Review-for-Chapter-1-川大计网部分习题答案

ReviewforChapter1R1.Whatisthedifferencebetweenahostandanendsystem?Listthetypesofendsystems.IsaWebserveranendsystem?Answer:Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.R3.Whatisaclientprogram?whatisaserverprogram?Doesaserverprogramrequestandreceiveservicesfromaclientprogram?Answer:Anetworkingprogramusuallyhastwoprograms,eachrunningonadifferenthost,communicatingwitheachother.Theprogramthatinitiatesthecommunicationistheclient.Typically,theclientprogramrequestsandreceivesservicesfromtheserverprogram.R8.Dail-upmodems,HFC,andDSLareallusedforresidentialaccess.Foreachoftheseaccesstechnologies,providearangeoftransmissionratesandcommentonwhetherthetransmissionrateissharedordedicated.Answer:Dialupmodems:upto56Kbps,bandwidthisdedicated;ISDN:upto128kbps,bandwidthisdedicated;ADSL:downstreamchannelis.5-8Mbps,upstreamchannelisupto1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthissharedR11.Whatadvantagedoesacircuit-switchednetworkhaveoverapacket-switchednetwork?WhatadvantagesdoesTDMhaveoverFDMinacircuit-switchednetwork?Answer:Acircuit-switchednetworkcanguaranteeacertainamountofend-to-endbandwidthforthedurationofacall.Mostpacket-switchednetworkstoday(includingtheInternet)cannotmakeanyend-to-endguaranteesforbandwidth.TDMhadtheadvantageinthatitrequireslessbandwidthasitusesthesamefrequency.R12.Supposethereisexactlyonepacketswitchbetweenasendinghostandareceivinghost.ThetransmissionratesbetweenthesendinghostandtheswitchandbetweentheswitchandthereceivinghostareR1andR2,respectively.Assumingthattheswitchusesstore-and-forwardpacketswitching,whatisthetotalend-to-enddelaytosendapacketoflengthL?(ignorequeuing,propagationdelay,andprocessingdelay.)Answer:Attimet0thesendinghostbeginstotransmit.Attimet1=L/R1,thesendinghostcompletestransmissionandtheentirepacketisreceivedattherouter(nopropagationdelay).Becausetherouterhastheentirepacketattimet1,itcanbegintotransmitthepackettothereceivinghostattimet1.Attimet2=t1+L/R2,theroutercompletestransmissionandtheentirepacketisreceivedatthereceivinghost(again,nopropagationdelay).Thus,theend-to-enddelayisL/R1+L/R2.R13.Whyisitsaidthatpacketswitchingemploysstatisticalmultiplexing?ContraststatisticalmultiplexingwiththemultiplexingthattakesplaceinTDM.Answer:Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.R15.Whatisthekeydistinguishingdifferencebetweenatier-1ISPandatier-2ISP?Answer:Atier-1ISPconnectstoallothertier-1ISPs;atier-2ISPconnectstoonlyafewofthetier-1ISPs.Also,atier-2ISPisacustomerofoneormoretier-1.R16.Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantandwhicharevariable?Answer:Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.R19.SupposeHostAwantstosendalargefiletoHostB.ThepathfromHostAtoHostBhasthreelinks,ofratesand.a.Assumingnoothertrafficinthenetwork,whatisthethroughputforthefiletransfer.b.Supposethefileis2millionbytes.Roughly,howlongwillittaketotransferthefiletoHostB?c.Repeat(a)and(b),butnowwithreducedto200kbps.1250,Rkbps=2500,Rkbps=31RMbps=2RAnswer:a)250kbpsb)64secondsc)200kbps;80secondsR23.WhatarethefivelayersintheInternetprotocolstack?Whataretheprincipalresponsibilitiesofeachoftheselayers?Answer:ThefivelayersintheInternetprotocolstackare–fromtoptobottom–theapplicationlayer,thetransportlayer,thenetworklayer,thelinklayer,andthephysicallayer.TheprincipalresponsibilitiesareoutlinedinSection1.5.1.R25.Whatisanapplication-layermessage?Atransport-layersegment?Anetwork-layerdatagram?Alink-layerframe?Answer:Application-layermessage:datawhichanapplicationwantstosendandpassedontothetransportlayer;transport-layersegment:generatedbythetransportlayerandencapsulatesapplication-layermessagewithtransportlayerheader;network-layerdatagram:encapsulatestransport-layersegmentwithanetwork-layerheader;link-layerframe:encapsulatesnetwork-layerdatagramwithalink-layerheader.R26.Whatisthedifferencebetweenavirus,aworm,andaTrojanhorse?Answer:a)VirusRequiressomeformofhumaninteractiontospread.Classicexample:E-mailviruses.b)WormsNouserreplicationneeded.WormininfectedhostscansIPaddressesandportnumbers,lookingforvulnerableprocessestoinfect.c)TrojanhorseHidden,deviouspartofsomeotherwiseusefulsoftware.ProblemsP2.Considerthecircuit-switchednetworkinFigure1.8.Recallthattherearencircuitsoneachlink.a.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogressatanyonetimeinthisnetwork?b.Supposethatallconnectionsarebetweentheswitchintheupper-left-handcornerandtheswitchinthelower-right-handcorner.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogress?Answer:a)Wecannconnectionsbetweeneachofthefourpair