1Stan KurkovskyComputer NetworksComputer NetworksNetwork Layer, Virtual Circuits Network Layer, Virtual Circuits and Datagram Networksand Datagram NetworksBased on Computer Networking, 4thEdition by Kurose and RossStan KurkovskyNetwork layerNetwork layer•• transport segment from sending to transport segment from sending to receiving host receiving host •• on sending side encapsulates on sending side encapsulates segments into datagramssegments into datagrams•• on rcving side, delivers segments on rcving side, delivers segments to transport layerto transport layer•• network layer protocols in network layer protocols in everyeveryhost, routerhost, router•• Router examines header fields in Router examines header fields in all IP datagrams passing through itall IP datagrams passing through itnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysical2Stan KurkovskyKey NetworkKey Network--Layer FunctionsLayer Functions••forwarding:forwarding:move packets move packets from router’s input to from router’s input to appropriate router outputappropriate router output•• Takes place inside a Takes place inside a routerrouter••analogy:analogy: process of process of planning trip from source planning trip from source to destto dest••routing:routing:determine route determine route taken by packets from taken by packets from source to destsource to dest•• Takes place between Takes place between routersrouters••Routing algorithmsRouting algorithms•• analogy:analogy: process of process of getting through single getting through single interchangeinterchange1230111value in arrivingpacket’s headerrouting algorithmlocal forwarding tableheader valueoutput link01000101011110013221Stan KurkovskyNetwork service modelNetwork service modelQ: What service modelfor “channel” transporting datagrams from sender to rcvr?Example services for individual datagrams:Example services for individual datagrams:•• Guaranteed deliveryGuaranteed delivery•• Guaranteed delivery with less than 40 msec delayGuaranteed delivery with less than 40 msec delayExample services for a flow of datagrams:Example services for a flow of datagrams:•• InIn--order datagram deliveryorder datagram delivery•• Guaranteed minimum bandwidth to flowGuaranteed minimum bandwidth to flow•• Restrictions on changes in interRestrictions on changes in inter--packet spacingpacket spacing3Stan KurkovskyNetwork layer service modelsNetwork layer service modelsNetworkArchitectureInternetATMATMATMATMServiceModelbest effortCBRVBRABRUBRBandwidthnoneconstantrateguaranteedrateguaranteed minimumnoneLossnoyesyesnonoOrdernoyesyesyesyesTimingnoyesyesnonoCongestionfeedbackno (inferredvia loss)nocongestionnocongestionyesnoGuarantees ?Stan KurkovskyNetwork layer connection and connectionNetwork layer connection and connection--less serviceless service•• Datagram network provides networkDatagram network provides network--layer connectionless servicelayer connectionless service•• VC network provides networkVC network provides network--layer connection servicelayer connection service•• Analogous to the transportAnalogous to the transport--layer services, but:layer services, but:•• Service: Service: hosthost--toto--hosthost••No choice: No choice: network provides one or the othernetwork provides one or the other••Implementation: Implementation: in the corein the core•• VC: ATM networksVC: ATM networks•• Asynchronous Transfer ModeAsynchronous Transfer Mode4Stan KurkovskyVirtual circuitsVirtual circuits“source“source--toto--dest path behaves much like telephone circuit”dest path behaves much like telephone circuit”•• performanceperformance--wisewise•• network actions along sourcenetwork actions along source--toto--dest pathdest path•• call setup, teardown for each call call setup, teardown for each call beforebeforedata can flowdata can flow•• each packet carries VC identifier (not destination host address)each packet carries VC identifier (not destination host address)••everyeveryrouter on sourcerouter on source--dest path maintains “state” for each passing dest path maintains “state” for each passing connectionconnection•• link, router resources (bandwidth, buffers) may be link, router resources (bandwidth, buffers) may be allocated allocated to VCto VCA VC consists of:A VC consists of:1.1. Path from source to destinationPath from source to destination2.2. VC numbers, one number for each link along pathVC numbers, one number for each link along path3.3. Entries in forwarding tables in routers along pathEntries in forwarding tables in routers along path•• Packet belonging to VC carries a VC number.Packet belonging to VC carries a VC number.•• VC number must be changed on each link.VC number must be changed on each link.•• New VC number comes from forwarding tableNew VC number comes from forwarding tableStan KurkovskyForwarding tableForwarding table122232123VC numberinterfacenumberIncoming interface Incoming VC # Outgoing interface Outgoing VC #1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …Forwarding table innorthwest router:Routers maintain connection state information!5Stan KurkovskyVirtual circuits: signaling protocolsVirtual circuits: signaling protocols•• used to setup, maintain, teardown VCused to setup, maintain, teardown VC•• used in ATM, frameused in ATM, frame--relay, X.25relay, X.25•• not used in today’s Internetnot used in today’s Internetapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysical1. Initiate call2. incoming call3. Accept call4. Call connected5. Data flow begins6. Receive dataStan KurkovskyDatagram networksDatagram networks•• no call setup at network layerno call setup at network layer•• routers: no state