英文阅读材料.docx

英文阅读材料.docx

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英文阅读材料

附录A英文阅读材料

本阅读材料引自BehrouzA.Forouzan所著DATACOMMUNICATIONSANDNETWORKINGSECONDEDITION,McGraw-HillCompanies,Inc.（清华大学影印版2001.4）。

在此，特对作者表达谢意！

CHAPTER24

TCP/IPProtocolSuite:

Part1

TheTransmissionControlProtocol/InternetworkingProtocol（TCP/IP）isasetofprotocols,oraprotocolsuite,thatdefineshowalltransmissionsareexchangedacrosstheInternet.Namedafteritstwomostpopularprotocols,TCP/IPhasbeeninactiveuseformanyyearsandhasdemonstrateditseffectivenessonaworldwidescale.

24.1OVERVIEWOFTCP/IP

In1969,aprojectwasfundedbytheAdvancedResearchProjectAgency（ARPA）,anarmoftheU.S.DepartmentofDefense.ARPAestablishedapacket-switchingnetworkofcomputerslinkedbypoint-to-pointleasedlinescalledAdvancedResearchProjectAgencyNetwork（ARPANET）thatprovidedabasisforearlyresearchintonetworking.TheconventionsdevelopedbyARPAtospecifyhowindividualcomputerscouldcommunicateacrossthatnetworkbecameTCP/IP.

Asnetworkingpossibilitiesgrewtoincludeothertypesoflinksanddevices,ARPAadaptedTCP/IPtothedemandsofthenewtechnology.AsinvolvementinTCP/IPgrew,thescopeofARPANETexpandeduntilitbecamethebackboneofaninternetworktodayreferredtoastheInternet.

TCP/IPandtheInternet

TCP/IPandtheconceptofinternetworkingdevelopedtogether,eachshapingthegrowthoftheother.Beforemovingmoredeeplyintotheprotocols,however,weneedtounderstandhowTCP/IPrelatestothephysicalentityofanyInternetitserves.

AninternetunderTCP/IPoperateslikeasinglenetworkconnectingmanycomputersofanysizeandtype.Internally,aninternet（or,morespecifically,theInternet）isaninterconnectionofindependentphysicalnetworks（suchasLANs）linkedtogetherbyinternetworkingdevices.Figure24.1showsthetopologyofapossibleInternet.Inthisexample,thelettersA,B,C,andsoonrepresenthosts.AhostinTCP/IPisacomputer.Thesolidcirclesinthefigure,numbered1,2,3,andsoon,areroutersorgateways.Thelargerovalscontainingromannumerals（I,II,III,etc.）representseparatephysicalnetworks.

Figure24.1AninternetaccordingtoTCP/IP

ToTCP/IP,thesameinternetappearsquitedifferently（seeagainFigure24,1）.TCP/IPconsidersallinterconnectedphysicalnetworkstobeonehugenetwork.Itconsidersallofthehoststobeconnectedtothislargerlogicalnetworkratherthantotheirindividualphysicalnetworks.

TCP/IPandOSI

TransmissionControlProtocol（TCP）wasdevelopedbeforetheOSImodel.Therefore,thelayersintheTCP/IPprotocoldonotmatchexactlywiththoseintheOSImodel.TheTCP/IPprotocolismadeoffivelayers:

physical,datalink,network,transport,andapplication.TheapplicationlayerinTCP/IPcanbeequatedwiththecombinationofsession,presentation,andapplicationlayersoftheOSImodel.

Atthetransportlayer,TCP/IPdefinestwoprotocolsTCPandUserDatagramProtocol（UDP）.Atthenetworklayer,themainprotocoldefinedbyTCP/IPisinternetworkingProtocol（IP）,althoughtherearesomeotherprotocolsthatsupportdatamovementinthislayer.

Atthephysicalanddatalinklayers,TCP/IPdoesnotdefineanyspecificprotocol.Itsupportsallofthestandardandproprietaryprotocolsdiscussedearlierinthisbook.AnetworkinaTCP/IPInternetworkingcanbealocalareanetwork（LAN）,ametropolitanareanetwork（MAN）,orawideareanetwork（WAN）.

Encapsulation

Figure24.2showstheencapsulationofdataunitsatdifferentlayersoftheTCP/IPprotocolsuite.Thedataunitcreatedattheapplicationlayeriscalledamessage.TCPorUDPcreatesadataunitthatiscalledeitherasegmentorauserdatagram.TheIPlayerinturnwillcreateadataunitcalledadatagram.ThemovementofthedatagramacrosstheinternetistheresponsibilityoftheTCP/IPprotocol.However,tobeabletomovephysicallyfromonenetworktoanother,thedatagrammustbeencapsulatedinaframeinthedatalinklayeroftheunderlyingnetworkandfinallytransmittedassignalsalongthetransmissionmedia.

Figure24.2TCP/IPandOSImodel

24.2NETWORKLAYER

Atthenetworklayer（or,moreaccurately,theinternetworklayer）,TCP/IPsupportstheinternetworkprotocol（IP）.IP,inturn,containsfoursupportingprotocols:

ARP,RARPICMP,andIGMP.Eachoftheseprotocolsisdescribedindetaillaterinthischapter.

InternetworkProtocol（IP）

IPisthetransmissionmechanismusedbytheTCP/IPprotocols.Itisanunreliableandconnectionlessdatagramprotocol---abest-effortdeliveryservice.Thetermbest-effortmeansthatIPprovidesnoerrorcheckingortracking.IPassumestheunreliabilityoftheunderlyinglayersanddoesitsbesttogetatransmissionthroughtoitsdestination,butwithnoguarantees.Aswehaveseeninpreviouschapters,transmissionsalongphysicalnetworkscanbedestroyedforanumberofreasons.Noisecancausebiterrorsduringtransmissionacrossamedium,acongestedroutermaydiscardadatagramifitisunabletorelayitbeforeatimelimitrunsout,routingquirkscanendinloopingandtheultimatedestructionofadatagram,anddisabledlinksmayleavenousablepathtothedestination.

Ifreliabilityisimportant,IPmustbepairedwithareliableprotocolsuchasTCP.Anexampleofamorecommonlyunderstoodbest-effortdeliveryserviceisthepostoffice.Thepostofficedoesitsbesttodeliverthemailbutdoesnotalwayssucceed.Ifanunregisteredletterislost,itisuptothesenderorwould-berecipienttodiscoverthelossandrectifytheproblem.Thepostofficeitselfdoesnotkeeptrackofeveryletterandcannotnotifyasenderoflossordamage.

Whentheletterisdelivered,thereceivermailsthepostcardbacktothesendertoindicatesuccess.Ifthesenderneverreceivesthepostcard,heorsheassumestheletterwaslostandsendsoutanothercopy.

IPtransportsdatainpacketscalleddatagrams（describedbelow）,eachofwhichistransportedseparately.Datagramsmaytravelalongdifferentroutesandmayarriveoutofsequenceorduplicated.IPdoesnotkeeptrackoftheroutesandhasnofacilityforreorderingdatagramsoncetheyarrive.Becauseitisaconnectionlessservice,IPdoesnotcreatevirtualcircuitsfordelivery.Thereisnocallsetuptoalertthereceivertoanincomingtransmission.

ThelimitedfunctionalityofIPshouldnotbeconsideredaweakness,however

IPprovidesbare-bonestransmissionfunctionsthatfreetheusertoaddonlythosefacilitiesnecessaryforagivenapplicationandtherebyallowsformaximumefficiency.

Datagram

PacketsintheIPlayerarecalleddatagrams.Figure24.3showstheIPdatagramformat.Adatagramisavariable-lengthpacket（upto65,536bytes）consistingoftwoparts:

headeranddata.Theheadercanbefrom20to60bytesandcontainsinformationessentialtoroutinganddelivery.ItiscustomaryinTCP/IPtoshowtheheaderinfourbytesections.Abriefdescriptionofeachfieldisinorder.

.Version.ThefirstfielddefinestheversionnumberoftheIP.Thecurrentversionis4（IPv4）,withabinaryvalueof0100.

.Headerlength（HLEN）.TheHLENfielddefinesthelengthoftheheaderinmultiplesoffourbytes.Thefourbitscanrepresentanumberbetween0and15,which,whenmultipliedby4,givesamaximumof60bytes.

.Servicetype.Theservicetypefielddefineshowthedatagramshouldbehandled.

Itincludesbitsthatdefinethepriorityofthedatagram.Italsocontainsbitsthatspecifythetypeofservicethesenderdesiressuchasthelevelofthroughput,reliability,anddelay.

.Totallength.ThetotallengthfielddefinesthetotallengthoftheIPdatagram.Itisatwo-bytefield（16bits）andcandefineupto65,535bytes.

.Identification.Theidentificationfieldisusedinfragmentation.Adatagram,

whenpassingthroughdifferentnetworks,maybedividedintofragmentstomatch

thenetworkframesize.Whenthishappens,eachfragmentisidentifiedwithasequencenumberinthisfield.

Figure24.3IPdamgram

.Flags.Thebitsintheflagsfielddealwithfragmentation（thedatagramcanorcannotbefragmented,canbethefirst,middle,orlastfragment,etc.）.

.Fragmentationoffset.Thefragmentationoffsetisapointerthatshowstheoffset

ofthedataintheoriginaldatagram（ifitisfragmented）.

.Timetolive.Thetime-to-livefielddefinesthenumberofhopsadatagramcan

travelbeforeitisdiscarded.Thesourcehost,whenitcreatesthedatagram,setsthisfieldtoaninitialvalue.Then,asthedatagramtravelsthroughtheinternet,routerbyrouter,eachrouterdecrementsthisvaluebyl.Ifthisvaluebecomes0beforethedatagramreachesitsfinaldestination,thedatagramisdiscarded.Thispreventsadatagramfromgoingbackandforthforeverbetweenrouters.

.Protocol.Theprotocolfielddefineswhichupper-layerprotocoldataareencapsulatedinthedatagram（TCP,UDP,ICMP,etc.）.

.Headerchecksum.Thisisa16-bitfieldusedtochecktheintegrityoftheheader,

nottherestofthepacket.

.Sourceaddress.Thesourceaddressfieldisafour-byte（32-bit）Internetaddress.Itidentifiestheoriginalsourceofthedatagram.

.Destinationaddress.Thedestinationaddressfieldisafour-byte（32-bit）Internetaddress.Itidentifiesthefinaldestinationofthedatagram.

.Options.TheoptionsfieldgivesmorefunctionalitytotheIPdatagram.Itcancarryfieldsthatcontrolrouting,timing,management,andalignment.

24.3ADDRESSING

Inaddiiiontothephysicaladdresses（containedonNICs）thatidentifyindividualdevices,theInternetrequiresanadditionaladdressingconvention:

anaddressthatidentifiestheconnectionofahosttoitsnetwork.

Eachinternetaddressconsistsoffourbytes（32bits）,defining