CS E551 : Computer Network Review

CSE4471: Computer Network Review Network Layers TCP/UDP IP Ethernet

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Internet Layers

application: supporting network applications ftp, smtp, http

transport: host-host data transfer tcp, udp

network: routing of datagrams from source to destination ip, routing protocols

link: data transfer between neighboring network elements ppp, ethernet

physical: bits “on the wire”

application

transport

network

link

physical

OSI Network Layers

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Layering: logical communication applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer: distributed “entities”

implement layer functions at each node

entities perform actions, exchange messages with peers

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Layering: physical communication applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

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Protocol layering and dataEach layer takes data from above adds header information to create new data unit passes new data unit to layer below

applicationtransportnetwork

linkphysical

source destination

MMMM

HtHtHnHtHnHl

MMMM

HtHtHnHtHnHl

messagesegmentdatagramframe

applicationtransportnetwork

linkphysical

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Internet structure: network of networks

roughly hierarchical national/international

backbone providers (NBPs) e.g. BBN/GTE, Sprint, AT&T,

IBM, UUNet interconnect (peer) with each

other privately, or at public Network Access Point (NAPs)

regional ISPs connect into NBPs

local ISP, company connect into regional ISPs

NBP A

NBP B

NAP NAP

regional ISP

regional ISP

localISP

localISP

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National Backbone Providere.g. Sprint US backbone network

TCP Transport Control Protocol Flow control and Responds to congestion Reliable In-order delivery “Nice” Protocol

TCP segment structure

source port # dest port #32 bits

applicationdata

(variable length)

sequence numberacknowledgement

numberrcvr window sizeptr urgent datachecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

Reliable Delivery Sender, Receiver keep track of bytes sent and

bytes received. Acks have an indication of next byte expected. Three duplicate acks considered a packet loss

- sender retransmits

TCP seq. #’s and ACKsSeq. #’s:

byte stream “number” of first byte in segment’s data

ACKs: seq # of next byte

expected from other side

cumulative ACKQ: how receiver handles

out-of-order segments A: TCP spec doesn’t

say, - up to implementer

Host A Host B

Seq=42, ACK=79, data = ‘C’

Seq=79, ACK=43, data = ‘C’

Seq=43, ACK=80

Usertypes

‘C’

host ACKsreceipt

of echoed‘C’

host ACKsreceipt of

‘C’, echoesback ‘C’

timesimple telnet scenario

TCP Flow Control Window based Sender cannot send more data than a window

without acknowledgements. Window is a minimum of receiver’s buffer and

‘congestion window’. After a window of data is transmitted, in

steady state, acks control sending rate.

Flow Control

UDP No reliability, flow control, congestion control. Sends data in a burst. Provides multiplexing and demultiplexing of

sources. Most multimedia applications using UDP

UDP: User Datagram Protocol [RFC 768]

“no frills,” “bare bones” Internet transport protocol

“best effort” service, UDP segments may be: lost delivered out of order to

app connectionless:

no handshaking between UDP sender, receiver

each UDP segment handled independently of others

Why is there a UDP? no connection

establishment (which can add delay)

simple: no connection state at sender, receiver

small segment header no congestion control: UDP

can blast away as fast as desired

UDP segment structure often used for streaming

multimedia apps loss tolerant rate sensitive

other UDP uses (why?): DNS SNMP

reliable transfer over UDP: add reliability at application layer application-specific error

recover!

source port # dest port #32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierInternet

checksumtime to

live32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgs fragment offset

upper layer

32 bit destination IP addressOptions (if any) E.g. timestamp,

record routetaken, pecifylist of routers to visit.

ICMP: Internet Control Message Protocol

used by hosts, routers, gateways to communication network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Routing in the Internet

The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider

Two-level routing: Intra-AS: administrator is responsible for choice: RIP,

OSPF Inter-AS: unique standard: BGP

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Link Layer

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Link Layer: setting the context two physically connected devices:

host-router, router-router, host-host unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

MMMM

HtHtHnHtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card

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Link Layer Services

Framing, link access: encapsulate datagram into frame, adding header, trailer implement channel access if shared medium, ‘physical addresses’ used in frame headers to identify

source, dest • different from IP address!

Reliable delivery between two physically connected devices: we learned how to do this already (chapter 3)! seldom used on low bit error link (fiber, some twisted

pair) wireless links: high error rates

• Q: why both link-level and end-end reliability?

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Link Layer Services (more) Flow Control:

pacing between sender and receivers Error Detection:

errors caused by signal attenuation, noise. receiver detects presence of errors:

• signals sender for retransmission or drops frame Error Correction:

receiver identifies and corrects bit error(s) without resorting to retransmission

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Multiple Access Links and Protocols

Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g,

Ethernet, Wavelan, etc.)

switched (e.g., switched Ethernet, ATM etc)

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

interference only one node can send successfully at a time

multiple access protocol: distributed algorithm that determines how stations share

channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:

• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance

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Ethernet: uses CSMA/CD

A: sense channel, if idle then {

transmit and monitor the channel; If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}

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Network Layer

Basic Functions

Representative Protocols

Security Vulnerability Examples

Application Providing services such as WWW to end-users

HTTP, SMTP, FTP JavaScript-based malware, Email spams

Transport End-to-end message transmission independent of the underlying network

TCP, UDP TCP SYN attack, UDP flooding attack

Network Routing IP, ICMP, RIP, OSPF, BGP IP spoofing,Black hole attack to RIP

Data Link Media access control Ethernet, Wi-Fi Eavesdropping attack

Physical Transmitting raw bit stream

Physical attack such as cut to cable

A Summary on Network Layers and Their Vulnerabilities

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Acknowledgement Part of the slides are from Kurose and Ross’s book

“Computer Networking: A Top-Down Approach”.