计算机网络系统方法英文版课后习题及解答.docx
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计算机网络系统方法英文版课后习题及解答
第一章
（1.21.3节）
5.Calculatethetotaltimerequiredtotransfera1,000-KBfileinthefollowingcases,assuminganRTTof100ms,apacketsizeof1-KBdata,andaninitial2×RTTof“handshaking”beforedataissent.
（a）Thebandwidthis1.5Mbps,anddatapacketscanbesentcontinuously.
（b）Thebandwidthis1.5Mbps,butafterwefinishsendingeachdatapacket
wemustwaitoneRTTbeforesendingthenext.
（c）Thebandwidthis“infinite,”meaningthatwetaketransmittimetobe
zero,andupto20packetscanbesentperRTT.
（d）Thebandwidthisinfinite,andduringthefirstRTTwecansendone
packet（21−1）,duringthesecondRTTwecansendtwopackets（22−1）,
duringthethirdwecansendfour（23−1）,andsoon.（Ajustificationfor
suchanexponentialincreasewillbegiveninChapter6.）
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×bandwidthproductfor
thelink.
（c）Whatisthesignificanceofhedelay×bandwidthproductcomputed
in（b）?
（d）AcameraonthelunarbasetakespicturesofEarthandsavesthemindigital
formattodisk.SupposeMissionControlonEarthwishestodownloadthe
mostcurrentimage,whichis25MB.Whatistheminimumamountof
timethatwillelapsebetweenwhentherequestforthedatagoesoutand
thetransferisfinished?
18.Calculatethelatency（fromfirstbitsenttolastbitreceived）forthefollowing:
（a）A10-MbpsEthernetwithasinglestore-and-forwardswitchinthepath,
andapacketsizeof5,000bits.Assumethateachlinkintroducesapropaga-
tiondelayof10µs,andthattheswitchbeginsretransmittingimmediately
afterithasfinishedreceivingthepacket.
（b）Sameas（a）butwiththreeswitches.
（c）Sameas（a）butassumetheswitchimplements“cut-through”switching:
it
isabletobeginretransmittingthepacketafterthefirst200bitshavebeen
received.
第二章
（除2.72.9节）
1.ShowtheNRZ,Manchester,andNRZIencodingsforthebitpatternshowninFigure2.46.AssumethattheNRZIsignalstartsoutlow.
23.ConsideranARQalgorithmrunningovera20-kmpoint-to-pointfiberlink.
（a）Computethepropagationdelayforthislink,assumingthatthespeedof
lightis2×108m/sinthefiber.
（b）SuggestasuitabletimeoutvaluefortheARQalgorithmtouse.
（c）WhymightitstillbepossiblefortheARQalgorithmtotimeoutand
retransmitaframe,giventhistimeoutvalue?
26.Thetextsuggeststhattheslidingwindowprotocolcanbeusedtoimplementflowcontrol.WecanimaginedoingthisbyhavingthereceiverdelayACKs,thatis,notsendtheACKuntilthereisfreebufferspacetoholdthenextframe.Indoingso,eachACKwouldsimultaneouslyacknowledgethereceiptofthelastframeandtellthesourcethatthereisnowfreebufferspaceavailabletoholdthenextframe.Explainwhyimplementingflowcontrolinthiswayisnotagoodidea.
44.LetAandBbetwostationsattemptingtotransmitonanEthernet.Eachhassteadyqueueofframesreadytosend;A’sframeswillbenumberedA1,A2,andsoon,andB’ssimilarly.LetT=51.2µsbetheexponentialbackoffbaseunit.SupposeAandBsimultaneouslyattempttosendframe1,collide,andhappentochoosebackofftimesof0×Tand1×T,respectively,meaningAwinstheraceandtransmitsA1whileBwaits.Attheendofthistransmission,BwillattempttoretransmitB1whileAwillattempttotransmitA2.Thesefirstattemptswillcollide,butnowAbacksoffforeither0×Tor1×T,whileBbacksofffortimeequaltooneof0×T,...,3×T.
（a）GivetheprobabilitythatAwinsthissecondbackoffraceimmediatelyafterthisfirst
collision,thatis,A’sfirstchoiceofbackofftimek×51.2islessthanB’s.
（b）SupposeAwinsthissecondbackoffrace.AtransmitsA3,andwhenitis
finished,AandBcollideagainasAtriestotransmitA4andBtriesonce
moretotransmitB1.GivetheprobabilitythatAwinsthisthirdbackoff
raceimmediatelyafterthefirstcollision.
（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节）
1.UsingtheexamplenetworkgiveninFigure3.30,givethevirtualcircuittablesforalltheswitchesaftereachofthefollowingconnectionsisestablished.Assumethatthesequenceofconnectionsiscumulative,thatis,thefirstconnectionisstillupwhenthesecondconnectionisestablished,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,that
is,theportsforB1aretobelabeled“A”and“B2.”
17.ConsiderhostsX,Y,Z,WandlearningbridgesB1,B2,B3,withinitiallyemptyforwardingtables,asinFigure3.36.
（a）SupposeXsendstoZ.WhichbridgeslearnwhereXis?
DoesY’snetwork
interfaceseethispacket?
（b）SupposeZnowsendstoX.WhichbridgeslearnwhereZ
is?
DoesY’snetworkinterfaceseethispacket?
（c）SupposeYnowsendstoX.WhichbridgeslearnwhereYis?
DoesZ’snet-
workinterfaceseethispacket?
（d）Finally,supposeZsendstoY.WhichbridgeslearnwhereZis?
DoesW’s
networkinterfaceseethispacket?
第四章
（4.14.24.3.14.3.54.5节）
4.SupposeaTCPmessagethatcontains2,048bytesofdataand20bytesofTCPheaderispassedtoIPfordeliveryacrosstwonetworksoftheInternet（i.e.,fromthesourcehosttoaroutertothedestinationhost）.Thefirstnetworkuses14-byteheadersandhasanMTUof1,024bytes;theseconduses8-byteheaderswithanMTUof512bytes.Eachnetwork’sMTUgivesthesizeofthelargestIPdatagramthatcanbecarriedinalinklayerframe.Givethesizesandoffsetsofthesequenceoffragmentsdeliveredtothenetworklayeratthedestinationhost.AssumeallIPheadersare20bytes.
21.SupposearouterhasbuiltuptheroutingtableshowninTable4.14.Theroutercandeliverpacketsdirectlyoverinterfaces0and1,oritcanforwardpacketstoroutersR2,R3,orR4.Describewhattherouterdoeswithapacketaddressedtoeachofthefollowingdestinations:
（a）128.96.39.10.
（b）128.96.40.12.
（c）128.96.40.151.
（d）192.4.153.17.
（e）192.4.153.90.
45.Table4.16isaroutingtableusingCIDR.Addressbytesareinhexadecimal.Thenotation“/12”inC4.50.0.0/12denotesanetmaskwith12leading1bits,thatis,FF.F0.0.0.Notethatthelastthreeentriescovereveryaddressandthusserveinlieuofadefaultroute.Statetowhatnexthopthefollowingwillbedelivered.
（a）C4.5E.13.87.
（b）C4.5E.22.09.
（c）C3.41.80.02.
（d）5E.43.91.12.
（e）C4.6D.31.2E.
（f）C4.6B.31.2E.
第五章
（5.15.2节）
10.Youarehiredtodesignareliablebyte-streamprotocolthatusesaslidingwindow（likeTCP）.Thisprotocolwillrunovera1-Gbpsnetwork.TheRTTofthenetworkis140ms,andthemaximumsegmentlifetimeis60seconds.HowmanybitswouldyouincludeintheAdvertisedWindowandSequenceNumfieldsofyourprotocolheader?
第六章
（6.5节）
第九章
（9.1节）
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