计算机网络复习要点Word格式文档下载.docx

计算机网络复习要点Word格式文档下载.docx

《计算机网络复习要点Word格式文档下载.docx》由会员分享，可在线阅读，更多相关《计算机网络复习要点Word格式文档下载.docx（11页珍藏版）》请在冰豆网上搜索。

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