《信息工程專業(yè)英語》課件第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.協(xié)議；草案；禮儀

stack n.棧；堆；堆疊

flourish n.興旺；茂盛；揮舞；炫耀

Layer n.層，階層

wire n.電線；金屬絲

encode vt.編碼，譯碼

orient vt.使適應；確定方向

host n.主機；主人；主持人

criss-crossed

縱橫交錯

microwaves 微波

opticalfibers 光纖

satellitelinks 衛(wèi)星鏈路

TCP(TransmissionControlProtocol) 傳輸控制協(xié)議

IP(InternetProtocol) 互聯(lián)協(xié)議

DoD(DepartmentofDefense) 國防部

OSI(OpenSystemInterconnectReferenceModel)

開放式系統(tǒng)互聯(lián)參考模型

LAN(LocalAreaNetwork) 局域網(wǎng)

WAN(WideAreaNetwork) 廣域網(wǎng)

4.1.1Exercises

1.PutthePhrasesintoEnglish

(1)參考模型； (2)應用層；

(3)傳輸層； (4)互聯(lián)層；

(5)網(wǎng)絡接入層； (6)物理電路；

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.

4.1.2參考譯文

互聯(lián)網(wǎng)(因特網(wǎng))歷史上和技術上的開放標準是傳輸控制協(xié)議/互聯(lián)協(xié)議(TCP/IP)。TCP/IP參考模型和TCP/IP協(xié)議使世界上任何地點、任何兩個計算機之間的數(shù)據(jù)通信成為可能。TCP/IP模型具有重要的歷史意義，正如那些曾讓電話、電力、鐵道、電視和錄像帶行業(yè)繁榮昌盛的標準一樣。

TCP/IP參考模型的分層

因為美國國防部曾想建立一種能在任何條件下，甚至在核戰(zhàn)爭條件下都能存活的網(wǎng)絡，所以創(chuàng)建了TCP/IP參考模型。為了進一步說明問題，讓我們想象在戰(zhàn)爭中的世界，縱橫交錯的不同類型的連接：電線、微波、光纖以及衛(wèi)星鏈路。然后設想你需要那個互聯(lián)網(wǎng)絡中任何一個節(jié)點或任何一個網(wǎng)絡條件的(在該情況下，這些節(jié)點或網(wǎng)絡興許已被戰(zhàn)爭摧毀了)信息或數(shù)據(jù)(以數(shù)據(jù)包的形式)流。

而國防部期望在任何條件下，從任何一點到另一點，每次都能獲得其數(shù)據(jù)包。正是這個極為困難的設計問題創(chuàng)造了

TCP/IP參考模型，而這個標準卻是互聯(lián)網(wǎng)發(fā)展起來的基礎。

當你在學習TCP/IP模型的各層時，請記住互聯(lián)網(wǎng)的原始意圖，它能夠幫助解釋某些事情的本質。TCP/IP模型有四層：應用層、傳輸層、網(wǎng)絡層和網(wǎng)絡接入層。

應用層

TCP/IP的設計者們認為，較高層協(xié)議應包括會話層和表示層的詳細信息。他們簡化了應用層，使之處理高層協(xié)議、聲明、編碼和會話控制。TCP/IP將所有與應用層相關的問題都綜合為一層，并確保這個數(shù)據(jù)被打包到下一層。該層也被稱為處理層。

傳輸層

典型地，傳輸層涉及可靠性、流量控制和重傳問題。其協(xié)議之一，傳輸控制協(xié)議(TCP),提供了完美的和靈活的方法以產(chǎn)生可靠的、通暢的網(wǎng)絡通信。TCP是一個面向連接的協(xié)議，它將應用層的信息打包，成為一種被人們稱為段的單元，以此支持信源和信宿間的對話。面向連接并不意味著正在通信的計算機之間存在著物理電路。它意味著在兩個主機之間必須來回傳送第四層的段，以便在可傳送數(shù)據(jù)之前建立邏輯連接。這層有時也被稱為主機到主機層。

網(wǎng)絡層

網(wǎng)絡層的作用是在互聯(lián)網(wǎng)上的任意網(wǎng)絡發(fā)出源數(shù)據(jù)包并將它們送至信宿，而這個過程與到信宿所走的路由和網(wǎng)絡無關。主宰這一層的特定協(xié)議稱為互聯(lián)協(xié)議(IP)。最佳路由確定和分組交換都發(fā)生在這層?？陕?lián)想一些郵政系統(tǒng)，如果你想寄一封信，你并不知道它是怎樣到達對方的(可能會有多條路由)，但你會關心它是否到達。

網(wǎng)絡接入層

這一層的名字是寬泛的，在某種程度上有點令人迷惑不解。它也被稱為主機到網(wǎng)絡層。有時它就像OSI模型中那樣用兩層表示。網(wǎng)絡接入層涉及到一個IP分組從一個部件實際穿過一條物理鏈路而到達直接相連的一個部件的所有問題。它包括局域網(wǎng)和廣域網(wǎng)的技術細節(jié)，以及OSI模型中物理層和數(shù)據(jù)鏈路層的所有細節(jié)。

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