计算机网络复习要点.docx
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计算机网络复习要点
Chapter1要点
1.从两个视角理解Internet的含义。
Nutsandboltsofcomputernetworks.Computernetworksconsistofendsystems,packetswitches,andcommunicationlinks.Endsystems—alsocalledhosts—includedesktopPCs,laptops,hand-heldnetworkdevices(includingcellphones,PDAs),sensors,andservers(suchasWebandmailservers).Justascitiesareinterconnectedbyanetworkofroadsandintersections,endsystemsofacomputernetworkareinterconnectedbyanetworkofcommunicationlinksandpacketswitches.Communicationlinkscanbewiredorwireless.
Distributedapplications.Acomputernetworkenablesdistributedapplications.Adistributedapplicationrunsonendsystemsandexchangesdataviathecomputernetwork.DistributedapplicationsincludeWebsurfing,e-mail,instantmessaging,Internetphone,distributedgames,peer-to-peerfilesharing,televisiondistribution,andvideoconferencing.NewdistributedapplicationscontinuetobeinventedanddeployedontheInternet.
2.协议的定义
Protocol.Aprotocoldefinestheformatandorderofmessagesexchangedbetweentwoormorecommunicationentities,aswellastheactionstakenonthetransmissionand/orreceiptofamessageorotherevent.Computernetworksmakeextensiveuseofprotocols.Figure1.2isanexample
3.两种基本的交换方式,各自特点。
Packetswitching.Whenoneendsystemsendsdatatoanotherendsystem,thesendingendsystembreaksthedataintochunks,calledpackets.Similartotheprocessofdeliveringpost-officemail,theInternettransportseachpacketseparately,routingapackettoitsdestinationusingadestinationaddressthatiswrittenintothepacket.Whenapacketswitchreceivesapacket,itusesthepacket’sdestinationaddresstodetermineonwhichlinkitshouldforwardthepacket.Thus,apacketswitchperforms“packetswitching,”forwardingincomingpacketstooutgoinglinkspacketbypacket.Also,packetswitchestypically“storeandforward”packets—thatis,beforeaswitchbeginstoforwardapacketonanoutgoinglink,firstitreceivesandstorestheentirepacket.
Circuit-switching.Computernetworksconstituteonemajorclassofcommunicationnetworks.Anothermajorclassofcommunicationnetworksistraditionaldigitaltelephonenetworks.Traditionaldigitaltelephonenetworksdonotusepacketswitchingtomovedatafromasourcetoadestination;theyuseatechniqueknownascircuitswitching.Incircuitswitching,beforetransmittingdatabetweentwoendsystems,thenetworkestablishesadedicatedend-to-endconnectionbetweentheendsystemsandreservesbandwidthineachlinkalongtheconnection.Thereservedconnectionbandwidthis“wasted”whenevertheendsystemsarenotsendingdata.
4.常见的传输介质分类。
Physicalmediaandaccessnetworks.Thecommunicationlinksinacomputernetworkmayhavedifferentphysicalmediatypes.Dial-upmodemlinks,DSL,andmostEthernetlinksaremadeofcopperwire.Cablelinksaremadeofcoaxialcable.Long-haulInternetbackbonelinksaremadeoffiberoptics.Inadditiontothesewiredlinks,thereisaplethoraofwirelesslinks,includingWi-Fi,Bluetooth,and
satellite.AnaccesslinkisalinkthatconnectstheendsystemtotheInternet.Accesslinkscanbecopperwire,coaxialcable,fiberopticsorwireless.AtremendousvarietyofmediatypescanbefoundontheInternet.
5.延迟分类
Transmissionandpropagationdelays.Transmissionandpropagationdelaysplayacriticalroleintheperformanceofmanydistributedapplications.
Thepropagationdelayoveralinkisthetimeittakesabittotravelfromoneendofthelinktotheother.Itisequaltothelengthofthelinkdividedbythepropagationspeedofthelink’sphysicalmedium.Thetransmissiondelayisaquantitythatrelatestopacketsandnotbits.
Thetransmissiondelayforalinkisequaltothenumberofbitsinthepacketdividedbythetransmissionrateofthelink.Itistheamountoftimeittakestopushthepacketontothelink.Onceabitispushedontoalink,itneedstopropagatetotheotherend.Thetotaldelayacrossalinkisequaltothesumofthetransmissiondelayandthepropagationdelay.
Queuingdelayandpacketloss.Manypacketscanarriveatapacketswitchroughlyatthesametime.Ifthesepacketsneedtobeforwardedonthesameoutboundlink,allbutonewillhaveto“queue,”thatis,waittobetransmitted.Thiswaitingintroducesaqueuingdelay.Furthermore,ifthequeueofpacketsbecomesverylarge,thepacketswitch’sbuffermaybecomeexhausted,causingpacketstobedroppedor“lost.”Queuingdelayandpacketlosscanseverelyimpacttheperformanceofanapplication.
6.TCP/IP五层名称
Atypicalcomputernetworkmakesuseofmanyprotocols—easilyhundreds.Todealwiththiscomplexity,theprotocolsareorganizedintolayers.Theseprotocollayersarearrangedina“stack.”Forexample,theInternetorganizesitsprotocolsintofivelayers—namely,fromtoptobottom:
applicationlayer,transportlayer,networklayer,linklayer,andphysicallayer.Theprotocolsoflayernusetheservicesprovidedbytheprotocolsatthelayern-1(thelayerbelow).
Encapsulation.Whenthesender-sideapplication-layerprocesspassesanapplication-leveldataunit(anapplicationmessage)tothetransportlayer,thatmessagebecomesthepayloadofthetransport-layersegment,whichalsocontainsadditionaltransport-layerheaderinformation,forexample,informationthatwillallowthetransportlayeratthereceiversidetodeliveramessagetothecorrectreceiver-sideapplication.Thetransportlayerpassesthetransport-layersegmenttothenetworklayer.Thesegmentbecomesthepayloadofthenetwork-layerdatagram,whichhasadditionalfieldsusedbythenetworklayer(forexample,theaddressofthereceiver).Finally,thenetworklayerdatagramispassedtothelinklayer,whichencapsulatesthedatagramwithinalink-layerframe.
7.了解ADSL,HFC,LAN,ISP,TDM,FDM,RFC
Chapter2要点
1.应用层的功能
Application-layerprotocol.InChapter1wenotedthat“Aprotocoldefinestheformatandtheorderofmessagesexchangedbetweentwoormorecommunicatingentities,aswellastheactionstakenonthetransmissionand/orreceiptofamessageorotherevent.”Inchapter2,wehaveseenhowprocessessendandreceivemessagesinanapplication-layerprotocol.Asareview,identifythemessagesexchangedandactionstakenbythefollowingprotocols:
HTTP,FTP,DNS,SMTP.
2.三种网络应用架构,各自特点
Client/server,peer-topeer,Hybrid.
Intheclient/serverparadigm,aclientprocessrequestsaservicebysendingoneormoremessagestoaserverprocess.Theserverprocessimplementsaservicebyreadingtheclientrequest,performingsomeaction(forexample,inthecaseofanHTTPserver,findingaWebpage),andsendingoneormoremessagesinreply(inthecaseofHTTP,returningtherequestedobject).
Inapeer-to-peerapproach,thetwoendsoftheprotocolareequals.
3.熟悉Socket功能
4.应用层协议的4个组成部分
5.应用层服务对网络的三个要求。
6.TCP、UDP各自提供的服务。
TwoservicesprovidedbytheInternet’stransportlayer:
reliable,congestioncontrolleddatatransfer(TCP),andunreliabledatatransfer(UDP).ThesearetheonlyservicesavailabletoanInternetapplicationtotransferdatafromoneprocesstoanotherremoteprocess.TheInternettransportlayerdoesnotprovideaminimumguaranteedtransferrate,oraboundonthedelayfromsourcetodestination.
7.理解各种应用层协议:
HTTP,SMTP,POP3,FTP,DNS
HTTP:
request/responseinteraction.TheHTTPprotocolisasimpleapplication-layerprotocol.Aclient(Webbrowser)makesarequestwithaGETmessage,andaWebserverprovidesareply(seeFigure2.6inyourtextbook).Thisisaclassicalclient/serverapproach.SinceHTTPusesTCPtoprovidereliabletransferoftheGETrequestfromclient-to-server,andthereplyfromserver-to-client,aTCPconnectionmustbesetup.ATCPsetuprequestissentfromtheTCPintheclienttotheTCPintheserver,withtheTCPserverreplyingtotheTCPclient.Followingthisexchange,theHTTPGETmessagecanbesentovertheTCPconnectionfromclient-to-server,andthereplyreceived(seeFigure2.7inyourtextbook).Withnon-persistentHTTP,anewTCPconnectionmustbesetupeachtimetheclientwantstocontacttheserver.WithpersistentHTTP,multipleHTTPGETmessagescanbesentoverasingleTCPconnection,resultinginperformancegainsfromnothavingtosetupanewTCPforeachoftheHTTPrequestsbeyondthefirst.
DNS:
coreinfrastructureimplementedasanapplication-layerprocess.TheDNSisanapplication-layerprotocol.Thename-to-IP-addresstranslationserviceisperformedattheDNSservers,justasanyapplicationprovidesaservicetoaclientviaaserver.ButtheDNSserviceisaveryspecialnetworkservice—withoutitthenetworkwouldbeunabletofunction.Yetitisimplementedinverymuchthesamewayasanyothernetworkapplication.
FTP:
separatecontrolanddata.FTPisaniceexampleofaprotocolthatseparatescontrolanddatamessages.AsshowninFigure2.15,controlanddatamessagesaresentoverseparateTCPconnections.Thislogicalandphysicalseparationofcontrolanddata(ratherthanmixingthetwotypesofmessages
inoneconnection)helpstomakethestructureofsuchanapplication“cleaner.”
8.P2P资源定位的三种方式
LocatinginformationinP2Psystems.WeidentifiedthreewaystolocateinformationinaP2Psystem:
queryflooding,directorysystems,andhybridsystems.AllexistingP2Psystemsuseoneoftheseapproaches.
9.了解:
Socket,HTTPcookies,SMTP,POP3,FTP,URL,RTT
Chapter3要点
1.传输层的功能
Whereasatransport-layerprotocolprovideslogicalcommunicationbetweenprocesses,anetwork-layerprotocolprovideslogicalcommunicationbetweenhosts.Thisdistinctionisimportantbutsubtle;Anapplicationprotocollivesonlyintheendsystemsandisnotprese