計算機網(wǎng)絡英文版課件：slide 05 The Network Layer – Part 1

TheNetworkLayer–Part1ContentsNetworkLayerDesignIssues(Chapter5.1)TheIPProtocol(Chapter5.6.1)IPAddresses(Chapter5.6.2)

InternetControlProtocols(Chapter5.6.3)IntroductionThenetworklayerisconcernedwithgettingpackets

fromthesourceallthewaytothedestination.Toachieveitsgoals,thenetworklayermustknowaboutthetopologyofthecommunicationsubnet(i.e.,thesetofallrouters)andchooseappropriatepathsthroughit.Itmustalsotakecaretochooseroutestoavoidoverloadingsomeofthecommunicationlinesandrouterswhileleavingothersidle.NetworkLayerDesignIssuesStore-and-ForwardPacketSwitchingServicesProvidedtotheTransportLayerImplementationofConnectionlessServiceImplementationofConnection-OrientedServiceComparisonofVirtual-CircuitandDatagramSubnetsStore-and-ForwardPacketSwitchingTheenvironmentofthenetworklayerprotocols.fig5-1ServicesProvidedtotheTransportLayerThenetworklayerserviceshavebeendesignedwiththefollowinggoalsinmind.TheservicesshouldbeindependentoftheroutertechnologyThetransportlayershouldbeshieldedfromthenumber,type,andtopologyoftherouterspresentThenetworkaddressesmadeavailabletothetransportlayershoulduseauniformnumberingplan,evenacrossLANsandWANsProvidingconnection-orientedserviceorconnectionlessservice?ImplementationofConnectionlessServiceRoutingwithinadiagramsubnet.ImplementationofConnection-OrientedServiceRoutingwithinavirtual-circuitsubnet.ComparisonofVirtual-CircuitandDatagramSubnetsComparisonofdatagramandvirtual-circuitsubnetsTheNetworkLayerintheInternet(1)TheIPProtocolIPAddressesInternetControlProtocolsCollectionofSubnetworksTheInternetisaninterconnectedcollectionofmanynetworksTheIPProtocolTheIPv4(InternetProtocol)header.numberof32-bitwordsintheheader3-bitprecedencefield;4TOSbits,andanunusedbitsetto0totallengthoftheIPdatagraminbytesuniquelyidentifieseachdatagramsentupperlimitonthenumberofroutersthroughwhichadatagramcanpasscalculatedoverIPheaderonlyAvariable-lengthlistofoptionalinformationOptionsdefined:securityandhandlingrestrictions,recordroute,timestamp,loosesourcerouting,strictsourceroutingTheseoptionsarerarelyusedandnotallhostsandrouterssupportalltheoptionsdon’tfragmentation.containstheoffsetofthisfragmentfromthebeginningoftheoriginaldatagram.turnedonforeachfragmentcomprisingadatagramexceptthefinalfragment.IPHeader4-bitversion4-bitheaderlength4-bittypeofservice(TOS)16-bittotallength(inbytes)16-bitidentification0DFMF13-bitfragmentoffset8-bittimetolive(TTL)8-bitprotocol16-bitheaderchecksum32-bitsourceIPaddress32-bitdestinationIPaddressoption(ifany)data20bytesExtractedfromTCP/IPIllustrated,Volume1,byW.RichardStevens,IPAddressesIPaddressformats.IPAddresses(2)SpecialIPaddresses.SubnetsFigure5-57

AcampusnetworkconsistingofLANsforvariousdepartments.Figure5-58

AclassBnetworksubnettedinto64subnets.NAT–NetworkAddressTranslationPlacementandoperationofaNATbox.InternetControlMessageProtocolTheprincipalICMPmessagetypes.5-61IPheaderICMPmessageIPdatagram20bytesICMPmessagesencapsulatedwithinanIPdatagram(contentsdependsontypeandcode)8-bytetype8-bytecode16-bytechecksum078151631ICMPmessageExtractedfromTCP/IPIllustrated,Volume1,byW.RichardStevens,typecodeDescriptionQueryError00echoreply(pingreply)*3destinationunreachable:0networkunreachable*1hostunreachable*2protocolunreachable*3portunreachable*4fragmentationneededbutdon't-fragmentbitset*5sourceroutefailed*6destinationnetworkunknown*7destinationhostunknown*8sourcehostisolated(obsolete)*9destinationnetworkadministrativelyprohibited*10destinationhostadministrativelyprohibited*11networkunreachableforTOS*12hostunreachableforTOS*13communicationadministrativelyprohibitedbyfiltering*14hostprecedenceviolation*15precedencecutoffineffect*40Sourcequench(elementaryflowcontrol.)*ICMPmessagetypes(part1)ICMPerrormessagealwayscontainstheIPheaderandthefirst8bytesoftheIPdatagramthatcausedtheICMPerrortobegeneratedTheICMP"hostunreachable"errormessageissentbyarouterwhenitreceivesanIPdatagramthatitcannotdeliverorforward.ExtractedfromTCP/IPIllustrated,Volume1,byW.RichardStevens,ICMPmessagetypes(part2)typecodeDescriptionQueryError5redirect(Section9.5):0redirectfornetwork*1redirectforhost*2redirectfortype-of-serviceandnetwork*3redirectfortype-of-serviceandhost*80echorequest(Pingrequest)*90routeradvertisement*100routersolicitation*11timeexceeded:0time-to-liveequals0duringtransit(Traceroute)*1time-to-liveequals0duringreassembly*12parameterproblem:0IPheaderbad(catchallerror)*1requiredoptionmissing*130timestamprequest*140timestampreply*150informationrequest(obsolete)*160informationreply(obsolete)*170addressmaskrequest*180addressmaskreply*TheICMPtimestamprequestallowsasystemtoqueryanotherforthecurrenttime.TheICMPaddressmaskrequestisintendedforadisklesssystemtoobtainitssubnetmaskExtractedfromTCP/IPIllustrated,Volume1,byW.RichardStevens,ARP–TheAddressResolutionProtocolThreeinterconnected/24networks:twoEthernetsandanFDDIring.ThespecialEthernetdestinationaddressofallonebitsmeansthebroadcast

address.EthernetdestinationaddrEthernetsourceaddrframetypehardtypeprottypeopsenderIPaddrtargetIPaddrSenderEthernetaddrtargetEthernetaddrEthernetheader28bytesARPrequest/reply662221126464protsizehardsizeFigure4.3FormatofARPrequestorreplypacketwhenusedonanEthernet=0x0806=1:forEthernet=0x0800:forIPaddress=1:ARPrequest;=2:ARPreply(2);=3:RARPrequest;=4:RARPreplyRARP,BOOTP,andDHCPRARP(ReverseAddressResolutionProtocol):forthedisklesssystemtoaskforIPaddress(inanRARPreply).usesadestinationaddressofall1s(limitedbroadcasting)toreachtheRARPserver,suchbroadcastsarenotforwardedbyrouters,soaRARPserverisneededoneachnetworkBOOTP:BOOTPusesUDPmessages,whichareforwardedoverroutersAseriousproblemwithBOOTPisthatitrequiresmanualconfigurationoftablesmappingIPaddresstoEthernetaddress.DHCP(DynamicHostConfigurationProtocol):DHCPallowsbothmanualIPaddressassignmentandautomaticassignment.DynamicHostConfigurationProtocolOper