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Instructor: Carey WilliamsonOffice: ICT 740Email: carey@cpsc.ucalgary.caClass Location: ICT 122Lectures: MWF 12:00 – 12:50Notes derived from “Computer Networking: A Top

Down Approach”, by Jim Kurose and Keith Ross,Addison-Wesley.

Slides are adapted from the book’s companion Web site,with changes by Anirban Mahanti and Carey Williamson.

Network Layer: Routing & Forwarding

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Key Network-Layer Functionsforwarding: move packets from router’s inputto appropriate router output

routing: determine the path taken by packetsas they flow from a sender to a receiver

Routing algorithms – run at routers to determine“paths”;

Routers have a forwarding table• Destination address-based in Datagram networks

• Virtual circuit number-based in VC Networks

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1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

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VC Networks: Connection setup

3rd important function in some networkarchitectures:

ATM, frame relay, X.25Before datagrams flow, two hosts andintervening routers establish virtualconnection

Routers get involvedNetwork and transport layer conn service:

Network: between two hostsTransport: between two processes

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Network service modelQ: What service model for the “channel”transporting datagrams from sender to receiver?

Example services forindividual datagrams:Guaranteed deliveryGuaranteed deliverywith less than 40 msecdelay

Example services for aflow of datagrams:In-order datagramdeliveryGuaranteed minimumbandwidth to flowRestrictions onchanges in inter-packet spacing

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Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteedminimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

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Network layer connection-orientedand connection-less serviceDatagram network provides network-layerconnectionless serviceVC network provides network-layerconnection-oriented serviceAnalogous to the transport-layer services,but:

Service: host-to-hostNo choice: network provides one or the otherImplementation: in the core

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Virtual circuits

call setup, teardown for each call before data can floweach packet carries VC identifier (not destination hostaddress)every router on source-dest path maintains “state” foreach passing connectionlink, router resources (bandwidth, buffers) may beallocated to VC

“source-to-dest path behaves much like telephonecircuit”

performance-wisenetwork actions along source-to-dest path

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VC implementation

A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along

path3. Entries in forwarding tables in routers along

pathPacket belonging to VC carries a VCnumber.VC number must be changed on each link.

New VC number comes from forwarding table

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Forwarding table12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 183 7 2 171 97 3 87… … … …

Forwarding table inNorthwest router:

Routers maintain connection state information!

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Virtual circuits: signaling protocols

used to setup, maintain, and teardown VCused in ATM, frame-relay, X.25not used in today’s Internet

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1. Initiate call 2. incoming call3. Accept call4. Call connected

5. Data flow begins 6. Receive data

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Datagram networksno call setup at network layerrouters: 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 takedifferent paths

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

1. Send data 2. Receive data

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Datagram or VC network: why?

Internetdata exchange amongcomputers

“elastic” service, no stricttiming req.

“smart” end systems(computers)

can adapt, performcontrol, error recoverysimple inside network,complexity at “edge”

many link typesdifferent characteristicsuniform service difficult

ATMevolved from telephonyhuman conversation:

strict timing, reliabilityrequirementsneed for guaranteedservice

“dumb” end systemstelephonescomplexity insidenetwork

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Inside a Router

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Router Architecture Overview

Two key router functions:run routing algorithms/protocol (RIP, OSPF, BGP)forwarding datagrams from incoming to outgoing link

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Input Port Functions

Decentralized switching:given datagram dest., lookup output portusing forwarding table in input portmemory (caching of entries)goal: complete input port processing at‘line speed’queuing: if datagrams arrive faster thanforwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernet

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Three types of switching fabrics

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Switching via MemoryFirst generation routers:traditional computers with switching under directcontrol of CPUpacket copied to system’s memoryspeed limited by memory bandwidth (2 buscrossings per datagram)

InputPort

OutputPort

Memory

System Bus

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Switching via a Bus

datagram from input port memoryto output port memory via a sharedbusbus contention: switching speedlimited by bus bandwidth1 Gbps bus, Cisco 1900: sufficientspeed for access and enterpriserouters (not regional or backbone)

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Switching via an InterconnectionNetwork

Overcomes bus bandwidth limitationsBanyan networks, other interconnection netsinitially developed to connect processors inmultiprocessorAdvanced design: fragmenting datagram into fixedlength cells, switch cells through the fabric.Cisco 12000: switches Gbps through theinterconnection network

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Output Ports

Buffering required when datagrams arrive fromfabric faster than the transmission rate

Need queue management policy (Drop-Tail, AQM)Also need packet scheduling policy (FCFS, WFQ)

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Output port queueing

buffering when arrival rate via switch exceedsoutput line speedqueueing (delay) and loss due to output portbuffer overflow!

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Input Port Queuing?

Fabric slower than input ports combined -> queueingmay occur at input queuesHead-of-the-Line (HOL) blocking: queued datagramat front of queue prevents others in queue frommoving forward (even though o/p port is free for theother datagram)

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Next Topic

Routing AlgorithmsRouting in the InternetReadings: Chapter 4 in the K&R book