《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(yīng)；確定方向

host n.主機(jī)；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(yīng)用層；

(3)傳輸層； (4)互聯(lián)層；

(5)網(wǎng)絡(luò)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(shù)上的開放標(biāo)準(zhǔn)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(jī)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(biāo)準(zhǔn)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(luò)，所以創(chuàng)建了TCP/IP參考模型。為了進(jìn)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(shè)想你需要那個互聯(lián)網(wǎng)絡(luò)中任何一個節(jié)點或任何一個網(wǎng)絡(luò)條件的(在該情況下，這些節(jié)點或網(wǎng)絡(luò)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(shè)計問題創(chuàng)造了

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

當(dāng)你在學(xué)習(xí)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(zhì)。TCP/IP模型有四層：應(yīng)用層、傳輸層、網(wǎng)絡(luò)層和網(wǎng)絡(luò)接入層。

應(yīng)用層

TCP/IP的設(shè)計者們認(rèn)為，較高層協(xié)議應(yīng)包括會話層和表示層的詳細(xì)信息。他們簡化了應(yīng)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(yīng)用層相關(guān)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(luò)通信。TCP是一個面向連接的協(xié)議，它將應(yīng)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(jī)之間存在著物理電路。它意味著在兩個主機(jī)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(jī)到主機(jī)層。

網(wǎng)絡(luò)層

網(wǎng)絡(luò)層的作用是在互聯(lián)網(wǎng)上的任意網(wǎng)絡(luò)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(luò)無關(guān)。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(dá)對方的(可能會有多條路由)，但你會關(guān)心它是否到達(dá)。

網(wǎng)絡(luò)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(jī)到網(wǎng)絡(luò)層。有時它就像OSI模型中那樣用兩層表示。網(wǎng)絡(luò)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(dá)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(shù)細(xì)節(jié)，以及OSI模型中物理層和數(shù)據(jù)鏈路層的所有細(xì)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