计算机网络期中考试题 A卷_Ans

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1、计算机网络期中考试题A卷基础概念题(10points)1.WhatarethefivelayersintheInternetprotocolstack?Ans:Applicationlayer,transportlayer,networklayer,linklayerandphysicallayer.2.Whattransportservicedoesanappneed?Ans:Dataloss,Timing,Throughput,Security.3.WesaythatFTPhasout-of-bandcontrol.Whatd

2、owemeanbythat?Ans:WemeanthatcommandsanddataflowacrossdifferentTCPconnections.4.WhynotcentralizeDNS?Ans:·Singlepointoffailure单点故障，该DNS服务器崩溃，整个因特网随之瘫痪·Trafficvolume通信容量，单个DNS服务器处理上亿请求·Distantcentralized远距离集中式数据库，导致严重时延·Maintenance维护：单个DNS服务器将不得不为所有的因特网主机保留记录，这个中央数据库十分庞大

3、，且需为每个新添加的主机频繁更新5.Whatiscookie?Ans:Cookie：某些网站为了辨别用户身份、进行session跟踪而储存在用户本地终端上的数据。用户首次访问站点时，可能需要提供一个用户标识。在后继访问中，浏览器向服务器传递一个cookie首部，供服务器识别该用户。Delay:(10points)ConsidertwohostsAandBconnectedbyasinglelinkofraterbits/sec.Thetwohostsareseparatedbydmeters.Signalpropagationis

4、pmeterspersecond.HostAissendingtohostBapacketofsizeSbits.a)(2pts)Whatisthepropagationdelay,tpr?tpr=d/pb)(2pts)Whatisthetransmissiondelay,ttr,ofthepacket?ttr=S/rc)(2pts)Ignoringanypossibleprocessingandqueueingdelays,whatistheend-to-enddelay?(d/p)+(S/r)d)(3pts)IfHostAst

5、artstotransmitthepacketattimet=0,whereisthepacketattimet=ttr?ThelastbitisjustleavingHostA.e)(3pts)Supposethattpr>tfr.Attimet=tfrwhereisthefirstbitofthepacket?ThefirstbitisonthelinkandhasnotreachedHostB.f)(3pts)Supposethattpr

6、t?Thefirstbithasreached(andlikelyreceivedby)HostB.Applicationlayer(15points)AnswerthefollowingquestionsregardingtheApplicationLayer.Http:1.a)WhatisHTTP?1eachandround-up(5pts)Ans:TheHyperTextTransferProtocolisastatelessapplicationlayerprotocol.HTTPisusedtotransferwebco

7、ntentbetweenabrowserapplication(client)andanHTTPserver.AllwebcontentisidentifiedbyaURL.HTTPisarequestresponseprotocolthatusesTCPforassureddelivery.HTTPusesASCIIencodedheaders.TheHTTPGETcommandretrievesHTMLfilesandotherobjects.TheGETheaderincludestheURLoftheobjectandot

8、heroptionalfieldssuchascapability,language,andsoon.Theresponseincludesaresponseheaderwitharesponsecode(code200isOKand404ispa