SYNACK

                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                        5 DataLink Layer 5-72

                                                        A day in the lifehellip HTTP requestreply

                                                        HTTPTCPIP

                                                        EthPhy

                                                        HTTP

                                                        HTTP request sent into TCP socket

                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                        IP datgram containing HTTP reply routed back to client

                                                        64233169105

                                                        web server

                                                        HTTPTCPIP

                                                        EthPhy

                                                        web server responds with HTTP reply (containing web page)

                                                        HTTP

                                                        HTTP

                                                        HTTPHTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        HTTP

                                                        web page finally () displayed

                                                        5 DataLink Layer 5-73

                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                        (except PHY) solid understanding of networking

                                                        principles practice hellip could stop here hellip but lots of

                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                        • Link Layer
                                                        • Link Layer Introduction
                                                        • Link layer context
                                                        • Encapsulation
                                                        • Link Layer Services
                                                        • Link Layer Services (more)
                                                        • Where is the link layer implemented
                                                        • Adaptors Communicating
                                                        • Slide 9
                                                        • Parity Checking
                                                        • Internet checksum (review)
                                                        • Checksumming Cyclic Redundancy Check
                                                        • Cyclic Redundancy Check
                                                        • Slide 14
                                                        • CRC Example
                                                        • Slide 16
                                                        • Multiple Access Links and Protocols
                                                        • Cable Network Architecture Overview
                                                        • Multiple Access protocols
                                                        • Ideal Multiple Access Protocol
                                                        • MAC Protocols a taxonomy
                                                        • Channel Partitioning MAC protocols TDMA
                                                        • Channel Partitioning MAC protocols FDMA
                                                        • Random Access Protocols
                                                        • ALOHA
                                                        • CSMA (Carrier Sense Multiple Access)
                                                        • CSMA collisions
                                                        • CSMACD (Collision Detection)
                                                        • CSMACD collision detection
                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                        • Slide 31
                                                        • Slide 32
                                                        • Summary of MAC protocols
                                                        • Slide 34
                                                        • MAC Addresses and ARP
                                                        • LAN Addresses and ARP
                                                        • LAN Address (more)
                                                        • ARP Address Resolution Protocol
                                                        • ARP protocol Same LAN (network)
                                                        • Addressing routing to another LAN
                                                        • Slide 41
                                                        • Slide 42
                                                        • Ethernet
                                                        • Star topology
                                                        • Ethernet Frame Structure
                                                        • Ethernet Frame Structure (more)
                                                        • Ethernet Unreliable connectionless
                                                        • Ethernet CSMACD algorithm
                                                        • Ethernetrsquos CSMACD (more)
                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                        • Manchester encoding
                                                        • Slide 52
                                                        • Hubs
                                                        • Switch
                                                        • Switch allows multiple simultaneous transmissions
                                                        • Switch Table
                                                        • Switch self-learning
                                                        • Switch frame filteringforwarding
                                                        • Self-learning forwarding example
                                                        • Interconnecting switches
                                                        • Self-learning multi-switch example
                                                        • Institutional network
                                                        • Switches vs Routers
                                                        • Slide 64
                                                        • Synthesis a day in the life of a web request
                                                        • A day in the life scenario
                                                        • A day in the lifehellip connecting to the Internet
                                                        • Slide 68
                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                        • A day in the lifehellip using DNS
                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                        • A day in the lifehellip HTTP requestreply
                                                        • Chapter 5 letrsquos take a breath

                                                          5 DataLink Layer 5-29

                                                          CSMACD collision detection

                                                          5 DataLink Layer 5-30

                                                          ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                          o share channel efficiently and fairly at high load

                                                          o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                          Random access MAC protocolso efficient at low load single node can fully

                                                          utilize channelo high load collision overhead

                                                          ldquotaking turnsrdquo protocolslook for best of both worlds

                                                          5 DataLink Layer 5-31

                                                          ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                          ldquoinvitesrdquo slave nodes to transmit in turn

                                                          typically used with ldquodumbrdquo slave devices

                                                          concernso polling overhead o latencyo single point of

                                                          failure (master)

                                                          master

                                                          slaves

                                                          poll

                                                          data

                                                          data

                                                          5 DataLink Layer 5-32

                                                          ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                          passed from one node to next sequentially

                                                          token message concerns

                                                          o token overhead o latencyo single point of failure

                                                          (token)

                                                          T

                                                          data

                                                          (nothingto send)

                                                          T

                                                          5 DataLink Layer 5-33

                                                          Summary of MAC protocols

                                                          channel partitioning by time frequency or codeo Time Division Frequency Division

                                                          random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                          hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                          taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                          5 DataLink Layer 5-34

                                                          Link Layer

                                                          51 Introduction and services

                                                          52 Error detection and correction

                                                          53Multiple access protocols

                                                          54 Link-Layer Addressing

                                                          55 Ethernet

                                                          56 Link-layer switches 59 A day in the life of

                                                          a web request

                                                          5 DataLink Layer 5-35

                                                          MAC Addresses and ARP

                                                          32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                          MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                          another physically-connected interface (in same network)

                                                          o 48 bit MAC addressbull burned in NIC ROM

                                                          5 DataLink Layer 5-36

                                                          LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                          Broadcast address =FF-FF-FF-FF-FF-FF

                                                          = adapter

                                                          1A-2F-BB-76-09-AD

                                                          58-23-D7-FA-20-B0

                                                          0C-C4-11-6F-E3-98

                                                          71-65-F7-2B-08-53

                                                          LAN(wired orwireless)

                                                          5 DataLink Layer 5-37

                                                          LAN Address (more)

                                                          MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                          space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                          o can move LAN card from one LAN to another

                                                          IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                          attached

                                                          5 DataLink Layer 5-38

                                                          ARP Address Resolution Protocol

                                                          Each IP node (host router) on LAN has ARP table

                                                          ARP table IPMAC address mappings for some LAN nodes

                                                          lt IP address MAC address TTLgto TTL (Time To Live)

                                                          time after which address mapping will be forgotten (typically 20 min)

                                                          Q given a nodersquos IP address how to determine its MAC address

                                                          1A-2F-BB-76-09-AD

                                                          0C-C4-11-6F-E3-98

                                                          71-65-F7-2B-08-53

                                                          LAN

                                                          137196723

                                                          137196778

                                                          137196714

                                                          137196788

                                                          5 DataLink Layer 5-39

                                                          ARP protocol Same LAN (network) A wants to send datagram

                                                          to B and Brsquos MAC address not in Arsquos ARP table

                                                          A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                          FF-FF-FF-FF-FFo all machines on LAN

                                                          receive ARP query B receives ARP packet

                                                          replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                          address (unicast)

                                                          A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                          ARP is ldquoplug-and-playrdquoo nodes create their

                                                          ARP tables without intervention from net administrator

                                                          5 DataLink Layer 5-40

                                                          Addressing routing to another LAN

                                                          R

                                                          1A-23-F9-CD-06-9B

                                                          222222222220111111111110

                                                          E6-E9-00-17-BB-4B

                                                          CC-49-DE-D0-AB-7D

                                                          111111111112

                                                          111111111111

                                                          A74-29-9C-E8-FF-55

                                                          222222222221

                                                          88-B2-2F-54-1A-0F

                                                          B222222222222

                                                          49-BD-D2-C7-56-2A

                                                          walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                          two ARP tables in router R one for each IP network (LAN)

                                                          LAN LAN

                                                          5 DataLink Layer 5-41

                                                          A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                          frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                          destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                          R

                                                          1A-23-F9-CD-06-9B

                                                          222222222220

                                                          111111111110

                                                          E6-E9-00-17-BB-4B

                                                          CC-49-DE-D0-AB-7D

                                                          111111111112

                                                          111111111111

                                                          A74-29-9C-E8-FF-55

                                                          222222222221

                                                          88-B2-2F-54-1A-0F

                                                          B222222222222

                                                          49-BD-D2-C7-56-2A

                                                          This is a really importantexample ndash make sure youunderstand

                                                          5 DataLink Layer 5-42

                                                          Link Layer

                                                          51 Introduction and services

                                                          52 Error detection and correction

                                                          53Multiple access protocols

                                                          54 Link-Layer Addressing

                                                          55 Ethernet

                                                          56 Link-layer switches 59 A day in the life of

                                                          a web request

                                                          5 DataLink Layer 5-43

                                                          Ethernet

                                                          ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                          Metcalfersquos Ethernetsketch

                                                          5 DataLink Layer 5-44

                                                          Star topology bus topology popular through mid 90s

                                                          o all nodes in same collision domain (can collide with each other)

                                                          today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                          (nodes do not collide with each other)

                                                          switch

                                                          bus coaxial cable star

                                                          5 DataLink Layer 5-45

                                                          Ethernet Frame Structure

                                                          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                          Preamble 7 bytes with pattern 10101010 followed by one

                                                          byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                          rates

                                                          5 DataLink Layer 5-46

                                                          Ethernet Frame Structure (more) Addresses 6 bytes

                                                          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                          o otherwise adapter discards frame

                                                          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                          CRC checked at receiver if error is detected frame is dropped

                                                          5 DataLink Layer 5-47

                                                          Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                          Ethernetrsquos MAC protocol CSMACD

                                                          5 DataLink Layer 5-48

                                                          Ethernet CSMACD algorithm

                                                          1 NIC receives datagram from network layer creates frame

                                                          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                          4 If NIC detects collision while transmitting aborts and sends jam signal

                                                          5 After aborting NIC enters exponential backoff

                                                          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                          (1 slot = 512 bit-times)

                                                          5 DataLink Layer 5-49

                                                          Ethernetrsquos CSMACD (more)

                                                          Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                          Exponential Backoff Goal adapt retransmission

                                                          attempts to estimated current loado heavy load random

                                                          wait will be longer first collision choose K

                                                          from 01 delay is K 512 bit transmission times

                                                          after second collision choose K from 0123hellip

                                                          after ten collisions choose K from 01234hellip1023

                                                          5 DataLink Layer 5-50

                                                          8023 Ethernet Standards Link amp Physical Layers

                                                          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                          Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                          applicationtransportnetwork

                                                          linkphysical

                                                          MAC protocoland frame format

                                                          100BASE-TX

                                                          100BASE-T4

                                                          100BASE-FX100BASE-T2

                                                          100BASE-SX 100BASE-BX

                                                          fiber physical layercopper (twisterpair) physical layer

                                                          5 DataLink Layer 5-51

                                                          Manchester encoding

                                                          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                          synchronize to each othero no need for a centralized global clock among nodes

                                                          Hey this is physical-layer stuff

                                                          5 DataLink Layer 5-52

                                                          Link Layer

                                                          51 Introduction and services

                                                          52 Error detection and correction

                                                          53 Multiple access protocols

                                                          54 Link-layer Addressing

                                                          55 Ethernet

                                                          56 Link-layer switches LANs

                                                          59 A day in the life of a web request

                                                          5 DataLink Layer 5-53

                                                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                          o bits coming in one link go out all other links at same rate

                                                          o all nodes connected to hub can collide with one another

                                                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                          twisted pair

                                                          hub

                                                          5 DataLink Layer 5-54

                                                          Switch link-layer device smarter than hubs take active

                                                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                          transparento hosts are unaware of presence of switches

                                                          plug-and-play self-learningo switches do not need to be configured

                                                          5 DataLink Layer 5-55

                                                          Switch allows multiple simultaneous transmissions

                                                          hosts have dedicated direct connection to switch

                                                          switches buffer packets Ethernet protocol used on

                                                          each incoming link but no collisions full duplexo each link is its own collision

                                                          domain switching A-to-Arsquo and B-

                                                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                          A

                                                          Arsquo

                                                          B

                                                          Brsquo

                                                          C

                                                          Crsquo

                                                          switch with six interfaces(123456)

                                                          1 23

                                                          45

                                                          6

                                                          5 DataLink Layer 5-56

                                                          Switch Table

                                                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                          A each switch has a switch table each entryo (MAC address of host interface

                                                          to reach host time stamp)

                                                          looks like a routing table Q how are entries created

                                                          maintained in switch table o something like a routing

                                                          protocol

                                                          A

                                                          Arsquo

                                                          B

                                                          Brsquo

                                                          C

                                                          Crsquo

                                                          switch with six interfaces(123456)

                                                          1 23

                                                          45

                                                          6

                                                          5 DataLink Layer 5-57

                                                          Switch self-learning

                                                          switch learns which hosts can be reached through which interfaceso when frame received

                                                          switch ldquolearnsrdquo location of sender incoming LAN segment

                                                          o records senderlocation pair in switch table

                                                          A

                                                          Arsquo

                                                          B

                                                          Brsquo

                                                          C

                                                          Crsquo

                                                          1 23

                                                          45

                                                          6

                                                          A Arsquo

                                                          Source ADest Arsquo

                                                          MAC addr interface TTL

                                                          Switch table (initially empty)

                                                          A 1 60

                                                          5 DataLink Layer 5-58

                                                          Switch frame filteringforwardingWhen frame received

                                                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                          then if dest on segment from which frame arrived

                                                          then drop the frame else forward the frame on interface indicated else flood

                                                          forward on all but the interface on which the frame arrived

                                                          5 DataLink Layer 5-59

                                                          Self-learning forwarding example

                                                          A

                                                          Arsquo

                                                          B

                                                          Brsquo

                                                          C

                                                          Crsquo

                                                          1 23

                                                          45

                                                          6

                                                          A Arsquo

                                                          Source ADest Arsquo

                                                          MAC addr interface TTL

                                                          Switch table (initially empty)

                                                          A 1 60

                                                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                          frame destination unknownflood

                                                          Arsquo A

                                                          destination A location known

                                                          Arsquo 4 60

                                                          selective send

                                                          5 DataLink Layer 5-60

                                                          Interconnecting switches

                                                          switches can be connected together

                                                          A

                                                          B

                                                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                          A self learning (works exactly the same as in single-switch case)

                                                          S1

                                                          C D

                                                          E

                                                          FS2

                                                          S4

                                                          S3

                                                          H

                                                          I

                                                          G

                                                          5 DataLink Layer 5-61

                                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                          A

                                                          B

                                                          S1

                                                          C D

                                                          E

                                                          FS2

                                                          S4

                                                          S3

                                                          H

                                                          I

                                                          G

                                                          1

                                                          2 3

                                                          34

                                                          5 DataLink Layer 5-62

                                                          Institutional network

                                                          to externalnetwork

                                                          router

                                                          IP subnet

                                                          mail server

                                                          web server

                                                          5 DataLink Layer 5-63

                                                          Switches vs Routers both store-and-forward devices

                                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                          routers maintain routing tables implement routing algorithms

                                                          switches maintain switch tables implement filtering learning algorithms

                                                          Switch

                                                          5 DataLink Layer 5-64

                                                          Link Layer

                                                          51 Introduction and services

                                                          52 Error detection and correction

                                                          53Multiple access protocols

                                                          54 Link-Layer Addressing

                                                          55 Ethernet

                                                          56 Link-layer switches 59 A day in the life of

                                                          a web request

                                                          5 DataLink Layer 5-65

                                                          Synthesis a day in the life of a web request

                                                          journey down protocol stack completeo application transport network link

                                                          putting-it-all-together synthesiso goal identify review understand protocols

                                                          (at all layers) involved in seemingly simple scenario requesting www page

                                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                          5 DataLink Layer 5-66

                                                          A day in the life scenario

                                                          Comcast network 68800013

                                                          Googlersquos network 64233160019 64233169105

                                                          web server

                                                          DNS server

                                                          school network 68802024

                                                          browser

                                                          web page

                                                          5 DataLink Layer 5-67

                                                          A day in the lifehellip connecting to the Internet

                                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                          router(runs DHCP)

                                                          DHCPUDP

                                                          IPEthPhy

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCPUDP

                                                          IPEthPhy

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCPDHCP

                                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                          5 DataLink Layer 5-68

                                                          A day in the lifehellip connecting to the Internet

                                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                          router(runs DHCP)

                                                          DHCPUDP

                                                          IPEthPhy

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCPUDP

                                                          IPEthPhy

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          DHCP

                                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                          DHCP client receives DHCP ACK reply

                                                          5 DataLink Layer 5-69

                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                          DNSUDP

                                                          IPEthPhy

                                                          DNS

                                                          DNS

                                                          DNS

                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                          ARP query

                                                          EthPhy

                                                          ARP

                                                          ARP

                                                          ARP reply

                                                          5 DataLink Layer 5-70

                                                          A day in the lifehellip using DNS

                                                          DNSUDP

                                                          IPEthPhy

                                                          DNS

                                                          DNS

                                                          DNS

                                                          DNS

                                                          DNS

                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                          demuxrsquoed to DNS server DNS server replies to

                                                          client with IP address of wwwgooglecom

                                                          Comcast network 68800013

                                                          DNS server

                                                          DNSUDP

                                                          IPEthPhy

                                                          DNS

                                                          DNS

                                                          DNS

                                                          DNS

                                                          5 DataLink Layer 5-71

                                                          A day in the lifehellip TCP connection carrying HTTP

                                                          HTTPTCPIP

                                                          EthPhy

                                                          HTTP

                                                          to send HTTP request client first opens TCP socket to web server

                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                          TCP connection established

                                                          64233169105

                                                          web server

                                                          SYN

                                                          SYN

                                                          SYN

                                                          SYN

                                                          TCPIP

                                                          EthPhy

                                                          SYN

                                                          SYN

                                                          SYN

                                                          SYNACK

                                                          SYNACK

                                                          SYNACK

                                                          SYNACK

                                                          SYNACK

                                                          SYNACK

                                                          SYNACK

                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                          5 DataLink Layer 5-72

                                                          A day in the lifehellip HTTP requestreply

                                                          HTTPTCPIP

                                                          EthPhy

                                                          HTTP

                                                          HTTP request sent into TCP socket

                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                          IP datgram containing HTTP reply routed back to client

                                                          64233169105

                                                          web server

                                                          HTTPTCPIP

                                                          EthPhy

                                                          web server responds with HTTP reply (containing web page)

                                                          HTTP

                                                          HTTP

                                                          HTTPHTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          HTTP

                                                          web page finally () displayed

                                                          5 DataLink Layer 5-73

                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                          (except PHY) solid understanding of networking

                                                          principles practice hellip could stop here hellip but lots of

                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                          • Link Layer
                                                          • Link Layer Introduction
                                                          • Link layer context
                                                          • Encapsulation
                                                          • Link Layer Services
                                                          • Link Layer Services (more)
                                                          • Where is the link layer implemented
                                                          • Adaptors Communicating
                                                          • Slide 9
                                                          • Parity Checking
                                                          • Internet checksum (review)
                                                          • Checksumming Cyclic Redundancy Check
                                                          • Cyclic Redundancy Check
                                                          • Slide 14
                                                          • CRC Example
                                                          • Slide 16
                                                          • Multiple Access Links and Protocols
                                                          • Cable Network Architecture Overview
                                                          • Multiple Access protocols
                                                          • Ideal Multiple Access Protocol
                                                          • MAC Protocols a taxonomy
                                                          • Channel Partitioning MAC protocols TDMA
                                                          • Channel Partitioning MAC protocols FDMA
                                                          • Random Access Protocols
                                                          • ALOHA
                                                          • CSMA (Carrier Sense Multiple Access)
                                                          • CSMA collisions
                                                          • CSMACD (Collision Detection)
                                                          • CSMACD collision detection
                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                          • Slide 31
                                                          • Slide 32
                                                          • Summary of MAC protocols
                                                          • Slide 34
                                                          • MAC Addresses and ARP
                                                          • LAN Addresses and ARP
                                                          • LAN Address (more)
                                                          • ARP Address Resolution Protocol
                                                          • ARP protocol Same LAN (network)
                                                          • Addressing routing to another LAN
                                                          • Slide 41
                                                          • Slide 42
                                                          • Ethernet
                                                          • Star topology
                                                          • Ethernet Frame Structure
                                                          • Ethernet Frame Structure (more)
                                                          • Ethernet Unreliable connectionless
                                                          • Ethernet CSMACD algorithm
                                                          • Ethernetrsquos CSMACD (more)
                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                          • Manchester encoding
                                                          • Slide 52
                                                          • Hubs
                                                          • Switch
                                                          • Switch allows multiple simultaneous transmissions
                                                          • Switch Table
                                                          • Switch self-learning
                                                          • Switch frame filteringforwarding
                                                          • Self-learning forwarding example
                                                          • Interconnecting switches
                                                          • Self-learning multi-switch example
                                                          • Institutional network
                                                          • Switches vs Routers
                                                          • Slide 64
                                                          • Synthesis a day in the life of a web request
                                                          • A day in the life scenario
                                                          • A day in the lifehellip connecting to the Internet
                                                          • Slide 68
                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                          • A day in the lifehellip using DNS
                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                          • A day in the lifehellip HTTP requestreply
                                                          • Chapter 5 letrsquos take a breath

                                                            5 DataLink Layer 5-30

                                                            ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols

                                                            o share channel efficiently and fairly at high load

                                                            o inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

                                                            Random access MAC protocolso efficient at low load single node can fully

                                                            utilize channelo high load collision overhead

                                                            ldquotaking turnsrdquo protocolslook for best of both worlds

                                                            5 DataLink Layer 5-31

                                                            ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                            ldquoinvitesrdquo slave nodes to transmit in turn

                                                            typically used with ldquodumbrdquo slave devices

                                                            concernso polling overhead o latencyo single point of

                                                            failure (master)

                                                            master

                                                            slaves

                                                            poll

                                                            data

                                                            data

                                                            5 DataLink Layer 5-32

                                                            ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                            passed from one node to next sequentially

                                                            token message concerns

                                                            o token overhead o latencyo single point of failure

                                                            (token)

                                                            T

                                                            data

                                                            (nothingto send)

                                                            T

                                                            5 DataLink Layer 5-33

                                                            Summary of MAC protocols

                                                            channel partitioning by time frequency or codeo Time Division Frequency Division

                                                            random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                            hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                            taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                            5 DataLink Layer 5-34

                                                            Link Layer

                                                            51 Introduction and services

                                                            52 Error detection and correction

                                                            53Multiple access protocols

                                                            54 Link-Layer Addressing

                                                            55 Ethernet

                                                            56 Link-layer switches 59 A day in the life of

                                                            a web request

                                                            5 DataLink Layer 5-35

                                                            MAC Addresses and ARP

                                                            32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                            MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                            another physically-connected interface (in same network)

                                                            o 48 bit MAC addressbull burned in NIC ROM

                                                            5 DataLink Layer 5-36

                                                            LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                            Broadcast address =FF-FF-FF-FF-FF-FF

                                                            = adapter

                                                            1A-2F-BB-76-09-AD

                                                            58-23-D7-FA-20-B0

                                                            0C-C4-11-6F-E3-98

                                                            71-65-F7-2B-08-53

                                                            LAN(wired orwireless)

                                                            5 DataLink Layer 5-37

                                                            LAN Address (more)

                                                            MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                            space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                            o can move LAN card from one LAN to another

                                                            IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                            attached

                                                            5 DataLink Layer 5-38

                                                            ARP Address Resolution Protocol

                                                            Each IP node (host router) on LAN has ARP table

                                                            ARP table IPMAC address mappings for some LAN nodes

                                                            lt IP address MAC address TTLgto TTL (Time To Live)

                                                            time after which address mapping will be forgotten (typically 20 min)

                                                            Q given a nodersquos IP address how to determine its MAC address

                                                            1A-2F-BB-76-09-AD

                                                            0C-C4-11-6F-E3-98

                                                            71-65-F7-2B-08-53

                                                            LAN

                                                            137196723

                                                            137196778

                                                            137196714

                                                            137196788

                                                            5 DataLink Layer 5-39

                                                            ARP protocol Same LAN (network) A wants to send datagram

                                                            to B and Brsquos MAC address not in Arsquos ARP table

                                                            A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                            FF-FF-FF-FF-FFo all machines on LAN

                                                            receive ARP query B receives ARP packet

                                                            replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                            address (unicast)

                                                            A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                            ARP is ldquoplug-and-playrdquoo nodes create their

                                                            ARP tables without intervention from net administrator

                                                            5 DataLink Layer 5-40

                                                            Addressing routing to another LAN

                                                            R

                                                            1A-23-F9-CD-06-9B

                                                            222222222220111111111110

                                                            E6-E9-00-17-BB-4B

                                                            CC-49-DE-D0-AB-7D

                                                            111111111112

                                                            111111111111

                                                            A74-29-9C-E8-FF-55

                                                            222222222221

                                                            88-B2-2F-54-1A-0F

                                                            B222222222222

                                                            49-BD-D2-C7-56-2A

                                                            walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                            two ARP tables in router R one for each IP network (LAN)

                                                            LAN LAN

                                                            5 DataLink Layer 5-41

                                                            A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                            frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                            destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                            R

                                                            1A-23-F9-CD-06-9B

                                                            222222222220

                                                            111111111110

                                                            E6-E9-00-17-BB-4B

                                                            CC-49-DE-D0-AB-7D

                                                            111111111112

                                                            111111111111

                                                            A74-29-9C-E8-FF-55

                                                            222222222221

                                                            88-B2-2F-54-1A-0F

                                                            B222222222222

                                                            49-BD-D2-C7-56-2A

                                                            This is a really importantexample ndash make sure youunderstand

                                                            5 DataLink Layer 5-42

                                                            Link Layer

                                                            51 Introduction and services

                                                            52 Error detection and correction

                                                            53Multiple access protocols

                                                            54 Link-Layer Addressing

                                                            55 Ethernet

                                                            56 Link-layer switches 59 A day in the life of

                                                            a web request

                                                            5 DataLink Layer 5-43

                                                            Ethernet

                                                            ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                            Metcalfersquos Ethernetsketch

                                                            5 DataLink Layer 5-44

                                                            Star topology bus topology popular through mid 90s

                                                            o all nodes in same collision domain (can collide with each other)

                                                            today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                            (nodes do not collide with each other)

                                                            switch

                                                            bus coaxial cable star

                                                            5 DataLink Layer 5-45

                                                            Ethernet Frame Structure

                                                            Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                            Preamble 7 bytes with pattern 10101010 followed by one

                                                            byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                            rates

                                                            5 DataLink Layer 5-46

                                                            Ethernet Frame Structure (more) Addresses 6 bytes

                                                            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                            o otherwise adapter discards frame

                                                            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                            CRC checked at receiver if error is detected frame is dropped

                                                            5 DataLink Layer 5-47

                                                            Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                            Ethernetrsquos MAC protocol CSMACD

                                                            5 DataLink Layer 5-48

                                                            Ethernet CSMACD algorithm

                                                            1 NIC receives datagram from network layer creates frame

                                                            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                            4 If NIC detects collision while transmitting aborts and sends jam signal

                                                            5 After aborting NIC enters exponential backoff

                                                            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                            (1 slot = 512 bit-times)

                                                            5 DataLink Layer 5-49

                                                            Ethernetrsquos CSMACD (more)

                                                            Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                            Exponential Backoff Goal adapt retransmission

                                                            attempts to estimated current loado heavy load random

                                                            wait will be longer first collision choose K

                                                            from 01 delay is K 512 bit transmission times

                                                            after second collision choose K from 0123hellip

                                                            after ten collisions choose K from 01234hellip1023

                                                            5 DataLink Layer 5-50

                                                            8023 Ethernet Standards Link amp Physical Layers

                                                            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                            Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                            applicationtransportnetwork

                                                            linkphysical

                                                            MAC protocoland frame format

                                                            100BASE-TX

                                                            100BASE-T4

                                                            100BASE-FX100BASE-T2

                                                            100BASE-SX 100BASE-BX

                                                            fiber physical layercopper (twisterpair) physical layer

                                                            5 DataLink Layer 5-51

                                                            Manchester encoding

                                                            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                            synchronize to each othero no need for a centralized global clock among nodes

                                                            Hey this is physical-layer stuff

                                                            5 DataLink Layer 5-52

                                                            Link Layer

                                                            51 Introduction and services

                                                            52 Error detection and correction

                                                            53 Multiple access protocols

                                                            54 Link-layer Addressing

                                                            55 Ethernet

                                                            56 Link-layer switches LANs

                                                            59 A day in the life of a web request

                                                            5 DataLink Layer 5-53

                                                            Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                            o bits coming in one link go out all other links at same rate

                                                            o all nodes connected to hub can collide with one another

                                                            o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                            twisted pair

                                                            hub

                                                            5 DataLink Layer 5-54

                                                            Switch link-layer device smarter than hubs take active

                                                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                            transparento hosts are unaware of presence of switches

                                                            plug-and-play self-learningo switches do not need to be configured

                                                            5 DataLink Layer 5-55

                                                            Switch allows multiple simultaneous transmissions

                                                            hosts have dedicated direct connection to switch

                                                            switches buffer packets Ethernet protocol used on

                                                            each incoming link but no collisions full duplexo each link is its own collision

                                                            domain switching A-to-Arsquo and B-

                                                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                            A

                                                            Arsquo

                                                            B

                                                            Brsquo

                                                            C

                                                            Crsquo

                                                            switch with six interfaces(123456)

                                                            1 23

                                                            45

                                                            6

                                                            5 DataLink Layer 5-56

                                                            Switch Table

                                                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                            A each switch has a switch table each entryo (MAC address of host interface

                                                            to reach host time stamp)

                                                            looks like a routing table Q how are entries created

                                                            maintained in switch table o something like a routing

                                                            protocol

                                                            A

                                                            Arsquo

                                                            B

                                                            Brsquo

                                                            C

                                                            Crsquo

                                                            switch with six interfaces(123456)

                                                            1 23

                                                            45

                                                            6

                                                            5 DataLink Layer 5-57

                                                            Switch self-learning

                                                            switch learns which hosts can be reached through which interfaceso when frame received

                                                            switch ldquolearnsrdquo location of sender incoming LAN segment

                                                            o records senderlocation pair in switch table

                                                            A

                                                            Arsquo

                                                            B

                                                            Brsquo

                                                            C

                                                            Crsquo

                                                            1 23

                                                            45

                                                            6

                                                            A Arsquo

                                                            Source ADest Arsquo

                                                            MAC addr interface TTL

                                                            Switch table (initially empty)

                                                            A 1 60

                                                            5 DataLink Layer 5-58

                                                            Switch frame filteringforwardingWhen frame received

                                                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                            then if dest on segment from which frame arrived

                                                            then drop the frame else forward the frame on interface indicated else flood

                                                            forward on all but the interface on which the frame arrived

                                                            5 DataLink Layer 5-59

                                                            Self-learning forwarding example

                                                            A

                                                            Arsquo

                                                            B

                                                            Brsquo

                                                            C

                                                            Crsquo

                                                            1 23

                                                            45

                                                            6

                                                            A Arsquo

                                                            Source ADest Arsquo

                                                            MAC addr interface TTL

                                                            Switch table (initially empty)

                                                            A 1 60

                                                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                            frame destination unknownflood

                                                            Arsquo A

                                                            destination A location known

                                                            Arsquo 4 60

                                                            selective send

                                                            5 DataLink Layer 5-60

                                                            Interconnecting switches

                                                            switches can be connected together

                                                            A

                                                            B

                                                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                            A self learning (works exactly the same as in single-switch case)

                                                            S1

                                                            C D

                                                            E

                                                            FS2

                                                            S4

                                                            S3

                                                            H

                                                            I

                                                            G

                                                            5 DataLink Layer 5-61

                                                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                            A

                                                            B

                                                            S1

                                                            C D

                                                            E

                                                            FS2

                                                            S4

                                                            S3

                                                            H

                                                            I

                                                            G

                                                            1

                                                            2 3

                                                            34

                                                            5 DataLink Layer 5-62

                                                            Institutional network

                                                            to externalnetwork

                                                            router

                                                            IP subnet

                                                            mail server

                                                            web server

                                                            5 DataLink Layer 5-63

                                                            Switches vs Routers both store-and-forward devices

                                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                            routers maintain routing tables implement routing algorithms

                                                            switches maintain switch tables implement filtering learning algorithms

                                                            Switch

                                                            5 DataLink Layer 5-64

                                                            Link Layer

                                                            51 Introduction and services

                                                            52 Error detection and correction

                                                            53Multiple access protocols

                                                            54 Link-Layer Addressing

                                                            55 Ethernet

                                                            56 Link-layer switches 59 A day in the life of

                                                            a web request

                                                            5 DataLink Layer 5-65

                                                            Synthesis a day in the life of a web request

                                                            journey down protocol stack completeo application transport network link

                                                            putting-it-all-together synthesiso goal identify review understand protocols

                                                            (at all layers) involved in seemingly simple scenario requesting www page

                                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                            5 DataLink Layer 5-66

                                                            A day in the life scenario

                                                            Comcast network 68800013

                                                            Googlersquos network 64233160019 64233169105

                                                            web server

                                                            DNS server

                                                            school network 68802024

                                                            browser

                                                            web page

                                                            5 DataLink Layer 5-67

                                                            A day in the lifehellip connecting to the Internet

                                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                            router(runs DHCP)

                                                            DHCPUDP

                                                            IPEthPhy

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCPUDP

                                                            IPEthPhy

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCPDHCP

                                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                            5 DataLink Layer 5-68

                                                            A day in the lifehellip connecting to the Internet

                                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                            router(runs DHCP)

                                                            DHCPUDP

                                                            IPEthPhy

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCPUDP

                                                            IPEthPhy

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            DHCP

                                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                            DHCP client receives DHCP ACK reply

                                                            5 DataLink Layer 5-69

                                                            A day in the lifehellip ARP (before DNS before HTTP)

                                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                                            DNSUDP

                                                            IPEthPhy

                                                            DNS

                                                            DNS

                                                            DNS

                                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                            ARP query

                                                            EthPhy

                                                            ARP

                                                            ARP

                                                            ARP reply

                                                            5 DataLink Layer 5-70

                                                            A day in the lifehellip using DNS

                                                            DNSUDP

                                                            IPEthPhy

                                                            DNS

                                                            DNS

                                                            DNS

                                                            DNS

                                                            DNS

                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                            demuxrsquoed to DNS server DNS server replies to

                                                            client with IP address of wwwgooglecom

                                                            Comcast network 68800013

                                                            DNS server

                                                            DNSUDP

                                                            IPEthPhy

                                                            DNS

                                                            DNS

                                                            DNS

                                                            DNS

                                                            5 DataLink Layer 5-71

                                                            A day in the lifehellip TCP connection carrying HTTP

                                                            HTTPTCPIP

                                                            EthPhy

                                                            HTTP

                                                            to send HTTP request client first opens TCP socket to web server

                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                            TCP connection established

                                                            64233169105

                                                            web server

                                                            SYN

                                                            SYN

                                                            SYN

                                                            SYN

                                                            TCPIP

                                                            EthPhy

                                                            SYN

                                                            SYN

                                                            SYN

                                                            SYNACK

                                                            SYNACK

                                                            SYNACK

                                                            SYNACK

                                                            SYNACK

                                                            SYNACK

                                                            SYNACK

                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                            5 DataLink Layer 5-72

                                                            A day in the lifehellip HTTP requestreply

                                                            HTTPTCPIP

                                                            EthPhy

                                                            HTTP

                                                            HTTP request sent into TCP socket

                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                            IP datgram containing HTTP reply routed back to client

                                                            64233169105

                                                            web server

                                                            HTTPTCPIP

                                                            EthPhy

                                                            web server responds with HTTP reply (containing web page)

                                                            HTTP

                                                            HTTP

                                                            HTTPHTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            HTTP

                                                            web page finally () displayed

                                                            5 DataLink Layer 5-73

                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                            (except PHY) solid understanding of networking

                                                            principles practice hellip could stop here hellip but lots of

                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                            • Link Layer
                                                            • Link Layer Introduction
                                                            • Link layer context
                                                            • Encapsulation
                                                            • Link Layer Services
                                                            • Link Layer Services (more)
                                                            • Where is the link layer implemented
                                                            • Adaptors Communicating
                                                            • Slide 9
                                                            • Parity Checking
                                                            • Internet checksum (review)
                                                            • Checksumming Cyclic Redundancy Check
                                                            • Cyclic Redundancy Check
                                                            • Slide 14
                                                            • CRC Example
                                                            • Slide 16
                                                            • Multiple Access Links and Protocols
                                                            • Cable Network Architecture Overview
                                                            • Multiple Access protocols
                                                            • Ideal Multiple Access Protocol
                                                            • MAC Protocols a taxonomy
                                                            • Channel Partitioning MAC protocols TDMA
                                                            • Channel Partitioning MAC protocols FDMA
                                                            • Random Access Protocols
                                                            • ALOHA
                                                            • CSMA (Carrier Sense Multiple Access)
                                                            • CSMA collisions
                                                            • CSMACD (Collision Detection)
                                                            • CSMACD collision detection
                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                            • Slide 31
                                                            • Slide 32
                                                            • Summary of MAC protocols
                                                            • Slide 34
                                                            • MAC Addresses and ARP
                                                            • LAN Addresses and ARP
                                                            • LAN Address (more)
                                                            • ARP Address Resolution Protocol
                                                            • ARP protocol Same LAN (network)
                                                            • Addressing routing to another LAN
                                                            • Slide 41
                                                            • Slide 42
                                                            • Ethernet
                                                            • Star topology
                                                            • Ethernet Frame Structure
                                                            • Ethernet Frame Structure (more)
                                                            • Ethernet Unreliable connectionless
                                                            • Ethernet CSMACD algorithm
                                                            • Ethernetrsquos CSMACD (more)
                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                            • Manchester encoding
                                                            • Slide 52
                                                            • Hubs
                                                            • Switch
                                                            • Switch allows multiple simultaneous transmissions
                                                            • Switch Table
                                                            • Switch self-learning
                                                            • Switch frame filteringforwarding
                                                            • Self-learning forwarding example
                                                            • Interconnecting switches
                                                            • Self-learning multi-switch example
                                                            • Institutional network
                                                            • Switches vs Routers
                                                            • Slide 64
                                                            • Synthesis a day in the life of a web request
                                                            • A day in the life scenario
                                                            • A day in the lifehellip connecting to the Internet
                                                            • Slide 68
                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                            • A day in the lifehellip using DNS
                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                            • A day in the lifehellip HTTP requestreply
                                                            • Chapter 5 letrsquos take a breath

                                                              5 DataLink Layer 5-31

                                                              ldquoTaking Turnsrdquo MAC protocolsPolling master node

                                                              ldquoinvitesrdquo slave nodes to transmit in turn

                                                              typically used with ldquodumbrdquo slave devices

                                                              concernso polling overhead o latencyo single point of

                                                              failure (master)

                                                              master

                                                              slaves

                                                              poll

                                                              data

                                                              data

                                                              5 DataLink Layer 5-32

                                                              ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                              passed from one node to next sequentially

                                                              token message concerns

                                                              o token overhead o latencyo single point of failure

                                                              (token)

                                                              T

                                                              data

                                                              (nothingto send)

                                                              T

                                                              5 DataLink Layer 5-33

                                                              Summary of MAC protocols

                                                              channel partitioning by time frequency or codeo Time Division Frequency Division

                                                              random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                              hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                              taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                              5 DataLink Layer 5-34

                                                              Link Layer

                                                              51 Introduction and services

                                                              52 Error detection and correction

                                                              53Multiple access protocols

                                                              54 Link-Layer Addressing

                                                              55 Ethernet

                                                              56 Link-layer switches 59 A day in the life of

                                                              a web request

                                                              5 DataLink Layer 5-35

                                                              MAC Addresses and ARP

                                                              32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                              MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                              another physically-connected interface (in same network)

                                                              o 48 bit MAC addressbull burned in NIC ROM

                                                              5 DataLink Layer 5-36

                                                              LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                              Broadcast address =FF-FF-FF-FF-FF-FF

                                                              = adapter

                                                              1A-2F-BB-76-09-AD

                                                              58-23-D7-FA-20-B0

                                                              0C-C4-11-6F-E3-98

                                                              71-65-F7-2B-08-53

                                                              LAN(wired orwireless)

                                                              5 DataLink Layer 5-37

                                                              LAN Address (more)

                                                              MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                              space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                              o can move LAN card from one LAN to another

                                                              IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                              attached

                                                              5 DataLink Layer 5-38

                                                              ARP Address Resolution Protocol

                                                              Each IP node (host router) on LAN has ARP table

                                                              ARP table IPMAC address mappings for some LAN nodes

                                                              lt IP address MAC address TTLgto TTL (Time To Live)

                                                              time after which address mapping will be forgotten (typically 20 min)

                                                              Q given a nodersquos IP address how to determine its MAC address

                                                              1A-2F-BB-76-09-AD

                                                              0C-C4-11-6F-E3-98

                                                              71-65-F7-2B-08-53

                                                              LAN

                                                              137196723

                                                              137196778

                                                              137196714

                                                              137196788

                                                              5 DataLink Layer 5-39

                                                              ARP protocol Same LAN (network) A wants to send datagram

                                                              to B and Brsquos MAC address not in Arsquos ARP table

                                                              A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                              FF-FF-FF-FF-FFo all machines on LAN

                                                              receive ARP query B receives ARP packet

                                                              replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                              address (unicast)

                                                              A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                              ARP is ldquoplug-and-playrdquoo nodes create their

                                                              ARP tables without intervention from net administrator

                                                              5 DataLink Layer 5-40

                                                              Addressing routing to another LAN

                                                              R

                                                              1A-23-F9-CD-06-9B

                                                              222222222220111111111110

                                                              E6-E9-00-17-BB-4B

                                                              CC-49-DE-D0-AB-7D

                                                              111111111112

                                                              111111111111

                                                              A74-29-9C-E8-FF-55

                                                              222222222221

                                                              88-B2-2F-54-1A-0F

                                                              B222222222222

                                                              49-BD-D2-C7-56-2A

                                                              walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                              two ARP tables in router R one for each IP network (LAN)

                                                              LAN LAN

                                                              5 DataLink Layer 5-41

                                                              A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                              frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                              destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                              R

                                                              1A-23-F9-CD-06-9B

                                                              222222222220

                                                              111111111110

                                                              E6-E9-00-17-BB-4B

                                                              CC-49-DE-D0-AB-7D

                                                              111111111112

                                                              111111111111

                                                              A74-29-9C-E8-FF-55

                                                              222222222221

                                                              88-B2-2F-54-1A-0F

                                                              B222222222222

                                                              49-BD-D2-C7-56-2A

                                                              This is a really importantexample ndash make sure youunderstand

                                                              5 DataLink Layer 5-42

                                                              Link Layer

                                                              51 Introduction and services

                                                              52 Error detection and correction

                                                              53Multiple access protocols

                                                              54 Link-Layer Addressing

                                                              55 Ethernet

                                                              56 Link-layer switches 59 A day in the life of

                                                              a web request

                                                              5 DataLink Layer 5-43

                                                              Ethernet

                                                              ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                              Metcalfersquos Ethernetsketch

                                                              5 DataLink Layer 5-44

                                                              Star topology bus topology popular through mid 90s

                                                              o all nodes in same collision domain (can collide with each other)

                                                              today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                              (nodes do not collide with each other)

                                                              switch

                                                              bus coaxial cable star

                                                              5 DataLink Layer 5-45

                                                              Ethernet Frame Structure

                                                              Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                              Preamble 7 bytes with pattern 10101010 followed by one

                                                              byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                              rates

                                                              5 DataLink Layer 5-46

                                                              Ethernet Frame Structure (more) Addresses 6 bytes

                                                              o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                              o otherwise adapter discards frame

                                                              Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                              CRC checked at receiver if error is detected frame is dropped

                                                              5 DataLink Layer 5-47

                                                              Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                              Ethernetrsquos MAC protocol CSMACD

                                                              5 DataLink Layer 5-48

                                                              Ethernet CSMACD algorithm

                                                              1 NIC receives datagram from network layer creates frame

                                                              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                              4 If NIC detects collision while transmitting aborts and sends jam signal

                                                              5 After aborting NIC enters exponential backoff

                                                              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                              (1 slot = 512 bit-times)

                                                              5 DataLink Layer 5-49

                                                              Ethernetrsquos CSMACD (more)

                                                              Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                              Exponential Backoff Goal adapt retransmission

                                                              attempts to estimated current loado heavy load random

                                                              wait will be longer first collision choose K

                                                              from 01 delay is K 512 bit transmission times

                                                              after second collision choose K from 0123hellip

                                                              after ten collisions choose K from 01234hellip1023

                                                              5 DataLink Layer 5-50

                                                              8023 Ethernet Standards Link amp Physical Layers

                                                              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                              Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                              applicationtransportnetwork

                                                              linkphysical

                                                              MAC protocoland frame format

                                                              100BASE-TX

                                                              100BASE-T4

                                                              100BASE-FX100BASE-T2

                                                              100BASE-SX 100BASE-BX

                                                              fiber physical layercopper (twisterpair) physical layer

                                                              5 DataLink Layer 5-51

                                                              Manchester encoding

                                                              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                              synchronize to each othero no need for a centralized global clock among nodes

                                                              Hey this is physical-layer stuff

                                                              5 DataLink Layer 5-52

                                                              Link Layer

                                                              51 Introduction and services

                                                              52 Error detection and correction

                                                              53 Multiple access protocols

                                                              54 Link-layer Addressing

                                                              55 Ethernet

                                                              56 Link-layer switches LANs

                                                              59 A day in the life of a web request

                                                              5 DataLink Layer 5-53

                                                              Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                              o bits coming in one link go out all other links at same rate

                                                              o all nodes connected to hub can collide with one another

                                                              o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                              twisted pair

                                                              hub

                                                              5 DataLink Layer 5-54

                                                              Switch link-layer device smarter than hubs take active

                                                              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                              transparento hosts are unaware of presence of switches

                                                              plug-and-play self-learningo switches do not need to be configured

                                                              5 DataLink Layer 5-55

                                                              Switch allows multiple simultaneous transmissions

                                                              hosts have dedicated direct connection to switch

                                                              switches buffer packets Ethernet protocol used on

                                                              each incoming link but no collisions full duplexo each link is its own collision

                                                              domain switching A-to-Arsquo and B-

                                                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                              A

                                                              Arsquo

                                                              B

                                                              Brsquo

                                                              C

                                                              Crsquo

                                                              switch with six interfaces(123456)

                                                              1 23

                                                              45

                                                              6

                                                              5 DataLink Layer 5-56

                                                              Switch Table

                                                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                              A each switch has a switch table each entryo (MAC address of host interface

                                                              to reach host time stamp)

                                                              looks like a routing table Q how are entries created

                                                              maintained in switch table o something like a routing

                                                              protocol

                                                              A

                                                              Arsquo

                                                              B

                                                              Brsquo

                                                              C

                                                              Crsquo

                                                              switch with six interfaces(123456)

                                                              1 23

                                                              45

                                                              6

                                                              5 DataLink Layer 5-57

                                                              Switch self-learning

                                                              switch learns which hosts can be reached through which interfaceso when frame received

                                                              switch ldquolearnsrdquo location of sender incoming LAN segment

                                                              o records senderlocation pair in switch table

                                                              A

                                                              Arsquo

                                                              B

                                                              Brsquo

                                                              C

                                                              Crsquo

                                                              1 23

                                                              45

                                                              6

                                                              A Arsquo

                                                              Source ADest Arsquo

                                                              MAC addr interface TTL

                                                              Switch table (initially empty)

                                                              A 1 60

                                                              5 DataLink Layer 5-58

                                                              Switch frame filteringforwardingWhen frame received

                                                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                              then if dest on segment from which frame arrived

                                                              then drop the frame else forward the frame on interface indicated else flood

                                                              forward on all but the interface on which the frame arrived

                                                              5 DataLink Layer 5-59

                                                              Self-learning forwarding example

                                                              A

                                                              Arsquo

                                                              B

                                                              Brsquo

                                                              C

                                                              Crsquo

                                                              1 23

                                                              45

                                                              6

                                                              A Arsquo

                                                              Source ADest Arsquo

                                                              MAC addr interface TTL

                                                              Switch table (initially empty)

                                                              A 1 60

                                                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                              frame destination unknownflood

                                                              Arsquo A

                                                              destination A location known

                                                              Arsquo 4 60

                                                              selective send

                                                              5 DataLink Layer 5-60

                                                              Interconnecting switches

                                                              switches can be connected together

                                                              A

                                                              B

                                                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                              A self learning (works exactly the same as in single-switch case)

                                                              S1

                                                              C D

                                                              E

                                                              FS2

                                                              S4

                                                              S3

                                                              H

                                                              I

                                                              G

                                                              5 DataLink Layer 5-61

                                                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                              A

                                                              B

                                                              S1

                                                              C D

                                                              E

                                                              FS2

                                                              S4

                                                              S3

                                                              H

                                                              I

                                                              G

                                                              1

                                                              2 3

                                                              34

                                                              5 DataLink Layer 5-62

                                                              Institutional network

                                                              to externalnetwork

                                                              router

                                                              IP subnet

                                                              mail server

                                                              web server

                                                              5 DataLink Layer 5-63

                                                              Switches vs Routers both store-and-forward devices

                                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                              routers maintain routing tables implement routing algorithms

                                                              switches maintain switch tables implement filtering learning algorithms

                                                              Switch

                                                              5 DataLink Layer 5-64

                                                              Link Layer

                                                              51 Introduction and services

                                                              52 Error detection and correction

                                                              53Multiple access protocols

                                                              54 Link-Layer Addressing

                                                              55 Ethernet

                                                              56 Link-layer switches 59 A day in the life of

                                                              a web request

                                                              5 DataLink Layer 5-65

                                                              Synthesis a day in the life of a web request

                                                              journey down protocol stack completeo application transport network link

                                                              putting-it-all-together synthesiso goal identify review understand protocols

                                                              (at all layers) involved in seemingly simple scenario requesting www page

                                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                              5 DataLink Layer 5-66

                                                              A day in the life scenario

                                                              Comcast network 68800013

                                                              Googlersquos network 64233160019 64233169105

                                                              web server

                                                              DNS server

                                                              school network 68802024

                                                              browser

                                                              web page

                                                              5 DataLink Layer 5-67

                                                              A day in the lifehellip connecting to the Internet

                                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                              router(runs DHCP)

                                                              DHCPUDP

                                                              IPEthPhy

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCPUDP

                                                              IPEthPhy

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCPDHCP

                                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                              5 DataLink Layer 5-68

                                                              A day in the lifehellip connecting to the Internet

                                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                              router(runs DHCP)

                                                              DHCPUDP

                                                              IPEthPhy

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCPUDP

                                                              IPEthPhy

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              DHCP

                                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                              DHCP client receives DHCP ACK reply

                                                              5 DataLink Layer 5-69

                                                              A day in the lifehellip ARP (before DNS before HTTP)

                                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                                              DNSUDP

                                                              IPEthPhy

                                                              DNS

                                                              DNS

                                                              DNS

                                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                              ARP query

                                                              EthPhy

                                                              ARP

                                                              ARP

                                                              ARP reply

                                                              5 DataLink Layer 5-70

                                                              A day in the lifehellip using DNS

                                                              DNSUDP

                                                              IPEthPhy

                                                              DNS

                                                              DNS

                                                              DNS

                                                              DNS

                                                              DNS

                                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                              demuxrsquoed to DNS server DNS server replies to

                                                              client with IP address of wwwgooglecom

                                                              Comcast network 68800013

                                                              DNS server

                                                              DNSUDP

                                                              IPEthPhy

                                                              DNS

                                                              DNS

                                                              DNS

                                                              DNS

                                                              5 DataLink Layer 5-71

                                                              A day in the lifehellip TCP connection carrying HTTP

                                                              HTTPTCPIP

                                                              EthPhy

                                                              HTTP

                                                              to send HTTP request client first opens TCP socket to web server

                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                              TCP connection established

                                                              64233169105

                                                              web server

                                                              SYN

                                                              SYN

                                                              SYN

                                                              SYN

                                                              TCPIP

                                                              EthPhy

                                                              SYN

                                                              SYN

                                                              SYN

                                                              SYNACK

                                                              SYNACK

                                                              SYNACK

                                                              SYNACK

                                                              SYNACK

                                                              SYNACK

                                                              SYNACK

                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                              5 DataLink Layer 5-72

                                                              A day in the lifehellip HTTP requestreply

                                                              HTTPTCPIP

                                                              EthPhy

                                                              HTTP

                                                              HTTP request sent into TCP socket

                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                              IP datgram containing HTTP reply routed back to client

                                                              64233169105

                                                              web server

                                                              HTTPTCPIP

                                                              EthPhy

                                                              web server responds with HTTP reply (containing web page)

                                                              HTTP

                                                              HTTP

                                                              HTTPHTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              HTTP

                                                              web page finally () displayed

                                                              5 DataLink Layer 5-73

                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                              (except PHY) solid understanding of networking

                                                              principles practice hellip could stop here hellip but lots of

                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                              • Link Layer
                                                              • Link Layer Introduction
                                                              • Link layer context
                                                              • Encapsulation
                                                              • Link Layer Services
                                                              • Link Layer Services (more)
                                                              • Where is the link layer implemented
                                                              • Adaptors Communicating
                                                              • Slide 9
                                                              • Parity Checking
                                                              • Internet checksum (review)
                                                              • Checksumming Cyclic Redundancy Check
                                                              • Cyclic Redundancy Check
                                                              • Slide 14
                                                              • CRC Example
                                                              • Slide 16
                                                              • Multiple Access Links and Protocols
                                                              • Cable Network Architecture Overview
                                                              • Multiple Access protocols
                                                              • Ideal Multiple Access Protocol
                                                              • MAC Protocols a taxonomy
                                                              • Channel Partitioning MAC protocols TDMA
                                                              • Channel Partitioning MAC protocols FDMA
                                                              • Random Access Protocols
                                                              • ALOHA
                                                              • CSMA (Carrier Sense Multiple Access)
                                                              • CSMA collisions
                                                              • CSMACD (Collision Detection)
                                                              • CSMACD collision detection
                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                              • Slide 31
                                                              • Slide 32
                                                              • Summary of MAC protocols
                                                              • Slide 34
                                                              • MAC Addresses and ARP
                                                              • LAN Addresses and ARP
                                                              • LAN Address (more)
                                                              • ARP Address Resolution Protocol
                                                              • ARP protocol Same LAN (network)
                                                              • Addressing routing to another LAN
                                                              • Slide 41
                                                              • Slide 42
                                                              • Ethernet
                                                              • Star topology
                                                              • Ethernet Frame Structure
                                                              • Ethernet Frame Structure (more)
                                                              • Ethernet Unreliable connectionless
                                                              • Ethernet CSMACD algorithm
                                                              • Ethernetrsquos CSMACD (more)
                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                              • Manchester encoding
                                                              • Slide 52
                                                              • Hubs
                                                              • Switch
                                                              • Switch allows multiple simultaneous transmissions
                                                              • Switch Table
                                                              • Switch self-learning
                                                              • Switch frame filteringforwarding
                                                              • Self-learning forwarding example
                                                              • Interconnecting switches
                                                              • Self-learning multi-switch example
                                                              • Institutional network
                                                              • Switches vs Routers
                                                              • Slide 64
                                                              • Synthesis a day in the life of a web request
                                                              • A day in the life scenario
                                                              • A day in the lifehellip connecting to the Internet
                                                              • Slide 68
                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                              • A day in the lifehellip using DNS
                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                              • A day in the lifehellip HTTP requestreply
                                                              • Chapter 5 letrsquos take a breath

                                                                5 DataLink Layer 5-32

                                                                ldquoTaking Turnsrdquo MAC protocolsToken passing control token

                                                                passed from one node to next sequentially

                                                                token message concerns

                                                                o token overhead o latencyo single point of failure

                                                                (token)

                                                                T

                                                                data

                                                                (nothingto send)

                                                                T

                                                                5 DataLink Layer 5-33

                                                                Summary of MAC protocols

                                                                channel partitioning by time frequency or codeo Time Division Frequency Division

                                                                random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                                hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                                taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                                5 DataLink Layer 5-34

                                                                Link Layer

                                                                51 Introduction and services

                                                                52 Error detection and correction

                                                                53Multiple access protocols

                                                                54 Link-Layer Addressing

                                                                55 Ethernet

                                                                56 Link-layer switches 59 A day in the life of

                                                                a web request

                                                                5 DataLink Layer 5-35

                                                                MAC Addresses and ARP

                                                                32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                                MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                                another physically-connected interface (in same network)

                                                                o 48 bit MAC addressbull burned in NIC ROM

                                                                5 DataLink Layer 5-36

                                                                LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                                Broadcast address =FF-FF-FF-FF-FF-FF

                                                                = adapter

                                                                1A-2F-BB-76-09-AD

                                                                58-23-D7-FA-20-B0

                                                                0C-C4-11-6F-E3-98

                                                                71-65-F7-2B-08-53

                                                                LAN(wired orwireless)

                                                                5 DataLink Layer 5-37

                                                                LAN Address (more)

                                                                MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                o can move LAN card from one LAN to another

                                                                IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                attached

                                                                5 DataLink Layer 5-38

                                                                ARP Address Resolution Protocol

                                                                Each IP node (host router) on LAN has ARP table

                                                                ARP table IPMAC address mappings for some LAN nodes

                                                                lt IP address MAC address TTLgto TTL (Time To Live)

                                                                time after which address mapping will be forgotten (typically 20 min)

                                                                Q given a nodersquos IP address how to determine its MAC address

                                                                1A-2F-BB-76-09-AD

                                                                0C-C4-11-6F-E3-98

                                                                71-65-F7-2B-08-53

                                                                LAN

                                                                137196723

                                                                137196778

                                                                137196714

                                                                137196788

                                                                5 DataLink Layer 5-39

                                                                ARP protocol Same LAN (network) A wants to send datagram

                                                                to B and Brsquos MAC address not in Arsquos ARP table

                                                                A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                FF-FF-FF-FF-FFo all machines on LAN

                                                                receive ARP query B receives ARP packet

                                                                replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                address (unicast)

                                                                A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                ARP is ldquoplug-and-playrdquoo nodes create their

                                                                ARP tables without intervention from net administrator

                                                                5 DataLink Layer 5-40

                                                                Addressing routing to another LAN

                                                                R

                                                                1A-23-F9-CD-06-9B

                                                                222222222220111111111110

                                                                E6-E9-00-17-BB-4B

                                                                CC-49-DE-D0-AB-7D

                                                                111111111112

                                                                111111111111

                                                                A74-29-9C-E8-FF-55

                                                                222222222221

                                                                88-B2-2F-54-1A-0F

                                                                B222222222222

                                                                49-BD-D2-C7-56-2A

                                                                walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                two ARP tables in router R one for each IP network (LAN)

                                                                LAN LAN

                                                                5 DataLink Layer 5-41

                                                                A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                R

                                                                1A-23-F9-CD-06-9B

                                                                222222222220

                                                                111111111110

                                                                E6-E9-00-17-BB-4B

                                                                CC-49-DE-D0-AB-7D

                                                                111111111112

                                                                111111111111

                                                                A74-29-9C-E8-FF-55

                                                                222222222221

                                                                88-B2-2F-54-1A-0F

                                                                B222222222222

                                                                49-BD-D2-C7-56-2A

                                                                This is a really importantexample ndash make sure youunderstand

                                                                5 DataLink Layer 5-42

                                                                Link Layer

                                                                51 Introduction and services

                                                                52 Error detection and correction

                                                                53Multiple access protocols

                                                                54 Link-Layer Addressing

                                                                55 Ethernet

                                                                56 Link-layer switches 59 A day in the life of

                                                                a web request

                                                                5 DataLink Layer 5-43

                                                                Ethernet

                                                                ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                Metcalfersquos Ethernetsketch

                                                                5 DataLink Layer 5-44

                                                                Star topology bus topology popular through mid 90s

                                                                o all nodes in same collision domain (can collide with each other)

                                                                today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                (nodes do not collide with each other)

                                                                switch

                                                                bus coaxial cable star

                                                                5 DataLink Layer 5-45

                                                                Ethernet Frame Structure

                                                                Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                Preamble 7 bytes with pattern 10101010 followed by one

                                                                byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                rates

                                                                5 DataLink Layer 5-46

                                                                Ethernet Frame Structure (more) Addresses 6 bytes

                                                                o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                o otherwise adapter discards frame

                                                                Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                CRC checked at receiver if error is detected frame is dropped

                                                                5 DataLink Layer 5-47

                                                                Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                Ethernetrsquos MAC protocol CSMACD

                                                                5 DataLink Layer 5-48

                                                                Ethernet CSMACD algorithm

                                                                1 NIC receives datagram from network layer creates frame

                                                                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                5 After aborting NIC enters exponential backoff

                                                                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                (1 slot = 512 bit-times)

                                                                5 DataLink Layer 5-49

                                                                Ethernetrsquos CSMACD (more)

                                                                Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                Exponential Backoff Goal adapt retransmission

                                                                attempts to estimated current loado heavy load random

                                                                wait will be longer first collision choose K

                                                                from 01 delay is K 512 bit transmission times

                                                                after second collision choose K from 0123hellip

                                                                after ten collisions choose K from 01234hellip1023

                                                                5 DataLink Layer 5-50

                                                                8023 Ethernet Standards Link amp Physical Layers

                                                                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                applicationtransportnetwork

                                                                linkphysical

                                                                MAC protocoland frame format

                                                                100BASE-TX

                                                                100BASE-T4

                                                                100BASE-FX100BASE-T2

                                                                100BASE-SX 100BASE-BX

                                                                fiber physical layercopper (twisterpair) physical layer

                                                                5 DataLink Layer 5-51

                                                                Manchester encoding

                                                                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                synchronize to each othero no need for a centralized global clock among nodes

                                                                Hey this is physical-layer stuff

                                                                5 DataLink Layer 5-52

                                                                Link Layer

                                                                51 Introduction and services

                                                                52 Error detection and correction

                                                                53 Multiple access protocols

                                                                54 Link-layer Addressing

                                                                55 Ethernet

                                                                56 Link-layer switches LANs

                                                                59 A day in the life of a web request

                                                                5 DataLink Layer 5-53

                                                                Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                o bits coming in one link go out all other links at same rate

                                                                o all nodes connected to hub can collide with one another

                                                                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                twisted pair

                                                                hub

                                                                5 DataLink Layer 5-54

                                                                Switch link-layer device smarter than hubs take active

                                                                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                transparento hosts are unaware of presence of switches

                                                                plug-and-play self-learningo switches do not need to be configured

                                                                5 DataLink Layer 5-55

                                                                Switch allows multiple simultaneous transmissions

                                                                hosts have dedicated direct connection to switch

                                                                switches buffer packets Ethernet protocol used on

                                                                each incoming link but no collisions full duplexo each link is its own collision

                                                                domain switching A-to-Arsquo and B-

                                                                to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                A

                                                                Arsquo

                                                                B

                                                                Brsquo

                                                                C

                                                                Crsquo

                                                                switch with six interfaces(123456)

                                                                1 23

                                                                45

                                                                6

                                                                5 DataLink Layer 5-56

                                                                Switch Table

                                                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                A each switch has a switch table each entryo (MAC address of host interface

                                                                to reach host time stamp)

                                                                looks like a routing table Q how are entries created

                                                                maintained in switch table o something like a routing

                                                                protocol

                                                                A

                                                                Arsquo

                                                                B

                                                                Brsquo

                                                                C

                                                                Crsquo

                                                                switch with six interfaces(123456)

                                                                1 23

                                                                45

                                                                6

                                                                5 DataLink Layer 5-57

                                                                Switch self-learning

                                                                switch learns which hosts can be reached through which interfaceso when frame received

                                                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                o records senderlocation pair in switch table

                                                                A

                                                                Arsquo

                                                                B

                                                                Brsquo

                                                                C

                                                                Crsquo

                                                                1 23

                                                                45

                                                                6

                                                                A Arsquo

                                                                Source ADest Arsquo

                                                                MAC addr interface TTL

                                                                Switch table (initially empty)

                                                                A 1 60

                                                                5 DataLink Layer 5-58

                                                                Switch frame filteringforwardingWhen frame received

                                                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                then if dest on segment from which frame arrived

                                                                then drop the frame else forward the frame on interface indicated else flood

                                                                forward on all but the interface on which the frame arrived

                                                                5 DataLink Layer 5-59

                                                                Self-learning forwarding example

                                                                A

                                                                Arsquo

                                                                B

                                                                Brsquo

                                                                C

                                                                Crsquo

                                                                1 23

                                                                45

                                                                6

                                                                A Arsquo

                                                                Source ADest Arsquo

                                                                MAC addr interface TTL

                                                                Switch table (initially empty)

                                                                A 1 60

                                                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                frame destination unknownflood

                                                                Arsquo A

                                                                destination A location known

                                                                Arsquo 4 60

                                                                selective send

                                                                5 DataLink Layer 5-60

                                                                Interconnecting switches

                                                                switches can be connected together

                                                                A

                                                                B

                                                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                A self learning (works exactly the same as in single-switch case)

                                                                S1

                                                                C D

                                                                E

                                                                FS2

                                                                S4

                                                                S3

                                                                H

                                                                I

                                                                G

                                                                5 DataLink Layer 5-61

                                                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                A

                                                                B

                                                                S1

                                                                C D

                                                                E

                                                                FS2

                                                                S4

                                                                S3

                                                                H

                                                                I

                                                                G

                                                                1

                                                                2 3

                                                                34

                                                                5 DataLink Layer 5-62

                                                                Institutional network

                                                                to externalnetwork

                                                                router

                                                                IP subnet

                                                                mail server

                                                                web server

                                                                5 DataLink Layer 5-63

                                                                Switches vs Routers both store-and-forward devices

                                                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                routers maintain routing tables implement routing algorithms

                                                                switches maintain switch tables implement filtering learning algorithms

                                                                Switch

                                                                5 DataLink Layer 5-64

                                                                Link Layer

                                                                51 Introduction and services

                                                                52 Error detection and correction

                                                                53Multiple access protocols

                                                                54 Link-Layer Addressing

                                                                55 Ethernet

                                                                56 Link-layer switches 59 A day in the life of

                                                                a web request

                                                                5 DataLink Layer 5-65

                                                                Synthesis a day in the life of a web request

                                                                journey down protocol stack completeo application transport network link

                                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                5 DataLink Layer 5-66

                                                                A day in the life scenario

                                                                Comcast network 68800013

                                                                Googlersquos network 64233160019 64233169105

                                                                web server

                                                                DNS server

                                                                school network 68802024

                                                                browser

                                                                web page

                                                                5 DataLink Layer 5-67

                                                                A day in the lifehellip connecting to the Internet

                                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                router(runs DHCP)

                                                                DHCPUDP

                                                                IPEthPhy

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCPUDP

                                                                IPEthPhy

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCPDHCP

                                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                5 DataLink Layer 5-68

                                                                A day in the lifehellip connecting to the Internet

                                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                router(runs DHCP)

                                                                DHCPUDP

                                                                IPEthPhy

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCPUDP

                                                                IPEthPhy

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                DHCP

                                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                DHCP client receives DHCP ACK reply

                                                                5 DataLink Layer 5-69

                                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                                DNSUDP

                                                                IPEthPhy

                                                                DNS

                                                                DNS

                                                                DNS

                                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                ARP query

                                                                EthPhy

                                                                ARP

                                                                ARP

                                                                ARP reply

                                                                5 DataLink Layer 5-70

                                                                A day in the lifehellip using DNS

                                                                DNSUDP

                                                                IPEthPhy

                                                                DNS

                                                                DNS

                                                                DNS

                                                                DNS

                                                                DNS

                                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                demuxrsquoed to DNS server DNS server replies to

                                                                client with IP address of wwwgooglecom

                                                                Comcast network 68800013

                                                                DNS server

                                                                DNSUDP

                                                                IPEthPhy

                                                                DNS

                                                                DNS

                                                                DNS

                                                                DNS

                                                                5 DataLink Layer 5-71

                                                                A day in the lifehellip TCP connection carrying HTTP

                                                                HTTPTCPIP

                                                                EthPhy

                                                                HTTP

                                                                to send HTTP request client first opens TCP socket to web server

                                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                TCP connection established

                                                                64233169105

                                                                web server

                                                                SYN

                                                                SYN

                                                                SYN

                                                                SYN

                                                                TCPIP

                                                                EthPhy

                                                                SYN

                                                                SYN

                                                                SYN

                                                                SYNACK

                                                                SYNACK

                                                                SYNACK

                                                                SYNACK

                                                                SYNACK

                                                                SYNACK

                                                                SYNACK

                                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                5 DataLink Layer 5-72

                                                                A day in the lifehellip HTTP requestreply

                                                                HTTPTCPIP

                                                                EthPhy

                                                                HTTP

                                                                HTTP request sent into TCP socket

                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                IP datgram containing HTTP reply routed back to client

                                                                64233169105

                                                                web server

                                                                HTTPTCPIP

                                                                EthPhy

                                                                web server responds with HTTP reply (containing web page)

                                                                HTTP

                                                                HTTP

                                                                HTTPHTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                HTTP

                                                                web page finally () displayed

                                                                5 DataLink Layer 5-73

                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                (except PHY) solid understanding of networking

                                                                principles practice hellip could stop here hellip but lots of

                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                • Link Layer
                                                                • Link Layer Introduction
                                                                • Link layer context
                                                                • Encapsulation
                                                                • Link Layer Services
                                                                • Link Layer Services (more)
                                                                • Where is the link layer implemented
                                                                • Adaptors Communicating
                                                                • Slide 9
                                                                • Parity Checking
                                                                • Internet checksum (review)
                                                                • Checksumming Cyclic Redundancy Check
                                                                • Cyclic Redundancy Check
                                                                • Slide 14
                                                                • CRC Example
                                                                • Slide 16
                                                                • Multiple Access Links and Protocols
                                                                • Cable Network Architecture Overview
                                                                • Multiple Access protocols
                                                                • Ideal Multiple Access Protocol
                                                                • MAC Protocols a taxonomy
                                                                • Channel Partitioning MAC protocols TDMA
                                                                • Channel Partitioning MAC protocols FDMA
                                                                • Random Access Protocols
                                                                • ALOHA
                                                                • CSMA (Carrier Sense Multiple Access)
                                                                • CSMA collisions
                                                                • CSMACD (Collision Detection)
                                                                • CSMACD collision detection
                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                • Slide 31
                                                                • Slide 32
                                                                • Summary of MAC protocols
                                                                • Slide 34
                                                                • MAC Addresses and ARP
                                                                • LAN Addresses and ARP
                                                                • LAN Address (more)
                                                                • ARP Address Resolution Protocol
                                                                • ARP protocol Same LAN (network)
                                                                • Addressing routing to another LAN
                                                                • Slide 41
                                                                • Slide 42
                                                                • Ethernet
                                                                • Star topology
                                                                • Ethernet Frame Structure
                                                                • Ethernet Frame Structure (more)
                                                                • Ethernet Unreliable connectionless
                                                                • Ethernet CSMACD algorithm
                                                                • Ethernetrsquos CSMACD (more)
                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                • Manchester encoding
                                                                • Slide 52
                                                                • Hubs
                                                                • Switch
                                                                • Switch allows multiple simultaneous transmissions
                                                                • Switch Table
                                                                • Switch self-learning
                                                                • Switch frame filteringforwarding
                                                                • Self-learning forwarding example
                                                                • Interconnecting switches
                                                                • Self-learning multi-switch example
                                                                • Institutional network
                                                                • Switches vs Routers
                                                                • Slide 64
                                                                • Synthesis a day in the life of a web request
                                                                • A day in the life scenario
                                                                • A day in the lifehellip connecting to the Internet
                                                                • Slide 68
                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                • A day in the lifehellip using DNS
                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                • A day in the lifehellip HTTP requestreply
                                                                • Chapter 5 letrsquos take a breath

                                                                  5 DataLink Layer 5-33

                                                                  Summary of MAC protocols

                                                                  channel partitioning by time frequency or codeo Time Division Frequency Division

                                                                  random access (dynamic) o ALOHA CSMA CSMACDo carrier sensing easy in some technologies (wire)

                                                                  hard in others (wireless)o CSMACD used in Etherneto CSMACA used in 80211

                                                                  taking turnso polling from central site token passingo Bluetooth FDDI IBM Token Ring

                                                                  5 DataLink Layer 5-34

                                                                  Link Layer

                                                                  51 Introduction and services

                                                                  52 Error detection and correction

                                                                  53Multiple access protocols

                                                                  54 Link-Layer Addressing

                                                                  55 Ethernet

                                                                  56 Link-layer switches 59 A day in the life of

                                                                  a web request

                                                                  5 DataLink Layer 5-35

                                                                  MAC Addresses and ARP

                                                                  32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                                  MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                                  another physically-connected interface (in same network)

                                                                  o 48 bit MAC addressbull burned in NIC ROM

                                                                  5 DataLink Layer 5-36

                                                                  LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                                  Broadcast address =FF-FF-FF-FF-FF-FF

                                                                  = adapter

                                                                  1A-2F-BB-76-09-AD

                                                                  58-23-D7-FA-20-B0

                                                                  0C-C4-11-6F-E3-98

                                                                  71-65-F7-2B-08-53

                                                                  LAN(wired orwireless)

                                                                  5 DataLink Layer 5-37

                                                                  LAN Address (more)

                                                                  MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                  space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                  o can move LAN card from one LAN to another

                                                                  IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                  attached

                                                                  5 DataLink Layer 5-38

                                                                  ARP Address Resolution Protocol

                                                                  Each IP node (host router) on LAN has ARP table

                                                                  ARP table IPMAC address mappings for some LAN nodes

                                                                  lt IP address MAC address TTLgto TTL (Time To Live)

                                                                  time after which address mapping will be forgotten (typically 20 min)

                                                                  Q given a nodersquos IP address how to determine its MAC address

                                                                  1A-2F-BB-76-09-AD

                                                                  0C-C4-11-6F-E3-98

                                                                  71-65-F7-2B-08-53

                                                                  LAN

                                                                  137196723

                                                                  137196778

                                                                  137196714

                                                                  137196788

                                                                  5 DataLink Layer 5-39

                                                                  ARP protocol Same LAN (network) A wants to send datagram

                                                                  to B and Brsquos MAC address not in Arsquos ARP table

                                                                  A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                  FF-FF-FF-FF-FFo all machines on LAN

                                                                  receive ARP query B receives ARP packet

                                                                  replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                  address (unicast)

                                                                  A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                  ARP is ldquoplug-and-playrdquoo nodes create their

                                                                  ARP tables without intervention from net administrator

                                                                  5 DataLink Layer 5-40

                                                                  Addressing routing to another LAN

                                                                  R

                                                                  1A-23-F9-CD-06-9B

                                                                  222222222220111111111110

                                                                  E6-E9-00-17-BB-4B

                                                                  CC-49-DE-D0-AB-7D

                                                                  111111111112

                                                                  111111111111

                                                                  A74-29-9C-E8-FF-55

                                                                  222222222221

                                                                  88-B2-2F-54-1A-0F

                                                                  B222222222222

                                                                  49-BD-D2-C7-56-2A

                                                                  walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                  two ARP tables in router R one for each IP network (LAN)

                                                                  LAN LAN

                                                                  5 DataLink Layer 5-41

                                                                  A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                  frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                  destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                  R

                                                                  1A-23-F9-CD-06-9B

                                                                  222222222220

                                                                  111111111110

                                                                  E6-E9-00-17-BB-4B

                                                                  CC-49-DE-D0-AB-7D

                                                                  111111111112

                                                                  111111111111

                                                                  A74-29-9C-E8-FF-55

                                                                  222222222221

                                                                  88-B2-2F-54-1A-0F

                                                                  B222222222222

                                                                  49-BD-D2-C7-56-2A

                                                                  This is a really importantexample ndash make sure youunderstand

                                                                  5 DataLink Layer 5-42

                                                                  Link Layer

                                                                  51 Introduction and services

                                                                  52 Error detection and correction

                                                                  53Multiple access protocols

                                                                  54 Link-Layer Addressing

                                                                  55 Ethernet

                                                                  56 Link-layer switches 59 A day in the life of

                                                                  a web request

                                                                  5 DataLink Layer 5-43

                                                                  Ethernet

                                                                  ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                  Metcalfersquos Ethernetsketch

                                                                  5 DataLink Layer 5-44

                                                                  Star topology bus topology popular through mid 90s

                                                                  o all nodes in same collision domain (can collide with each other)

                                                                  today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                  (nodes do not collide with each other)

                                                                  switch

                                                                  bus coaxial cable star

                                                                  5 DataLink Layer 5-45

                                                                  Ethernet Frame Structure

                                                                  Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                  Preamble 7 bytes with pattern 10101010 followed by one

                                                                  byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                  rates

                                                                  5 DataLink Layer 5-46

                                                                  Ethernet Frame Structure (more) Addresses 6 bytes

                                                                  o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                  o otherwise adapter discards frame

                                                                  Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                  CRC checked at receiver if error is detected frame is dropped

                                                                  5 DataLink Layer 5-47

                                                                  Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                  and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                  send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                  gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                  Ethernetrsquos MAC protocol CSMACD

                                                                  5 DataLink Layer 5-48

                                                                  Ethernet CSMACD algorithm

                                                                  1 NIC receives datagram from network layer creates frame

                                                                  2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                  3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                  4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                  5 After aborting NIC enters exponential backoff

                                                                  after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                  (1 slot = 512 bit-times)

                                                                  5 DataLink Layer 5-49

                                                                  Ethernetrsquos CSMACD (more)

                                                                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                  Exponential Backoff Goal adapt retransmission

                                                                  attempts to estimated current loado heavy load random

                                                                  wait will be longer first collision choose K

                                                                  from 01 delay is K 512 bit transmission times

                                                                  after second collision choose K from 0123hellip

                                                                  after ten collisions choose K from 01234hellip1023

                                                                  5 DataLink Layer 5-50

                                                                  8023 Ethernet Standards Link amp Physical Layers

                                                                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                  applicationtransportnetwork

                                                                  linkphysical

                                                                  MAC protocoland frame format

                                                                  100BASE-TX

                                                                  100BASE-T4

                                                                  100BASE-FX100BASE-T2

                                                                  100BASE-SX 100BASE-BX

                                                                  fiber physical layercopper (twisterpair) physical layer

                                                                  5 DataLink Layer 5-51

                                                                  Manchester encoding

                                                                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                  synchronize to each othero no need for a centralized global clock among nodes

                                                                  Hey this is physical-layer stuff

                                                                  5 DataLink Layer 5-52

                                                                  Link Layer

                                                                  51 Introduction and services

                                                                  52 Error detection and correction

                                                                  53 Multiple access protocols

                                                                  54 Link-layer Addressing

                                                                  55 Ethernet

                                                                  56 Link-layer switches LANs

                                                                  59 A day in the life of a web request

                                                                  5 DataLink Layer 5-53

                                                                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                  o bits coming in one link go out all other links at same rate

                                                                  o all nodes connected to hub can collide with one another

                                                                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                  twisted pair

                                                                  hub

                                                                  5 DataLink Layer 5-54

                                                                  Switch link-layer device smarter than hubs take active

                                                                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                  transparento hosts are unaware of presence of switches

                                                                  plug-and-play self-learningo switches do not need to be configured

                                                                  5 DataLink Layer 5-55

                                                                  Switch allows multiple simultaneous transmissions

                                                                  hosts have dedicated direct connection to switch

                                                                  switches buffer packets Ethernet protocol used on

                                                                  each incoming link but no collisions full duplexo each link is its own collision

                                                                  domain switching A-to-Arsquo and B-

                                                                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                  A

                                                                  Arsquo

                                                                  B

                                                                  Brsquo

                                                                  C

                                                                  Crsquo

                                                                  switch with six interfaces(123456)

                                                                  1 23

                                                                  45

                                                                  6

                                                                  5 DataLink Layer 5-56

                                                                  Switch Table

                                                                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                  A each switch has a switch table each entryo (MAC address of host interface

                                                                  to reach host time stamp)

                                                                  looks like a routing table Q how are entries created

                                                                  maintained in switch table o something like a routing

                                                                  protocol

                                                                  A

                                                                  Arsquo

                                                                  B

                                                                  Brsquo

                                                                  C

                                                                  Crsquo

                                                                  switch with six interfaces(123456)

                                                                  1 23

                                                                  45

                                                                  6

                                                                  5 DataLink Layer 5-57

                                                                  Switch self-learning

                                                                  switch learns which hosts can be reached through which interfaceso when frame received

                                                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                  o records senderlocation pair in switch table

                                                                  A

                                                                  Arsquo

                                                                  B

                                                                  Brsquo

                                                                  C

                                                                  Crsquo

                                                                  1 23

                                                                  45

                                                                  6

                                                                  A Arsquo

                                                                  Source ADest Arsquo

                                                                  MAC addr interface TTL

                                                                  Switch table (initially empty)

                                                                  A 1 60

                                                                  5 DataLink Layer 5-58

                                                                  Switch frame filteringforwardingWhen frame received

                                                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                  then if dest on segment from which frame arrived

                                                                  then drop the frame else forward the frame on interface indicated else flood

                                                                  forward on all but the interface on which the frame arrived

                                                                  5 DataLink Layer 5-59

                                                                  Self-learning forwarding example

                                                                  A

                                                                  Arsquo

                                                                  B

                                                                  Brsquo

                                                                  C

                                                                  Crsquo

                                                                  1 23

                                                                  45

                                                                  6

                                                                  A Arsquo

                                                                  Source ADest Arsquo

                                                                  MAC addr interface TTL

                                                                  Switch table (initially empty)

                                                                  A 1 60

                                                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                  frame destination unknownflood

                                                                  Arsquo A

                                                                  destination A location known

                                                                  Arsquo 4 60

                                                                  selective send

                                                                  5 DataLink Layer 5-60

                                                                  Interconnecting switches

                                                                  switches can be connected together

                                                                  A

                                                                  B

                                                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                  A self learning (works exactly the same as in single-switch case)

                                                                  S1

                                                                  C D

                                                                  E

                                                                  FS2

                                                                  S4

                                                                  S3

                                                                  H

                                                                  I

                                                                  G

                                                                  5 DataLink Layer 5-61

                                                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                  A

                                                                  B

                                                                  S1

                                                                  C D

                                                                  E

                                                                  FS2

                                                                  S4

                                                                  S3

                                                                  H

                                                                  I

                                                                  G

                                                                  1

                                                                  2 3

                                                                  34

                                                                  5 DataLink Layer 5-62

                                                                  Institutional network

                                                                  to externalnetwork

                                                                  router

                                                                  IP subnet

                                                                  mail server

                                                                  web server

                                                                  5 DataLink Layer 5-63

                                                                  Switches vs Routers both store-and-forward devices

                                                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                  routers maintain routing tables implement routing algorithms

                                                                  switches maintain switch tables implement filtering learning algorithms

                                                                  Switch

                                                                  5 DataLink Layer 5-64

                                                                  Link Layer

                                                                  51 Introduction and services

                                                                  52 Error detection and correction

                                                                  53Multiple access protocols

                                                                  54 Link-Layer Addressing

                                                                  55 Ethernet

                                                                  56 Link-layer switches 59 A day in the life of

                                                                  a web request

                                                                  5 DataLink Layer 5-65

                                                                  Synthesis a day in the life of a web request

                                                                  journey down protocol stack completeo application transport network link

                                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                  5 DataLink Layer 5-66

                                                                  A day in the life scenario

                                                                  Comcast network 68800013

                                                                  Googlersquos network 64233160019 64233169105

                                                                  web server

                                                                  DNS server

                                                                  school network 68802024

                                                                  browser

                                                                  web page

                                                                  5 DataLink Layer 5-67

                                                                  A day in the lifehellip connecting to the Internet

                                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                  router(runs DHCP)

                                                                  DHCPUDP

                                                                  IPEthPhy

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCPUDP

                                                                  IPEthPhy

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCPDHCP

                                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                  5 DataLink Layer 5-68

                                                                  A day in the lifehellip connecting to the Internet

                                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                  router(runs DHCP)

                                                                  DHCPUDP

                                                                  IPEthPhy

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCPUDP

                                                                  IPEthPhy

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  DHCP

                                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                  DHCP client receives DHCP ACK reply

                                                                  5 DataLink Layer 5-69

                                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                                  DNSUDP

                                                                  IPEthPhy

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                  ARP query

                                                                  EthPhy

                                                                  ARP

                                                                  ARP

                                                                  ARP reply

                                                                  5 DataLink Layer 5-70

                                                                  A day in the lifehellip using DNS

                                                                  DNSUDP

                                                                  IPEthPhy

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                  demuxrsquoed to DNS server DNS server replies to

                                                                  client with IP address of wwwgooglecom

                                                                  Comcast network 68800013

                                                                  DNS server

                                                                  DNSUDP

                                                                  IPEthPhy

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  DNS

                                                                  5 DataLink Layer 5-71

                                                                  A day in the lifehellip TCP connection carrying HTTP

                                                                  HTTPTCPIP

                                                                  EthPhy

                                                                  HTTP

                                                                  to send HTTP request client first opens TCP socket to web server

                                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                  TCP connection established

                                                                  64233169105

                                                                  web server

                                                                  SYN

                                                                  SYN

                                                                  SYN

                                                                  SYN

                                                                  TCPIP

                                                                  EthPhy

                                                                  SYN

                                                                  SYN

                                                                  SYN

                                                                  SYNACK

                                                                  SYNACK

                                                                  SYNACK

                                                                  SYNACK

                                                                  SYNACK

                                                                  SYNACK

                                                                  SYNACK

                                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                  5 DataLink Layer 5-72

                                                                  A day in the lifehellip HTTP requestreply

                                                                  HTTPTCPIP

                                                                  EthPhy

                                                                  HTTP

                                                                  HTTP request sent into TCP socket

                                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                                  IP datgram containing HTTP reply routed back to client

                                                                  64233169105

                                                                  web server

                                                                  HTTPTCPIP

                                                                  EthPhy

                                                                  web server responds with HTTP reply (containing web page)

                                                                  HTTP

                                                                  HTTP

                                                                  HTTPHTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  HTTP

                                                                  web page finally () displayed

                                                                  5 DataLink Layer 5-73

                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                  (except PHY) solid understanding of networking

                                                                  principles practice hellip could stop here hellip but lots of

                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                  • Link Layer
                                                                  • Link Layer Introduction
                                                                  • Link layer context
                                                                  • Encapsulation
                                                                  • Link Layer Services
                                                                  • Link Layer Services (more)
                                                                  • Where is the link layer implemented
                                                                  • Adaptors Communicating
                                                                  • Slide 9
                                                                  • Parity Checking
                                                                  • Internet checksum (review)
                                                                  • Checksumming Cyclic Redundancy Check
                                                                  • Cyclic Redundancy Check
                                                                  • Slide 14
                                                                  • CRC Example
                                                                  • Slide 16
                                                                  • Multiple Access Links and Protocols
                                                                  • Cable Network Architecture Overview
                                                                  • Multiple Access protocols
                                                                  • Ideal Multiple Access Protocol
                                                                  • MAC Protocols a taxonomy
                                                                  • Channel Partitioning MAC protocols TDMA
                                                                  • Channel Partitioning MAC protocols FDMA
                                                                  • Random Access Protocols
                                                                  • ALOHA
                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                  • CSMA collisions
                                                                  • CSMACD (Collision Detection)
                                                                  • CSMACD collision detection
                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                  • Slide 31
                                                                  • Slide 32
                                                                  • Summary of MAC protocols
                                                                  • Slide 34
                                                                  • MAC Addresses and ARP
                                                                  • LAN Addresses and ARP
                                                                  • LAN Address (more)
                                                                  • ARP Address Resolution Protocol
                                                                  • ARP protocol Same LAN (network)
                                                                  • Addressing routing to another LAN
                                                                  • Slide 41
                                                                  • Slide 42
                                                                  • Ethernet
                                                                  • Star topology
                                                                  • Ethernet Frame Structure
                                                                  • Ethernet Frame Structure (more)
                                                                  • Ethernet Unreliable connectionless
                                                                  • Ethernet CSMACD algorithm
                                                                  • Ethernetrsquos CSMACD (more)
                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                  • Manchester encoding
                                                                  • Slide 52
                                                                  • Hubs
                                                                  • Switch
                                                                  • Switch allows multiple simultaneous transmissions
                                                                  • Switch Table
                                                                  • Switch self-learning
                                                                  • Switch frame filteringforwarding
                                                                  • Self-learning forwarding example
                                                                  • Interconnecting switches
                                                                  • Self-learning multi-switch example
                                                                  • Institutional network
                                                                  • Switches vs Routers
                                                                  • Slide 64
                                                                  • Synthesis a day in the life of a web request
                                                                  • A day in the life scenario
                                                                  • A day in the lifehellip connecting to the Internet
                                                                  • Slide 68
                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                  • A day in the lifehellip using DNS
                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                  • A day in the lifehellip HTTP requestreply
                                                                  • Chapter 5 letrsquos take a breath

                                                                    5 DataLink Layer 5-34

                                                                    Link Layer

                                                                    51 Introduction and services

                                                                    52 Error detection and correction

                                                                    53Multiple access protocols

                                                                    54 Link-Layer Addressing

                                                                    55 Ethernet

                                                                    56 Link-layer switches 59 A day in the life of

                                                                    a web request

                                                                    5 DataLink Layer 5-35

                                                                    MAC Addresses and ARP

                                                                    32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                                    MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                                    another physically-connected interface (in same network)

                                                                    o 48 bit MAC addressbull burned in NIC ROM

                                                                    5 DataLink Layer 5-36

                                                                    LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                                    Broadcast address =FF-FF-FF-FF-FF-FF

                                                                    = adapter

                                                                    1A-2F-BB-76-09-AD

                                                                    58-23-D7-FA-20-B0

                                                                    0C-C4-11-6F-E3-98

                                                                    71-65-F7-2B-08-53

                                                                    LAN(wired orwireless)

                                                                    5 DataLink Layer 5-37

                                                                    LAN Address (more)

                                                                    MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                    space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                    o can move LAN card from one LAN to another

                                                                    IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                    attached

                                                                    5 DataLink Layer 5-38

                                                                    ARP Address Resolution Protocol

                                                                    Each IP node (host router) on LAN has ARP table

                                                                    ARP table IPMAC address mappings for some LAN nodes

                                                                    lt IP address MAC address TTLgto TTL (Time To Live)

                                                                    time after which address mapping will be forgotten (typically 20 min)

                                                                    Q given a nodersquos IP address how to determine its MAC address

                                                                    1A-2F-BB-76-09-AD

                                                                    0C-C4-11-6F-E3-98

                                                                    71-65-F7-2B-08-53

                                                                    LAN

                                                                    137196723

                                                                    137196778

                                                                    137196714

                                                                    137196788

                                                                    5 DataLink Layer 5-39

                                                                    ARP protocol Same LAN (network) A wants to send datagram

                                                                    to B and Brsquos MAC address not in Arsquos ARP table

                                                                    A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                    FF-FF-FF-FF-FFo all machines on LAN

                                                                    receive ARP query B receives ARP packet

                                                                    replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                    address (unicast)

                                                                    A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                    ARP is ldquoplug-and-playrdquoo nodes create their

                                                                    ARP tables without intervention from net administrator

                                                                    5 DataLink Layer 5-40

                                                                    Addressing routing to another LAN

                                                                    R

                                                                    1A-23-F9-CD-06-9B

                                                                    222222222220111111111110

                                                                    E6-E9-00-17-BB-4B

                                                                    CC-49-DE-D0-AB-7D

                                                                    111111111112

                                                                    111111111111

                                                                    A74-29-9C-E8-FF-55

                                                                    222222222221

                                                                    88-B2-2F-54-1A-0F

                                                                    B222222222222

                                                                    49-BD-D2-C7-56-2A

                                                                    walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                    two ARP tables in router R one for each IP network (LAN)

                                                                    LAN LAN

                                                                    5 DataLink Layer 5-41

                                                                    A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                    frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                    destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                    R

                                                                    1A-23-F9-CD-06-9B

                                                                    222222222220

                                                                    111111111110

                                                                    E6-E9-00-17-BB-4B

                                                                    CC-49-DE-D0-AB-7D

                                                                    111111111112

                                                                    111111111111

                                                                    A74-29-9C-E8-FF-55

                                                                    222222222221

                                                                    88-B2-2F-54-1A-0F

                                                                    B222222222222

                                                                    49-BD-D2-C7-56-2A

                                                                    This is a really importantexample ndash make sure youunderstand

                                                                    5 DataLink Layer 5-42

                                                                    Link Layer

                                                                    51 Introduction and services

                                                                    52 Error detection and correction

                                                                    53Multiple access protocols

                                                                    54 Link-Layer Addressing

                                                                    55 Ethernet

                                                                    56 Link-layer switches 59 A day in the life of

                                                                    a web request

                                                                    5 DataLink Layer 5-43

                                                                    Ethernet

                                                                    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                    Metcalfersquos Ethernetsketch

                                                                    5 DataLink Layer 5-44

                                                                    Star topology bus topology popular through mid 90s

                                                                    o all nodes in same collision domain (can collide with each other)

                                                                    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                    (nodes do not collide with each other)

                                                                    switch

                                                                    bus coaxial cable star

                                                                    5 DataLink Layer 5-45

                                                                    Ethernet Frame Structure

                                                                    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                    Preamble 7 bytes with pattern 10101010 followed by one

                                                                    byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                    rates

                                                                    5 DataLink Layer 5-46

                                                                    Ethernet Frame Structure (more) Addresses 6 bytes

                                                                    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                    o otherwise adapter discards frame

                                                                    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                    CRC checked at receiver if error is detected frame is dropped

                                                                    5 DataLink Layer 5-47

                                                                    Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                    Ethernetrsquos MAC protocol CSMACD

                                                                    5 DataLink Layer 5-48

                                                                    Ethernet CSMACD algorithm

                                                                    1 NIC receives datagram from network layer creates frame

                                                                    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                    4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                    5 After aborting NIC enters exponential backoff

                                                                    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                    (1 slot = 512 bit-times)

                                                                    5 DataLink Layer 5-49

                                                                    Ethernetrsquos CSMACD (more)

                                                                    Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                    Exponential Backoff Goal adapt retransmission

                                                                    attempts to estimated current loado heavy load random

                                                                    wait will be longer first collision choose K

                                                                    from 01 delay is K 512 bit transmission times

                                                                    after second collision choose K from 0123hellip

                                                                    after ten collisions choose K from 01234hellip1023

                                                                    5 DataLink Layer 5-50

                                                                    8023 Ethernet Standards Link amp Physical Layers

                                                                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                    applicationtransportnetwork

                                                                    linkphysical

                                                                    MAC protocoland frame format

                                                                    100BASE-TX

                                                                    100BASE-T4

                                                                    100BASE-FX100BASE-T2

                                                                    100BASE-SX 100BASE-BX

                                                                    fiber physical layercopper (twisterpair) physical layer

                                                                    5 DataLink Layer 5-51

                                                                    Manchester encoding

                                                                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                    synchronize to each othero no need for a centralized global clock among nodes

                                                                    Hey this is physical-layer stuff

                                                                    5 DataLink Layer 5-52

                                                                    Link Layer

                                                                    51 Introduction and services

                                                                    52 Error detection and correction

                                                                    53 Multiple access protocols

                                                                    54 Link-layer Addressing

                                                                    55 Ethernet

                                                                    56 Link-layer switches LANs

                                                                    59 A day in the life of a web request

                                                                    5 DataLink Layer 5-53

                                                                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                    o bits coming in one link go out all other links at same rate

                                                                    o all nodes connected to hub can collide with one another

                                                                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                    twisted pair

                                                                    hub

                                                                    5 DataLink Layer 5-54

                                                                    Switch link-layer device smarter than hubs take active

                                                                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                    transparento hosts are unaware of presence of switches

                                                                    plug-and-play self-learningo switches do not need to be configured

                                                                    5 DataLink Layer 5-55

                                                                    Switch allows multiple simultaneous transmissions

                                                                    hosts have dedicated direct connection to switch

                                                                    switches buffer packets Ethernet protocol used on

                                                                    each incoming link but no collisions full duplexo each link is its own collision

                                                                    domain switching A-to-Arsquo and B-

                                                                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                    A

                                                                    Arsquo

                                                                    B

                                                                    Brsquo

                                                                    C

                                                                    Crsquo

                                                                    switch with six interfaces(123456)

                                                                    1 23

                                                                    45

                                                                    6

                                                                    5 DataLink Layer 5-56

                                                                    Switch Table

                                                                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                    A each switch has a switch table each entryo (MAC address of host interface

                                                                    to reach host time stamp)

                                                                    looks like a routing table Q how are entries created

                                                                    maintained in switch table o something like a routing

                                                                    protocol

                                                                    A

                                                                    Arsquo

                                                                    B

                                                                    Brsquo

                                                                    C

                                                                    Crsquo

                                                                    switch with six interfaces(123456)

                                                                    1 23

                                                                    45

                                                                    6

                                                                    5 DataLink Layer 5-57

                                                                    Switch self-learning

                                                                    switch learns which hosts can be reached through which interfaceso when frame received

                                                                    switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                    o records senderlocation pair in switch table

                                                                    A

                                                                    Arsquo

                                                                    B

                                                                    Brsquo

                                                                    C

                                                                    Crsquo

                                                                    1 23

                                                                    45

                                                                    6

                                                                    A Arsquo

                                                                    Source ADest Arsquo

                                                                    MAC addr interface TTL

                                                                    Switch table (initially empty)

                                                                    A 1 60

                                                                    5 DataLink Layer 5-58

                                                                    Switch frame filteringforwardingWhen frame received

                                                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                    then if dest on segment from which frame arrived

                                                                    then drop the frame else forward the frame on interface indicated else flood

                                                                    forward on all but the interface on which the frame arrived

                                                                    5 DataLink Layer 5-59

                                                                    Self-learning forwarding example

                                                                    A

                                                                    Arsquo

                                                                    B

                                                                    Brsquo

                                                                    C

                                                                    Crsquo

                                                                    1 23

                                                                    45

                                                                    6

                                                                    A Arsquo

                                                                    Source ADest Arsquo

                                                                    MAC addr interface TTL

                                                                    Switch table (initially empty)

                                                                    A 1 60

                                                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                    frame destination unknownflood

                                                                    Arsquo A

                                                                    destination A location known

                                                                    Arsquo 4 60

                                                                    selective send

                                                                    5 DataLink Layer 5-60

                                                                    Interconnecting switches

                                                                    switches can be connected together

                                                                    A

                                                                    B

                                                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                    A self learning (works exactly the same as in single-switch case)

                                                                    S1

                                                                    C D

                                                                    E

                                                                    FS2

                                                                    S4

                                                                    S3

                                                                    H

                                                                    I

                                                                    G

                                                                    5 DataLink Layer 5-61

                                                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                    A

                                                                    B

                                                                    S1

                                                                    C D

                                                                    E

                                                                    FS2

                                                                    S4

                                                                    S3

                                                                    H

                                                                    I

                                                                    G

                                                                    1

                                                                    2 3

                                                                    34

                                                                    5 DataLink Layer 5-62

                                                                    Institutional network

                                                                    to externalnetwork

                                                                    router

                                                                    IP subnet

                                                                    mail server

                                                                    web server

                                                                    5 DataLink Layer 5-63

                                                                    Switches vs Routers both store-and-forward devices

                                                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                    routers maintain routing tables implement routing algorithms

                                                                    switches maintain switch tables implement filtering learning algorithms

                                                                    Switch

                                                                    5 DataLink Layer 5-64

                                                                    Link Layer

                                                                    51 Introduction and services

                                                                    52 Error detection and correction

                                                                    53Multiple access protocols

                                                                    54 Link-Layer Addressing

                                                                    55 Ethernet

                                                                    56 Link-layer switches 59 A day in the life of

                                                                    a web request

                                                                    5 DataLink Layer 5-65

                                                                    Synthesis a day in the life of a web request

                                                                    journey down protocol stack completeo application transport network link

                                                                    putting-it-all-together synthesiso goal identify review understand protocols

                                                                    (at all layers) involved in seemingly simple scenario requesting www page

                                                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                    5 DataLink Layer 5-66

                                                                    A day in the life scenario

                                                                    Comcast network 68800013

                                                                    Googlersquos network 64233160019 64233169105

                                                                    web server

                                                                    DNS server

                                                                    school network 68802024

                                                                    browser

                                                                    web page

                                                                    5 DataLink Layer 5-67

                                                                    A day in the lifehellip connecting to the Internet

                                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                    router(runs DHCP)

                                                                    DHCPUDP

                                                                    IPEthPhy

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCPUDP

                                                                    IPEthPhy

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCPDHCP

                                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                    5 DataLink Layer 5-68

                                                                    A day in the lifehellip connecting to the Internet

                                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                    router(runs DHCP)

                                                                    DHCPUDP

                                                                    IPEthPhy

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCPUDP

                                                                    IPEthPhy

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    DHCP

                                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                    DHCP client receives DHCP ACK reply

                                                                    5 DataLink Layer 5-69

                                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                                    DNSUDP

                                                                    IPEthPhy

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                    ARP query

                                                                    EthPhy

                                                                    ARP

                                                                    ARP

                                                                    ARP reply

                                                                    5 DataLink Layer 5-70

                                                                    A day in the lifehellip using DNS

                                                                    DNSUDP

                                                                    IPEthPhy

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                    demuxrsquoed to DNS server DNS server replies to

                                                                    client with IP address of wwwgooglecom

                                                                    Comcast network 68800013

                                                                    DNS server

                                                                    DNSUDP

                                                                    IPEthPhy

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    DNS

                                                                    5 DataLink Layer 5-71

                                                                    A day in the lifehellip TCP connection carrying HTTP

                                                                    HTTPTCPIP

                                                                    EthPhy

                                                                    HTTP

                                                                    to send HTTP request client first opens TCP socket to web server

                                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                    TCP connection established

                                                                    64233169105

                                                                    web server

                                                                    SYN

                                                                    SYN

                                                                    SYN

                                                                    SYN

                                                                    TCPIP

                                                                    EthPhy

                                                                    SYN

                                                                    SYN

                                                                    SYN

                                                                    SYNACK

                                                                    SYNACK

                                                                    SYNACK

                                                                    SYNACK

                                                                    SYNACK

                                                                    SYNACK

                                                                    SYNACK

                                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                    5 DataLink Layer 5-72

                                                                    A day in the lifehellip HTTP requestreply

                                                                    HTTPTCPIP

                                                                    EthPhy

                                                                    HTTP

                                                                    HTTP request sent into TCP socket

                                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                                    IP datgram containing HTTP reply routed back to client

                                                                    64233169105

                                                                    web server

                                                                    HTTPTCPIP

                                                                    EthPhy

                                                                    web server responds with HTTP reply (containing web page)

                                                                    HTTP

                                                                    HTTP

                                                                    HTTPHTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    HTTP

                                                                    web page finally () displayed

                                                                    5 DataLink Layer 5-73

                                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                    (except PHY) solid understanding of networking

                                                                    principles practice hellip could stop here hellip but lots of

                                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                    • Link Layer
                                                                    • Link Layer Introduction
                                                                    • Link layer context
                                                                    • Encapsulation
                                                                    • Link Layer Services
                                                                    • Link Layer Services (more)
                                                                    • Where is the link layer implemented
                                                                    • Adaptors Communicating
                                                                    • Slide 9
                                                                    • Parity Checking
                                                                    • Internet checksum (review)
                                                                    • Checksumming Cyclic Redundancy Check
                                                                    • Cyclic Redundancy Check
                                                                    • Slide 14
                                                                    • CRC Example
                                                                    • Slide 16
                                                                    • Multiple Access Links and Protocols
                                                                    • Cable Network Architecture Overview
                                                                    • Multiple Access protocols
                                                                    • Ideal Multiple Access Protocol
                                                                    • MAC Protocols a taxonomy
                                                                    • Channel Partitioning MAC protocols TDMA
                                                                    • Channel Partitioning MAC protocols FDMA
                                                                    • Random Access Protocols
                                                                    • ALOHA
                                                                    • CSMA (Carrier Sense Multiple Access)
                                                                    • CSMA collisions
                                                                    • CSMACD (Collision Detection)
                                                                    • CSMACD collision detection
                                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                                    • Slide 31
                                                                    • Slide 32
                                                                    • Summary of MAC protocols
                                                                    • Slide 34
                                                                    • MAC Addresses and ARP
                                                                    • LAN Addresses and ARP
                                                                    • LAN Address (more)
                                                                    • ARP Address Resolution Protocol
                                                                    • ARP protocol Same LAN (network)
                                                                    • Addressing routing to another LAN
                                                                    • Slide 41
                                                                    • Slide 42
                                                                    • Ethernet
                                                                    • Star topology
                                                                    • Ethernet Frame Structure
                                                                    • Ethernet Frame Structure (more)
                                                                    • Ethernet Unreliable connectionless
                                                                    • Ethernet CSMACD algorithm
                                                                    • Ethernetrsquos CSMACD (more)
                                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                                    • Manchester encoding
                                                                    • Slide 52
                                                                    • Hubs
                                                                    • Switch
                                                                    • Switch allows multiple simultaneous transmissions
                                                                    • Switch Table
                                                                    • Switch self-learning
                                                                    • Switch frame filteringforwarding
                                                                    • Self-learning forwarding example
                                                                    • Interconnecting switches
                                                                    • Self-learning multi-switch example
                                                                    • Institutional network
                                                                    • Switches vs Routers
                                                                    • Slide 64
                                                                    • Synthesis a day in the life of a web request
                                                                    • A day in the life scenario
                                                                    • A day in the lifehellip connecting to the Internet
                                                                    • Slide 68
                                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                                    • A day in the lifehellip using DNS
                                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                                    • A day in the lifehellip HTTP requestreply
                                                                    • Chapter 5 letrsquos take a breath

                                                                      5 DataLink Layer 5-35

                                                                      MAC Addresses and ARP

                                                                      32-bit IP address o network-layer addresso used to get datagram to destination IP subnet

                                                                      MAC (or LAN or physical or Ethernet) address o function get frame from one interface to

                                                                      another physically-connected interface (in same network)

                                                                      o 48 bit MAC addressbull burned in NIC ROM

                                                                      5 DataLink Layer 5-36

                                                                      LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                                      Broadcast address =FF-FF-FF-FF-FF-FF

                                                                      = adapter

                                                                      1A-2F-BB-76-09-AD

                                                                      58-23-D7-FA-20-B0

                                                                      0C-C4-11-6F-E3-98

                                                                      71-65-F7-2B-08-53

                                                                      LAN(wired orwireless)

                                                                      5 DataLink Layer 5-37

                                                                      LAN Address (more)

                                                                      MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                      space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                      o can move LAN card from one LAN to another

                                                                      IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                      attached

                                                                      5 DataLink Layer 5-38

                                                                      ARP Address Resolution Protocol

                                                                      Each IP node (host router) on LAN has ARP table

                                                                      ARP table IPMAC address mappings for some LAN nodes

                                                                      lt IP address MAC address TTLgto TTL (Time To Live)

                                                                      time after which address mapping will be forgotten (typically 20 min)

                                                                      Q given a nodersquos IP address how to determine its MAC address

                                                                      1A-2F-BB-76-09-AD

                                                                      0C-C4-11-6F-E3-98

                                                                      71-65-F7-2B-08-53

                                                                      LAN

                                                                      137196723

                                                                      137196778

                                                                      137196714

                                                                      137196788

                                                                      5 DataLink Layer 5-39

                                                                      ARP protocol Same LAN (network) A wants to send datagram

                                                                      to B and Brsquos MAC address not in Arsquos ARP table

                                                                      A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                      FF-FF-FF-FF-FFo all machines on LAN

                                                                      receive ARP query B receives ARP packet

                                                                      replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                      address (unicast)

                                                                      A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                      ARP is ldquoplug-and-playrdquoo nodes create their

                                                                      ARP tables without intervention from net administrator

                                                                      5 DataLink Layer 5-40

                                                                      Addressing routing to another LAN

                                                                      R

                                                                      1A-23-F9-CD-06-9B

                                                                      222222222220111111111110

                                                                      E6-E9-00-17-BB-4B

                                                                      CC-49-DE-D0-AB-7D

                                                                      111111111112

                                                                      111111111111

                                                                      A74-29-9C-E8-FF-55

                                                                      222222222221

                                                                      88-B2-2F-54-1A-0F

                                                                      B222222222222

                                                                      49-BD-D2-C7-56-2A

                                                                      walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                      two ARP tables in router R one for each IP network (LAN)

                                                                      LAN LAN

                                                                      5 DataLink Layer 5-41

                                                                      A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                      frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                      destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                      R

                                                                      1A-23-F9-CD-06-9B

                                                                      222222222220

                                                                      111111111110

                                                                      E6-E9-00-17-BB-4B

                                                                      CC-49-DE-D0-AB-7D

                                                                      111111111112

                                                                      111111111111

                                                                      A74-29-9C-E8-FF-55

                                                                      222222222221

                                                                      88-B2-2F-54-1A-0F

                                                                      B222222222222

                                                                      49-BD-D2-C7-56-2A

                                                                      This is a really importantexample ndash make sure youunderstand

                                                                      5 DataLink Layer 5-42

                                                                      Link Layer

                                                                      51 Introduction and services

                                                                      52 Error detection and correction

                                                                      53Multiple access protocols

                                                                      54 Link-Layer Addressing

                                                                      55 Ethernet

                                                                      56 Link-layer switches 59 A day in the life of

                                                                      a web request

                                                                      5 DataLink Layer 5-43

                                                                      Ethernet

                                                                      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                      Metcalfersquos Ethernetsketch

                                                                      5 DataLink Layer 5-44

                                                                      Star topology bus topology popular through mid 90s

                                                                      o all nodes in same collision domain (can collide with each other)

                                                                      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                      (nodes do not collide with each other)

                                                                      switch

                                                                      bus coaxial cable star

                                                                      5 DataLink Layer 5-45

                                                                      Ethernet Frame Structure

                                                                      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                      Preamble 7 bytes with pattern 10101010 followed by one

                                                                      byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                      rates

                                                                      5 DataLink Layer 5-46

                                                                      Ethernet Frame Structure (more) Addresses 6 bytes

                                                                      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                      o otherwise adapter discards frame

                                                                      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                      CRC checked at receiver if error is detected frame is dropped

                                                                      5 DataLink Layer 5-47

                                                                      Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                      Ethernetrsquos MAC protocol CSMACD

                                                                      5 DataLink Layer 5-48

                                                                      Ethernet CSMACD algorithm

                                                                      1 NIC receives datagram from network layer creates frame

                                                                      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                      4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                      5 After aborting NIC enters exponential backoff

                                                                      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                      (1 slot = 512 bit-times)

                                                                      5 DataLink Layer 5-49

                                                                      Ethernetrsquos CSMACD (more)

                                                                      Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                      Exponential Backoff Goal adapt retransmission

                                                                      attempts to estimated current loado heavy load random

                                                                      wait will be longer first collision choose K

                                                                      from 01 delay is K 512 bit transmission times

                                                                      after second collision choose K from 0123hellip

                                                                      after ten collisions choose K from 01234hellip1023

                                                                      5 DataLink Layer 5-50

                                                                      8023 Ethernet Standards Link amp Physical Layers

                                                                      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                      Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                      applicationtransportnetwork

                                                                      linkphysical

                                                                      MAC protocoland frame format

                                                                      100BASE-TX

                                                                      100BASE-T4

                                                                      100BASE-FX100BASE-T2

                                                                      100BASE-SX 100BASE-BX

                                                                      fiber physical layercopper (twisterpair) physical layer

                                                                      5 DataLink Layer 5-51

                                                                      Manchester encoding

                                                                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                      synchronize to each othero no need for a centralized global clock among nodes

                                                                      Hey this is physical-layer stuff

                                                                      5 DataLink Layer 5-52

                                                                      Link Layer

                                                                      51 Introduction and services

                                                                      52 Error detection and correction

                                                                      53 Multiple access protocols

                                                                      54 Link-layer Addressing

                                                                      55 Ethernet

                                                                      56 Link-layer switches LANs

                                                                      59 A day in the life of a web request

                                                                      5 DataLink Layer 5-53

                                                                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                      o bits coming in one link go out all other links at same rate

                                                                      o all nodes connected to hub can collide with one another

                                                                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                      twisted pair

                                                                      hub

                                                                      5 DataLink Layer 5-54

                                                                      Switch link-layer device smarter than hubs take active

                                                                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                      transparento hosts are unaware of presence of switches

                                                                      plug-and-play self-learningo switches do not need to be configured

                                                                      5 DataLink Layer 5-55

                                                                      Switch allows multiple simultaneous transmissions

                                                                      hosts have dedicated direct connection to switch

                                                                      switches buffer packets Ethernet protocol used on

                                                                      each incoming link but no collisions full duplexo each link is its own collision

                                                                      domain switching A-to-Arsquo and B-

                                                                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                      A

                                                                      Arsquo

                                                                      B

                                                                      Brsquo

                                                                      C

                                                                      Crsquo

                                                                      switch with six interfaces(123456)

                                                                      1 23

                                                                      45

                                                                      6

                                                                      5 DataLink Layer 5-56

                                                                      Switch Table

                                                                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                      A each switch has a switch table each entryo (MAC address of host interface

                                                                      to reach host time stamp)

                                                                      looks like a routing table Q how are entries created

                                                                      maintained in switch table o something like a routing

                                                                      protocol

                                                                      A

                                                                      Arsquo

                                                                      B

                                                                      Brsquo

                                                                      C

                                                                      Crsquo

                                                                      switch with six interfaces(123456)

                                                                      1 23

                                                                      45

                                                                      6

                                                                      5 DataLink Layer 5-57

                                                                      Switch self-learning

                                                                      switch learns which hosts can be reached through which interfaceso when frame received

                                                                      switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                      o records senderlocation pair in switch table

                                                                      A

                                                                      Arsquo

                                                                      B

                                                                      Brsquo

                                                                      C

                                                                      Crsquo

                                                                      1 23

                                                                      45

                                                                      6

                                                                      A Arsquo

                                                                      Source ADest Arsquo

                                                                      MAC addr interface TTL

                                                                      Switch table (initially empty)

                                                                      A 1 60

                                                                      5 DataLink Layer 5-58

                                                                      Switch frame filteringforwardingWhen frame received

                                                                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                      then if dest on segment from which frame arrived

                                                                      then drop the frame else forward the frame on interface indicated else flood

                                                                      forward on all but the interface on which the frame arrived

                                                                      5 DataLink Layer 5-59

                                                                      Self-learning forwarding example

                                                                      A

                                                                      Arsquo

                                                                      B

                                                                      Brsquo

                                                                      C

                                                                      Crsquo

                                                                      1 23

                                                                      45

                                                                      6

                                                                      A Arsquo

                                                                      Source ADest Arsquo

                                                                      MAC addr interface TTL

                                                                      Switch table (initially empty)

                                                                      A 1 60

                                                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                      frame destination unknownflood

                                                                      Arsquo A

                                                                      destination A location known

                                                                      Arsquo 4 60

                                                                      selective send

                                                                      5 DataLink Layer 5-60

                                                                      Interconnecting switches

                                                                      switches can be connected together

                                                                      A

                                                                      B

                                                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                      A self learning (works exactly the same as in single-switch case)

                                                                      S1

                                                                      C D

                                                                      E

                                                                      FS2

                                                                      S4

                                                                      S3

                                                                      H

                                                                      I

                                                                      G

                                                                      5 DataLink Layer 5-61

                                                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                      A

                                                                      B

                                                                      S1

                                                                      C D

                                                                      E

                                                                      FS2

                                                                      S4

                                                                      S3

                                                                      H

                                                                      I

                                                                      G

                                                                      1

                                                                      2 3

                                                                      34

                                                                      5 DataLink Layer 5-62

                                                                      Institutional network

                                                                      to externalnetwork

                                                                      router

                                                                      IP subnet

                                                                      mail server

                                                                      web server

                                                                      5 DataLink Layer 5-63

                                                                      Switches vs Routers both store-and-forward devices

                                                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                      routers maintain routing tables implement routing algorithms

                                                                      switches maintain switch tables implement filtering learning algorithms

                                                                      Switch

                                                                      5 DataLink Layer 5-64

                                                                      Link Layer

                                                                      51 Introduction and services

                                                                      52 Error detection and correction

                                                                      53Multiple access protocols

                                                                      54 Link-Layer Addressing

                                                                      55 Ethernet

                                                                      56 Link-layer switches 59 A day in the life of

                                                                      a web request

                                                                      5 DataLink Layer 5-65

                                                                      Synthesis a day in the life of a web request

                                                                      journey down protocol stack completeo application transport network link

                                                                      putting-it-all-together synthesiso goal identify review understand protocols

                                                                      (at all layers) involved in seemingly simple scenario requesting www page

                                                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                      5 DataLink Layer 5-66

                                                                      A day in the life scenario

                                                                      Comcast network 68800013

                                                                      Googlersquos network 64233160019 64233169105

                                                                      web server

                                                                      DNS server

                                                                      school network 68802024

                                                                      browser

                                                                      web page

                                                                      5 DataLink Layer 5-67

                                                                      A day in the lifehellip connecting to the Internet

                                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                      router(runs DHCP)

                                                                      DHCPUDP

                                                                      IPEthPhy

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCPUDP

                                                                      IPEthPhy

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCPDHCP

                                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                      5 DataLink Layer 5-68

                                                                      A day in the lifehellip connecting to the Internet

                                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                      router(runs DHCP)

                                                                      DHCPUDP

                                                                      IPEthPhy

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCPUDP

                                                                      IPEthPhy

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      DHCP

                                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                      DHCP client receives DHCP ACK reply

                                                                      5 DataLink Layer 5-69

                                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                                      DNSUDP

                                                                      IPEthPhy

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                      ARP query

                                                                      EthPhy

                                                                      ARP

                                                                      ARP

                                                                      ARP reply

                                                                      5 DataLink Layer 5-70

                                                                      A day in the lifehellip using DNS

                                                                      DNSUDP

                                                                      IPEthPhy

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                      demuxrsquoed to DNS server DNS server replies to

                                                                      client with IP address of wwwgooglecom

                                                                      Comcast network 68800013

                                                                      DNS server

                                                                      DNSUDP

                                                                      IPEthPhy

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      DNS

                                                                      5 DataLink Layer 5-71

                                                                      A day in the lifehellip TCP connection carrying HTTP

                                                                      HTTPTCPIP

                                                                      EthPhy

                                                                      HTTP

                                                                      to send HTTP request client first opens TCP socket to web server

                                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                      TCP connection established

                                                                      64233169105

                                                                      web server

                                                                      SYN

                                                                      SYN

                                                                      SYN

                                                                      SYN

                                                                      TCPIP

                                                                      EthPhy

                                                                      SYN

                                                                      SYN

                                                                      SYN

                                                                      SYNACK

                                                                      SYNACK

                                                                      SYNACK

                                                                      SYNACK

                                                                      SYNACK

                                                                      SYNACK

                                                                      SYNACK

                                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                      5 DataLink Layer 5-72

                                                                      A day in the lifehellip HTTP requestreply

                                                                      HTTPTCPIP

                                                                      EthPhy

                                                                      HTTP

                                                                      HTTP request sent into TCP socket

                                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                                      IP datgram containing HTTP reply routed back to client

                                                                      64233169105

                                                                      web server

                                                                      HTTPTCPIP

                                                                      EthPhy

                                                                      web server responds with HTTP reply (containing web page)

                                                                      HTTP

                                                                      HTTP

                                                                      HTTPHTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      HTTP

                                                                      web page finally () displayed

                                                                      5 DataLink Layer 5-73

                                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                      (except PHY) solid understanding of networking

                                                                      principles practice hellip could stop here hellip but lots of

                                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                      • Link Layer
                                                                      • Link Layer Introduction
                                                                      • Link layer context
                                                                      • Encapsulation
                                                                      • Link Layer Services
                                                                      • Link Layer Services (more)
                                                                      • Where is the link layer implemented
                                                                      • Adaptors Communicating
                                                                      • Slide 9
                                                                      • Parity Checking
                                                                      • Internet checksum (review)
                                                                      • Checksumming Cyclic Redundancy Check
                                                                      • Cyclic Redundancy Check
                                                                      • Slide 14
                                                                      • CRC Example
                                                                      • Slide 16
                                                                      • Multiple Access Links and Protocols
                                                                      • Cable Network Architecture Overview
                                                                      • Multiple Access protocols
                                                                      • Ideal Multiple Access Protocol
                                                                      • MAC Protocols a taxonomy
                                                                      • Channel Partitioning MAC protocols TDMA
                                                                      • Channel Partitioning MAC protocols FDMA
                                                                      • Random Access Protocols
                                                                      • ALOHA
                                                                      • CSMA (Carrier Sense Multiple Access)
                                                                      • CSMA collisions
                                                                      • CSMACD (Collision Detection)
                                                                      • CSMACD collision detection
                                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                                      • Slide 31
                                                                      • Slide 32
                                                                      • Summary of MAC protocols
                                                                      • Slide 34
                                                                      • MAC Addresses and ARP
                                                                      • LAN Addresses and ARP
                                                                      • LAN Address (more)
                                                                      • ARP Address Resolution Protocol
                                                                      • ARP protocol Same LAN (network)
                                                                      • Addressing routing to another LAN
                                                                      • Slide 41
                                                                      • Slide 42
                                                                      • Ethernet
                                                                      • Star topology
                                                                      • Ethernet Frame Structure
                                                                      • Ethernet Frame Structure (more)
                                                                      • Ethernet Unreliable connectionless
                                                                      • Ethernet CSMACD algorithm
                                                                      • Ethernetrsquos CSMACD (more)
                                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                                      • Manchester encoding
                                                                      • Slide 52
                                                                      • Hubs
                                                                      • Switch
                                                                      • Switch allows multiple simultaneous transmissions
                                                                      • Switch Table
                                                                      • Switch self-learning
                                                                      • Switch frame filteringforwarding
                                                                      • Self-learning forwarding example
                                                                      • Interconnecting switches
                                                                      • Self-learning multi-switch example
                                                                      • Institutional network
                                                                      • Switches vs Routers
                                                                      • Slide 64
                                                                      • Synthesis a day in the life of a web request
                                                                      • A day in the life scenario
                                                                      • A day in the lifehellip connecting to the Internet
                                                                      • Slide 68
                                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                                      • A day in the lifehellip using DNS
                                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                                      • A day in the lifehellip HTTP requestreply
                                                                      • Chapter 5 letrsquos take a breath

                                                                        5 DataLink Layer 5-36

                                                                        LAN Addresses and ARPEach adapter on LAN has unique LAN address

                                                                        Broadcast address =FF-FF-FF-FF-FF-FF

                                                                        = adapter

                                                                        1A-2F-BB-76-09-AD

                                                                        58-23-D7-FA-20-B0

                                                                        0C-C4-11-6F-E3-98

                                                                        71-65-F7-2B-08-53

                                                                        LAN(wired orwireless)

                                                                        5 DataLink Layer 5-37

                                                                        LAN Address (more)

                                                                        MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                        space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                        o can move LAN card from one LAN to another

                                                                        IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                        attached

                                                                        5 DataLink Layer 5-38

                                                                        ARP Address Resolution Protocol

                                                                        Each IP node (host router) on LAN has ARP table

                                                                        ARP table IPMAC address mappings for some LAN nodes

                                                                        lt IP address MAC address TTLgto TTL (Time To Live)

                                                                        time after which address mapping will be forgotten (typically 20 min)

                                                                        Q given a nodersquos IP address how to determine its MAC address

                                                                        1A-2F-BB-76-09-AD

                                                                        0C-C4-11-6F-E3-98

                                                                        71-65-F7-2B-08-53

                                                                        LAN

                                                                        137196723

                                                                        137196778

                                                                        137196714

                                                                        137196788

                                                                        5 DataLink Layer 5-39

                                                                        ARP protocol Same LAN (network) A wants to send datagram

                                                                        to B and Brsquos MAC address not in Arsquos ARP table

                                                                        A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                        FF-FF-FF-FF-FFo all machines on LAN

                                                                        receive ARP query B receives ARP packet

                                                                        replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                        address (unicast)

                                                                        A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                        ARP is ldquoplug-and-playrdquoo nodes create their

                                                                        ARP tables without intervention from net administrator

                                                                        5 DataLink Layer 5-40

                                                                        Addressing routing to another LAN

                                                                        R

                                                                        1A-23-F9-CD-06-9B

                                                                        222222222220111111111110

                                                                        E6-E9-00-17-BB-4B

                                                                        CC-49-DE-D0-AB-7D

                                                                        111111111112

                                                                        111111111111

                                                                        A74-29-9C-E8-FF-55

                                                                        222222222221

                                                                        88-B2-2F-54-1A-0F

                                                                        B222222222222

                                                                        49-BD-D2-C7-56-2A

                                                                        walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                        two ARP tables in router R one for each IP network (LAN)

                                                                        LAN LAN

                                                                        5 DataLink Layer 5-41

                                                                        A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                        frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                        destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                        R

                                                                        1A-23-F9-CD-06-9B

                                                                        222222222220

                                                                        111111111110

                                                                        E6-E9-00-17-BB-4B

                                                                        CC-49-DE-D0-AB-7D

                                                                        111111111112

                                                                        111111111111

                                                                        A74-29-9C-E8-FF-55

                                                                        222222222221

                                                                        88-B2-2F-54-1A-0F

                                                                        B222222222222

                                                                        49-BD-D2-C7-56-2A

                                                                        This is a really importantexample ndash make sure youunderstand

                                                                        5 DataLink Layer 5-42

                                                                        Link Layer

                                                                        51 Introduction and services

                                                                        52 Error detection and correction

                                                                        53Multiple access protocols

                                                                        54 Link-Layer Addressing

                                                                        55 Ethernet

                                                                        56 Link-layer switches 59 A day in the life of

                                                                        a web request

                                                                        5 DataLink Layer 5-43

                                                                        Ethernet

                                                                        ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                        Metcalfersquos Ethernetsketch

                                                                        5 DataLink Layer 5-44

                                                                        Star topology bus topology popular through mid 90s

                                                                        o all nodes in same collision domain (can collide with each other)

                                                                        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                        (nodes do not collide with each other)

                                                                        switch

                                                                        bus coaxial cable star

                                                                        5 DataLink Layer 5-45

                                                                        Ethernet Frame Structure

                                                                        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                        Preamble 7 bytes with pattern 10101010 followed by one

                                                                        byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                        rates

                                                                        5 DataLink Layer 5-46

                                                                        Ethernet Frame Structure (more) Addresses 6 bytes

                                                                        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                        o otherwise adapter discards frame

                                                                        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                        CRC checked at receiver if error is detected frame is dropped

                                                                        5 DataLink Layer 5-47

                                                                        Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                        Ethernetrsquos MAC protocol CSMACD

                                                                        5 DataLink Layer 5-48

                                                                        Ethernet CSMACD algorithm

                                                                        1 NIC receives datagram from network layer creates frame

                                                                        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                        4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                        5 After aborting NIC enters exponential backoff

                                                                        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                        (1 slot = 512 bit-times)

                                                                        5 DataLink Layer 5-49

                                                                        Ethernetrsquos CSMACD (more)

                                                                        Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                        Exponential Backoff Goal adapt retransmission

                                                                        attempts to estimated current loado heavy load random

                                                                        wait will be longer first collision choose K

                                                                        from 01 delay is K 512 bit transmission times

                                                                        after second collision choose K from 0123hellip

                                                                        after ten collisions choose K from 01234hellip1023

                                                                        5 DataLink Layer 5-50

                                                                        8023 Ethernet Standards Link amp Physical Layers

                                                                        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                        Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                        applicationtransportnetwork

                                                                        linkphysical

                                                                        MAC protocoland frame format

                                                                        100BASE-TX

                                                                        100BASE-T4

                                                                        100BASE-FX100BASE-T2

                                                                        100BASE-SX 100BASE-BX

                                                                        fiber physical layercopper (twisterpair) physical layer

                                                                        5 DataLink Layer 5-51

                                                                        Manchester encoding

                                                                        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                        synchronize to each othero no need for a centralized global clock among nodes

                                                                        Hey this is physical-layer stuff

                                                                        5 DataLink Layer 5-52

                                                                        Link Layer

                                                                        51 Introduction and services

                                                                        52 Error detection and correction

                                                                        53 Multiple access protocols

                                                                        54 Link-layer Addressing

                                                                        55 Ethernet

                                                                        56 Link-layer switches LANs

                                                                        59 A day in the life of a web request

                                                                        5 DataLink Layer 5-53

                                                                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                        o bits coming in one link go out all other links at same rate

                                                                        o all nodes connected to hub can collide with one another

                                                                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                        twisted pair

                                                                        hub

                                                                        5 DataLink Layer 5-54

                                                                        Switch link-layer device smarter than hubs take active

                                                                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                        transparento hosts are unaware of presence of switches

                                                                        plug-and-play self-learningo switches do not need to be configured

                                                                        5 DataLink Layer 5-55

                                                                        Switch allows multiple simultaneous transmissions

                                                                        hosts have dedicated direct connection to switch

                                                                        switches buffer packets Ethernet protocol used on

                                                                        each incoming link but no collisions full duplexo each link is its own collision

                                                                        domain switching A-to-Arsquo and B-

                                                                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                        A

                                                                        Arsquo

                                                                        B

                                                                        Brsquo

                                                                        C

                                                                        Crsquo

                                                                        switch with six interfaces(123456)

                                                                        1 23

                                                                        45

                                                                        6

                                                                        5 DataLink Layer 5-56

                                                                        Switch Table

                                                                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                        A each switch has a switch table each entryo (MAC address of host interface

                                                                        to reach host time stamp)

                                                                        looks like a routing table Q how are entries created

                                                                        maintained in switch table o something like a routing

                                                                        protocol

                                                                        A

                                                                        Arsquo

                                                                        B

                                                                        Brsquo

                                                                        C

                                                                        Crsquo

                                                                        switch with six interfaces(123456)

                                                                        1 23

                                                                        45

                                                                        6

                                                                        5 DataLink Layer 5-57

                                                                        Switch self-learning

                                                                        switch learns which hosts can be reached through which interfaceso when frame received

                                                                        switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                        o records senderlocation pair in switch table

                                                                        A

                                                                        Arsquo

                                                                        B

                                                                        Brsquo

                                                                        C

                                                                        Crsquo

                                                                        1 23

                                                                        45

                                                                        6

                                                                        A Arsquo

                                                                        Source ADest Arsquo

                                                                        MAC addr interface TTL

                                                                        Switch table (initially empty)

                                                                        A 1 60

                                                                        5 DataLink Layer 5-58

                                                                        Switch frame filteringforwardingWhen frame received

                                                                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                        then if dest on segment from which frame arrived

                                                                        then drop the frame else forward the frame on interface indicated else flood

                                                                        forward on all but the interface on which the frame arrived

                                                                        5 DataLink Layer 5-59

                                                                        Self-learning forwarding example

                                                                        A

                                                                        Arsquo

                                                                        B

                                                                        Brsquo

                                                                        C

                                                                        Crsquo

                                                                        1 23

                                                                        45

                                                                        6

                                                                        A Arsquo

                                                                        Source ADest Arsquo

                                                                        MAC addr interface TTL

                                                                        Switch table (initially empty)

                                                                        A 1 60

                                                                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                        frame destination unknownflood

                                                                        Arsquo A

                                                                        destination A location known

                                                                        Arsquo 4 60

                                                                        selective send

                                                                        5 DataLink Layer 5-60

                                                                        Interconnecting switches

                                                                        switches can be connected together

                                                                        A

                                                                        B

                                                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                        A self learning (works exactly the same as in single-switch case)

                                                                        S1

                                                                        C D

                                                                        E

                                                                        FS2

                                                                        S4

                                                                        S3

                                                                        H

                                                                        I

                                                                        G

                                                                        5 DataLink Layer 5-61

                                                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                        A

                                                                        B

                                                                        S1

                                                                        C D

                                                                        E

                                                                        FS2

                                                                        S4

                                                                        S3

                                                                        H

                                                                        I

                                                                        G

                                                                        1

                                                                        2 3

                                                                        34

                                                                        5 DataLink Layer 5-62

                                                                        Institutional network

                                                                        to externalnetwork

                                                                        router

                                                                        IP subnet

                                                                        mail server

                                                                        web server

                                                                        5 DataLink Layer 5-63

                                                                        Switches vs Routers both store-and-forward devices

                                                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                        routers maintain routing tables implement routing algorithms

                                                                        switches maintain switch tables implement filtering learning algorithms

                                                                        Switch

                                                                        5 DataLink Layer 5-64

                                                                        Link Layer

                                                                        51 Introduction and services

                                                                        52 Error detection and correction

                                                                        53Multiple access protocols

                                                                        54 Link-Layer Addressing

                                                                        55 Ethernet

                                                                        56 Link-layer switches 59 A day in the life of

                                                                        a web request

                                                                        5 DataLink Layer 5-65

                                                                        Synthesis a day in the life of a web request

                                                                        journey down protocol stack completeo application transport network link

                                                                        putting-it-all-together synthesiso goal identify review understand protocols

                                                                        (at all layers) involved in seemingly simple scenario requesting www page

                                                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                        5 DataLink Layer 5-66

                                                                        A day in the life scenario

                                                                        Comcast network 68800013

                                                                        Googlersquos network 64233160019 64233169105

                                                                        web server

                                                                        DNS server

                                                                        school network 68802024

                                                                        browser

                                                                        web page

                                                                        5 DataLink Layer 5-67

                                                                        A day in the lifehellip connecting to the Internet

                                                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                        router(runs DHCP)

                                                                        DHCPUDP

                                                                        IPEthPhy

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCPUDP

                                                                        IPEthPhy

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCPDHCP

                                                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                        5 DataLink Layer 5-68

                                                                        A day in the lifehellip connecting to the Internet

                                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                        router(runs DHCP)

                                                                        DHCPUDP

                                                                        IPEthPhy

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCPUDP

                                                                        IPEthPhy

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        DHCP

                                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                        DHCP client receives DHCP ACK reply

                                                                        5 DataLink Layer 5-69

                                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                                        DNSUDP

                                                                        IPEthPhy

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                        ARP query

                                                                        EthPhy

                                                                        ARP

                                                                        ARP

                                                                        ARP reply

                                                                        5 DataLink Layer 5-70

                                                                        A day in the lifehellip using DNS

                                                                        DNSUDP

                                                                        IPEthPhy

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                        demuxrsquoed to DNS server DNS server replies to

                                                                        client with IP address of wwwgooglecom

                                                                        Comcast network 68800013

                                                                        DNS server

                                                                        DNSUDP

                                                                        IPEthPhy

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        DNS

                                                                        5 DataLink Layer 5-71

                                                                        A day in the lifehellip TCP connection carrying HTTP

                                                                        HTTPTCPIP

                                                                        EthPhy

                                                                        HTTP

                                                                        to send HTTP request client first opens TCP socket to web server

                                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                        TCP connection established

                                                                        64233169105

                                                                        web server

                                                                        SYN

                                                                        SYN

                                                                        SYN

                                                                        SYN

                                                                        TCPIP

                                                                        EthPhy

                                                                        SYN

                                                                        SYN

                                                                        SYN

                                                                        SYNACK

                                                                        SYNACK

                                                                        SYNACK

                                                                        SYNACK

                                                                        SYNACK

                                                                        SYNACK

                                                                        SYNACK

                                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                        5 DataLink Layer 5-72

                                                                        A day in the lifehellip HTTP requestreply

                                                                        HTTPTCPIP

                                                                        EthPhy

                                                                        HTTP

                                                                        HTTP request sent into TCP socket

                                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                                        IP datgram containing HTTP reply routed back to client

                                                                        64233169105

                                                                        web server

                                                                        HTTPTCPIP

                                                                        EthPhy

                                                                        web server responds with HTTP reply (containing web page)

                                                                        HTTP

                                                                        HTTP

                                                                        HTTPHTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        HTTP

                                                                        web page finally () displayed

                                                                        5 DataLink Layer 5-73

                                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                        (except PHY) solid understanding of networking

                                                                        principles practice hellip could stop here hellip but lots of

                                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                        • Link Layer
                                                                        • Link Layer Introduction
                                                                        • Link layer context
                                                                        • Encapsulation
                                                                        • Link Layer Services
                                                                        • Link Layer Services (more)
                                                                        • Where is the link layer implemented
                                                                        • Adaptors Communicating
                                                                        • Slide 9
                                                                        • Parity Checking
                                                                        • Internet checksum (review)
                                                                        • Checksumming Cyclic Redundancy Check
                                                                        • Cyclic Redundancy Check
                                                                        • Slide 14
                                                                        • CRC Example
                                                                        • Slide 16
                                                                        • Multiple Access Links and Protocols
                                                                        • Cable Network Architecture Overview
                                                                        • Multiple Access protocols
                                                                        • Ideal Multiple Access Protocol
                                                                        • MAC Protocols a taxonomy
                                                                        • Channel Partitioning MAC protocols TDMA
                                                                        • Channel Partitioning MAC protocols FDMA
                                                                        • Random Access Protocols
                                                                        • ALOHA
                                                                        • CSMA (Carrier Sense Multiple Access)
                                                                        • CSMA collisions
                                                                        • CSMACD (Collision Detection)
                                                                        • CSMACD collision detection
                                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                                        • Slide 31
                                                                        • Slide 32
                                                                        • Summary of MAC protocols
                                                                        • Slide 34
                                                                        • MAC Addresses and ARP
                                                                        • LAN Addresses and ARP
                                                                        • LAN Address (more)
                                                                        • ARP Address Resolution Protocol
                                                                        • ARP protocol Same LAN (network)
                                                                        • Addressing routing to another LAN
                                                                        • Slide 41
                                                                        • Slide 42
                                                                        • Ethernet
                                                                        • Star topology
                                                                        • Ethernet Frame Structure
                                                                        • Ethernet Frame Structure (more)
                                                                        • Ethernet Unreliable connectionless
                                                                        • Ethernet CSMACD algorithm
                                                                        • Ethernetrsquos CSMACD (more)
                                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                                        • Manchester encoding
                                                                        • Slide 52
                                                                        • Hubs
                                                                        • Switch
                                                                        • Switch allows multiple simultaneous transmissions
                                                                        • Switch Table
                                                                        • Switch self-learning
                                                                        • Switch frame filteringforwarding
                                                                        • Self-learning forwarding example
                                                                        • Interconnecting switches
                                                                        • Self-learning multi-switch example
                                                                        • Institutional network
                                                                        • Switches vs Routers
                                                                        • Slide 64
                                                                        • Synthesis a day in the life of a web request
                                                                        • A day in the life scenario
                                                                        • A day in the lifehellip connecting to the Internet
                                                                        • Slide 68
                                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                                        • A day in the lifehellip using DNS
                                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                                        • A day in the lifehellip HTTP requestreply
                                                                        • Chapter 5 letrsquos take a breath

                                                                          5 DataLink Layer 5-37

                                                                          LAN Address (more)

                                                                          MAC address allocation administered by IEEE manufacturer buys portion of MAC address

                                                                          space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability

                                                                          o can move LAN card from one LAN to another

                                                                          IP hierarchical address NOT portableo address depends on IP subnet to which node is

                                                                          attached

                                                                          5 DataLink Layer 5-38

                                                                          ARP Address Resolution Protocol

                                                                          Each IP node (host router) on LAN has ARP table

                                                                          ARP table IPMAC address mappings for some LAN nodes

                                                                          lt IP address MAC address TTLgto TTL (Time To Live)

                                                                          time after which address mapping will be forgotten (typically 20 min)

                                                                          Q given a nodersquos IP address how to determine its MAC address

                                                                          1A-2F-BB-76-09-AD

                                                                          0C-C4-11-6F-E3-98

                                                                          71-65-F7-2B-08-53

                                                                          LAN

                                                                          137196723

                                                                          137196778

                                                                          137196714

                                                                          137196788

                                                                          5 DataLink Layer 5-39

                                                                          ARP protocol Same LAN (network) A wants to send datagram

                                                                          to B and Brsquos MAC address not in Arsquos ARP table

                                                                          A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                          FF-FF-FF-FF-FFo all machines on LAN

                                                                          receive ARP query B receives ARP packet

                                                                          replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                          address (unicast)

                                                                          A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                          ARP is ldquoplug-and-playrdquoo nodes create their

                                                                          ARP tables without intervention from net administrator

                                                                          5 DataLink Layer 5-40

                                                                          Addressing routing to another LAN

                                                                          R

                                                                          1A-23-F9-CD-06-9B

                                                                          222222222220111111111110

                                                                          E6-E9-00-17-BB-4B

                                                                          CC-49-DE-D0-AB-7D

                                                                          111111111112

                                                                          111111111111

                                                                          A74-29-9C-E8-FF-55

                                                                          222222222221

                                                                          88-B2-2F-54-1A-0F

                                                                          B222222222222

                                                                          49-BD-D2-C7-56-2A

                                                                          walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                          two ARP tables in router R one for each IP network (LAN)

                                                                          LAN LAN

                                                                          5 DataLink Layer 5-41

                                                                          A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                          frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                          destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                          R

                                                                          1A-23-F9-CD-06-9B

                                                                          222222222220

                                                                          111111111110

                                                                          E6-E9-00-17-BB-4B

                                                                          CC-49-DE-D0-AB-7D

                                                                          111111111112

                                                                          111111111111

                                                                          A74-29-9C-E8-FF-55

                                                                          222222222221

                                                                          88-B2-2F-54-1A-0F

                                                                          B222222222222

                                                                          49-BD-D2-C7-56-2A

                                                                          This is a really importantexample ndash make sure youunderstand

                                                                          5 DataLink Layer 5-42

                                                                          Link Layer

                                                                          51 Introduction and services

                                                                          52 Error detection and correction

                                                                          53Multiple access protocols

                                                                          54 Link-Layer Addressing

                                                                          55 Ethernet

                                                                          56 Link-layer switches 59 A day in the life of

                                                                          a web request

                                                                          5 DataLink Layer 5-43

                                                                          Ethernet

                                                                          ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                          Metcalfersquos Ethernetsketch

                                                                          5 DataLink Layer 5-44

                                                                          Star topology bus topology popular through mid 90s

                                                                          o all nodes in same collision domain (can collide with each other)

                                                                          today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                          (nodes do not collide with each other)

                                                                          switch

                                                                          bus coaxial cable star

                                                                          5 DataLink Layer 5-45

                                                                          Ethernet Frame Structure

                                                                          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                          Preamble 7 bytes with pattern 10101010 followed by one

                                                                          byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                          rates

                                                                          5 DataLink Layer 5-46

                                                                          Ethernet Frame Structure (more) Addresses 6 bytes

                                                                          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                          o otherwise adapter discards frame

                                                                          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                          CRC checked at receiver if error is detected frame is dropped

                                                                          5 DataLink Layer 5-47

                                                                          Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                          Ethernetrsquos MAC protocol CSMACD

                                                                          5 DataLink Layer 5-48

                                                                          Ethernet CSMACD algorithm

                                                                          1 NIC receives datagram from network layer creates frame

                                                                          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                          4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                          5 After aborting NIC enters exponential backoff

                                                                          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                          (1 slot = 512 bit-times)

                                                                          5 DataLink Layer 5-49

                                                                          Ethernetrsquos CSMACD (more)

                                                                          Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                          Exponential Backoff Goal adapt retransmission

                                                                          attempts to estimated current loado heavy load random

                                                                          wait will be longer first collision choose K

                                                                          from 01 delay is K 512 bit transmission times

                                                                          after second collision choose K from 0123hellip

                                                                          after ten collisions choose K from 01234hellip1023

                                                                          5 DataLink Layer 5-50

                                                                          8023 Ethernet Standards Link amp Physical Layers

                                                                          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                          Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                          applicationtransportnetwork

                                                                          linkphysical

                                                                          MAC protocoland frame format

                                                                          100BASE-TX

                                                                          100BASE-T4

                                                                          100BASE-FX100BASE-T2

                                                                          100BASE-SX 100BASE-BX

                                                                          fiber physical layercopper (twisterpair) physical layer

                                                                          5 DataLink Layer 5-51

                                                                          Manchester encoding

                                                                          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                          synchronize to each othero no need for a centralized global clock among nodes

                                                                          Hey this is physical-layer stuff

                                                                          5 DataLink Layer 5-52

                                                                          Link Layer

                                                                          51 Introduction and services

                                                                          52 Error detection and correction

                                                                          53 Multiple access protocols

                                                                          54 Link-layer Addressing

                                                                          55 Ethernet

                                                                          56 Link-layer switches LANs

                                                                          59 A day in the life of a web request

                                                                          5 DataLink Layer 5-53

                                                                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                          o bits coming in one link go out all other links at same rate

                                                                          o all nodes connected to hub can collide with one another

                                                                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                          twisted pair

                                                                          hub

                                                                          5 DataLink Layer 5-54

                                                                          Switch link-layer device smarter than hubs take active

                                                                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                          transparento hosts are unaware of presence of switches

                                                                          plug-and-play self-learningo switches do not need to be configured

                                                                          5 DataLink Layer 5-55

                                                                          Switch allows multiple simultaneous transmissions

                                                                          hosts have dedicated direct connection to switch

                                                                          switches buffer packets Ethernet protocol used on

                                                                          each incoming link but no collisions full duplexo each link is its own collision

                                                                          domain switching A-to-Arsquo and B-

                                                                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                          A

                                                                          Arsquo

                                                                          B

                                                                          Brsquo

                                                                          C

                                                                          Crsquo

                                                                          switch with six interfaces(123456)

                                                                          1 23

                                                                          45

                                                                          6

                                                                          5 DataLink Layer 5-56

                                                                          Switch Table

                                                                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                          A each switch has a switch table each entryo (MAC address of host interface

                                                                          to reach host time stamp)

                                                                          looks like a routing table Q how are entries created

                                                                          maintained in switch table o something like a routing

                                                                          protocol

                                                                          A

                                                                          Arsquo

                                                                          B

                                                                          Brsquo

                                                                          C

                                                                          Crsquo

                                                                          switch with six interfaces(123456)

                                                                          1 23

                                                                          45

                                                                          6

                                                                          5 DataLink Layer 5-57

                                                                          Switch self-learning

                                                                          switch learns which hosts can be reached through which interfaceso when frame received

                                                                          switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                          o records senderlocation pair in switch table

                                                                          A

                                                                          Arsquo

                                                                          B

                                                                          Brsquo

                                                                          C

                                                                          Crsquo

                                                                          1 23

                                                                          45

                                                                          6

                                                                          A Arsquo

                                                                          Source ADest Arsquo

                                                                          MAC addr interface TTL

                                                                          Switch table (initially empty)

                                                                          A 1 60

                                                                          5 DataLink Layer 5-58

                                                                          Switch frame filteringforwardingWhen frame received

                                                                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                          then if dest on segment from which frame arrived

                                                                          then drop the frame else forward the frame on interface indicated else flood

                                                                          forward on all but the interface on which the frame arrived

                                                                          5 DataLink Layer 5-59

                                                                          Self-learning forwarding example

                                                                          A

                                                                          Arsquo

                                                                          B

                                                                          Brsquo

                                                                          C

                                                                          Crsquo

                                                                          1 23

                                                                          45

                                                                          6

                                                                          A Arsquo

                                                                          Source ADest Arsquo

                                                                          MAC addr interface TTL

                                                                          Switch table (initially empty)

                                                                          A 1 60

                                                                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                          frame destination unknownflood

                                                                          Arsquo A

                                                                          destination A location known

                                                                          Arsquo 4 60

                                                                          selective send

                                                                          5 DataLink Layer 5-60

                                                                          Interconnecting switches

                                                                          switches can be connected together

                                                                          A

                                                                          B

                                                                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                          A self learning (works exactly the same as in single-switch case)

                                                                          S1

                                                                          C D

                                                                          E

                                                                          FS2

                                                                          S4

                                                                          S3

                                                                          H

                                                                          I

                                                                          G

                                                                          5 DataLink Layer 5-61

                                                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                          A

                                                                          B

                                                                          S1

                                                                          C D

                                                                          E

                                                                          FS2

                                                                          S4

                                                                          S3

                                                                          H

                                                                          I

                                                                          G

                                                                          1

                                                                          2 3

                                                                          34

                                                                          5 DataLink Layer 5-62

                                                                          Institutional network

                                                                          to externalnetwork

                                                                          router

                                                                          IP subnet

                                                                          mail server

                                                                          web server

                                                                          5 DataLink Layer 5-63

                                                                          Switches vs Routers both store-and-forward devices

                                                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                          routers maintain routing tables implement routing algorithms

                                                                          switches maintain switch tables implement filtering learning algorithms

                                                                          Switch

                                                                          5 DataLink Layer 5-64

                                                                          Link Layer

                                                                          51 Introduction and services

                                                                          52 Error detection and correction

                                                                          53Multiple access protocols

                                                                          54 Link-Layer Addressing

                                                                          55 Ethernet

                                                                          56 Link-layer switches 59 A day in the life of

                                                                          a web request

                                                                          5 DataLink Layer 5-65

                                                                          Synthesis a day in the life of a web request

                                                                          journey down protocol stack completeo application transport network link

                                                                          putting-it-all-together synthesiso goal identify review understand protocols

                                                                          (at all layers) involved in seemingly simple scenario requesting www page

                                                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                          5 DataLink Layer 5-66

                                                                          A day in the life scenario

                                                                          Comcast network 68800013

                                                                          Googlersquos network 64233160019 64233169105

                                                                          web server

                                                                          DNS server

                                                                          school network 68802024

                                                                          browser

                                                                          web page

                                                                          5 DataLink Layer 5-67

                                                                          A day in the lifehellip connecting to the Internet

                                                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                          router(runs DHCP)

                                                                          DHCPUDP

                                                                          IPEthPhy

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCPUDP

                                                                          IPEthPhy

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCPDHCP

                                                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                          5 DataLink Layer 5-68

                                                                          A day in the lifehellip connecting to the Internet

                                                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                          router(runs DHCP)

                                                                          DHCPUDP

                                                                          IPEthPhy

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCPUDP

                                                                          IPEthPhy

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          DHCP

                                                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                          DHCP client receives DHCP ACK reply

                                                                          5 DataLink Layer 5-69

                                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                                          DNSUDP

                                                                          IPEthPhy

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                          ARP query

                                                                          EthPhy

                                                                          ARP

                                                                          ARP

                                                                          ARP reply

                                                                          5 DataLink Layer 5-70

                                                                          A day in the lifehellip using DNS

                                                                          DNSUDP

                                                                          IPEthPhy

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                          demuxrsquoed to DNS server DNS server replies to

                                                                          client with IP address of wwwgooglecom

                                                                          Comcast network 68800013

                                                                          DNS server

                                                                          DNSUDP

                                                                          IPEthPhy

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          DNS

                                                                          5 DataLink Layer 5-71

                                                                          A day in the lifehellip TCP connection carrying HTTP

                                                                          HTTPTCPIP

                                                                          EthPhy

                                                                          HTTP

                                                                          to send HTTP request client first opens TCP socket to web server

                                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                          TCP connection established

                                                                          64233169105

                                                                          web server

                                                                          SYN

                                                                          SYN

                                                                          SYN

                                                                          SYN

                                                                          TCPIP

                                                                          EthPhy

                                                                          SYN

                                                                          SYN

                                                                          SYN

                                                                          SYNACK

                                                                          SYNACK

                                                                          SYNACK

                                                                          SYNACK

                                                                          SYNACK

                                                                          SYNACK

                                                                          SYNACK

                                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                          5 DataLink Layer 5-72

                                                                          A day in the lifehellip HTTP requestreply

                                                                          HTTPTCPIP

                                                                          EthPhy

                                                                          HTTP

                                                                          HTTP request sent into TCP socket

                                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                                          IP datgram containing HTTP reply routed back to client

                                                                          64233169105

                                                                          web server

                                                                          HTTPTCPIP

                                                                          EthPhy

                                                                          web server responds with HTTP reply (containing web page)

                                                                          HTTP

                                                                          HTTP

                                                                          HTTPHTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          HTTP

                                                                          web page finally () displayed

                                                                          5 DataLink Layer 5-73

                                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                          (except PHY) solid understanding of networking

                                                                          principles practice hellip could stop here hellip but lots of

                                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                          • Link Layer
                                                                          • Link Layer Introduction
                                                                          • Link layer context
                                                                          • Encapsulation
                                                                          • Link Layer Services
                                                                          • Link Layer Services (more)
                                                                          • Where is the link layer implemented
                                                                          • Adaptors Communicating
                                                                          • Slide 9
                                                                          • Parity Checking
                                                                          • Internet checksum (review)
                                                                          • Checksumming Cyclic Redundancy Check
                                                                          • Cyclic Redundancy Check
                                                                          • Slide 14
                                                                          • CRC Example
                                                                          • Slide 16
                                                                          • Multiple Access Links and Protocols
                                                                          • Cable Network Architecture Overview
                                                                          • Multiple Access protocols
                                                                          • Ideal Multiple Access Protocol
                                                                          • MAC Protocols a taxonomy
                                                                          • Channel Partitioning MAC protocols TDMA
                                                                          • Channel Partitioning MAC protocols FDMA
                                                                          • Random Access Protocols
                                                                          • ALOHA
                                                                          • CSMA (Carrier Sense Multiple Access)
                                                                          • CSMA collisions
                                                                          • CSMACD (Collision Detection)
                                                                          • CSMACD collision detection
                                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                                          • Slide 31
                                                                          • Slide 32
                                                                          • Summary of MAC protocols
                                                                          • Slide 34
                                                                          • MAC Addresses and ARP
                                                                          • LAN Addresses and ARP
                                                                          • LAN Address (more)
                                                                          • ARP Address Resolution Protocol
                                                                          • ARP protocol Same LAN (network)
                                                                          • Addressing routing to another LAN
                                                                          • Slide 41
                                                                          • Slide 42
                                                                          • Ethernet
                                                                          • Star topology
                                                                          • Ethernet Frame Structure
                                                                          • Ethernet Frame Structure (more)
                                                                          • Ethernet Unreliable connectionless
                                                                          • Ethernet CSMACD algorithm
                                                                          • Ethernetrsquos CSMACD (more)
                                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                                          • Manchester encoding
                                                                          • Slide 52
                                                                          • Hubs
                                                                          • Switch
                                                                          • Switch allows multiple simultaneous transmissions
                                                                          • Switch Table
                                                                          • Switch self-learning
                                                                          • Switch frame filteringforwarding
                                                                          • Self-learning forwarding example
                                                                          • Interconnecting switches
                                                                          • Self-learning multi-switch example
                                                                          • Institutional network
                                                                          • Switches vs Routers
                                                                          • Slide 64
                                                                          • Synthesis a day in the life of a web request
                                                                          • A day in the life scenario
                                                                          • A day in the lifehellip connecting to the Internet
                                                                          • Slide 68
                                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                                          • A day in the lifehellip using DNS
                                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                                          • A day in the lifehellip HTTP requestreply
                                                                          • Chapter 5 letrsquos take a breath

                                                                            5 DataLink Layer 5-38

                                                                            ARP Address Resolution Protocol

                                                                            Each IP node (host router) on LAN has ARP table

                                                                            ARP table IPMAC address mappings for some LAN nodes

                                                                            lt IP address MAC address TTLgto TTL (Time To Live)

                                                                            time after which address mapping will be forgotten (typically 20 min)

                                                                            Q given a nodersquos IP address how to determine its MAC address

                                                                            1A-2F-BB-76-09-AD

                                                                            0C-C4-11-6F-E3-98

                                                                            71-65-F7-2B-08-53

                                                                            LAN

                                                                            137196723

                                                                            137196778

                                                                            137196714

                                                                            137196788

                                                                            5 DataLink Layer 5-39

                                                                            ARP protocol Same LAN (network) A wants to send datagram

                                                                            to B and Brsquos MAC address not in Arsquos ARP table

                                                                            A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                            FF-FF-FF-FF-FFo all machines on LAN

                                                                            receive ARP query B receives ARP packet

                                                                            replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                            address (unicast)

                                                                            A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                            ARP is ldquoplug-and-playrdquoo nodes create their

                                                                            ARP tables without intervention from net administrator

                                                                            5 DataLink Layer 5-40

                                                                            Addressing routing to another LAN

                                                                            R

                                                                            1A-23-F9-CD-06-9B

                                                                            222222222220111111111110

                                                                            E6-E9-00-17-BB-4B

                                                                            CC-49-DE-D0-AB-7D

                                                                            111111111112

                                                                            111111111111

                                                                            A74-29-9C-E8-FF-55

                                                                            222222222221

                                                                            88-B2-2F-54-1A-0F

                                                                            B222222222222

                                                                            49-BD-D2-C7-56-2A

                                                                            walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                            two ARP tables in router R one for each IP network (LAN)

                                                                            LAN LAN

                                                                            5 DataLink Layer 5-41

                                                                            A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                            frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                            destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                            R

                                                                            1A-23-F9-CD-06-9B

                                                                            222222222220

                                                                            111111111110

                                                                            E6-E9-00-17-BB-4B

                                                                            CC-49-DE-D0-AB-7D

                                                                            111111111112

                                                                            111111111111

                                                                            A74-29-9C-E8-FF-55

                                                                            222222222221

                                                                            88-B2-2F-54-1A-0F

                                                                            B222222222222

                                                                            49-BD-D2-C7-56-2A

                                                                            This is a really importantexample ndash make sure youunderstand

                                                                            5 DataLink Layer 5-42

                                                                            Link Layer

                                                                            51 Introduction and services

                                                                            52 Error detection and correction

                                                                            53Multiple access protocols

                                                                            54 Link-Layer Addressing

                                                                            55 Ethernet

                                                                            56 Link-layer switches 59 A day in the life of

                                                                            a web request

                                                                            5 DataLink Layer 5-43

                                                                            Ethernet

                                                                            ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                            Metcalfersquos Ethernetsketch

                                                                            5 DataLink Layer 5-44

                                                                            Star topology bus topology popular through mid 90s

                                                                            o all nodes in same collision domain (can collide with each other)

                                                                            today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                            (nodes do not collide with each other)

                                                                            switch

                                                                            bus coaxial cable star

                                                                            5 DataLink Layer 5-45

                                                                            Ethernet Frame Structure

                                                                            Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                            Preamble 7 bytes with pattern 10101010 followed by one

                                                                            byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                            rates

                                                                            5 DataLink Layer 5-46

                                                                            Ethernet Frame Structure (more) Addresses 6 bytes

                                                                            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                            o otherwise adapter discards frame

                                                                            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                            CRC checked at receiver if error is detected frame is dropped

                                                                            5 DataLink Layer 5-47

                                                                            Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                            Ethernetrsquos MAC protocol CSMACD

                                                                            5 DataLink Layer 5-48

                                                                            Ethernet CSMACD algorithm

                                                                            1 NIC receives datagram from network layer creates frame

                                                                            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                            4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                            5 After aborting NIC enters exponential backoff

                                                                            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                            (1 slot = 512 bit-times)

                                                                            5 DataLink Layer 5-49

                                                                            Ethernetrsquos CSMACD (more)

                                                                            Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                            Exponential Backoff Goal adapt retransmission

                                                                            attempts to estimated current loado heavy load random

                                                                            wait will be longer first collision choose K

                                                                            from 01 delay is K 512 bit transmission times

                                                                            after second collision choose K from 0123hellip

                                                                            after ten collisions choose K from 01234hellip1023

                                                                            5 DataLink Layer 5-50

                                                                            8023 Ethernet Standards Link amp Physical Layers

                                                                            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                            Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                            applicationtransportnetwork

                                                                            linkphysical

                                                                            MAC protocoland frame format

                                                                            100BASE-TX

                                                                            100BASE-T4

                                                                            100BASE-FX100BASE-T2

                                                                            100BASE-SX 100BASE-BX

                                                                            fiber physical layercopper (twisterpair) physical layer

                                                                            5 DataLink Layer 5-51

                                                                            Manchester encoding

                                                                            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                            synchronize to each othero no need for a centralized global clock among nodes

                                                                            Hey this is physical-layer stuff

                                                                            5 DataLink Layer 5-52

                                                                            Link Layer

                                                                            51 Introduction and services

                                                                            52 Error detection and correction

                                                                            53 Multiple access protocols

                                                                            54 Link-layer Addressing

                                                                            55 Ethernet

                                                                            56 Link-layer switches LANs

                                                                            59 A day in the life of a web request

                                                                            5 DataLink Layer 5-53

                                                                            Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                            o bits coming in one link go out all other links at same rate

                                                                            o all nodes connected to hub can collide with one another

                                                                            o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                            twisted pair

                                                                            hub

                                                                            5 DataLink Layer 5-54

                                                                            Switch link-layer device smarter than hubs take active

                                                                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                            transparento hosts are unaware of presence of switches

                                                                            plug-and-play self-learningo switches do not need to be configured

                                                                            5 DataLink Layer 5-55

                                                                            Switch allows multiple simultaneous transmissions

                                                                            hosts have dedicated direct connection to switch

                                                                            switches buffer packets Ethernet protocol used on

                                                                            each incoming link but no collisions full duplexo each link is its own collision

                                                                            domain switching A-to-Arsquo and B-

                                                                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                            A

                                                                            Arsquo

                                                                            B

                                                                            Brsquo

                                                                            C

                                                                            Crsquo

                                                                            switch with six interfaces(123456)

                                                                            1 23

                                                                            45

                                                                            6

                                                                            5 DataLink Layer 5-56

                                                                            Switch Table

                                                                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                            A each switch has a switch table each entryo (MAC address of host interface

                                                                            to reach host time stamp)

                                                                            looks like a routing table Q how are entries created

                                                                            maintained in switch table o something like a routing

                                                                            protocol

                                                                            A

                                                                            Arsquo

                                                                            B

                                                                            Brsquo

                                                                            C

                                                                            Crsquo

                                                                            switch with six interfaces(123456)

                                                                            1 23

                                                                            45

                                                                            6

                                                                            5 DataLink Layer 5-57

                                                                            Switch self-learning

                                                                            switch learns which hosts can be reached through which interfaceso when frame received

                                                                            switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                            o records senderlocation pair in switch table

                                                                            A

                                                                            Arsquo

                                                                            B

                                                                            Brsquo

                                                                            C

                                                                            Crsquo

                                                                            1 23

                                                                            45

                                                                            6

                                                                            A Arsquo

                                                                            Source ADest Arsquo

                                                                            MAC addr interface TTL

                                                                            Switch table (initially empty)

                                                                            A 1 60

                                                                            5 DataLink Layer 5-58

                                                                            Switch frame filteringforwardingWhen frame received

                                                                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                            then if dest on segment from which frame arrived

                                                                            then drop the frame else forward the frame on interface indicated else flood

                                                                            forward on all but the interface on which the frame arrived

                                                                            5 DataLink Layer 5-59

                                                                            Self-learning forwarding example

                                                                            A

                                                                            Arsquo

                                                                            B

                                                                            Brsquo

                                                                            C

                                                                            Crsquo

                                                                            1 23

                                                                            45

                                                                            6

                                                                            A Arsquo

                                                                            Source ADest Arsquo

                                                                            MAC addr interface TTL

                                                                            Switch table (initially empty)

                                                                            A 1 60

                                                                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                            frame destination unknownflood

                                                                            Arsquo A

                                                                            destination A location known

                                                                            Arsquo 4 60

                                                                            selective send

                                                                            5 DataLink Layer 5-60

                                                                            Interconnecting switches

                                                                            switches can be connected together

                                                                            A

                                                                            B

                                                                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                            A self learning (works exactly the same as in single-switch case)

                                                                            S1

                                                                            C D

                                                                            E

                                                                            FS2

                                                                            S4

                                                                            S3

                                                                            H

                                                                            I

                                                                            G

                                                                            5 DataLink Layer 5-61

                                                                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                            A

                                                                            B

                                                                            S1

                                                                            C D

                                                                            E

                                                                            FS2

                                                                            S4

                                                                            S3

                                                                            H

                                                                            I

                                                                            G

                                                                            1

                                                                            2 3

                                                                            34

                                                                            5 DataLink Layer 5-62

                                                                            Institutional network

                                                                            to externalnetwork

                                                                            router

                                                                            IP subnet

                                                                            mail server

                                                                            web server

                                                                            5 DataLink Layer 5-63

                                                                            Switches vs Routers both store-and-forward devices

                                                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                            routers maintain routing tables implement routing algorithms

                                                                            switches maintain switch tables implement filtering learning algorithms

                                                                            Switch

                                                                            5 DataLink Layer 5-64

                                                                            Link Layer

                                                                            51 Introduction and services

                                                                            52 Error detection and correction

                                                                            53Multiple access protocols

                                                                            54 Link-Layer Addressing

                                                                            55 Ethernet

                                                                            56 Link-layer switches 59 A day in the life of

                                                                            a web request

                                                                            5 DataLink Layer 5-65

                                                                            Synthesis a day in the life of a web request

                                                                            journey down protocol stack completeo application transport network link

                                                                            putting-it-all-together synthesiso goal identify review understand protocols

                                                                            (at all layers) involved in seemingly simple scenario requesting www page

                                                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                            5 DataLink Layer 5-66

                                                                            A day in the life scenario

                                                                            Comcast network 68800013

                                                                            Googlersquos network 64233160019 64233169105

                                                                            web server

                                                                            DNS server

                                                                            school network 68802024

                                                                            browser

                                                                            web page

                                                                            5 DataLink Layer 5-67

                                                                            A day in the lifehellip connecting to the Internet

                                                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                            router(runs DHCP)

                                                                            DHCPUDP

                                                                            IPEthPhy

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCPUDP

                                                                            IPEthPhy

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCPDHCP

                                                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                            5 DataLink Layer 5-68

                                                                            A day in the lifehellip connecting to the Internet

                                                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                            router(runs DHCP)

                                                                            DHCPUDP

                                                                            IPEthPhy

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCPUDP

                                                                            IPEthPhy

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            DHCP

                                                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                            DHCP client receives DHCP ACK reply

                                                                            5 DataLink Layer 5-69

                                                                            A day in the lifehellip ARP (before DNS before HTTP)

                                                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                                                            DNSUDP

                                                                            IPEthPhy

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                            ARP query

                                                                            EthPhy

                                                                            ARP

                                                                            ARP

                                                                            ARP reply

                                                                            5 DataLink Layer 5-70

                                                                            A day in the lifehellip using DNS

                                                                            DNSUDP

                                                                            IPEthPhy

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                            demuxrsquoed to DNS server DNS server replies to

                                                                            client with IP address of wwwgooglecom

                                                                            Comcast network 68800013

                                                                            DNS server

                                                                            DNSUDP

                                                                            IPEthPhy

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            DNS

                                                                            5 DataLink Layer 5-71

                                                                            A day in the lifehellip TCP connection carrying HTTP

                                                                            HTTPTCPIP

                                                                            EthPhy

                                                                            HTTP

                                                                            to send HTTP request client first opens TCP socket to web server

                                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                            TCP connection established

                                                                            64233169105

                                                                            web server

                                                                            SYN

                                                                            SYN

                                                                            SYN

                                                                            SYN

                                                                            TCPIP

                                                                            EthPhy

                                                                            SYN

                                                                            SYN

                                                                            SYN

                                                                            SYNACK

                                                                            SYNACK

                                                                            SYNACK

                                                                            SYNACK

                                                                            SYNACK

                                                                            SYNACK

                                                                            SYNACK

                                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                            5 DataLink Layer 5-72

                                                                            A day in the lifehellip HTTP requestreply

                                                                            HTTPTCPIP

                                                                            EthPhy

                                                                            HTTP

                                                                            HTTP request sent into TCP socket

                                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                                            IP datgram containing HTTP reply routed back to client

                                                                            64233169105

                                                                            web server

                                                                            HTTPTCPIP

                                                                            EthPhy

                                                                            web server responds with HTTP reply (containing web page)

                                                                            HTTP

                                                                            HTTP

                                                                            HTTPHTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            HTTP

                                                                            web page finally () displayed

                                                                            5 DataLink Layer 5-73

                                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                            (except PHY) solid understanding of networking

                                                                            principles practice hellip could stop here hellip but lots of

                                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                            • Link Layer
                                                                            • Link Layer Introduction
                                                                            • Link layer context
                                                                            • Encapsulation
                                                                            • Link Layer Services
                                                                            • Link Layer Services (more)
                                                                            • Where is the link layer implemented
                                                                            • Adaptors Communicating
                                                                            • Slide 9
                                                                            • Parity Checking
                                                                            • Internet checksum (review)
                                                                            • Checksumming Cyclic Redundancy Check
                                                                            • Cyclic Redundancy Check
                                                                            • Slide 14
                                                                            • CRC Example
                                                                            • Slide 16
                                                                            • Multiple Access Links and Protocols
                                                                            • Cable Network Architecture Overview
                                                                            • Multiple Access protocols
                                                                            • Ideal Multiple Access Protocol
                                                                            • MAC Protocols a taxonomy
                                                                            • Channel Partitioning MAC protocols TDMA
                                                                            • Channel Partitioning MAC protocols FDMA
                                                                            • Random Access Protocols
                                                                            • ALOHA
                                                                            • CSMA (Carrier Sense Multiple Access)
                                                                            • CSMA collisions
                                                                            • CSMACD (Collision Detection)
                                                                            • CSMACD collision detection
                                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                                            • Slide 31
                                                                            • Slide 32
                                                                            • Summary of MAC protocols
                                                                            • Slide 34
                                                                            • MAC Addresses and ARP
                                                                            • LAN Addresses and ARP
                                                                            • LAN Address (more)
                                                                            • ARP Address Resolution Protocol
                                                                            • ARP protocol Same LAN (network)
                                                                            • Addressing routing to another LAN
                                                                            • Slide 41
                                                                            • Slide 42
                                                                            • Ethernet
                                                                            • Star topology
                                                                            • Ethernet Frame Structure
                                                                            • Ethernet Frame Structure (more)
                                                                            • Ethernet Unreliable connectionless
                                                                            • Ethernet CSMACD algorithm
                                                                            • Ethernetrsquos CSMACD (more)
                                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                                            • Manchester encoding
                                                                            • Slide 52
                                                                            • Hubs
                                                                            • Switch
                                                                            • Switch allows multiple simultaneous transmissions
                                                                            • Switch Table
                                                                            • Switch self-learning
                                                                            • Switch frame filteringforwarding
                                                                            • Self-learning forwarding example
                                                                            • Interconnecting switches
                                                                            • Self-learning multi-switch example
                                                                            • Institutional network
                                                                            • Switches vs Routers
                                                                            • Slide 64
                                                                            • Synthesis a day in the life of a web request
                                                                            • A day in the life scenario
                                                                            • A day in the lifehellip connecting to the Internet
                                                                            • Slide 68
                                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                                            • A day in the lifehellip using DNS
                                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                                            • A day in the lifehellip HTTP requestreply
                                                                            • Chapter 5 letrsquos take a breath

                                                                              5 DataLink Layer 5-39

                                                                              ARP protocol Same LAN (network) A wants to send datagram

                                                                              to B and Brsquos MAC address not in Arsquos ARP table

                                                                              A broadcasts ARP query packet containing Bs IP address o dest MAC address = FF-

                                                                              FF-FF-FF-FF-FFo all machines on LAN

                                                                              receive ARP query B receives ARP packet

                                                                              replies to A with its (Bs) MAC addresso frame sent to Arsquos MAC

                                                                              address (unicast)

                                                                              A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

                                                                              ARP is ldquoplug-and-playrdquoo nodes create their

                                                                              ARP tables without intervention from net administrator

                                                                              5 DataLink Layer 5-40

                                                                              Addressing routing to another LAN

                                                                              R

                                                                              1A-23-F9-CD-06-9B

                                                                              222222222220111111111110

                                                                              E6-E9-00-17-BB-4B

                                                                              CC-49-DE-D0-AB-7D

                                                                              111111111112

                                                                              111111111111

                                                                              A74-29-9C-E8-FF-55

                                                                              222222222221

                                                                              88-B2-2F-54-1A-0F

                                                                              B222222222222

                                                                              49-BD-D2-C7-56-2A

                                                                              walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                              two ARP tables in router R one for each IP network (LAN)

                                                                              LAN LAN

                                                                              5 DataLink Layer 5-41

                                                                              A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                              frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                              destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                              R

                                                                              1A-23-F9-CD-06-9B

                                                                              222222222220

                                                                              111111111110

                                                                              E6-E9-00-17-BB-4B

                                                                              CC-49-DE-D0-AB-7D

                                                                              111111111112

                                                                              111111111111

                                                                              A74-29-9C-E8-FF-55

                                                                              222222222221

                                                                              88-B2-2F-54-1A-0F

                                                                              B222222222222

                                                                              49-BD-D2-C7-56-2A

                                                                              This is a really importantexample ndash make sure youunderstand

                                                                              5 DataLink Layer 5-42

                                                                              Link Layer

                                                                              51 Introduction and services

                                                                              52 Error detection and correction

                                                                              53Multiple access protocols

                                                                              54 Link-Layer Addressing

                                                                              55 Ethernet

                                                                              56 Link-layer switches 59 A day in the life of

                                                                              a web request

                                                                              5 DataLink Layer 5-43

                                                                              Ethernet

                                                                              ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                              Metcalfersquos Ethernetsketch

                                                                              5 DataLink Layer 5-44

                                                                              Star topology bus topology popular through mid 90s

                                                                              o all nodes in same collision domain (can collide with each other)

                                                                              today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                              (nodes do not collide with each other)

                                                                              switch

                                                                              bus coaxial cable star

                                                                              5 DataLink Layer 5-45

                                                                              Ethernet Frame Structure

                                                                              Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                              Preamble 7 bytes with pattern 10101010 followed by one

                                                                              byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                              rates

                                                                              5 DataLink Layer 5-46

                                                                              Ethernet Frame Structure (more) Addresses 6 bytes

                                                                              o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                              o otherwise adapter discards frame

                                                                              Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                              CRC checked at receiver if error is detected frame is dropped

                                                                              5 DataLink Layer 5-47

                                                                              Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                              Ethernetrsquos MAC protocol CSMACD

                                                                              5 DataLink Layer 5-48

                                                                              Ethernet CSMACD algorithm

                                                                              1 NIC receives datagram from network layer creates frame

                                                                              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                              4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                              5 After aborting NIC enters exponential backoff

                                                                              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                              (1 slot = 512 bit-times)

                                                                              5 DataLink Layer 5-49

                                                                              Ethernetrsquos CSMACD (more)

                                                                              Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                              Exponential Backoff Goal adapt retransmission

                                                                              attempts to estimated current loado heavy load random

                                                                              wait will be longer first collision choose K

                                                                              from 01 delay is K 512 bit transmission times

                                                                              after second collision choose K from 0123hellip

                                                                              after ten collisions choose K from 01234hellip1023

                                                                              5 DataLink Layer 5-50

                                                                              8023 Ethernet Standards Link amp Physical Layers

                                                                              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                              Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                              applicationtransportnetwork

                                                                              linkphysical

                                                                              MAC protocoland frame format

                                                                              100BASE-TX

                                                                              100BASE-T4

                                                                              100BASE-FX100BASE-T2

                                                                              100BASE-SX 100BASE-BX

                                                                              fiber physical layercopper (twisterpair) physical layer

                                                                              5 DataLink Layer 5-51

                                                                              Manchester encoding

                                                                              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                              synchronize to each othero no need for a centralized global clock among nodes

                                                                              Hey this is physical-layer stuff

                                                                              5 DataLink Layer 5-52

                                                                              Link Layer

                                                                              51 Introduction and services

                                                                              52 Error detection and correction

                                                                              53 Multiple access protocols

                                                                              54 Link-layer Addressing

                                                                              55 Ethernet

                                                                              56 Link-layer switches LANs

                                                                              59 A day in the life of a web request

                                                                              5 DataLink Layer 5-53

                                                                              Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                              o bits coming in one link go out all other links at same rate

                                                                              o all nodes connected to hub can collide with one another

                                                                              o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                              twisted pair

                                                                              hub

                                                                              5 DataLink Layer 5-54

                                                                              Switch link-layer device smarter than hubs take active

                                                                              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                              transparento hosts are unaware of presence of switches

                                                                              plug-and-play self-learningo switches do not need to be configured

                                                                              5 DataLink Layer 5-55

                                                                              Switch allows multiple simultaneous transmissions

                                                                              hosts have dedicated direct connection to switch

                                                                              switches buffer packets Ethernet protocol used on

                                                                              each incoming link but no collisions full duplexo each link is its own collision

                                                                              domain switching A-to-Arsquo and B-

                                                                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                              A

                                                                              Arsquo

                                                                              B

                                                                              Brsquo

                                                                              C

                                                                              Crsquo

                                                                              switch with six interfaces(123456)

                                                                              1 23

                                                                              45

                                                                              6

                                                                              5 DataLink Layer 5-56

                                                                              Switch Table

                                                                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                              A each switch has a switch table each entryo (MAC address of host interface

                                                                              to reach host time stamp)

                                                                              looks like a routing table Q how are entries created

                                                                              maintained in switch table o something like a routing

                                                                              protocol

                                                                              A

                                                                              Arsquo

                                                                              B

                                                                              Brsquo

                                                                              C

                                                                              Crsquo

                                                                              switch with six interfaces(123456)

                                                                              1 23

                                                                              45

                                                                              6

                                                                              5 DataLink Layer 5-57

                                                                              Switch self-learning

                                                                              switch learns which hosts can be reached through which interfaceso when frame received

                                                                              switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                              o records senderlocation pair in switch table

                                                                              A

                                                                              Arsquo

                                                                              B

                                                                              Brsquo

                                                                              C

                                                                              Crsquo

                                                                              1 23

                                                                              45

                                                                              6

                                                                              A Arsquo

                                                                              Source ADest Arsquo

                                                                              MAC addr interface TTL

                                                                              Switch table (initially empty)

                                                                              A 1 60

                                                                              5 DataLink Layer 5-58

                                                                              Switch frame filteringforwardingWhen frame received

                                                                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                              then if dest on segment from which frame arrived

                                                                              then drop the frame else forward the frame on interface indicated else flood

                                                                              forward on all but the interface on which the frame arrived

                                                                              5 DataLink Layer 5-59

                                                                              Self-learning forwarding example

                                                                              A

                                                                              Arsquo

                                                                              B

                                                                              Brsquo

                                                                              C

                                                                              Crsquo

                                                                              1 23

                                                                              45

                                                                              6

                                                                              A Arsquo

                                                                              Source ADest Arsquo

                                                                              MAC addr interface TTL

                                                                              Switch table (initially empty)

                                                                              A 1 60

                                                                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                              frame destination unknownflood

                                                                              Arsquo A

                                                                              destination A location known

                                                                              Arsquo 4 60

                                                                              selective send

                                                                              5 DataLink Layer 5-60

                                                                              Interconnecting switches

                                                                              switches can be connected together

                                                                              A

                                                                              B

                                                                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                              A self learning (works exactly the same as in single-switch case)

                                                                              S1

                                                                              C D

                                                                              E

                                                                              FS2

                                                                              S4

                                                                              S3

                                                                              H

                                                                              I

                                                                              G

                                                                              5 DataLink Layer 5-61

                                                                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                              A

                                                                              B

                                                                              S1

                                                                              C D

                                                                              E

                                                                              FS2

                                                                              S4

                                                                              S3

                                                                              H

                                                                              I

                                                                              G

                                                                              1

                                                                              2 3

                                                                              34

                                                                              5 DataLink Layer 5-62

                                                                              Institutional network

                                                                              to externalnetwork

                                                                              router

                                                                              IP subnet

                                                                              mail server

                                                                              web server

                                                                              5 DataLink Layer 5-63

                                                                              Switches vs Routers both store-and-forward devices

                                                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                              routers maintain routing tables implement routing algorithms

                                                                              switches maintain switch tables implement filtering learning algorithms

                                                                              Switch

                                                                              5 DataLink Layer 5-64

                                                                              Link Layer

                                                                              51 Introduction and services

                                                                              52 Error detection and correction

                                                                              53Multiple access protocols

                                                                              54 Link-Layer Addressing

                                                                              55 Ethernet

                                                                              56 Link-layer switches 59 A day in the life of

                                                                              a web request

                                                                              5 DataLink Layer 5-65

                                                                              Synthesis a day in the life of a web request

                                                                              journey down protocol stack completeo application transport network link

                                                                              putting-it-all-together synthesiso goal identify review understand protocols

                                                                              (at all layers) involved in seemingly simple scenario requesting www page

                                                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                              5 DataLink Layer 5-66

                                                                              A day in the life scenario

                                                                              Comcast network 68800013

                                                                              Googlersquos network 64233160019 64233169105

                                                                              web server

                                                                              DNS server

                                                                              school network 68802024

                                                                              browser

                                                                              web page

                                                                              5 DataLink Layer 5-67

                                                                              A day in the lifehellip connecting to the Internet

                                                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                              router(runs DHCP)

                                                                              DHCPUDP

                                                                              IPEthPhy

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCPUDP

                                                                              IPEthPhy

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCPDHCP

                                                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                              5 DataLink Layer 5-68

                                                                              A day in the lifehellip connecting to the Internet

                                                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                              router(runs DHCP)

                                                                              DHCPUDP

                                                                              IPEthPhy

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCPUDP

                                                                              IPEthPhy

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              DHCP

                                                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                              DHCP client receives DHCP ACK reply

                                                                              5 DataLink Layer 5-69

                                                                              A day in the lifehellip ARP (before DNS before HTTP)

                                                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                                                              DNSUDP

                                                                              IPEthPhy

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                              ARP query

                                                                              EthPhy

                                                                              ARP

                                                                              ARP

                                                                              ARP reply

                                                                              5 DataLink Layer 5-70

                                                                              A day in the lifehellip using DNS

                                                                              DNSUDP

                                                                              IPEthPhy

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                              demuxrsquoed to DNS server DNS server replies to

                                                                              client with IP address of wwwgooglecom

                                                                              Comcast network 68800013

                                                                              DNS server

                                                                              DNSUDP

                                                                              IPEthPhy

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              DNS

                                                                              5 DataLink Layer 5-71

                                                                              A day in the lifehellip TCP connection carrying HTTP

                                                                              HTTPTCPIP

                                                                              EthPhy

                                                                              HTTP

                                                                              to send HTTP request client first opens TCP socket to web server

                                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                              TCP connection established

                                                                              64233169105

                                                                              web server

                                                                              SYN

                                                                              SYN

                                                                              SYN

                                                                              SYN

                                                                              TCPIP

                                                                              EthPhy

                                                                              SYN

                                                                              SYN

                                                                              SYN

                                                                              SYNACK

                                                                              SYNACK

                                                                              SYNACK

                                                                              SYNACK

                                                                              SYNACK

                                                                              SYNACK

                                                                              SYNACK

                                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                              5 DataLink Layer 5-72

                                                                              A day in the lifehellip HTTP requestreply

                                                                              HTTPTCPIP

                                                                              EthPhy

                                                                              HTTP

                                                                              HTTP request sent into TCP socket

                                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                                              IP datgram containing HTTP reply routed back to client

                                                                              64233169105

                                                                              web server

                                                                              HTTPTCPIP

                                                                              EthPhy

                                                                              web server responds with HTTP reply (containing web page)

                                                                              HTTP

                                                                              HTTP

                                                                              HTTPHTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              HTTP

                                                                              web page finally () displayed

                                                                              5 DataLink Layer 5-73

                                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                              (except PHY) solid understanding of networking

                                                                              principles practice hellip could stop here hellip but lots of

                                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                              • Link Layer
                                                                              • Link Layer Introduction
                                                                              • Link layer context
                                                                              • Encapsulation
                                                                              • Link Layer Services
                                                                              • Link Layer Services (more)
                                                                              • Where is the link layer implemented
                                                                              • Adaptors Communicating
                                                                              • Slide 9
                                                                              • Parity Checking
                                                                              • Internet checksum (review)
                                                                              • Checksumming Cyclic Redundancy Check
                                                                              • Cyclic Redundancy Check
                                                                              • Slide 14
                                                                              • CRC Example
                                                                              • Slide 16
                                                                              • Multiple Access Links and Protocols
                                                                              • Cable Network Architecture Overview
                                                                              • Multiple Access protocols
                                                                              • Ideal Multiple Access Protocol
                                                                              • MAC Protocols a taxonomy
                                                                              • Channel Partitioning MAC protocols TDMA
                                                                              • Channel Partitioning MAC protocols FDMA
                                                                              • Random Access Protocols
                                                                              • ALOHA
                                                                              • CSMA (Carrier Sense Multiple Access)
                                                                              • CSMA collisions
                                                                              • CSMACD (Collision Detection)
                                                                              • CSMACD collision detection
                                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                                              • Slide 31
                                                                              • Slide 32
                                                                              • Summary of MAC protocols
                                                                              • Slide 34
                                                                              • MAC Addresses and ARP
                                                                              • LAN Addresses and ARP
                                                                              • LAN Address (more)
                                                                              • ARP Address Resolution Protocol
                                                                              • ARP protocol Same LAN (network)
                                                                              • Addressing routing to another LAN
                                                                              • Slide 41
                                                                              • Slide 42
                                                                              • Ethernet
                                                                              • Star topology
                                                                              • Ethernet Frame Structure
                                                                              • Ethernet Frame Structure (more)
                                                                              • Ethernet Unreliable connectionless
                                                                              • Ethernet CSMACD algorithm
                                                                              • Ethernetrsquos CSMACD (more)
                                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                                              • Manchester encoding
                                                                              • Slide 52
                                                                              • Hubs
                                                                              • Switch
                                                                              • Switch allows multiple simultaneous transmissions
                                                                              • Switch Table
                                                                              • Switch self-learning
                                                                              • Switch frame filteringforwarding
                                                                              • Self-learning forwarding example
                                                                              • Interconnecting switches
                                                                              • Self-learning multi-switch example
                                                                              • Institutional network
                                                                              • Switches vs Routers
                                                                              • Slide 64
                                                                              • Synthesis a day in the life of a web request
                                                                              • A day in the life scenario
                                                                              • A day in the lifehellip connecting to the Internet
                                                                              • Slide 68
                                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                                              • A day in the lifehellip using DNS
                                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                                              • A day in the lifehellip HTTP requestreply
                                                                              • Chapter 5 letrsquos take a breath

                                                                                5 DataLink Layer 5-40

                                                                                Addressing routing to another LAN

                                                                                R

                                                                                1A-23-F9-CD-06-9B

                                                                                222222222220111111111110

                                                                                E6-E9-00-17-BB-4B

                                                                                CC-49-DE-D0-AB-7D

                                                                                111111111112

                                                                                111111111111

                                                                                A74-29-9C-E8-FF-55

                                                                                222222222221

                                                                                88-B2-2F-54-1A-0F

                                                                                B222222222222

                                                                                49-BD-D2-C7-56-2A

                                                                                walkthrough send datagram from A to B via R assume A knows Brsquos IP address

                                                                                two ARP tables in router R one for each IP network (LAN)

                                                                                LAN LAN

                                                                                5 DataLink Layer 5-41

                                                                                A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                                frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                                destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                                R

                                                                                1A-23-F9-CD-06-9B

                                                                                222222222220

                                                                                111111111110

                                                                                E6-E9-00-17-BB-4B

                                                                                CC-49-DE-D0-AB-7D

                                                                                111111111112

                                                                                111111111111

                                                                                A74-29-9C-E8-FF-55

                                                                                222222222221

                                                                                88-B2-2F-54-1A-0F

                                                                                B222222222222

                                                                                49-BD-D2-C7-56-2A

                                                                                This is a really importantexample ndash make sure youunderstand

                                                                                5 DataLink Layer 5-42

                                                                                Link Layer

                                                                                51 Introduction and services

                                                                                52 Error detection and correction

                                                                                53Multiple access protocols

                                                                                54 Link-Layer Addressing

                                                                                55 Ethernet

                                                                                56 Link-layer switches 59 A day in the life of

                                                                                a web request

                                                                                5 DataLink Layer 5-43

                                                                                Ethernet

                                                                                ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                                Metcalfersquos Ethernetsketch

                                                                                5 DataLink Layer 5-44

                                                                                Star topology bus topology popular through mid 90s

                                                                                o all nodes in same collision domain (can collide with each other)

                                                                                today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                                (nodes do not collide with each other)

                                                                                switch

                                                                                bus coaxial cable star

                                                                                5 DataLink Layer 5-45

                                                                                Ethernet Frame Structure

                                                                                Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                Preamble 7 bytes with pattern 10101010 followed by one

                                                                                byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                rates

                                                                                5 DataLink Layer 5-46

                                                                                Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                o otherwise adapter discards frame

                                                                                Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                CRC checked at receiver if error is detected frame is dropped

                                                                                5 DataLink Layer 5-47

                                                                                Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                Ethernetrsquos MAC protocol CSMACD

                                                                                5 DataLink Layer 5-48

                                                                                Ethernet CSMACD algorithm

                                                                                1 NIC receives datagram from network layer creates frame

                                                                                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                5 After aborting NIC enters exponential backoff

                                                                                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                (1 slot = 512 bit-times)

                                                                                5 DataLink Layer 5-49

                                                                                Ethernetrsquos CSMACD (more)

                                                                                Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                Exponential Backoff Goal adapt retransmission

                                                                                attempts to estimated current loado heavy load random

                                                                                wait will be longer first collision choose K

                                                                                from 01 delay is K 512 bit transmission times

                                                                                after second collision choose K from 0123hellip

                                                                                after ten collisions choose K from 01234hellip1023

                                                                                5 DataLink Layer 5-50

                                                                                8023 Ethernet Standards Link amp Physical Layers

                                                                                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                applicationtransportnetwork

                                                                                linkphysical

                                                                                MAC protocoland frame format

                                                                                100BASE-TX

                                                                                100BASE-T4

                                                                                100BASE-FX100BASE-T2

                                                                                100BASE-SX 100BASE-BX

                                                                                fiber physical layercopper (twisterpair) physical layer

                                                                                5 DataLink Layer 5-51

                                                                                Manchester encoding

                                                                                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                synchronize to each othero no need for a centralized global clock among nodes

                                                                                Hey this is physical-layer stuff

                                                                                5 DataLink Layer 5-52

                                                                                Link Layer

                                                                                51 Introduction and services

                                                                                52 Error detection and correction

                                                                                53 Multiple access protocols

                                                                                54 Link-layer Addressing

                                                                                55 Ethernet

                                                                                56 Link-layer switches LANs

                                                                                59 A day in the life of a web request

                                                                                5 DataLink Layer 5-53

                                                                                Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                o bits coming in one link go out all other links at same rate

                                                                                o all nodes connected to hub can collide with one another

                                                                                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                twisted pair

                                                                                hub

                                                                                5 DataLink Layer 5-54

                                                                                Switch link-layer device smarter than hubs take active

                                                                                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                transparento hosts are unaware of presence of switches

                                                                                plug-and-play self-learningo switches do not need to be configured

                                                                                5 DataLink Layer 5-55

                                                                                Switch allows multiple simultaneous transmissions

                                                                                hosts have dedicated direct connection to switch

                                                                                switches buffer packets Ethernet protocol used on

                                                                                each incoming link but no collisions full duplexo each link is its own collision

                                                                                domain switching A-to-Arsquo and B-

                                                                                to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                A

                                                                                Arsquo

                                                                                B

                                                                                Brsquo

                                                                                C

                                                                                Crsquo

                                                                                switch with six interfaces(123456)

                                                                                1 23

                                                                                45

                                                                                6

                                                                                5 DataLink Layer 5-56

                                                                                Switch Table

                                                                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                A each switch has a switch table each entryo (MAC address of host interface

                                                                                to reach host time stamp)

                                                                                looks like a routing table Q how are entries created

                                                                                maintained in switch table o something like a routing

                                                                                protocol

                                                                                A

                                                                                Arsquo

                                                                                B

                                                                                Brsquo

                                                                                C

                                                                                Crsquo

                                                                                switch with six interfaces(123456)

                                                                                1 23

                                                                                45

                                                                                6

                                                                                5 DataLink Layer 5-57

                                                                                Switch self-learning

                                                                                switch learns which hosts can be reached through which interfaceso when frame received

                                                                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                o records senderlocation pair in switch table

                                                                                A

                                                                                Arsquo

                                                                                B

                                                                                Brsquo

                                                                                C

                                                                                Crsquo

                                                                                1 23

                                                                                45

                                                                                6

                                                                                A Arsquo

                                                                                Source ADest Arsquo

                                                                                MAC addr interface TTL

                                                                                Switch table (initially empty)

                                                                                A 1 60

                                                                                5 DataLink Layer 5-58

                                                                                Switch frame filteringforwardingWhen frame received

                                                                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                then if dest on segment from which frame arrived

                                                                                then drop the frame else forward the frame on interface indicated else flood

                                                                                forward on all but the interface on which the frame arrived

                                                                                5 DataLink Layer 5-59

                                                                                Self-learning forwarding example

                                                                                A

                                                                                Arsquo

                                                                                B

                                                                                Brsquo

                                                                                C

                                                                                Crsquo

                                                                                1 23

                                                                                45

                                                                                6

                                                                                A Arsquo

                                                                                Source ADest Arsquo

                                                                                MAC addr interface TTL

                                                                                Switch table (initially empty)

                                                                                A 1 60

                                                                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                frame destination unknownflood

                                                                                Arsquo A

                                                                                destination A location known

                                                                                Arsquo 4 60

                                                                                selective send

                                                                                5 DataLink Layer 5-60

                                                                                Interconnecting switches

                                                                                switches can be connected together

                                                                                A

                                                                                B

                                                                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                A self learning (works exactly the same as in single-switch case)

                                                                                S1

                                                                                C D

                                                                                E

                                                                                FS2

                                                                                S4

                                                                                S3

                                                                                H

                                                                                I

                                                                                G

                                                                                5 DataLink Layer 5-61

                                                                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                A

                                                                                B

                                                                                S1

                                                                                C D

                                                                                E

                                                                                FS2

                                                                                S4

                                                                                S3

                                                                                H

                                                                                I

                                                                                G

                                                                                1

                                                                                2 3

                                                                                34

                                                                                5 DataLink Layer 5-62

                                                                                Institutional network

                                                                                to externalnetwork

                                                                                router

                                                                                IP subnet

                                                                                mail server

                                                                                web server

                                                                                5 DataLink Layer 5-63

                                                                                Switches vs Routers both store-and-forward devices

                                                                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                routers maintain routing tables implement routing algorithms

                                                                                switches maintain switch tables implement filtering learning algorithms

                                                                                Switch

                                                                                5 DataLink Layer 5-64

                                                                                Link Layer

                                                                                51 Introduction and services

                                                                                52 Error detection and correction

                                                                                53Multiple access protocols

                                                                                54 Link-Layer Addressing

                                                                                55 Ethernet

                                                                                56 Link-layer switches 59 A day in the life of

                                                                                a web request

                                                                                5 DataLink Layer 5-65

                                                                                Synthesis a day in the life of a web request

                                                                                journey down protocol stack completeo application transport network link

                                                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                5 DataLink Layer 5-66

                                                                                A day in the life scenario

                                                                                Comcast network 68800013

                                                                                Googlersquos network 64233160019 64233169105

                                                                                web server

                                                                                DNS server

                                                                                school network 68802024

                                                                                browser

                                                                                web page

                                                                                5 DataLink Layer 5-67

                                                                                A day in the lifehellip connecting to the Internet

                                                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                router(runs DHCP)

                                                                                DHCPUDP

                                                                                IPEthPhy

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCPUDP

                                                                                IPEthPhy

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCPDHCP

                                                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                5 DataLink Layer 5-68

                                                                                A day in the lifehellip connecting to the Internet

                                                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                router(runs DHCP)

                                                                                DHCPUDP

                                                                                IPEthPhy

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCPUDP

                                                                                IPEthPhy

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                DHCP

                                                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                DHCP client receives DHCP ACK reply

                                                                                5 DataLink Layer 5-69

                                                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                DNSUDP

                                                                                IPEthPhy

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                ARP query

                                                                                EthPhy

                                                                                ARP

                                                                                ARP

                                                                                ARP reply

                                                                                5 DataLink Layer 5-70

                                                                                A day in the lifehellip using DNS

                                                                                DNSUDP

                                                                                IPEthPhy

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                demuxrsquoed to DNS server DNS server replies to

                                                                                client with IP address of wwwgooglecom

                                                                                Comcast network 68800013

                                                                                DNS server

                                                                                DNSUDP

                                                                                IPEthPhy

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                DNS

                                                                                5 DataLink Layer 5-71

                                                                                A day in the lifehellip TCP connection carrying HTTP

                                                                                HTTPTCPIP

                                                                                EthPhy

                                                                                HTTP

                                                                                to send HTTP request client first opens TCP socket to web server

                                                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                TCP connection established

                                                                                64233169105

                                                                                web server

                                                                                SYN

                                                                                SYN

                                                                                SYN

                                                                                SYN

                                                                                TCPIP

                                                                                EthPhy

                                                                                SYN

                                                                                SYN

                                                                                SYN

                                                                                SYNACK

                                                                                SYNACK

                                                                                SYNACK

                                                                                SYNACK

                                                                                SYNACK

                                                                                SYNACK

                                                                                SYNACK

                                                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                5 DataLink Layer 5-72

                                                                                A day in the lifehellip HTTP requestreply

                                                                                HTTPTCPIP

                                                                                EthPhy

                                                                                HTTP

                                                                                HTTP request sent into TCP socket

                                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                                IP datgram containing HTTP reply routed back to client

                                                                                64233169105

                                                                                web server

                                                                                HTTPTCPIP

                                                                                EthPhy

                                                                                web server responds with HTTP reply (containing web page)

                                                                                HTTP

                                                                                HTTP

                                                                                HTTPHTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                HTTP

                                                                                web page finally () displayed

                                                                                5 DataLink Layer 5-73

                                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                (except PHY) solid understanding of networking

                                                                                principles practice hellip could stop here hellip but lots of

                                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                • Link Layer
                                                                                • Link Layer Introduction
                                                                                • Link layer context
                                                                                • Encapsulation
                                                                                • Link Layer Services
                                                                                • Link Layer Services (more)
                                                                                • Where is the link layer implemented
                                                                                • Adaptors Communicating
                                                                                • Slide 9
                                                                                • Parity Checking
                                                                                • Internet checksum (review)
                                                                                • Checksumming Cyclic Redundancy Check
                                                                                • Cyclic Redundancy Check
                                                                                • Slide 14
                                                                                • CRC Example
                                                                                • Slide 16
                                                                                • Multiple Access Links and Protocols
                                                                                • Cable Network Architecture Overview
                                                                                • Multiple Access protocols
                                                                                • Ideal Multiple Access Protocol
                                                                                • MAC Protocols a taxonomy
                                                                                • Channel Partitioning MAC protocols TDMA
                                                                                • Channel Partitioning MAC protocols FDMA
                                                                                • Random Access Protocols
                                                                                • ALOHA
                                                                                • CSMA (Carrier Sense Multiple Access)
                                                                                • CSMA collisions
                                                                                • CSMACD (Collision Detection)
                                                                                • CSMACD collision detection
                                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                                • Slide 31
                                                                                • Slide 32
                                                                                • Summary of MAC protocols
                                                                                • Slide 34
                                                                                • MAC Addresses and ARP
                                                                                • LAN Addresses and ARP
                                                                                • LAN Address (more)
                                                                                • ARP Address Resolution Protocol
                                                                                • ARP protocol Same LAN (network)
                                                                                • Addressing routing to another LAN
                                                                                • Slide 41
                                                                                • Slide 42
                                                                                • Ethernet
                                                                                • Star topology
                                                                                • Ethernet Frame Structure
                                                                                • Ethernet Frame Structure (more)
                                                                                • Ethernet Unreliable connectionless
                                                                                • Ethernet CSMACD algorithm
                                                                                • Ethernetrsquos CSMACD (more)
                                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                                • Manchester encoding
                                                                                • Slide 52
                                                                                • Hubs
                                                                                • Switch
                                                                                • Switch allows multiple simultaneous transmissions
                                                                                • Switch Table
                                                                                • Switch self-learning
                                                                                • Switch frame filteringforwarding
                                                                                • Self-learning forwarding example
                                                                                • Interconnecting switches
                                                                                • Self-learning multi-switch example
                                                                                • Institutional network
                                                                                • Switches vs Routers
                                                                                • Slide 64
                                                                                • Synthesis a day in the life of a web request
                                                                                • A day in the life scenario
                                                                                • A day in the lifehellip connecting to the Internet
                                                                                • Slide 68
                                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                • A day in the lifehellip using DNS
                                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                                • A day in the lifehellip HTTP requestreply
                                                                                • Chapter 5 letrsquos take a breath

                                                                                  5 DataLink Layer 5-41

                                                                                  A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest

                                                                                  frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its

                                                                                  destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B

                                                                                  R

                                                                                  1A-23-F9-CD-06-9B

                                                                                  222222222220

                                                                                  111111111110

                                                                                  E6-E9-00-17-BB-4B

                                                                                  CC-49-DE-D0-AB-7D

                                                                                  111111111112

                                                                                  111111111111

                                                                                  A74-29-9C-E8-FF-55

                                                                                  222222222221

                                                                                  88-B2-2F-54-1A-0F

                                                                                  B222222222222

                                                                                  49-BD-D2-C7-56-2A

                                                                                  This is a really importantexample ndash make sure youunderstand

                                                                                  5 DataLink Layer 5-42

                                                                                  Link Layer

                                                                                  51 Introduction and services

                                                                                  52 Error detection and correction

                                                                                  53Multiple access protocols

                                                                                  54 Link-Layer Addressing

                                                                                  55 Ethernet

                                                                                  56 Link-layer switches 59 A day in the life of

                                                                                  a web request

                                                                                  5 DataLink Layer 5-43

                                                                                  Ethernet

                                                                                  ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                                  Metcalfersquos Ethernetsketch

                                                                                  5 DataLink Layer 5-44

                                                                                  Star topology bus topology popular through mid 90s

                                                                                  o all nodes in same collision domain (can collide with each other)

                                                                                  today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                                  (nodes do not collide with each other)

                                                                                  switch

                                                                                  bus coaxial cable star

                                                                                  5 DataLink Layer 5-45

                                                                                  Ethernet Frame Structure

                                                                                  Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                  Preamble 7 bytes with pattern 10101010 followed by one

                                                                                  byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                  rates

                                                                                  5 DataLink Layer 5-46

                                                                                  Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                  o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                  o otherwise adapter discards frame

                                                                                  Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                  CRC checked at receiver if error is detected frame is dropped

                                                                                  5 DataLink Layer 5-47

                                                                                  Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                  and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                  send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                  gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                  Ethernetrsquos MAC protocol CSMACD

                                                                                  5 DataLink Layer 5-48

                                                                                  Ethernet CSMACD algorithm

                                                                                  1 NIC receives datagram from network layer creates frame

                                                                                  2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                  3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                  4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                  5 After aborting NIC enters exponential backoff

                                                                                  after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                  (1 slot = 512 bit-times)

                                                                                  5 DataLink Layer 5-49

                                                                                  Ethernetrsquos CSMACD (more)

                                                                                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                  Exponential Backoff Goal adapt retransmission

                                                                                  attempts to estimated current loado heavy load random

                                                                                  wait will be longer first collision choose K

                                                                                  from 01 delay is K 512 bit transmission times

                                                                                  after second collision choose K from 0123hellip

                                                                                  after ten collisions choose K from 01234hellip1023

                                                                                  5 DataLink Layer 5-50

                                                                                  8023 Ethernet Standards Link amp Physical Layers

                                                                                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                  applicationtransportnetwork

                                                                                  linkphysical

                                                                                  MAC protocoland frame format

                                                                                  100BASE-TX

                                                                                  100BASE-T4

                                                                                  100BASE-FX100BASE-T2

                                                                                  100BASE-SX 100BASE-BX

                                                                                  fiber physical layercopper (twisterpair) physical layer

                                                                                  5 DataLink Layer 5-51

                                                                                  Manchester encoding

                                                                                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                  synchronize to each othero no need for a centralized global clock among nodes

                                                                                  Hey this is physical-layer stuff

                                                                                  5 DataLink Layer 5-52

                                                                                  Link Layer

                                                                                  51 Introduction and services

                                                                                  52 Error detection and correction

                                                                                  53 Multiple access protocols

                                                                                  54 Link-layer Addressing

                                                                                  55 Ethernet

                                                                                  56 Link-layer switches LANs

                                                                                  59 A day in the life of a web request

                                                                                  5 DataLink Layer 5-53

                                                                                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                  o bits coming in one link go out all other links at same rate

                                                                                  o all nodes connected to hub can collide with one another

                                                                                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                  twisted pair

                                                                                  hub

                                                                                  5 DataLink Layer 5-54

                                                                                  Switch link-layer device smarter than hubs take active

                                                                                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                  transparento hosts are unaware of presence of switches

                                                                                  plug-and-play self-learningo switches do not need to be configured

                                                                                  5 DataLink Layer 5-55

                                                                                  Switch allows multiple simultaneous transmissions

                                                                                  hosts have dedicated direct connection to switch

                                                                                  switches buffer packets Ethernet protocol used on

                                                                                  each incoming link but no collisions full duplexo each link is its own collision

                                                                                  domain switching A-to-Arsquo and B-

                                                                                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                  A

                                                                                  Arsquo

                                                                                  B

                                                                                  Brsquo

                                                                                  C

                                                                                  Crsquo

                                                                                  switch with six interfaces(123456)

                                                                                  1 23

                                                                                  45

                                                                                  6

                                                                                  5 DataLink Layer 5-56

                                                                                  Switch Table

                                                                                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                  A each switch has a switch table each entryo (MAC address of host interface

                                                                                  to reach host time stamp)

                                                                                  looks like a routing table Q how are entries created

                                                                                  maintained in switch table o something like a routing

                                                                                  protocol

                                                                                  A

                                                                                  Arsquo

                                                                                  B

                                                                                  Brsquo

                                                                                  C

                                                                                  Crsquo

                                                                                  switch with six interfaces(123456)

                                                                                  1 23

                                                                                  45

                                                                                  6

                                                                                  5 DataLink Layer 5-57

                                                                                  Switch self-learning

                                                                                  switch learns which hosts can be reached through which interfaceso when frame received

                                                                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                  o records senderlocation pair in switch table

                                                                                  A

                                                                                  Arsquo

                                                                                  B

                                                                                  Brsquo

                                                                                  C

                                                                                  Crsquo

                                                                                  1 23

                                                                                  45

                                                                                  6

                                                                                  A Arsquo

                                                                                  Source ADest Arsquo

                                                                                  MAC addr interface TTL

                                                                                  Switch table (initially empty)

                                                                                  A 1 60

                                                                                  5 DataLink Layer 5-58

                                                                                  Switch frame filteringforwardingWhen frame received

                                                                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                  then if dest on segment from which frame arrived

                                                                                  then drop the frame else forward the frame on interface indicated else flood

                                                                                  forward on all but the interface on which the frame arrived

                                                                                  5 DataLink Layer 5-59

                                                                                  Self-learning forwarding example

                                                                                  A

                                                                                  Arsquo

                                                                                  B

                                                                                  Brsquo

                                                                                  C

                                                                                  Crsquo

                                                                                  1 23

                                                                                  45

                                                                                  6

                                                                                  A Arsquo

                                                                                  Source ADest Arsquo

                                                                                  MAC addr interface TTL

                                                                                  Switch table (initially empty)

                                                                                  A 1 60

                                                                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                  frame destination unknownflood

                                                                                  Arsquo A

                                                                                  destination A location known

                                                                                  Arsquo 4 60

                                                                                  selective send

                                                                                  5 DataLink Layer 5-60

                                                                                  Interconnecting switches

                                                                                  switches can be connected together

                                                                                  A

                                                                                  B

                                                                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                  A self learning (works exactly the same as in single-switch case)

                                                                                  S1

                                                                                  C D

                                                                                  E

                                                                                  FS2

                                                                                  S4

                                                                                  S3

                                                                                  H

                                                                                  I

                                                                                  G

                                                                                  5 DataLink Layer 5-61

                                                                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                  A

                                                                                  B

                                                                                  S1

                                                                                  C D

                                                                                  E

                                                                                  FS2

                                                                                  S4

                                                                                  S3

                                                                                  H

                                                                                  I

                                                                                  G

                                                                                  1

                                                                                  2 3

                                                                                  34

                                                                                  5 DataLink Layer 5-62

                                                                                  Institutional network

                                                                                  to externalnetwork

                                                                                  router

                                                                                  IP subnet

                                                                                  mail server

                                                                                  web server

                                                                                  5 DataLink Layer 5-63

                                                                                  Switches vs Routers both store-and-forward devices

                                                                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                  routers maintain routing tables implement routing algorithms

                                                                                  switches maintain switch tables implement filtering learning algorithms

                                                                                  Switch

                                                                                  5 DataLink Layer 5-64

                                                                                  Link Layer

                                                                                  51 Introduction and services

                                                                                  52 Error detection and correction

                                                                                  53Multiple access protocols

                                                                                  54 Link-Layer Addressing

                                                                                  55 Ethernet

                                                                                  56 Link-layer switches 59 A day in the life of

                                                                                  a web request

                                                                                  5 DataLink Layer 5-65

                                                                                  Synthesis a day in the life of a web request

                                                                                  journey down protocol stack completeo application transport network link

                                                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                  5 DataLink Layer 5-66

                                                                                  A day in the life scenario

                                                                                  Comcast network 68800013

                                                                                  Googlersquos network 64233160019 64233169105

                                                                                  web server

                                                                                  DNS server

                                                                                  school network 68802024

                                                                                  browser

                                                                                  web page

                                                                                  5 DataLink Layer 5-67

                                                                                  A day in the lifehellip connecting to the Internet

                                                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                  router(runs DHCP)

                                                                                  DHCPUDP

                                                                                  IPEthPhy

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCPUDP

                                                                                  IPEthPhy

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCPDHCP

                                                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                  5 DataLink Layer 5-68

                                                                                  A day in the lifehellip connecting to the Internet

                                                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                  router(runs DHCP)

                                                                                  DHCPUDP

                                                                                  IPEthPhy

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCPUDP

                                                                                  IPEthPhy

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  DHCP

                                                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                  DHCP client receives DHCP ACK reply

                                                                                  5 DataLink Layer 5-69

                                                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                  DNSUDP

                                                                                  IPEthPhy

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                  ARP query

                                                                                  EthPhy

                                                                                  ARP

                                                                                  ARP

                                                                                  ARP reply

                                                                                  5 DataLink Layer 5-70

                                                                                  A day in the lifehellip using DNS

                                                                                  DNSUDP

                                                                                  IPEthPhy

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                  demuxrsquoed to DNS server DNS server replies to

                                                                                  client with IP address of wwwgooglecom

                                                                                  Comcast network 68800013

                                                                                  DNS server

                                                                                  DNSUDP

                                                                                  IPEthPhy

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  DNS

                                                                                  5 DataLink Layer 5-71

                                                                                  A day in the lifehellip TCP connection carrying HTTP

                                                                                  HTTPTCPIP

                                                                                  EthPhy

                                                                                  HTTP

                                                                                  to send HTTP request client first opens TCP socket to web server

                                                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                  TCP connection established

                                                                                  64233169105

                                                                                  web server

                                                                                  SYN

                                                                                  SYN

                                                                                  SYN

                                                                                  SYN

                                                                                  TCPIP

                                                                                  EthPhy

                                                                                  SYN

                                                                                  SYN

                                                                                  SYN

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  SYNACK

                                                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                  5 DataLink Layer 5-72

                                                                                  A day in the lifehellip HTTP requestreply

                                                                                  HTTPTCPIP

                                                                                  EthPhy

                                                                                  HTTP

                                                                                  HTTP request sent into TCP socket

                                                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                                                  IP datgram containing HTTP reply routed back to client

                                                                                  64233169105

                                                                                  web server

                                                                                  HTTPTCPIP

                                                                                  EthPhy

                                                                                  web server responds with HTTP reply (containing web page)

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTPHTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  HTTP

                                                                                  web page finally () displayed

                                                                                  5 DataLink Layer 5-73

                                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                  (except PHY) solid understanding of networking

                                                                                  principles practice hellip could stop here hellip but lots of

                                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                  • Link Layer
                                                                                  • Link Layer Introduction
                                                                                  • Link layer context
                                                                                  • Encapsulation
                                                                                  • Link Layer Services
                                                                                  • Link Layer Services (more)
                                                                                  • Where is the link layer implemented
                                                                                  • Adaptors Communicating
                                                                                  • Slide 9
                                                                                  • Parity Checking
                                                                                  • Internet checksum (review)
                                                                                  • Checksumming Cyclic Redundancy Check
                                                                                  • Cyclic Redundancy Check
                                                                                  • Slide 14
                                                                                  • CRC Example
                                                                                  • Slide 16
                                                                                  • Multiple Access Links and Protocols
                                                                                  • Cable Network Architecture Overview
                                                                                  • Multiple Access protocols
                                                                                  • Ideal Multiple Access Protocol
                                                                                  • MAC Protocols a taxonomy
                                                                                  • Channel Partitioning MAC protocols TDMA
                                                                                  • Channel Partitioning MAC protocols FDMA
                                                                                  • Random Access Protocols
                                                                                  • ALOHA
                                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                                  • CSMA collisions
                                                                                  • CSMACD (Collision Detection)
                                                                                  • CSMACD collision detection
                                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                                  • Slide 31
                                                                                  • Slide 32
                                                                                  • Summary of MAC protocols
                                                                                  • Slide 34
                                                                                  • MAC Addresses and ARP
                                                                                  • LAN Addresses and ARP
                                                                                  • LAN Address (more)
                                                                                  • ARP Address Resolution Protocol
                                                                                  • ARP protocol Same LAN (network)
                                                                                  • Addressing routing to another LAN
                                                                                  • Slide 41
                                                                                  • Slide 42
                                                                                  • Ethernet
                                                                                  • Star topology
                                                                                  • Ethernet Frame Structure
                                                                                  • Ethernet Frame Structure (more)
                                                                                  • Ethernet Unreliable connectionless
                                                                                  • Ethernet CSMACD algorithm
                                                                                  • Ethernetrsquos CSMACD (more)
                                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                                  • Manchester encoding
                                                                                  • Slide 52
                                                                                  • Hubs
                                                                                  • Switch
                                                                                  • Switch allows multiple simultaneous transmissions
                                                                                  • Switch Table
                                                                                  • Switch self-learning
                                                                                  • Switch frame filteringforwarding
                                                                                  • Self-learning forwarding example
                                                                                  • Interconnecting switches
                                                                                  • Self-learning multi-switch example
                                                                                  • Institutional network
                                                                                  • Switches vs Routers
                                                                                  • Slide 64
                                                                                  • Synthesis a day in the life of a web request
                                                                                  • A day in the life scenario
                                                                                  • A day in the lifehellip connecting to the Internet
                                                                                  • Slide 68
                                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                  • A day in the lifehellip using DNS
                                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                                  • A day in the lifehellip HTTP requestreply
                                                                                  • Chapter 5 letrsquos take a breath

                                                                                    5 DataLink Layer 5-42

                                                                                    Link Layer

                                                                                    51 Introduction and services

                                                                                    52 Error detection and correction

                                                                                    53Multiple access protocols

                                                                                    54 Link-Layer Addressing

                                                                                    55 Ethernet

                                                                                    56 Link-layer switches 59 A day in the life of

                                                                                    a web request

                                                                                    5 DataLink Layer 5-43

                                                                                    Ethernet

                                                                                    ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                                    Metcalfersquos Ethernetsketch

                                                                                    5 DataLink Layer 5-44

                                                                                    Star topology bus topology popular through mid 90s

                                                                                    o all nodes in same collision domain (can collide with each other)

                                                                                    today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                                    (nodes do not collide with each other)

                                                                                    switch

                                                                                    bus coaxial cable star

                                                                                    5 DataLink Layer 5-45

                                                                                    Ethernet Frame Structure

                                                                                    Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                    Preamble 7 bytes with pattern 10101010 followed by one

                                                                                    byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                    rates

                                                                                    5 DataLink Layer 5-46

                                                                                    Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                    o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                    o otherwise adapter discards frame

                                                                                    Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                    CRC checked at receiver if error is detected frame is dropped

                                                                                    5 DataLink Layer 5-47

                                                                                    Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                    and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                    send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                    gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                    Ethernetrsquos MAC protocol CSMACD

                                                                                    5 DataLink Layer 5-48

                                                                                    Ethernet CSMACD algorithm

                                                                                    1 NIC receives datagram from network layer creates frame

                                                                                    2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                    3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                    4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                    5 After aborting NIC enters exponential backoff

                                                                                    after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                    (1 slot = 512 bit-times)

                                                                                    5 DataLink Layer 5-49

                                                                                    Ethernetrsquos CSMACD (more)

                                                                                    Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                    Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                    Exponential Backoff Goal adapt retransmission

                                                                                    attempts to estimated current loado heavy load random

                                                                                    wait will be longer first collision choose K

                                                                                    from 01 delay is K 512 bit transmission times

                                                                                    after second collision choose K from 0123hellip

                                                                                    after ten collisions choose K from 01234hellip1023

                                                                                    5 DataLink Layer 5-50

                                                                                    8023 Ethernet Standards Link amp Physical Layers

                                                                                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                    applicationtransportnetwork

                                                                                    linkphysical

                                                                                    MAC protocoland frame format

                                                                                    100BASE-TX

                                                                                    100BASE-T4

                                                                                    100BASE-FX100BASE-T2

                                                                                    100BASE-SX 100BASE-BX

                                                                                    fiber physical layercopper (twisterpair) physical layer

                                                                                    5 DataLink Layer 5-51

                                                                                    Manchester encoding

                                                                                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                    synchronize to each othero no need for a centralized global clock among nodes

                                                                                    Hey this is physical-layer stuff

                                                                                    5 DataLink Layer 5-52

                                                                                    Link Layer

                                                                                    51 Introduction and services

                                                                                    52 Error detection and correction

                                                                                    53 Multiple access protocols

                                                                                    54 Link-layer Addressing

                                                                                    55 Ethernet

                                                                                    56 Link-layer switches LANs

                                                                                    59 A day in the life of a web request

                                                                                    5 DataLink Layer 5-53

                                                                                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                    o bits coming in one link go out all other links at same rate

                                                                                    o all nodes connected to hub can collide with one another

                                                                                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                    twisted pair

                                                                                    hub

                                                                                    5 DataLink Layer 5-54

                                                                                    Switch link-layer device smarter than hubs take active

                                                                                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                    transparento hosts are unaware of presence of switches

                                                                                    plug-and-play self-learningo switches do not need to be configured

                                                                                    5 DataLink Layer 5-55

                                                                                    Switch allows multiple simultaneous transmissions

                                                                                    hosts have dedicated direct connection to switch

                                                                                    switches buffer packets Ethernet protocol used on

                                                                                    each incoming link but no collisions full duplexo each link is its own collision

                                                                                    domain switching A-to-Arsquo and B-

                                                                                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                    A

                                                                                    Arsquo

                                                                                    B

                                                                                    Brsquo

                                                                                    C

                                                                                    Crsquo

                                                                                    switch with six interfaces(123456)

                                                                                    1 23

                                                                                    45

                                                                                    6

                                                                                    5 DataLink Layer 5-56

                                                                                    Switch Table

                                                                                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                    A each switch has a switch table each entryo (MAC address of host interface

                                                                                    to reach host time stamp)

                                                                                    looks like a routing table Q how are entries created

                                                                                    maintained in switch table o something like a routing

                                                                                    protocol

                                                                                    A

                                                                                    Arsquo

                                                                                    B

                                                                                    Brsquo

                                                                                    C

                                                                                    Crsquo

                                                                                    switch with six interfaces(123456)

                                                                                    1 23

                                                                                    45

                                                                                    6

                                                                                    5 DataLink Layer 5-57

                                                                                    Switch self-learning

                                                                                    switch learns which hosts can be reached through which interfaceso when frame received

                                                                                    switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                    o records senderlocation pair in switch table

                                                                                    A

                                                                                    Arsquo

                                                                                    B

                                                                                    Brsquo

                                                                                    C

                                                                                    Crsquo

                                                                                    1 23

                                                                                    45

                                                                                    6

                                                                                    A Arsquo

                                                                                    Source ADest Arsquo

                                                                                    MAC addr interface TTL

                                                                                    Switch table (initially empty)

                                                                                    A 1 60

                                                                                    5 DataLink Layer 5-58

                                                                                    Switch frame filteringforwardingWhen frame received

                                                                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                    then if dest on segment from which frame arrived

                                                                                    then drop the frame else forward the frame on interface indicated else flood

                                                                                    forward on all but the interface on which the frame arrived

                                                                                    5 DataLink Layer 5-59

                                                                                    Self-learning forwarding example

                                                                                    A

                                                                                    Arsquo

                                                                                    B

                                                                                    Brsquo

                                                                                    C

                                                                                    Crsquo

                                                                                    1 23

                                                                                    45

                                                                                    6

                                                                                    A Arsquo

                                                                                    Source ADest Arsquo

                                                                                    MAC addr interface TTL

                                                                                    Switch table (initially empty)

                                                                                    A 1 60

                                                                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                    frame destination unknownflood

                                                                                    Arsquo A

                                                                                    destination A location known

                                                                                    Arsquo 4 60

                                                                                    selective send

                                                                                    5 DataLink Layer 5-60

                                                                                    Interconnecting switches

                                                                                    switches can be connected together

                                                                                    A

                                                                                    B

                                                                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                    A self learning (works exactly the same as in single-switch case)

                                                                                    S1

                                                                                    C D

                                                                                    E

                                                                                    FS2

                                                                                    S4

                                                                                    S3

                                                                                    H

                                                                                    I

                                                                                    G

                                                                                    5 DataLink Layer 5-61

                                                                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                    A

                                                                                    B

                                                                                    S1

                                                                                    C D

                                                                                    E

                                                                                    FS2

                                                                                    S4

                                                                                    S3

                                                                                    H

                                                                                    I

                                                                                    G

                                                                                    1

                                                                                    2 3

                                                                                    34

                                                                                    5 DataLink Layer 5-62

                                                                                    Institutional network

                                                                                    to externalnetwork

                                                                                    router

                                                                                    IP subnet

                                                                                    mail server

                                                                                    web server

                                                                                    5 DataLink Layer 5-63

                                                                                    Switches vs Routers both store-and-forward devices

                                                                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                    routers maintain routing tables implement routing algorithms

                                                                                    switches maintain switch tables implement filtering learning algorithms

                                                                                    Switch

                                                                                    5 DataLink Layer 5-64

                                                                                    Link Layer

                                                                                    51 Introduction and services

                                                                                    52 Error detection and correction

                                                                                    53Multiple access protocols

                                                                                    54 Link-Layer Addressing

                                                                                    55 Ethernet

                                                                                    56 Link-layer switches 59 A day in the life of

                                                                                    a web request

                                                                                    5 DataLink Layer 5-65

                                                                                    Synthesis a day in the life of a web request

                                                                                    journey down protocol stack completeo application transport network link

                                                                                    putting-it-all-together synthesiso goal identify review understand protocols

                                                                                    (at all layers) involved in seemingly simple scenario requesting www page

                                                                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                    5 DataLink Layer 5-66

                                                                                    A day in the life scenario

                                                                                    Comcast network 68800013

                                                                                    Googlersquos network 64233160019 64233169105

                                                                                    web server

                                                                                    DNS server

                                                                                    school network 68802024

                                                                                    browser

                                                                                    web page

                                                                                    5 DataLink Layer 5-67

                                                                                    A day in the lifehellip connecting to the Internet

                                                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                    router(runs DHCP)

                                                                                    DHCPUDP

                                                                                    IPEthPhy

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCPUDP

                                                                                    IPEthPhy

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCPDHCP

                                                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                    5 DataLink Layer 5-68

                                                                                    A day in the lifehellip connecting to the Internet

                                                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                    router(runs DHCP)

                                                                                    DHCPUDP

                                                                                    IPEthPhy

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCPUDP

                                                                                    IPEthPhy

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    DHCP

                                                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                    DHCP client receives DHCP ACK reply

                                                                                    5 DataLink Layer 5-69

                                                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                    DNSUDP

                                                                                    IPEthPhy

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                    ARP query

                                                                                    EthPhy

                                                                                    ARP

                                                                                    ARP

                                                                                    ARP reply

                                                                                    5 DataLink Layer 5-70

                                                                                    A day in the lifehellip using DNS

                                                                                    DNSUDP

                                                                                    IPEthPhy

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                    demuxrsquoed to DNS server DNS server replies to

                                                                                    client with IP address of wwwgooglecom

                                                                                    Comcast network 68800013

                                                                                    DNS server

                                                                                    DNSUDP

                                                                                    IPEthPhy

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    DNS

                                                                                    5 DataLink Layer 5-71

                                                                                    A day in the lifehellip TCP connection carrying HTTP

                                                                                    HTTPTCPIP

                                                                                    EthPhy

                                                                                    HTTP

                                                                                    to send HTTP request client first opens TCP socket to web server

                                                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                    TCP connection established

                                                                                    64233169105

                                                                                    web server

                                                                                    SYN

                                                                                    SYN

                                                                                    SYN

                                                                                    SYN

                                                                                    TCPIP

                                                                                    EthPhy

                                                                                    SYN

                                                                                    SYN

                                                                                    SYN

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    SYNACK

                                                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                    5 DataLink Layer 5-72

                                                                                    A day in the lifehellip HTTP requestreply

                                                                                    HTTPTCPIP

                                                                                    EthPhy

                                                                                    HTTP

                                                                                    HTTP request sent into TCP socket

                                                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                                                    IP datgram containing HTTP reply routed back to client

                                                                                    64233169105

                                                                                    web server

                                                                                    HTTPTCPIP

                                                                                    EthPhy

                                                                                    web server responds with HTTP reply (containing web page)

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTPHTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    HTTP

                                                                                    web page finally () displayed

                                                                                    5 DataLink Layer 5-73

                                                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                    (except PHY) solid understanding of networking

                                                                                    principles practice hellip could stop here hellip but lots of

                                                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                    • Link Layer
                                                                                    • Link Layer Introduction
                                                                                    • Link layer context
                                                                                    • Encapsulation
                                                                                    • Link Layer Services
                                                                                    • Link Layer Services (more)
                                                                                    • Where is the link layer implemented
                                                                                    • Adaptors Communicating
                                                                                    • Slide 9
                                                                                    • Parity Checking
                                                                                    • Internet checksum (review)
                                                                                    • Checksumming Cyclic Redundancy Check
                                                                                    • Cyclic Redundancy Check
                                                                                    • Slide 14
                                                                                    • CRC Example
                                                                                    • Slide 16
                                                                                    • Multiple Access Links and Protocols
                                                                                    • Cable Network Architecture Overview
                                                                                    • Multiple Access protocols
                                                                                    • Ideal Multiple Access Protocol
                                                                                    • MAC Protocols a taxonomy
                                                                                    • Channel Partitioning MAC protocols TDMA
                                                                                    • Channel Partitioning MAC protocols FDMA
                                                                                    • Random Access Protocols
                                                                                    • ALOHA
                                                                                    • CSMA (Carrier Sense Multiple Access)
                                                                                    • CSMA collisions
                                                                                    • CSMACD (Collision Detection)
                                                                                    • CSMACD collision detection
                                                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                                                    • Slide 31
                                                                                    • Slide 32
                                                                                    • Summary of MAC protocols
                                                                                    • Slide 34
                                                                                    • MAC Addresses and ARP
                                                                                    • LAN Addresses and ARP
                                                                                    • LAN Address (more)
                                                                                    • ARP Address Resolution Protocol
                                                                                    • ARP protocol Same LAN (network)
                                                                                    • Addressing routing to another LAN
                                                                                    • Slide 41
                                                                                    • Slide 42
                                                                                    • Ethernet
                                                                                    • Star topology
                                                                                    • Ethernet Frame Structure
                                                                                    • Ethernet Frame Structure (more)
                                                                                    • Ethernet Unreliable connectionless
                                                                                    • Ethernet CSMACD algorithm
                                                                                    • Ethernetrsquos CSMACD (more)
                                                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                                                    • Manchester encoding
                                                                                    • Slide 52
                                                                                    • Hubs
                                                                                    • Switch
                                                                                    • Switch allows multiple simultaneous transmissions
                                                                                    • Switch Table
                                                                                    • Switch self-learning
                                                                                    • Switch frame filteringforwarding
                                                                                    • Self-learning forwarding example
                                                                                    • Interconnecting switches
                                                                                    • Self-learning multi-switch example
                                                                                    • Institutional network
                                                                                    • Switches vs Routers
                                                                                    • Slide 64
                                                                                    • Synthesis a day in the life of a web request
                                                                                    • A day in the life scenario
                                                                                    • A day in the lifehellip connecting to the Internet
                                                                                    • Slide 68
                                                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                    • A day in the lifehellip using DNS
                                                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                                                    • A day in the lifehellip HTTP requestreply
                                                                                    • Chapter 5 letrsquos take a breath

                                                                                      5 DataLink Layer 5-43

                                                                                      Ethernet

                                                                                      ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps

                                                                                      Metcalfersquos Ethernetsketch

                                                                                      5 DataLink Layer 5-44

                                                                                      Star topology bus topology popular through mid 90s

                                                                                      o all nodes in same collision domain (can collide with each other)

                                                                                      today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                                      (nodes do not collide with each other)

                                                                                      switch

                                                                                      bus coaxial cable star

                                                                                      5 DataLink Layer 5-45

                                                                                      Ethernet Frame Structure

                                                                                      Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                      Preamble 7 bytes with pattern 10101010 followed by one

                                                                                      byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                      rates

                                                                                      5 DataLink Layer 5-46

                                                                                      Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                      o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                      o otherwise adapter discards frame

                                                                                      Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                      CRC checked at receiver if error is detected frame is dropped

                                                                                      5 DataLink Layer 5-47

                                                                                      Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                      and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                      send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                      gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                      Ethernetrsquos MAC protocol CSMACD

                                                                                      5 DataLink Layer 5-48

                                                                                      Ethernet CSMACD algorithm

                                                                                      1 NIC receives datagram from network layer creates frame

                                                                                      2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                      3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                      4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                      5 After aborting NIC enters exponential backoff

                                                                                      after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                      (1 slot = 512 bit-times)

                                                                                      5 DataLink Layer 5-49

                                                                                      Ethernetrsquos CSMACD (more)

                                                                                      Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                      Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                      Exponential Backoff Goal adapt retransmission

                                                                                      attempts to estimated current loado heavy load random

                                                                                      wait will be longer first collision choose K

                                                                                      from 01 delay is K 512 bit transmission times

                                                                                      after second collision choose K from 0123hellip

                                                                                      after ten collisions choose K from 01234hellip1023

                                                                                      5 DataLink Layer 5-50

                                                                                      8023 Ethernet Standards Link amp Physical Layers

                                                                                      many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                      Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                      applicationtransportnetwork

                                                                                      linkphysical

                                                                                      MAC protocoland frame format

                                                                                      100BASE-TX

                                                                                      100BASE-T4

                                                                                      100BASE-FX100BASE-T2

                                                                                      100BASE-SX 100BASE-BX

                                                                                      fiber physical layercopper (twisterpair) physical layer

                                                                                      5 DataLink Layer 5-51

                                                                                      Manchester encoding

                                                                                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                      synchronize to each othero no need for a centralized global clock among nodes

                                                                                      Hey this is physical-layer stuff

                                                                                      5 DataLink Layer 5-52

                                                                                      Link Layer

                                                                                      51 Introduction and services

                                                                                      52 Error detection and correction

                                                                                      53 Multiple access protocols

                                                                                      54 Link-layer Addressing

                                                                                      55 Ethernet

                                                                                      56 Link-layer switches LANs

                                                                                      59 A day in the life of a web request

                                                                                      5 DataLink Layer 5-53

                                                                                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                      o bits coming in one link go out all other links at same rate

                                                                                      o all nodes connected to hub can collide with one another

                                                                                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                      twisted pair

                                                                                      hub

                                                                                      5 DataLink Layer 5-54

                                                                                      Switch link-layer device smarter than hubs take active

                                                                                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                      transparento hosts are unaware of presence of switches

                                                                                      plug-and-play self-learningo switches do not need to be configured

                                                                                      5 DataLink Layer 5-55

                                                                                      Switch allows multiple simultaneous transmissions

                                                                                      hosts have dedicated direct connection to switch

                                                                                      switches buffer packets Ethernet protocol used on

                                                                                      each incoming link but no collisions full duplexo each link is its own collision

                                                                                      domain switching A-to-Arsquo and B-

                                                                                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                      A

                                                                                      Arsquo

                                                                                      B

                                                                                      Brsquo

                                                                                      C

                                                                                      Crsquo

                                                                                      switch with six interfaces(123456)

                                                                                      1 23

                                                                                      45

                                                                                      6

                                                                                      5 DataLink Layer 5-56

                                                                                      Switch Table

                                                                                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                      A each switch has a switch table each entryo (MAC address of host interface

                                                                                      to reach host time stamp)

                                                                                      looks like a routing table Q how are entries created

                                                                                      maintained in switch table o something like a routing

                                                                                      protocol

                                                                                      A

                                                                                      Arsquo

                                                                                      B

                                                                                      Brsquo

                                                                                      C

                                                                                      Crsquo

                                                                                      switch with six interfaces(123456)

                                                                                      1 23

                                                                                      45

                                                                                      6

                                                                                      5 DataLink Layer 5-57

                                                                                      Switch self-learning

                                                                                      switch learns which hosts can be reached through which interfaceso when frame received

                                                                                      switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                      o records senderlocation pair in switch table

                                                                                      A

                                                                                      Arsquo

                                                                                      B

                                                                                      Brsquo

                                                                                      C

                                                                                      Crsquo

                                                                                      1 23

                                                                                      45

                                                                                      6

                                                                                      A Arsquo

                                                                                      Source ADest Arsquo

                                                                                      MAC addr interface TTL

                                                                                      Switch table (initially empty)

                                                                                      A 1 60

                                                                                      5 DataLink Layer 5-58

                                                                                      Switch frame filteringforwardingWhen frame received

                                                                                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                      then if dest on segment from which frame arrived

                                                                                      then drop the frame else forward the frame on interface indicated else flood

                                                                                      forward on all but the interface on which the frame arrived

                                                                                      5 DataLink Layer 5-59

                                                                                      Self-learning forwarding example

                                                                                      A

                                                                                      Arsquo

                                                                                      B

                                                                                      Brsquo

                                                                                      C

                                                                                      Crsquo

                                                                                      1 23

                                                                                      45

                                                                                      6

                                                                                      A Arsquo

                                                                                      Source ADest Arsquo

                                                                                      MAC addr interface TTL

                                                                                      Switch table (initially empty)

                                                                                      A 1 60

                                                                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                      frame destination unknownflood

                                                                                      Arsquo A

                                                                                      destination A location known

                                                                                      Arsquo 4 60

                                                                                      selective send

                                                                                      5 DataLink Layer 5-60

                                                                                      Interconnecting switches

                                                                                      switches can be connected together

                                                                                      A

                                                                                      B

                                                                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                      A self learning (works exactly the same as in single-switch case)

                                                                                      S1

                                                                                      C D

                                                                                      E

                                                                                      FS2

                                                                                      S4

                                                                                      S3

                                                                                      H

                                                                                      I

                                                                                      G

                                                                                      5 DataLink Layer 5-61

                                                                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                      A

                                                                                      B

                                                                                      S1

                                                                                      C D

                                                                                      E

                                                                                      FS2

                                                                                      S4

                                                                                      S3

                                                                                      H

                                                                                      I

                                                                                      G

                                                                                      1

                                                                                      2 3

                                                                                      34

                                                                                      5 DataLink Layer 5-62

                                                                                      Institutional network

                                                                                      to externalnetwork

                                                                                      router

                                                                                      IP subnet

                                                                                      mail server

                                                                                      web server

                                                                                      5 DataLink Layer 5-63

                                                                                      Switches vs Routers both store-and-forward devices

                                                                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                      routers maintain routing tables implement routing algorithms

                                                                                      switches maintain switch tables implement filtering learning algorithms

                                                                                      Switch

                                                                                      5 DataLink Layer 5-64

                                                                                      Link Layer

                                                                                      51 Introduction and services

                                                                                      52 Error detection and correction

                                                                                      53Multiple access protocols

                                                                                      54 Link-Layer Addressing

                                                                                      55 Ethernet

                                                                                      56 Link-layer switches 59 A day in the life of

                                                                                      a web request

                                                                                      5 DataLink Layer 5-65

                                                                                      Synthesis a day in the life of a web request

                                                                                      journey down protocol stack completeo application transport network link

                                                                                      putting-it-all-together synthesiso goal identify review understand protocols

                                                                                      (at all layers) involved in seemingly simple scenario requesting www page

                                                                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                      5 DataLink Layer 5-66

                                                                                      A day in the life scenario

                                                                                      Comcast network 68800013

                                                                                      Googlersquos network 64233160019 64233169105

                                                                                      web server

                                                                                      DNS server

                                                                                      school network 68802024

                                                                                      browser

                                                                                      web page

                                                                                      5 DataLink Layer 5-67

                                                                                      A day in the lifehellip connecting to the Internet

                                                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                      router(runs DHCP)

                                                                                      DHCPUDP

                                                                                      IPEthPhy

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCPUDP

                                                                                      IPEthPhy

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCPDHCP

                                                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                      5 DataLink Layer 5-68

                                                                                      A day in the lifehellip connecting to the Internet

                                                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                      router(runs DHCP)

                                                                                      DHCPUDP

                                                                                      IPEthPhy

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCPUDP

                                                                                      IPEthPhy

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      DHCP

                                                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                      DHCP client receives DHCP ACK reply

                                                                                      5 DataLink Layer 5-69

                                                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                      DNSUDP

                                                                                      IPEthPhy

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                      ARP query

                                                                                      EthPhy

                                                                                      ARP

                                                                                      ARP

                                                                                      ARP reply

                                                                                      5 DataLink Layer 5-70

                                                                                      A day in the lifehellip using DNS

                                                                                      DNSUDP

                                                                                      IPEthPhy

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                      demuxrsquoed to DNS server DNS server replies to

                                                                                      client with IP address of wwwgooglecom

                                                                                      Comcast network 68800013

                                                                                      DNS server

                                                                                      DNSUDP

                                                                                      IPEthPhy

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      DNS

                                                                                      5 DataLink Layer 5-71

                                                                                      A day in the lifehellip TCP connection carrying HTTP

                                                                                      HTTPTCPIP

                                                                                      EthPhy

                                                                                      HTTP

                                                                                      to send HTTP request client first opens TCP socket to web server

                                                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                      TCP connection established

                                                                                      64233169105

                                                                                      web server

                                                                                      SYN

                                                                                      SYN

                                                                                      SYN

                                                                                      SYN

                                                                                      TCPIP

                                                                                      EthPhy

                                                                                      SYN

                                                                                      SYN

                                                                                      SYN

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      SYNACK

                                                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                      5 DataLink Layer 5-72

                                                                                      A day in the lifehellip HTTP requestreply

                                                                                      HTTPTCPIP

                                                                                      EthPhy

                                                                                      HTTP

                                                                                      HTTP request sent into TCP socket

                                                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                                                      IP datgram containing HTTP reply routed back to client

                                                                                      64233169105

                                                                                      web server

                                                                                      HTTPTCPIP

                                                                                      EthPhy

                                                                                      web server responds with HTTP reply (containing web page)

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTPHTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      HTTP

                                                                                      web page finally () displayed

                                                                                      5 DataLink Layer 5-73

                                                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                      (except PHY) solid understanding of networking

                                                                                      principles practice hellip could stop here hellip but lots of

                                                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                      • Link Layer
                                                                                      • Link Layer Introduction
                                                                                      • Link layer context
                                                                                      • Encapsulation
                                                                                      • Link Layer Services
                                                                                      • Link Layer Services (more)
                                                                                      • Where is the link layer implemented
                                                                                      • Adaptors Communicating
                                                                                      • Slide 9
                                                                                      • Parity Checking
                                                                                      • Internet checksum (review)
                                                                                      • Checksumming Cyclic Redundancy Check
                                                                                      • Cyclic Redundancy Check
                                                                                      • Slide 14
                                                                                      • CRC Example
                                                                                      • Slide 16
                                                                                      • Multiple Access Links and Protocols
                                                                                      • Cable Network Architecture Overview
                                                                                      • Multiple Access protocols
                                                                                      • Ideal Multiple Access Protocol
                                                                                      • MAC Protocols a taxonomy
                                                                                      • Channel Partitioning MAC protocols TDMA
                                                                                      • Channel Partitioning MAC protocols FDMA
                                                                                      • Random Access Protocols
                                                                                      • ALOHA
                                                                                      • CSMA (Carrier Sense Multiple Access)
                                                                                      • CSMA collisions
                                                                                      • CSMACD (Collision Detection)
                                                                                      • CSMACD collision detection
                                                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                                                      • Slide 31
                                                                                      • Slide 32
                                                                                      • Summary of MAC protocols
                                                                                      • Slide 34
                                                                                      • MAC Addresses and ARP
                                                                                      • LAN Addresses and ARP
                                                                                      • LAN Address (more)
                                                                                      • ARP Address Resolution Protocol
                                                                                      • ARP protocol Same LAN (network)
                                                                                      • Addressing routing to another LAN
                                                                                      • Slide 41
                                                                                      • Slide 42
                                                                                      • Ethernet
                                                                                      • Star topology
                                                                                      • Ethernet Frame Structure
                                                                                      • Ethernet Frame Structure (more)
                                                                                      • Ethernet Unreliable connectionless
                                                                                      • Ethernet CSMACD algorithm
                                                                                      • Ethernetrsquos CSMACD (more)
                                                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                                                      • Manchester encoding
                                                                                      • Slide 52
                                                                                      • Hubs
                                                                                      • Switch
                                                                                      • Switch allows multiple simultaneous transmissions
                                                                                      • Switch Table
                                                                                      • Switch self-learning
                                                                                      • Switch frame filteringforwarding
                                                                                      • Self-learning forwarding example
                                                                                      • Interconnecting switches
                                                                                      • Self-learning multi-switch example
                                                                                      • Institutional network
                                                                                      • Switches vs Routers
                                                                                      • Slide 64
                                                                                      • Synthesis a day in the life of a web request
                                                                                      • A day in the life scenario
                                                                                      • A day in the lifehellip connecting to the Internet
                                                                                      • Slide 68
                                                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                      • A day in the lifehellip using DNS
                                                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                                                      • A day in the lifehellip HTTP requestreply
                                                                                      • Chapter 5 letrsquos take a breath

                                                                                        5 DataLink Layer 5-44

                                                                                        Star topology bus topology popular through mid 90s

                                                                                        o all nodes in same collision domain (can collide with each other)

                                                                                        today star topology prevailso active switch in centero each ldquospokerdquo runs a (separate) Ethernet protocol

                                                                                        (nodes do not collide with each other)

                                                                                        switch

                                                                                        bus coaxial cable star

                                                                                        5 DataLink Layer 5-45

                                                                                        Ethernet Frame Structure

                                                                                        Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                        Preamble 7 bytes with pattern 10101010 followed by one

                                                                                        byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                        rates

                                                                                        5 DataLink Layer 5-46

                                                                                        Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                        o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                        o otherwise adapter discards frame

                                                                                        Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                        CRC checked at receiver if error is detected frame is dropped

                                                                                        5 DataLink Layer 5-47

                                                                                        Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                        and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                        send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                        gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                        Ethernetrsquos MAC protocol CSMACD

                                                                                        5 DataLink Layer 5-48

                                                                                        Ethernet CSMACD algorithm

                                                                                        1 NIC receives datagram from network layer creates frame

                                                                                        2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                        3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                        4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                        5 After aborting NIC enters exponential backoff

                                                                                        after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                        (1 slot = 512 bit-times)

                                                                                        5 DataLink Layer 5-49

                                                                                        Ethernetrsquos CSMACD (more)

                                                                                        Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                        Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                        Exponential Backoff Goal adapt retransmission

                                                                                        attempts to estimated current loado heavy load random

                                                                                        wait will be longer first collision choose K

                                                                                        from 01 delay is K 512 bit transmission times

                                                                                        after second collision choose K from 0123hellip

                                                                                        after ten collisions choose K from 01234hellip1023

                                                                                        5 DataLink Layer 5-50

                                                                                        8023 Ethernet Standards Link amp Physical Layers

                                                                                        many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                        Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                        applicationtransportnetwork

                                                                                        linkphysical

                                                                                        MAC protocoland frame format

                                                                                        100BASE-TX

                                                                                        100BASE-T4

                                                                                        100BASE-FX100BASE-T2

                                                                                        100BASE-SX 100BASE-BX

                                                                                        fiber physical layercopper (twisterpair) physical layer

                                                                                        5 DataLink Layer 5-51

                                                                                        Manchester encoding

                                                                                        used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                        synchronize to each othero no need for a centralized global clock among nodes

                                                                                        Hey this is physical-layer stuff

                                                                                        5 DataLink Layer 5-52

                                                                                        Link Layer

                                                                                        51 Introduction and services

                                                                                        52 Error detection and correction

                                                                                        53 Multiple access protocols

                                                                                        54 Link-layer Addressing

                                                                                        55 Ethernet

                                                                                        56 Link-layer switches LANs

                                                                                        59 A day in the life of a web request

                                                                                        5 DataLink Layer 5-53

                                                                                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                        o bits coming in one link go out all other links at same rate

                                                                                        o all nodes connected to hub can collide with one another

                                                                                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                        twisted pair

                                                                                        hub

                                                                                        5 DataLink Layer 5-54

                                                                                        Switch link-layer device smarter than hubs take active

                                                                                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                        transparento hosts are unaware of presence of switches

                                                                                        plug-and-play self-learningo switches do not need to be configured

                                                                                        5 DataLink Layer 5-55

                                                                                        Switch allows multiple simultaneous transmissions

                                                                                        hosts have dedicated direct connection to switch

                                                                                        switches buffer packets Ethernet protocol used on

                                                                                        each incoming link but no collisions full duplexo each link is its own collision

                                                                                        domain switching A-to-Arsquo and B-

                                                                                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                        A

                                                                                        Arsquo

                                                                                        B

                                                                                        Brsquo

                                                                                        C

                                                                                        Crsquo

                                                                                        switch with six interfaces(123456)

                                                                                        1 23

                                                                                        45

                                                                                        6

                                                                                        5 DataLink Layer 5-56

                                                                                        Switch Table

                                                                                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                        A each switch has a switch table each entryo (MAC address of host interface

                                                                                        to reach host time stamp)

                                                                                        looks like a routing table Q how are entries created

                                                                                        maintained in switch table o something like a routing

                                                                                        protocol

                                                                                        A

                                                                                        Arsquo

                                                                                        B

                                                                                        Brsquo

                                                                                        C

                                                                                        Crsquo

                                                                                        switch with six interfaces(123456)

                                                                                        1 23

                                                                                        45

                                                                                        6

                                                                                        5 DataLink Layer 5-57

                                                                                        Switch self-learning

                                                                                        switch learns which hosts can be reached through which interfaceso when frame received

                                                                                        switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                        o records senderlocation pair in switch table

                                                                                        A

                                                                                        Arsquo

                                                                                        B

                                                                                        Brsquo

                                                                                        C

                                                                                        Crsquo

                                                                                        1 23

                                                                                        45

                                                                                        6

                                                                                        A Arsquo

                                                                                        Source ADest Arsquo

                                                                                        MAC addr interface TTL

                                                                                        Switch table (initially empty)

                                                                                        A 1 60

                                                                                        5 DataLink Layer 5-58

                                                                                        Switch frame filteringforwardingWhen frame received

                                                                                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                        then if dest on segment from which frame arrived

                                                                                        then drop the frame else forward the frame on interface indicated else flood

                                                                                        forward on all but the interface on which the frame arrived

                                                                                        5 DataLink Layer 5-59

                                                                                        Self-learning forwarding example

                                                                                        A

                                                                                        Arsquo

                                                                                        B

                                                                                        Brsquo

                                                                                        C

                                                                                        Crsquo

                                                                                        1 23

                                                                                        45

                                                                                        6

                                                                                        A Arsquo

                                                                                        Source ADest Arsquo

                                                                                        MAC addr interface TTL

                                                                                        Switch table (initially empty)

                                                                                        A 1 60

                                                                                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                        frame destination unknownflood

                                                                                        Arsquo A

                                                                                        destination A location known

                                                                                        Arsquo 4 60

                                                                                        selective send

                                                                                        5 DataLink Layer 5-60

                                                                                        Interconnecting switches

                                                                                        switches can be connected together

                                                                                        A

                                                                                        B

                                                                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                        A self learning (works exactly the same as in single-switch case)

                                                                                        S1

                                                                                        C D

                                                                                        E

                                                                                        FS2

                                                                                        S4

                                                                                        S3

                                                                                        H

                                                                                        I

                                                                                        G

                                                                                        5 DataLink Layer 5-61

                                                                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                        A

                                                                                        B

                                                                                        S1

                                                                                        C D

                                                                                        E

                                                                                        FS2

                                                                                        S4

                                                                                        S3

                                                                                        H

                                                                                        I

                                                                                        G

                                                                                        1

                                                                                        2 3

                                                                                        34

                                                                                        5 DataLink Layer 5-62

                                                                                        Institutional network

                                                                                        to externalnetwork

                                                                                        router

                                                                                        IP subnet

                                                                                        mail server

                                                                                        web server

                                                                                        5 DataLink Layer 5-63

                                                                                        Switches vs Routers both store-and-forward devices

                                                                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                        routers maintain routing tables implement routing algorithms

                                                                                        switches maintain switch tables implement filtering learning algorithms

                                                                                        Switch

                                                                                        5 DataLink Layer 5-64

                                                                                        Link Layer

                                                                                        51 Introduction and services

                                                                                        52 Error detection and correction

                                                                                        53Multiple access protocols

                                                                                        54 Link-Layer Addressing

                                                                                        55 Ethernet

                                                                                        56 Link-layer switches 59 A day in the life of

                                                                                        a web request

                                                                                        5 DataLink Layer 5-65

                                                                                        Synthesis a day in the life of a web request

                                                                                        journey down protocol stack completeo application transport network link

                                                                                        putting-it-all-together synthesiso goal identify review understand protocols

                                                                                        (at all layers) involved in seemingly simple scenario requesting www page

                                                                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                        5 DataLink Layer 5-66

                                                                                        A day in the life scenario

                                                                                        Comcast network 68800013

                                                                                        Googlersquos network 64233160019 64233169105

                                                                                        web server

                                                                                        DNS server

                                                                                        school network 68802024

                                                                                        browser

                                                                                        web page

                                                                                        5 DataLink Layer 5-67

                                                                                        A day in the lifehellip connecting to the Internet

                                                                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                        router(runs DHCP)

                                                                                        DHCPUDP

                                                                                        IPEthPhy

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCPUDP

                                                                                        IPEthPhy

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCPDHCP

                                                                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                        5 DataLink Layer 5-68

                                                                                        A day in the lifehellip connecting to the Internet

                                                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                        router(runs DHCP)

                                                                                        DHCPUDP

                                                                                        IPEthPhy

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCPUDP

                                                                                        IPEthPhy

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        DHCP

                                                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                        DHCP client receives DHCP ACK reply

                                                                                        5 DataLink Layer 5-69

                                                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                        DNSUDP

                                                                                        IPEthPhy

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                        ARP query

                                                                                        EthPhy

                                                                                        ARP

                                                                                        ARP

                                                                                        ARP reply

                                                                                        5 DataLink Layer 5-70

                                                                                        A day in the lifehellip using DNS

                                                                                        DNSUDP

                                                                                        IPEthPhy

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                        demuxrsquoed to DNS server DNS server replies to

                                                                                        client with IP address of wwwgooglecom

                                                                                        Comcast network 68800013

                                                                                        DNS server

                                                                                        DNSUDP

                                                                                        IPEthPhy

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        DNS

                                                                                        5 DataLink Layer 5-71

                                                                                        A day in the lifehellip TCP connection carrying HTTP

                                                                                        HTTPTCPIP

                                                                                        EthPhy

                                                                                        HTTP

                                                                                        to send HTTP request client first opens TCP socket to web server

                                                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                        TCP connection established

                                                                                        64233169105

                                                                                        web server

                                                                                        SYN

                                                                                        SYN

                                                                                        SYN

                                                                                        SYN

                                                                                        TCPIP

                                                                                        EthPhy

                                                                                        SYN

                                                                                        SYN

                                                                                        SYN

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        SYNACK

                                                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                        5 DataLink Layer 5-72

                                                                                        A day in the lifehellip HTTP requestreply

                                                                                        HTTPTCPIP

                                                                                        EthPhy

                                                                                        HTTP

                                                                                        HTTP request sent into TCP socket

                                                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                                                        IP datgram containing HTTP reply routed back to client

                                                                                        64233169105

                                                                                        web server

                                                                                        HTTPTCPIP

                                                                                        EthPhy

                                                                                        web server responds with HTTP reply (containing web page)

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTPHTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        HTTP

                                                                                        web page finally () displayed

                                                                                        5 DataLink Layer 5-73

                                                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                        (except PHY) solid understanding of networking

                                                                                        principles practice hellip could stop here hellip but lots of

                                                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                        • Link Layer
                                                                                        • Link Layer Introduction
                                                                                        • Link layer context
                                                                                        • Encapsulation
                                                                                        • Link Layer Services
                                                                                        • Link Layer Services (more)
                                                                                        • Where is the link layer implemented
                                                                                        • Adaptors Communicating
                                                                                        • Slide 9
                                                                                        • Parity Checking
                                                                                        • Internet checksum (review)
                                                                                        • Checksumming Cyclic Redundancy Check
                                                                                        • Cyclic Redundancy Check
                                                                                        • Slide 14
                                                                                        • CRC Example
                                                                                        • Slide 16
                                                                                        • Multiple Access Links and Protocols
                                                                                        • Cable Network Architecture Overview
                                                                                        • Multiple Access protocols
                                                                                        • Ideal Multiple Access Protocol
                                                                                        • MAC Protocols a taxonomy
                                                                                        • Channel Partitioning MAC protocols TDMA
                                                                                        • Channel Partitioning MAC protocols FDMA
                                                                                        • Random Access Protocols
                                                                                        • ALOHA
                                                                                        • CSMA (Carrier Sense Multiple Access)
                                                                                        • CSMA collisions
                                                                                        • CSMACD (Collision Detection)
                                                                                        • CSMACD collision detection
                                                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                                                        • Slide 31
                                                                                        • Slide 32
                                                                                        • Summary of MAC protocols
                                                                                        • Slide 34
                                                                                        • MAC Addresses and ARP
                                                                                        • LAN Addresses and ARP
                                                                                        • LAN Address (more)
                                                                                        • ARP Address Resolution Protocol
                                                                                        • ARP protocol Same LAN (network)
                                                                                        • Addressing routing to another LAN
                                                                                        • Slide 41
                                                                                        • Slide 42
                                                                                        • Ethernet
                                                                                        • Star topology
                                                                                        • Ethernet Frame Structure
                                                                                        • Ethernet Frame Structure (more)
                                                                                        • Ethernet Unreliable connectionless
                                                                                        • Ethernet CSMACD algorithm
                                                                                        • Ethernetrsquos CSMACD (more)
                                                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                                                        • Manchester encoding
                                                                                        • Slide 52
                                                                                        • Hubs
                                                                                        • Switch
                                                                                        • Switch allows multiple simultaneous transmissions
                                                                                        • Switch Table
                                                                                        • Switch self-learning
                                                                                        • Switch frame filteringforwarding
                                                                                        • Self-learning forwarding example
                                                                                        • Interconnecting switches
                                                                                        • Self-learning multi-switch example
                                                                                        • Institutional network
                                                                                        • Switches vs Routers
                                                                                        • Slide 64
                                                                                        • Synthesis a day in the life of a web request
                                                                                        • A day in the life scenario
                                                                                        • A day in the lifehellip connecting to the Internet
                                                                                        • Slide 68
                                                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                        • A day in the lifehellip using DNS
                                                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                                                        • A day in the lifehellip HTTP requestreply
                                                                                        • Chapter 5 letrsquos take a breath

                                                                                          5 DataLink Layer 5-45

                                                                                          Ethernet Frame Structure

                                                                                          Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

                                                                                          Preamble 7 bytes with pattern 10101010 followed by one

                                                                                          byte with pattern 10101011 used to synchronize receiver amp sender clock

                                                                                          rates

                                                                                          5 DataLink Layer 5-46

                                                                                          Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                          o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                          o otherwise adapter discards frame

                                                                                          Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                          CRC checked at receiver if error is detected frame is dropped

                                                                                          5 DataLink Layer 5-47

                                                                                          Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                          and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                          send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                          gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                          Ethernetrsquos MAC protocol CSMACD

                                                                                          5 DataLink Layer 5-48

                                                                                          Ethernet CSMACD algorithm

                                                                                          1 NIC receives datagram from network layer creates frame

                                                                                          2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                          3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                          4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                          5 After aborting NIC enters exponential backoff

                                                                                          after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                          (1 slot = 512 bit-times)

                                                                                          5 DataLink Layer 5-49

                                                                                          Ethernetrsquos CSMACD (more)

                                                                                          Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                          Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                          Exponential Backoff Goal adapt retransmission

                                                                                          attempts to estimated current loado heavy load random

                                                                                          wait will be longer first collision choose K

                                                                                          from 01 delay is K 512 bit transmission times

                                                                                          after second collision choose K from 0123hellip

                                                                                          after ten collisions choose K from 01234hellip1023

                                                                                          5 DataLink Layer 5-50

                                                                                          8023 Ethernet Standards Link amp Physical Layers

                                                                                          many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                          Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                          applicationtransportnetwork

                                                                                          linkphysical

                                                                                          MAC protocoland frame format

                                                                                          100BASE-TX

                                                                                          100BASE-T4

                                                                                          100BASE-FX100BASE-T2

                                                                                          100BASE-SX 100BASE-BX

                                                                                          fiber physical layercopper (twisterpair) physical layer

                                                                                          5 DataLink Layer 5-51

                                                                                          Manchester encoding

                                                                                          used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                          synchronize to each othero no need for a centralized global clock among nodes

                                                                                          Hey this is physical-layer stuff

                                                                                          5 DataLink Layer 5-52

                                                                                          Link Layer

                                                                                          51 Introduction and services

                                                                                          52 Error detection and correction

                                                                                          53 Multiple access protocols

                                                                                          54 Link-layer Addressing

                                                                                          55 Ethernet

                                                                                          56 Link-layer switches LANs

                                                                                          59 A day in the life of a web request

                                                                                          5 DataLink Layer 5-53

                                                                                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                          o bits coming in one link go out all other links at same rate

                                                                                          o all nodes connected to hub can collide with one another

                                                                                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                          twisted pair

                                                                                          hub

                                                                                          5 DataLink Layer 5-54

                                                                                          Switch link-layer device smarter than hubs take active

                                                                                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                          transparento hosts are unaware of presence of switches

                                                                                          plug-and-play self-learningo switches do not need to be configured

                                                                                          5 DataLink Layer 5-55

                                                                                          Switch allows multiple simultaneous transmissions

                                                                                          hosts have dedicated direct connection to switch

                                                                                          switches buffer packets Ethernet protocol used on

                                                                                          each incoming link but no collisions full duplexo each link is its own collision

                                                                                          domain switching A-to-Arsquo and B-

                                                                                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                          A

                                                                                          Arsquo

                                                                                          B

                                                                                          Brsquo

                                                                                          C

                                                                                          Crsquo

                                                                                          switch with six interfaces(123456)

                                                                                          1 23

                                                                                          45

                                                                                          6

                                                                                          5 DataLink Layer 5-56

                                                                                          Switch Table

                                                                                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                          A each switch has a switch table each entryo (MAC address of host interface

                                                                                          to reach host time stamp)

                                                                                          looks like a routing table Q how are entries created

                                                                                          maintained in switch table o something like a routing

                                                                                          protocol

                                                                                          A

                                                                                          Arsquo

                                                                                          B

                                                                                          Brsquo

                                                                                          C

                                                                                          Crsquo

                                                                                          switch with six interfaces(123456)

                                                                                          1 23

                                                                                          45

                                                                                          6

                                                                                          5 DataLink Layer 5-57

                                                                                          Switch self-learning

                                                                                          switch learns which hosts can be reached through which interfaceso when frame received

                                                                                          switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                          o records senderlocation pair in switch table

                                                                                          A

                                                                                          Arsquo

                                                                                          B

                                                                                          Brsquo

                                                                                          C

                                                                                          Crsquo

                                                                                          1 23

                                                                                          45

                                                                                          6

                                                                                          A Arsquo

                                                                                          Source ADest Arsquo

                                                                                          MAC addr interface TTL

                                                                                          Switch table (initially empty)

                                                                                          A 1 60

                                                                                          5 DataLink Layer 5-58

                                                                                          Switch frame filteringforwardingWhen frame received

                                                                                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                          then if dest on segment from which frame arrived

                                                                                          then drop the frame else forward the frame on interface indicated else flood

                                                                                          forward on all but the interface on which the frame arrived

                                                                                          5 DataLink Layer 5-59

                                                                                          Self-learning forwarding example

                                                                                          A

                                                                                          Arsquo

                                                                                          B

                                                                                          Brsquo

                                                                                          C

                                                                                          Crsquo

                                                                                          1 23

                                                                                          45

                                                                                          6

                                                                                          A Arsquo

                                                                                          Source ADest Arsquo

                                                                                          MAC addr interface TTL

                                                                                          Switch table (initially empty)

                                                                                          A 1 60

                                                                                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                          frame destination unknownflood

                                                                                          Arsquo A

                                                                                          destination A location known

                                                                                          Arsquo 4 60

                                                                                          selective send

                                                                                          5 DataLink Layer 5-60

                                                                                          Interconnecting switches

                                                                                          switches can be connected together

                                                                                          A

                                                                                          B

                                                                                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                          A self learning (works exactly the same as in single-switch case)

                                                                                          S1

                                                                                          C D

                                                                                          E

                                                                                          FS2

                                                                                          S4

                                                                                          S3

                                                                                          H

                                                                                          I

                                                                                          G

                                                                                          5 DataLink Layer 5-61

                                                                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                          A

                                                                                          B

                                                                                          S1

                                                                                          C D

                                                                                          E

                                                                                          FS2

                                                                                          S4

                                                                                          S3

                                                                                          H

                                                                                          I

                                                                                          G

                                                                                          1

                                                                                          2 3

                                                                                          34

                                                                                          5 DataLink Layer 5-62

                                                                                          Institutional network

                                                                                          to externalnetwork

                                                                                          router

                                                                                          IP subnet

                                                                                          mail server

                                                                                          web server

                                                                                          5 DataLink Layer 5-63

                                                                                          Switches vs Routers both store-and-forward devices

                                                                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                          routers maintain routing tables implement routing algorithms

                                                                                          switches maintain switch tables implement filtering learning algorithms

                                                                                          Switch

                                                                                          5 DataLink Layer 5-64

                                                                                          Link Layer

                                                                                          51 Introduction and services

                                                                                          52 Error detection and correction

                                                                                          53Multiple access protocols

                                                                                          54 Link-Layer Addressing

                                                                                          55 Ethernet

                                                                                          56 Link-layer switches 59 A day in the life of

                                                                                          a web request

                                                                                          5 DataLink Layer 5-65

                                                                                          Synthesis a day in the life of a web request

                                                                                          journey down protocol stack completeo application transport network link

                                                                                          putting-it-all-together synthesiso goal identify review understand protocols

                                                                                          (at all layers) involved in seemingly simple scenario requesting www page

                                                                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                          5 DataLink Layer 5-66

                                                                                          A day in the life scenario

                                                                                          Comcast network 68800013

                                                                                          Googlersquos network 64233160019 64233169105

                                                                                          web server

                                                                                          DNS server

                                                                                          school network 68802024

                                                                                          browser

                                                                                          web page

                                                                                          5 DataLink Layer 5-67

                                                                                          A day in the lifehellip connecting to the Internet

                                                                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                          router(runs DHCP)

                                                                                          DHCPUDP

                                                                                          IPEthPhy

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCPUDP

                                                                                          IPEthPhy

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCPDHCP

                                                                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                          5 DataLink Layer 5-68

                                                                                          A day in the lifehellip connecting to the Internet

                                                                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                          router(runs DHCP)

                                                                                          DHCPUDP

                                                                                          IPEthPhy

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCPUDP

                                                                                          IPEthPhy

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          DHCP

                                                                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                          DHCP client receives DHCP ACK reply

                                                                                          5 DataLink Layer 5-69

                                                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                          DNSUDP

                                                                                          IPEthPhy

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                          ARP query

                                                                                          EthPhy

                                                                                          ARP

                                                                                          ARP

                                                                                          ARP reply

                                                                                          5 DataLink Layer 5-70

                                                                                          A day in the lifehellip using DNS

                                                                                          DNSUDP

                                                                                          IPEthPhy

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                          demuxrsquoed to DNS server DNS server replies to

                                                                                          client with IP address of wwwgooglecom

                                                                                          Comcast network 68800013

                                                                                          DNS server

                                                                                          DNSUDP

                                                                                          IPEthPhy

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          DNS

                                                                                          5 DataLink Layer 5-71

                                                                                          A day in the lifehellip TCP connection carrying HTTP

                                                                                          HTTPTCPIP

                                                                                          EthPhy

                                                                                          HTTP

                                                                                          to send HTTP request client first opens TCP socket to web server

                                                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                          TCP connection established

                                                                                          64233169105

                                                                                          web server

                                                                                          SYN

                                                                                          SYN

                                                                                          SYN

                                                                                          SYN

                                                                                          TCPIP

                                                                                          EthPhy

                                                                                          SYN

                                                                                          SYN

                                                                                          SYN

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          SYNACK

                                                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                          5 DataLink Layer 5-72

                                                                                          A day in the lifehellip HTTP requestreply

                                                                                          HTTPTCPIP

                                                                                          EthPhy

                                                                                          HTTP

                                                                                          HTTP request sent into TCP socket

                                                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                                                          IP datgram containing HTTP reply routed back to client

                                                                                          64233169105

                                                                                          web server

                                                                                          HTTPTCPIP

                                                                                          EthPhy

                                                                                          web server responds with HTTP reply (containing web page)

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTPHTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          HTTP

                                                                                          web page finally () displayed

                                                                                          5 DataLink Layer 5-73

                                                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                          (except PHY) solid understanding of networking

                                                                                          principles practice hellip could stop here hellip but lots of

                                                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                          • Link Layer
                                                                                          • Link Layer Introduction
                                                                                          • Link layer context
                                                                                          • Encapsulation
                                                                                          • Link Layer Services
                                                                                          • Link Layer Services (more)
                                                                                          • Where is the link layer implemented
                                                                                          • Adaptors Communicating
                                                                                          • Slide 9
                                                                                          • Parity Checking
                                                                                          • Internet checksum (review)
                                                                                          • Checksumming Cyclic Redundancy Check
                                                                                          • Cyclic Redundancy Check
                                                                                          • Slide 14
                                                                                          • CRC Example
                                                                                          • Slide 16
                                                                                          • Multiple Access Links and Protocols
                                                                                          • Cable Network Architecture Overview
                                                                                          • Multiple Access protocols
                                                                                          • Ideal Multiple Access Protocol
                                                                                          • MAC Protocols a taxonomy
                                                                                          • Channel Partitioning MAC protocols TDMA
                                                                                          • Channel Partitioning MAC protocols FDMA
                                                                                          • Random Access Protocols
                                                                                          • ALOHA
                                                                                          • CSMA (Carrier Sense Multiple Access)
                                                                                          • CSMA collisions
                                                                                          • CSMACD (Collision Detection)
                                                                                          • CSMACD collision detection
                                                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                                                          • Slide 31
                                                                                          • Slide 32
                                                                                          • Summary of MAC protocols
                                                                                          • Slide 34
                                                                                          • MAC Addresses and ARP
                                                                                          • LAN Addresses and ARP
                                                                                          • LAN Address (more)
                                                                                          • ARP Address Resolution Protocol
                                                                                          • ARP protocol Same LAN (network)
                                                                                          • Addressing routing to another LAN
                                                                                          • Slide 41
                                                                                          • Slide 42
                                                                                          • Ethernet
                                                                                          • Star topology
                                                                                          • Ethernet Frame Structure
                                                                                          • Ethernet Frame Structure (more)
                                                                                          • Ethernet Unreliable connectionless
                                                                                          • Ethernet CSMACD algorithm
                                                                                          • Ethernetrsquos CSMACD (more)
                                                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                                                          • Manchester encoding
                                                                                          • Slide 52
                                                                                          • Hubs
                                                                                          • Switch
                                                                                          • Switch allows multiple simultaneous transmissions
                                                                                          • Switch Table
                                                                                          • Switch self-learning
                                                                                          • Switch frame filteringforwarding
                                                                                          • Self-learning forwarding example
                                                                                          • Interconnecting switches
                                                                                          • Self-learning multi-switch example
                                                                                          • Institutional network
                                                                                          • Switches vs Routers
                                                                                          • Slide 64
                                                                                          • Synthesis a day in the life of a web request
                                                                                          • A day in the life scenario
                                                                                          • A day in the lifehellip connecting to the Internet
                                                                                          • Slide 68
                                                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                          • A day in the lifehellip using DNS
                                                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                                                          • A day in the lifehellip HTTP requestreply
                                                                                          • Chapter 5 letrsquos take a breath

                                                                                            5 DataLink Layer 5-46

                                                                                            Ethernet Frame Structure (more) Addresses 6 bytes

                                                                                            o if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

                                                                                            o otherwise adapter discards frame

                                                                                            Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)

                                                                                            CRC checked at receiver if error is detected frame is dropped

                                                                                            5 DataLink Layer 5-47

                                                                                            Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                            and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                            send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                            gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                            Ethernetrsquos MAC protocol CSMACD

                                                                                            5 DataLink Layer 5-48

                                                                                            Ethernet CSMACD algorithm

                                                                                            1 NIC receives datagram from network layer creates frame

                                                                                            2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                            3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                            4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                            5 After aborting NIC enters exponential backoff

                                                                                            after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                            (1 slot = 512 bit-times)

                                                                                            5 DataLink Layer 5-49

                                                                                            Ethernetrsquos CSMACD (more)

                                                                                            Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                            Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                            Exponential Backoff Goal adapt retransmission

                                                                                            attempts to estimated current loado heavy load random

                                                                                            wait will be longer first collision choose K

                                                                                            from 01 delay is K 512 bit transmission times

                                                                                            after second collision choose K from 0123hellip

                                                                                            after ten collisions choose K from 01234hellip1023

                                                                                            5 DataLink Layer 5-50

                                                                                            8023 Ethernet Standards Link amp Physical Layers

                                                                                            many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                            Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                            applicationtransportnetwork

                                                                                            linkphysical

                                                                                            MAC protocoland frame format

                                                                                            100BASE-TX

                                                                                            100BASE-T4

                                                                                            100BASE-FX100BASE-T2

                                                                                            100BASE-SX 100BASE-BX

                                                                                            fiber physical layercopper (twisterpair) physical layer

                                                                                            5 DataLink Layer 5-51

                                                                                            Manchester encoding

                                                                                            used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                            synchronize to each othero no need for a centralized global clock among nodes

                                                                                            Hey this is physical-layer stuff

                                                                                            5 DataLink Layer 5-52

                                                                                            Link Layer

                                                                                            51 Introduction and services

                                                                                            52 Error detection and correction

                                                                                            53 Multiple access protocols

                                                                                            54 Link-layer Addressing

                                                                                            55 Ethernet

                                                                                            56 Link-layer switches LANs

                                                                                            59 A day in the life of a web request

                                                                                            5 DataLink Layer 5-53

                                                                                            Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                            o bits coming in one link go out all other links at same rate

                                                                                            o all nodes connected to hub can collide with one another

                                                                                            o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                            twisted pair

                                                                                            hub

                                                                                            5 DataLink Layer 5-54

                                                                                            Switch link-layer device smarter than hubs take active

                                                                                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                            transparento hosts are unaware of presence of switches

                                                                                            plug-and-play self-learningo switches do not need to be configured

                                                                                            5 DataLink Layer 5-55

                                                                                            Switch allows multiple simultaneous transmissions

                                                                                            hosts have dedicated direct connection to switch

                                                                                            switches buffer packets Ethernet protocol used on

                                                                                            each incoming link but no collisions full duplexo each link is its own collision

                                                                                            domain switching A-to-Arsquo and B-

                                                                                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                            A

                                                                                            Arsquo

                                                                                            B

                                                                                            Brsquo

                                                                                            C

                                                                                            Crsquo

                                                                                            switch with six interfaces(123456)

                                                                                            1 23

                                                                                            45

                                                                                            6

                                                                                            5 DataLink Layer 5-56

                                                                                            Switch Table

                                                                                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                            A each switch has a switch table each entryo (MAC address of host interface

                                                                                            to reach host time stamp)

                                                                                            looks like a routing table Q how are entries created

                                                                                            maintained in switch table o something like a routing

                                                                                            protocol

                                                                                            A

                                                                                            Arsquo

                                                                                            B

                                                                                            Brsquo

                                                                                            C

                                                                                            Crsquo

                                                                                            switch with six interfaces(123456)

                                                                                            1 23

                                                                                            45

                                                                                            6

                                                                                            5 DataLink Layer 5-57

                                                                                            Switch self-learning

                                                                                            switch learns which hosts can be reached through which interfaceso when frame received

                                                                                            switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                            o records senderlocation pair in switch table

                                                                                            A

                                                                                            Arsquo

                                                                                            B

                                                                                            Brsquo

                                                                                            C

                                                                                            Crsquo

                                                                                            1 23

                                                                                            45

                                                                                            6

                                                                                            A Arsquo

                                                                                            Source ADest Arsquo

                                                                                            MAC addr interface TTL

                                                                                            Switch table (initially empty)

                                                                                            A 1 60

                                                                                            5 DataLink Layer 5-58

                                                                                            Switch frame filteringforwardingWhen frame received

                                                                                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                            then if dest on segment from which frame arrived

                                                                                            then drop the frame else forward the frame on interface indicated else flood

                                                                                            forward on all but the interface on which the frame arrived

                                                                                            5 DataLink Layer 5-59

                                                                                            Self-learning forwarding example

                                                                                            A

                                                                                            Arsquo

                                                                                            B

                                                                                            Brsquo

                                                                                            C

                                                                                            Crsquo

                                                                                            1 23

                                                                                            45

                                                                                            6

                                                                                            A Arsquo

                                                                                            Source ADest Arsquo

                                                                                            MAC addr interface TTL

                                                                                            Switch table (initially empty)

                                                                                            A 1 60

                                                                                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                            frame destination unknownflood

                                                                                            Arsquo A

                                                                                            destination A location known

                                                                                            Arsquo 4 60

                                                                                            selective send

                                                                                            5 DataLink Layer 5-60

                                                                                            Interconnecting switches

                                                                                            switches can be connected together

                                                                                            A

                                                                                            B

                                                                                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                            A self learning (works exactly the same as in single-switch case)

                                                                                            S1

                                                                                            C D

                                                                                            E

                                                                                            FS2

                                                                                            S4

                                                                                            S3

                                                                                            H

                                                                                            I

                                                                                            G

                                                                                            5 DataLink Layer 5-61

                                                                                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                            A

                                                                                            B

                                                                                            S1

                                                                                            C D

                                                                                            E

                                                                                            FS2

                                                                                            S4

                                                                                            S3

                                                                                            H

                                                                                            I

                                                                                            G

                                                                                            1

                                                                                            2 3

                                                                                            34

                                                                                            5 DataLink Layer 5-62

                                                                                            Institutional network

                                                                                            to externalnetwork

                                                                                            router

                                                                                            IP subnet

                                                                                            mail server

                                                                                            web server

                                                                                            5 DataLink Layer 5-63

                                                                                            Switches vs Routers both store-and-forward devices

                                                                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                            routers maintain routing tables implement routing algorithms

                                                                                            switches maintain switch tables implement filtering learning algorithms

                                                                                            Switch

                                                                                            5 DataLink Layer 5-64

                                                                                            Link Layer

                                                                                            51 Introduction and services

                                                                                            52 Error detection and correction

                                                                                            53Multiple access protocols

                                                                                            54 Link-Layer Addressing

                                                                                            55 Ethernet

                                                                                            56 Link-layer switches 59 A day in the life of

                                                                                            a web request

                                                                                            5 DataLink Layer 5-65

                                                                                            Synthesis a day in the life of a web request

                                                                                            journey down protocol stack completeo application transport network link

                                                                                            putting-it-all-together synthesiso goal identify review understand protocols

                                                                                            (at all layers) involved in seemingly simple scenario requesting www page

                                                                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                            5 DataLink Layer 5-66

                                                                                            A day in the life scenario

                                                                                            Comcast network 68800013

                                                                                            Googlersquos network 64233160019 64233169105

                                                                                            web server

                                                                                            DNS server

                                                                                            school network 68802024

                                                                                            browser

                                                                                            web page

                                                                                            5 DataLink Layer 5-67

                                                                                            A day in the lifehellip connecting to the Internet

                                                                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                            router(runs DHCP)

                                                                                            DHCPUDP

                                                                                            IPEthPhy

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCPUDP

                                                                                            IPEthPhy

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCPDHCP

                                                                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                            5 DataLink Layer 5-68

                                                                                            A day in the lifehellip connecting to the Internet

                                                                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                            router(runs DHCP)

                                                                                            DHCPUDP

                                                                                            IPEthPhy

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCPUDP

                                                                                            IPEthPhy

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            DHCP

                                                                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                            DHCP client receives DHCP ACK reply

                                                                                            5 DataLink Layer 5-69

                                                                                            A day in the lifehellip ARP (before DNS before HTTP)

                                                                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                            DNSUDP

                                                                                            IPEthPhy

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                            ARP query

                                                                                            EthPhy

                                                                                            ARP

                                                                                            ARP

                                                                                            ARP reply

                                                                                            5 DataLink Layer 5-70

                                                                                            A day in the lifehellip using DNS

                                                                                            DNSUDP

                                                                                            IPEthPhy

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                            demuxrsquoed to DNS server DNS server replies to

                                                                                            client with IP address of wwwgooglecom

                                                                                            Comcast network 68800013

                                                                                            DNS server

                                                                                            DNSUDP

                                                                                            IPEthPhy

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            DNS

                                                                                            5 DataLink Layer 5-71

                                                                                            A day in the lifehellip TCP connection carrying HTTP

                                                                                            HTTPTCPIP

                                                                                            EthPhy

                                                                                            HTTP

                                                                                            to send HTTP request client first opens TCP socket to web server

                                                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                            TCP connection established

                                                                                            64233169105

                                                                                            web server

                                                                                            SYN

                                                                                            SYN

                                                                                            SYN

                                                                                            SYN

                                                                                            TCPIP

                                                                                            EthPhy

                                                                                            SYN

                                                                                            SYN

                                                                                            SYN

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            SYNACK

                                                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                            5 DataLink Layer 5-72

                                                                                            A day in the lifehellip HTTP requestreply

                                                                                            HTTPTCPIP

                                                                                            EthPhy

                                                                                            HTTP

                                                                                            HTTP request sent into TCP socket

                                                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                                                            IP datgram containing HTTP reply routed back to client

                                                                                            64233169105

                                                                                            web server

                                                                                            HTTPTCPIP

                                                                                            EthPhy

                                                                                            web server responds with HTTP reply (containing web page)

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTPHTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            HTTP

                                                                                            web page finally () displayed

                                                                                            5 DataLink Layer 5-73

                                                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                            (except PHY) solid understanding of networking

                                                                                            principles practice hellip could stop here hellip but lots of

                                                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                            • Link Layer
                                                                                            • Link Layer Introduction
                                                                                            • Link layer context
                                                                                            • Encapsulation
                                                                                            • Link Layer Services
                                                                                            • Link Layer Services (more)
                                                                                            • Where is the link layer implemented
                                                                                            • Adaptors Communicating
                                                                                            • Slide 9
                                                                                            • Parity Checking
                                                                                            • Internet checksum (review)
                                                                                            • Checksumming Cyclic Redundancy Check
                                                                                            • Cyclic Redundancy Check
                                                                                            • Slide 14
                                                                                            • CRC Example
                                                                                            • Slide 16
                                                                                            • Multiple Access Links and Protocols
                                                                                            • Cable Network Architecture Overview
                                                                                            • Multiple Access protocols
                                                                                            • Ideal Multiple Access Protocol
                                                                                            • MAC Protocols a taxonomy
                                                                                            • Channel Partitioning MAC protocols TDMA
                                                                                            • Channel Partitioning MAC protocols FDMA
                                                                                            • Random Access Protocols
                                                                                            • ALOHA
                                                                                            • CSMA (Carrier Sense Multiple Access)
                                                                                            • CSMA collisions
                                                                                            • CSMACD (Collision Detection)
                                                                                            • CSMACD collision detection
                                                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                                                            • Slide 31
                                                                                            • Slide 32
                                                                                            • Summary of MAC protocols
                                                                                            • Slide 34
                                                                                            • MAC Addresses and ARP
                                                                                            • LAN Addresses and ARP
                                                                                            • LAN Address (more)
                                                                                            • ARP Address Resolution Protocol
                                                                                            • ARP protocol Same LAN (network)
                                                                                            • Addressing routing to another LAN
                                                                                            • Slide 41
                                                                                            • Slide 42
                                                                                            • Ethernet
                                                                                            • Star topology
                                                                                            • Ethernet Frame Structure
                                                                                            • Ethernet Frame Structure (more)
                                                                                            • Ethernet Unreliable connectionless
                                                                                            • Ethernet CSMACD algorithm
                                                                                            • Ethernetrsquos CSMACD (more)
                                                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                                                            • Manchester encoding
                                                                                            • Slide 52
                                                                                            • Hubs
                                                                                            • Switch
                                                                                            • Switch allows multiple simultaneous transmissions
                                                                                            • Switch Table
                                                                                            • Switch self-learning
                                                                                            • Switch frame filteringforwarding
                                                                                            • Self-learning forwarding example
                                                                                            • Interconnecting switches
                                                                                            • Self-learning multi-switch example
                                                                                            • Institutional network
                                                                                            • Switches vs Routers
                                                                                            • Slide 64
                                                                                            • Synthesis a day in the life of a web request
                                                                                            • A day in the life scenario
                                                                                            • A day in the lifehellip connecting to the Internet
                                                                                            • Slide 68
                                                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                            • A day in the lifehellip using DNS
                                                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                                                            • A day in the lifehellip HTTP requestreply
                                                                                            • Chapter 5 letrsquos take a breath

                                                                                              5 DataLink Layer 5-47

                                                                                              Ethernet Unreliable connectionless connectionless No handshaking between sending

                                                                                              and receiving NICs Unreliable (best effort) receiving NIC doesnrsquot

                                                                                              send acks or nacks to sending NICo stream of datagrams passed to network layer can have

                                                                                              gaps (missing datagrams)o gaps will be filled if app is using TCPo otherwise app will see gaps

                                                                                              Ethernetrsquos MAC protocol CSMACD

                                                                                              5 DataLink Layer 5-48

                                                                                              Ethernet CSMACD algorithm

                                                                                              1 NIC receives datagram from network layer creates frame

                                                                                              2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                              3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                              4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                              5 After aborting NIC enters exponential backoff

                                                                                              after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                              (1 slot = 512 bit-times)

                                                                                              5 DataLink Layer 5-49

                                                                                              Ethernetrsquos CSMACD (more)

                                                                                              Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                              Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                              Exponential Backoff Goal adapt retransmission

                                                                                              attempts to estimated current loado heavy load random

                                                                                              wait will be longer first collision choose K

                                                                                              from 01 delay is K 512 bit transmission times

                                                                                              after second collision choose K from 0123hellip

                                                                                              after ten collisions choose K from 01234hellip1023

                                                                                              5 DataLink Layer 5-50

                                                                                              8023 Ethernet Standards Link amp Physical Layers

                                                                                              many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                              Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                              applicationtransportnetwork

                                                                                              linkphysical

                                                                                              MAC protocoland frame format

                                                                                              100BASE-TX

                                                                                              100BASE-T4

                                                                                              100BASE-FX100BASE-T2

                                                                                              100BASE-SX 100BASE-BX

                                                                                              fiber physical layercopper (twisterpair) physical layer

                                                                                              5 DataLink Layer 5-51

                                                                                              Manchester encoding

                                                                                              used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                              synchronize to each othero no need for a centralized global clock among nodes

                                                                                              Hey this is physical-layer stuff

                                                                                              5 DataLink Layer 5-52

                                                                                              Link Layer

                                                                                              51 Introduction and services

                                                                                              52 Error detection and correction

                                                                                              53 Multiple access protocols

                                                                                              54 Link-layer Addressing

                                                                                              55 Ethernet

                                                                                              56 Link-layer switches LANs

                                                                                              59 A day in the life of a web request

                                                                                              5 DataLink Layer 5-53

                                                                                              Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                              o bits coming in one link go out all other links at same rate

                                                                                              o all nodes connected to hub can collide with one another

                                                                                              o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                              twisted pair

                                                                                              hub

                                                                                              5 DataLink Layer 5-54

                                                                                              Switch link-layer device smarter than hubs take active

                                                                                              roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                              forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                              transparento hosts are unaware of presence of switches

                                                                                              plug-and-play self-learningo switches do not need to be configured

                                                                                              5 DataLink Layer 5-55

                                                                                              Switch allows multiple simultaneous transmissions

                                                                                              hosts have dedicated direct connection to switch

                                                                                              switches buffer packets Ethernet protocol used on

                                                                                              each incoming link but no collisions full duplexo each link is its own collision

                                                                                              domain switching A-to-Arsquo and B-

                                                                                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                              A

                                                                                              Arsquo

                                                                                              B

                                                                                              Brsquo

                                                                                              C

                                                                                              Crsquo

                                                                                              switch with six interfaces(123456)

                                                                                              1 23

                                                                                              45

                                                                                              6

                                                                                              5 DataLink Layer 5-56

                                                                                              Switch Table

                                                                                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                              A each switch has a switch table each entryo (MAC address of host interface

                                                                                              to reach host time stamp)

                                                                                              looks like a routing table Q how are entries created

                                                                                              maintained in switch table o something like a routing

                                                                                              protocol

                                                                                              A

                                                                                              Arsquo

                                                                                              B

                                                                                              Brsquo

                                                                                              C

                                                                                              Crsquo

                                                                                              switch with six interfaces(123456)

                                                                                              1 23

                                                                                              45

                                                                                              6

                                                                                              5 DataLink Layer 5-57

                                                                                              Switch self-learning

                                                                                              switch learns which hosts can be reached through which interfaceso when frame received

                                                                                              switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                              o records senderlocation pair in switch table

                                                                                              A

                                                                                              Arsquo

                                                                                              B

                                                                                              Brsquo

                                                                                              C

                                                                                              Crsquo

                                                                                              1 23

                                                                                              45

                                                                                              6

                                                                                              A Arsquo

                                                                                              Source ADest Arsquo

                                                                                              MAC addr interface TTL

                                                                                              Switch table (initially empty)

                                                                                              A 1 60

                                                                                              5 DataLink Layer 5-58

                                                                                              Switch frame filteringforwardingWhen frame received

                                                                                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                              then if dest on segment from which frame arrived

                                                                                              then drop the frame else forward the frame on interface indicated else flood

                                                                                              forward on all but the interface on which the frame arrived

                                                                                              5 DataLink Layer 5-59

                                                                                              Self-learning forwarding example

                                                                                              A

                                                                                              Arsquo

                                                                                              B

                                                                                              Brsquo

                                                                                              C

                                                                                              Crsquo

                                                                                              1 23

                                                                                              45

                                                                                              6

                                                                                              A Arsquo

                                                                                              Source ADest Arsquo

                                                                                              MAC addr interface TTL

                                                                                              Switch table (initially empty)

                                                                                              A 1 60

                                                                                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                              frame destination unknownflood

                                                                                              Arsquo A

                                                                                              destination A location known

                                                                                              Arsquo 4 60

                                                                                              selective send

                                                                                              5 DataLink Layer 5-60

                                                                                              Interconnecting switches

                                                                                              switches can be connected together

                                                                                              A

                                                                                              B

                                                                                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                              A self learning (works exactly the same as in single-switch case)

                                                                                              S1

                                                                                              C D

                                                                                              E

                                                                                              FS2

                                                                                              S4

                                                                                              S3

                                                                                              H

                                                                                              I

                                                                                              G

                                                                                              5 DataLink Layer 5-61

                                                                                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                              A

                                                                                              B

                                                                                              S1

                                                                                              C D

                                                                                              E

                                                                                              FS2

                                                                                              S4

                                                                                              S3

                                                                                              H

                                                                                              I

                                                                                              G

                                                                                              1

                                                                                              2 3

                                                                                              34

                                                                                              5 DataLink Layer 5-62

                                                                                              Institutional network

                                                                                              to externalnetwork

                                                                                              router

                                                                                              IP subnet

                                                                                              mail server

                                                                                              web server

                                                                                              5 DataLink Layer 5-63

                                                                                              Switches vs Routers both store-and-forward devices

                                                                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                              routers maintain routing tables implement routing algorithms

                                                                                              switches maintain switch tables implement filtering learning algorithms

                                                                                              Switch

                                                                                              5 DataLink Layer 5-64

                                                                                              Link Layer

                                                                                              51 Introduction and services

                                                                                              52 Error detection and correction

                                                                                              53Multiple access protocols

                                                                                              54 Link-Layer Addressing

                                                                                              55 Ethernet

                                                                                              56 Link-layer switches 59 A day in the life of

                                                                                              a web request

                                                                                              5 DataLink Layer 5-65

                                                                                              Synthesis a day in the life of a web request

                                                                                              journey down protocol stack completeo application transport network link

                                                                                              putting-it-all-together synthesiso goal identify review understand protocols

                                                                                              (at all layers) involved in seemingly simple scenario requesting www page

                                                                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                              5 DataLink Layer 5-66

                                                                                              A day in the life scenario

                                                                                              Comcast network 68800013

                                                                                              Googlersquos network 64233160019 64233169105

                                                                                              web server

                                                                                              DNS server

                                                                                              school network 68802024

                                                                                              browser

                                                                                              web page

                                                                                              5 DataLink Layer 5-67

                                                                                              A day in the lifehellip connecting to the Internet

                                                                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                              router(runs DHCP)

                                                                                              DHCPUDP

                                                                                              IPEthPhy

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCPUDP

                                                                                              IPEthPhy

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCPDHCP

                                                                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                              5 DataLink Layer 5-68

                                                                                              A day in the lifehellip connecting to the Internet

                                                                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                              router(runs DHCP)

                                                                                              DHCPUDP

                                                                                              IPEthPhy

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCPUDP

                                                                                              IPEthPhy

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              DHCP

                                                                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                              DHCP client receives DHCP ACK reply

                                                                                              5 DataLink Layer 5-69

                                                                                              A day in the lifehellip ARP (before DNS before HTTP)

                                                                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                              DNSUDP

                                                                                              IPEthPhy

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                              ARP query

                                                                                              EthPhy

                                                                                              ARP

                                                                                              ARP

                                                                                              ARP reply

                                                                                              5 DataLink Layer 5-70

                                                                                              A day in the lifehellip using DNS

                                                                                              DNSUDP

                                                                                              IPEthPhy

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                              demuxrsquoed to DNS server DNS server replies to

                                                                                              client with IP address of wwwgooglecom

                                                                                              Comcast network 68800013

                                                                                              DNS server

                                                                                              DNSUDP

                                                                                              IPEthPhy

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              DNS

                                                                                              5 DataLink Layer 5-71

                                                                                              A day in the lifehellip TCP connection carrying HTTP

                                                                                              HTTPTCPIP

                                                                                              EthPhy

                                                                                              HTTP

                                                                                              to send HTTP request client first opens TCP socket to web server

                                                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                              TCP connection established

                                                                                              64233169105

                                                                                              web server

                                                                                              SYN

                                                                                              SYN

                                                                                              SYN

                                                                                              SYN

                                                                                              TCPIP

                                                                                              EthPhy

                                                                                              SYN

                                                                                              SYN

                                                                                              SYN

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              SYNACK

                                                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                              5 DataLink Layer 5-72

                                                                                              A day in the lifehellip HTTP requestreply

                                                                                              HTTPTCPIP

                                                                                              EthPhy

                                                                                              HTTP

                                                                                              HTTP request sent into TCP socket

                                                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                                                              IP datgram containing HTTP reply routed back to client

                                                                                              64233169105

                                                                                              web server

                                                                                              HTTPTCPIP

                                                                                              EthPhy

                                                                                              web server responds with HTTP reply (containing web page)

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTPHTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              HTTP

                                                                                              web page finally () displayed

                                                                                              5 DataLink Layer 5-73

                                                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                              (except PHY) solid understanding of networking

                                                                                              principles practice hellip could stop here hellip but lots of

                                                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                              • Link Layer
                                                                                              • Link Layer Introduction
                                                                                              • Link layer context
                                                                                              • Encapsulation
                                                                                              • Link Layer Services
                                                                                              • Link Layer Services (more)
                                                                                              • Where is the link layer implemented
                                                                                              • Adaptors Communicating
                                                                                              • Slide 9
                                                                                              • Parity Checking
                                                                                              • Internet checksum (review)
                                                                                              • Checksumming Cyclic Redundancy Check
                                                                                              • Cyclic Redundancy Check
                                                                                              • Slide 14
                                                                                              • CRC Example
                                                                                              • Slide 16
                                                                                              • Multiple Access Links and Protocols
                                                                                              • Cable Network Architecture Overview
                                                                                              • Multiple Access protocols
                                                                                              • Ideal Multiple Access Protocol
                                                                                              • MAC Protocols a taxonomy
                                                                                              • Channel Partitioning MAC protocols TDMA
                                                                                              • Channel Partitioning MAC protocols FDMA
                                                                                              • Random Access Protocols
                                                                                              • ALOHA
                                                                                              • CSMA (Carrier Sense Multiple Access)
                                                                                              • CSMA collisions
                                                                                              • CSMACD (Collision Detection)
                                                                                              • CSMACD collision detection
                                                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                                                              • Slide 31
                                                                                              • Slide 32
                                                                                              • Summary of MAC protocols
                                                                                              • Slide 34
                                                                                              • MAC Addresses and ARP
                                                                                              • LAN Addresses and ARP
                                                                                              • LAN Address (more)
                                                                                              • ARP Address Resolution Protocol
                                                                                              • ARP protocol Same LAN (network)
                                                                                              • Addressing routing to another LAN
                                                                                              • Slide 41
                                                                                              • Slide 42
                                                                                              • Ethernet
                                                                                              • Star topology
                                                                                              • Ethernet Frame Structure
                                                                                              • Ethernet Frame Structure (more)
                                                                                              • Ethernet Unreliable connectionless
                                                                                              • Ethernet CSMACD algorithm
                                                                                              • Ethernetrsquos CSMACD (more)
                                                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                                                              • Manchester encoding
                                                                                              • Slide 52
                                                                                              • Hubs
                                                                                              • Switch
                                                                                              • Switch allows multiple simultaneous transmissions
                                                                                              • Switch Table
                                                                                              • Switch self-learning
                                                                                              • Switch frame filteringforwarding
                                                                                              • Self-learning forwarding example
                                                                                              • Interconnecting switches
                                                                                              • Self-learning multi-switch example
                                                                                              • Institutional network
                                                                                              • Switches vs Routers
                                                                                              • Slide 64
                                                                                              • Synthesis a day in the life of a web request
                                                                                              • A day in the life scenario
                                                                                              • A day in the lifehellip connecting to the Internet
                                                                                              • Slide 68
                                                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                              • A day in the lifehellip using DNS
                                                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                                                              • A day in the lifehellip HTTP requestreply
                                                                                              • Chapter 5 letrsquos take a breath

                                                                                                5 DataLink Layer 5-48

                                                                                                Ethernet CSMACD algorithm

                                                                                                1 NIC receives datagram from network layer creates frame

                                                                                                2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

                                                                                                3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

                                                                                                4 If NIC detects collision while transmitting aborts and sends jam signal

                                                                                                5 After aborting NIC enters exponential backoff

                                                                                                after m-th collision NIC waits K slots of time and then returns to Step 2 where K is a random value in 0 1 2 hellip 2m-1

                                                                                                (1 slot = 512 bit-times)

                                                                                                5 DataLink Layer 5-49

                                                                                                Ethernetrsquos CSMACD (more)

                                                                                                Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                                Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                                Exponential Backoff Goal adapt retransmission

                                                                                                attempts to estimated current loado heavy load random

                                                                                                wait will be longer first collision choose K

                                                                                                from 01 delay is K 512 bit transmission times

                                                                                                after second collision choose K from 0123hellip

                                                                                                after ten collisions choose K from 01234hellip1023

                                                                                                5 DataLink Layer 5-50

                                                                                                8023 Ethernet Standards Link amp Physical Layers

                                                                                                many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                                Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                                applicationtransportnetwork

                                                                                                linkphysical

                                                                                                MAC protocoland frame format

                                                                                                100BASE-TX

                                                                                                100BASE-T4

                                                                                                100BASE-FX100BASE-T2

                                                                                                100BASE-SX 100BASE-BX

                                                                                                fiber physical layercopper (twisterpair) physical layer

                                                                                                5 DataLink Layer 5-51

                                                                                                Manchester encoding

                                                                                                used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                                synchronize to each othero no need for a centralized global clock among nodes

                                                                                                Hey this is physical-layer stuff

                                                                                                5 DataLink Layer 5-52

                                                                                                Link Layer

                                                                                                51 Introduction and services

                                                                                                52 Error detection and correction

                                                                                                53 Multiple access protocols

                                                                                                54 Link-layer Addressing

                                                                                                55 Ethernet

                                                                                                56 Link-layer switches LANs

                                                                                                59 A day in the life of a web request

                                                                                                5 DataLink Layer 5-53

                                                                                                Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                o bits coming in one link go out all other links at same rate

                                                                                                o all nodes connected to hub can collide with one another

                                                                                                o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                twisted pair

                                                                                                hub

                                                                                                5 DataLink Layer 5-54

                                                                                                Switch link-layer device smarter than hubs take active

                                                                                                roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                transparento hosts are unaware of presence of switches

                                                                                                plug-and-play self-learningo switches do not need to be configured

                                                                                                5 DataLink Layer 5-55

                                                                                                Switch allows multiple simultaneous transmissions

                                                                                                hosts have dedicated direct connection to switch

                                                                                                switches buffer packets Ethernet protocol used on

                                                                                                each incoming link but no collisions full duplexo each link is its own collision

                                                                                                domain switching A-to-Arsquo and B-

                                                                                                to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                A

                                                                                                Arsquo

                                                                                                B

                                                                                                Brsquo

                                                                                                C

                                                                                                Crsquo

                                                                                                switch with six interfaces(123456)

                                                                                                1 23

                                                                                                45

                                                                                                6

                                                                                                5 DataLink Layer 5-56

                                                                                                Switch Table

                                                                                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                A each switch has a switch table each entryo (MAC address of host interface

                                                                                                to reach host time stamp)

                                                                                                looks like a routing table Q how are entries created

                                                                                                maintained in switch table o something like a routing

                                                                                                protocol

                                                                                                A

                                                                                                Arsquo

                                                                                                B

                                                                                                Brsquo

                                                                                                C

                                                                                                Crsquo

                                                                                                switch with six interfaces(123456)

                                                                                                1 23

                                                                                                45

                                                                                                6

                                                                                                5 DataLink Layer 5-57

                                                                                                Switch self-learning

                                                                                                switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                o records senderlocation pair in switch table

                                                                                                A

                                                                                                Arsquo

                                                                                                B

                                                                                                Brsquo

                                                                                                C

                                                                                                Crsquo

                                                                                                1 23

                                                                                                45

                                                                                                6

                                                                                                A Arsquo

                                                                                                Source ADest Arsquo

                                                                                                MAC addr interface TTL

                                                                                                Switch table (initially empty)

                                                                                                A 1 60

                                                                                                5 DataLink Layer 5-58

                                                                                                Switch frame filteringforwardingWhen frame received

                                                                                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                then if dest on segment from which frame arrived

                                                                                                then drop the frame else forward the frame on interface indicated else flood

                                                                                                forward on all but the interface on which the frame arrived

                                                                                                5 DataLink Layer 5-59

                                                                                                Self-learning forwarding example

                                                                                                A

                                                                                                Arsquo

                                                                                                B

                                                                                                Brsquo

                                                                                                C

                                                                                                Crsquo

                                                                                                1 23

                                                                                                45

                                                                                                6

                                                                                                A Arsquo

                                                                                                Source ADest Arsquo

                                                                                                MAC addr interface TTL

                                                                                                Switch table (initially empty)

                                                                                                A 1 60

                                                                                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                frame destination unknownflood

                                                                                                Arsquo A

                                                                                                destination A location known

                                                                                                Arsquo 4 60

                                                                                                selective send

                                                                                                5 DataLink Layer 5-60

                                                                                                Interconnecting switches

                                                                                                switches can be connected together

                                                                                                A

                                                                                                B

                                                                                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                A self learning (works exactly the same as in single-switch case)

                                                                                                S1

                                                                                                C D

                                                                                                E

                                                                                                FS2

                                                                                                S4

                                                                                                S3

                                                                                                H

                                                                                                I

                                                                                                G

                                                                                                5 DataLink Layer 5-61

                                                                                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                A

                                                                                                B

                                                                                                S1

                                                                                                C D

                                                                                                E

                                                                                                FS2

                                                                                                S4

                                                                                                S3

                                                                                                H

                                                                                                I

                                                                                                G

                                                                                                1

                                                                                                2 3

                                                                                                34

                                                                                                5 DataLink Layer 5-62

                                                                                                Institutional network

                                                                                                to externalnetwork

                                                                                                router

                                                                                                IP subnet

                                                                                                mail server

                                                                                                web server

                                                                                                5 DataLink Layer 5-63

                                                                                                Switches vs Routers both store-and-forward devices

                                                                                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                routers maintain routing tables implement routing algorithms

                                                                                                switches maintain switch tables implement filtering learning algorithms

                                                                                                Switch

                                                                                                5 DataLink Layer 5-64

                                                                                                Link Layer

                                                                                                51 Introduction and services

                                                                                                52 Error detection and correction

                                                                                                53Multiple access protocols

                                                                                                54 Link-Layer Addressing

                                                                                                55 Ethernet

                                                                                                56 Link-layer switches 59 A day in the life of

                                                                                                a web request

                                                                                                5 DataLink Layer 5-65

                                                                                                Synthesis a day in the life of a web request

                                                                                                journey down protocol stack completeo application transport network link

                                                                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                5 DataLink Layer 5-66

                                                                                                A day in the life scenario

                                                                                                Comcast network 68800013

                                                                                                Googlersquos network 64233160019 64233169105

                                                                                                web server

                                                                                                DNS server

                                                                                                school network 68802024

                                                                                                browser

                                                                                                web page

                                                                                                5 DataLink Layer 5-67

                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                router(runs DHCP)

                                                                                                DHCPUDP

                                                                                                IPEthPhy

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCPUDP

                                                                                                IPEthPhy

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCPDHCP

                                                                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                5 DataLink Layer 5-68

                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                router(runs DHCP)

                                                                                                DHCPUDP

                                                                                                IPEthPhy

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCPUDP

                                                                                                IPEthPhy

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                DHCP

                                                                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                DHCP client receives DHCP ACK reply

                                                                                                5 DataLink Layer 5-69

                                                                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                DNSUDP

                                                                                                IPEthPhy

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                ARP query

                                                                                                EthPhy

                                                                                                ARP

                                                                                                ARP

                                                                                                ARP reply

                                                                                                5 DataLink Layer 5-70

                                                                                                A day in the lifehellip using DNS

                                                                                                DNSUDP

                                                                                                IPEthPhy

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                demuxrsquoed to DNS server DNS server replies to

                                                                                                client with IP address of wwwgooglecom

                                                                                                Comcast network 68800013

                                                                                                DNS server

                                                                                                DNSUDP

                                                                                                IPEthPhy

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                DNS

                                                                                                5 DataLink Layer 5-71

                                                                                                A day in the lifehellip TCP connection carrying HTTP

                                                                                                HTTPTCPIP

                                                                                                EthPhy

                                                                                                HTTP

                                                                                                to send HTTP request client first opens TCP socket to web server

                                                                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                TCP connection established

                                                                                                64233169105

                                                                                                web server

                                                                                                SYN

                                                                                                SYN

                                                                                                SYN

                                                                                                SYN

                                                                                                TCPIP

                                                                                                EthPhy

                                                                                                SYN

                                                                                                SYN

                                                                                                SYN

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                SYNACK

                                                                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                5 DataLink Layer 5-72

                                                                                                A day in the lifehellip HTTP requestreply

                                                                                                HTTPTCPIP

                                                                                                EthPhy

                                                                                                HTTP

                                                                                                HTTP request sent into TCP socket

                                                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                IP datgram containing HTTP reply routed back to client

                                                                                                64233169105

                                                                                                web server

                                                                                                HTTPTCPIP

                                                                                                EthPhy

                                                                                                web server responds with HTTP reply (containing web page)

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTPHTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                HTTP

                                                                                                web page finally () displayed

                                                                                                5 DataLink Layer 5-73

                                                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                (except PHY) solid understanding of networking

                                                                                                principles practice hellip could stop here hellip but lots of

                                                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                • Link Layer
                                                                                                • Link Layer Introduction
                                                                                                • Link layer context
                                                                                                • Encapsulation
                                                                                                • Link Layer Services
                                                                                                • Link Layer Services (more)
                                                                                                • Where is the link layer implemented
                                                                                                • Adaptors Communicating
                                                                                                • Slide 9
                                                                                                • Parity Checking
                                                                                                • Internet checksum (review)
                                                                                                • Checksumming Cyclic Redundancy Check
                                                                                                • Cyclic Redundancy Check
                                                                                                • Slide 14
                                                                                                • CRC Example
                                                                                                • Slide 16
                                                                                                • Multiple Access Links and Protocols
                                                                                                • Cable Network Architecture Overview
                                                                                                • Multiple Access protocols
                                                                                                • Ideal Multiple Access Protocol
                                                                                                • MAC Protocols a taxonomy
                                                                                                • Channel Partitioning MAC protocols TDMA
                                                                                                • Channel Partitioning MAC protocols FDMA
                                                                                                • Random Access Protocols
                                                                                                • ALOHA
                                                                                                • CSMA (Carrier Sense Multiple Access)
                                                                                                • CSMA collisions
                                                                                                • CSMACD (Collision Detection)
                                                                                                • CSMACD collision detection
                                                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                                                • Slide 31
                                                                                                • Slide 32
                                                                                                • Summary of MAC protocols
                                                                                                • Slide 34
                                                                                                • MAC Addresses and ARP
                                                                                                • LAN Addresses and ARP
                                                                                                • LAN Address (more)
                                                                                                • ARP Address Resolution Protocol
                                                                                                • ARP protocol Same LAN (network)
                                                                                                • Addressing routing to another LAN
                                                                                                • Slide 41
                                                                                                • Slide 42
                                                                                                • Ethernet
                                                                                                • Star topology
                                                                                                • Ethernet Frame Structure
                                                                                                • Ethernet Frame Structure (more)
                                                                                                • Ethernet Unreliable connectionless
                                                                                                • Ethernet CSMACD algorithm
                                                                                                • Ethernetrsquos CSMACD (more)
                                                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                • Manchester encoding
                                                                                                • Slide 52
                                                                                                • Hubs
                                                                                                • Switch
                                                                                                • Switch allows multiple simultaneous transmissions
                                                                                                • Switch Table
                                                                                                • Switch self-learning
                                                                                                • Switch frame filteringforwarding
                                                                                                • Self-learning forwarding example
                                                                                                • Interconnecting switches
                                                                                                • Self-learning multi-switch example
                                                                                                • Institutional network
                                                                                                • Switches vs Routers
                                                                                                • Slide 64
                                                                                                • Synthesis a day in the life of a web request
                                                                                                • A day in the life scenario
                                                                                                • A day in the lifehellip connecting to the Internet
                                                                                                • Slide 68
                                                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                • A day in the lifehellip using DNS
                                                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                                                • A day in the lifehellip HTTP requestreply
                                                                                                • Chapter 5 letrsquos take a breath

                                                                                                  5 DataLink Layer 5-49

                                                                                                  Ethernetrsquos CSMACD (more)

                                                                                                  Jam Signal make sure all other transmitters are aware of collision 48 bits

                                                                                                  Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

                                                                                                  Exponential Backoff Goal adapt retransmission

                                                                                                  attempts to estimated current loado heavy load random

                                                                                                  wait will be longer first collision choose K

                                                                                                  from 01 delay is K 512 bit transmission times

                                                                                                  after second collision choose K from 0123hellip

                                                                                                  after ten collisions choose K from 01234hellip1023

                                                                                                  5 DataLink Layer 5-50

                                                                                                  8023 Ethernet Standards Link amp Physical Layers

                                                                                                  many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                                  Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                                  applicationtransportnetwork

                                                                                                  linkphysical

                                                                                                  MAC protocoland frame format

                                                                                                  100BASE-TX

                                                                                                  100BASE-T4

                                                                                                  100BASE-FX100BASE-T2

                                                                                                  100BASE-SX 100BASE-BX

                                                                                                  fiber physical layercopper (twisterpair) physical layer

                                                                                                  5 DataLink Layer 5-51

                                                                                                  Manchester encoding

                                                                                                  used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                                  synchronize to each othero no need for a centralized global clock among nodes

                                                                                                  Hey this is physical-layer stuff

                                                                                                  5 DataLink Layer 5-52

                                                                                                  Link Layer

                                                                                                  51 Introduction and services

                                                                                                  52 Error detection and correction

                                                                                                  53 Multiple access protocols

                                                                                                  54 Link-layer Addressing

                                                                                                  55 Ethernet

                                                                                                  56 Link-layer switches LANs

                                                                                                  59 A day in the life of a web request

                                                                                                  5 DataLink Layer 5-53

                                                                                                  Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                  o bits coming in one link go out all other links at same rate

                                                                                                  o all nodes connected to hub can collide with one another

                                                                                                  o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                  twisted pair

                                                                                                  hub

                                                                                                  5 DataLink Layer 5-54

                                                                                                  Switch link-layer device smarter than hubs take active

                                                                                                  roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                  forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                  transparento hosts are unaware of presence of switches

                                                                                                  plug-and-play self-learningo switches do not need to be configured

                                                                                                  5 DataLink Layer 5-55

                                                                                                  Switch allows multiple simultaneous transmissions

                                                                                                  hosts have dedicated direct connection to switch

                                                                                                  switches buffer packets Ethernet protocol used on

                                                                                                  each incoming link but no collisions full duplexo each link is its own collision

                                                                                                  domain switching A-to-Arsquo and B-

                                                                                                  to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                  A

                                                                                                  Arsquo

                                                                                                  B

                                                                                                  Brsquo

                                                                                                  C

                                                                                                  Crsquo

                                                                                                  switch with six interfaces(123456)

                                                                                                  1 23

                                                                                                  45

                                                                                                  6

                                                                                                  5 DataLink Layer 5-56

                                                                                                  Switch Table

                                                                                                  Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                  A each switch has a switch table each entryo (MAC address of host interface

                                                                                                  to reach host time stamp)

                                                                                                  looks like a routing table Q how are entries created

                                                                                                  maintained in switch table o something like a routing

                                                                                                  protocol

                                                                                                  A

                                                                                                  Arsquo

                                                                                                  B

                                                                                                  Brsquo

                                                                                                  C

                                                                                                  Crsquo

                                                                                                  switch with six interfaces(123456)

                                                                                                  1 23

                                                                                                  45

                                                                                                  6

                                                                                                  5 DataLink Layer 5-57

                                                                                                  Switch self-learning

                                                                                                  switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                  o records senderlocation pair in switch table

                                                                                                  A

                                                                                                  Arsquo

                                                                                                  B

                                                                                                  Brsquo

                                                                                                  C

                                                                                                  Crsquo

                                                                                                  1 23

                                                                                                  45

                                                                                                  6

                                                                                                  A Arsquo

                                                                                                  Source ADest Arsquo

                                                                                                  MAC addr interface TTL

                                                                                                  Switch table (initially empty)

                                                                                                  A 1 60

                                                                                                  5 DataLink Layer 5-58

                                                                                                  Switch frame filteringforwardingWhen frame received

                                                                                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                  then if dest on segment from which frame arrived

                                                                                                  then drop the frame else forward the frame on interface indicated else flood

                                                                                                  forward on all but the interface on which the frame arrived

                                                                                                  5 DataLink Layer 5-59

                                                                                                  Self-learning forwarding example

                                                                                                  A

                                                                                                  Arsquo

                                                                                                  B

                                                                                                  Brsquo

                                                                                                  C

                                                                                                  Crsquo

                                                                                                  1 23

                                                                                                  45

                                                                                                  6

                                                                                                  A Arsquo

                                                                                                  Source ADest Arsquo

                                                                                                  MAC addr interface TTL

                                                                                                  Switch table (initially empty)

                                                                                                  A 1 60

                                                                                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                  frame destination unknownflood

                                                                                                  Arsquo A

                                                                                                  destination A location known

                                                                                                  Arsquo 4 60

                                                                                                  selective send

                                                                                                  5 DataLink Layer 5-60

                                                                                                  Interconnecting switches

                                                                                                  switches can be connected together

                                                                                                  A

                                                                                                  B

                                                                                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                  A self learning (works exactly the same as in single-switch case)

                                                                                                  S1

                                                                                                  C D

                                                                                                  E

                                                                                                  FS2

                                                                                                  S4

                                                                                                  S3

                                                                                                  H

                                                                                                  I

                                                                                                  G

                                                                                                  5 DataLink Layer 5-61

                                                                                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                  A

                                                                                                  B

                                                                                                  S1

                                                                                                  C D

                                                                                                  E

                                                                                                  FS2

                                                                                                  S4

                                                                                                  S3

                                                                                                  H

                                                                                                  I

                                                                                                  G

                                                                                                  1

                                                                                                  2 3

                                                                                                  34

                                                                                                  5 DataLink Layer 5-62

                                                                                                  Institutional network

                                                                                                  to externalnetwork

                                                                                                  router

                                                                                                  IP subnet

                                                                                                  mail server

                                                                                                  web server

                                                                                                  5 DataLink Layer 5-63

                                                                                                  Switches vs Routers both store-and-forward devices

                                                                                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                  routers maintain routing tables implement routing algorithms

                                                                                                  switches maintain switch tables implement filtering learning algorithms

                                                                                                  Switch

                                                                                                  5 DataLink Layer 5-64

                                                                                                  Link Layer

                                                                                                  51 Introduction and services

                                                                                                  52 Error detection and correction

                                                                                                  53Multiple access protocols

                                                                                                  54 Link-Layer Addressing

                                                                                                  55 Ethernet

                                                                                                  56 Link-layer switches 59 A day in the life of

                                                                                                  a web request

                                                                                                  5 DataLink Layer 5-65

                                                                                                  Synthesis a day in the life of a web request

                                                                                                  journey down protocol stack completeo application transport network link

                                                                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                  5 DataLink Layer 5-66

                                                                                                  A day in the life scenario

                                                                                                  Comcast network 68800013

                                                                                                  Googlersquos network 64233160019 64233169105

                                                                                                  web server

                                                                                                  DNS server

                                                                                                  school network 68802024

                                                                                                  browser

                                                                                                  web page

                                                                                                  5 DataLink Layer 5-67

                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                  router(runs DHCP)

                                                                                                  DHCPUDP

                                                                                                  IPEthPhy

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCPUDP

                                                                                                  IPEthPhy

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCPDHCP

                                                                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                  5 DataLink Layer 5-68

                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                  router(runs DHCP)

                                                                                                  DHCPUDP

                                                                                                  IPEthPhy

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCPUDP

                                                                                                  IPEthPhy

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  DHCP

                                                                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                  DHCP client receives DHCP ACK reply

                                                                                                  5 DataLink Layer 5-69

                                                                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                  DNSUDP

                                                                                                  IPEthPhy

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                  ARP query

                                                                                                  EthPhy

                                                                                                  ARP

                                                                                                  ARP

                                                                                                  ARP reply

                                                                                                  5 DataLink Layer 5-70

                                                                                                  A day in the lifehellip using DNS

                                                                                                  DNSUDP

                                                                                                  IPEthPhy

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                  demuxrsquoed to DNS server DNS server replies to

                                                                                                  client with IP address of wwwgooglecom

                                                                                                  Comcast network 68800013

                                                                                                  DNS server

                                                                                                  DNSUDP

                                                                                                  IPEthPhy

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  DNS

                                                                                                  5 DataLink Layer 5-71

                                                                                                  A day in the lifehellip TCP connection carrying HTTP

                                                                                                  HTTPTCPIP

                                                                                                  EthPhy

                                                                                                  HTTP

                                                                                                  to send HTTP request client first opens TCP socket to web server

                                                                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                  TCP connection established

                                                                                                  64233169105

                                                                                                  web server

                                                                                                  SYN

                                                                                                  SYN

                                                                                                  SYN

                                                                                                  SYN

                                                                                                  TCPIP

                                                                                                  EthPhy

                                                                                                  SYN

                                                                                                  SYN

                                                                                                  SYN

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  SYNACK

                                                                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                  5 DataLink Layer 5-72

                                                                                                  A day in the lifehellip HTTP requestreply

                                                                                                  HTTPTCPIP

                                                                                                  EthPhy

                                                                                                  HTTP

                                                                                                  HTTP request sent into TCP socket

                                                                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                  IP datgram containing HTTP reply routed back to client

                                                                                                  64233169105

                                                                                                  web server

                                                                                                  HTTPTCPIP

                                                                                                  EthPhy

                                                                                                  web server responds with HTTP reply (containing web page)

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTPHTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  HTTP

                                                                                                  web page finally () displayed

                                                                                                  5 DataLink Layer 5-73

                                                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                  (except PHY) solid understanding of networking

                                                                                                  principles practice hellip could stop here hellip but lots of

                                                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                  • Link Layer
                                                                                                  • Link Layer Introduction
                                                                                                  • Link layer context
                                                                                                  • Encapsulation
                                                                                                  • Link Layer Services
                                                                                                  • Link Layer Services (more)
                                                                                                  • Where is the link layer implemented
                                                                                                  • Adaptors Communicating
                                                                                                  • Slide 9
                                                                                                  • Parity Checking
                                                                                                  • Internet checksum (review)
                                                                                                  • Checksumming Cyclic Redundancy Check
                                                                                                  • Cyclic Redundancy Check
                                                                                                  • Slide 14
                                                                                                  • CRC Example
                                                                                                  • Slide 16
                                                                                                  • Multiple Access Links and Protocols
                                                                                                  • Cable Network Architecture Overview
                                                                                                  • Multiple Access protocols
                                                                                                  • Ideal Multiple Access Protocol
                                                                                                  • MAC Protocols a taxonomy
                                                                                                  • Channel Partitioning MAC protocols TDMA
                                                                                                  • Channel Partitioning MAC protocols FDMA
                                                                                                  • Random Access Protocols
                                                                                                  • ALOHA
                                                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                                                  • CSMA collisions
                                                                                                  • CSMACD (Collision Detection)
                                                                                                  • CSMACD collision detection
                                                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                                                  • Slide 31
                                                                                                  • Slide 32
                                                                                                  • Summary of MAC protocols
                                                                                                  • Slide 34
                                                                                                  • MAC Addresses and ARP
                                                                                                  • LAN Addresses and ARP
                                                                                                  • LAN Address (more)
                                                                                                  • ARP Address Resolution Protocol
                                                                                                  • ARP protocol Same LAN (network)
                                                                                                  • Addressing routing to another LAN
                                                                                                  • Slide 41
                                                                                                  • Slide 42
                                                                                                  • Ethernet
                                                                                                  • Star topology
                                                                                                  • Ethernet Frame Structure
                                                                                                  • Ethernet Frame Structure (more)
                                                                                                  • Ethernet Unreliable connectionless
                                                                                                  • Ethernet CSMACD algorithm
                                                                                                  • Ethernetrsquos CSMACD (more)
                                                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                  • Manchester encoding
                                                                                                  • Slide 52
                                                                                                  • Hubs
                                                                                                  • Switch
                                                                                                  • Switch allows multiple simultaneous transmissions
                                                                                                  • Switch Table
                                                                                                  • Switch self-learning
                                                                                                  • Switch frame filteringforwarding
                                                                                                  • Self-learning forwarding example
                                                                                                  • Interconnecting switches
                                                                                                  • Self-learning multi-switch example
                                                                                                  • Institutional network
                                                                                                  • Switches vs Routers
                                                                                                  • Slide 64
                                                                                                  • Synthesis a day in the life of a web request
                                                                                                  • A day in the life scenario
                                                                                                  • A day in the lifehellip connecting to the Internet
                                                                                                  • Slide 68
                                                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                  • A day in the lifehellip using DNS
                                                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                                                  • A day in the lifehellip HTTP requestreply
                                                                                                  • Chapter 5 letrsquos take a breath

                                                                                                    5 DataLink Layer 5-50

                                                                                                    8023 Ethernet Standards Link amp Physical Layers

                                                                                                    many different Ethernet standardso common MAC protocol and frame formato different speeds 2 Mbps 10 Mbps 100

                                                                                                    Mbps 1Gbps 10G bpso different physical layer media fiber cable

                                                                                                    applicationtransportnetwork

                                                                                                    linkphysical

                                                                                                    MAC protocoland frame format

                                                                                                    100BASE-TX

                                                                                                    100BASE-T4

                                                                                                    100BASE-FX100BASE-T2

                                                                                                    100BASE-SX 100BASE-BX

                                                                                                    fiber physical layercopper (twisterpair) physical layer

                                                                                                    5 DataLink Layer 5-51

                                                                                                    Manchester encoding

                                                                                                    used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                                    synchronize to each othero no need for a centralized global clock among nodes

                                                                                                    Hey this is physical-layer stuff

                                                                                                    5 DataLink Layer 5-52

                                                                                                    Link Layer

                                                                                                    51 Introduction and services

                                                                                                    52 Error detection and correction

                                                                                                    53 Multiple access protocols

                                                                                                    54 Link-layer Addressing

                                                                                                    55 Ethernet

                                                                                                    56 Link-layer switches LANs

                                                                                                    59 A day in the life of a web request

                                                                                                    5 DataLink Layer 5-53

                                                                                                    Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                    o bits coming in one link go out all other links at same rate

                                                                                                    o all nodes connected to hub can collide with one another

                                                                                                    o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                    twisted pair

                                                                                                    hub

                                                                                                    5 DataLink Layer 5-54

                                                                                                    Switch link-layer device smarter than hubs take active

                                                                                                    roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                    forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                    transparento hosts are unaware of presence of switches

                                                                                                    plug-and-play self-learningo switches do not need to be configured

                                                                                                    5 DataLink Layer 5-55

                                                                                                    Switch allows multiple simultaneous transmissions

                                                                                                    hosts have dedicated direct connection to switch

                                                                                                    switches buffer packets Ethernet protocol used on

                                                                                                    each incoming link but no collisions full duplexo each link is its own collision

                                                                                                    domain switching A-to-Arsquo and B-

                                                                                                    to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                    A

                                                                                                    Arsquo

                                                                                                    B

                                                                                                    Brsquo

                                                                                                    C

                                                                                                    Crsquo

                                                                                                    switch with six interfaces(123456)

                                                                                                    1 23

                                                                                                    45

                                                                                                    6

                                                                                                    5 DataLink Layer 5-56

                                                                                                    Switch Table

                                                                                                    Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                    A each switch has a switch table each entryo (MAC address of host interface

                                                                                                    to reach host time stamp)

                                                                                                    looks like a routing table Q how are entries created

                                                                                                    maintained in switch table o something like a routing

                                                                                                    protocol

                                                                                                    A

                                                                                                    Arsquo

                                                                                                    B

                                                                                                    Brsquo

                                                                                                    C

                                                                                                    Crsquo

                                                                                                    switch with six interfaces(123456)

                                                                                                    1 23

                                                                                                    45

                                                                                                    6

                                                                                                    5 DataLink Layer 5-57

                                                                                                    Switch self-learning

                                                                                                    switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                    switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                    o records senderlocation pair in switch table

                                                                                                    A

                                                                                                    Arsquo

                                                                                                    B

                                                                                                    Brsquo

                                                                                                    C

                                                                                                    Crsquo

                                                                                                    1 23

                                                                                                    45

                                                                                                    6

                                                                                                    A Arsquo

                                                                                                    Source ADest Arsquo

                                                                                                    MAC addr interface TTL

                                                                                                    Switch table (initially empty)

                                                                                                    A 1 60

                                                                                                    5 DataLink Layer 5-58

                                                                                                    Switch frame filteringforwardingWhen frame received

                                                                                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                    then if dest on segment from which frame arrived

                                                                                                    then drop the frame else forward the frame on interface indicated else flood

                                                                                                    forward on all but the interface on which the frame arrived

                                                                                                    5 DataLink Layer 5-59

                                                                                                    Self-learning forwarding example

                                                                                                    A

                                                                                                    Arsquo

                                                                                                    B

                                                                                                    Brsquo

                                                                                                    C

                                                                                                    Crsquo

                                                                                                    1 23

                                                                                                    45

                                                                                                    6

                                                                                                    A Arsquo

                                                                                                    Source ADest Arsquo

                                                                                                    MAC addr interface TTL

                                                                                                    Switch table (initially empty)

                                                                                                    A 1 60

                                                                                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                    frame destination unknownflood

                                                                                                    Arsquo A

                                                                                                    destination A location known

                                                                                                    Arsquo 4 60

                                                                                                    selective send

                                                                                                    5 DataLink Layer 5-60

                                                                                                    Interconnecting switches

                                                                                                    switches can be connected together

                                                                                                    A

                                                                                                    B

                                                                                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                    A self learning (works exactly the same as in single-switch case)

                                                                                                    S1

                                                                                                    C D

                                                                                                    E

                                                                                                    FS2

                                                                                                    S4

                                                                                                    S3

                                                                                                    H

                                                                                                    I

                                                                                                    G

                                                                                                    5 DataLink Layer 5-61

                                                                                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                    A

                                                                                                    B

                                                                                                    S1

                                                                                                    C D

                                                                                                    E

                                                                                                    FS2

                                                                                                    S4

                                                                                                    S3

                                                                                                    H

                                                                                                    I

                                                                                                    G

                                                                                                    1

                                                                                                    2 3

                                                                                                    34

                                                                                                    5 DataLink Layer 5-62

                                                                                                    Institutional network

                                                                                                    to externalnetwork

                                                                                                    router

                                                                                                    IP subnet

                                                                                                    mail server

                                                                                                    web server

                                                                                                    5 DataLink Layer 5-63

                                                                                                    Switches vs Routers both store-and-forward devices

                                                                                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                    routers maintain routing tables implement routing algorithms

                                                                                                    switches maintain switch tables implement filtering learning algorithms

                                                                                                    Switch

                                                                                                    5 DataLink Layer 5-64

                                                                                                    Link Layer

                                                                                                    51 Introduction and services

                                                                                                    52 Error detection and correction

                                                                                                    53Multiple access protocols

                                                                                                    54 Link-Layer Addressing

                                                                                                    55 Ethernet

                                                                                                    56 Link-layer switches 59 A day in the life of

                                                                                                    a web request

                                                                                                    5 DataLink Layer 5-65

                                                                                                    Synthesis a day in the life of a web request

                                                                                                    journey down protocol stack completeo application transport network link

                                                                                                    putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                    (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                    5 DataLink Layer 5-66

                                                                                                    A day in the life scenario

                                                                                                    Comcast network 68800013

                                                                                                    Googlersquos network 64233160019 64233169105

                                                                                                    web server

                                                                                                    DNS server

                                                                                                    school network 68802024

                                                                                                    browser

                                                                                                    web page

                                                                                                    5 DataLink Layer 5-67

                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                    router(runs DHCP)

                                                                                                    DHCPUDP

                                                                                                    IPEthPhy

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCPUDP

                                                                                                    IPEthPhy

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCPDHCP

                                                                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                    5 DataLink Layer 5-68

                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                    router(runs DHCP)

                                                                                                    DHCPUDP

                                                                                                    IPEthPhy

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCPUDP

                                                                                                    IPEthPhy

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    DHCP

                                                                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                    DHCP client receives DHCP ACK reply

                                                                                                    5 DataLink Layer 5-69

                                                                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                    DNSUDP

                                                                                                    IPEthPhy

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                    ARP query

                                                                                                    EthPhy

                                                                                                    ARP

                                                                                                    ARP

                                                                                                    ARP reply

                                                                                                    5 DataLink Layer 5-70

                                                                                                    A day in the lifehellip using DNS

                                                                                                    DNSUDP

                                                                                                    IPEthPhy

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                    demuxrsquoed to DNS server DNS server replies to

                                                                                                    client with IP address of wwwgooglecom

                                                                                                    Comcast network 68800013

                                                                                                    DNS server

                                                                                                    DNSUDP

                                                                                                    IPEthPhy

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    DNS

                                                                                                    5 DataLink Layer 5-71

                                                                                                    A day in the lifehellip TCP connection carrying HTTP

                                                                                                    HTTPTCPIP

                                                                                                    EthPhy

                                                                                                    HTTP

                                                                                                    to send HTTP request client first opens TCP socket to web server

                                                                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                    TCP connection established

                                                                                                    64233169105

                                                                                                    web server

                                                                                                    SYN

                                                                                                    SYN

                                                                                                    SYN

                                                                                                    SYN

                                                                                                    TCPIP

                                                                                                    EthPhy

                                                                                                    SYN

                                                                                                    SYN

                                                                                                    SYN

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    SYNACK

                                                                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                    5 DataLink Layer 5-72

                                                                                                    A day in the lifehellip HTTP requestreply

                                                                                                    HTTPTCPIP

                                                                                                    EthPhy

                                                                                                    HTTP

                                                                                                    HTTP request sent into TCP socket

                                                                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                    IP datgram containing HTTP reply routed back to client

                                                                                                    64233169105

                                                                                                    web server

                                                                                                    HTTPTCPIP

                                                                                                    EthPhy

                                                                                                    web server responds with HTTP reply (containing web page)

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTPHTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    HTTP

                                                                                                    web page finally () displayed

                                                                                                    5 DataLink Layer 5-73

                                                                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                    (except PHY) solid understanding of networking

                                                                                                    principles practice hellip could stop here hellip but lots of

                                                                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                    • Link Layer
                                                                                                    • Link Layer Introduction
                                                                                                    • Link layer context
                                                                                                    • Encapsulation
                                                                                                    • Link Layer Services
                                                                                                    • Link Layer Services (more)
                                                                                                    • Where is the link layer implemented
                                                                                                    • Adaptors Communicating
                                                                                                    • Slide 9
                                                                                                    • Parity Checking
                                                                                                    • Internet checksum (review)
                                                                                                    • Checksumming Cyclic Redundancy Check
                                                                                                    • Cyclic Redundancy Check
                                                                                                    • Slide 14
                                                                                                    • CRC Example
                                                                                                    • Slide 16
                                                                                                    • Multiple Access Links and Protocols
                                                                                                    • Cable Network Architecture Overview
                                                                                                    • Multiple Access protocols
                                                                                                    • Ideal Multiple Access Protocol
                                                                                                    • MAC Protocols a taxonomy
                                                                                                    • Channel Partitioning MAC protocols TDMA
                                                                                                    • Channel Partitioning MAC protocols FDMA
                                                                                                    • Random Access Protocols
                                                                                                    • ALOHA
                                                                                                    • CSMA (Carrier Sense Multiple Access)
                                                                                                    • CSMA collisions
                                                                                                    • CSMACD (Collision Detection)
                                                                                                    • CSMACD collision detection
                                                                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                                                                    • Slide 31
                                                                                                    • Slide 32
                                                                                                    • Summary of MAC protocols
                                                                                                    • Slide 34
                                                                                                    • MAC Addresses and ARP
                                                                                                    • LAN Addresses and ARP
                                                                                                    • LAN Address (more)
                                                                                                    • ARP Address Resolution Protocol
                                                                                                    • ARP protocol Same LAN (network)
                                                                                                    • Addressing routing to another LAN
                                                                                                    • Slide 41
                                                                                                    • Slide 42
                                                                                                    • Ethernet
                                                                                                    • Star topology
                                                                                                    • Ethernet Frame Structure
                                                                                                    • Ethernet Frame Structure (more)
                                                                                                    • Ethernet Unreliable connectionless
                                                                                                    • Ethernet CSMACD algorithm
                                                                                                    • Ethernetrsquos CSMACD (more)
                                                                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                    • Manchester encoding
                                                                                                    • Slide 52
                                                                                                    • Hubs
                                                                                                    • Switch
                                                                                                    • Switch allows multiple simultaneous transmissions
                                                                                                    • Switch Table
                                                                                                    • Switch self-learning
                                                                                                    • Switch frame filteringforwarding
                                                                                                    • Self-learning forwarding example
                                                                                                    • Interconnecting switches
                                                                                                    • Self-learning multi-switch example
                                                                                                    • Institutional network
                                                                                                    • Switches vs Routers
                                                                                                    • Slide 64
                                                                                                    • Synthesis a day in the life of a web request
                                                                                                    • A day in the life scenario
                                                                                                    • A day in the lifehellip connecting to the Internet
                                                                                                    • Slide 68
                                                                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                    • A day in the lifehellip using DNS
                                                                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                                                                    • A day in the lifehellip HTTP requestreply
                                                                                                    • Chapter 5 letrsquos take a breath

                                                                                                      5 DataLink Layer 5-51

                                                                                                      Manchester encoding

                                                                                                      used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to

                                                                                                      synchronize to each othero no need for a centralized global clock among nodes

                                                                                                      Hey this is physical-layer stuff

                                                                                                      5 DataLink Layer 5-52

                                                                                                      Link Layer

                                                                                                      51 Introduction and services

                                                                                                      52 Error detection and correction

                                                                                                      53 Multiple access protocols

                                                                                                      54 Link-layer Addressing

                                                                                                      55 Ethernet

                                                                                                      56 Link-layer switches LANs

                                                                                                      59 A day in the life of a web request

                                                                                                      5 DataLink Layer 5-53

                                                                                                      Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                      o bits coming in one link go out all other links at same rate

                                                                                                      o all nodes connected to hub can collide with one another

                                                                                                      o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                      twisted pair

                                                                                                      hub

                                                                                                      5 DataLink Layer 5-54

                                                                                                      Switch link-layer device smarter than hubs take active

                                                                                                      roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                      forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                      transparento hosts are unaware of presence of switches

                                                                                                      plug-and-play self-learningo switches do not need to be configured

                                                                                                      5 DataLink Layer 5-55

                                                                                                      Switch allows multiple simultaneous transmissions

                                                                                                      hosts have dedicated direct connection to switch

                                                                                                      switches buffer packets Ethernet protocol used on

                                                                                                      each incoming link but no collisions full duplexo each link is its own collision

                                                                                                      domain switching A-to-Arsquo and B-

                                                                                                      to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                      A

                                                                                                      Arsquo

                                                                                                      B

                                                                                                      Brsquo

                                                                                                      C

                                                                                                      Crsquo

                                                                                                      switch with six interfaces(123456)

                                                                                                      1 23

                                                                                                      45

                                                                                                      6

                                                                                                      5 DataLink Layer 5-56

                                                                                                      Switch Table

                                                                                                      Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                      A each switch has a switch table each entryo (MAC address of host interface

                                                                                                      to reach host time stamp)

                                                                                                      looks like a routing table Q how are entries created

                                                                                                      maintained in switch table o something like a routing

                                                                                                      protocol

                                                                                                      A

                                                                                                      Arsquo

                                                                                                      B

                                                                                                      Brsquo

                                                                                                      C

                                                                                                      Crsquo

                                                                                                      switch with six interfaces(123456)

                                                                                                      1 23

                                                                                                      45

                                                                                                      6

                                                                                                      5 DataLink Layer 5-57

                                                                                                      Switch self-learning

                                                                                                      switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                      switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                      o records senderlocation pair in switch table

                                                                                                      A

                                                                                                      Arsquo

                                                                                                      B

                                                                                                      Brsquo

                                                                                                      C

                                                                                                      Crsquo

                                                                                                      1 23

                                                                                                      45

                                                                                                      6

                                                                                                      A Arsquo

                                                                                                      Source ADest Arsquo

                                                                                                      MAC addr interface TTL

                                                                                                      Switch table (initially empty)

                                                                                                      A 1 60

                                                                                                      5 DataLink Layer 5-58

                                                                                                      Switch frame filteringforwardingWhen frame received

                                                                                                      1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                      then if dest on segment from which frame arrived

                                                                                                      then drop the frame else forward the frame on interface indicated else flood

                                                                                                      forward on all but the interface on which the frame arrived

                                                                                                      5 DataLink Layer 5-59

                                                                                                      Self-learning forwarding example

                                                                                                      A

                                                                                                      Arsquo

                                                                                                      B

                                                                                                      Brsquo

                                                                                                      C

                                                                                                      Crsquo

                                                                                                      1 23

                                                                                                      45

                                                                                                      6

                                                                                                      A Arsquo

                                                                                                      Source ADest Arsquo

                                                                                                      MAC addr interface TTL

                                                                                                      Switch table (initially empty)

                                                                                                      A 1 60

                                                                                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                      frame destination unknownflood

                                                                                                      Arsquo A

                                                                                                      destination A location known

                                                                                                      Arsquo 4 60

                                                                                                      selective send

                                                                                                      5 DataLink Layer 5-60

                                                                                                      Interconnecting switches

                                                                                                      switches can be connected together

                                                                                                      A

                                                                                                      B

                                                                                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                      A self learning (works exactly the same as in single-switch case)

                                                                                                      S1

                                                                                                      C D

                                                                                                      E

                                                                                                      FS2

                                                                                                      S4

                                                                                                      S3

                                                                                                      H

                                                                                                      I

                                                                                                      G

                                                                                                      5 DataLink Layer 5-61

                                                                                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                      A

                                                                                                      B

                                                                                                      S1

                                                                                                      C D

                                                                                                      E

                                                                                                      FS2

                                                                                                      S4

                                                                                                      S3

                                                                                                      H

                                                                                                      I

                                                                                                      G

                                                                                                      1

                                                                                                      2 3

                                                                                                      34

                                                                                                      5 DataLink Layer 5-62

                                                                                                      Institutional network

                                                                                                      to externalnetwork

                                                                                                      router

                                                                                                      IP subnet

                                                                                                      mail server

                                                                                                      web server

                                                                                                      5 DataLink Layer 5-63

                                                                                                      Switches vs Routers both store-and-forward devices

                                                                                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                      routers maintain routing tables implement routing algorithms

                                                                                                      switches maintain switch tables implement filtering learning algorithms

                                                                                                      Switch

                                                                                                      5 DataLink Layer 5-64

                                                                                                      Link Layer

                                                                                                      51 Introduction and services

                                                                                                      52 Error detection and correction

                                                                                                      53Multiple access protocols

                                                                                                      54 Link-Layer Addressing

                                                                                                      55 Ethernet

                                                                                                      56 Link-layer switches 59 A day in the life of

                                                                                                      a web request

                                                                                                      5 DataLink Layer 5-65

                                                                                                      Synthesis a day in the life of a web request

                                                                                                      journey down protocol stack completeo application transport network link

                                                                                                      putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                      (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                      5 DataLink Layer 5-66

                                                                                                      A day in the life scenario

                                                                                                      Comcast network 68800013

                                                                                                      Googlersquos network 64233160019 64233169105

                                                                                                      web server

                                                                                                      DNS server

                                                                                                      school network 68802024

                                                                                                      browser

                                                                                                      web page

                                                                                                      5 DataLink Layer 5-67

                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                      router(runs DHCP)

                                                                                                      DHCPUDP

                                                                                                      IPEthPhy

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCPUDP

                                                                                                      IPEthPhy

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCPDHCP

                                                                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                      5 DataLink Layer 5-68

                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                      router(runs DHCP)

                                                                                                      DHCPUDP

                                                                                                      IPEthPhy

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCPUDP

                                                                                                      IPEthPhy

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      DHCP

                                                                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                      DHCP client receives DHCP ACK reply

                                                                                                      5 DataLink Layer 5-69

                                                                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                      DNSUDP

                                                                                                      IPEthPhy

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                      ARP query

                                                                                                      EthPhy

                                                                                                      ARP

                                                                                                      ARP

                                                                                                      ARP reply

                                                                                                      5 DataLink Layer 5-70

                                                                                                      A day in the lifehellip using DNS

                                                                                                      DNSUDP

                                                                                                      IPEthPhy

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                      demuxrsquoed to DNS server DNS server replies to

                                                                                                      client with IP address of wwwgooglecom

                                                                                                      Comcast network 68800013

                                                                                                      DNS server

                                                                                                      DNSUDP

                                                                                                      IPEthPhy

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      DNS

                                                                                                      5 DataLink Layer 5-71

                                                                                                      A day in the lifehellip TCP connection carrying HTTP

                                                                                                      HTTPTCPIP

                                                                                                      EthPhy

                                                                                                      HTTP

                                                                                                      to send HTTP request client first opens TCP socket to web server

                                                                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                      TCP connection established

                                                                                                      64233169105

                                                                                                      web server

                                                                                                      SYN

                                                                                                      SYN

                                                                                                      SYN

                                                                                                      SYN

                                                                                                      TCPIP

                                                                                                      EthPhy

                                                                                                      SYN

                                                                                                      SYN

                                                                                                      SYN

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      SYNACK

                                                                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                      5 DataLink Layer 5-72

                                                                                                      A day in the lifehellip HTTP requestreply

                                                                                                      HTTPTCPIP

                                                                                                      EthPhy

                                                                                                      HTTP

                                                                                                      HTTP request sent into TCP socket

                                                                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                      IP datgram containing HTTP reply routed back to client

                                                                                                      64233169105

                                                                                                      web server

                                                                                                      HTTPTCPIP

                                                                                                      EthPhy

                                                                                                      web server responds with HTTP reply (containing web page)

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTPHTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      HTTP

                                                                                                      web page finally () displayed

                                                                                                      5 DataLink Layer 5-73

                                                                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                      (except PHY) solid understanding of networking

                                                                                                      principles practice hellip could stop here hellip but lots of

                                                                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                      • Link Layer
                                                                                                      • Link Layer Introduction
                                                                                                      • Link layer context
                                                                                                      • Encapsulation
                                                                                                      • Link Layer Services
                                                                                                      • Link Layer Services (more)
                                                                                                      • Where is the link layer implemented
                                                                                                      • Adaptors Communicating
                                                                                                      • Slide 9
                                                                                                      • Parity Checking
                                                                                                      • Internet checksum (review)
                                                                                                      • Checksumming Cyclic Redundancy Check
                                                                                                      • Cyclic Redundancy Check
                                                                                                      • Slide 14
                                                                                                      • CRC Example
                                                                                                      • Slide 16
                                                                                                      • Multiple Access Links and Protocols
                                                                                                      • Cable Network Architecture Overview
                                                                                                      • Multiple Access protocols
                                                                                                      • Ideal Multiple Access Protocol
                                                                                                      • MAC Protocols a taxonomy
                                                                                                      • Channel Partitioning MAC protocols TDMA
                                                                                                      • Channel Partitioning MAC protocols FDMA
                                                                                                      • Random Access Protocols
                                                                                                      • ALOHA
                                                                                                      • CSMA (Carrier Sense Multiple Access)
                                                                                                      • CSMA collisions
                                                                                                      • CSMACD (Collision Detection)
                                                                                                      • CSMACD collision detection
                                                                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                                                                      • Slide 31
                                                                                                      • Slide 32
                                                                                                      • Summary of MAC protocols
                                                                                                      • Slide 34
                                                                                                      • MAC Addresses and ARP
                                                                                                      • LAN Addresses and ARP
                                                                                                      • LAN Address (more)
                                                                                                      • ARP Address Resolution Protocol
                                                                                                      • ARP protocol Same LAN (network)
                                                                                                      • Addressing routing to another LAN
                                                                                                      • Slide 41
                                                                                                      • Slide 42
                                                                                                      • Ethernet
                                                                                                      • Star topology
                                                                                                      • Ethernet Frame Structure
                                                                                                      • Ethernet Frame Structure (more)
                                                                                                      • Ethernet Unreliable connectionless
                                                                                                      • Ethernet CSMACD algorithm
                                                                                                      • Ethernetrsquos CSMACD (more)
                                                                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                      • Manchester encoding
                                                                                                      • Slide 52
                                                                                                      • Hubs
                                                                                                      • Switch
                                                                                                      • Switch allows multiple simultaneous transmissions
                                                                                                      • Switch Table
                                                                                                      • Switch self-learning
                                                                                                      • Switch frame filteringforwarding
                                                                                                      • Self-learning forwarding example
                                                                                                      • Interconnecting switches
                                                                                                      • Self-learning multi-switch example
                                                                                                      • Institutional network
                                                                                                      • Switches vs Routers
                                                                                                      • Slide 64
                                                                                                      • Synthesis a day in the life of a web request
                                                                                                      • A day in the life scenario
                                                                                                      • A day in the lifehellip connecting to the Internet
                                                                                                      • Slide 68
                                                                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                      • A day in the lifehellip using DNS
                                                                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                                                                      • A day in the lifehellip HTTP requestreply
                                                                                                      • Chapter 5 letrsquos take a breath

                                                                                                        5 DataLink Layer 5-52

                                                                                                        Link Layer

                                                                                                        51 Introduction and services

                                                                                                        52 Error detection and correction

                                                                                                        53 Multiple access protocols

                                                                                                        54 Link-layer Addressing

                                                                                                        55 Ethernet

                                                                                                        56 Link-layer switches LANs

                                                                                                        59 A day in the life of a web request

                                                                                                        5 DataLink Layer 5-53

                                                                                                        Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                        o bits coming in one link go out all other links at same rate

                                                                                                        o all nodes connected to hub can collide with one another

                                                                                                        o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                        twisted pair

                                                                                                        hub

                                                                                                        5 DataLink Layer 5-54

                                                                                                        Switch link-layer device smarter than hubs take active

                                                                                                        roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                        forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                        transparento hosts are unaware of presence of switches

                                                                                                        plug-and-play self-learningo switches do not need to be configured

                                                                                                        5 DataLink Layer 5-55

                                                                                                        Switch allows multiple simultaneous transmissions

                                                                                                        hosts have dedicated direct connection to switch

                                                                                                        switches buffer packets Ethernet protocol used on

                                                                                                        each incoming link but no collisions full duplexo each link is its own collision

                                                                                                        domain switching A-to-Arsquo and B-

                                                                                                        to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                        A

                                                                                                        Arsquo

                                                                                                        B

                                                                                                        Brsquo

                                                                                                        C

                                                                                                        Crsquo

                                                                                                        switch with six interfaces(123456)

                                                                                                        1 23

                                                                                                        45

                                                                                                        6

                                                                                                        5 DataLink Layer 5-56

                                                                                                        Switch Table

                                                                                                        Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                        A each switch has a switch table each entryo (MAC address of host interface

                                                                                                        to reach host time stamp)

                                                                                                        looks like a routing table Q how are entries created

                                                                                                        maintained in switch table o something like a routing

                                                                                                        protocol

                                                                                                        A

                                                                                                        Arsquo

                                                                                                        B

                                                                                                        Brsquo

                                                                                                        C

                                                                                                        Crsquo

                                                                                                        switch with six interfaces(123456)

                                                                                                        1 23

                                                                                                        45

                                                                                                        6

                                                                                                        5 DataLink Layer 5-57

                                                                                                        Switch self-learning

                                                                                                        switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                        switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                        o records senderlocation pair in switch table

                                                                                                        A

                                                                                                        Arsquo

                                                                                                        B

                                                                                                        Brsquo

                                                                                                        C

                                                                                                        Crsquo

                                                                                                        1 23

                                                                                                        45

                                                                                                        6

                                                                                                        A Arsquo

                                                                                                        Source ADest Arsquo

                                                                                                        MAC addr interface TTL

                                                                                                        Switch table (initially empty)

                                                                                                        A 1 60

                                                                                                        5 DataLink Layer 5-58

                                                                                                        Switch frame filteringforwardingWhen frame received

                                                                                                        1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                        then if dest on segment from which frame arrived

                                                                                                        then drop the frame else forward the frame on interface indicated else flood

                                                                                                        forward on all but the interface on which the frame arrived

                                                                                                        5 DataLink Layer 5-59

                                                                                                        Self-learning forwarding example

                                                                                                        A

                                                                                                        Arsquo

                                                                                                        B

                                                                                                        Brsquo

                                                                                                        C

                                                                                                        Crsquo

                                                                                                        1 23

                                                                                                        45

                                                                                                        6

                                                                                                        A Arsquo

                                                                                                        Source ADest Arsquo

                                                                                                        MAC addr interface TTL

                                                                                                        Switch table (initially empty)

                                                                                                        A 1 60

                                                                                                        A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                        frame destination unknownflood

                                                                                                        Arsquo A

                                                                                                        destination A location known

                                                                                                        Arsquo 4 60

                                                                                                        selective send

                                                                                                        5 DataLink Layer 5-60

                                                                                                        Interconnecting switches

                                                                                                        switches can be connected together

                                                                                                        A

                                                                                                        B

                                                                                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                        A self learning (works exactly the same as in single-switch case)

                                                                                                        S1

                                                                                                        C D

                                                                                                        E

                                                                                                        FS2

                                                                                                        S4

                                                                                                        S3

                                                                                                        H

                                                                                                        I

                                                                                                        G

                                                                                                        5 DataLink Layer 5-61

                                                                                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                        A

                                                                                                        B

                                                                                                        S1

                                                                                                        C D

                                                                                                        E

                                                                                                        FS2

                                                                                                        S4

                                                                                                        S3

                                                                                                        H

                                                                                                        I

                                                                                                        G

                                                                                                        1

                                                                                                        2 3

                                                                                                        34

                                                                                                        5 DataLink Layer 5-62

                                                                                                        Institutional network

                                                                                                        to externalnetwork

                                                                                                        router

                                                                                                        IP subnet

                                                                                                        mail server

                                                                                                        web server

                                                                                                        5 DataLink Layer 5-63

                                                                                                        Switches vs Routers both store-and-forward devices

                                                                                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                        routers maintain routing tables implement routing algorithms

                                                                                                        switches maintain switch tables implement filtering learning algorithms

                                                                                                        Switch

                                                                                                        5 DataLink Layer 5-64

                                                                                                        Link Layer

                                                                                                        51 Introduction and services

                                                                                                        52 Error detection and correction

                                                                                                        53Multiple access protocols

                                                                                                        54 Link-Layer Addressing

                                                                                                        55 Ethernet

                                                                                                        56 Link-layer switches 59 A day in the life of

                                                                                                        a web request

                                                                                                        5 DataLink Layer 5-65

                                                                                                        Synthesis a day in the life of a web request

                                                                                                        journey down protocol stack completeo application transport network link

                                                                                                        putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                        (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                        5 DataLink Layer 5-66

                                                                                                        A day in the life scenario

                                                                                                        Comcast network 68800013

                                                                                                        Googlersquos network 64233160019 64233169105

                                                                                                        web server

                                                                                                        DNS server

                                                                                                        school network 68802024

                                                                                                        browser

                                                                                                        web page

                                                                                                        5 DataLink Layer 5-67

                                                                                                        A day in the lifehellip connecting to the Internet

                                                                                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                        router(runs DHCP)

                                                                                                        DHCPUDP

                                                                                                        IPEthPhy

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCPUDP

                                                                                                        IPEthPhy

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCPDHCP

                                                                                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                        5 DataLink Layer 5-68

                                                                                                        A day in the lifehellip connecting to the Internet

                                                                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                        router(runs DHCP)

                                                                                                        DHCPUDP

                                                                                                        IPEthPhy

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCPUDP

                                                                                                        IPEthPhy

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        DHCP

                                                                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                        DHCP client receives DHCP ACK reply

                                                                                                        5 DataLink Layer 5-69

                                                                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                        DNSUDP

                                                                                                        IPEthPhy

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                        ARP query

                                                                                                        EthPhy

                                                                                                        ARP

                                                                                                        ARP

                                                                                                        ARP reply

                                                                                                        5 DataLink Layer 5-70

                                                                                                        A day in the lifehellip using DNS

                                                                                                        DNSUDP

                                                                                                        IPEthPhy

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                        demuxrsquoed to DNS server DNS server replies to

                                                                                                        client with IP address of wwwgooglecom

                                                                                                        Comcast network 68800013

                                                                                                        DNS server

                                                                                                        DNSUDP

                                                                                                        IPEthPhy

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        DNS

                                                                                                        5 DataLink Layer 5-71

                                                                                                        A day in the lifehellip TCP connection carrying HTTP

                                                                                                        HTTPTCPIP

                                                                                                        EthPhy

                                                                                                        HTTP

                                                                                                        to send HTTP request client first opens TCP socket to web server

                                                                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                        TCP connection established

                                                                                                        64233169105

                                                                                                        web server

                                                                                                        SYN

                                                                                                        SYN

                                                                                                        SYN

                                                                                                        SYN

                                                                                                        TCPIP

                                                                                                        EthPhy

                                                                                                        SYN

                                                                                                        SYN

                                                                                                        SYN

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        SYNACK

                                                                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                        5 DataLink Layer 5-72

                                                                                                        A day in the lifehellip HTTP requestreply

                                                                                                        HTTPTCPIP

                                                                                                        EthPhy

                                                                                                        HTTP

                                                                                                        HTTP request sent into TCP socket

                                                                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                        IP datgram containing HTTP reply routed back to client

                                                                                                        64233169105

                                                                                                        web server

                                                                                                        HTTPTCPIP

                                                                                                        EthPhy

                                                                                                        web server responds with HTTP reply (containing web page)

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTPHTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        HTTP

                                                                                                        web page finally () displayed

                                                                                                        5 DataLink Layer 5-73

                                                                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                        (except PHY) solid understanding of networking

                                                                                                        principles practice hellip could stop here hellip but lots of

                                                                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                        • Link Layer
                                                                                                        • Link Layer Introduction
                                                                                                        • Link layer context
                                                                                                        • Encapsulation
                                                                                                        • Link Layer Services
                                                                                                        • Link Layer Services (more)
                                                                                                        • Where is the link layer implemented
                                                                                                        • Adaptors Communicating
                                                                                                        • Slide 9
                                                                                                        • Parity Checking
                                                                                                        • Internet checksum (review)
                                                                                                        • Checksumming Cyclic Redundancy Check
                                                                                                        • Cyclic Redundancy Check
                                                                                                        • Slide 14
                                                                                                        • CRC Example
                                                                                                        • Slide 16
                                                                                                        • Multiple Access Links and Protocols
                                                                                                        • Cable Network Architecture Overview
                                                                                                        • Multiple Access protocols
                                                                                                        • Ideal Multiple Access Protocol
                                                                                                        • MAC Protocols a taxonomy
                                                                                                        • Channel Partitioning MAC protocols TDMA
                                                                                                        • Channel Partitioning MAC protocols FDMA
                                                                                                        • Random Access Protocols
                                                                                                        • ALOHA
                                                                                                        • CSMA (Carrier Sense Multiple Access)
                                                                                                        • CSMA collisions
                                                                                                        • CSMACD (Collision Detection)
                                                                                                        • CSMACD collision detection
                                                                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                                                                        • Slide 31
                                                                                                        • Slide 32
                                                                                                        • Summary of MAC protocols
                                                                                                        • Slide 34
                                                                                                        • MAC Addresses and ARP
                                                                                                        • LAN Addresses and ARP
                                                                                                        • LAN Address (more)
                                                                                                        • ARP Address Resolution Protocol
                                                                                                        • ARP protocol Same LAN (network)
                                                                                                        • Addressing routing to another LAN
                                                                                                        • Slide 41
                                                                                                        • Slide 42
                                                                                                        • Ethernet
                                                                                                        • Star topology
                                                                                                        • Ethernet Frame Structure
                                                                                                        • Ethernet Frame Structure (more)
                                                                                                        • Ethernet Unreliable connectionless
                                                                                                        • Ethernet CSMACD algorithm
                                                                                                        • Ethernetrsquos CSMACD (more)
                                                                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                        • Manchester encoding
                                                                                                        • Slide 52
                                                                                                        • Hubs
                                                                                                        • Switch
                                                                                                        • Switch allows multiple simultaneous transmissions
                                                                                                        • Switch Table
                                                                                                        • Switch self-learning
                                                                                                        • Switch frame filteringforwarding
                                                                                                        • Self-learning forwarding example
                                                                                                        • Interconnecting switches
                                                                                                        • Self-learning multi-switch example
                                                                                                        • Institutional network
                                                                                                        • Switches vs Routers
                                                                                                        • Slide 64
                                                                                                        • Synthesis a day in the life of a web request
                                                                                                        • A day in the life scenario
                                                                                                        • A day in the lifehellip connecting to the Internet
                                                                                                        • Slide 68
                                                                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                        • A day in the lifehellip using DNS
                                                                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                                                                        • A day in the lifehellip HTTP requestreply
                                                                                                        • Chapter 5 letrsquos take a breath

                                                                                                          5 DataLink Layer 5-53

                                                                                                          Hubsphysical-layer (ldquodumbrdquo) repeaters

                                                                                                          o bits coming in one link go out all other links at same rate

                                                                                                          o all nodes connected to hub can collide with one another

                                                                                                          o no frame bufferingo no CSMACD at hub host NICs detect collisions

                                                                                                          twisted pair

                                                                                                          hub

                                                                                                          5 DataLink Layer 5-54

                                                                                                          Switch link-layer device smarter than hubs take active

                                                                                                          roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                          forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                          transparento hosts are unaware of presence of switches

                                                                                                          plug-and-play self-learningo switches do not need to be configured

                                                                                                          5 DataLink Layer 5-55

                                                                                                          Switch allows multiple simultaneous transmissions

                                                                                                          hosts have dedicated direct connection to switch

                                                                                                          switches buffer packets Ethernet protocol used on

                                                                                                          each incoming link but no collisions full duplexo each link is its own collision

                                                                                                          domain switching A-to-Arsquo and B-

                                                                                                          to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                          A

                                                                                                          Arsquo

                                                                                                          B

                                                                                                          Brsquo

                                                                                                          C

                                                                                                          Crsquo

                                                                                                          switch with six interfaces(123456)

                                                                                                          1 23

                                                                                                          45

                                                                                                          6

                                                                                                          5 DataLink Layer 5-56

                                                                                                          Switch Table

                                                                                                          Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                          A each switch has a switch table each entryo (MAC address of host interface

                                                                                                          to reach host time stamp)

                                                                                                          looks like a routing table Q how are entries created

                                                                                                          maintained in switch table o something like a routing

                                                                                                          protocol

                                                                                                          A

                                                                                                          Arsquo

                                                                                                          B

                                                                                                          Brsquo

                                                                                                          C

                                                                                                          Crsquo

                                                                                                          switch with six interfaces(123456)

                                                                                                          1 23

                                                                                                          45

                                                                                                          6

                                                                                                          5 DataLink Layer 5-57

                                                                                                          Switch self-learning

                                                                                                          switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                          switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                          o records senderlocation pair in switch table

                                                                                                          A

                                                                                                          Arsquo

                                                                                                          B

                                                                                                          Brsquo

                                                                                                          C

                                                                                                          Crsquo

                                                                                                          1 23

                                                                                                          45

                                                                                                          6

                                                                                                          A Arsquo

                                                                                                          Source ADest Arsquo

                                                                                                          MAC addr interface TTL

                                                                                                          Switch table (initially empty)

                                                                                                          A 1 60

                                                                                                          5 DataLink Layer 5-58

                                                                                                          Switch frame filteringforwardingWhen frame received

                                                                                                          1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                          then if dest on segment from which frame arrived

                                                                                                          then drop the frame else forward the frame on interface indicated else flood

                                                                                                          forward on all but the interface on which the frame arrived

                                                                                                          5 DataLink Layer 5-59

                                                                                                          Self-learning forwarding example

                                                                                                          A

                                                                                                          Arsquo

                                                                                                          B

                                                                                                          Brsquo

                                                                                                          C

                                                                                                          Crsquo

                                                                                                          1 23

                                                                                                          45

                                                                                                          6

                                                                                                          A Arsquo

                                                                                                          Source ADest Arsquo

                                                                                                          MAC addr interface TTL

                                                                                                          Switch table (initially empty)

                                                                                                          A 1 60

                                                                                                          A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                          frame destination unknownflood

                                                                                                          Arsquo A

                                                                                                          destination A location known

                                                                                                          Arsquo 4 60

                                                                                                          selective send

                                                                                                          5 DataLink Layer 5-60

                                                                                                          Interconnecting switches

                                                                                                          switches can be connected together

                                                                                                          A

                                                                                                          B

                                                                                                          Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                          A self learning (works exactly the same as in single-switch case)

                                                                                                          S1

                                                                                                          C D

                                                                                                          E

                                                                                                          FS2

                                                                                                          S4

                                                                                                          S3

                                                                                                          H

                                                                                                          I

                                                                                                          G

                                                                                                          5 DataLink Layer 5-61

                                                                                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                          A

                                                                                                          B

                                                                                                          S1

                                                                                                          C D

                                                                                                          E

                                                                                                          FS2

                                                                                                          S4

                                                                                                          S3

                                                                                                          H

                                                                                                          I

                                                                                                          G

                                                                                                          1

                                                                                                          2 3

                                                                                                          34

                                                                                                          5 DataLink Layer 5-62

                                                                                                          Institutional network

                                                                                                          to externalnetwork

                                                                                                          router

                                                                                                          IP subnet

                                                                                                          mail server

                                                                                                          web server

                                                                                                          5 DataLink Layer 5-63

                                                                                                          Switches vs Routers both store-and-forward devices

                                                                                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                          routers maintain routing tables implement routing algorithms

                                                                                                          switches maintain switch tables implement filtering learning algorithms

                                                                                                          Switch

                                                                                                          5 DataLink Layer 5-64

                                                                                                          Link Layer

                                                                                                          51 Introduction and services

                                                                                                          52 Error detection and correction

                                                                                                          53Multiple access protocols

                                                                                                          54 Link-Layer Addressing

                                                                                                          55 Ethernet

                                                                                                          56 Link-layer switches 59 A day in the life of

                                                                                                          a web request

                                                                                                          5 DataLink Layer 5-65

                                                                                                          Synthesis a day in the life of a web request

                                                                                                          journey down protocol stack completeo application transport network link

                                                                                                          putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                          (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                          5 DataLink Layer 5-66

                                                                                                          A day in the life scenario

                                                                                                          Comcast network 68800013

                                                                                                          Googlersquos network 64233160019 64233169105

                                                                                                          web server

                                                                                                          DNS server

                                                                                                          school network 68802024

                                                                                                          browser

                                                                                                          web page

                                                                                                          5 DataLink Layer 5-67

                                                                                                          A day in the lifehellip connecting to the Internet

                                                                                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                          router(runs DHCP)

                                                                                                          DHCPUDP

                                                                                                          IPEthPhy

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCPUDP

                                                                                                          IPEthPhy

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCPDHCP

                                                                                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                          5 DataLink Layer 5-68

                                                                                                          A day in the lifehellip connecting to the Internet

                                                                                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                          router(runs DHCP)

                                                                                                          DHCPUDP

                                                                                                          IPEthPhy

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCPUDP

                                                                                                          IPEthPhy

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          DHCP

                                                                                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                          DHCP client receives DHCP ACK reply

                                                                                                          5 DataLink Layer 5-69

                                                                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                          DNSUDP

                                                                                                          IPEthPhy

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                          ARP query

                                                                                                          EthPhy

                                                                                                          ARP

                                                                                                          ARP

                                                                                                          ARP reply

                                                                                                          5 DataLink Layer 5-70

                                                                                                          A day in the lifehellip using DNS

                                                                                                          DNSUDP

                                                                                                          IPEthPhy

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                          demuxrsquoed to DNS server DNS server replies to

                                                                                                          client with IP address of wwwgooglecom

                                                                                                          Comcast network 68800013

                                                                                                          DNS server

                                                                                                          DNSUDP

                                                                                                          IPEthPhy

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          DNS

                                                                                                          5 DataLink Layer 5-71

                                                                                                          A day in the lifehellip TCP connection carrying HTTP

                                                                                                          HTTPTCPIP

                                                                                                          EthPhy

                                                                                                          HTTP

                                                                                                          to send HTTP request client first opens TCP socket to web server

                                                                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                          TCP connection established

                                                                                                          64233169105

                                                                                                          web server

                                                                                                          SYN

                                                                                                          SYN

                                                                                                          SYN

                                                                                                          SYN

                                                                                                          TCPIP

                                                                                                          EthPhy

                                                                                                          SYN

                                                                                                          SYN

                                                                                                          SYN

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          SYNACK

                                                                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                          5 DataLink Layer 5-72

                                                                                                          A day in the lifehellip HTTP requestreply

                                                                                                          HTTPTCPIP

                                                                                                          EthPhy

                                                                                                          HTTP

                                                                                                          HTTP request sent into TCP socket

                                                                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                          IP datgram containing HTTP reply routed back to client

                                                                                                          64233169105

                                                                                                          web server

                                                                                                          HTTPTCPIP

                                                                                                          EthPhy

                                                                                                          web server responds with HTTP reply (containing web page)

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTPHTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          HTTP

                                                                                                          web page finally () displayed

                                                                                                          5 DataLink Layer 5-73

                                                                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                          (except PHY) solid understanding of networking

                                                                                                          principles practice hellip could stop here hellip but lots of

                                                                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                          • Link Layer
                                                                                                          • Link Layer Introduction
                                                                                                          • Link layer context
                                                                                                          • Encapsulation
                                                                                                          • Link Layer Services
                                                                                                          • Link Layer Services (more)
                                                                                                          • Where is the link layer implemented
                                                                                                          • Adaptors Communicating
                                                                                                          • Slide 9
                                                                                                          • Parity Checking
                                                                                                          • Internet checksum (review)
                                                                                                          • Checksumming Cyclic Redundancy Check
                                                                                                          • Cyclic Redundancy Check
                                                                                                          • Slide 14
                                                                                                          • CRC Example
                                                                                                          • Slide 16
                                                                                                          • Multiple Access Links and Protocols
                                                                                                          • Cable Network Architecture Overview
                                                                                                          • Multiple Access protocols
                                                                                                          • Ideal Multiple Access Protocol
                                                                                                          • MAC Protocols a taxonomy
                                                                                                          • Channel Partitioning MAC protocols TDMA
                                                                                                          • Channel Partitioning MAC protocols FDMA
                                                                                                          • Random Access Protocols
                                                                                                          • ALOHA
                                                                                                          • CSMA (Carrier Sense Multiple Access)
                                                                                                          • CSMA collisions
                                                                                                          • CSMACD (Collision Detection)
                                                                                                          • CSMACD collision detection
                                                                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                                                                          • Slide 31
                                                                                                          • Slide 32
                                                                                                          • Summary of MAC protocols
                                                                                                          • Slide 34
                                                                                                          • MAC Addresses and ARP
                                                                                                          • LAN Addresses and ARP
                                                                                                          • LAN Address (more)
                                                                                                          • ARP Address Resolution Protocol
                                                                                                          • ARP protocol Same LAN (network)
                                                                                                          • Addressing routing to another LAN
                                                                                                          • Slide 41
                                                                                                          • Slide 42
                                                                                                          • Ethernet
                                                                                                          • Star topology
                                                                                                          • Ethernet Frame Structure
                                                                                                          • Ethernet Frame Structure (more)
                                                                                                          • Ethernet Unreliable connectionless
                                                                                                          • Ethernet CSMACD algorithm
                                                                                                          • Ethernetrsquos CSMACD (more)
                                                                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                          • Manchester encoding
                                                                                                          • Slide 52
                                                                                                          • Hubs
                                                                                                          • Switch
                                                                                                          • Switch allows multiple simultaneous transmissions
                                                                                                          • Switch Table
                                                                                                          • Switch self-learning
                                                                                                          • Switch frame filteringforwarding
                                                                                                          • Self-learning forwarding example
                                                                                                          • Interconnecting switches
                                                                                                          • Self-learning multi-switch example
                                                                                                          • Institutional network
                                                                                                          • Switches vs Routers
                                                                                                          • Slide 64
                                                                                                          • Synthesis a day in the life of a web request
                                                                                                          • A day in the life scenario
                                                                                                          • A day in the lifehellip connecting to the Internet
                                                                                                          • Slide 68
                                                                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                          • A day in the lifehellip using DNS
                                                                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                                                                          • A day in the lifehellip HTTP requestreply
                                                                                                          • Chapter 5 letrsquos take a breath

                                                                                                            5 DataLink Layer 5-54

                                                                                                            Switch link-layer device smarter than hubs take active

                                                                                                            roleo store forward Ethernet frameso examine incoming framersquos MAC address selectively

                                                                                                            forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

                                                                                                            transparento hosts are unaware of presence of switches

                                                                                                            plug-and-play self-learningo switches do not need to be configured

                                                                                                            5 DataLink Layer 5-55

                                                                                                            Switch allows multiple simultaneous transmissions

                                                                                                            hosts have dedicated direct connection to switch

                                                                                                            switches buffer packets Ethernet protocol used on

                                                                                                            each incoming link but no collisions full duplexo each link is its own collision

                                                                                                            domain switching A-to-Arsquo and B-

                                                                                                            to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                            A

                                                                                                            Arsquo

                                                                                                            B

                                                                                                            Brsquo

                                                                                                            C

                                                                                                            Crsquo

                                                                                                            switch with six interfaces(123456)

                                                                                                            1 23

                                                                                                            45

                                                                                                            6

                                                                                                            5 DataLink Layer 5-56

                                                                                                            Switch Table

                                                                                                            Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                            A each switch has a switch table each entryo (MAC address of host interface

                                                                                                            to reach host time stamp)

                                                                                                            looks like a routing table Q how are entries created

                                                                                                            maintained in switch table o something like a routing

                                                                                                            protocol

                                                                                                            A

                                                                                                            Arsquo

                                                                                                            B

                                                                                                            Brsquo

                                                                                                            C

                                                                                                            Crsquo

                                                                                                            switch with six interfaces(123456)

                                                                                                            1 23

                                                                                                            45

                                                                                                            6

                                                                                                            5 DataLink Layer 5-57

                                                                                                            Switch self-learning

                                                                                                            switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                            switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                            o records senderlocation pair in switch table

                                                                                                            A

                                                                                                            Arsquo

                                                                                                            B

                                                                                                            Brsquo

                                                                                                            C

                                                                                                            Crsquo

                                                                                                            1 23

                                                                                                            45

                                                                                                            6

                                                                                                            A Arsquo

                                                                                                            Source ADest Arsquo

                                                                                                            MAC addr interface TTL

                                                                                                            Switch table (initially empty)

                                                                                                            A 1 60

                                                                                                            5 DataLink Layer 5-58

                                                                                                            Switch frame filteringforwardingWhen frame received

                                                                                                            1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                            then if dest on segment from which frame arrived

                                                                                                            then drop the frame else forward the frame on interface indicated else flood

                                                                                                            forward on all but the interface on which the frame arrived

                                                                                                            5 DataLink Layer 5-59

                                                                                                            Self-learning forwarding example

                                                                                                            A

                                                                                                            Arsquo

                                                                                                            B

                                                                                                            Brsquo

                                                                                                            C

                                                                                                            Crsquo

                                                                                                            1 23

                                                                                                            45

                                                                                                            6

                                                                                                            A Arsquo

                                                                                                            Source ADest Arsquo

                                                                                                            MAC addr interface TTL

                                                                                                            Switch table (initially empty)

                                                                                                            A 1 60

                                                                                                            A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                            frame destination unknownflood

                                                                                                            Arsquo A

                                                                                                            destination A location known

                                                                                                            Arsquo 4 60

                                                                                                            selective send

                                                                                                            5 DataLink Layer 5-60

                                                                                                            Interconnecting switches

                                                                                                            switches can be connected together

                                                                                                            A

                                                                                                            B

                                                                                                            Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                            A self learning (works exactly the same as in single-switch case)

                                                                                                            S1

                                                                                                            C D

                                                                                                            E

                                                                                                            FS2

                                                                                                            S4

                                                                                                            S3

                                                                                                            H

                                                                                                            I

                                                                                                            G

                                                                                                            5 DataLink Layer 5-61

                                                                                                            Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                            Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                            A

                                                                                                            B

                                                                                                            S1

                                                                                                            C D

                                                                                                            E

                                                                                                            FS2

                                                                                                            S4

                                                                                                            S3

                                                                                                            H

                                                                                                            I

                                                                                                            G

                                                                                                            1

                                                                                                            2 3

                                                                                                            34

                                                                                                            5 DataLink Layer 5-62

                                                                                                            Institutional network

                                                                                                            to externalnetwork

                                                                                                            router

                                                                                                            IP subnet

                                                                                                            mail server

                                                                                                            web server

                                                                                                            5 DataLink Layer 5-63

                                                                                                            Switches vs Routers both store-and-forward devices

                                                                                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                            routers maintain routing tables implement routing algorithms

                                                                                                            switches maintain switch tables implement filtering learning algorithms

                                                                                                            Switch

                                                                                                            5 DataLink Layer 5-64

                                                                                                            Link Layer

                                                                                                            51 Introduction and services

                                                                                                            52 Error detection and correction

                                                                                                            53Multiple access protocols

                                                                                                            54 Link-Layer Addressing

                                                                                                            55 Ethernet

                                                                                                            56 Link-layer switches 59 A day in the life of

                                                                                                            a web request

                                                                                                            5 DataLink Layer 5-65

                                                                                                            Synthesis a day in the life of a web request

                                                                                                            journey down protocol stack completeo application transport network link

                                                                                                            putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                            (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                            5 DataLink Layer 5-66

                                                                                                            A day in the life scenario

                                                                                                            Comcast network 68800013

                                                                                                            Googlersquos network 64233160019 64233169105

                                                                                                            web server

                                                                                                            DNS server

                                                                                                            school network 68802024

                                                                                                            browser

                                                                                                            web page

                                                                                                            5 DataLink Layer 5-67

                                                                                                            A day in the lifehellip connecting to the Internet

                                                                                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                            router(runs DHCP)

                                                                                                            DHCPUDP

                                                                                                            IPEthPhy

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCPUDP

                                                                                                            IPEthPhy

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCPDHCP

                                                                                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                            5 DataLink Layer 5-68

                                                                                                            A day in the lifehellip connecting to the Internet

                                                                                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                            router(runs DHCP)

                                                                                                            DHCPUDP

                                                                                                            IPEthPhy

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCPUDP

                                                                                                            IPEthPhy

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            DHCP

                                                                                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                            DHCP client receives DHCP ACK reply

                                                                                                            5 DataLink Layer 5-69

                                                                                                            A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                            DNSUDP

                                                                                                            IPEthPhy

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                            ARP query

                                                                                                            EthPhy

                                                                                                            ARP

                                                                                                            ARP

                                                                                                            ARP reply

                                                                                                            5 DataLink Layer 5-70

                                                                                                            A day in the lifehellip using DNS

                                                                                                            DNSUDP

                                                                                                            IPEthPhy

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                            demuxrsquoed to DNS server DNS server replies to

                                                                                                            client with IP address of wwwgooglecom

                                                                                                            Comcast network 68800013

                                                                                                            DNS server

                                                                                                            DNSUDP

                                                                                                            IPEthPhy

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            DNS

                                                                                                            5 DataLink Layer 5-71

                                                                                                            A day in the lifehellip TCP connection carrying HTTP

                                                                                                            HTTPTCPIP

                                                                                                            EthPhy

                                                                                                            HTTP

                                                                                                            to send HTTP request client first opens TCP socket to web server

                                                                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                            TCP connection established

                                                                                                            64233169105

                                                                                                            web server

                                                                                                            SYN

                                                                                                            SYN

                                                                                                            SYN

                                                                                                            SYN

                                                                                                            TCPIP

                                                                                                            EthPhy

                                                                                                            SYN

                                                                                                            SYN

                                                                                                            SYN

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            SYNACK

                                                                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                            5 DataLink Layer 5-72

                                                                                                            A day in the lifehellip HTTP requestreply

                                                                                                            HTTPTCPIP

                                                                                                            EthPhy

                                                                                                            HTTP

                                                                                                            HTTP request sent into TCP socket

                                                                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                            IP datgram containing HTTP reply routed back to client

                                                                                                            64233169105

                                                                                                            web server

                                                                                                            HTTPTCPIP

                                                                                                            EthPhy

                                                                                                            web server responds with HTTP reply (containing web page)

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTPHTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            HTTP

                                                                                                            web page finally () displayed

                                                                                                            5 DataLink Layer 5-73

                                                                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                            (except PHY) solid understanding of networking

                                                                                                            principles practice hellip could stop here hellip but lots of

                                                                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                            • Link Layer
                                                                                                            • Link Layer Introduction
                                                                                                            • Link layer context
                                                                                                            • Encapsulation
                                                                                                            • Link Layer Services
                                                                                                            • Link Layer Services (more)
                                                                                                            • Where is the link layer implemented
                                                                                                            • Adaptors Communicating
                                                                                                            • Slide 9
                                                                                                            • Parity Checking
                                                                                                            • Internet checksum (review)
                                                                                                            • Checksumming Cyclic Redundancy Check
                                                                                                            • Cyclic Redundancy Check
                                                                                                            • Slide 14
                                                                                                            • CRC Example
                                                                                                            • Slide 16
                                                                                                            • Multiple Access Links and Protocols
                                                                                                            • Cable Network Architecture Overview
                                                                                                            • Multiple Access protocols
                                                                                                            • Ideal Multiple Access Protocol
                                                                                                            • MAC Protocols a taxonomy
                                                                                                            • Channel Partitioning MAC protocols TDMA
                                                                                                            • Channel Partitioning MAC protocols FDMA
                                                                                                            • Random Access Protocols
                                                                                                            • ALOHA
                                                                                                            • CSMA (Carrier Sense Multiple Access)
                                                                                                            • CSMA collisions
                                                                                                            • CSMACD (Collision Detection)
                                                                                                            • CSMACD collision detection
                                                                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                                                                            • Slide 31
                                                                                                            • Slide 32
                                                                                                            • Summary of MAC protocols
                                                                                                            • Slide 34
                                                                                                            • MAC Addresses and ARP
                                                                                                            • LAN Addresses and ARP
                                                                                                            • LAN Address (more)
                                                                                                            • ARP Address Resolution Protocol
                                                                                                            • ARP protocol Same LAN (network)
                                                                                                            • Addressing routing to another LAN
                                                                                                            • Slide 41
                                                                                                            • Slide 42
                                                                                                            • Ethernet
                                                                                                            • Star topology
                                                                                                            • Ethernet Frame Structure
                                                                                                            • Ethernet Frame Structure (more)
                                                                                                            • Ethernet Unreliable connectionless
                                                                                                            • Ethernet CSMACD algorithm
                                                                                                            • Ethernetrsquos CSMACD (more)
                                                                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                            • Manchester encoding
                                                                                                            • Slide 52
                                                                                                            • Hubs
                                                                                                            • Switch
                                                                                                            • Switch allows multiple simultaneous transmissions
                                                                                                            • Switch Table
                                                                                                            • Switch self-learning
                                                                                                            • Switch frame filteringforwarding
                                                                                                            • Self-learning forwarding example
                                                                                                            • Interconnecting switches
                                                                                                            • Self-learning multi-switch example
                                                                                                            • Institutional network
                                                                                                            • Switches vs Routers
                                                                                                            • Slide 64
                                                                                                            • Synthesis a day in the life of a web request
                                                                                                            • A day in the life scenario
                                                                                                            • A day in the lifehellip connecting to the Internet
                                                                                                            • Slide 68
                                                                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                            • A day in the lifehellip using DNS
                                                                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                                                                            • A day in the lifehellip HTTP requestreply
                                                                                                            • Chapter 5 letrsquos take a breath

                                                                                                              5 DataLink Layer 5-55

                                                                                                              Switch allows multiple simultaneous transmissions

                                                                                                              hosts have dedicated direct connection to switch

                                                                                                              switches buffer packets Ethernet protocol used on

                                                                                                              each incoming link but no collisions full duplexo each link is its own collision

                                                                                                              domain switching A-to-Arsquo and B-

                                                                                                              to-Brsquo simultaneously without collisions o not possible with dumb hub

                                                                                                              A

                                                                                                              Arsquo

                                                                                                              B

                                                                                                              Brsquo

                                                                                                              C

                                                                                                              Crsquo

                                                                                                              switch with six interfaces(123456)

                                                                                                              1 23

                                                                                                              45

                                                                                                              6

                                                                                                              5 DataLink Layer 5-56

                                                                                                              Switch Table

                                                                                                              Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                              A each switch has a switch table each entryo (MAC address of host interface

                                                                                                              to reach host time stamp)

                                                                                                              looks like a routing table Q how are entries created

                                                                                                              maintained in switch table o something like a routing

                                                                                                              protocol

                                                                                                              A

                                                                                                              Arsquo

                                                                                                              B

                                                                                                              Brsquo

                                                                                                              C

                                                                                                              Crsquo

                                                                                                              switch with six interfaces(123456)

                                                                                                              1 23

                                                                                                              45

                                                                                                              6

                                                                                                              5 DataLink Layer 5-57

                                                                                                              Switch self-learning

                                                                                                              switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                              switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                              o records senderlocation pair in switch table

                                                                                                              A

                                                                                                              Arsquo

                                                                                                              B

                                                                                                              Brsquo

                                                                                                              C

                                                                                                              Crsquo

                                                                                                              1 23

                                                                                                              45

                                                                                                              6

                                                                                                              A Arsquo

                                                                                                              Source ADest Arsquo

                                                                                                              MAC addr interface TTL

                                                                                                              Switch table (initially empty)

                                                                                                              A 1 60

                                                                                                              5 DataLink Layer 5-58

                                                                                                              Switch frame filteringforwardingWhen frame received

                                                                                                              1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                              then if dest on segment from which frame arrived

                                                                                                              then drop the frame else forward the frame on interface indicated else flood

                                                                                                              forward on all but the interface on which the frame arrived

                                                                                                              5 DataLink Layer 5-59

                                                                                                              Self-learning forwarding example

                                                                                                              A

                                                                                                              Arsquo

                                                                                                              B

                                                                                                              Brsquo

                                                                                                              C

                                                                                                              Crsquo

                                                                                                              1 23

                                                                                                              45

                                                                                                              6

                                                                                                              A Arsquo

                                                                                                              Source ADest Arsquo

                                                                                                              MAC addr interface TTL

                                                                                                              Switch table (initially empty)

                                                                                                              A 1 60

                                                                                                              A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                              frame destination unknownflood

                                                                                                              Arsquo A

                                                                                                              destination A location known

                                                                                                              Arsquo 4 60

                                                                                                              selective send

                                                                                                              5 DataLink Layer 5-60

                                                                                                              Interconnecting switches

                                                                                                              switches can be connected together

                                                                                                              A

                                                                                                              B

                                                                                                              Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                              A self learning (works exactly the same as in single-switch case)

                                                                                                              S1

                                                                                                              C D

                                                                                                              E

                                                                                                              FS2

                                                                                                              S4

                                                                                                              S3

                                                                                                              H

                                                                                                              I

                                                                                                              G

                                                                                                              5 DataLink Layer 5-61

                                                                                                              Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                              Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                              A

                                                                                                              B

                                                                                                              S1

                                                                                                              C D

                                                                                                              E

                                                                                                              FS2

                                                                                                              S4

                                                                                                              S3

                                                                                                              H

                                                                                                              I

                                                                                                              G

                                                                                                              1

                                                                                                              2 3

                                                                                                              34

                                                                                                              5 DataLink Layer 5-62

                                                                                                              Institutional network

                                                                                                              to externalnetwork

                                                                                                              router

                                                                                                              IP subnet

                                                                                                              mail server

                                                                                                              web server

                                                                                                              5 DataLink Layer 5-63

                                                                                                              Switches vs Routers both store-and-forward devices

                                                                                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                              routers maintain routing tables implement routing algorithms

                                                                                                              switches maintain switch tables implement filtering learning algorithms

                                                                                                              Switch

                                                                                                              5 DataLink Layer 5-64

                                                                                                              Link Layer

                                                                                                              51 Introduction and services

                                                                                                              52 Error detection and correction

                                                                                                              53Multiple access protocols

                                                                                                              54 Link-Layer Addressing

                                                                                                              55 Ethernet

                                                                                                              56 Link-layer switches 59 A day in the life of

                                                                                                              a web request

                                                                                                              5 DataLink Layer 5-65

                                                                                                              Synthesis a day in the life of a web request

                                                                                                              journey down protocol stack completeo application transport network link

                                                                                                              putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                              (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                              5 DataLink Layer 5-66

                                                                                                              A day in the life scenario

                                                                                                              Comcast network 68800013

                                                                                                              Googlersquos network 64233160019 64233169105

                                                                                                              web server

                                                                                                              DNS server

                                                                                                              school network 68802024

                                                                                                              browser

                                                                                                              web page

                                                                                                              5 DataLink Layer 5-67

                                                                                                              A day in the lifehellip connecting to the Internet

                                                                                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                              router(runs DHCP)

                                                                                                              DHCPUDP

                                                                                                              IPEthPhy

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCPUDP

                                                                                                              IPEthPhy

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCPDHCP

                                                                                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                              5 DataLink Layer 5-68

                                                                                                              A day in the lifehellip connecting to the Internet

                                                                                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                              router(runs DHCP)

                                                                                                              DHCPUDP

                                                                                                              IPEthPhy

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCPUDP

                                                                                                              IPEthPhy

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              DHCP

                                                                                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                              DHCP client receives DHCP ACK reply

                                                                                                              5 DataLink Layer 5-69

                                                                                                              A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                              DNSUDP

                                                                                                              IPEthPhy

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                              ARP query

                                                                                                              EthPhy

                                                                                                              ARP

                                                                                                              ARP

                                                                                                              ARP reply

                                                                                                              5 DataLink Layer 5-70

                                                                                                              A day in the lifehellip using DNS

                                                                                                              DNSUDP

                                                                                                              IPEthPhy

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                              demuxrsquoed to DNS server DNS server replies to

                                                                                                              client with IP address of wwwgooglecom

                                                                                                              Comcast network 68800013

                                                                                                              DNS server

                                                                                                              DNSUDP

                                                                                                              IPEthPhy

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              DNS

                                                                                                              5 DataLink Layer 5-71

                                                                                                              A day in the lifehellip TCP connection carrying HTTP

                                                                                                              HTTPTCPIP

                                                                                                              EthPhy

                                                                                                              HTTP

                                                                                                              to send HTTP request client first opens TCP socket to web server

                                                                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                              TCP connection established

                                                                                                              64233169105

                                                                                                              web server

                                                                                                              SYN

                                                                                                              SYN

                                                                                                              SYN

                                                                                                              SYN

                                                                                                              TCPIP

                                                                                                              EthPhy

                                                                                                              SYN

                                                                                                              SYN

                                                                                                              SYN

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              SYNACK

                                                                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                              5 DataLink Layer 5-72

                                                                                                              A day in the lifehellip HTTP requestreply

                                                                                                              HTTPTCPIP

                                                                                                              EthPhy

                                                                                                              HTTP

                                                                                                              HTTP request sent into TCP socket

                                                                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                              IP datgram containing HTTP reply routed back to client

                                                                                                              64233169105

                                                                                                              web server

                                                                                                              HTTPTCPIP

                                                                                                              EthPhy

                                                                                                              web server responds with HTTP reply (containing web page)

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTPHTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              HTTP

                                                                                                              web page finally () displayed

                                                                                                              5 DataLink Layer 5-73

                                                                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                              (except PHY) solid understanding of networking

                                                                                                              principles practice hellip could stop here hellip but lots of

                                                                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                              • Link Layer
                                                                                                              • Link Layer Introduction
                                                                                                              • Link layer context
                                                                                                              • Encapsulation
                                                                                                              • Link Layer Services
                                                                                                              • Link Layer Services (more)
                                                                                                              • Where is the link layer implemented
                                                                                                              • Adaptors Communicating
                                                                                                              • Slide 9
                                                                                                              • Parity Checking
                                                                                                              • Internet checksum (review)
                                                                                                              • Checksumming Cyclic Redundancy Check
                                                                                                              • Cyclic Redundancy Check
                                                                                                              • Slide 14
                                                                                                              • CRC Example
                                                                                                              • Slide 16
                                                                                                              • Multiple Access Links and Protocols
                                                                                                              • Cable Network Architecture Overview
                                                                                                              • Multiple Access protocols
                                                                                                              • Ideal Multiple Access Protocol
                                                                                                              • MAC Protocols a taxonomy
                                                                                                              • Channel Partitioning MAC protocols TDMA
                                                                                                              • Channel Partitioning MAC protocols FDMA
                                                                                                              • Random Access Protocols
                                                                                                              • ALOHA
                                                                                                              • CSMA (Carrier Sense Multiple Access)
                                                                                                              • CSMA collisions
                                                                                                              • CSMACD (Collision Detection)
                                                                                                              • CSMACD collision detection
                                                                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                                                                              • Slide 31
                                                                                                              • Slide 32
                                                                                                              • Summary of MAC protocols
                                                                                                              • Slide 34
                                                                                                              • MAC Addresses and ARP
                                                                                                              • LAN Addresses and ARP
                                                                                                              • LAN Address (more)
                                                                                                              • ARP Address Resolution Protocol
                                                                                                              • ARP protocol Same LAN (network)
                                                                                                              • Addressing routing to another LAN
                                                                                                              • Slide 41
                                                                                                              • Slide 42
                                                                                                              • Ethernet
                                                                                                              • Star topology
                                                                                                              • Ethernet Frame Structure
                                                                                                              • Ethernet Frame Structure (more)
                                                                                                              • Ethernet Unreliable connectionless
                                                                                                              • Ethernet CSMACD algorithm
                                                                                                              • Ethernetrsquos CSMACD (more)
                                                                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                              • Manchester encoding
                                                                                                              • Slide 52
                                                                                                              • Hubs
                                                                                                              • Switch
                                                                                                              • Switch allows multiple simultaneous transmissions
                                                                                                              • Switch Table
                                                                                                              • Switch self-learning
                                                                                                              • Switch frame filteringforwarding
                                                                                                              • Self-learning forwarding example
                                                                                                              • Interconnecting switches
                                                                                                              • Self-learning multi-switch example
                                                                                                              • Institutional network
                                                                                                              • Switches vs Routers
                                                                                                              • Slide 64
                                                                                                              • Synthesis a day in the life of a web request
                                                                                                              • A day in the life scenario
                                                                                                              • A day in the lifehellip connecting to the Internet
                                                                                                              • Slide 68
                                                                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                              • A day in the lifehellip using DNS
                                                                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                                                                              • A day in the lifehellip HTTP requestreply
                                                                                                              • Chapter 5 letrsquos take a breath

                                                                                                                5 DataLink Layer 5-56

                                                                                                                Switch Table

                                                                                                                Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5

                                                                                                                A each switch has a switch table each entryo (MAC address of host interface

                                                                                                                to reach host time stamp)

                                                                                                                looks like a routing table Q how are entries created

                                                                                                                maintained in switch table o something like a routing

                                                                                                                protocol

                                                                                                                A

                                                                                                                Arsquo

                                                                                                                B

                                                                                                                Brsquo

                                                                                                                C

                                                                                                                Crsquo

                                                                                                                switch with six interfaces(123456)

                                                                                                                1 23

                                                                                                                45

                                                                                                                6

                                                                                                                5 DataLink Layer 5-57

                                                                                                                Switch self-learning

                                                                                                                switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                                switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                                o records senderlocation pair in switch table

                                                                                                                A

                                                                                                                Arsquo

                                                                                                                B

                                                                                                                Brsquo

                                                                                                                C

                                                                                                                Crsquo

                                                                                                                1 23

                                                                                                                45

                                                                                                                6

                                                                                                                A Arsquo

                                                                                                                Source ADest Arsquo

                                                                                                                MAC addr interface TTL

                                                                                                                Switch table (initially empty)

                                                                                                                A 1 60

                                                                                                                5 DataLink Layer 5-58

                                                                                                                Switch frame filteringforwardingWhen frame received

                                                                                                                1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                                then if dest on segment from which frame arrived

                                                                                                                then drop the frame else forward the frame on interface indicated else flood

                                                                                                                forward on all but the interface on which the frame arrived

                                                                                                                5 DataLink Layer 5-59

                                                                                                                Self-learning forwarding example

                                                                                                                A

                                                                                                                Arsquo

                                                                                                                B

                                                                                                                Brsquo

                                                                                                                C

                                                                                                                Crsquo

                                                                                                                1 23

                                                                                                                45

                                                                                                                6

                                                                                                                A Arsquo

                                                                                                                Source ADest Arsquo

                                                                                                                MAC addr interface TTL

                                                                                                                Switch table (initially empty)

                                                                                                                A 1 60

                                                                                                                A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                                frame destination unknownflood

                                                                                                                Arsquo A

                                                                                                                destination A location known

                                                                                                                Arsquo 4 60

                                                                                                                selective send

                                                                                                                5 DataLink Layer 5-60

                                                                                                                Interconnecting switches

                                                                                                                switches can be connected together

                                                                                                                A

                                                                                                                B

                                                                                                                Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                                A self learning (works exactly the same as in single-switch case)

                                                                                                                S1

                                                                                                                C D

                                                                                                                E

                                                                                                                FS2

                                                                                                                S4

                                                                                                                S3

                                                                                                                H

                                                                                                                I

                                                                                                                G

                                                                                                                5 DataLink Layer 5-61

                                                                                                                Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                A

                                                                                                                B

                                                                                                                S1

                                                                                                                C D

                                                                                                                E

                                                                                                                FS2

                                                                                                                S4

                                                                                                                S3

                                                                                                                H

                                                                                                                I

                                                                                                                G

                                                                                                                1

                                                                                                                2 3

                                                                                                                34

                                                                                                                5 DataLink Layer 5-62

                                                                                                                Institutional network

                                                                                                                to externalnetwork

                                                                                                                router

                                                                                                                IP subnet

                                                                                                                mail server

                                                                                                                web server

                                                                                                                5 DataLink Layer 5-63

                                                                                                                Switches vs Routers both store-and-forward devices

                                                                                                                o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                routers maintain routing tables implement routing algorithms

                                                                                                                switches maintain switch tables implement filtering learning algorithms

                                                                                                                Switch

                                                                                                                5 DataLink Layer 5-64

                                                                                                                Link Layer

                                                                                                                51 Introduction and services

                                                                                                                52 Error detection and correction

                                                                                                                53Multiple access protocols

                                                                                                                54 Link-Layer Addressing

                                                                                                                55 Ethernet

                                                                                                                56 Link-layer switches 59 A day in the life of

                                                                                                                a web request

                                                                                                                5 DataLink Layer 5-65

                                                                                                                Synthesis a day in the life of a web request

                                                                                                                journey down protocol stack completeo application transport network link

                                                                                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                5 DataLink Layer 5-66

                                                                                                                A day in the life scenario

                                                                                                                Comcast network 68800013

                                                                                                                Googlersquos network 64233160019 64233169105

                                                                                                                web server

                                                                                                                DNS server

                                                                                                                school network 68802024

                                                                                                                browser

                                                                                                                web page

                                                                                                                5 DataLink Layer 5-67

                                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                router(runs DHCP)

                                                                                                                DHCPUDP

                                                                                                                IPEthPhy

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCPUDP

                                                                                                                IPEthPhy

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCPDHCP

                                                                                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                5 DataLink Layer 5-68

                                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                router(runs DHCP)

                                                                                                                DHCPUDP

                                                                                                                IPEthPhy

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCPUDP

                                                                                                                IPEthPhy

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                DHCP

                                                                                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                DHCP client receives DHCP ACK reply

                                                                                                                5 DataLink Layer 5-69

                                                                                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                DNSUDP

                                                                                                                IPEthPhy

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                ARP query

                                                                                                                EthPhy

                                                                                                                ARP

                                                                                                                ARP

                                                                                                                ARP reply

                                                                                                                5 DataLink Layer 5-70

                                                                                                                A day in the lifehellip using DNS

                                                                                                                DNSUDP

                                                                                                                IPEthPhy

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                demuxrsquoed to DNS server DNS server replies to

                                                                                                                client with IP address of wwwgooglecom

                                                                                                                Comcast network 68800013

                                                                                                                DNS server

                                                                                                                DNSUDP

                                                                                                                IPEthPhy

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                DNS

                                                                                                                5 DataLink Layer 5-71

                                                                                                                A day in the lifehellip TCP connection carrying HTTP

                                                                                                                HTTPTCPIP

                                                                                                                EthPhy

                                                                                                                HTTP

                                                                                                                to send HTTP request client first opens TCP socket to web server

                                                                                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                TCP connection established

                                                                                                                64233169105

                                                                                                                web server

                                                                                                                SYN

                                                                                                                SYN

                                                                                                                SYN

                                                                                                                SYN

                                                                                                                TCPIP

                                                                                                                EthPhy

                                                                                                                SYN

                                                                                                                SYN

                                                                                                                SYN

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                SYNACK

                                                                                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                5 DataLink Layer 5-72

                                                                                                                A day in the lifehellip HTTP requestreply

                                                                                                                HTTPTCPIP

                                                                                                                EthPhy

                                                                                                                HTTP

                                                                                                                HTTP request sent into TCP socket

                                                                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                IP datgram containing HTTP reply routed back to client

                                                                                                                64233169105

                                                                                                                web server

                                                                                                                HTTPTCPIP

                                                                                                                EthPhy

                                                                                                                web server responds with HTTP reply (containing web page)

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTPHTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                HTTP

                                                                                                                web page finally () displayed

                                                                                                                5 DataLink Layer 5-73

                                                                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                (except PHY) solid understanding of networking

                                                                                                                principles practice hellip could stop here hellip but lots of

                                                                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                • Link Layer
                                                                                                                • Link Layer Introduction
                                                                                                                • Link layer context
                                                                                                                • Encapsulation
                                                                                                                • Link Layer Services
                                                                                                                • Link Layer Services (more)
                                                                                                                • Where is the link layer implemented
                                                                                                                • Adaptors Communicating
                                                                                                                • Slide 9
                                                                                                                • Parity Checking
                                                                                                                • Internet checksum (review)
                                                                                                                • Checksumming Cyclic Redundancy Check
                                                                                                                • Cyclic Redundancy Check
                                                                                                                • Slide 14
                                                                                                                • CRC Example
                                                                                                                • Slide 16
                                                                                                                • Multiple Access Links and Protocols
                                                                                                                • Cable Network Architecture Overview
                                                                                                                • Multiple Access protocols
                                                                                                                • Ideal Multiple Access Protocol
                                                                                                                • MAC Protocols a taxonomy
                                                                                                                • Channel Partitioning MAC protocols TDMA
                                                                                                                • Channel Partitioning MAC protocols FDMA
                                                                                                                • Random Access Protocols
                                                                                                                • ALOHA
                                                                                                                • CSMA (Carrier Sense Multiple Access)
                                                                                                                • CSMA collisions
                                                                                                                • CSMACD (Collision Detection)
                                                                                                                • CSMACD collision detection
                                                                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                • Slide 31
                                                                                                                • Slide 32
                                                                                                                • Summary of MAC protocols
                                                                                                                • Slide 34
                                                                                                                • MAC Addresses and ARP
                                                                                                                • LAN Addresses and ARP
                                                                                                                • LAN Address (more)
                                                                                                                • ARP Address Resolution Protocol
                                                                                                                • ARP protocol Same LAN (network)
                                                                                                                • Addressing routing to another LAN
                                                                                                                • Slide 41
                                                                                                                • Slide 42
                                                                                                                • Ethernet
                                                                                                                • Star topology
                                                                                                                • Ethernet Frame Structure
                                                                                                                • Ethernet Frame Structure (more)
                                                                                                                • Ethernet Unreliable connectionless
                                                                                                                • Ethernet CSMACD algorithm
                                                                                                                • Ethernetrsquos CSMACD (more)
                                                                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                • Manchester encoding
                                                                                                                • Slide 52
                                                                                                                • Hubs
                                                                                                                • Switch
                                                                                                                • Switch allows multiple simultaneous transmissions
                                                                                                                • Switch Table
                                                                                                                • Switch self-learning
                                                                                                                • Switch frame filteringforwarding
                                                                                                                • Self-learning forwarding example
                                                                                                                • Interconnecting switches
                                                                                                                • Self-learning multi-switch example
                                                                                                                • Institutional network
                                                                                                                • Switches vs Routers
                                                                                                                • Slide 64
                                                                                                                • Synthesis a day in the life of a web request
                                                                                                                • A day in the life scenario
                                                                                                                • A day in the lifehellip connecting to the Internet
                                                                                                                • Slide 68
                                                                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                • A day in the lifehellip using DNS
                                                                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                • A day in the lifehellip HTTP requestreply
                                                                                                                • Chapter 5 letrsquos take a breath

                                                                                                                  5 DataLink Layer 5-57

                                                                                                                  Switch self-learning

                                                                                                                  switch learns which hosts can be reached through which interfaceso when frame received

                                                                                                                  switch ldquolearnsrdquo location of sender incoming LAN segment

                                                                                                                  o records senderlocation pair in switch table

                                                                                                                  A

                                                                                                                  Arsquo

                                                                                                                  B

                                                                                                                  Brsquo

                                                                                                                  C

                                                                                                                  Crsquo

                                                                                                                  1 23

                                                                                                                  45

                                                                                                                  6

                                                                                                                  A Arsquo

                                                                                                                  Source ADest Arsquo

                                                                                                                  MAC addr interface TTL

                                                                                                                  Switch table (initially empty)

                                                                                                                  A 1 60

                                                                                                                  5 DataLink Layer 5-58

                                                                                                                  Switch frame filteringforwardingWhen frame received

                                                                                                                  1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                                  then if dest on segment from which frame arrived

                                                                                                                  then drop the frame else forward the frame on interface indicated else flood

                                                                                                                  forward on all but the interface on which the frame arrived

                                                                                                                  5 DataLink Layer 5-59

                                                                                                                  Self-learning forwarding example

                                                                                                                  A

                                                                                                                  Arsquo

                                                                                                                  B

                                                                                                                  Brsquo

                                                                                                                  C

                                                                                                                  Crsquo

                                                                                                                  1 23

                                                                                                                  45

                                                                                                                  6

                                                                                                                  A Arsquo

                                                                                                                  Source ADest Arsquo

                                                                                                                  MAC addr interface TTL

                                                                                                                  Switch table (initially empty)

                                                                                                                  A 1 60

                                                                                                                  A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                                  frame destination unknownflood

                                                                                                                  Arsquo A

                                                                                                                  destination A location known

                                                                                                                  Arsquo 4 60

                                                                                                                  selective send

                                                                                                                  5 DataLink Layer 5-60

                                                                                                                  Interconnecting switches

                                                                                                                  switches can be connected together

                                                                                                                  A

                                                                                                                  B

                                                                                                                  Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                                  A self learning (works exactly the same as in single-switch case)

                                                                                                                  S1

                                                                                                                  C D

                                                                                                                  E

                                                                                                                  FS2

                                                                                                                  S4

                                                                                                                  S3

                                                                                                                  H

                                                                                                                  I

                                                                                                                  G

                                                                                                                  5 DataLink Layer 5-61

                                                                                                                  Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                  Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                  A

                                                                                                                  B

                                                                                                                  S1

                                                                                                                  C D

                                                                                                                  E

                                                                                                                  FS2

                                                                                                                  S4

                                                                                                                  S3

                                                                                                                  H

                                                                                                                  I

                                                                                                                  G

                                                                                                                  1

                                                                                                                  2 3

                                                                                                                  34

                                                                                                                  5 DataLink Layer 5-62

                                                                                                                  Institutional network

                                                                                                                  to externalnetwork

                                                                                                                  router

                                                                                                                  IP subnet

                                                                                                                  mail server

                                                                                                                  web server

                                                                                                                  5 DataLink Layer 5-63

                                                                                                                  Switches vs Routers both store-and-forward devices

                                                                                                                  o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                  routers maintain routing tables implement routing algorithms

                                                                                                                  switches maintain switch tables implement filtering learning algorithms

                                                                                                                  Switch

                                                                                                                  5 DataLink Layer 5-64

                                                                                                                  Link Layer

                                                                                                                  51 Introduction and services

                                                                                                                  52 Error detection and correction

                                                                                                                  53Multiple access protocols

                                                                                                                  54 Link-Layer Addressing

                                                                                                                  55 Ethernet

                                                                                                                  56 Link-layer switches 59 A day in the life of

                                                                                                                  a web request

                                                                                                                  5 DataLink Layer 5-65

                                                                                                                  Synthesis a day in the life of a web request

                                                                                                                  journey down protocol stack completeo application transport network link

                                                                                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                  5 DataLink Layer 5-66

                                                                                                                  A day in the life scenario

                                                                                                                  Comcast network 68800013

                                                                                                                  Googlersquos network 64233160019 64233169105

                                                                                                                  web server

                                                                                                                  DNS server

                                                                                                                  school network 68802024

                                                                                                                  browser

                                                                                                                  web page

                                                                                                                  5 DataLink Layer 5-67

                                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                  router(runs DHCP)

                                                                                                                  DHCPUDP

                                                                                                                  IPEthPhy

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCPUDP

                                                                                                                  IPEthPhy

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCPDHCP

                                                                                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                  5 DataLink Layer 5-68

                                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                  router(runs DHCP)

                                                                                                                  DHCPUDP

                                                                                                                  IPEthPhy

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCPUDP

                                                                                                                  IPEthPhy

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  DHCP

                                                                                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                  DHCP client receives DHCP ACK reply

                                                                                                                  5 DataLink Layer 5-69

                                                                                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                  DNSUDP

                                                                                                                  IPEthPhy

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                  ARP query

                                                                                                                  EthPhy

                                                                                                                  ARP

                                                                                                                  ARP

                                                                                                                  ARP reply

                                                                                                                  5 DataLink Layer 5-70

                                                                                                                  A day in the lifehellip using DNS

                                                                                                                  DNSUDP

                                                                                                                  IPEthPhy

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                  demuxrsquoed to DNS server DNS server replies to

                                                                                                                  client with IP address of wwwgooglecom

                                                                                                                  Comcast network 68800013

                                                                                                                  DNS server

                                                                                                                  DNSUDP

                                                                                                                  IPEthPhy

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  DNS

                                                                                                                  5 DataLink Layer 5-71

                                                                                                                  A day in the lifehellip TCP connection carrying HTTP

                                                                                                                  HTTPTCPIP

                                                                                                                  EthPhy

                                                                                                                  HTTP

                                                                                                                  to send HTTP request client first opens TCP socket to web server

                                                                                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                  TCP connection established

                                                                                                                  64233169105

                                                                                                                  web server

                                                                                                                  SYN

                                                                                                                  SYN

                                                                                                                  SYN

                                                                                                                  SYN

                                                                                                                  TCPIP

                                                                                                                  EthPhy

                                                                                                                  SYN

                                                                                                                  SYN

                                                                                                                  SYN

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  SYNACK

                                                                                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                  5 DataLink Layer 5-72

                                                                                                                  A day in the lifehellip HTTP requestreply

                                                                                                                  HTTPTCPIP

                                                                                                                  EthPhy

                                                                                                                  HTTP

                                                                                                                  HTTP request sent into TCP socket

                                                                                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                  IP datgram containing HTTP reply routed back to client

                                                                                                                  64233169105

                                                                                                                  web server

                                                                                                                  HTTPTCPIP

                                                                                                                  EthPhy

                                                                                                                  web server responds with HTTP reply (containing web page)

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTPHTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  HTTP

                                                                                                                  web page finally () displayed

                                                                                                                  5 DataLink Layer 5-73

                                                                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                  (except PHY) solid understanding of networking

                                                                                                                  principles practice hellip could stop here hellip but lots of

                                                                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                  • Link Layer
                                                                                                                  • Link Layer Introduction
                                                                                                                  • Link layer context
                                                                                                                  • Encapsulation
                                                                                                                  • Link Layer Services
                                                                                                                  • Link Layer Services (more)
                                                                                                                  • Where is the link layer implemented
                                                                                                                  • Adaptors Communicating
                                                                                                                  • Slide 9
                                                                                                                  • Parity Checking
                                                                                                                  • Internet checksum (review)
                                                                                                                  • Checksumming Cyclic Redundancy Check
                                                                                                                  • Cyclic Redundancy Check
                                                                                                                  • Slide 14
                                                                                                                  • CRC Example
                                                                                                                  • Slide 16
                                                                                                                  • Multiple Access Links and Protocols
                                                                                                                  • Cable Network Architecture Overview
                                                                                                                  • Multiple Access protocols
                                                                                                                  • Ideal Multiple Access Protocol
                                                                                                                  • MAC Protocols a taxonomy
                                                                                                                  • Channel Partitioning MAC protocols TDMA
                                                                                                                  • Channel Partitioning MAC protocols FDMA
                                                                                                                  • Random Access Protocols
                                                                                                                  • ALOHA
                                                                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                                                                  • CSMA collisions
                                                                                                                  • CSMACD (Collision Detection)
                                                                                                                  • CSMACD collision detection
                                                                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                  • Slide 31
                                                                                                                  • Slide 32
                                                                                                                  • Summary of MAC protocols
                                                                                                                  • Slide 34
                                                                                                                  • MAC Addresses and ARP
                                                                                                                  • LAN Addresses and ARP
                                                                                                                  • LAN Address (more)
                                                                                                                  • ARP Address Resolution Protocol
                                                                                                                  • ARP protocol Same LAN (network)
                                                                                                                  • Addressing routing to another LAN
                                                                                                                  • Slide 41
                                                                                                                  • Slide 42
                                                                                                                  • Ethernet
                                                                                                                  • Star topology
                                                                                                                  • Ethernet Frame Structure
                                                                                                                  • Ethernet Frame Structure (more)
                                                                                                                  • Ethernet Unreliable connectionless
                                                                                                                  • Ethernet CSMACD algorithm
                                                                                                                  • Ethernetrsquos CSMACD (more)
                                                                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                  • Manchester encoding
                                                                                                                  • Slide 52
                                                                                                                  • Hubs
                                                                                                                  • Switch
                                                                                                                  • Switch allows multiple simultaneous transmissions
                                                                                                                  • Switch Table
                                                                                                                  • Switch self-learning
                                                                                                                  • Switch frame filteringforwarding
                                                                                                                  • Self-learning forwarding example
                                                                                                                  • Interconnecting switches
                                                                                                                  • Self-learning multi-switch example
                                                                                                                  • Institutional network
                                                                                                                  • Switches vs Routers
                                                                                                                  • Slide 64
                                                                                                                  • Synthesis a day in the life of a web request
                                                                                                                  • A day in the life scenario
                                                                                                                  • A day in the lifehellip connecting to the Internet
                                                                                                                  • Slide 68
                                                                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                  • A day in the lifehellip using DNS
                                                                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                  • A day in the lifehellip HTTP requestreply
                                                                                                                  • Chapter 5 letrsquos take a breath

                                                                                                                    5 DataLink Layer 5-58

                                                                                                                    Switch frame filteringforwardingWhen frame received

                                                                                                                    1 record (in switch table) link associated with sending host2 index switch table using MAC dest address3 if entry found for destination

                                                                                                                    then if dest on segment from which frame arrived

                                                                                                                    then drop the frame else forward the frame on interface indicated else flood

                                                                                                                    forward on all but the interface on which the frame arrived

                                                                                                                    5 DataLink Layer 5-59

                                                                                                                    Self-learning forwarding example

                                                                                                                    A

                                                                                                                    Arsquo

                                                                                                                    B

                                                                                                                    Brsquo

                                                                                                                    C

                                                                                                                    Crsquo

                                                                                                                    1 23

                                                                                                                    45

                                                                                                                    6

                                                                                                                    A Arsquo

                                                                                                                    Source ADest Arsquo

                                                                                                                    MAC addr interface TTL

                                                                                                                    Switch table (initially empty)

                                                                                                                    A 1 60

                                                                                                                    A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                                    frame destination unknownflood

                                                                                                                    Arsquo A

                                                                                                                    destination A location known

                                                                                                                    Arsquo 4 60

                                                                                                                    selective send

                                                                                                                    5 DataLink Layer 5-60

                                                                                                                    Interconnecting switches

                                                                                                                    switches can be connected together

                                                                                                                    A

                                                                                                                    B

                                                                                                                    Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                                    A self learning (works exactly the same as in single-switch case)

                                                                                                                    S1

                                                                                                                    C D

                                                                                                                    E

                                                                                                                    FS2

                                                                                                                    S4

                                                                                                                    S3

                                                                                                                    H

                                                                                                                    I

                                                                                                                    G

                                                                                                                    5 DataLink Layer 5-61

                                                                                                                    Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                    Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                    A

                                                                                                                    B

                                                                                                                    S1

                                                                                                                    C D

                                                                                                                    E

                                                                                                                    FS2

                                                                                                                    S4

                                                                                                                    S3

                                                                                                                    H

                                                                                                                    I

                                                                                                                    G

                                                                                                                    1

                                                                                                                    2 3

                                                                                                                    34

                                                                                                                    5 DataLink Layer 5-62

                                                                                                                    Institutional network

                                                                                                                    to externalnetwork

                                                                                                                    router

                                                                                                                    IP subnet

                                                                                                                    mail server

                                                                                                                    web server

                                                                                                                    5 DataLink Layer 5-63

                                                                                                                    Switches vs Routers both store-and-forward devices

                                                                                                                    o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                    routers maintain routing tables implement routing algorithms

                                                                                                                    switches maintain switch tables implement filtering learning algorithms

                                                                                                                    Switch

                                                                                                                    5 DataLink Layer 5-64

                                                                                                                    Link Layer

                                                                                                                    51 Introduction and services

                                                                                                                    52 Error detection and correction

                                                                                                                    53Multiple access protocols

                                                                                                                    54 Link-Layer Addressing

                                                                                                                    55 Ethernet

                                                                                                                    56 Link-layer switches 59 A day in the life of

                                                                                                                    a web request

                                                                                                                    5 DataLink Layer 5-65

                                                                                                                    Synthesis a day in the life of a web request

                                                                                                                    journey down protocol stack completeo application transport network link

                                                                                                                    putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                    (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                    o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                    5 DataLink Layer 5-66

                                                                                                                    A day in the life scenario

                                                                                                                    Comcast network 68800013

                                                                                                                    Googlersquos network 64233160019 64233169105

                                                                                                                    web server

                                                                                                                    DNS server

                                                                                                                    school network 68802024

                                                                                                                    browser

                                                                                                                    web page

                                                                                                                    5 DataLink Layer 5-67

                                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                    router(runs DHCP)

                                                                                                                    DHCPUDP

                                                                                                                    IPEthPhy

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCPUDP

                                                                                                                    IPEthPhy

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCPDHCP

                                                                                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                    5 DataLink Layer 5-68

                                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                    router(runs DHCP)

                                                                                                                    DHCPUDP

                                                                                                                    IPEthPhy

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCPUDP

                                                                                                                    IPEthPhy

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    DHCP

                                                                                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                    DHCP client receives DHCP ACK reply

                                                                                                                    5 DataLink Layer 5-69

                                                                                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                    DNSUDP

                                                                                                                    IPEthPhy

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                    ARP query

                                                                                                                    EthPhy

                                                                                                                    ARP

                                                                                                                    ARP

                                                                                                                    ARP reply

                                                                                                                    5 DataLink Layer 5-70

                                                                                                                    A day in the lifehellip using DNS

                                                                                                                    DNSUDP

                                                                                                                    IPEthPhy

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                    demuxrsquoed to DNS server DNS server replies to

                                                                                                                    client with IP address of wwwgooglecom

                                                                                                                    Comcast network 68800013

                                                                                                                    DNS server

                                                                                                                    DNSUDP

                                                                                                                    IPEthPhy

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    DNS

                                                                                                                    5 DataLink Layer 5-71

                                                                                                                    A day in the lifehellip TCP connection carrying HTTP

                                                                                                                    HTTPTCPIP

                                                                                                                    EthPhy

                                                                                                                    HTTP

                                                                                                                    to send HTTP request client first opens TCP socket to web server

                                                                                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                    TCP connection established

                                                                                                                    64233169105

                                                                                                                    web server

                                                                                                                    SYN

                                                                                                                    SYN

                                                                                                                    SYN

                                                                                                                    SYN

                                                                                                                    TCPIP

                                                                                                                    EthPhy

                                                                                                                    SYN

                                                                                                                    SYN

                                                                                                                    SYN

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    SYNACK

                                                                                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                    5 DataLink Layer 5-72

                                                                                                                    A day in the lifehellip HTTP requestreply

                                                                                                                    HTTPTCPIP

                                                                                                                    EthPhy

                                                                                                                    HTTP

                                                                                                                    HTTP request sent into TCP socket

                                                                                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                    IP datgram containing HTTP reply routed back to client

                                                                                                                    64233169105

                                                                                                                    web server

                                                                                                                    HTTPTCPIP

                                                                                                                    EthPhy

                                                                                                                    web server responds with HTTP reply (containing web page)

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTPHTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    HTTP

                                                                                                                    web page finally () displayed

                                                                                                                    5 DataLink Layer 5-73

                                                                                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                    (except PHY) solid understanding of networking

                                                                                                                    principles practice hellip could stop here hellip but lots of

                                                                                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                    • Link Layer
                                                                                                                    • Link Layer Introduction
                                                                                                                    • Link layer context
                                                                                                                    • Encapsulation
                                                                                                                    • Link Layer Services
                                                                                                                    • Link Layer Services (more)
                                                                                                                    • Where is the link layer implemented
                                                                                                                    • Adaptors Communicating
                                                                                                                    • Slide 9
                                                                                                                    • Parity Checking
                                                                                                                    • Internet checksum (review)
                                                                                                                    • Checksumming Cyclic Redundancy Check
                                                                                                                    • Cyclic Redundancy Check
                                                                                                                    • Slide 14
                                                                                                                    • CRC Example
                                                                                                                    • Slide 16
                                                                                                                    • Multiple Access Links and Protocols
                                                                                                                    • Cable Network Architecture Overview
                                                                                                                    • Multiple Access protocols
                                                                                                                    • Ideal Multiple Access Protocol
                                                                                                                    • MAC Protocols a taxonomy
                                                                                                                    • Channel Partitioning MAC protocols TDMA
                                                                                                                    • Channel Partitioning MAC protocols FDMA
                                                                                                                    • Random Access Protocols
                                                                                                                    • ALOHA
                                                                                                                    • CSMA (Carrier Sense Multiple Access)
                                                                                                                    • CSMA collisions
                                                                                                                    • CSMACD (Collision Detection)
                                                                                                                    • CSMACD collision detection
                                                                                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                    • Slide 31
                                                                                                                    • Slide 32
                                                                                                                    • Summary of MAC protocols
                                                                                                                    • Slide 34
                                                                                                                    • MAC Addresses and ARP
                                                                                                                    • LAN Addresses and ARP
                                                                                                                    • LAN Address (more)
                                                                                                                    • ARP Address Resolution Protocol
                                                                                                                    • ARP protocol Same LAN (network)
                                                                                                                    • Addressing routing to another LAN
                                                                                                                    • Slide 41
                                                                                                                    • Slide 42
                                                                                                                    • Ethernet
                                                                                                                    • Star topology
                                                                                                                    • Ethernet Frame Structure
                                                                                                                    • Ethernet Frame Structure (more)
                                                                                                                    • Ethernet Unreliable connectionless
                                                                                                                    • Ethernet CSMACD algorithm
                                                                                                                    • Ethernetrsquos CSMACD (more)
                                                                                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                    • Manchester encoding
                                                                                                                    • Slide 52
                                                                                                                    • Hubs
                                                                                                                    • Switch
                                                                                                                    • Switch allows multiple simultaneous transmissions
                                                                                                                    • Switch Table
                                                                                                                    • Switch self-learning
                                                                                                                    • Switch frame filteringforwarding
                                                                                                                    • Self-learning forwarding example
                                                                                                                    • Interconnecting switches
                                                                                                                    • Self-learning multi-switch example
                                                                                                                    • Institutional network
                                                                                                                    • Switches vs Routers
                                                                                                                    • Slide 64
                                                                                                                    • Synthesis a day in the life of a web request
                                                                                                                    • A day in the life scenario
                                                                                                                    • A day in the lifehellip connecting to the Internet
                                                                                                                    • Slide 68
                                                                                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                    • A day in the lifehellip using DNS
                                                                                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                    • A day in the lifehellip HTTP requestreply
                                                                                                                    • Chapter 5 letrsquos take a breath

                                                                                                                      5 DataLink Layer 5-59

                                                                                                                      Self-learning forwarding example

                                                                                                                      A

                                                                                                                      Arsquo

                                                                                                                      B

                                                                                                                      Brsquo

                                                                                                                      C

                                                                                                                      Crsquo

                                                                                                                      1 23

                                                                                                                      45

                                                                                                                      6

                                                                                                                      A Arsquo

                                                                                                                      Source ADest Arsquo

                                                                                                                      MAC addr interface TTL

                                                                                                                      Switch table (initially empty)

                                                                                                                      A 1 60

                                                                                                                      A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

                                                                                                                      frame destination unknownflood

                                                                                                                      Arsquo A

                                                                                                                      destination A location known

                                                                                                                      Arsquo 4 60

                                                                                                                      selective send

                                                                                                                      5 DataLink Layer 5-60

                                                                                                                      Interconnecting switches

                                                                                                                      switches can be connected together

                                                                                                                      A

                                                                                                                      B

                                                                                                                      Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                                      A self learning (works exactly the same as in single-switch case)

                                                                                                                      S1

                                                                                                                      C D

                                                                                                                      E

                                                                                                                      FS2

                                                                                                                      S4

                                                                                                                      S3

                                                                                                                      H

                                                                                                                      I

                                                                                                                      G

                                                                                                                      5 DataLink Layer 5-61

                                                                                                                      Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                      Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                      A

                                                                                                                      B

                                                                                                                      S1

                                                                                                                      C D

                                                                                                                      E

                                                                                                                      FS2

                                                                                                                      S4

                                                                                                                      S3

                                                                                                                      H

                                                                                                                      I

                                                                                                                      G

                                                                                                                      1

                                                                                                                      2 3

                                                                                                                      34

                                                                                                                      5 DataLink Layer 5-62

                                                                                                                      Institutional network

                                                                                                                      to externalnetwork

                                                                                                                      router

                                                                                                                      IP subnet

                                                                                                                      mail server

                                                                                                                      web server

                                                                                                                      5 DataLink Layer 5-63

                                                                                                                      Switches vs Routers both store-and-forward devices

                                                                                                                      o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                      routers maintain routing tables implement routing algorithms

                                                                                                                      switches maintain switch tables implement filtering learning algorithms

                                                                                                                      Switch

                                                                                                                      5 DataLink Layer 5-64

                                                                                                                      Link Layer

                                                                                                                      51 Introduction and services

                                                                                                                      52 Error detection and correction

                                                                                                                      53Multiple access protocols

                                                                                                                      54 Link-Layer Addressing

                                                                                                                      55 Ethernet

                                                                                                                      56 Link-layer switches 59 A day in the life of

                                                                                                                      a web request

                                                                                                                      5 DataLink Layer 5-65

                                                                                                                      Synthesis a day in the life of a web request

                                                                                                                      journey down protocol stack completeo application transport network link

                                                                                                                      putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                      (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                      o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                      5 DataLink Layer 5-66

                                                                                                                      A day in the life scenario

                                                                                                                      Comcast network 68800013

                                                                                                                      Googlersquos network 64233160019 64233169105

                                                                                                                      web server

                                                                                                                      DNS server

                                                                                                                      school network 68802024

                                                                                                                      browser

                                                                                                                      web page

                                                                                                                      5 DataLink Layer 5-67

                                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                      router(runs DHCP)

                                                                                                                      DHCPUDP

                                                                                                                      IPEthPhy

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCPUDP

                                                                                                                      IPEthPhy

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCPDHCP

                                                                                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                      5 DataLink Layer 5-68

                                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                      router(runs DHCP)

                                                                                                                      DHCPUDP

                                                                                                                      IPEthPhy

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCPUDP

                                                                                                                      IPEthPhy

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      DHCP

                                                                                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                      DHCP client receives DHCP ACK reply

                                                                                                                      5 DataLink Layer 5-69

                                                                                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                      DNSUDP

                                                                                                                      IPEthPhy

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                      ARP query

                                                                                                                      EthPhy

                                                                                                                      ARP

                                                                                                                      ARP

                                                                                                                      ARP reply

                                                                                                                      5 DataLink Layer 5-70

                                                                                                                      A day in the lifehellip using DNS

                                                                                                                      DNSUDP

                                                                                                                      IPEthPhy

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                      demuxrsquoed to DNS server DNS server replies to

                                                                                                                      client with IP address of wwwgooglecom

                                                                                                                      Comcast network 68800013

                                                                                                                      DNS server

                                                                                                                      DNSUDP

                                                                                                                      IPEthPhy

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      DNS

                                                                                                                      5 DataLink Layer 5-71

                                                                                                                      A day in the lifehellip TCP connection carrying HTTP

                                                                                                                      HTTPTCPIP

                                                                                                                      EthPhy

                                                                                                                      HTTP

                                                                                                                      to send HTTP request client first opens TCP socket to web server

                                                                                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                      TCP connection established

                                                                                                                      64233169105

                                                                                                                      web server

                                                                                                                      SYN

                                                                                                                      SYN

                                                                                                                      SYN

                                                                                                                      SYN

                                                                                                                      TCPIP

                                                                                                                      EthPhy

                                                                                                                      SYN

                                                                                                                      SYN

                                                                                                                      SYN

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      SYNACK

                                                                                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                      5 DataLink Layer 5-72

                                                                                                                      A day in the lifehellip HTTP requestreply

                                                                                                                      HTTPTCPIP

                                                                                                                      EthPhy

                                                                                                                      HTTP

                                                                                                                      HTTP request sent into TCP socket

                                                                                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                      IP datgram containing HTTP reply routed back to client

                                                                                                                      64233169105

                                                                                                                      web server

                                                                                                                      HTTPTCPIP

                                                                                                                      EthPhy

                                                                                                                      web server responds with HTTP reply (containing web page)

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTPHTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      HTTP

                                                                                                                      web page finally () displayed

                                                                                                                      5 DataLink Layer 5-73

                                                                                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                      (except PHY) solid understanding of networking

                                                                                                                      principles practice hellip could stop here hellip but lots of

                                                                                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                      • Link Layer
                                                                                                                      • Link Layer Introduction
                                                                                                                      • Link layer context
                                                                                                                      • Encapsulation
                                                                                                                      • Link Layer Services
                                                                                                                      • Link Layer Services (more)
                                                                                                                      • Where is the link layer implemented
                                                                                                                      • Adaptors Communicating
                                                                                                                      • Slide 9
                                                                                                                      • Parity Checking
                                                                                                                      • Internet checksum (review)
                                                                                                                      • Checksumming Cyclic Redundancy Check
                                                                                                                      • Cyclic Redundancy Check
                                                                                                                      • Slide 14
                                                                                                                      • CRC Example
                                                                                                                      • Slide 16
                                                                                                                      • Multiple Access Links and Protocols
                                                                                                                      • Cable Network Architecture Overview
                                                                                                                      • Multiple Access protocols
                                                                                                                      • Ideal Multiple Access Protocol
                                                                                                                      • MAC Protocols a taxonomy
                                                                                                                      • Channel Partitioning MAC protocols TDMA
                                                                                                                      • Channel Partitioning MAC protocols FDMA
                                                                                                                      • Random Access Protocols
                                                                                                                      • ALOHA
                                                                                                                      • CSMA (Carrier Sense Multiple Access)
                                                                                                                      • CSMA collisions
                                                                                                                      • CSMACD (Collision Detection)
                                                                                                                      • CSMACD collision detection
                                                                                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                      • Slide 31
                                                                                                                      • Slide 32
                                                                                                                      • Summary of MAC protocols
                                                                                                                      • Slide 34
                                                                                                                      • MAC Addresses and ARP
                                                                                                                      • LAN Addresses and ARP
                                                                                                                      • LAN Address (more)
                                                                                                                      • ARP Address Resolution Protocol
                                                                                                                      • ARP protocol Same LAN (network)
                                                                                                                      • Addressing routing to another LAN
                                                                                                                      • Slide 41
                                                                                                                      • Slide 42
                                                                                                                      • Ethernet
                                                                                                                      • Star topology
                                                                                                                      • Ethernet Frame Structure
                                                                                                                      • Ethernet Frame Structure (more)
                                                                                                                      • Ethernet Unreliable connectionless
                                                                                                                      • Ethernet CSMACD algorithm
                                                                                                                      • Ethernetrsquos CSMACD (more)
                                                                                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                      • Manchester encoding
                                                                                                                      • Slide 52
                                                                                                                      • Hubs
                                                                                                                      • Switch
                                                                                                                      • Switch allows multiple simultaneous transmissions
                                                                                                                      • Switch Table
                                                                                                                      • Switch self-learning
                                                                                                                      • Switch frame filteringforwarding
                                                                                                                      • Self-learning forwarding example
                                                                                                                      • Interconnecting switches
                                                                                                                      • Self-learning multi-switch example
                                                                                                                      • Institutional network
                                                                                                                      • Switches vs Routers
                                                                                                                      • Slide 64
                                                                                                                      • Synthesis a day in the life of a web request
                                                                                                                      • A day in the life scenario
                                                                                                                      • A day in the lifehellip connecting to the Internet
                                                                                                                      • Slide 68
                                                                                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                      • A day in the lifehellip using DNS
                                                                                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                      • A day in the lifehellip HTTP requestreply
                                                                                                                      • Chapter 5 letrsquos take a breath

                                                                                                                        5 DataLink Layer 5-60

                                                                                                                        Interconnecting switches

                                                                                                                        switches can be connected together

                                                                                                                        A

                                                                                                                        B

                                                                                                                        Q sending from A to I - how does S1 know to forward frame destined to F via S4 and S3

                                                                                                                        A self learning (works exactly the same as in single-switch case)

                                                                                                                        S1

                                                                                                                        C D

                                                                                                                        E

                                                                                                                        FS2

                                                                                                                        S4

                                                                                                                        S3

                                                                                                                        H

                                                                                                                        I

                                                                                                                        G

                                                                                                                        5 DataLink Layer 5-61

                                                                                                                        Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                        Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                        A

                                                                                                                        B

                                                                                                                        S1

                                                                                                                        C D

                                                                                                                        E

                                                                                                                        FS2

                                                                                                                        S4

                                                                                                                        S3

                                                                                                                        H

                                                                                                                        I

                                                                                                                        G

                                                                                                                        1

                                                                                                                        2 3

                                                                                                                        34

                                                                                                                        5 DataLink Layer 5-62

                                                                                                                        Institutional network

                                                                                                                        to externalnetwork

                                                                                                                        router

                                                                                                                        IP subnet

                                                                                                                        mail server

                                                                                                                        web server

                                                                                                                        5 DataLink Layer 5-63

                                                                                                                        Switches vs Routers both store-and-forward devices

                                                                                                                        o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                        routers maintain routing tables implement routing algorithms

                                                                                                                        switches maintain switch tables implement filtering learning algorithms

                                                                                                                        Switch

                                                                                                                        5 DataLink Layer 5-64

                                                                                                                        Link Layer

                                                                                                                        51 Introduction and services

                                                                                                                        52 Error detection and correction

                                                                                                                        53Multiple access protocols

                                                                                                                        54 Link-Layer Addressing

                                                                                                                        55 Ethernet

                                                                                                                        56 Link-layer switches 59 A day in the life of

                                                                                                                        a web request

                                                                                                                        5 DataLink Layer 5-65

                                                                                                                        Synthesis a day in the life of a web request

                                                                                                                        journey down protocol stack completeo application transport network link

                                                                                                                        putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                        (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                        o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                        5 DataLink Layer 5-66

                                                                                                                        A day in the life scenario

                                                                                                                        Comcast network 68800013

                                                                                                                        Googlersquos network 64233160019 64233169105

                                                                                                                        web server

                                                                                                                        DNS server

                                                                                                                        school network 68802024

                                                                                                                        browser

                                                                                                                        web page

                                                                                                                        5 DataLink Layer 5-67

                                                                                                                        A day in the lifehellip connecting to the Internet

                                                                                                                        connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                        router(runs DHCP)

                                                                                                                        DHCPUDP

                                                                                                                        IPEthPhy

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCPUDP

                                                                                                                        IPEthPhy

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCPDHCP

                                                                                                                        DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                        Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                        5 DataLink Layer 5-68

                                                                                                                        A day in the lifehellip connecting to the Internet

                                                                                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                        router(runs DHCP)

                                                                                                                        DHCPUDP

                                                                                                                        IPEthPhy

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCPUDP

                                                                                                                        IPEthPhy

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        DHCP

                                                                                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                        DHCP client receives DHCP ACK reply

                                                                                                                        5 DataLink Layer 5-69

                                                                                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                        DNSUDP

                                                                                                                        IPEthPhy

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                        ARP query

                                                                                                                        EthPhy

                                                                                                                        ARP

                                                                                                                        ARP

                                                                                                                        ARP reply

                                                                                                                        5 DataLink Layer 5-70

                                                                                                                        A day in the lifehellip using DNS

                                                                                                                        DNSUDP

                                                                                                                        IPEthPhy

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                        demuxrsquoed to DNS server DNS server replies to

                                                                                                                        client with IP address of wwwgooglecom

                                                                                                                        Comcast network 68800013

                                                                                                                        DNS server

                                                                                                                        DNSUDP

                                                                                                                        IPEthPhy

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        DNS

                                                                                                                        5 DataLink Layer 5-71

                                                                                                                        A day in the lifehellip TCP connection carrying HTTP

                                                                                                                        HTTPTCPIP

                                                                                                                        EthPhy

                                                                                                                        HTTP

                                                                                                                        to send HTTP request client first opens TCP socket to web server

                                                                                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                        TCP connection established

                                                                                                                        64233169105

                                                                                                                        web server

                                                                                                                        SYN

                                                                                                                        SYN

                                                                                                                        SYN

                                                                                                                        SYN

                                                                                                                        TCPIP

                                                                                                                        EthPhy

                                                                                                                        SYN

                                                                                                                        SYN

                                                                                                                        SYN

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        SYNACK

                                                                                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                        5 DataLink Layer 5-72

                                                                                                                        A day in the lifehellip HTTP requestreply

                                                                                                                        HTTPTCPIP

                                                                                                                        EthPhy

                                                                                                                        HTTP

                                                                                                                        HTTP request sent into TCP socket

                                                                                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                        IP datgram containing HTTP reply routed back to client

                                                                                                                        64233169105

                                                                                                                        web server

                                                                                                                        HTTPTCPIP

                                                                                                                        EthPhy

                                                                                                                        web server responds with HTTP reply (containing web page)

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTPHTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        HTTP

                                                                                                                        web page finally () displayed

                                                                                                                        5 DataLink Layer 5-73

                                                                                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                        (except PHY) solid understanding of networking

                                                                                                                        principles practice hellip could stop here hellip but lots of

                                                                                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                        • Link Layer
                                                                                                                        • Link Layer Introduction
                                                                                                                        • Link layer context
                                                                                                                        • Encapsulation
                                                                                                                        • Link Layer Services
                                                                                                                        • Link Layer Services (more)
                                                                                                                        • Where is the link layer implemented
                                                                                                                        • Adaptors Communicating
                                                                                                                        • Slide 9
                                                                                                                        • Parity Checking
                                                                                                                        • Internet checksum (review)
                                                                                                                        • Checksumming Cyclic Redundancy Check
                                                                                                                        • Cyclic Redundancy Check
                                                                                                                        • Slide 14
                                                                                                                        • CRC Example
                                                                                                                        • Slide 16
                                                                                                                        • Multiple Access Links and Protocols
                                                                                                                        • Cable Network Architecture Overview
                                                                                                                        • Multiple Access protocols
                                                                                                                        • Ideal Multiple Access Protocol
                                                                                                                        • MAC Protocols a taxonomy
                                                                                                                        • Channel Partitioning MAC protocols TDMA
                                                                                                                        • Channel Partitioning MAC protocols FDMA
                                                                                                                        • Random Access Protocols
                                                                                                                        • ALOHA
                                                                                                                        • CSMA (Carrier Sense Multiple Access)
                                                                                                                        • CSMA collisions
                                                                                                                        • CSMACD (Collision Detection)
                                                                                                                        • CSMACD collision detection
                                                                                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                        • Slide 31
                                                                                                                        • Slide 32
                                                                                                                        • Summary of MAC protocols
                                                                                                                        • Slide 34
                                                                                                                        • MAC Addresses and ARP
                                                                                                                        • LAN Addresses and ARP
                                                                                                                        • LAN Address (more)
                                                                                                                        • ARP Address Resolution Protocol
                                                                                                                        • ARP protocol Same LAN (network)
                                                                                                                        • Addressing routing to another LAN
                                                                                                                        • Slide 41
                                                                                                                        • Slide 42
                                                                                                                        • Ethernet
                                                                                                                        • Star topology
                                                                                                                        • Ethernet Frame Structure
                                                                                                                        • Ethernet Frame Structure (more)
                                                                                                                        • Ethernet Unreliable connectionless
                                                                                                                        • Ethernet CSMACD algorithm
                                                                                                                        • Ethernetrsquos CSMACD (more)
                                                                                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                        • Manchester encoding
                                                                                                                        • Slide 52
                                                                                                                        • Hubs
                                                                                                                        • Switch
                                                                                                                        • Switch allows multiple simultaneous transmissions
                                                                                                                        • Switch Table
                                                                                                                        • Switch self-learning
                                                                                                                        • Switch frame filteringforwarding
                                                                                                                        • Self-learning forwarding example
                                                                                                                        • Interconnecting switches
                                                                                                                        • Self-learning multi-switch example
                                                                                                                        • Institutional network
                                                                                                                        • Switches vs Routers
                                                                                                                        • Slide 64
                                                                                                                        • Synthesis a day in the life of a web request
                                                                                                                        • A day in the life scenario
                                                                                                                        • A day in the lifehellip connecting to the Internet
                                                                                                                        • Slide 68
                                                                                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                        • A day in the lifehellip using DNS
                                                                                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                        • A day in the lifehellip HTTP requestreply
                                                                                                                        • Chapter 5 letrsquos take a breath

                                                                                                                          5 DataLink Layer 5-61

                                                                                                                          Self-learning multi-switch exampleSuppose A sends frame to I I responds to A

                                                                                                                          Q show switch tables and packet forwarding in S1 S2 S3 S4

                                                                                                                          A

                                                                                                                          B

                                                                                                                          S1

                                                                                                                          C D

                                                                                                                          E

                                                                                                                          FS2

                                                                                                                          S4

                                                                                                                          S3

                                                                                                                          H

                                                                                                                          I

                                                                                                                          G

                                                                                                                          1

                                                                                                                          2 3

                                                                                                                          34

                                                                                                                          5 DataLink Layer 5-62

                                                                                                                          Institutional network

                                                                                                                          to externalnetwork

                                                                                                                          router

                                                                                                                          IP subnet

                                                                                                                          mail server

                                                                                                                          web server

                                                                                                                          5 DataLink Layer 5-63

                                                                                                                          Switches vs Routers both store-and-forward devices

                                                                                                                          o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                          routers maintain routing tables implement routing algorithms

                                                                                                                          switches maintain switch tables implement filtering learning algorithms

                                                                                                                          Switch

                                                                                                                          5 DataLink Layer 5-64

                                                                                                                          Link Layer

                                                                                                                          51 Introduction and services

                                                                                                                          52 Error detection and correction

                                                                                                                          53Multiple access protocols

                                                                                                                          54 Link-Layer Addressing

                                                                                                                          55 Ethernet

                                                                                                                          56 Link-layer switches 59 A day in the life of

                                                                                                                          a web request

                                                                                                                          5 DataLink Layer 5-65

                                                                                                                          Synthesis a day in the life of a web request

                                                                                                                          journey down protocol stack completeo application transport network link

                                                                                                                          putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                          (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                          o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                          5 DataLink Layer 5-66

                                                                                                                          A day in the life scenario

                                                                                                                          Comcast network 68800013

                                                                                                                          Googlersquos network 64233160019 64233169105

                                                                                                                          web server

                                                                                                                          DNS server

                                                                                                                          school network 68802024

                                                                                                                          browser

                                                                                                                          web page

                                                                                                                          5 DataLink Layer 5-67

                                                                                                                          A day in the lifehellip connecting to the Internet

                                                                                                                          connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                          router(runs DHCP)

                                                                                                                          DHCPUDP

                                                                                                                          IPEthPhy

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCPUDP

                                                                                                                          IPEthPhy

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCPDHCP

                                                                                                                          DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                          Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                          5 DataLink Layer 5-68

                                                                                                                          A day in the lifehellip connecting to the Internet

                                                                                                                          DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                          router(runs DHCP)

                                                                                                                          DHCPUDP

                                                                                                                          IPEthPhy

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCPUDP

                                                                                                                          IPEthPhy

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          DHCP

                                                                                                                          encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                          Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                          DHCP client receives DHCP ACK reply

                                                                                                                          5 DataLink Layer 5-69

                                                                                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                          DNSUDP

                                                                                                                          IPEthPhy

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                          ARP query

                                                                                                                          EthPhy

                                                                                                                          ARP

                                                                                                                          ARP

                                                                                                                          ARP reply

                                                                                                                          5 DataLink Layer 5-70

                                                                                                                          A day in the lifehellip using DNS

                                                                                                                          DNSUDP

                                                                                                                          IPEthPhy

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                          demuxrsquoed to DNS server DNS server replies to

                                                                                                                          client with IP address of wwwgooglecom

                                                                                                                          Comcast network 68800013

                                                                                                                          DNS server

                                                                                                                          DNSUDP

                                                                                                                          IPEthPhy

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          DNS

                                                                                                                          5 DataLink Layer 5-71

                                                                                                                          A day in the lifehellip TCP connection carrying HTTP

                                                                                                                          HTTPTCPIP

                                                                                                                          EthPhy

                                                                                                                          HTTP

                                                                                                                          to send HTTP request client first opens TCP socket to web server

                                                                                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                          TCP connection established

                                                                                                                          64233169105

                                                                                                                          web server

                                                                                                                          SYN

                                                                                                                          SYN

                                                                                                                          SYN

                                                                                                                          SYN

                                                                                                                          TCPIP

                                                                                                                          EthPhy

                                                                                                                          SYN

                                                                                                                          SYN

                                                                                                                          SYN

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          SYNACK

                                                                                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                          5 DataLink Layer 5-72

                                                                                                                          A day in the lifehellip HTTP requestreply

                                                                                                                          HTTPTCPIP

                                                                                                                          EthPhy

                                                                                                                          HTTP

                                                                                                                          HTTP request sent into TCP socket

                                                                                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                          IP datgram containing HTTP reply routed back to client

                                                                                                                          64233169105

                                                                                                                          web server

                                                                                                                          HTTPTCPIP

                                                                                                                          EthPhy

                                                                                                                          web server responds with HTTP reply (containing web page)

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTPHTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          HTTP

                                                                                                                          web page finally () displayed

                                                                                                                          5 DataLink Layer 5-73

                                                                                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                          (except PHY) solid understanding of networking

                                                                                                                          principles practice hellip could stop here hellip but lots of

                                                                                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                          • Link Layer
                                                                                                                          • Link Layer Introduction
                                                                                                                          • Link layer context
                                                                                                                          • Encapsulation
                                                                                                                          • Link Layer Services
                                                                                                                          • Link Layer Services (more)
                                                                                                                          • Where is the link layer implemented
                                                                                                                          • Adaptors Communicating
                                                                                                                          • Slide 9
                                                                                                                          • Parity Checking
                                                                                                                          • Internet checksum (review)
                                                                                                                          • Checksumming Cyclic Redundancy Check
                                                                                                                          • Cyclic Redundancy Check
                                                                                                                          • Slide 14
                                                                                                                          • CRC Example
                                                                                                                          • Slide 16
                                                                                                                          • Multiple Access Links and Protocols
                                                                                                                          • Cable Network Architecture Overview
                                                                                                                          • Multiple Access protocols
                                                                                                                          • Ideal Multiple Access Protocol
                                                                                                                          • MAC Protocols a taxonomy
                                                                                                                          • Channel Partitioning MAC protocols TDMA
                                                                                                                          • Channel Partitioning MAC protocols FDMA
                                                                                                                          • Random Access Protocols
                                                                                                                          • ALOHA
                                                                                                                          • CSMA (Carrier Sense Multiple Access)
                                                                                                                          • CSMA collisions
                                                                                                                          • CSMACD (Collision Detection)
                                                                                                                          • CSMACD collision detection
                                                                                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                          • Slide 31
                                                                                                                          • Slide 32
                                                                                                                          • Summary of MAC protocols
                                                                                                                          • Slide 34
                                                                                                                          • MAC Addresses and ARP
                                                                                                                          • LAN Addresses and ARP
                                                                                                                          • LAN Address (more)
                                                                                                                          • ARP Address Resolution Protocol
                                                                                                                          • ARP protocol Same LAN (network)
                                                                                                                          • Addressing routing to another LAN
                                                                                                                          • Slide 41
                                                                                                                          • Slide 42
                                                                                                                          • Ethernet
                                                                                                                          • Star topology
                                                                                                                          • Ethernet Frame Structure
                                                                                                                          • Ethernet Frame Structure (more)
                                                                                                                          • Ethernet Unreliable connectionless
                                                                                                                          • Ethernet CSMACD algorithm
                                                                                                                          • Ethernetrsquos CSMACD (more)
                                                                                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                          • Manchester encoding
                                                                                                                          • Slide 52
                                                                                                                          • Hubs
                                                                                                                          • Switch
                                                                                                                          • Switch allows multiple simultaneous transmissions
                                                                                                                          • Switch Table
                                                                                                                          • Switch self-learning
                                                                                                                          • Switch frame filteringforwarding
                                                                                                                          • Self-learning forwarding example
                                                                                                                          • Interconnecting switches
                                                                                                                          • Self-learning multi-switch example
                                                                                                                          • Institutional network
                                                                                                                          • Switches vs Routers
                                                                                                                          • Slide 64
                                                                                                                          • Synthesis a day in the life of a web request
                                                                                                                          • A day in the life scenario
                                                                                                                          • A day in the lifehellip connecting to the Internet
                                                                                                                          • Slide 68
                                                                                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                          • A day in the lifehellip using DNS
                                                                                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                          • A day in the lifehellip HTTP requestreply
                                                                                                                          • Chapter 5 letrsquos take a breath

                                                                                                                            5 DataLink Layer 5-62

                                                                                                                            Institutional network

                                                                                                                            to externalnetwork

                                                                                                                            router

                                                                                                                            IP subnet

                                                                                                                            mail server

                                                                                                                            web server

                                                                                                                            5 DataLink Layer 5-63

                                                                                                                            Switches vs Routers both store-and-forward devices

                                                                                                                            o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                            routers maintain routing tables implement routing algorithms

                                                                                                                            switches maintain switch tables implement filtering learning algorithms

                                                                                                                            Switch

                                                                                                                            5 DataLink Layer 5-64

                                                                                                                            Link Layer

                                                                                                                            51 Introduction and services

                                                                                                                            52 Error detection and correction

                                                                                                                            53Multiple access protocols

                                                                                                                            54 Link-Layer Addressing

                                                                                                                            55 Ethernet

                                                                                                                            56 Link-layer switches 59 A day in the life of

                                                                                                                            a web request

                                                                                                                            5 DataLink Layer 5-65

                                                                                                                            Synthesis a day in the life of a web request

                                                                                                                            journey down protocol stack completeo application transport network link

                                                                                                                            putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                            (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                            o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                            5 DataLink Layer 5-66

                                                                                                                            A day in the life scenario

                                                                                                                            Comcast network 68800013

                                                                                                                            Googlersquos network 64233160019 64233169105

                                                                                                                            web server

                                                                                                                            DNS server

                                                                                                                            school network 68802024

                                                                                                                            browser

                                                                                                                            web page

                                                                                                                            5 DataLink Layer 5-67

                                                                                                                            A day in the lifehellip connecting to the Internet

                                                                                                                            connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                            router(runs DHCP)

                                                                                                                            DHCPUDP

                                                                                                                            IPEthPhy

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCPUDP

                                                                                                                            IPEthPhy

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCPDHCP

                                                                                                                            DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                            Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                            5 DataLink Layer 5-68

                                                                                                                            A day in the lifehellip connecting to the Internet

                                                                                                                            DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                            router(runs DHCP)

                                                                                                                            DHCPUDP

                                                                                                                            IPEthPhy

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCPUDP

                                                                                                                            IPEthPhy

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            DHCP

                                                                                                                            encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                            Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                            DHCP client receives DHCP ACK reply

                                                                                                                            5 DataLink Layer 5-69

                                                                                                                            A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                            before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                            DNSUDP

                                                                                                                            IPEthPhy

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                            ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                            ARP query

                                                                                                                            EthPhy

                                                                                                                            ARP

                                                                                                                            ARP

                                                                                                                            ARP reply

                                                                                                                            5 DataLink Layer 5-70

                                                                                                                            A day in the lifehellip using DNS

                                                                                                                            DNSUDP

                                                                                                                            IPEthPhy

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                            demuxrsquoed to DNS server DNS server replies to

                                                                                                                            client with IP address of wwwgooglecom

                                                                                                                            Comcast network 68800013

                                                                                                                            DNS server

                                                                                                                            DNSUDP

                                                                                                                            IPEthPhy

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            DNS

                                                                                                                            5 DataLink Layer 5-71

                                                                                                                            A day in the lifehellip TCP connection carrying HTTP

                                                                                                                            HTTPTCPIP

                                                                                                                            EthPhy

                                                                                                                            HTTP

                                                                                                                            to send HTTP request client first opens TCP socket to web server

                                                                                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                            TCP connection established

                                                                                                                            64233169105

                                                                                                                            web server

                                                                                                                            SYN

                                                                                                                            SYN

                                                                                                                            SYN

                                                                                                                            SYN

                                                                                                                            TCPIP

                                                                                                                            EthPhy

                                                                                                                            SYN

                                                                                                                            SYN

                                                                                                                            SYN

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            SYNACK

                                                                                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                            5 DataLink Layer 5-72

                                                                                                                            A day in the lifehellip HTTP requestreply

                                                                                                                            HTTPTCPIP

                                                                                                                            EthPhy

                                                                                                                            HTTP

                                                                                                                            HTTP request sent into TCP socket

                                                                                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                            IP datgram containing HTTP reply routed back to client

                                                                                                                            64233169105

                                                                                                                            web server

                                                                                                                            HTTPTCPIP

                                                                                                                            EthPhy

                                                                                                                            web server responds with HTTP reply (containing web page)

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTPHTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            HTTP

                                                                                                                            web page finally () displayed

                                                                                                                            5 DataLink Layer 5-73

                                                                                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                            (except PHY) solid understanding of networking

                                                                                                                            principles practice hellip could stop here hellip but lots of

                                                                                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                            • Link Layer
                                                                                                                            • Link Layer Introduction
                                                                                                                            • Link layer context
                                                                                                                            • Encapsulation
                                                                                                                            • Link Layer Services
                                                                                                                            • Link Layer Services (more)
                                                                                                                            • Where is the link layer implemented
                                                                                                                            • Adaptors Communicating
                                                                                                                            • Slide 9
                                                                                                                            • Parity Checking
                                                                                                                            • Internet checksum (review)
                                                                                                                            • Checksumming Cyclic Redundancy Check
                                                                                                                            • Cyclic Redundancy Check
                                                                                                                            • Slide 14
                                                                                                                            • CRC Example
                                                                                                                            • Slide 16
                                                                                                                            • Multiple Access Links and Protocols
                                                                                                                            • Cable Network Architecture Overview
                                                                                                                            • Multiple Access protocols
                                                                                                                            • Ideal Multiple Access Protocol
                                                                                                                            • MAC Protocols a taxonomy
                                                                                                                            • Channel Partitioning MAC protocols TDMA
                                                                                                                            • Channel Partitioning MAC protocols FDMA
                                                                                                                            • Random Access Protocols
                                                                                                                            • ALOHA
                                                                                                                            • CSMA (Carrier Sense Multiple Access)
                                                                                                                            • CSMA collisions
                                                                                                                            • CSMACD (Collision Detection)
                                                                                                                            • CSMACD collision detection
                                                                                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                            • Slide 31
                                                                                                                            • Slide 32
                                                                                                                            • Summary of MAC protocols
                                                                                                                            • Slide 34
                                                                                                                            • MAC Addresses and ARP
                                                                                                                            • LAN Addresses and ARP
                                                                                                                            • LAN Address (more)
                                                                                                                            • ARP Address Resolution Protocol
                                                                                                                            • ARP protocol Same LAN (network)
                                                                                                                            • Addressing routing to another LAN
                                                                                                                            • Slide 41
                                                                                                                            • Slide 42
                                                                                                                            • Ethernet
                                                                                                                            • Star topology
                                                                                                                            • Ethernet Frame Structure
                                                                                                                            • Ethernet Frame Structure (more)
                                                                                                                            • Ethernet Unreliable connectionless
                                                                                                                            • Ethernet CSMACD algorithm
                                                                                                                            • Ethernetrsquos CSMACD (more)
                                                                                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                            • Manchester encoding
                                                                                                                            • Slide 52
                                                                                                                            • Hubs
                                                                                                                            • Switch
                                                                                                                            • Switch allows multiple simultaneous transmissions
                                                                                                                            • Switch Table
                                                                                                                            • Switch self-learning
                                                                                                                            • Switch frame filteringforwarding
                                                                                                                            • Self-learning forwarding example
                                                                                                                            • Interconnecting switches
                                                                                                                            • Self-learning multi-switch example
                                                                                                                            • Institutional network
                                                                                                                            • Switches vs Routers
                                                                                                                            • Slide 64
                                                                                                                            • Synthesis a day in the life of a web request
                                                                                                                            • A day in the life scenario
                                                                                                                            • A day in the lifehellip connecting to the Internet
                                                                                                                            • Slide 68
                                                                                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                            • A day in the lifehellip using DNS
                                                                                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                            • A day in the lifehellip HTTP requestreply
                                                                                                                            • Chapter 5 letrsquos take a breath

                                                                                                                              5 DataLink Layer 5-63

                                                                                                                              Switches vs Routers both store-and-forward devices

                                                                                                                              o routers network layer devices (examine network layer headers)o switches are link layer devices

                                                                                                                              routers maintain routing tables implement routing algorithms

                                                                                                                              switches maintain switch tables implement filtering learning algorithms

                                                                                                                              Switch

                                                                                                                              5 DataLink Layer 5-64

                                                                                                                              Link Layer

                                                                                                                              51 Introduction and services

                                                                                                                              52 Error detection and correction

                                                                                                                              53Multiple access protocols

                                                                                                                              54 Link-Layer Addressing

                                                                                                                              55 Ethernet

                                                                                                                              56 Link-layer switches 59 A day in the life of

                                                                                                                              a web request

                                                                                                                              5 DataLink Layer 5-65

                                                                                                                              Synthesis a day in the life of a web request

                                                                                                                              journey down protocol stack completeo application transport network link

                                                                                                                              putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                              (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                              o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                              5 DataLink Layer 5-66

                                                                                                                              A day in the life scenario

                                                                                                                              Comcast network 68800013

                                                                                                                              Googlersquos network 64233160019 64233169105

                                                                                                                              web server

                                                                                                                              DNS server

                                                                                                                              school network 68802024

                                                                                                                              browser

                                                                                                                              web page

                                                                                                                              5 DataLink Layer 5-67

                                                                                                                              A day in the lifehellip connecting to the Internet

                                                                                                                              connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                              router(runs DHCP)

                                                                                                                              DHCPUDP

                                                                                                                              IPEthPhy

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCPUDP

                                                                                                                              IPEthPhy

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCPDHCP

                                                                                                                              DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                              Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                              5 DataLink Layer 5-68

                                                                                                                              A day in the lifehellip connecting to the Internet

                                                                                                                              DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                              router(runs DHCP)

                                                                                                                              DHCPUDP

                                                                                                                              IPEthPhy

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCPUDP

                                                                                                                              IPEthPhy

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              DHCP

                                                                                                                              encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                              Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                              DHCP client receives DHCP ACK reply

                                                                                                                              5 DataLink Layer 5-69

                                                                                                                              A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                              before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                              DNSUDP

                                                                                                                              IPEthPhy

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                              ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                              ARP query

                                                                                                                              EthPhy

                                                                                                                              ARP

                                                                                                                              ARP

                                                                                                                              ARP reply

                                                                                                                              5 DataLink Layer 5-70

                                                                                                                              A day in the lifehellip using DNS

                                                                                                                              DNSUDP

                                                                                                                              IPEthPhy

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                              IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                              demuxrsquoed to DNS server DNS server replies to

                                                                                                                              client with IP address of wwwgooglecom

                                                                                                                              Comcast network 68800013

                                                                                                                              DNS server

                                                                                                                              DNSUDP

                                                                                                                              IPEthPhy

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              DNS

                                                                                                                              5 DataLink Layer 5-71

                                                                                                                              A day in the lifehellip TCP connection carrying HTTP

                                                                                                                              HTTPTCPIP

                                                                                                                              EthPhy

                                                                                                                              HTTP

                                                                                                                              to send HTTP request client first opens TCP socket to web server

                                                                                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                              TCP connection established

                                                                                                                              64233169105

                                                                                                                              web server

                                                                                                                              SYN

                                                                                                                              SYN

                                                                                                                              SYN

                                                                                                                              SYN

                                                                                                                              TCPIP

                                                                                                                              EthPhy

                                                                                                                              SYN

                                                                                                                              SYN

                                                                                                                              SYN

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              SYNACK

                                                                                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                              5 DataLink Layer 5-72

                                                                                                                              A day in the lifehellip HTTP requestreply

                                                                                                                              HTTPTCPIP

                                                                                                                              EthPhy

                                                                                                                              HTTP

                                                                                                                              HTTP request sent into TCP socket

                                                                                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                              IP datgram containing HTTP reply routed back to client

                                                                                                                              64233169105

                                                                                                                              web server

                                                                                                                              HTTPTCPIP

                                                                                                                              EthPhy

                                                                                                                              web server responds with HTTP reply (containing web page)

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTPHTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              HTTP

                                                                                                                              web page finally () displayed

                                                                                                                              5 DataLink Layer 5-73

                                                                                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                              (except PHY) solid understanding of networking

                                                                                                                              principles practice hellip could stop here hellip but lots of

                                                                                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                              • Link Layer
                                                                                                                              • Link Layer Introduction
                                                                                                                              • Link layer context
                                                                                                                              • Encapsulation
                                                                                                                              • Link Layer Services
                                                                                                                              • Link Layer Services (more)
                                                                                                                              • Where is the link layer implemented
                                                                                                                              • Adaptors Communicating
                                                                                                                              • Slide 9
                                                                                                                              • Parity Checking
                                                                                                                              • Internet checksum (review)
                                                                                                                              • Checksumming Cyclic Redundancy Check
                                                                                                                              • Cyclic Redundancy Check
                                                                                                                              • Slide 14
                                                                                                                              • CRC Example
                                                                                                                              • Slide 16
                                                                                                                              • Multiple Access Links and Protocols
                                                                                                                              • Cable Network Architecture Overview
                                                                                                                              • Multiple Access protocols
                                                                                                                              • Ideal Multiple Access Protocol
                                                                                                                              • MAC Protocols a taxonomy
                                                                                                                              • Channel Partitioning MAC protocols TDMA
                                                                                                                              • Channel Partitioning MAC protocols FDMA
                                                                                                                              • Random Access Protocols
                                                                                                                              • ALOHA
                                                                                                                              • CSMA (Carrier Sense Multiple Access)
                                                                                                                              • CSMA collisions
                                                                                                                              • CSMACD (Collision Detection)
                                                                                                                              • CSMACD collision detection
                                                                                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                              • Slide 31
                                                                                                                              • Slide 32
                                                                                                                              • Summary of MAC protocols
                                                                                                                              • Slide 34
                                                                                                                              • MAC Addresses and ARP
                                                                                                                              • LAN Addresses and ARP
                                                                                                                              • LAN Address (more)
                                                                                                                              • ARP Address Resolution Protocol
                                                                                                                              • ARP protocol Same LAN (network)
                                                                                                                              • Addressing routing to another LAN
                                                                                                                              • Slide 41
                                                                                                                              • Slide 42
                                                                                                                              • Ethernet
                                                                                                                              • Star topology
                                                                                                                              • Ethernet Frame Structure
                                                                                                                              • Ethernet Frame Structure (more)
                                                                                                                              • Ethernet Unreliable connectionless
                                                                                                                              • Ethernet CSMACD algorithm
                                                                                                                              • Ethernetrsquos CSMACD (more)
                                                                                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                              • Manchester encoding
                                                                                                                              • Slide 52
                                                                                                                              • Hubs
                                                                                                                              • Switch
                                                                                                                              • Switch allows multiple simultaneous transmissions
                                                                                                                              • Switch Table
                                                                                                                              • Switch self-learning
                                                                                                                              • Switch frame filteringforwarding
                                                                                                                              • Self-learning forwarding example
                                                                                                                              • Interconnecting switches
                                                                                                                              • Self-learning multi-switch example
                                                                                                                              • Institutional network
                                                                                                                              • Switches vs Routers
                                                                                                                              • Slide 64
                                                                                                                              • Synthesis a day in the life of a web request
                                                                                                                              • A day in the life scenario
                                                                                                                              • A day in the lifehellip connecting to the Internet
                                                                                                                              • Slide 68
                                                                                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                              • A day in the lifehellip using DNS
                                                                                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                              • A day in the lifehellip HTTP requestreply
                                                                                                                              • Chapter 5 letrsquos take a breath

                                                                                                                                5 DataLink Layer 5-64

                                                                                                                                Link Layer

                                                                                                                                51 Introduction and services

                                                                                                                                52 Error detection and correction

                                                                                                                                53Multiple access protocols

                                                                                                                                54 Link-Layer Addressing

                                                                                                                                55 Ethernet

                                                                                                                                56 Link-layer switches 59 A day in the life of

                                                                                                                                a web request

                                                                                                                                5 DataLink Layer 5-65

                                                                                                                                Synthesis a day in the life of a web request

                                                                                                                                journey down protocol stack completeo application transport network link

                                                                                                                                putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                                (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                                o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                                5 DataLink Layer 5-66

                                                                                                                                A day in the life scenario

                                                                                                                                Comcast network 68800013

                                                                                                                                Googlersquos network 64233160019 64233169105

                                                                                                                                web server

                                                                                                                                DNS server

                                                                                                                                school network 68802024

                                                                                                                                browser

                                                                                                                                web page

                                                                                                                                5 DataLink Layer 5-67

                                                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                                                connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                                router(runs DHCP)

                                                                                                                                DHCPUDP

                                                                                                                                IPEthPhy

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCPUDP

                                                                                                                                IPEthPhy

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCPDHCP

                                                                                                                                DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                                Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                                5 DataLink Layer 5-68

                                                                                                                                A day in the lifehellip connecting to the Internet

                                                                                                                                DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                                router(runs DHCP)

                                                                                                                                DHCPUDP

                                                                                                                                IPEthPhy

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCPUDP

                                                                                                                                IPEthPhy

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                DHCP

                                                                                                                                encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                                Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                                DHCP client receives DHCP ACK reply

                                                                                                                                5 DataLink Layer 5-69

                                                                                                                                A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                DNSUDP

                                                                                                                                IPEthPhy

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                ARP query

                                                                                                                                EthPhy

                                                                                                                                ARP

                                                                                                                                ARP

                                                                                                                                ARP reply

                                                                                                                                5 DataLink Layer 5-70

                                                                                                                                A day in the lifehellip using DNS

                                                                                                                                DNSUDP

                                                                                                                                IPEthPhy

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                demuxrsquoed to DNS server DNS server replies to

                                                                                                                                client with IP address of wwwgooglecom

                                                                                                                                Comcast network 68800013

                                                                                                                                DNS server

                                                                                                                                DNSUDP

                                                                                                                                IPEthPhy

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                DNS

                                                                                                                                5 DataLink Layer 5-71

                                                                                                                                A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                HTTPTCPIP

                                                                                                                                EthPhy

                                                                                                                                HTTP

                                                                                                                                to send HTTP request client first opens TCP socket to web server

                                                                                                                                TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                TCP connection established

                                                                                                                                64233169105

                                                                                                                                web server

                                                                                                                                SYN

                                                                                                                                SYN

                                                                                                                                SYN

                                                                                                                                SYN

                                                                                                                                TCPIP

                                                                                                                                EthPhy

                                                                                                                                SYN

                                                                                                                                SYN

                                                                                                                                SYN

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                SYNACK

                                                                                                                                web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                5 DataLink Layer 5-72

                                                                                                                                A day in the lifehellip HTTP requestreply

                                                                                                                                HTTPTCPIP

                                                                                                                                EthPhy

                                                                                                                                HTTP

                                                                                                                                HTTP request sent into TCP socket

                                                                                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                IP datgram containing HTTP reply routed back to client

                                                                                                                                64233169105

                                                                                                                                web server

                                                                                                                                HTTPTCPIP

                                                                                                                                EthPhy

                                                                                                                                web server responds with HTTP reply (containing web page)

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTPHTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                HTTP

                                                                                                                                web page finally () displayed

                                                                                                                                5 DataLink Layer 5-73

                                                                                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                (except PHY) solid understanding of networking

                                                                                                                                principles practice hellip could stop here hellip but lots of

                                                                                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                • Link Layer
                                                                                                                                • Link Layer Introduction
                                                                                                                                • Link layer context
                                                                                                                                • Encapsulation
                                                                                                                                • Link Layer Services
                                                                                                                                • Link Layer Services (more)
                                                                                                                                • Where is the link layer implemented
                                                                                                                                • Adaptors Communicating
                                                                                                                                • Slide 9
                                                                                                                                • Parity Checking
                                                                                                                                • Internet checksum (review)
                                                                                                                                • Checksumming Cyclic Redundancy Check
                                                                                                                                • Cyclic Redundancy Check
                                                                                                                                • Slide 14
                                                                                                                                • CRC Example
                                                                                                                                • Slide 16
                                                                                                                                • Multiple Access Links and Protocols
                                                                                                                                • Cable Network Architecture Overview
                                                                                                                                • Multiple Access protocols
                                                                                                                                • Ideal Multiple Access Protocol
                                                                                                                                • MAC Protocols a taxonomy
                                                                                                                                • Channel Partitioning MAC protocols TDMA
                                                                                                                                • Channel Partitioning MAC protocols FDMA
                                                                                                                                • Random Access Protocols
                                                                                                                                • ALOHA
                                                                                                                                • CSMA (Carrier Sense Multiple Access)
                                                                                                                                • CSMA collisions
                                                                                                                                • CSMACD (Collision Detection)
                                                                                                                                • CSMACD collision detection
                                                                                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                • Slide 31
                                                                                                                                • Slide 32
                                                                                                                                • Summary of MAC protocols
                                                                                                                                • Slide 34
                                                                                                                                • MAC Addresses and ARP
                                                                                                                                • LAN Addresses and ARP
                                                                                                                                • LAN Address (more)
                                                                                                                                • ARP Address Resolution Protocol
                                                                                                                                • ARP protocol Same LAN (network)
                                                                                                                                • Addressing routing to another LAN
                                                                                                                                • Slide 41
                                                                                                                                • Slide 42
                                                                                                                                • Ethernet
                                                                                                                                • Star topology
                                                                                                                                • Ethernet Frame Structure
                                                                                                                                • Ethernet Frame Structure (more)
                                                                                                                                • Ethernet Unreliable connectionless
                                                                                                                                • Ethernet CSMACD algorithm
                                                                                                                                • Ethernetrsquos CSMACD (more)
                                                                                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                • Manchester encoding
                                                                                                                                • Slide 52
                                                                                                                                • Hubs
                                                                                                                                • Switch
                                                                                                                                • Switch allows multiple simultaneous transmissions
                                                                                                                                • Switch Table
                                                                                                                                • Switch self-learning
                                                                                                                                • Switch frame filteringforwarding
                                                                                                                                • Self-learning forwarding example
                                                                                                                                • Interconnecting switches
                                                                                                                                • Self-learning multi-switch example
                                                                                                                                • Institutional network
                                                                                                                                • Switches vs Routers
                                                                                                                                • Slide 64
                                                                                                                                • Synthesis a day in the life of a web request
                                                                                                                                • A day in the life scenario
                                                                                                                                • A day in the lifehellip connecting to the Internet
                                                                                                                                • Slide 68
                                                                                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                • A day in the lifehellip using DNS
                                                                                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                • A day in the lifehellip HTTP requestreply
                                                                                                                                • Chapter 5 letrsquos take a breath

                                                                                                                                  5 DataLink Layer 5-65

                                                                                                                                  Synthesis a day in the life of a web request

                                                                                                                                  journey down protocol stack completeo application transport network link

                                                                                                                                  putting-it-all-together synthesiso goal identify review understand protocols

                                                                                                                                  (at all layers) involved in seemingly simple scenario requesting www page

                                                                                                                                  o scenario student attaches laptop to campus network requestsreceives wwwgooglecom

                                                                                                                                  5 DataLink Layer 5-66

                                                                                                                                  A day in the life scenario

                                                                                                                                  Comcast network 68800013

                                                                                                                                  Googlersquos network 64233160019 64233169105

                                                                                                                                  web server

                                                                                                                                  DNS server

                                                                                                                                  school network 68802024

                                                                                                                                  browser

                                                                                                                                  web page

                                                                                                                                  5 DataLink Layer 5-67

                                                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                                                  connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                                  router(runs DHCP)

                                                                                                                                  DHCPUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCPUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCPDHCP

                                                                                                                                  DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                                  Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                                  5 DataLink Layer 5-68

                                                                                                                                  A day in the lifehellip connecting to the Internet

                                                                                                                                  DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                                  router(runs DHCP)

                                                                                                                                  DHCPUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCPUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  DHCP

                                                                                                                                  encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                                  Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                                  DHCP client receives DHCP ACK reply

                                                                                                                                  5 DataLink Layer 5-69

                                                                                                                                  A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                  before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                  DNSUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                  ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                  ARP query

                                                                                                                                  EthPhy

                                                                                                                                  ARP

                                                                                                                                  ARP

                                                                                                                                  ARP reply

                                                                                                                                  5 DataLink Layer 5-70

                                                                                                                                  A day in the lifehellip using DNS

                                                                                                                                  DNSUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                  IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                  demuxrsquoed to DNS server DNS server replies to

                                                                                                                                  client with IP address of wwwgooglecom

                                                                                                                                  Comcast network 68800013

                                                                                                                                  DNS server

                                                                                                                                  DNSUDP

                                                                                                                                  IPEthPhy

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  DNS

                                                                                                                                  5 DataLink Layer 5-71

                                                                                                                                  A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                  HTTPTCPIP

                                                                                                                                  EthPhy

                                                                                                                                  HTTP

                                                                                                                                  to send HTTP request client first opens TCP socket to web server

                                                                                                                                  TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                  TCP connection established

                                                                                                                                  64233169105

                                                                                                                                  web server

                                                                                                                                  SYN

                                                                                                                                  SYN

                                                                                                                                  SYN

                                                                                                                                  SYN

                                                                                                                                  TCPIP

                                                                                                                                  EthPhy

                                                                                                                                  SYN

                                                                                                                                  SYN

                                                                                                                                  SYN

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  SYNACK

                                                                                                                                  web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                  5 DataLink Layer 5-72

                                                                                                                                  A day in the lifehellip HTTP requestreply

                                                                                                                                  HTTPTCPIP

                                                                                                                                  EthPhy

                                                                                                                                  HTTP

                                                                                                                                  HTTP request sent into TCP socket

                                                                                                                                  IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                  IP datgram containing HTTP reply routed back to client

                                                                                                                                  64233169105

                                                                                                                                  web server

                                                                                                                                  HTTPTCPIP

                                                                                                                                  EthPhy

                                                                                                                                  web server responds with HTTP reply (containing web page)

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTPHTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  HTTP

                                                                                                                                  web page finally () displayed

                                                                                                                                  5 DataLink Layer 5-73

                                                                                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                  (except PHY) solid understanding of networking

                                                                                                                                  principles practice hellip could stop here hellip but lots of

                                                                                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                  • Link Layer
                                                                                                                                  • Link Layer Introduction
                                                                                                                                  • Link layer context
                                                                                                                                  • Encapsulation
                                                                                                                                  • Link Layer Services
                                                                                                                                  • Link Layer Services (more)
                                                                                                                                  • Where is the link layer implemented
                                                                                                                                  • Adaptors Communicating
                                                                                                                                  • Slide 9
                                                                                                                                  • Parity Checking
                                                                                                                                  • Internet checksum (review)
                                                                                                                                  • Checksumming Cyclic Redundancy Check
                                                                                                                                  • Cyclic Redundancy Check
                                                                                                                                  • Slide 14
                                                                                                                                  • CRC Example
                                                                                                                                  • Slide 16
                                                                                                                                  • Multiple Access Links and Protocols
                                                                                                                                  • Cable Network Architecture Overview
                                                                                                                                  • Multiple Access protocols
                                                                                                                                  • Ideal Multiple Access Protocol
                                                                                                                                  • MAC Protocols a taxonomy
                                                                                                                                  • Channel Partitioning MAC protocols TDMA
                                                                                                                                  • Channel Partitioning MAC protocols FDMA
                                                                                                                                  • Random Access Protocols
                                                                                                                                  • ALOHA
                                                                                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                                                                                  • CSMA collisions
                                                                                                                                  • CSMACD (Collision Detection)
                                                                                                                                  • CSMACD collision detection
                                                                                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                  • Slide 31
                                                                                                                                  • Slide 32
                                                                                                                                  • Summary of MAC protocols
                                                                                                                                  • Slide 34
                                                                                                                                  • MAC Addresses and ARP
                                                                                                                                  • LAN Addresses and ARP
                                                                                                                                  • LAN Address (more)
                                                                                                                                  • ARP Address Resolution Protocol
                                                                                                                                  • ARP protocol Same LAN (network)
                                                                                                                                  • Addressing routing to another LAN
                                                                                                                                  • Slide 41
                                                                                                                                  • Slide 42
                                                                                                                                  • Ethernet
                                                                                                                                  • Star topology
                                                                                                                                  • Ethernet Frame Structure
                                                                                                                                  • Ethernet Frame Structure (more)
                                                                                                                                  • Ethernet Unreliable connectionless
                                                                                                                                  • Ethernet CSMACD algorithm
                                                                                                                                  • Ethernetrsquos CSMACD (more)
                                                                                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                  • Manchester encoding
                                                                                                                                  • Slide 52
                                                                                                                                  • Hubs
                                                                                                                                  • Switch
                                                                                                                                  • Switch allows multiple simultaneous transmissions
                                                                                                                                  • Switch Table
                                                                                                                                  • Switch self-learning
                                                                                                                                  • Switch frame filteringforwarding
                                                                                                                                  • Self-learning forwarding example
                                                                                                                                  • Interconnecting switches
                                                                                                                                  • Self-learning multi-switch example
                                                                                                                                  • Institutional network
                                                                                                                                  • Switches vs Routers
                                                                                                                                  • Slide 64
                                                                                                                                  • Synthesis a day in the life of a web request
                                                                                                                                  • A day in the life scenario
                                                                                                                                  • A day in the lifehellip connecting to the Internet
                                                                                                                                  • Slide 68
                                                                                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                  • A day in the lifehellip using DNS
                                                                                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                  • A day in the lifehellip HTTP requestreply
                                                                                                                                  • Chapter 5 letrsquos take a breath

                                                                                                                                    5 DataLink Layer 5-66

                                                                                                                                    A day in the life scenario

                                                                                                                                    Comcast network 68800013

                                                                                                                                    Googlersquos network 64233160019 64233169105

                                                                                                                                    web server

                                                                                                                                    DNS server

                                                                                                                                    school network 68802024

                                                                                                                                    browser

                                                                                                                                    web page

                                                                                                                                    5 DataLink Layer 5-67

                                                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                                                    connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                                    router(runs DHCP)

                                                                                                                                    DHCPUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCPUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCPDHCP

                                                                                                                                    DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                                    Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                                    5 DataLink Layer 5-68

                                                                                                                                    A day in the lifehellip connecting to the Internet

                                                                                                                                    DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                                    router(runs DHCP)

                                                                                                                                    DHCPUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCPUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    DHCP

                                                                                                                                    encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                                    Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                                    DHCP client receives DHCP ACK reply

                                                                                                                                    5 DataLink Layer 5-69

                                                                                                                                    A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                    before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                    DNSUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                    ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                    ARP query

                                                                                                                                    EthPhy

                                                                                                                                    ARP

                                                                                                                                    ARP

                                                                                                                                    ARP reply

                                                                                                                                    5 DataLink Layer 5-70

                                                                                                                                    A day in the lifehellip using DNS

                                                                                                                                    DNSUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                    IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                    demuxrsquoed to DNS server DNS server replies to

                                                                                                                                    client with IP address of wwwgooglecom

                                                                                                                                    Comcast network 68800013

                                                                                                                                    DNS server

                                                                                                                                    DNSUDP

                                                                                                                                    IPEthPhy

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    DNS

                                                                                                                                    5 DataLink Layer 5-71

                                                                                                                                    A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                    HTTPTCPIP

                                                                                                                                    EthPhy

                                                                                                                                    HTTP

                                                                                                                                    to send HTTP request client first opens TCP socket to web server

                                                                                                                                    TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                    TCP connection established

                                                                                                                                    64233169105

                                                                                                                                    web server

                                                                                                                                    SYN

                                                                                                                                    SYN

                                                                                                                                    SYN

                                                                                                                                    SYN

                                                                                                                                    TCPIP

                                                                                                                                    EthPhy

                                                                                                                                    SYN

                                                                                                                                    SYN

                                                                                                                                    SYN

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    SYNACK

                                                                                                                                    web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                    5 DataLink Layer 5-72

                                                                                                                                    A day in the lifehellip HTTP requestreply

                                                                                                                                    HTTPTCPIP

                                                                                                                                    EthPhy

                                                                                                                                    HTTP

                                                                                                                                    HTTP request sent into TCP socket

                                                                                                                                    IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                    IP datgram containing HTTP reply routed back to client

                                                                                                                                    64233169105

                                                                                                                                    web server

                                                                                                                                    HTTPTCPIP

                                                                                                                                    EthPhy

                                                                                                                                    web server responds with HTTP reply (containing web page)

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTPHTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    HTTP

                                                                                                                                    web page finally () displayed

                                                                                                                                    5 DataLink Layer 5-73

                                                                                                                                    Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                    (except PHY) solid understanding of networking

                                                                                                                                    principles practice hellip could stop here hellip but lots of

                                                                                                                                    interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                    programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                    • Link Layer
                                                                                                                                    • Link Layer Introduction
                                                                                                                                    • Link layer context
                                                                                                                                    • Encapsulation
                                                                                                                                    • Link Layer Services
                                                                                                                                    • Link Layer Services (more)
                                                                                                                                    • Where is the link layer implemented
                                                                                                                                    • Adaptors Communicating
                                                                                                                                    • Slide 9
                                                                                                                                    • Parity Checking
                                                                                                                                    • Internet checksum (review)
                                                                                                                                    • Checksumming Cyclic Redundancy Check
                                                                                                                                    • Cyclic Redundancy Check
                                                                                                                                    • Slide 14
                                                                                                                                    • CRC Example
                                                                                                                                    • Slide 16
                                                                                                                                    • Multiple Access Links and Protocols
                                                                                                                                    • Cable Network Architecture Overview
                                                                                                                                    • Multiple Access protocols
                                                                                                                                    • Ideal Multiple Access Protocol
                                                                                                                                    • MAC Protocols a taxonomy
                                                                                                                                    • Channel Partitioning MAC protocols TDMA
                                                                                                                                    • Channel Partitioning MAC protocols FDMA
                                                                                                                                    • Random Access Protocols
                                                                                                                                    • ALOHA
                                                                                                                                    • CSMA (Carrier Sense Multiple Access)
                                                                                                                                    • CSMA collisions
                                                                                                                                    • CSMACD (Collision Detection)
                                                                                                                                    • CSMACD collision detection
                                                                                                                                    • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                    • Slide 31
                                                                                                                                    • Slide 32
                                                                                                                                    • Summary of MAC protocols
                                                                                                                                    • Slide 34
                                                                                                                                    • MAC Addresses and ARP
                                                                                                                                    • LAN Addresses and ARP
                                                                                                                                    • LAN Address (more)
                                                                                                                                    • ARP Address Resolution Protocol
                                                                                                                                    • ARP protocol Same LAN (network)
                                                                                                                                    • Addressing routing to another LAN
                                                                                                                                    • Slide 41
                                                                                                                                    • Slide 42
                                                                                                                                    • Ethernet
                                                                                                                                    • Star topology
                                                                                                                                    • Ethernet Frame Structure
                                                                                                                                    • Ethernet Frame Structure (more)
                                                                                                                                    • Ethernet Unreliable connectionless
                                                                                                                                    • Ethernet CSMACD algorithm
                                                                                                                                    • Ethernetrsquos CSMACD (more)
                                                                                                                                    • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                    • Manchester encoding
                                                                                                                                    • Slide 52
                                                                                                                                    • Hubs
                                                                                                                                    • Switch
                                                                                                                                    • Switch allows multiple simultaneous transmissions
                                                                                                                                    • Switch Table
                                                                                                                                    • Switch self-learning
                                                                                                                                    • Switch frame filteringforwarding
                                                                                                                                    • Self-learning forwarding example
                                                                                                                                    • Interconnecting switches
                                                                                                                                    • Self-learning multi-switch example
                                                                                                                                    • Institutional network
                                                                                                                                    • Switches vs Routers
                                                                                                                                    • Slide 64
                                                                                                                                    • Synthesis a day in the life of a web request
                                                                                                                                    • A day in the life scenario
                                                                                                                                    • A day in the lifehellip connecting to the Internet
                                                                                                                                    • Slide 68
                                                                                                                                    • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                    • A day in the lifehellip using DNS
                                                                                                                                    • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                    • A day in the lifehellip HTTP requestreply
                                                                                                                                    • Chapter 5 letrsquos take a breath

                                                                                                                                      5 DataLink Layer 5-67

                                                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                                                      connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

                                                                                                                                      router(runs DHCP)

                                                                                                                                      DHCPUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCPUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCPDHCP

                                                                                                                                      DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8023 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

                                                                                                                                      Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

                                                                                                                                      5 DataLink Layer 5-68

                                                                                                                                      A day in the lifehellip connecting to the Internet

                                                                                                                                      DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                                      router(runs DHCP)

                                                                                                                                      DHCPUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCPUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      DHCP

                                                                                                                                      encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                                      Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                                      DHCP client receives DHCP ACK reply

                                                                                                                                      5 DataLink Layer 5-69

                                                                                                                                      A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                      before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                      DNSUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                      ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                      ARP query

                                                                                                                                      EthPhy

                                                                                                                                      ARP

                                                                                                                                      ARP

                                                                                                                                      ARP reply

                                                                                                                                      5 DataLink Layer 5-70

                                                                                                                                      A day in the lifehellip using DNS

                                                                                                                                      DNSUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                      IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                      demuxrsquoed to DNS server DNS server replies to

                                                                                                                                      client with IP address of wwwgooglecom

                                                                                                                                      Comcast network 68800013

                                                                                                                                      DNS server

                                                                                                                                      DNSUDP

                                                                                                                                      IPEthPhy

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      DNS

                                                                                                                                      5 DataLink Layer 5-71

                                                                                                                                      A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                      HTTPTCPIP

                                                                                                                                      EthPhy

                                                                                                                                      HTTP

                                                                                                                                      to send HTTP request client first opens TCP socket to web server

                                                                                                                                      TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                      TCP connection established

                                                                                                                                      64233169105

                                                                                                                                      web server

                                                                                                                                      SYN

                                                                                                                                      SYN

                                                                                                                                      SYN

                                                                                                                                      SYN

                                                                                                                                      TCPIP

                                                                                                                                      EthPhy

                                                                                                                                      SYN

                                                                                                                                      SYN

                                                                                                                                      SYN

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      SYNACK

                                                                                                                                      web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                      5 DataLink Layer 5-72

                                                                                                                                      A day in the lifehellip HTTP requestreply

                                                                                                                                      HTTPTCPIP

                                                                                                                                      EthPhy

                                                                                                                                      HTTP

                                                                                                                                      HTTP request sent into TCP socket

                                                                                                                                      IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                      IP datgram containing HTTP reply routed back to client

                                                                                                                                      64233169105

                                                                                                                                      web server

                                                                                                                                      HTTPTCPIP

                                                                                                                                      EthPhy

                                                                                                                                      web server responds with HTTP reply (containing web page)

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTPHTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      HTTP

                                                                                                                                      web page finally () displayed

                                                                                                                                      5 DataLink Layer 5-73

                                                                                                                                      Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                      (except PHY) solid understanding of networking

                                                                                                                                      principles practice hellip could stop here hellip but lots of

                                                                                                                                      interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                      programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                      • Link Layer
                                                                                                                                      • Link Layer Introduction
                                                                                                                                      • Link layer context
                                                                                                                                      • Encapsulation
                                                                                                                                      • Link Layer Services
                                                                                                                                      • Link Layer Services (more)
                                                                                                                                      • Where is the link layer implemented
                                                                                                                                      • Adaptors Communicating
                                                                                                                                      • Slide 9
                                                                                                                                      • Parity Checking
                                                                                                                                      • Internet checksum (review)
                                                                                                                                      • Checksumming Cyclic Redundancy Check
                                                                                                                                      • Cyclic Redundancy Check
                                                                                                                                      • Slide 14
                                                                                                                                      • CRC Example
                                                                                                                                      • Slide 16
                                                                                                                                      • Multiple Access Links and Protocols
                                                                                                                                      • Cable Network Architecture Overview
                                                                                                                                      • Multiple Access protocols
                                                                                                                                      • Ideal Multiple Access Protocol
                                                                                                                                      • MAC Protocols a taxonomy
                                                                                                                                      • Channel Partitioning MAC protocols TDMA
                                                                                                                                      • Channel Partitioning MAC protocols FDMA
                                                                                                                                      • Random Access Protocols
                                                                                                                                      • ALOHA
                                                                                                                                      • CSMA (Carrier Sense Multiple Access)
                                                                                                                                      • CSMA collisions
                                                                                                                                      • CSMACD (Collision Detection)
                                                                                                                                      • CSMACD collision detection
                                                                                                                                      • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                      • Slide 31
                                                                                                                                      • Slide 32
                                                                                                                                      • Summary of MAC protocols
                                                                                                                                      • Slide 34
                                                                                                                                      • MAC Addresses and ARP
                                                                                                                                      • LAN Addresses and ARP
                                                                                                                                      • LAN Address (more)
                                                                                                                                      • ARP Address Resolution Protocol
                                                                                                                                      • ARP protocol Same LAN (network)
                                                                                                                                      • Addressing routing to another LAN
                                                                                                                                      • Slide 41
                                                                                                                                      • Slide 42
                                                                                                                                      • Ethernet
                                                                                                                                      • Star topology
                                                                                                                                      • Ethernet Frame Structure
                                                                                                                                      • Ethernet Frame Structure (more)
                                                                                                                                      • Ethernet Unreliable connectionless
                                                                                                                                      • Ethernet CSMACD algorithm
                                                                                                                                      • Ethernetrsquos CSMACD (more)
                                                                                                                                      • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                      • Manchester encoding
                                                                                                                                      • Slide 52
                                                                                                                                      • Hubs
                                                                                                                                      • Switch
                                                                                                                                      • Switch allows multiple simultaneous transmissions
                                                                                                                                      • Switch Table
                                                                                                                                      • Switch self-learning
                                                                                                                                      • Switch frame filteringforwarding
                                                                                                                                      • Self-learning forwarding example
                                                                                                                                      • Interconnecting switches
                                                                                                                                      • Self-learning multi-switch example
                                                                                                                                      • Institutional network
                                                                                                                                      • Switches vs Routers
                                                                                                                                      • Slide 64
                                                                                                                                      • Synthesis a day in the life of a web request
                                                                                                                                      • A day in the life scenario
                                                                                                                                      • A day in the lifehellip connecting to the Internet
                                                                                                                                      • Slide 68
                                                                                                                                      • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                      • A day in the lifehellip using DNS
                                                                                                                                      • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                      • A day in the lifehellip HTTP requestreply
                                                                                                                                      • Chapter 5 letrsquos take a breath

                                                                                                                                        5 DataLink Layer 5-68

                                                                                                                                        A day in the lifehellip connecting to the Internet

                                                                                                                                        DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

                                                                                                                                        router(runs DHCP)

                                                                                                                                        DHCPUDP

                                                                                                                                        IPEthPhy

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCPUDP

                                                                                                                                        IPEthPhy

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        DHCP

                                                                                                                                        encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

                                                                                                                                        Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

                                                                                                                                        DHCP client receives DHCP ACK reply

                                                                                                                                        5 DataLink Layer 5-69

                                                                                                                                        A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                        before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                        DNSUDP

                                                                                                                                        IPEthPhy

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                        ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                        ARP query

                                                                                                                                        EthPhy

                                                                                                                                        ARP

                                                                                                                                        ARP

                                                                                                                                        ARP reply

                                                                                                                                        5 DataLink Layer 5-70

                                                                                                                                        A day in the lifehellip using DNS

                                                                                                                                        DNSUDP

                                                                                                                                        IPEthPhy

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                        IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                        demuxrsquoed to DNS server DNS server replies to

                                                                                                                                        client with IP address of wwwgooglecom

                                                                                                                                        Comcast network 68800013

                                                                                                                                        DNS server

                                                                                                                                        DNSUDP

                                                                                                                                        IPEthPhy

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        DNS

                                                                                                                                        5 DataLink Layer 5-71

                                                                                                                                        A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                        HTTPTCPIP

                                                                                                                                        EthPhy

                                                                                                                                        HTTP

                                                                                                                                        to send HTTP request client first opens TCP socket to web server

                                                                                                                                        TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                        TCP connection established

                                                                                                                                        64233169105

                                                                                                                                        web server

                                                                                                                                        SYN

                                                                                                                                        SYN

                                                                                                                                        SYN

                                                                                                                                        SYN

                                                                                                                                        TCPIP

                                                                                                                                        EthPhy

                                                                                                                                        SYN

                                                                                                                                        SYN

                                                                                                                                        SYN

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        SYNACK

                                                                                                                                        web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                        5 DataLink Layer 5-72

                                                                                                                                        A day in the lifehellip HTTP requestreply

                                                                                                                                        HTTPTCPIP

                                                                                                                                        EthPhy

                                                                                                                                        HTTP

                                                                                                                                        HTTP request sent into TCP socket

                                                                                                                                        IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                        IP datgram containing HTTP reply routed back to client

                                                                                                                                        64233169105

                                                                                                                                        web server

                                                                                                                                        HTTPTCPIP

                                                                                                                                        EthPhy

                                                                                                                                        web server responds with HTTP reply (containing web page)

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTPHTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        HTTP

                                                                                                                                        web page finally () displayed

                                                                                                                                        5 DataLink Layer 5-73

                                                                                                                                        Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                        (except PHY) solid understanding of networking

                                                                                                                                        principles practice hellip could stop here hellip but lots of

                                                                                                                                        interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                        programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                        • Link Layer
                                                                                                                                        • Link Layer Introduction
                                                                                                                                        • Link layer context
                                                                                                                                        • Encapsulation
                                                                                                                                        • Link Layer Services
                                                                                                                                        • Link Layer Services (more)
                                                                                                                                        • Where is the link layer implemented
                                                                                                                                        • Adaptors Communicating
                                                                                                                                        • Slide 9
                                                                                                                                        • Parity Checking
                                                                                                                                        • Internet checksum (review)
                                                                                                                                        • Checksumming Cyclic Redundancy Check
                                                                                                                                        • Cyclic Redundancy Check
                                                                                                                                        • Slide 14
                                                                                                                                        • CRC Example
                                                                                                                                        • Slide 16
                                                                                                                                        • Multiple Access Links and Protocols
                                                                                                                                        • Cable Network Architecture Overview
                                                                                                                                        • Multiple Access protocols
                                                                                                                                        • Ideal Multiple Access Protocol
                                                                                                                                        • MAC Protocols a taxonomy
                                                                                                                                        • Channel Partitioning MAC protocols TDMA
                                                                                                                                        • Channel Partitioning MAC protocols FDMA
                                                                                                                                        • Random Access Protocols
                                                                                                                                        • ALOHA
                                                                                                                                        • CSMA (Carrier Sense Multiple Access)
                                                                                                                                        • CSMA collisions
                                                                                                                                        • CSMACD (Collision Detection)
                                                                                                                                        • CSMACD collision detection
                                                                                                                                        • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                        • Slide 31
                                                                                                                                        • Slide 32
                                                                                                                                        • Summary of MAC protocols
                                                                                                                                        • Slide 34
                                                                                                                                        • MAC Addresses and ARP
                                                                                                                                        • LAN Addresses and ARP
                                                                                                                                        • LAN Address (more)
                                                                                                                                        • ARP Address Resolution Protocol
                                                                                                                                        • ARP protocol Same LAN (network)
                                                                                                                                        • Addressing routing to another LAN
                                                                                                                                        • Slide 41
                                                                                                                                        • Slide 42
                                                                                                                                        • Ethernet
                                                                                                                                        • Star topology
                                                                                                                                        • Ethernet Frame Structure
                                                                                                                                        • Ethernet Frame Structure (more)
                                                                                                                                        • Ethernet Unreliable connectionless
                                                                                                                                        • Ethernet CSMACD algorithm
                                                                                                                                        • Ethernetrsquos CSMACD (more)
                                                                                                                                        • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                        • Manchester encoding
                                                                                                                                        • Slide 52
                                                                                                                                        • Hubs
                                                                                                                                        • Switch
                                                                                                                                        • Switch allows multiple simultaneous transmissions
                                                                                                                                        • Switch Table
                                                                                                                                        • Switch self-learning
                                                                                                                                        • Switch frame filteringforwarding
                                                                                                                                        • Self-learning forwarding example
                                                                                                                                        • Interconnecting switches
                                                                                                                                        • Self-learning multi-switch example
                                                                                                                                        • Institutional network
                                                                                                                                        • Switches vs Routers
                                                                                                                                        • Slide 64
                                                                                                                                        • Synthesis a day in the life of a web request
                                                                                                                                        • A day in the life scenario
                                                                                                                                        • A day in the lifehellip connecting to the Internet
                                                                                                                                        • Slide 68
                                                                                                                                        • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                        • A day in the lifehellip using DNS
                                                                                                                                        • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                        • A day in the lifehellip HTTP requestreply
                                                                                                                                        • Chapter 5 letrsquos take a breath

                                                                                                                                          5 DataLink Layer 5-69

                                                                                                                                          A day in the lifehellip ARP (before DNS before HTTP)

                                                                                                                                          before sending HTTP request need IP address of wwwgooglecom DNS

                                                                                                                                          DNSUDP

                                                                                                                                          IPEthPhy

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

                                                                                                                                          ARP query broadcast received by router which replies with ARP reply giving MAC address of router interface client now knows MAC address of first hop router so can now send frame containing DNS query

                                                                                                                                          ARP query

                                                                                                                                          EthPhy

                                                                                                                                          ARP

                                                                                                                                          ARP

                                                                                                                                          ARP reply

                                                                                                                                          5 DataLink Layer 5-70

                                                                                                                                          A day in the lifehellip using DNS

                                                                                                                                          DNSUDP

                                                                                                                                          IPEthPhy

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                          IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                          demuxrsquoed to DNS server DNS server replies to

                                                                                                                                          client with IP address of wwwgooglecom

                                                                                                                                          Comcast network 68800013

                                                                                                                                          DNS server

                                                                                                                                          DNSUDP

                                                                                                                                          IPEthPhy

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          DNS

                                                                                                                                          5 DataLink Layer 5-71

                                                                                                                                          A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                          HTTPTCPIP

                                                                                                                                          EthPhy

                                                                                                                                          HTTP

                                                                                                                                          to send HTTP request client first opens TCP socket to web server

                                                                                                                                          TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                          TCP connection established

                                                                                                                                          64233169105

                                                                                                                                          web server

                                                                                                                                          SYN

                                                                                                                                          SYN

                                                                                                                                          SYN

                                                                                                                                          SYN

                                                                                                                                          TCPIP

                                                                                                                                          EthPhy

                                                                                                                                          SYN

                                                                                                                                          SYN

                                                                                                                                          SYN

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          SYNACK

                                                                                                                                          web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                          5 DataLink Layer 5-72

                                                                                                                                          A day in the lifehellip HTTP requestreply

                                                                                                                                          HTTPTCPIP

                                                                                                                                          EthPhy

                                                                                                                                          HTTP

                                                                                                                                          HTTP request sent into TCP socket

                                                                                                                                          IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                          IP datgram containing HTTP reply routed back to client

                                                                                                                                          64233169105

                                                                                                                                          web server

                                                                                                                                          HTTPTCPIP

                                                                                                                                          EthPhy

                                                                                                                                          web server responds with HTTP reply (containing web page)

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTPHTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          HTTP

                                                                                                                                          web page finally () displayed

                                                                                                                                          5 DataLink Layer 5-73

                                                                                                                                          Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                          (except PHY) solid understanding of networking

                                                                                                                                          principles practice hellip could stop here hellip but lots of

                                                                                                                                          interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                          programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                          • Link Layer
                                                                                                                                          • Link Layer Introduction
                                                                                                                                          • Link layer context
                                                                                                                                          • Encapsulation
                                                                                                                                          • Link Layer Services
                                                                                                                                          • Link Layer Services (more)
                                                                                                                                          • Where is the link layer implemented
                                                                                                                                          • Adaptors Communicating
                                                                                                                                          • Slide 9
                                                                                                                                          • Parity Checking
                                                                                                                                          • Internet checksum (review)
                                                                                                                                          • Checksumming Cyclic Redundancy Check
                                                                                                                                          • Cyclic Redundancy Check
                                                                                                                                          • Slide 14
                                                                                                                                          • CRC Example
                                                                                                                                          • Slide 16
                                                                                                                                          • Multiple Access Links and Protocols
                                                                                                                                          • Cable Network Architecture Overview
                                                                                                                                          • Multiple Access protocols
                                                                                                                                          • Ideal Multiple Access Protocol
                                                                                                                                          • MAC Protocols a taxonomy
                                                                                                                                          • Channel Partitioning MAC protocols TDMA
                                                                                                                                          • Channel Partitioning MAC protocols FDMA
                                                                                                                                          • Random Access Protocols
                                                                                                                                          • ALOHA
                                                                                                                                          • CSMA (Carrier Sense Multiple Access)
                                                                                                                                          • CSMA collisions
                                                                                                                                          • CSMACD (Collision Detection)
                                                                                                                                          • CSMACD collision detection
                                                                                                                                          • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                          • Slide 31
                                                                                                                                          • Slide 32
                                                                                                                                          • Summary of MAC protocols
                                                                                                                                          • Slide 34
                                                                                                                                          • MAC Addresses and ARP
                                                                                                                                          • LAN Addresses and ARP
                                                                                                                                          • LAN Address (more)
                                                                                                                                          • ARP Address Resolution Protocol
                                                                                                                                          • ARP protocol Same LAN (network)
                                                                                                                                          • Addressing routing to another LAN
                                                                                                                                          • Slide 41
                                                                                                                                          • Slide 42
                                                                                                                                          • Ethernet
                                                                                                                                          • Star topology
                                                                                                                                          • Ethernet Frame Structure
                                                                                                                                          • Ethernet Frame Structure (more)
                                                                                                                                          • Ethernet Unreliable connectionless
                                                                                                                                          • Ethernet CSMACD algorithm
                                                                                                                                          • Ethernetrsquos CSMACD (more)
                                                                                                                                          • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                          • Manchester encoding
                                                                                                                                          • Slide 52
                                                                                                                                          • Hubs
                                                                                                                                          • Switch
                                                                                                                                          • Switch allows multiple simultaneous transmissions
                                                                                                                                          • Switch Table
                                                                                                                                          • Switch self-learning
                                                                                                                                          • Switch frame filteringforwarding
                                                                                                                                          • Self-learning forwarding example
                                                                                                                                          • Interconnecting switches
                                                                                                                                          • Self-learning multi-switch example
                                                                                                                                          • Institutional network
                                                                                                                                          • Switches vs Routers
                                                                                                                                          • Slide 64
                                                                                                                                          • Synthesis a day in the life of a web request
                                                                                                                                          • A day in the life scenario
                                                                                                                                          • A day in the lifehellip connecting to the Internet
                                                                                                                                          • Slide 68
                                                                                                                                          • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                          • A day in the lifehellip using DNS
                                                                                                                                          • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                          • A day in the lifehellip HTTP requestreply
                                                                                                                                          • Chapter 5 letrsquos take a breath

                                                                                                                                            5 DataLink Layer 5-70

                                                                                                                                            A day in the lifehellip using DNS

                                                                                                                                            DNSUDP

                                                                                                                                            IPEthPhy

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

                                                                                                                                            IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF andor BGP routing protocols) to DNS server

                                                                                                                                            demuxrsquoed to DNS server DNS server replies to

                                                                                                                                            client with IP address of wwwgooglecom

                                                                                                                                            Comcast network 68800013

                                                                                                                                            DNS server

                                                                                                                                            DNSUDP

                                                                                                                                            IPEthPhy

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            DNS

                                                                                                                                            5 DataLink Layer 5-71

                                                                                                                                            A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                            HTTPTCPIP

                                                                                                                                            EthPhy

                                                                                                                                            HTTP

                                                                                                                                            to send HTTP request client first opens TCP socket to web server

                                                                                                                                            TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                            TCP connection established

                                                                                                                                            64233169105

                                                                                                                                            web server

                                                                                                                                            SYN

                                                                                                                                            SYN

                                                                                                                                            SYN

                                                                                                                                            SYN

                                                                                                                                            TCPIP

                                                                                                                                            EthPhy

                                                                                                                                            SYN

                                                                                                                                            SYN

                                                                                                                                            SYN

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            SYNACK

                                                                                                                                            web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                            5 DataLink Layer 5-72

                                                                                                                                            A day in the lifehellip HTTP requestreply

                                                                                                                                            HTTPTCPIP

                                                                                                                                            EthPhy

                                                                                                                                            HTTP

                                                                                                                                            HTTP request sent into TCP socket

                                                                                                                                            IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                            IP datgram containing HTTP reply routed back to client

                                                                                                                                            64233169105

                                                                                                                                            web server

                                                                                                                                            HTTPTCPIP

                                                                                                                                            EthPhy

                                                                                                                                            web server responds with HTTP reply (containing web page)

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTPHTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            HTTP

                                                                                                                                            web page finally () displayed

                                                                                                                                            5 DataLink Layer 5-73

                                                                                                                                            Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                            (except PHY) solid understanding of networking

                                                                                                                                            principles practice hellip could stop here hellip but lots of

                                                                                                                                            interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                            programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                            • Link Layer
                                                                                                                                            • Link Layer Introduction
                                                                                                                                            • Link layer context
                                                                                                                                            • Encapsulation
                                                                                                                                            • Link Layer Services
                                                                                                                                            • Link Layer Services (more)
                                                                                                                                            • Where is the link layer implemented
                                                                                                                                            • Adaptors Communicating
                                                                                                                                            • Slide 9
                                                                                                                                            • Parity Checking
                                                                                                                                            • Internet checksum (review)
                                                                                                                                            • Checksumming Cyclic Redundancy Check
                                                                                                                                            • Cyclic Redundancy Check
                                                                                                                                            • Slide 14
                                                                                                                                            • CRC Example
                                                                                                                                            • Slide 16
                                                                                                                                            • Multiple Access Links and Protocols
                                                                                                                                            • Cable Network Architecture Overview
                                                                                                                                            • Multiple Access protocols
                                                                                                                                            • Ideal Multiple Access Protocol
                                                                                                                                            • MAC Protocols a taxonomy
                                                                                                                                            • Channel Partitioning MAC protocols TDMA
                                                                                                                                            • Channel Partitioning MAC protocols FDMA
                                                                                                                                            • Random Access Protocols
                                                                                                                                            • ALOHA
                                                                                                                                            • CSMA (Carrier Sense Multiple Access)
                                                                                                                                            • CSMA collisions
                                                                                                                                            • CSMACD (Collision Detection)
                                                                                                                                            • CSMACD collision detection
                                                                                                                                            • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                            • Slide 31
                                                                                                                                            • Slide 32
                                                                                                                                            • Summary of MAC protocols
                                                                                                                                            • Slide 34
                                                                                                                                            • MAC Addresses and ARP
                                                                                                                                            • LAN Addresses and ARP
                                                                                                                                            • LAN Address (more)
                                                                                                                                            • ARP Address Resolution Protocol
                                                                                                                                            • ARP protocol Same LAN (network)
                                                                                                                                            • Addressing routing to another LAN
                                                                                                                                            • Slide 41
                                                                                                                                            • Slide 42
                                                                                                                                            • Ethernet
                                                                                                                                            • Star topology
                                                                                                                                            • Ethernet Frame Structure
                                                                                                                                            • Ethernet Frame Structure (more)
                                                                                                                                            • Ethernet Unreliable connectionless
                                                                                                                                            • Ethernet CSMACD algorithm
                                                                                                                                            • Ethernetrsquos CSMACD (more)
                                                                                                                                            • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                            • Manchester encoding
                                                                                                                                            • Slide 52
                                                                                                                                            • Hubs
                                                                                                                                            • Switch
                                                                                                                                            • Switch allows multiple simultaneous transmissions
                                                                                                                                            • Switch Table
                                                                                                                                            • Switch self-learning
                                                                                                                                            • Switch frame filteringforwarding
                                                                                                                                            • Self-learning forwarding example
                                                                                                                                            • Interconnecting switches
                                                                                                                                            • Self-learning multi-switch example
                                                                                                                                            • Institutional network
                                                                                                                                            • Switches vs Routers
                                                                                                                                            • Slide 64
                                                                                                                                            • Synthesis a day in the life of a web request
                                                                                                                                            • A day in the life scenario
                                                                                                                                            • A day in the lifehellip connecting to the Internet
                                                                                                                                            • Slide 68
                                                                                                                                            • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                            • A day in the lifehellip using DNS
                                                                                                                                            • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                            • A day in the lifehellip HTTP requestreply
                                                                                                                                            • Chapter 5 letrsquos take a breath

                                                                                                                                              5 DataLink Layer 5-71

                                                                                                                                              A day in the lifehellip TCP connection carrying HTTP

                                                                                                                                              HTTPTCPIP

                                                                                                                                              EthPhy

                                                                                                                                              HTTP

                                                                                                                                              to send HTTP request client first opens TCP socket to web server

                                                                                                                                              TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

                                                                                                                                              TCP connection established

                                                                                                                                              64233169105

                                                                                                                                              web server

                                                                                                                                              SYN

                                                                                                                                              SYN

                                                                                                                                              SYN

                                                                                                                                              SYN

                                                                                                                                              TCPIP

                                                                                                                                              EthPhy

                                                                                                                                              SYN

                                                                                                                                              SYN

                                                                                                                                              SYN

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              SYNACK

                                                                                                                                              web server responds with TCP SYNACK (step 2 in 3-way handshake)

                                                                                                                                              5 DataLink Layer 5-72

                                                                                                                                              A day in the lifehellip HTTP requestreply

                                                                                                                                              HTTPTCPIP

                                                                                                                                              EthPhy

                                                                                                                                              HTTP

                                                                                                                                              HTTP request sent into TCP socket

                                                                                                                                              IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                              IP datgram containing HTTP reply routed back to client

                                                                                                                                              64233169105

                                                                                                                                              web server

                                                                                                                                              HTTPTCPIP

                                                                                                                                              EthPhy

                                                                                                                                              web server responds with HTTP reply (containing web page)

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTPHTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              HTTP

                                                                                                                                              web page finally () displayed

                                                                                                                                              5 DataLink Layer 5-73

                                                                                                                                              Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                              (except PHY) solid understanding of networking

                                                                                                                                              principles practice hellip could stop here hellip but lots of

                                                                                                                                              interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                              programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                              • Link Layer
                                                                                                                                              • Link Layer Introduction
                                                                                                                                              • Link layer context
                                                                                                                                              • Encapsulation
                                                                                                                                              • Link Layer Services
                                                                                                                                              • Link Layer Services (more)
                                                                                                                                              • Where is the link layer implemented
                                                                                                                                              • Adaptors Communicating
                                                                                                                                              • Slide 9
                                                                                                                                              • Parity Checking
                                                                                                                                              • Internet checksum (review)
                                                                                                                                              • Checksumming Cyclic Redundancy Check
                                                                                                                                              • Cyclic Redundancy Check
                                                                                                                                              • Slide 14
                                                                                                                                              • CRC Example
                                                                                                                                              • Slide 16
                                                                                                                                              • Multiple Access Links and Protocols
                                                                                                                                              • Cable Network Architecture Overview
                                                                                                                                              • Multiple Access protocols
                                                                                                                                              • Ideal Multiple Access Protocol
                                                                                                                                              • MAC Protocols a taxonomy
                                                                                                                                              • Channel Partitioning MAC protocols TDMA
                                                                                                                                              • Channel Partitioning MAC protocols FDMA
                                                                                                                                              • Random Access Protocols
                                                                                                                                              • ALOHA
                                                                                                                                              • CSMA (Carrier Sense Multiple Access)
                                                                                                                                              • CSMA collisions
                                                                                                                                              • CSMACD (Collision Detection)
                                                                                                                                              • CSMACD collision detection
                                                                                                                                              • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                              • Slide 31
                                                                                                                                              • Slide 32
                                                                                                                                              • Summary of MAC protocols
                                                                                                                                              • Slide 34
                                                                                                                                              • MAC Addresses and ARP
                                                                                                                                              • LAN Addresses and ARP
                                                                                                                                              • LAN Address (more)
                                                                                                                                              • ARP Address Resolution Protocol
                                                                                                                                              • ARP protocol Same LAN (network)
                                                                                                                                              • Addressing routing to another LAN
                                                                                                                                              • Slide 41
                                                                                                                                              • Slide 42
                                                                                                                                              • Ethernet
                                                                                                                                              • Star topology
                                                                                                                                              • Ethernet Frame Structure
                                                                                                                                              • Ethernet Frame Structure (more)
                                                                                                                                              • Ethernet Unreliable connectionless
                                                                                                                                              • Ethernet CSMACD algorithm
                                                                                                                                              • Ethernetrsquos CSMACD (more)
                                                                                                                                              • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                              • Manchester encoding
                                                                                                                                              • Slide 52
                                                                                                                                              • Hubs
                                                                                                                                              • Switch
                                                                                                                                              • Switch allows multiple simultaneous transmissions
                                                                                                                                              • Switch Table
                                                                                                                                              • Switch self-learning
                                                                                                                                              • Switch frame filteringforwarding
                                                                                                                                              • Self-learning forwarding example
                                                                                                                                              • Interconnecting switches
                                                                                                                                              • Self-learning multi-switch example
                                                                                                                                              • Institutional network
                                                                                                                                              • Switches vs Routers
                                                                                                                                              • Slide 64
                                                                                                                                              • Synthesis a day in the life of a web request
                                                                                                                                              • A day in the life scenario
                                                                                                                                              • A day in the lifehellip connecting to the Internet
                                                                                                                                              • Slide 68
                                                                                                                                              • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                              • A day in the lifehellip using DNS
                                                                                                                                              • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                              • A day in the lifehellip HTTP requestreply
                                                                                                                                              • Chapter 5 letrsquos take a breath

                                                                                                                                                5 DataLink Layer 5-72

                                                                                                                                                A day in the lifehellip HTTP requestreply

                                                                                                                                                HTTPTCPIP

                                                                                                                                                EthPhy

                                                                                                                                                HTTP

                                                                                                                                                HTTP request sent into TCP socket

                                                                                                                                                IP datagram containing HTTP request routed to wwwgooglecom

                                                                                                                                                IP datgram containing HTTP reply routed back to client

                                                                                                                                                64233169105

                                                                                                                                                web server

                                                                                                                                                HTTPTCPIP

                                                                                                                                                EthPhy

                                                                                                                                                web server responds with HTTP reply (containing web page)

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTPHTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                HTTP

                                                                                                                                                web page finally () displayed

                                                                                                                                                5 DataLink Layer 5-73

                                                                                                                                                Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                                (except PHY) solid understanding of networking

                                                                                                                                                principles practice hellip could stop here hellip but lots of

                                                                                                                                                interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                                programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                                • Link Layer
                                                                                                                                                • Link Layer Introduction
                                                                                                                                                • Link layer context
                                                                                                                                                • Encapsulation
                                                                                                                                                • Link Layer Services
                                                                                                                                                • Link Layer Services (more)
                                                                                                                                                • Where is the link layer implemented
                                                                                                                                                • Adaptors Communicating
                                                                                                                                                • Slide 9
                                                                                                                                                • Parity Checking
                                                                                                                                                • Internet checksum (review)
                                                                                                                                                • Checksumming Cyclic Redundancy Check
                                                                                                                                                • Cyclic Redundancy Check
                                                                                                                                                • Slide 14
                                                                                                                                                • CRC Example
                                                                                                                                                • Slide 16
                                                                                                                                                • Multiple Access Links and Protocols
                                                                                                                                                • Cable Network Architecture Overview
                                                                                                                                                • Multiple Access protocols
                                                                                                                                                • Ideal Multiple Access Protocol
                                                                                                                                                • MAC Protocols a taxonomy
                                                                                                                                                • Channel Partitioning MAC protocols TDMA
                                                                                                                                                • Channel Partitioning MAC protocols FDMA
                                                                                                                                                • Random Access Protocols
                                                                                                                                                • ALOHA
                                                                                                                                                • CSMA (Carrier Sense Multiple Access)
                                                                                                                                                • CSMA collisions
                                                                                                                                                • CSMACD (Collision Detection)
                                                                                                                                                • CSMACD collision detection
                                                                                                                                                • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                                • Slide 31
                                                                                                                                                • Slide 32
                                                                                                                                                • Summary of MAC protocols
                                                                                                                                                • Slide 34
                                                                                                                                                • MAC Addresses and ARP
                                                                                                                                                • LAN Addresses and ARP
                                                                                                                                                • LAN Address (more)
                                                                                                                                                • ARP Address Resolution Protocol
                                                                                                                                                • ARP protocol Same LAN (network)
                                                                                                                                                • Addressing routing to another LAN
                                                                                                                                                • Slide 41
                                                                                                                                                • Slide 42
                                                                                                                                                • Ethernet
                                                                                                                                                • Star topology
                                                                                                                                                • Ethernet Frame Structure
                                                                                                                                                • Ethernet Frame Structure (more)
                                                                                                                                                • Ethernet Unreliable connectionless
                                                                                                                                                • Ethernet CSMACD algorithm
                                                                                                                                                • Ethernetrsquos CSMACD (more)
                                                                                                                                                • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                                • Manchester encoding
                                                                                                                                                • Slide 52
                                                                                                                                                • Hubs
                                                                                                                                                • Switch
                                                                                                                                                • Switch allows multiple simultaneous transmissions
                                                                                                                                                • Switch Table
                                                                                                                                                • Switch self-learning
                                                                                                                                                • Switch frame filteringforwarding
                                                                                                                                                • Self-learning forwarding example
                                                                                                                                                • Interconnecting switches
                                                                                                                                                • Self-learning multi-switch example
                                                                                                                                                • Institutional network
                                                                                                                                                • Switches vs Routers
                                                                                                                                                • Slide 64
                                                                                                                                                • Synthesis a day in the life of a web request
                                                                                                                                                • A day in the life scenario
                                                                                                                                                • A day in the lifehellip connecting to the Internet
                                                                                                                                                • Slide 68
                                                                                                                                                • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                                • A day in the lifehellip using DNS
                                                                                                                                                • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                                • A day in the lifehellip HTTP requestreply
                                                                                                                                                • Chapter 5 letrsquos take a breath

                                                                                                                                                  5 DataLink Layer 5-73

                                                                                                                                                  Chapter 5 letrsquos take a breath journey down protocol stack complete

                                                                                                                                                  (except PHY) solid understanding of networking

                                                                                                                                                  principles practice hellip could stop here hellip but lots of

                                                                                                                                                  interesting topicso Internetworking (CSE 678 TCPIP socket

                                                                                                                                                  programming)o Wireless (ECE xxx)o Multimedia (CSE 679)o Security (CSE 651)

                                                                                                                                                  • Link Layer
                                                                                                                                                  • Link Layer Introduction
                                                                                                                                                  • Link layer context
                                                                                                                                                  • Encapsulation
                                                                                                                                                  • Link Layer Services
                                                                                                                                                  • Link Layer Services (more)
                                                                                                                                                  • Where is the link layer implemented
                                                                                                                                                  • Adaptors Communicating
                                                                                                                                                  • Slide 9
                                                                                                                                                  • Parity Checking
                                                                                                                                                  • Internet checksum (review)
                                                                                                                                                  • Checksumming Cyclic Redundancy Check
                                                                                                                                                  • Cyclic Redundancy Check
                                                                                                                                                  • Slide 14
                                                                                                                                                  • CRC Example
                                                                                                                                                  • Slide 16
                                                                                                                                                  • Multiple Access Links and Protocols
                                                                                                                                                  • Cable Network Architecture Overview
                                                                                                                                                  • Multiple Access protocols
                                                                                                                                                  • Ideal Multiple Access Protocol
                                                                                                                                                  • MAC Protocols a taxonomy
                                                                                                                                                  • Channel Partitioning MAC protocols TDMA
                                                                                                                                                  • Channel Partitioning MAC protocols FDMA
                                                                                                                                                  • Random Access Protocols
                                                                                                                                                  • ALOHA
                                                                                                                                                  • CSMA (Carrier Sense Multiple Access)
                                                                                                                                                  • CSMA collisions
                                                                                                                                                  • CSMACD (Collision Detection)
                                                                                                                                                  • CSMACD collision detection
                                                                                                                                                  • ldquoTaking Turnsrdquo MAC protocols
                                                                                                                                                  • Slide 31
                                                                                                                                                  • Slide 32
                                                                                                                                                  • Summary of MAC protocols
                                                                                                                                                  • Slide 34
                                                                                                                                                  • MAC Addresses and ARP
                                                                                                                                                  • LAN Addresses and ARP
                                                                                                                                                  • LAN Address (more)
                                                                                                                                                  • ARP Address Resolution Protocol
                                                                                                                                                  • ARP protocol Same LAN (network)
                                                                                                                                                  • Addressing routing to another LAN
                                                                                                                                                  • Slide 41
                                                                                                                                                  • Slide 42
                                                                                                                                                  • Ethernet
                                                                                                                                                  • Star topology
                                                                                                                                                  • Ethernet Frame Structure
                                                                                                                                                  • Ethernet Frame Structure (more)
                                                                                                                                                  • Ethernet Unreliable connectionless
                                                                                                                                                  • Ethernet CSMACD algorithm
                                                                                                                                                  • Ethernetrsquos CSMACD (more)
                                                                                                                                                  • 8023 Ethernet Standards Link amp Physical Layers
                                                                                                                                                  • Manchester encoding
                                                                                                                                                  • Slide 52
                                                                                                                                                  • Hubs
                                                                                                                                                  • Switch
                                                                                                                                                  • Switch allows multiple simultaneous transmissions
                                                                                                                                                  • Switch Table
                                                                                                                                                  • Switch self-learning
                                                                                                                                                  • Switch frame filteringforwarding
                                                                                                                                                  • Self-learning forwarding example
                                                                                                                                                  • Interconnecting switches
                                                                                                                                                  • Self-learning multi-switch example
                                                                                                                                                  • Institutional network
                                                                                                                                                  • Switches vs Routers
                                                                                                                                                  • Slide 64
                                                                                                                                                  • Synthesis a day in the life of a web request
                                                                                                                                                  • A day in the life scenario
                                                                                                                                                  • A day in the lifehellip connecting to the Internet
                                                                                                                                                  • Slide 68
                                                                                                                                                  • A day in the lifehellip ARP (before DNS before HTTP)
                                                                                                                                                  • A day in the lifehellip using DNS
                                                                                                                                                  • A day in the lifehellip TCP connection carrying HTTP
                                                                                                                                                  • A day in the lifehellip HTTP requestreply
                                                                                                                                                  • Chapter 5 letrsquos take a breath

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