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

L14-2

Lesson14CommunicationsProtocolsText2BasicRequirementsofProtocols

KeyPoints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)isneedtoforwardm