《信息工程专业英语》课件第4章

Unit4TCP/IPProtocol4.1Text4.2ReadingMaterials

4.1Text

TCP/IPArchitecture

ThehistoricalandtechnicalopenstandardoftheinternetisTransmissionControlProtocol/InternetProtocol(TCP/IP).TheTCP/IPreferencemodelandtheTCP/IPprotocolstackmakedatacommunicationpossiblebetweenanytwocomputers,anywhereintheworld.TheTCP/IPmodelhashistoricalimportance,justlikethestandardsthatallowedthetelephone,electricalpower,railroad,television,andvideotapeindustriestoflourish.

TheLayersoftheTCP/IPReferenceModel

TheU.S.DepartmentofDefense(DoD)createdtheTCP/IPreferencemodelbecauseitwantedanetworkthatcouldsurviveanycondition,evenanuclearwar.Toillustratefurther,imagineaworldatwar,criss-crossedbydifferentkindsofconnections:wires,microwaves,opticalfibers,andsatellitelinks.Thenimaginethatyouneedinformation/data(intheformofpackets)toflow,regardlessoftheconditionofanyparticularnodeornetworkontheinternetwork(which,inthiscase,mighthavebeendestroyedbythewar).

TheDoDwantsitspacketstogetthrougheverytime,underanycondition,fromanyonepointtoanyotherpoint.ItwasthisverydifficultdesignproblemthatbroughtaboutthecreationoftheTCP/IPreferencemodel,whichisthestandardonwhichtheInternethasgrown.

AsyoureadabouttheTCP/IPmodellayers,keepinmindtheoriginalintentoftheinternet;itcanhelpexplainwhycertainthingsareastheyare.TheTCP/IPmodelhasfourlayers:theapplicationlayer,thetransportlayer,theinternetlayer,andthenetworkaccesslayer.

ApplicationLayer

ThedesignersofTCP/IPfeltthatthehigher-levelprotocolsshouldincludethesessionandpresentationlayerdetails.Theysimplycreatedanapplicationlayerthathandleshigh-levelprotocols,issuesofrepresentation,encoding,anddialogcontrol.TheTCP/IPcombinesallapplication-relatedissuesintoonelayer,andensuresthisdataisproperlypackagedforthenextlayer.Thisisalsoreferredtoastheprocesslayer.

TransportLayer

Thetransportlayertypicallydealswiththeissuesofthereliability,flowcontrol,andretransmission.Oneofitsprotocols,thetransmissioncontrolprotocol(TCP),providesexcellentandflexiblewaystocreatereliable,well-flowing,networkcommunications.TCPisaconnection-orientedprotocol.Itsupportsdialoguesbetweensourceanddestinationwhilepackagingapplicationlayerinformationintounitscalledsegments.

Connection-orienteddoesnotmeanthataphysicalcircuitexistsbetweenthecommunicatingcomputers(thatwouldbecircuitswitching).ItdoesmeanthatLayer4segmentsmusttravelbackandforthbetweentwohoststosetupalogicalconnectionbeforedatacanbesent.Thislayerisalsosometimescalledthehost-to-hostlayer.

InternetLayer

Thepurposeoftheinternetlayeristosendsourcepacketsformanynetworkontheinternetworkandhavethemarriveatthedestinationindependentofthepathandnetworkstheylooktogetthere.ThespecificprotocolthatgovernsthislayeriscalledtheinternetProtocol(IP).Bestpathdeterminationandpacketswitchingoccuratthislayer.Thinkofitintermsofthepostalsystem.Whenyoumailaletter,youdonotknowhowitgetsthere(therearevariouspossibleroutes),butyoudocarethatitarrives.

NetworkAccessLayer

Thenameofthislayerisbroadandsomewhatconfusing.Itisalsocalledthehost-to-networklayer.Sometimes,it’sshownastwolayers,asintheOSImodel.ThenetworkaccesslayerisconcernedwithalltheissuesthatanIPpacketrequirestoactuallycrossaphysicallinkfromonedevicetoadirectlyconnectedone.ItincludestheLANandWANtechnologydetails,andallthedetailsintheOSIphysicalanddatalinklayers.

Technicalwordsandphrases

protocol n.协议；草案；礼仪

stack n.栈；堆；堆叠

flourish n.兴旺；茂盛；挥舞；炫耀

Layer n.层，阶层

wire n.电线；金属丝

encode vt.编码，译码

orient vt.使适应；确定方向

host n.主机；主人；主持人

criss-crossed

纵横交错

microwaves 微波

opticalfibers 光纤

satellitelinks 卫星链路

TCP(TransmissionControlProtocol) 传输控制协议

IP(InternetProtocol) 互联协议

DoD(DepartmentofDefense) 国防部

OSI(OpenSystemInterconnectReferenceModel)

开放式系统互联参考模型

LAN(LocalAreaNetwork) 局域网

WAN(WideAreaNetwork) 广域网

4.1.1Exercises

1.PutthePhrasesintoEnglish

(1)参考模型； (2)应用层；

(3)传输层； (4)互联层；

(5)网络接入层； (6)物理电路；

2.PutthePhrasesintoChinese

(1)protocolstack;

(2)datacommunication;

(3)electricalpower; (4)keepinmind;

(5)sourcepackets; (6)pathdetermination;

(7)packetswitching; (8)postalsystem.

3.Translation

(1)TheTCP/IPreferencemodelandtheTCP/IPprotocolstackmakedatacommunicationpossiblebetweenanytwocomputers,anywhereintheworld.

(2)Toillustratefurther,imagineaworldatwar,criss-crossedbydifferentkindsofconnections:wires,microwaves,opticalfibers,andsatellitelinks.

(3)TheTCP/IPcombinesallapplication-relatedissuesintoonelayer,andensuresthisdataisproperlypackagedforthenextlayer.

(4)Connection-orienteddoesnotmeanthataphysicalcircuitexistsbetweenthecommunicatingcomputers(thatwouldbecircuitswitching).ItdoesmeanthatLayer4segmentsmusttravelbackandforthbetweentwohoststosetupalogicalconnectionbeforedatacanbesent.

(5)Whenyoumailaletter,youdonotknowhowitgetsthere(therearevariouspossibleroutes),butyoudocarethatitarrives.

4.1.2参考译文

互联网(因特网)历史上和技术上的开放标准是传输控制协议/互联协议(TCP/IP)。TCP/IP参考模型和TCP/IP协议使世界上任何地点、任何两个计算机之间的数据通信成为可能。TCP/IP模型具有重要的历史意义，正如那些曾让电话、电力、铁道、电视和录像带行业繁荣昌盛的标准一样。

TCP/IP参考模型的分层

因为美国国防部曾想建立一种能在任何条件下，甚至在核战争条件下都能存活的网络，所以创建了TCP/IP参考模型。为了进一步说明问题，让我们想象在战争中的世界，纵横交错的不同类型的连接：电线、微波、光纤以及卫星链路。然后设想你需要那个互联网络中任何一个节点或任何一个网络条件的(在该情况下，这些节点或网络兴许已被战争摧毁了)信息或数据(以数据包的形式)流。

而国防部期望在任何条件下，从任何一点到另一点，每次都能获得其数据包。正是这个极为困难的设计问题创造了

TCP/IP参考模型，而这个标准却是互联网发展起来的基础。

当你在学习TCP/IP模型的各层时，请记住互联网的原始意图，它能够帮助解释某些事情的本质。TCP/IP模型有四层：应用层、传输层、网络层和网络接入层。

应用层

TCP/IP的设计者们认为，较高层协议应包括会话层和表示层的详细信息。他们简化了应用层，使之处理高层协议、声明、编码和会话控制。TCP/IP将所有与应用层相关的问题都综合为一层，并确保这个数据被打包到下一层。该层也被称为处理层。

传输层

典型地，传输层涉及可靠性、流量控制和重传问题。其协议之一，传输控制协议(TCP),提供了完美的和灵活的方法以产生可靠的、通畅的网络通信。TCP是一个面向连接的协议，它将应用层的信息打包，成为一种被人们称为段的单元，以此支持信源和信宿间的对话。面向连接并不意味着正在通信的计算机之间存在着物理电路。它意味着在两个主机之间必须来回传送第四层的段，以便在可传送数据之前建立逻辑连接。这层有时也被称为主机到主机层。

网络层

网络层的作用是在互联网上的任意网络发出源数据包并将它们送至信宿，而这个过程与到信宿所走的路由和网络无关。主宰这一层的特定协议称为互联协议(IP)。最佳路由确定和分组交换都发生在这层。可联想一些邮政系统，如果你想寄一封信，你并不知道它是怎样到达对方的(可能会有多条路由)，但你会关心它是否到达。

网络接入层

这一层的名字是宽泛的，在某种程度上有点令人迷惑不解。它也被称为主机到网络层。有时它就像OSI模型中那样用两层表示。网络接入层涉及到一个IP分组从一个部件实际穿过一条物理链路而到达直接相连的一个部件的所有问题。它包括局域网和广域网的技术细节，以及OSI模型中物理层和数据链路层的所有细节。

4.2ReadingMaterials

4.2.1TheOSIArchitecture

TheOSIreferencemodel,releasedin1984,wasthedescriptiveschemetheycreated.Itprovidedvendorswithasetofstandardsthatcouldenablegreatercompatibilityandinteroperabilitybetweenthevarioustypesofnetworktechnologiesthatwereproducedbymanycompaniesaroundtheworld.

TheOSIreferencemodelistheprimarymodelfornetworkcommunications.AprimaryobjectiveoftheOSIreferencemodelistoacceleratethedevelopmentoffuturenetworkingproducts.Althoughthereareothermodelsinexistence,mostnetworkvendorstodayrelatetheirproductstotheOSIreferencemodel,especiallywhentheywanttoeducateusersontheuseoftheirproducts.Theyconsideritthebesttoolavailabletoteachpeopleaboutsendingandreceivingdataonanetwork.

TheOSIreferencemodelallowsyoutoviewthenetworkfunctionsthatoccurateachlayer.Moreimportantly,theOSIreferencemodelisaframeworkyoucanusetounderstandhowinformationtravelsthroughoutanetwork.Inaddition,theOSIreferencemodelcanbeusedtovisualizehowinformation,ordatapackets,travelsfromapplicationprograms,throughanetworkmedium,tootherapplicationprogramsthatarelocatedinanothercomputeronanetwork,evenifthesenderandthereceiverhavedifferenttypesofnetworkmedia.

IntheOSIreferencemodel,therearesevennumberedlayers.Eachlayerillustratesaparticularnetworkfunction.Thisseparationofnetworkingfunctionsiscalledlayering.Dividingthenetworkintothesesevenlayersprovidesthefollowingadvantages:

•Itbreaksnetworkcommunicationintosmaller,simplerpartsthatareeasiertodevelop.

•Itfacilitatesstandardizationofnetworkcomponentstoallowmultiple-vendordevelopmentandsupport.

•Itbreaksnetworkcommunicationintosmallerpartstomakelearningiteasiertounderstand.

TheprocessofmovinginformationbetweencomputersisdividedintosevensmallerandmoremanageablestepsintheOSIreferencemodel.Eachofthesevensmallerproblemsisrepresentedbyitsownlayerinthemodel.ThesevenlayersoftheOSIreferencemodelare:

•Layer7:theapplicationlayer

•Layer6:thepresentationlayer

•Layer5:thesessionlayer

•Layer4:thetransportlayer

•Layer3:thenetworklayer

•Layer2:thedatalinklayer

•Layer1:thephysicallayer

Nowlet’stakealookateachlayerintheOSIreferencemodel,asshowninFig4.1.

Fig4.1OSIreferencemodel

TheUpperLayers

ThethreeupperlayersoftheOSIreferencemodelarereferredtoastheapplicationlayers.Fig.4.2showstheupperlayersandprovidesinformationontheirfunctionalitywithsomeexamples.

Fig4.2ThebasicfunctionofApplicationLayers,PresentationLayersandSessionLayers

Layer7:TheApplicationLayer

TheapplicationlayeristheOSIlayerthatisclosesttotheuser.Itprovidesnetworkservices,suchasfileaccessandprinting,totheuser’sapplications.ItdiffersfromtheotherlayersinthatitdoesnotprovideservicestoanyotherOSIlayer,butrather,onlytoapplicationsoutsidetheOSImodel.Theapplicationlayerestablishestheavailabilityofintendedcommunicationpartners.Italsosynchronizesandestablishesanagreementonproceduresforerrorrecoveryandcontrolofdataintegrity.IfyouwanttorememberLayer7inasfewwordsaspossible,thinkofthebrowsers.

Layer6:ThePresentationLayer

Thepresentationlayerensuresthattheinformationthattheapplicationlayerofonesystemsendsoutisreadablebytheapplicationlayerofanothersystem.Ifnecessary,thepresentationlayertranslatesbetweenmultipledataformatsbyusingacommonformat.Thislayeralsoisresponsibleforcompressionandencryption.IfyouwanttothinkofLayer6inasfewwordsaspossible,thinkofacommondataformat.

Layer5:TheSessionLayer

Asitsnameimplies,thesessionlayerestablishes,manages,andterminatessessionsbetweentwocommunicatinghosts.Thesessionlayerprovidesitsservicestothepresentationlayer.Italsosynchronizesdialoguebetweenthetwohosts’presentationlayersandmanagestheirdataexchange.Inadditiontosessionregulation,thesessionlayeroffersprovisionsforefficientdatatransfer,classofservice,andexceptionreportingofsessionlayer,presentationlayer,andapplicationlayerproblems.IfyouwanttorememberLayer5inasfewwordsaspossible,thinkofdialoguesandconversations.

TheLowerLayers

ThefourlowerlayersoftheOSImodeldefinehowdataistransferredacrossaphysicalwirethroughinternetworkingdevices,tothedesiredendstation,andfinallytotheapplication.Fig4.3summarizesthebasicfunctionofthesefourlayers.

Fig4.3ThebasicfunctionofTransportLayers,NetworkLayers,DataLinkLayersandPhysicalLayers

Layer4:theTransportLayer

Thetransportlayersegmentsdatafromthesendinghost’ssystemandreassemblesthedataintoadatastreamonthereceivinghost’ssystem.Theboundarybetweenthetransportlayerandthesessionlayercanbethoughtofastheboundarybetweenapplicationprotocolsanddata-flowprotocols.Whereastheapplication,presentation,andsessionlayersareconcernedwithapplicationissues,thelowerfourlayersareconcernedwithdatatransportissues.

Thetransportlayerattemptstoprovideadatatransportservicethatshieldstheupperlayersfromtransportimplementationdetails.Specifically,suchissueashowreliabletransportbetweentwohostsisaccomplishedintheconcernofthetransportlayer.Inprovidingcommunicationservice,thetransportlayerestablishes,maintains,andproperlyterminatesconnection-orientedcircuits.Inprovidingreliableservice,transporterrordetection-and-recoveryandinformationflowcontrolareused.IfyouwanttorememberLayer4inasfewwordsaspossible,thinkoftheflowcontrolandreliability.

Layer3:ThenetworkLayer

Thenetworklayerisacomplexlayerthatprovidesconnectivityandpathselectionbetweentwohostsystemsthatmightbelocatedongeographicallyseparatednetworks.IfyouwanttorememberLayer3inasfewwordsaspossible,thinkofpathselection,routing,andlogicaladdressing.

Layer2:TheDataLinkLayer

Thedatalinklayerprovidesthetransitofdataacrossaphysicallink.Insodoing,thedatalinklayerisconcernedwithphysical(asopposedtological)addressing,network(sometimescalledlogical)topology,networkmediaaccess,anderrordetection.IfyouwanttorememberLayer2inasfewwordsaspossible,thinkofframesandmediaaccesscontrol.

Layer1:ThePhysicalLayer

Thephysicallayerdefinestheelectrical,mechanical,procedural,andfunctionalspecificationsforactivating,maintaining,anddeactivatingthephysicallinkbetweenendsystems.Suchcharacteristicsasvoltagelevels,timingofvoltagechanges,physicaldatarates,maximumtransmissiondistances,physicalconnectors,andother,similar,attributesaredefinedbyphysicallayerspecifications.IfyouwanttorememberLayer1inasfewwordsaspossible,thinkofsignalsandmedia.

4.2.2TCPandIPDetails

TCP/IPProtocolGraph

ThediagramshowninFig4.4iscalledaprotocolgraph.Attheapplicationlayer,youseedifferentnetworktasksyoumightnotrecognizebut,asauseroftheInternet,probablyuseeveryday.Theseapplicationsincludethefollowing:

FTP——FileTransferProtocol

HTTP——HypertextTransferProtocol

SMTP——SimpleMailTransferProtocol

DNS——DomainNameSystem

TFTP——TrivialFileTransferProtocol

Fig4.4Aprotocolgraph

TheTCP/IPmodelemphasizesmaximumflexibility,attheapplicationlayer,fordevelopersofsoftware.Thetransportlayersupportstwoprotocols:TransmissionControlProtocol(TCP)andUserDatagramProtocol(UDP).Thelowestlayer,thenetworkaccesslayer,referstotheparticularLANorWANtechnologythatisbeingused.

IntheTCP/IPmodel,regardlessofwhichapplicationrequestingnetworkservices,andregardlessofwhichtransportprotocolused,thereisonlyonenetworkprotocol:InternetProtocol(IP).Thisisadeliberatedesigndecision.IPservesasauniversalprotocolthatallowsanycomputer,anywhere,tocommunicateatanytime.

IfyoucomparetheOSImodelandtheTCP/IPmodel,youcannoticethattheyhavesimilaritiesanddifferences(seeFig4.5).

Fig4.5TCP/IPmodelandOSImodel

Examplesincludethefollowing:

Similarities:

Bothhavelayers.

Bothhaveapplicationlayers,althoughtheyincludeverydifferentservices.

Bothhavecomparabletransportandnetworkandnetworklayers.

Packet-switched(notcircuit-switched)technologyisassumed.

Networkingprofessionalsneedtoknowbothmodels.

Similarities:

Bothhavelayers.

Bothhaveapplicationlayers,althoughtheyincludeverydifferentservices.

Bothhavecomparabletransportandnetworkandnetworklayers.

Packet-switched(notcircuit-switched)technologyisassumed.

Networkingprofessionalsneedtoknowbothmodels.

TCP/IPprotocolsarethestandardsaroundwhichtheinternetwasdeveloped,sotheTCP/IPmodelgainscredibilityjustbecauseofitsprotocols.Incontrast,networkstypicallyaren’tbuiltwiththeOSIprotocols,althoughtheOSImodelisusedasaguide.

Manynetworkingprofessionalshavedifferentopinionsonwhichmodeltouse.Youshouldbecomefamiliarwithboth.YoucanusetheOSImodelasthemicroscopethroughwhichtoanalyzenetworks,butyoualsocanusetheTCP/IPprotocolsincommonsituation.Rememberthatthereisadifferencebetweenamodel(thatis,layers,interfaces,andprotocolspecifications)andanactualprotocolthatisusedinnetworking.

4.2.3Anti-TerrorTechnology

Duetofactofopeningnetworks,connectingpartners,andusingapublicdomain,suchasInternet,securityrisksincreaseconsiderably.

Securityproceduresinclude:

Entityauthentication:Thismechanismallowsidentityverificationbycomparingidentificationinformationprovidedbyentitytothecontentofaknownandtrustedinformationrepository.Thisinformationmanytaketheformofsomethingtheuserknows,somethingtheuserhas,orsomethingtheuseris.Forstrongerverification,morethanoneofthesecharacteristicsmanyberequired.

Accesscontrollistsandsecuritylabels:Accesscontrollistsareaformofinformationrepositorythatcontainsdatarelativetotherightsandpermissionsofaccessgrantedtoeachauthenticatedidentityknowntothesystem.Securitylabelingprovidesamechanismtoenhanceorrefinethelevelsofcontrolimposedonaresourceorentity.Thisisdonebydefiningspecificcontrolsonthelabeltagitself.

Encipherment/decipherment:Cryptographyisthemechanismusedtoensureconfidentiality.Itisalsousedquitefrequentlyincomplementingothermechanismstoprovidetotalsecuritysolutions.

Modificationdetectioncodesandmessageauthenticationcodes:Dataintegrityissupportedbytheuseofsomesortofcheckingcode.Threemethodsofcalculatingthecheckingcodeareincommonuse:CyclicRedundancyCheck(CRC),ModificationDetectionCodes(MDCs),andMessageAuthenticationCodes(MACs).

Digitalsignature:Inadditiontodataintegrity,nonrepudiationservicessuchasdigitalsignaturearebecomingmoreimportanttomanycustomers.Digitalsignaturesprovideproofofdataorigin(tellsrecipientwhosentthedata)and/orproofofdelivery(areceiptfo