《计算机网络英文版课件蒋 ch5 network》由会员分享，可在线阅读，更多相关《计算机网络英文版课件蒋 ch5 network（183页珍藏版）》请在金锄头文库上搜索。
1、5TheNetworkLayer NetworkLayer NetworkLayerDesignIssues Issues Datagram VirtualCircuit RoutingAlgorithms ShortestPathRouting Flooding DistanceVectorRouting LinkStateRouting HierarchicalRouting CongestionControlAlgorithms GeneralPrinciples CongestionPrevention CongestionControl LoadShedding QualityofS
2、ervice Internetworking NetworkLayerintheInternet Internet sNetworkLayer IPv4Header ICMP IPAddresses RoutingTable NAT ARP DHCP OSPF BGP IPv6 HomeWork 1 2 Overview Networklayeristhelowestlayerthatdealswithend to endtransmissionNetworklayermustknowabouttopologyAvoidoverloadingsomeofthecommunicationline
3、sandrouterswhileleavingothersidle 5 1NetworkLayerDesignIssues Issues NetworkLayerDesignIssues ServicesProvidedtoTransportLayerInternalOrganizationofNetworkLayer Store and ForwardPacketSwitching ServicesProvidedtoTransportLayer DesignGoalThetransportlayershouldbeshieldedfromthenumber type andtopology
4、oftherouterspresentThenetworkaddressesmadeavailabletothetransportlayershoulduseauniformnumberingplan evenacrossLANsandWANsTwoclassesserviceConnection OrientedService VirtualCircuit ConnectionlessService Datagram Datagram Datagram 1 Routingwithinadatagramsubnet Datagram 2 Transport Network Datalink P
5、rocessP1hasalongmessageforP2 Datagram 3 A B C A B C VirtualCircuit VirtualCircuit 1 Routingwithinavirtual circuitsubnet VirtualCircuit 2 A B C A B C vc1 vc2 vc1 A 1 2 4 Bvc2 A 1 3 5 C Virtual Circuitvs Datagram 5 4 5 2RoutingAlgorithms ResponsibilityofRoutingAlgorithms Responsiblefordecidingwhichout
6、putlineanincomingpacketshouldbetransmittedonIfthesubnetusesdatagramsinternally thisdecisionmustbemadeanewforeveryarrivingdatapacketsincethebestroutemayhavechangedsincelasttimeIfthesubnetusesvirtualcircuitsinternally routingdecisionsaremadeonlywhenanewvirtualcircuitisbeingsetup Thereafter datapackets
7、justfollowthepreviously establishedroute Sessionrouting Forwarding Routing GoalsfortheRoutingAlgorithm CorrectnessSimplicityRobustnessStabilityFairnessOptimalityMinimizingmeanpacketdelayMaximizingtotalnetworkthroughput ClassificationofRoutingAlgorithms Nonadaptivealgorithms StaticRouting Donotbaseth
8、eirroutingdecisionsonmeasurementsorestimatesofthecurrenttrafficandtopologyThechoiceoftheroutetouseiscomputedinadvance off line anddownloadedtotherouterswhenthenetworkisbootedAdaptivealgorithmsChangetheirroutingdecisionstoreflectchangesinthetopology andusuallythetrafficaswell TheOptimalityPrinciple I
9、frouterJisontheoptimalpathfromrouterItorouterK thentheoptimalpathfromJtoKalsofallsalongthesamerouteIfarouterxbetterthanr2existedfromJtoK itcouldbeconcatenatedwithr1toimprovetheroutefromItoKContradictingourstatementthatr1r2isoptimal I K J r1 r2 rx r1 r2r1 rx SinkTree Optimalroutesfromallsourcestoades
10、tinationformatreerootedatthedestinationDoesnotcontainanyloops soeachpacketwillbedeliveredwithinafinitenumberofhopsThegoalofroutingalgorithmsDiscoverandusethesinktreesforallroutersDifferentroutersmayhavedifferentideasaboutthecurrenttopologysometime ExampleofSinkTree a Asubnet b AsinktreeforrouterB Sh
11、ortestPathRouting ShortestPathRouting BuildaweightedanddirectedgraphofsubnetNodesrepresentroutersArcsrepresentscommunicationlinesWeightHopsGeographicdistanceinkilometersMeanqueueingandtransmissiondelayDeterminedbyhourlytest somestandardtestpacketAfunctionofthedistance bandwidth averagetraffic commun
12、icationcost meanqueuelength measureddelay andotherfactorsDijkstraAlgorithmFindstheshortestpathbetweenagivenpairofrouters Flooding Flooding NonetworkinforequiredIncomingpacketsretransmittedoneverylinkexceptincominglinkEventuallyanumberofcopieswillarriveatdestination 1 4 6 2 3 5 FloodingExample Step1
13、1 2 4 3 5 6 FloodingExample Step2 1 2 4 3 5 6 FloodingExample Step3 TechniquesforDampingtheFlood ProblemGeneratesvastnumbersofduplicatepacketsaninfinitenumberunlesssomemeasuresaretakentodamptheprocessMeasuresTheheaderofeachpacketcontainsahopcounterCounterisdecrementedateachhopWhenthecounterreachesze
14、ro discardthepacketSenderinitializesthehopcounterasthelengthofthepathfromsourcetodestination orthesubnetdiameterSourceputsasequencenumberineachpacketEachrouterrecordsmaximalseqpersourcetellingwhichseqhavealreadybeenseenSelectivefloodingEachrouterrecordsreceiveddataitemsindatabaseEverydataitemscontai
15、nsaversionnumberOlnynewdataitemswillbeflooded SelectiveFloodingExample Step1 4 6 2 3 5 1 SelectiveFloodingExample Step2 2 4 3 5 6 1 SelectiveFloodingExample Step3 1 2 4 3 5 6 PropertiesofFlooding AllpossibleroutesaretriedVeryrobustAtleastonepacketwillhavetakenminimumhopcountrouteAllnodesarevisitedUs
16、efultodistributeroutinginformation DistanceVectorRouting DistanceVectorRouting RoutedbyrumorEachroutermaintainsatableThebestknowndistancetoeachdestinationThedistancemightbenumberofhops timedelayinmilliseconds WhichlinetouseThetableisupdatedbyexchanginginformationwithneighborsSendRoutinginformation Destination Distance Periodically RIPRoutingInformationProtocol 30seconds Whenevertablechanges triggeredupdate ReceiveRoutinginformationUpdatelocaltableifreceiveabetterrouteRefreshexistingroutesDeleter
