计算机网络英文版课件：4-10 Internet路由协议

Internet路由协议

RIP(RoutingInformationProtocol)distancevectoralgorithmincludedinBSD-UNIXDistributionin1982distancemetric:#ofhops(max=15hops)RIP:ExampleRIP:ExampleRIPadvertisementsdistancevectors:exchangedamongneighborsevery30secviaResponseMessage(alsocalledadvertisement)eachadvertisement:listofupto25destinationsubnetswithinASRIP:LinkFailureandRecovery

Ifnoadvertisementheardafter180sec-->neighbor/linkdeclareddeadroutesvianeighborinvalidatednewadvertisementssenttoneighborsneighborsinturnsendoutnewadvertisements(iftableschanged)linkfailureinfoquickly(?)propagatestoentirenetpoisonreverseusedtopreventping-pongloops(infinitedistance=16hops)RIPTableprocessingRIProutingtablesmanagedbyapplication-levelprocesscalledroute-d(daemon)advertisementssentinUDPpackets,periodicallyrepeatedOSPF(OpenShortestPathFirst)“open”:publiclyavailableusesLinkStatealgorithmLSpacketdisseminationtopologymapateachnoderoutecomputationusingDijkstra’salgorithmOSPFadvertisementcarriesoneentryperneighborrouteradvertisementsdisseminatedtoentireAS(viaflooding)carriedinOSPFmessagesdirectlyoverIP(ratherthanTCPorUDPOSPF“advanced”features(notinRIP)security:allOSPFmessagesauthenticated(topreventmaliciousintrusion)multiplesame-costpathsallowed(onlyonepathinRIP)Foreachlink,multiplecostmetricsfordifferentTOS(e.g.,satellitelinkcostset“low”forbesteffort;highforrealtime)integrateduni-andmulticastsupport:MulticastOSPF(MOSPF)usessametopologydatabaseasOSPFhierarchicalOSPFinlargedomains.HierarchicalOSPFtwo-levelhierarchy:localarea,backbone.Link-stateadvertisementsonlyinareaeachnodeshasdetailedareatopology;onlyknowdirection(shortestpath)tonetsinotherareas.areaborderrouters:

“summarize”distancestonetsinownarea,advertisetootherAreaBorderrouters.backbonerouters:runOSPFroutinglimitedtobackbone.boundaryrouters:connecttootherAS’s.HierarchicalOSPFInternetinter-ASrouting:BGPBGP(BorderGatewayProtocol):

thedefactostandardBGPprovideseachASameansto:ObtainsubnetreachabilityinformationfromneighboringASs.PropagatereachabilityinformationtoallAS-internalrouters.Determine“good”routestosubnetsbasedonreachabilityinformationandpolicy.allowssubnettoadvertiseitsexistencetorestofInternet:“Iamhere”BGPbasicspairsofrouters(BGPpeers)exchangeroutinginfooversemi-permanentTCPconnections:BGPsessionsBGPsessionsneednotcorrespondtophysicallinks.whenAS2advertisesaprefixtoAS1:AS2promisesitwillforwarddatagramstowardsthatprefix.AS2canaggregateprefixesinitsadvertisementDistributingreachabilityinfousingeBGPsessionbetween3aand1c,AS3sendsprefixreachabilityinfotoAS1.1ccanthenuseiBGPdodistributenewprefixinfotoallroutersinAS11bcanthenre-advertisenewreachabilityinfotoAS2over1b-to-2aeBGPsessionwhenrouterlearnsofnewprefix,itcreatesentryforprefixinitsforwardingtable.WhydifferentIntra-andInter-ASrouting?

Policy:

Inter-AS:adminwantscontroloverhowitstrafficrouted,whoroutesthroughitsnet.Intra-AS:singleadmin,sonopolicydecisionsneededScale: