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
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