计算机网络专业英语 课件 L14-2 Text 2 Basic Requirements of Protocols

L14-2

Lesson14CommunicationsProtocolsText2BasicRequirementsofProtocolsKeyPoints要点CRC、MTU-freeRelevant/relevancePiece\unit\part\area\fieldAddress、地址映射Reject、refuse理解课文Messagesaresentandreceivedoncommunicatingsystemstoestablishcommunications.Protocolsshouldthereforespecifyrulesgoverningthetransmission.Ingeneral,muchofthefollowingshouldbeaddressed解决:DataFormatsforDataExchangeTheexchangeddigitalmessagebitstringsaredividedinfieldsandeachfieldcarriesinformationrelevanttotheprotocol.Conceptuallythebitstringisdividedintotwopartscalledtheheaderareaandthedataarea.Theactualmessageisstoredinthedataarea,sotheheaderareacontainsthefieldswithmorerelevancetotheprotocol.Bitstringslongerthanthemaximumtransmissionunit(MTU)aredividedinpieces块ofappropriatesize.AddressFormatsforDataExchangeAddressesareusedtoidentifyboththesenderandtheintendedreceiver(s).Theaddressesarestoredintheheaderareaofthebitstrings,allowingthereceiverstodeterminewhetherthebitstringsareintendedforthemselvesandshouldbeprocessedorshouldbeignored.Aconnectionbetweenasenderandareceivercanbeidentifiedusinganaddresspair(senderaddress,receiveraddress).Usually,someaddressvalueshavespecialmeanings.Anall1saddresscouldbetakentomeananaddressingofallstationsonthenetwork,sosendingtothisaddresswouldresultinabroadcastonthelocalnetwork.Therulesdescribingthemeaningsoftheaddressvaluearecollectivelycalledanaddressingscheme寻址方案.AddressMappingSometimesprotocolsneedtomapaddressesofoneschemeonaddressesofanotherscheme,forinstance,totranslatealogicalIPaddressspecifiedbytheapplicationtoanEthernethardwareaddress.Thisisreferredtoasaddressmapping.RoutingWhensystemsarenotdirectlyconnected,intermediarysystemsalongtheroutetotheintendedreceiver(s)isneedtoforwardmessagesonbehalfofthesender.OntheInternet,thenetworksareconnectedusingrouters.Thiswayofconnectingnetworksiscalledinternetworking.CRC(CyclicRedundancyCheck)Detectionoftransmissionerrorsisnecessaryonnetworkswhichcannotguaranteeerror-freeoperation运行.Inacommonapproach,CRCsofthedataareaareaddedtotheendofpackets,makingitpossibleforthereceivertodetectdifferencescausedbyerrors.ThereceiverrejectsthepacketsonCRCdifferencesandarrangessomehowforretransmission.从语气上看：refuse属普通用词；而reject则属于正式用词，从意思上看：refuse表示“拒绝”，强调态度的肯定和坚决；而reject则强调摈弃、不采用或不使用的行动，只能是及物动词。AcknowledgementsAcknowledgementsofcorrectreceptionofpacketsarerequiredforconnection-orientedcommunication.Acknowledgementsaresentfromreceiversbacktotheirrespectivesenders.LossofInformation-TimeoutsandRetriesPacketsmaybelostonthenetworkorsufferfromlongdelays.Tocopewiththis,undersomeprotocols,asendermayexpectanacknowledgementofcorrectreceptionfromthereceiverwithinacertainamountoftime.Ontimeouts,thesendermustassumethepacketwasnotreceivedandretransmitit.Incaseofapermanentlybrokenlink,theretransmissionhasnoeffectsothenumberofretransmissionsislimited.Exceedingtheretrylimitisconsideredanerror.AccessControlDirectionofinformationflowneedstobeaddressed解决

iftransmissionscanonlyoccurinonedirectionatatimeasonhalf-duplexlinks.ThisisknownasMediaAccessControl(MAC).Arrangementshavetobemadetoaccommodate适应

thecasewhentwopartieswanttogaincontrolatthesametime.SequenceControllingWehaveseenthatlongbitstringsaredividedinpieces,andthensentonthenetworkindividually.Thepiecesmaygetlostordelayedortakedifferentroutestotheirdestinationonsometypesofnetworks.Asaresult，piecesmayarriveoutofsequence.Retransmissionscanresultinduplicate重复的pieces.Throughmarksonthepieceswithsequenceinformationbythesender,thereceivercandeterminewhatwaslostorduplicated,askfornecessaryretransmissionsandreassembletheoriginalmessage.FlowControlFlowcontrolisneededwhenthesendertransmitsfasterthanthereceiv