计算机网络复习-课件chapterip

NetworkLayer4-1Chapter4

NetworkLayerComputerNetworking:ATopDownApproach

4thedition.

JimKurose,KeithRoss

Addison-Wesley,July2007.

NetworkLayer4-2Chapter4:NetworkLayerChaptergoals:

understandprinciplesbehindnetworklayerservices:networklayerservicemodelsforwardingversusroutinghowarouterworksrouting(pathselection)dealingwithscaleadvancedtopics:IPv6,mobilityinstantiation,implementationintheInternetNetworkLayer4-3Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-4Networklayertransportsegmentfromsendingtoreceivinghostonsendingsideencapsulatessegmentsintodatagramsonrcvingside,deliverssegmentstotransportlayernetworklayerprotocolsineveryhost,routerrouterexaminesheaderfieldsinallIPdatagramspassingthroughitapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalnetworkdatalinkphysicalNetworkLayer4-6TwoKeyNetwork-LayerFunctionsforwarding:movepacketsfromrouter’sinputtoappropriaterouteroutputrouting:determineroutetakenbypacketsfromsourcetodest.routingalgorithmsanalogy:routing:processofplanningtripfromsourcetodestforwarding:processofgettingthroughsingleinterchangeNetworkLayer4-71230111valueinarrivingpacket’sheaderroutingalgorithmlocalforwardingtableheadervalueoutputlink01000101011110013221InterplaybetweenroutingandforwardingNetworkLayer4-8Connectionsetup3rdimportantfunctioninsomenetworkarchitectures:ATM,framerelay,X.25beforedatagramsflow,twoendhostsandinterveningroutersestablishvirtualconnectionroutersgetinvolvednetworkvstransportlayerconnectionservice:network:betweentwohosts(mayalsoinvolveinterveningroutersincaseofVCs)transport:betweentwoprocessesNetworkLayer4-9NetworkservicemodelQ:Whatservicemodelfor“channel”transportingdatagramsfromsendertoreceiver?Exampleservicesforindividualdatagrams:guaranteeddeliveryguaranteeddeliverywithlessthan40msecdelayExampleservicesforaflowofdatagrams:in-orderdatagramdeliveryguaranteedminimumbandwidthtoflowrestrictionsonchangesininter-packetspacingNetworkLayer4-10Networklayerservicemodels:NetworkArchitectureInternetATMATMATMATMServiceModelbesteffortCBRVBRABRUBRBandwidthnoneconstantrateguaranteedrateguaranteedminimumnoneLossnoyesyesnonoOrdernoyesyesyesyesTimingnoyesyesnonoCongestionfeedbackno(inferredvialoss)nocongestionnocongestionyesnoGuarantees?NetworkLayer4-11Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-12Networklayerconnectionandconnection-lessservicedatagramnetworkprovidesnetwork-layerconnectionlessserviceVCnetworkprovidesnetwork-layerconnectionserviceanalogoustothetransport-layerservices,but:service:host-to-hostnochoice:networkprovidesoneortheotherimplementation:innetworkcoreNetworkLayer4-13Virtualcircuitscallsetup,teardownforeachcallbeforedatacanfloweachpacketcarriesVCidentifier(notdestinationhostaddress)everyrouteronsource-destpathmaintains“state”foreachpassingconnectionlink,routerresources(bandwidth,buffers)maybeallocatedtoVC(dedicatedresources=predictableservice)“source-to-destpathbehavesmuchliketelephonecircuit”performance-wisenetworkactionsalongsource-to-destpathNetworkLayer4-14VCimplementationaVCconsistsof:pathfromsourcetodestinationVCnumbers,onenumberforeachlinkalongpathentriesinforwardingtablesinroutersalongpathpacketbelongingtoVCcarriesVCnumber(ratherthandestaddress)VCnumbercanbechangedoneachlink.NewVCnumbercomesfromforwardingtableNetworkLayer4-15Forwardingtable122232123VCnumberinterfacenumberinginterfaceingVC#OutgoinginterfaceOutgoingVC#11232226311837217197387…………Forwardingtableinnorthwestrouter:Routersmaintainconnectionstateinformation!NetworkLayer4-16Virtualcircuits:signalingprotocolsusedtosetup,maintainteardownVCusedinATM,frame-relay,X.25notusedintoday’sInternetapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Initiatecall2.ingcall3.Acceptcall4.Callconnected5.Dataflowbegins6.ReceivedataNetworkLayer4-17Datagramnetworksnocallsetupatnetworklayerrouters:nostateaboutend-to-endconnectionsnonetwork-levelconceptof“connection”packetsforwardedusingdestinationhostaddresspacketsbetweensamesource-destpairmaytakedifferentpathsapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Senddata2.ReceivedataNetworkLayer4-18Forwardingtable

DestinationAddressRange

LinkInterface11001000000101110001000000000000through01100100000010111000101111111111111001000000101110001100000000000through11100100000010111000110001111111111001000000101110001100100000000through211001000000101110001111111111111otherwise34billionpossibleentriesNetworkLayer4-19Longestprefixmatching

PrefixMatch

LinkInterface110010000001011100010011001000000101110001100011100100000010111000112otherwise3DA:11001000000101110001100010101010ExamplesDA:11001000000101110001011010100001Whichinterface?Whichinterface?NetworkLayer4-20DatagramorVCnetwork:why?Internet(datagram)dataexchangeamongcomputers“elastic”service,nostricttimingreq.“smart”endsystems(computers)canadapt,performcontrol,errorrecoverysimpleinsidenetwork,complexityat“edge”manylinktypesdifferentcharacteristicsuniformservicedifficultATM(VC)evolvedfromtelephonyhumanconversation:stricttiming,reliabilityrequirementsneedforguaranteedservice“dumb”endsystemstelephonescomplexityinsidenetworkNetworkLayer4-21Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouterandhowtoroute4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-22RouterArchitectureOverviewTwokeyrouterfunctions:runroutingalgorithms/protocol(RIP,OSPF,BGP)forwardingdatagramsfromingtooutgoinglinkroutingprotocltable3input3switchinginputoutputforwardingRoutingtableoutput……11133122223——network2——datalink1——physicalphyDatalinknetwork

swtching

inputphyDatalink

outputswitchingNetworkLayer4-26InputPortFunctionsDecentralizedswitching:

givendatagramdest.,lookupoutputportusingforwardingtableininputportmemorygoal:completeinputportprocessingat‘linespeed’queuing:ifdatagramsarrivefasterthanforwardingrateintoswitchfabricPhysicallayer:bit-levelreceptionDatalinklayer:e.g.,Ethernetseechapter5NetworkLayer4-27ThreetypesofswitchingfabricsNetworkLayer4-28SwitchingViaMemoryFirstgenerationrouters:traditionalcomputerswithswitchingunderdirectcontrolofCPUpacketcopiedtosystem’smemoryspeedlimitedbymemorybandwidth(2buscrossingsperdatagram)InputPortOutputPortMemorySystemBusNetworkLayer4-29SwitchingViaaBusdatagramfrominputportmemorytooutputportmemoryviaasharedbusbuscontention:switchingspeedlimitedbybusbandwidth32Gbpsbus,Cisco5600:sufficientspeedforaccessandenterpriseroutersNetworkLayer4-30SwitchingViaAnInterconnectionNetworkebusbandwidthlimitationsSwitchFabric:2nbuses,n:input;n:outputadvanceddesign:fragmentingdatagramintofixedlengthcells,switchcellsthroughthefabric.Cisco12000:switches60GbpsthroughtheinterconnectionnetworkNetworkLayer4-31OutputPortsBufferingrequiredwhendatagramsarrivefromfabricfasterthanthetransmissionrateSchedulingdisciplinechoosesamongqueueddatagramsfortransmissionNetworkLayer4-32Outputportqueueingbufferingwhenarrivalrateviaswitchexceedsoutputlinespeedqueueing(delay)andlossduetooutputportbufferoverflow!NetworkLayer4-33Howmuchbuffering?RFC3439ruleofthumb:averagebufferingequalto“typical”RTT(say250msec)timeslinkcapacityCe.g.,C=10Gpslink:2.5GbitbufferRecentmendation:withNflows,bufferingequaltoRTTC.NNetworkLayer4-34InputPortQueuingFabricslowerthaninputportscombined->queueingmayoccuratinputqueuesHead-of-the-Line(HOL)blocking:queueddatagramatfrontofqueuepreventsothersinqueuefrommovingforwardqueueingdelayandlossduetoinputbufferoverflow!fromAtoBNetwork网络网络routerABRoutingtableDestnexthopRoutingtableDestnexthopRoutingtableDestnexthopStoreandforwardDirectdeliver:sourceanddestinationareinthesamenetwork.Indirectdeliver:sourceanddestinationareinthedifferentnetwork.NetworkLayer4-37Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-39TheInternetNetworklayerforwardingtableHost,routernetworklayerfunctions:RoutingprotocolspathselectionRIP,OSPF,BGPIPprotocoladdressingconventionsdatagramformatpackethandlingconventionsICMPprotocolerrorreportingrouter“signaling”Transportlayer:TCP,UDPLinklayerphysicallayerNetworklayerNetworkLayer4-40Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-41IPdatagramformatverlength32bitsdata(variablelength,typicallyaTCPorUDPsegment)16-bitidentifierheaderchecksumtimetolive32bitsourceIPaddressIPprotocolversionnumberheaderlength(bytes)maxnumberremaininghops(decrementedateachrouter)forfragmentation/reassemblytotaldatagramlength(bytes)upperlayerprotocoltodeliverpayloadtohead.lentypeofservice“type”ofdataflgsfragmentoffsetupperlayer32bitdestinationIPaddressOptions(ifany)E.g.timestamp,recordroutetaken,specifylistofrouterstovisit.howmuchoverheadwithTCP?20bytesofTCP20bytesofIP=40bytes+applayeroverheadIPheaderRFC791identification:flag:

MF：MoreFragments；(the1stbit)DF：Don’tFragment；(the2ndbit)Fragmentoffset8BitasaunitNetworkLayer4-45IPFragmentation&ReassemblynetworklinkshaveMTU(max.transfersize)-largestpossiblelink-levelframe.differentlinktypes,differentMTUslargeIPdatagramdivided(“fragmented”)withinnetonedatagramesseveraldatagrams“reassembled”onlyatfinaldestinationIPheaderbitsusedtoidentify,orderrelatedfragmentsfragmentation:in:onelargedatagramout:3smallerdatagramsreassemblyprotocol

MTU（Byte)TokenRing（16M）17914 TokenRing

（4M)4464FDDI4352ethernet

1500X.25576NetworkLayer4-47IPFragmentationandReassemblyID=xoffset=0fragflag=0length=4000ID=xoffset=0fragflag=1length=1500ID=xoffset=185fragflag=1length=1500ID=xoffset=370fragflag=0length=1040OnelargedatagramesseveralsmallerdatagramsExample4000bytedatagramMTU=1500bytes1480bytesin

datafieldoffset=1480/8NetworkLayer4-48Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingICMPIPv64.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-49IPAddressing:introductionIPaddress:32-bitidentifierforhost,routerinterface

interface:connectionbetweenhost/routerandphysicallinkrouter’stypicallyhavemultipleinterfaceshosttypicallyhasoneinterfaceIPaddressesassociatedwitheachinterface7=11011111000000010000000100000001223111“dotteddecimalnotation”100011111000000010001001100100001stByte=1282ndByte=1433rdByte=1374thByte=14444SpecialIp1，broadcastip：Thesubnetbroadcast2455hostsinthesamesubnetreceiveit.Limitedbroadcast55,allhostsreceiveit.2，multicastip：D，–55IGMP(01-00-5e-00-00-00~01-00-5e-ff-ff-ff)3，“0”：Host+networkHost=0,networkaddress;Network=0,hostaddress;

4，loopback~55；PrivateIP

-55 -55 -55

NAT:NetworkAddressTranslationUsedinlanSubnet:Why?dlutCollege2libraryCollege101014563………subnetsubnet

subnetnetworkR1R3R2Subnets:onemoreexampleNetworkLayer4-58Subnets/24/24/24RecipeTodeterminethesubnets,detacheachinterfacefromitshostorrouter,creatingislandsofisolatednetworks.Eachisolatednetworkiscalledasubnet.Subnetmask:/24NetworkLayer4-59SubnetsHowmany?7

Subnetmask:

A:(/8);

B:(/16);

C:(/24)

networknumberhostnumbersubnetnumbernetworknumberhostnumbersubnetpart(highorderbits)hostpart(loworderbits)Thebitsofnetwork+subnetare1.NetworkLayer4-62ISP'sblock11001000000101110001000000000000/20Organization0110010000001011100010000

00000000/23Organization111001000000101110001001000000000/23Organization211001000000101110001010000000000/23...…..….….Organization711001000000101110001111000000000/23

Question:Subnetmaskis24HostIp:41/27Cumpute:1,networkaddressandHostaddress?2,howmanysubnetsarecreatedatmost?ComputethemIp:networkid+hostid;Networkaddress:networkid+0;Hostaddress:0+hostid;2241111…..1110000041:24111110001=17

7;24241111…..11100000Eightsubnets:；2；4；6；28；60；92；24；NetworkLayer4-65Hierarchicaladdressing:routeaggregation“Sendmeanythingwithaddressesbeginning/20”/23/23/23Fly-By-Night-ISPOrganization0Organization7InternetOrganization1ISPs-R-Us“Sendmeanythingwithaddressesbeginning/16”/23Organization2......Hierarchicaladdressingallowsefficientadvertisementofroutinginformation:NetworkLayer4-66Hierarchicaladdressing:morespecificroutesISPs-R-UshasamorespecificroutetoOrganization1“Sendmeanythingwithaddressesbeginning/20”/23/23/23Fly-By-Night-ISPOrganization0Organization7InternetOrganization1ISPs-R-Us“Sendmeanythingwithaddressesbeginning/16or/23”/23Organization2......NetworkLayer4-67IPaddresses:howtogetone?Q:HowdoesahostgetIPaddress?hard-codedbysystemadmininafileWindows:control-panel->network->configuration->tcp/ip->propertiesUNIX:/etc/rc.configDHCP:

DynamicHostConfigurationProtocol:dynamicallygetaddressfromasserver“plug-and-play”NetworkLayer4-68DHCP:DynamicHostConfigurationProtocolGoal:allowhosttodynamicallyobtainitsIPaddressfromnetworkserverwhenitjoinsnetworkCanrenewitsleaseonaddressinuseAllowsreuseofaddresses(onlyholdaddresswhileconnectedan“on”)Supportformobileuserswhowanttojoinnetwork(moreshortly)DHCPoverview:hostbroadcasts“DHCPdiscover”msgDHCPserverrespondswith“DHCPoffer”msghostrequestsIPaddress:“DHCPrequest”msgDHCPserversendsaddress:“DHCPack”msgNetworkLayer4-69DHCPclient-serverscenario7ABE

DHCP

server

arrivingDHCPclientneedsaddressinthisnetworkNetworkLayer4-70DHCPclient-serverscenarioDHCPserver:arrivingclienttimeDHCPdiscoversrc:,68dest.:55,67yiaddr:transactionID:654DHCPoffersrc:,67dest:55,68yiaddrr:transactionID:654Lifetime:3600secsDHCPrequestsrc:,68dest::55,67yiaddrr:transactionID:655Lifetime:3600secsDHCPACKsrc:,67dest:55,68yiaddrr:transactionID:655Lifetime:3600secsNetworkLayer4-71Q:HowdoesanISPgetblockofaddresses?A:ICANN:InternetCorporationforAssigned

NamesandNumbers互联网名称与数字地址分配机构allocatesaddressesmanagesDNSassignsdomainnames,resolvesdisputesInordertosolvetheshortageofIPaddress1，CIDR无分类编址2,NAT3，IPv6NetworkLayer4-73IPaddressing:CIDRCIDR:

ClasslessInterDomainRoutingsubnetportionofaddressofarbitrarylength:VLSM(VariableLengthSubnetMasking)addressformat:a.b.c.d/x,wherexis#bitsinsubnetportionofaddress1100100000010111

0001000000000000subnetparthostpart/23AsetofIPaddressassignments.5-59IPaddressing:CIDRNetworkLayer4-74e.g./20(212addrs)1000000000001110

00100000000000001000000000001110

00100000000000011000000000001110

00100000000000101000000000001110

00100000000000111000000000001110

00100000000001001000000000001110

00100000000001011000000000001110

00101111111110111000000000001110

00101111111111001000000000001110

00101111111111011000000000001110

00101111111111101000000000001110

0010111111111111Thesame20-bitprefix“min”“max”VLSM#ofIPaddrsw.r.t.lengthofnetworkmask:masklength#ofaddrs

/27 32/26 64/25 128/24 256/23 512/22 1,024/21 2,048/20 4,096/19 8,192/18 16,384/17 32,768/16 65,536/15 131,072/14 262,144/13 524,288NetworkLayer4-76Howtoidentify?Explicit明确:VLSMImplicit含蓄:ClassfulNetworkLayer4-77NetworkLayer4-78NAT:NetworkAddressTranslationlocalnetwork(e.g.,homenetwork)10.0.0/24restofInternetDatagramswithsourceordestinationinthisnetworkhave10.0.0/24addressforsource,destination(asusual)AlldatagramsleavinglocalnetworkhavesamesinglesourceNATIPaddress:,differentsourceportnumbersNetworkLayer4-79NAT:NetworkAddressTranslationMotivation:localnetworkusesjustoneIPaddressasfarasoutsideworldisconcerned:rangeofaddressesnotneededfromISP:justoneIPaddressforalldevicescanchangeaddressesofdevicesinlocalnetworkwithoutnotifyingoutsideworldcanchangeISPwithoutchangingaddressesofdevicesinlocalnetworkdevicesinsidelocalnetnotexplicitlyaddressable,visiblebyoutsideworld(asecurityplus).NetworkLayer4-80NAT:NetworkAddressTranslationImplementation:NATroutermust:

outgoingdatagrams:replace(sourceIPaddress,port#)ofeveryoutgoingdatagramto(NATIPaddress,newport#)...remoteclients/serverswillrespondusing(NATIPaddress,newport#)asdestinationaddr.

remember(inNATtranslationtable)every(sourceIPaddress,port#)to(NATIPaddress,newport#)translationpair

ingdatagrams:replace(NATIPaddress,newport#)indestfieldsofeveryingdatagramwithcorresponding(sourceIPaddress,port#)storedinNATtableNetworkLayer4-81NAT:NetworkAddressTranslationS:,3345D:86,801:hostsendsdatagramto86,80NATtranslationtableWANsideaddrLANsideaddr,500,3345…………S:86,80D:,33454S:,5001D:86,8022:NATrouterchangesdatagramsourceaddrfrom,3345to,5001,updatestableS:86,80D:,500133:Replyarrivesdest.address:,50014:NATrouterchangesdatagramdestaddrfrom,5001to,3345

NetworkLayer4-82NAT:NetworkAddressTranslation16-bitport-numberfield:60,000simultaneousconnectionswithasingleLAN-sideaddress!NATiscontroversial:routersshouldonlyprocessuptolayer3violatesend-to-endargumentNATpossibilitymustbetakenintoaccountbyappdesigners,eg,P2PapplicationsaddressshortageshouldinsteadbesolvedbyIPv6NATtraversal穿透

isacoveralltermusedtodescribethevariousmethodsemployedtosetupandmaintainaTCP/IPnetworkorUDPconnectionsthatcrossNATgateways.ThesetechniquesareneededinmostpeertopeerandVoIPapplicationsNetworkLayer4-84NATtraversalproblemclientwantstoconnecttoserverwithaddressserveraddresslocaltoLAN(clientcan’tuseitasdestinationaddr)onlyoneexternallyvisibleNATtedaddress:solution1:staticallyconfigureNATtoforwardingconnectionrequestsatgivenporttoservere.g.,(,port2500)alwaysforwardedtoport25000NATrouterClient?NetworkLayer4-85NATtraversalproblemsolution2:UniversalPlugandPlay(UPnP)InternetGatewayDevice(IGD)Protocol.AllowsNATtedhostto:learnpublicIPaddress()add/removeportmappings(withleasetimes)i.e.,automatestaticNATportmapconfigurationNATrouterIGDNetworkLayer4-86NATtraversalproblemsolution3:relaying(usedinSkype)NATedclientestablishesconnectiontorelayExternalclientconnectstorelayrelaybridgespacketsbetweentoconnectionsClientNATrouter1.connectiontorelayinitiatedbyNATtedhost2.connectiontorelayinitiatedbyclient3.relayingestablishedNetworkLayer4-87Chapter4:NetworkLayer4.1Introduction4.2Virtualcircuitanddatagramnetworks4.3What’sinsidearouter4.4IP:InternetProtocolDatagramformatIPv4addressingIPv6ICMP4.5RoutingalgorithmsLinkstateDistanceVectorHierarchicalrouting4.6RoutingintheInternetRIPOSPFBGP4.7BroadcastandmulticastroutingNetworkLayer4-88IPv6Initialmotivation:

32-bitaddressspacesoontobecompletelyallocated.Additionalmotivation:headerformathelpsspeedprocessing/forwardingheaderchangestofacilitateQoSIPv6datagramformat:

fixed-length40byteheadernofragmentationallowedIPv6(IPVersion6),又称为IPng(IPNextGeneration)，是IPv4的下一个版本。IPv6的标准文档是RFC2460RFC2461–RFC2466讨论更多的有关IPv6的细节IPv6–overviewIPv6–overviewIPv6把IP地址的长度增加到了16个字节IPv6简化了IP分组的首部格式IPv6增强了对进一步扩展的支持IPv6增强了对QoS(QualityofService)的支持IPv6增强了对安全的支持IPv6增加了对Any