Spring 2007 1

Routing & Switching

Umar KalimDept. of Communication Systems

Engineering

umar.kalim@niit.edu.pkhttp://www.niit.edu.pk/~umarkalim

20/03/2007

Ref: CSci5211 Univ. of Minnesota

Spring 2007 2

AgendaVirtual Circuit Switching ModelDatagram Switching ModelRouter Tables - OverviewLongest Prefix Match

– ARPICMP

Spring 2007 3

Virtual Circuit vs. Datagram Objective of both: move packets through routers from source to

destination Datagram Model:

– Routing: determine next hop to each destination a priori

– Forwarding: destination address in packet header, used at each hop to look up for next hop routes may change during “session”

– analogy: driving, asking directions at every corner gas station, or based on the road signs at every turn

Virtual Circuit Model: – Routing: determine a path from source to each

destination – “Call” Set-up: fixed path (“virtual circuit”) set up at

“call” setup time, remains fixed thru “call” – Data Forwarding: each packet carries “tag” or “label”

(virtual circuit id, VCI), which determines next hop– routers maintain ”per-call” state

Spring 2007 4

Virtual Circuit SwitchingExplicit connection setup (and tear-down)

phaseSubsequence packets follow same circuitSometimes called connection-oriented

modelstill packet switching, not circuit

switching!Analogy:

phone call

Each switch maintains a VC table

2

0

1

2

3

0

1

2

3

0

13

0

1

2

3

Host A Host B

Switch 3

Switch 2Switch 1

75

4

11

Spring 2007 5

Datagram Switching

No connection setup phaseEach packet forwarded independently Sometimes called connectionless model

Analogy: postal system

Each switch maintains a forwarding (routing) table

0

132

0

1 3

2

013

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Spring 2007 6

Forwarding Tables: VC vs. Datagram

Virtual Circuit Forwarding Table

a.k.a. VC (Translation) Table

(switch 1, port 2)

Datagram Forwarding Table

(switch 1)

Address PortA 2C 3F 1G 1… …

VC In VC Out Port Out

5 11 16 8 1

… … …

Spring 2007 7

More on Virtual Circuits

call setup/teardown for each call before data can flow

– need special control protocol: “signaling” – every router on source-dest path maintains

“state” (VCI translation table) for each passing call

– VCI translation table at routers along the path of a call “weaving together” a “logical connection” for the call

link, router resources (bandwidth, buffers) may be reserved and allocated to each VC

– to get “circuit-like” performance

“source-to-dest path behaves much like telephone circuit” (but actually over packet network)

Spring 2007 9

Virtual Circuit Setup/TeardownCall Set-Up:

Source: select a path from source to destination– Use routing table (which provides a “map of

network”) Source: send VC setup request control (“signaling”) packet

– Specify path for the call, and also the (initial) output VCI – perhaps also resources to be reserved, if supported

Each router along the path:– Determine output port and choose a (local) output VCI for

the call need to ensure that no two distinct VCs leaving the

same output port have the same VCI!– Update VCI translation table (“forwarding table”)

add an entry, establishing an mapping between incoming VCI & port no. and outgoing VCI & port no. for the call

Call Tear-Down: similar, but remove entry instead

Spring 2007 10

During data packet forwarding phase, input VCI is used to look up the table, and is “swapped” w/ output VCI (VCI translation, or “label swapping”)

VCI translation table (aka “forwarding table”), built at call set-up phase

1

2

13

1

2 2

1

four “calls” going thru the router, each entry corresponding one call

green call

purple call

blue call

orange call

Spring 2007 11

Virtual Circuit: Example

0

13

2

0

1 3

2

0

13

2

511

4

7

Router 3

Host B

Router 2

Host A

Router 1

Router 4

“call” from host A to host B along path: host A router 1 router 2 router 3 host B

•each router along path maintains an entry for the call in its VCI translation table• the entries piece together a “logical connection” for the call

Spring 2007 12

Virtual Circuit Model: Pros and Cons

Full RTT for connection setup– before sending first data packet.

Setup request carries full destination address– each data packet contains only a small

identifierIf a switch or a link in a connection fails

– new connection needs to be established.Provides opportunity to reserve

resources.

Spring 2007 13

ATM Networks

Study for Reference

Spring 2007 14

Datagram Networks: the Internet model

no call setup at network layer routers: no state about end-to-end connections

– no network-level concept of “connection” packets forwarded using destination host address

– packets between same source-dest pair may take different paths, when intermediate routes change!

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

1. Send data 2. Receive data

Spring 2007 15

Datagram ModelThere is no round trip delay waiting for

connection setup; a host can send data as soon as it is ready.

Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up.

Since packets are treated independently, it is possible to route around link and node failures.

Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.

Spring 2007 16

Network Layer Service Models:

Internet model being extended: MPLS, Diffserv

Spring 2007 17

Datagram or VC: Why?Internet data exchange among

computers

– “elastic” service, no strict timing req.

“smart” end systems (computers)

– can adapt, perform control, error recovery

– simple inside network, complexity at “edge”

many link types

– different characteristics– uniform service difficult

ATM evolved from telephony human conversation:

– strict timing, reliability requirements

– need for guaranteed service

“dumb” end systems– telephones– complexity inside

networkMPLS evolve from ATM

– traffic engineering, fast path restoration (a priori “backup” paths)

Spring 2007 25

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 as server– “plug-and-play”

Spring 2007 26

DHCP: Dynamic Host Configuration Protocol

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

connected an “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

Spring 2007 27

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

Spring 2007 28

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

Spring 2007 29

IP Addresses: How to Get One? …

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

A: gets 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

Spring 2007 30

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

Spring 2007 31

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, …)

Spring 2007 32

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>

– Try out “netstat –rn” command

Spring 2007 33

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 (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

Spring 2007 34

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

Spring 2007 35

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

Spring 2007 36

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?

Spring 2007 37

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

Dest. Net. next router Nhops

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

forwarding table in A

Spring 2007 38

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

Spring 2007 39

MAC (Physical) Addresses 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)

Spring 2007 40

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!

Spring 2007 41

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 20 min)

try out “arp –a” command

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

Spring 2007 42

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

Spring 2007 43

ARP Messages

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

Spring 2007 44

ARP Request & Response Processing

The requester broadcasts ARP requestThe 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?

Spring 2007 45

ARP Operation Illustration

Spring 2007 46

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

Dest. Net. next router Nhops

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

forwarding table in A

Spring 2007 47

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

Spring 2007 48

Forwarding to Another LAN:Interaction of IP and Data Link

Layerwalkthrough: 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

Spring 2007 49

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

Spring 2007 55

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

Spring 2007 56

ICMP Message Transport & Usage

ICMP messages carried in IP datagramsTreated 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

Spring 2007 57

Questions?

That’s all for today!