計(jì)算機(jī)網(wǎng)絡(luò)系統(tǒng)方法英文版課后習(xí)題及解答

第一章(1.21.3節(jié))5.Calculatethetotaltimerequiredtotransfera1,000-KB?leinthefollowingcases,assuminganRTTof100ms,apacketsizeof1-KBdata,andaninitial2×RTTof“handshaking”beforedataissent.(a)Thebandwidthis1.5Mbps,anddatapacketscanbesentcontinuously.(b)Thebandwidthis1.5Mbps,butafterwe?nishsendingeachdatapacketwemustwaitoneRTTbeforesendingthenext.(c)Thebandwidthis“in?nite,”meaningthatwetaketransmittimetobezero,andupto20packetscanbesentperRTT.(d)Thebandwidthisin?nite,andduringthe?rstRTTwecansendonepacket(21?1),duringthesecondRTTwecansendtwopackets(22?1),duringthethirdwecansendfour(23?1),andsoon.(Ajusti?cationforsuchanexponentialincreasewillbegiveninChapter6.)7.Considerapoint-to-pointlink2kminlength.Atwhatbandwidthwouldpropagationdelay(ataspeedof2×108m/sec)equaltransmitdelayfor100-bytepackets?Whatabout512-bytepackets?13.How“wide”isabitona1-Gbpslink?Howlongisabitincopperwire,wherethespeedofpropagationis2.3×108m/s?15.Supposea100-Mbpspoint-to-pointlinkisbeingsetupbetweenEarthandanewlunarcolony.ThedistancefromthemoontoEarthisapproximately385,000km,anddatatravelsoverthelinkatthespeedoflight—3×108m/s.(a)CalculatetheminimumRTTforthelink.(b)UsingtheRTTasthedelay,calculatethedelay×bandwidthproductforthelink.(c)Whatisthesigni?canceofhedelay×bandwidthproductcomputedin(b)?(d)AcameraonthelunarbasetakespicturesofEarthandsavesthemindigitalformattodisk.SupposeMissionControlonEarthwishestodownloadthemostcurrentimage,whichis25MB.Whatistheminimumamountoftimethatwillelapsebetweenwhentherequestforthedatagoesoutandthetransferis?nished?18.Calculatethelatency(from?rstbitsenttolastbitreceived)forthefollowing:(a)A10-MbpsEthernetwithasinglestore-and-forwardswitchinthepath,andapacketsizeof5,000bits.Assumethateachlinkintroducesapropaga-tiondelayof10μs,andthattheswitchbeginsretransmittingimmediatelyafterithas?nishedreceivingthepacket.(b)Sameas(a)butwiththreeswitches.(c)Sameas(a)butassumetheswitchimplements“cut-through”switching:itisabletobeginretransmittingthepacketafterthe?rst200bitshavebeenreceived.第二章（除2.72.9節(jié)）1.ShowtheNRZ,Manchester,andNRZIencodingsforthebitpatternshowninFigure2.46.AssumethattheNRZIsignalstartsoutlow.23.ConsideranARQalgorithmrunningovera20-kmpoint-to-point?berlink.(a)Computethepropagationdelayforthislink,assumingthatthespeedoflightis2×108m/sinthe?ber.(b)SuggestasuitabletimeoutvaluefortheARQalgorithmtouse.(c)WhymightitstillbepossiblefortheARQalgorithmtotimeoutandretransmitaframe,giventhistimeoutvalue?26.Thetextsuggeststhattheslidingwindowprotocolcanbeusedtoimplement?owcontrol.WecanimaginedoingthisbyhavingthereceiverdelayACKs,thatis,notsendtheACKuntilthereisfreebufferspacetoholdthenextframe.Indoingso,eachACKwouldsimultaneouslyacknowledgethereceiptofthelastframeandtellthesourcethatthereisnowfreebufferspaceavailabletoholdthenextframe.Explainwhyimplementing?owcontrolinthiswayisnotagoodidea.44.LetAandBbetwostationsattemptingtotransmitonanEthernet.Eachhassteadyqueueofframesreadytosend;A’sframeswillbenumberedA1,A2,andsoon,andB’ssimilarly.LetT=51.2μsbetheexponentialbackoffbaseunit.SupposeAandBsimultaneouslyattempttosendframe1,collide,andhappentochoosebackofftimesof0×Tand1×T,respectively,meaningAwinstheraceandtransmitsA1whileBwaits.Attheendofthistransmission,BwillattempttoretransmitB1whileAwillattempttotransmitA2.These?rstattemptswillcollide,butnowAbacksoffforeither0×Tor1×T,whileBbacksofffortimeequaltooneof0×T,...,3×T.(a)GivetheprobabilitythatAwinsthissecondbackoffraceimmediatelyafterthis?rstcollision,thatis,A’s?rstchoiceofbackofftimek×51.2islessthanB’s.(b)SupposeAwinsthissecondbackoffrace.AtransmitsA3,andwhenitis?nished,AandBcollideagainasAtriestotransmitA4andBtriesoncemoretotransmitB1.GivetheprobabilitythatAwinsthisthirdbackoffraceimmediatelyafterthe?rstcollision.(c)GiveareasonablelowerboundfortheprobabilitythatAwinsallthere-mainingbackoffraces.(d)WhatthenhappenstotheframeB1?ThisscenarioisknownastheEthernetcaptureeffect.48.Repeatthepreviousexercise,nowwiththeassumptionthatEthernetisp-persistentwithp=0.33(thatis,awaitingstationtransmitsimmediatelywithprobabilitypwhenthelinegoesidle,andotherwisedefersone51.2-μsslottimeandrepeatstheprocess).Yourtimelineshouldmeetcriterion(1)ofthepreviousproblem,butinlieuofcriterion(2),youshouldshowatleastonecollisionandatleastonerunoffourdeferralsonanidleline.Again,notethatmanysolutionsarepossible.第三章（3.13.2節(jié)）1.UsingtheexamplenetworkgiveninFigure3.30,givethevirtualcircuittablesforalltheswitchesaftereachofthefollowingconnectionsisestablished.Assumethatthesequenceofconnectionsiscumulative,thatis,the?rstconnectionisstillupwhenthesecondconnectionisestablished,andsoon.AlsoassumethattheVCIassignmentalwayspicksthelowestunusedVCIoneachlink,startingwith0.(a)HostAconnectstohostB.(b)HostCconnectstohostG.(c)HostEconnectstohostI.(d)HostDconnectstohostB.(e)HostFconnectstohostJ.(f)HostHconnectstohostA.3.ForthenetworkgiveninFigure3.31,givethedatagramforwardingtableforeachnode.Thelinksarelabeledwithrelativecosts;yourtablesshouldforwardeachpacketviathelowest-costpathtoitsdestination.5.ConsiderthevirtualcircuitswitchesinFigure3.33.Table3.6lists,foreachswitch,whatport,VCI(orVCI,interface)pairsareconnectedtoother.Connectionsarebidirectional.Listallendpoint-to-endpointconnections.13.GiventheextendedLANshowninFigure3.34,indicatewhichportsarenotselectedbythespanningtreealgorithm.15.ConsiderthearrangementoflearningbridgesshowninFigure3.35.Assumingallareinitiallyempty,givetheforwardingtablesforeachofthebridgesB1–B4afterthefollowingtransmissions:■AsendstoC.■CsendstoA.■DsendstoC.Identifyportswiththeuniqueneighborreacheddirectlyfromthatport,thatis,theportsforB1aretobelabeled“A”and“B2.”17.ConsiderhostsX,Y,Z,WandlearningbridgesB1,B2,B3,withinitiallyemptyforwardingtables,asinFigure3.36.(a)SupposeXsendstoZ.WhichbridgeslearnwhereXis?DoesY’snetworkinterfaceseethispacket?(b)SupposeZnowsendstoX.WhichbridgeslearnwhereZis?DoesY’snetworkinterfaceseethispacket?(c)SupposeYnowsendstoX.WhichbridgeslearnwhereYis?DoesZ’snet-workinterfaceseethispacket?(d)Finally,supposeZsendstoY.WhichbridgeslearnwhereZis?DoesW’snetworkinterfaceseethispacket?第四章（4.14.24.3.14.3.54.5節(jié)）4.SupposeaTCPmessagethatcontains2,048bytesofdataand20bytesofTCPheaderispassedtoIPfordeliveryacrosstwonetworksoftheInternet(i.e.,fromthesourcehosttoaroutertothedestinationhost).The?rstnetworkuses14-byteheadersandhasanMTUof1,024bytes;theseconduses8-byteheaderswithanMTUof512bytes.Eachnetwork’sMTUgivesthesizeofthelargestIPdatagramthatcanbecarriedinalinklayerframe.Givethesizesandoffsetsofthesequenceoffragmentsdeliveredtothenetworklayeratthedestinationhost.AssumeallIPheadersare20bytes.21.SupposearouterhasbuiltuptheroutingtableshowninTable4.14.Theroutercandeliverpacketsdirectlyoverinterfaces0and1,oritcanforwardpacketstoroutersR2,R3,orR4.Describewhattherouterdoeswithapacketaddressedtoeachofthefollowingdestinations:(a)