计算机网络期中考试试题_2013_答案

第-1-页/共4页中山大学软件学院2010级软件工程专业(2012学年秋季学期)《SE-301计算机网络》期中试题(A卷)(考试形式：闭卷考试时间:2小时)《中山大学授予学士学位工作细则》第六条考试作弊不授予学士学位方向：姓名：______学号：1.(6points)What’sthedifferencebetweenHTTPandFTP?HTTP是带内连接，而FTP是带外连接；（3points）HTTP是无状态的，而FTP是有状态的（在会话期间需要ID密码）。(3points)端口号、协议命令、返回码(一点一分，最多两分)3.(8points)Whatarethechanges(oradvantages)ofIPv6,comparetoIPv4?/*(a).headerformathelpsspeedprocessing/forwarding(3points)(b).headerchangestofacilitateQoS(3points)*/1.扩大地址容量（128bits）2.简单高效的首部（各种其它解释）3.不允许分片重组；4.取消校验和。答1点3分，2点5分，3点7分，4点8分。4．(8points)HowcanUDPdetectbiterrorforareceivedsegment?Pleasedescribesuchmechanism./*使用checksum（1分）sender:checksumisaddition(1’scomplementsum)ofsegmentcontents.putschecksumvalueintoUDPchecksumfield（3分）receiver:computechecksumofreceivedsegment.checkifcomputedchecksumequalschecksumfieldvalue（3分）note:whenaddingnumbers,acarryoutfromthemostsignificantbitneedstobeaddedtotheresult（1分）*/检查和（2分）发送方：求和（溢出要回卷），补1运算得到checksum(3分)接收方：所有字段求和（包括checksum），不全为1则有bit-error(3分)6.(10points)Considerane-commercesitethatwantstokeepapurchaserecordforeachofitscustomers.Describehowthiscanbedonewithcookies.a.Whentheuserfirstvisitsthesite,thesitereturnsacookienumber.Thiscookienumberisstoredontheuser’shostandismanagedbythebrowser.b.Duringeachsubsequentvisit(andpurchase),thebrowsersendsthecookienumberbacktothesite.Thusthesiteknowswhenthisuser(moreprecisely,thisbrowser)isvisitingthesite.警示第-2-页/共4页1.(6points)Inourrdtprotocols,whydidweneedtointroducetimers?Tohandlelossesinthechannel.(3points)ifnotreceivedwithinthedurationofthetimerforthepacket,thepacket(oritsACKorNACK)isassumedtohavebeenlost.(3points)2.(9points)InstallandcompiletheJavaprogramsTCPClientandUDPClientononehostandTCPServerandUDPServeronanotherhost.a.SupposeyourunTCPClientbeforeyourunTCPServer.Whathappens?Why?b.SupposeyourunUDPClientbeforeyourunUDPServer.Whathappens?Why?c.Whathappensifyouusedifferentportnumbersfortheclientandserversides?a)theclientwillattempttomakeaTCPconnectionwithanon-existentserverprocess.ATCPconnectionwillnotbemade.(3points)b)UDPClientdoesn'testablishaTCPconnectionwiththeserver.Thus,everythingshouldworkfineifyoufirstrunUDPClient,thenrunUDPServer,andthentypesomeinputintothekeyboard.(3points)c)theclientwillattempttoestablishaTCPconnectionwiththewrongprocessoranon-existentprocess.Errorswilloccur.(3points)（这一问觉得可以有不同理解，所以按自己理解的答就行）3.(10points)Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantandwhicharevariable?Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.(1.5pointsperdelay)Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.(1pointperdelay)4.(12points)Consideradatagramnetworkusing8-bithostaddresses.Supposearouteruseslongestprefixmatchingandhasthefollowingforwardingtable:PrefixMatchInterface111111otherwise0123Foreachofthefourinterfaces,givetheassociatedrangeofdestinationhostaddressesandthenumberofaddressesintherange.DestinationAddressRangeLinkInterface10000000through10111111(64addresses)11000000through11011111(32addresses)11100000through11111111(32addresses)00000000through01111111(128addresses)0123每行：范围2分，范围大小1分。5.(15points)Supposetwohosts,AandB,areseparatedby10,000kmandareconnectedbyadirectlinkofR=2Mbps.Supposethepropagationspeedoverthe第-3-页/共4页linkis2.5*108m/s.a)Calculatethebandwidth-delayproduct,R*dprop.(5points)b)Considersendingafileof400,000bitsfromHostAtoHostB.Supposethefileissentcontinuouslyasonelargemessage.What’sthemaximumnumberofbitsthatwillbeinthelinkatanygiventime?(5points)c)Whatisthewidth(inmeters)ofabitinthelink?Isitlongerthanafootballfield(105mlongwithFIFAstandard)?(5points)a)R*dprop=R*d/s=2Mbps*10000km/2.5*108m/s=80Kb.b)Since400,000bits80kb,thusthemaximumnumberofbitsthatwillbeinthelinkisR*dprop=80Kb.c)X=20000km/(R*dprop)=10000km/80Kb=125m105m6.(16points)AssumingTCPRenoistheprotocolexperiencingthebehaviorshownabove,answerthefollowingquestions.Inallcases,youshouldprovideashortdiscussionjustifyingyouranswer.a.IdentifytheintervalsoftimewhenTCPslowstartisoperating.TCPslowstartisoperatingintheintervals[1,6]and[23,26](2points)b.IdentifytheintervalsoftimewhenTCPcongestionavoidanceisoperating.TCPcongestionavoidanceisoperatingintheintervals[6,16]and[17,22](2points)c.Afterthe16thtransmissionround,issegmentlossdetectedbytripleduplicateACKorbyatimeout？Afterthe16thtransmissionround,packetlossisrecognizedbyatripleduplicateACK.Iftherewasatimeout,thecongestionwindowsizewouldhavedroppedto1.(2第-4-页/共4页points)d.Afterthe22ndtransmissionround,issegmentlossdetectedbyatripleduplicateACKorbyatimeout?Afterthe22ndtransmissionround,segmentlossisdetectedduetotimeout,andhencethecongestionwindowsizeissetto1.(2points)e.WhatistheinitialvalueofThresholdatthefirsttransmissionround?Thethresholdisinitially32,sinceitisatthiswindowsizethatslowtartstopsandcongestionavoidancebegins.(2points)f.WhatisthevalueofThresholdatthe18thtransmissionround?Thethresholdissettohalfthevalueofthecongestionwindowwhenpacketlossisdetected.Whenlossisdetecteddu