計算機學(xué)校-英文講座

Chapter4-1Networklayer1NetworklayerChapter4:NetworkLayerChaptergoals:

understandprinciplesbehindnetworklayerservices:networklayerservicemodelsforwardingversusroutinghowarouterworksrouting(pathselection)dealingwithscaleadvancedtopics:IPv6,mobilityinstantiation,implementationintheInternet2NetworklayerChapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingVLSM/CIDRICMPIPv6IPsec4.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7Broadcastandmulticastrouting3NetworklayerChapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7Broadcastandmulticastrouting4NetworklayerNetworklayertransportsegmentfromsendingtoreceivinghostonsendingsideencapsulatessegmentsintodatagramsonrcvingside,deliverssegmentstotransportlayernetworklayerprotocolsineveryhost,routerrouterexaminesheaderfieldsinallIPdatagramspassingthroughitapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysical5NetworklayerTwoKeyNetwork-LayerFunctionsforwarding:movepacketsfromrouter’sinputtoappropriaterouteroutputrouting:determineroutetakenbypacketsfromsourcetodest.routingalgorithmsanalogy:routing:processofplanningtripfromsourcetodestforwarding:processofgettingthroughsingleinterchange6Networklayer1230111valueinarrivingpacket’sheaderroutingalgorithmlocalforwardingtableheadervalueoutputlink01000101011110013221Interplaybetweenroutingandforwarding7NetworklayerConnectionsetup3rdimportantfunctioninsomenetworkarchitectures:ATM,framerelay,X.25beforedatagramsflow,twoendhostsandinterveningroutersestablishvirtualconnectionroutersgetinvolvednetworkvstransportlayerconnectionservice:network:betweentwohosts(mayalsoinvolveinterveningroutersincaseofVCs)transport:betweentwoprocesses8NetworklayerNetworkservicemodelQ:Whatservicemodelfor“channel”transportingdatagramsfromsendertoreceiver?Exampleservicesforindividualdatagrams:guaranteeddeliveryguaranteeddeliverywithlessthan40msecdelayExampleservicesforaflowofdatagrams:in-orderdatagramdeliveryguaranteedminimumbandwidthtoflowrestrictionsonchangesininter-packetspacing9NetworklayerNetworklayerservicemodels:NetworkArchitectureInternetATMATMATMATMServiceModelbesteffortCBRVBRABRUBRBandwidthnoneconstantrateguaranteedrateguaranteedminimumnoneLossnoyesyesnonoOrdernoyesyesyesyesTimingnoyesyesnonoCongestionfeedbackno(inferredvialoss)nocongestionnocongestionyesnoGuarantees?10NetworklayerChapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7Broadcastandmulticastrouting11NetworklayerNetworklayerconnectionandconnection-lessservicedatagramnetworkprovidesnetwork-layerconnectionlessserviceVCnetworkprovidesnetwork-layerconnectionserviceanalogoustothetransport-layerservices,but:service:host-to-hostnochoice:networkprovidesoneortheotherimplementation:innetworkcore12NetworklayerVirtualcircuitscallsetup,teardownforeachcallbeforedatacanfloweachpacketcarriesVCidentifier(notdestinationhostaddress)everyrouteronsource-destpathmaintains“state”foreachpassingconnectionlink,routerresources(bandwidth,buffers)maybeallocatedtoVC(dedicatedresources=predictableservice)“source-to-destpathbehavesmuchliketelephonecircuit”performance-wisenetworkactionsalongsource-to-destpath13NetworklayerVCimplementationaVCconsistsof:pathfromsourcetodestinationVCnumbers,onenumberforeachlinkalongpathentriesinforwardingtablesinroutersalongpathpacketbelongingtoVCcarriesVCnumber(ratherthandestaddress)VCnumbercanbechangedoneachlink.NewVCnumbercomesfromforwardingtable14NetworklayerForwardingtable122232123VCnumberinterfacenumberIncominginterfaceIncomingVC#OutgoinginterfaceOutgoingVC#11232226311837217197387…………Forwardingtableinnorthwestrouter:Routersmaintainconnectionstateinformation!15NetworklayerVirtualcircuits:signalingprotocolsusedtosetup,maintainteardownVCusedinATM,frame-relay,X.25notusedintoday’sInternetapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Initiatecall2.incomingcall3.Acceptcall4.Callconnected5.Dataflowbegins6.Receivedata16NetworklayerDatagramnetworksnocallsetupatnetworklayerrouters:nostateaboutend-to-endconnectionsnonetwork-levelconceptof“connection”packetsforwardedusingdestinationhostaddresspacketsbetweensamesource-destpairmaytakedifferentpathsapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Senddata2.Receivedata17NetworklayerForwardingtable

DestinationAddressRange

LinkInterface11001000000101110001000000000000through01100100000010111000101111111111111001000000101110001100000000000through11100100000010111000110001111111111001000000101110001100100000000through211001000000101110001111111111111otherwise34billionpossibleentries18NetworklayerLongestprefixmatching

PrefixMatch

LinkInterface110010000001011100010011001000000101110001100011100100000010111000112otherwise3DA:11001000000101110001100010101010ExamplesDA:11001000000101110001011010100001Whichinterface?Whichinterface?19NetworklayerDatagramorVCnetwork:why?Internet(datagram)dataexchangeamongcomputers“elastic”service,nostricttimingreq.“smart”endsystems(computers)canadapt,performcontrol,errorrecoverysimpleinsidenetwork,complexityat“edge”manylinktypesdifferentcharacteristicsuniformservicedifficultATM(VC)evolvedfromtelephonyhumanconversation:stricttiming,reliabilityrequirementsneedforguaranteedservice“dumb”endsystemstelephonescomplexityinsidenetwork20NetworklayerNetworkLayerDesignIssuesStore-and-ForwardPacketSwitchingAhostwithapackettosendtransmitsittothenearestrouter,eitheronitsownLANoroverapoint-to-pointlinktothecarrier.Thepacketisstoredthereuntilithasfullyarrivedsothechecksumcanbeverified.Thenitisforwardedtothenextrouteralongthepathuntilitreachesthedestinationhost,whereitisdelivered.Theenvironmentofthenetworklayerprotocols21NetworklayerImplementationofConnectionlessServiceRoutingwithinadatagramsubnet.22NetworklayerImplementationofConnection-orientedServiceRoutingwithinavirtual-circuitsubnet.23NetworklayerComparisonofvirtualcircuitanddatagram24NetworklayerChapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7Broadcastandmulticastrouting25NetworklayerRouterArchitectureOverviewTwokeyrouterfunctions:

runroutingalgorithms/protocol(RIP,OSPF,BGP)forwardingdatagramsfromincomingtooutgoinglink26NetworklayerInputPortFunctionsDecentralizedswitching:

givendatagramdest.,lookupoutputportusingforwardingtableininputportmemorygoal:completeinputportprocessingat‘linespeed’queuing:ifdatagramsarrivefasterthanforwardingrateintoswitchfabricPhysicallayer:bit-levelreceptionDatalinklayer:e.g.,Ethernetseechapter527NetworklayerThreetypesofswitchingfabrics28NetworklayerSwitchingViaMemoryFirstgenerationrouters:traditionalcomputerswithswitchingunderdirectcontrolofCPUpacketcopiedtosystem’smemoryspeedlimitedbymemorybandwidth(2buscrossingsperdatagram)InputPortOutputPortMemorySystemBus29NetworklayerSwitchingViaaBusdatagramfrominputportmemorytooutputportmemoryviaasharedbusbuscontention:switchingspeedlimitedbybusbandwidth32Gbpsbus,Cisco5600:sufficientspeedforaccessandenterpriserouters30NetworklayerSwitchingViaAnInterconnectionNetworkovercomebusbandwidthlimitationsBanyannetworks,otherinterconnectionnetsinitiallydevelopedtoconnectprocessorsinmultiprocessoradvanceddesign:fragmentingdatagramintofixedlengthcells,switchcellsthroughthefabric.Cisco12000:switches60Gbpsthroughtheinterconnectionnetwork31NetworklayerOutputPortsBufferingrequiredwhendatagra