一个互联网的内部工作指南外文翻译.docx

一个互联网的内部工作指南外文翻译.docx

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一个互联网的内部工作指南外文翻译

毕业论文（设计）

外文翻译

外文原文

AnInsider’sGuidetotheInternet

DavidD.Clark

M.I.T.ComputerScienceandArtificialIntelligenceLaboratory

Version2.07/25/04

AlmosteveryonehasheardoftheInternet.Wecruisetheweb,wewatchthevaluationofInternetcompaniesonthestockmarket,andwereadthepundits’predictionsaboutwhatwillhappennext.Butnotmanypeopleactuallyunderstandwhatitisandhowitworks.Takeawaythehype,andthebasicoperationoftheInternetisrathersimple.Here,inafewpages,isanoverviewofhowitworksinside,andwhyitworksthewayitdoes.

Don’tforget—theInternetisnottheWorldWideWeb,ore-mail.TheInternetiswhatis“underneath”them,andmakesthemallhappen.ThispaperdescribeswhattheInternetitselfis,andalsotellswhatactuallyhappens,forexample,whenyouclickonalinkinaWebpage.

1.IntroductiontotheInternet

TheInternetisacommunicationsfacilitydesignedtoconnectcomputerstogethersothattheycanexchangedigitalinformation.Forthispurpose,theInternetprovidesabasiccommunicationservicethatconveysunitsofinformation,calledpackets,fromasourcecomputerattachedtotheInternettooneormoredestinationcomputersattachedtotheInternet.Additionally,theInternetprovidessupportingservicessuchasthenamingoftheattachedcomputers.Anumberofhigh-levelservicesorapplicationshavebeendesignedandimplementedmakinguseofthisbasiccommunicationservice,includingtheWorldWideWeb,Internete-mail,theInternet"newsgroups",distributionofaudioandvideoinformation,andfiletransferand"login"betweendistantcomputers.ThedesignoftheInternetissuchthatnewhigh-levelservicescanbedesignedanddeployedinthefuture.

TheInternetdiffersinimportantwaysfromthenetworksinothercommunicationsindustriessuchastelephone,radioortelevision.Inthoseindustries,thecommunicationsinfrastructure--wires,fibers,transmissiontowersandsoon—hasbeenputinplacetoserveaspecificapplication.Itmayseemobviousthatthetelephonesystemwasdesignedtocarrytelephonecalls,buttheInternethadnosuchclearpurpose.TounderstandtheroleoftheInternet,considerthepersonalcomputer,orPC.ThePCwasnotdesignedforoneapplication,suchaswordprocessingorspreadsheets,butisinsteadageneral-purposedevice,specializedtooneuseoranotherbythelateradditionofsoftware.TheInternetisanetworkdesignedtoconnectcomputerstogether,andsharesthissamedesigngoalofgenerality.TheInternetisanetworkdesignedtosupportarangeofapplications,dependingonwhatsoftwareisloadedintotheattachedcomputers,andwhatusethatsoftwaremakesoftheInternet.Manycommunicationpatternsarepossible:

betweenpairsofcomputers,fromaservertomanyclients,oramongagroupofco-operatingcomputers.TheInternetisdesignedtosupportallthesemodes.

TheInternetisnotaspecificcommunication“technology”,suchasfiberopticsorradio.Itmakesuseoftheseandothertechnologiesinordertogetpacketsfromplacetoplace.ItwasintentionallydesignedtoallowasmanytechnologiesaspossibletobeexploitedaspartoftheInternet,andtoincorporatenewtechnologiesastheyareinvented.IntheearlydaysoftheInternet,itwasdeployedusingtechnologies

（e.g.telephonecircuits）originallydesignedandinstalledforotherpurposes.AstheInternethasmatured,weseethedesignofcommunicationtechnologiessuchasEthernetand802.11wirelessthataretailoredspecificallytotheneedsoftheInternet—theyweredesignedfromthegrounduptocarrypackets.

2.Separationoffunction

IftheInternetisnotaspecificcommunicationstechnology,norforaspecificpurpose,whatisit?

Technically,itscoreisaverysimpleandminimalspecificationthatdescribesitsbasiccommunicationmodel.Figure1providesaframeworkthatishelpfulinunderstandinghowtheInternetisdefined.Atthetopofthefigure,thereisawiderangeofapplications.Atthebottomisawiderangeoftechnologiesforwideareaandlocalareacommunications.ThedesigngoaloftheInternetwastoallowthiswiderangeofapplicationstotakeadvantageofallthesetechnologies.

TheheartoftheInternetisthedefinitionofaverysimpleservicemodelbetweentheapplicationsandthetechnologies.Thedesignerofeachapplicationdoesnotneedtoknowthedetailsofeachtechnology,butonlythisbasiccommunicationservice.Thedesignerofeachtechnologymustsupportthisservice,butneednotknowabouttheindividualapplications.Inthisway,thedetailsoftheapplicationsandthedetailsofthetechnologiesareseparated,sothateachcanevolveindependently.

2.1.ThebasiccommunicationmodeloftheInternet

Thebasicservicemodelforpacketdeliveryisverysimple.Itcontainstwoparts:

theaddressesandthedeliverycontract.Toimplementaddressing,theInternethasnumbersthatidentifyendpoints,similartothetelephonesystem,andthesenderidentifiesthedestinationofacommunicationusingthesenumbers.ThedeliverycontractspecifieswhatthesendercanexpectwhenithandsdataovertotheInternetfordelivery.TheoriginaldeliverycontractoftheInternetisthattheInternetwilldoitsbesttodeliverallthedatagiventoitforcarriage,butmakesnocommitmentastodatarate,deliverydelay,orlossrates.Thisserviceiscalledthebesteffortdeliverymodel.

Thisveryindefiniteandnon-committaldeliverycontracthasbothbenefitandrisk.Thebenefitisthatalmostanyunderlyingtechnologycanimplementit.Theriskofthisvaguecontractisthatapplicationscannotbesuccessfullybuiltontopofit.However,thedemonstratedrangeofapplicationsthathavebeendeployedovertheInternetsuggeststhatitisadequateinpractice.Asisdiscussedbelow,thissimpleservicemodeldoeshavelimits,anditisbeingextendedtodealwithnewobjectivessuchasrealtimedeliveryofaudioandvideo.

2.2.Layering,notintegration.

ThedesignapproachoftheInternetisacommononeinComputerScience:

provideasimplifiedviewofcomplextechnologybyhidingthattechnologyunderneathaninterfacethatprovidesanabstractionoftheunderlyingtechnology.Thisapproachisoftencalledlayering.Incontrast,networkssuchasthetelephonesystemaremoreintegrated.Inthetelephonesystem,designersofthelowleveltechnology,knowingthatthepurposeistocarrytelephonecalls,makedecisionsthatoptimizethatgoalinallpartsofthesystem.TheInternetisnotoptimizedtoanyoneapplication;ratherthegoalisgenerality,flexibilityandevolvability.Innovationcanoccuratthetechnologylevelindependentofinnovationattheapplicationlevel,andthisisoneofthemeanstoinsurethattheInternetcanevolverapidlyenoughtokeeppacewiththerateofinnovationinthecomputerindustry.

2.3.Protocols

ThewordprotocolisusedtorefertotheconventionsandstandardsthatdefinehoweachlayeroftheInternetoperates.TheInternetlayerdiscussedaboveisspecifiedinadocumentthatdefinestheformatofthepacketheaders,thecontrolmessagesthatcanbesent,andsoon.ThissetofdefinitionsiscalledtheInternetProtocol,orIP.

DifferentbodieshavecreatedtheprotocolsthatspecifythedifferentpartsoftheInternet.TheInternetEngineeringTaskForce,anopenworkinggroupthathasgrownupalongwiththeInternet,createdtheInternetProtocolandtheotherprotocolsthatdefinethebasiccommunicationserviceoftheInternet.Thisgroupalsodevelopedtheprotocolsforearlyapplicationssuchase-mail.Someprotocolsaredefinedbyacademicandindustryconsortia;forexampletheprotocolsthatspecifytheWorldWideWebaremostlydevelopedbytheWorldWideWebConsortium（theW3C）hostedattheComputerScienceandArtificialIntelligencelaboratoryatMIT.Theseprotocols,oncedeveloped,arethenusedasthebasisofproductsthataresoldtothevariousentitiesinvolvedinthedeploymentandoperationoftheInternet.

3.Forwardingdata—theInternetlayer

3.1.Thepacketmodel

DatacarriedacrosstheInternetisorganizedintopackets,whichareindependentunitsofdata,nomorethansomespecifiedlength（1000to2000bytesistypical）,completewithdeliveryinformationattached.AnapplicationprogramonacomputerthatneedstodeliverdatatoanothercomputerinvokessoftwarethatbreaksthatdataintosomenumberofpacketsandtransmitsthesepacketsoneatatimeintotheInternet.（ThemostcommonversionofthesoftwarethatdoesthisiscalledTransmissionControlProtocol,orTCP;itisdiscussedbelow.）

TheInternetconsistsofaseriesofcommunicationlinksconnectedbyrelaypointscalledrouters.Figure2illustratesthisconceptualrepresentation.Asfigure3illustrates,thecommunicationlinksthatconnectroutersintheInternetcanbeofmanysorts,asemphasizedbythehourglass.Theyallsharethebasicfunctionthattheycantransportapacketfromoneroutertoanother.Ateachrouter,thedeliveryinformationinthepacket,calledtheheader,isexamined,andbasedonthedestinationaddress,adeterminationismadeastowheretosendthepacketnext.ThisprocessingandforwardingofpacketsisthebasiccommunicationserviceoftheInternet.

Typically,arouterisacomputer,eithergeneralpurposeorspeciallydesignedforthisrole,runningsoftwareandhardwarethatimplementstheforwardingfunctions.Ahigh-performancerouterusedintheinterioroftheInternetmaybeaveryexpensiveandsophisticateddevice,whilearouterusedinasmallbusinessoratotherpointsneartheedgeofthenetworkmaybeasmallunitcostinglessthanahundreddollars.Whateverthepricean