Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 1 Switching and Forwarding Network Layer Part I Switching and Forwarding.

Csci 183/183W/232: Computer Networks

Forwarding and Routing Network Layer Part I

1

Switching and ForwardingNetwork Layer Part I

• Switching and Forwarding– Generic Router Architecture– Forwarding Tables:

• Bridges/Layer 2 Switches; VLAN• Routers and Layer 3 Switches

• Forwarding in Layer 3 (Network Layer) – Network Layer Functions – Network Service Models: VC vs. Datagram

• ATM and IP Datagram Forwarding– IP Addressing

• Network vs. host: address blocks, longest prefix matching• Address allocation and DHCP

– IP Datagram Forwarding Model and ARP Protocol– IP and ICMP Protocols, IP Fragmentation and Re-assembly

Readings: Textbook: Chapter 4: Section 4.1;



2

Routing & Forwarding:Logical View of a Router

A

ED

CB

F

22

13

1

1

2

53

5



3

IP Addressing: Basics• Globally unique (for “public” IP

addresses)• IP address: 32-bit identifier for host, router

interface • Interface: connection between host/router and

physical link– router’s typically have multiple interfaces– host may have multiple interfaces– IP addresses associated with each interface

• Dot notation (for ease of human reading)223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11



4

IP Addressing: Network vs. Host

• Two-level hierarchy – network part (high order

bits)– host part (low order bits)

• What’s a network ? (from IP address perspective)

– device interfaces with same network part of IP address

– can physically reach each other without intervening router

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

multi-accessLAN

point-to-point link



5

“Classful” IP Addressing

32 bits

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

77 15 23 31

• Disadvantage: inefficient use of address space; address space exhaustion

• e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network



6

Classless Addressing: CIDR CIDR: Classless InterDomain Routing• Network portion of address is of arbitrary

length• Addresses allocated in contiguous blocks

– Number of addresses assigned always power of 2

• Address format: a.b.c.d/x– x is number of bits in network portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23



7

Representation of Address Blocks

• “Human Readable” address format: a.b.c.d/x– x is number of bits in network portion of address, the

network portion is also called the network prefix• machine representation of a network (addr block):

using a combination of – first IP of address blocks of the network– network mask ( x “1”’s followed by 32-x “0”’s

11001000 00010111 00010000 00000000network mask:

network w/ address block: 200.23.16.0/23

11111111 11111111 11111110 00000000

first IP address of address block:



8

More Examples Three Address Blocks: First IP address: 11001000 00010111 00010000 00000000 Network mask: 11111111 11111111 11111000 00000000 First IP address: 11001000 00010111 00011000 00000000 Last IP address: 11001000 00010111 00011000 11111111

what is the network prefix? 11001000 00010111 00011000

First IP address: 11001000 00010111 00011001 00000000 Last IP address: 11001000 00010111 00011111 11111111

what is the network prefix? 11001000 00010111 00011

Given an IP address, whichnetwork (or address block) does it belong to?

Example 2:11001000 00010111 00011000 10101010

Example 1: 11001000 00010111 00010110 10100001

Use longest prefix matching!



9

Another Example• Consider a datagram network using 32-bit host addresses,

suppose a router has four links, numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows:

Destination Addr RangeLink Interface

11100000 00000000 00000000 00000000 through 011100000 11111111 11111111 11111111

11100001 00000000 00000000 00000000 through 111100001 00000000 11111111 11111111

11100001 00000001 00000000 00000000 through 211100001 11111111 11111111 11111111

O.W. 3

Provide the forwarding table – a table containing the network prefix and the outgoing interface.



10

IP Addresses: How to Get One?

Q: How does host get IP address?

• “static” assigned: i.e., hard-coded in a file– Wintel: control-panel->network->configuration->tcp/ip-

>properties– UNIX: /etc/rc.config

• Dynamically assigned: using DHCP (Dynamic Host Configuration Protocol)– dynamically get address from a server– “plug-and-play”



11

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network DHCP server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected as

“on”)Support for mobile users who want to join network (more shortly)

DHCP overview:– host broadcasts “DHCP discover” msg– DHCP server responds with “DHCP offer” msg– host requests IP address: “DHCP request” msg– DHCP server sends address: “DHCP ack” msg



12

DHCP Client-Server Scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

DHCP server

arriving DHCP client needsaddress in thisnetwork



13

DHCP Client-Server ScenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs



14

IP Addresses: How to Get One? …

Q: How does a network get network part of IP addr?

A: gets an allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23



15

IP Addressing: the Last Word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned Names and Numbers– allocates addresses– manages DNS– assigns domain names, resolves disputes



16

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

10.0.0.0/8 has been reserved for private networks!



17


• Motivation: local network uses just one IP address as far as outside world is concerned:– no need to be allocated range of addresses from ISP: - just

one IP address is used for all devices– can change addresses of devices in local network without

notifying outside world– can change ISP without changing addresses of devices in

local network– devices inside local net not explicitly addressable, visible

by outside world (a security plus).



18


Implementation: NAT router must:

– outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

– remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

– incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table



19


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345



20


• 16-bit port-number field: – 60,000 simultaneous connections with a single LAN-

side address!

• NAT is controversial:– routers should only process up to layer 3– violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

– address shortage should instead be solved by IPv6



21

IP Forwarding & IP/ICMP Protocol

Networklayer routing

table

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Data Link layer (Ethernet, WiFi, PPP, …)

Physical Layer (SONET, …)



22

IP Service Model and Datagram Forwarding

• Connectionless (datagram-based)– Each datagram carries source and destination

• Best-effort delivery (unreliable service)– packets may be lost– packets can be delivered out of order– duplicate copies of a packet may be delivered– packets can be delayed for a long time

• Forwarding and IP address– forwarding based on network id

• Delivers packet to the appropriate network• Once on destination network, direct delivery using host id

• IP destination-based next-hop forwarding paradigm– Each host/router has IP forwarding table

• Entries like <network prefix, next-hop, output interface>



23

IP Datagram Format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (32-bit words)

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 address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

• 20 bytes of TCP• 20 bytes of IP• = 40 bytes +

app layer overhead



24

IP Datagram Forwarding Model

IP datagram:

miscfields

sourceIP addr

destIP addr data

• datagram remains unchanged, as it travels source to destination

• addr fields of interest here

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

forwarding table in A



25

IP Forwarding Table4 billion possible entries! (in reality, far less, but can still have millions of “routes”)

forwarding table entry format destination network next-hop (IP address) link interface (1st IP address , network mask ) 11001000 00010111 00010000 00000000, 200.23.16.1 0 11111111 11111111 11111000 00000000

11001000 00010111 00011000 00000000, - (direct) 1 11111111 11111111 11111111 00000000

11001000 00010111 00011001 00000000, 200.23.25.6 2 11111111 11111111 11111000 00000000

otherwise 128.30.0.1 3



26

Forwarding Table Lookupusing Longest Prefix Matching

Prefix Match Next Hop Link Interface

11001000 00010111 00010 200.23.16.1 0 11001000 00010111 00011000 - 1 11001000 00010111 00011 200.23.25.6 2 otherwise 128.30.0.1 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?



27

IP Forwarding: Destination in Same Net

Starting at A, send IP datagram addressed to B:

• look up net. address of B in forwarding table

• find B is on same net. as A• link layer will send datagram

directly to B inside link-layer frame– B and A are directly

connected

miscfields223.1.1.1223.1.1.3data


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE




28

IP Datagram Forwarding on Same LAN:Interaction of IP and data link layers

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

• look up net. address of B, find B on same net. as A

• link layer send datagram to B inside link-layer frame

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address



29

MAC (Physical) Addresses -- Revisited• used to get frames from one interface to another physically-

connected interface (same physical network, i.e., p2p or LAN)• 48 bit MAC address (for most LANs)

– fixed for each adaptor, burned in the adapter ROM– MAC address allocation administered by IEEE

• 1st bit: 0 unicast, 1 multicast.• all 1’s : broadcast

• MAC flat address -> portability – can move LAN card from one LAN to another

• MAC addressing operations on a LAN:– each adaptor on the LAN “sees” all frames– accept a frame if dest. MAC address matches its own MAC address– accept all broadcast (MAC= all1’s) frames– accept all frames if set in “promiscuous” mode– can configure to accept certain multicast addresses (first bit = 1)



30

MAC vs. IP Addresses32-bit IP address: • network-layer address, logical

– i.e., not bound to any physical device, can be re-assigned• IP hierarchical address NOT portable

– depends on IP network to which an interface is attached– when move to another IP network, IP address re-assigned

• used to get IP packets to destination IP network – Recall how IP datagram forwarding is performed

• IP network is “virtual,” actually packet delivery done by the underlying physical networks– from source host to destination host, hop-by-hop via IP routers – over each link, different link layer protocol used, with its own

frame headers, and source and destination MAC addresses • Underlying physical networks do not understand IP protocol

and datagram format!



31

ARP: Address Resolution Protocol

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

• ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; timer>– timer: time after which

address mapping will be forgotten (typically 15 min)

Question: how to determineMAC address of Bknowing B’s IP address?



32

ARP Protocol

• A wants to send datagram to B, and A knows B’s IP address.

• A looks up B’s MAC address in its ARP table

• Suppose B’s MAC address is not in A’s ARP table.

• A broadcasts (why?) ARP query packet, containing B's IP address – all machines on LAN

receive ARP query

• B receives ARP packet, replies to A with its (B's) MAC address– frame sent to A’s MAC

address (unicast)• A caches (saves) IP-to-MAC

address pair in its ARP table until information becomes old (times out) – soft state: information

that times out (goes away) unless refreshed

• ARP is “plug-and-play”:– nodes create their ARP

tables without intervention from net administrator



33

ARP Messages

Hardware Address Type: e.g., EthernetProtocol address Type: e.g., IPOperation: ARP request or ARP response



34

ARP Request & Response Processing

• The requester broadcasts ARP request• The target node unicasts (why?) ARP reply to

requester – With its physical address– Adds the requester into its ARP table (why?)

• On receiving the response, requester– updates its table, sets timer

• Other nodes upon receiving the ARP request– Refresh the requester entry if already there– No action otherwise (why?)

• Some questions to think about:– Shall requester buffer IP datagram while performing ARP?– What shall requester do if never receive any ARP

response?



35

ARP Operation Illustration



36

IP Forwarding: Destination in Diff. Net

Starting at A, dest. E:• look up network address of

E in forwarding table• E on different network

– A, E not directly attached

• routing table: next hop router to E is 223.1.1.4

• link layer sends datagram to router 223.1.1.4 inside link-layer frame

• datagram arrives at 223.1.1.4

• continued…..

miscfields223.1.1.1223.1.2.3 data


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE




37

IP Forwarding: Destination in Diff. Net …

Arriving at 223.1.4, destined for 223.1.2.2

• look up network address of E in router’s forwarding table

• E on same network as router’s interface 223.1.2.9 – router, E directly attached

• link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9

• datagram arrives at 223.1.2.2!!! (hooray!)

miscfields223.1.1.1223.1.2.3 data

Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

forwarding table in router



38

Forwarding to Another LAN:Interaction of IP and Data Link Layer

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

• Two ARP tables in router R, one for each IP network (LAN)• In routing table at source host, find router 111.111.111.110• In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB



39

• A creates datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110• A creates link-layer frame with R's MAC address as dest,

frame contains A-to-B IP datagram• A’s data link layer sends frame • R’s data link layer receives frame • R removes IP datagram from Ethernet frame, sees its

destined to B• R uses ARP to get B’s physical layer address • R creates frame containing A-to-B IP datagram sends to B

A RB



40

IP Datagram Format Again

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 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 address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

• 20 bytes of TCP• 20 bytes of IP• = 40 bytes +

app layer overhead



41

Fields in IP Datagram• IP protocol version: current version is 4, IPv4, new: IPv6• Header length: number of 32-bit words in the header• Type of Service:

– 3-bit priority,e.g, delay, throughput, reliability bits, …

• Total length: including header (maximum 65535 bytes)• Identification: all fragments of a packet have same

identification• Flags: don’t fragment, more fragments• Fragment offset: where in the original packet (count in 8

byte units)• Time to live: maximum life time of a packet• Protocol Type: e.g., ICMP, TCP, UDP etc• IP Option: non-default processing, e.g., IP source routing

option, etc.



42

IP Fragmentation & Reassembly: Why

• network links have MTU (maximum transmission unit) - largest possible data gram.– different link types,

different MTUs • large IP datagram divided

(“fragmented”) within net– one datagram

becomes several datagrams

– “reassembled” only at final destination

– IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly



43

IP Fragmentation & Reassembly: How• An IP datagram is chopped by a router into smaller

pieces if– datagram size is greater than network MTU– Don’t fragment option is not set

• Each datagram has unique datagram identification– Generated by source hosts

– All fragments of a packet carry original datagram id

• All fragments except the last have more flag set– Fragment offset and Length fields are modified appropriately

• Fragments of IP packet can be further fragmented by other routers along the way to destination !

• Reassembly only done at destination host (why?)– Use IP datagram id, fragment offset, fragment flags.

Length



44

IP Fragmentation and Reassembly: Exp

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example• 4000 byte

datagram• MTU = 1500

bytes



45

ICMP: Internet Control Message Protocol

• used by hosts, routers, gateways to communicate 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



46

ICMP Message Transport & Usage

• ICMP messages carried in IP datagrams• Treated like any other datagrams

– But no error message sent if ICMP message causes error

• Message sent to the source– 8 bytes of the original header included

• ICMP Usage (non-error, informational): Examples– Testing reachability: ICMP echo request/reply

• ping

– Tracing route to a destination: Time-to-live field• traceroute

– Path MTU discovery• Don’t fragment bit

– IP direct (for hosts only): inform hosts of better routes

Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 1 Switching and Forwarding Network Layer Part I Switching and Forwarding.

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