text7_TCP_IP协议簇

TechnicalEnglishForCommunicationEngineering2020/1/10TheTCP/IPProtocolSuiteText7Basedonprotocolresearchanddevelopmentconductedon(关于)itsexperimentalpacket-switchednetwork,ARPANET,theU.S.DepartmentofDefense(DOD)hasissued（发行）asetofmilitarystandardsforcomputer-communicationsprotocols.Althoughtherearefiveoftheseprotocols,theentiresetisknownbythenamesoftwoofthem:transmission-controlprotocol(TCP)andinternetprotocol(IP).TheseprotocolsareinwidespreadusewithintheU.S.defensecommunity.Butwhatismoreinterestingisthattheyhavesteadilybuiltupafollowinginthecommercialarenaduringatimewhenmuchattentionhasbeenfocusedontheinternationalstandardsbasedontheopensystemsinterconnection(OSI)model.TherearehundredsofvendorsthatprovideTCP/IPproducts,andthesearethemostwidelyavailableandmostwidelyusedsetofstandardizedcomputer-communicationsprotocols.舞台、竞技场TheTCP/IParchitecture,liketheOSImodel,islayered.InthecaseofTCP/IP,fourlayersareinvolved:networkaccess,internet,host-to-host,andprocess.ThisarchitectureiscomparedwiththatofOSIinFigure7.1.Thenetwork-accesslayercontainstheprotocolsthatprovideaccesstoacommunicationsnetwork,suchasalocalareanetwork(LAN).Protocolsatthislayerarebetween流程管理acommunicationsnodeandanattachedhost.TheTCP/IPsuitedoesnotincludeanyuniqueprotocolsatthislayer.Rather(不过),theprotocolappropriatefor（适于）aparticularnetwork(e.g.,Ethernet,IEEE802,X.25)isused.Theinternetlayerconsistsoftheproceduresrequiredtoallowdatatotraversemultiplenetworkbetweenhosts.Thus,itmustprovidearouting（路由）连接的InstituteofElectricalandElectronicsEngineers：美国电气与电子工程师协会横穿、穿越function.Thisprotocolsisimplementedwithinhostsandrouters.Arouterisaprocessorconnectingtwonetworks,whoseprimaryfunctionistorelaydatabetweennetworksusinganinternetworkprotocol（网际协议）.Theprotocolatthelayeristheinternetprotocol.AtypicaluseofIPistoconnectmultipleLANswithinthesamebuildingortoconnectLANsatdifferentsitesthroughawideareapacket-switchingnetwork.Thehost-to-hostlayer（到主机层）providesthelogicforassuringthatdataexchangedbetweenhostsarereliablydelivered.Itisalsoresponsiblefordirectingincomingdatatotheintendedapplication.Theprotocolatthislayeristhetransmission-controlprotocol.Finally,theprocesslayercontainsprotocolsforspecificuserapplications.Foreachdifferenttypeofapplication,suchasfiletransfer,aprotocolisneededthatsupportsthatapplication.ThreesuchprotocolsareincludedintheTCP/IPprotocolsuite:SMTP,FTP,andTELNET.SimpleMessageTransferProtocol,简单邮件传输协议；FileTransferProtocol,文件传送[输]协议；远程登录用于远程联接服务的标准协议或者实现此协议的软件OperationofTCPandIPFigure7.2indicateshowtheTCP/IPprotocolsareconfiguredforcommunications.Tomakeclearthatthetotalcommunicationsfacilitymayconsistofmultiplenetworks,theconstituentnetworksareusuallyreferredtoassubnetworks.Somesortofnetwork-accessprotocol(NAP)suchastheEthernetlogic,isusedtoconnectacomputertoasubnetwork.组成部分、成分Thisprotocolenablesthehosttosenddataacrossthesubnetworktoanotherhostor,inthecaseof（在…情况下）ahostonanothersubnetwork,toarouter.IPisimplementedinalltheendsystemsandrouters.Itactsasarelaytomoveablockofdatafromonehost,throughoneormorerouters,toanotherhost.TCPisimplementedonlyintheendsystems:itkeepstrackoftheblocksofdatatoassurethatallaredeliveredreliablytotheappropriateapplications.实施、应用Forsuccessfulcommunication,everyentityintheoverallsystemmusthaveauniqueaddress.Actually,twolevelsofaddressingareneeded.Eachhostonasubnetworkmusthaveauniqueglobalinternetaddress:thisallowsthedatatobedeliveredtotheproperhost.Eachprocesswithinahostmusthaveanaddressthatisuniquewithinthehost:thisallowsthehost-to-hostprotocol(TCP)todeliverdatatotheproperprocess.Theselatteraddressesareknownasports.寻址Letustraceasimpleoperation.Supposethataprocess,associatedwithport1athostA,wishestosendamessagetoanotherprocess,associatedwithport2athostB.TheprocessatAhands（递交）themessagedowntoTCPwithinstructionstosendittohostB,port2.TCPhandsthemessagedowntoIPwithinstructionstosendittohostB.NotethatIPneednotbetoldtheidentityofthedestinationport.AllitneedstoknowisthatthedataareintendedforhostB.Next,IPhandsthemessagedowntothenetwork-accesslayer(e.g.,Ethernetlogic)withinstructionstosendittorouterX(thefirsthoponthewaytoB).Tocontrolthisoperation,controlinformationaswellasuserdatamustbetransmitted,assuggestedinFigure7.3.Letussaythatthesendingprocess驿站、中继站generatesablockofdataandpassesthistoTCP.TCPmaybreakthisblockintosmallerpiecestomakeitmoremanageable.Toeachofthesepieces,TCPappendscontrolinformationknownastheTCPheader(TCPH),formingaTCPsegment.Thecontrolinformationistobeusedbythepeer（对等）TCP-protocolentityathostB.Thefollowingareexamplesofitemsthatareincludedinthisheader:附加、添加条目、项目•Destinationport:WhentheTCPentityatBreceivesthesegment,itmustknowtowhichportthedataaretobedelivered.•Sequencenumber:TCPsequentiallynumbersthesegmentsthatitsendstoaparticulardestinationport,sothatiftheyarriveoutoforder,theTCPentityatBcanreorderthem.•Checksum:ThesendingTCPincludesacodethatisafunctionofthecontentsoftheremainderofthe重新排序连续计数校验和剩余segment.ThereceivingTCPperformsthesamecalculationandcomparestheresultwiththeincomingcode.Adiscrepancyresultsiftherehasbe