川大林锋计网第三次作业.doc

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1、ReviewQuestions:3.ConsideraTCPconnectionbetweenHostAandHostB.SupposethattheTCPsegmentstravelingfromHostAtoHostBhavesourceportnumberxanddestinationportnumbery.WhatarethesourceanddestinationportnumbersforthesegmentstravelingfromHostBtoHostA?Thesourceportnumberisy,thed

2、estinationportnumberisx.5.WhyisitthatvoiceandvideotrafficisoftensentoverTCPratherthanUDPintoday’sInternet.(Hint:TheanswerwearelookingforhasnothingtodowithTCP’scongestion-controlmechanism.)因为UDP虽然传输速度快，不需要建立对等连接，但是容易丢包，而且数据包顺序容易混乱；TCP传输更稳定，文件没有丢失或者损失。6.Isitpossiblefo

3、ranapplicationtoenjoyreliabledatatransferevenwhentheapplicationrunsoverUDP?Ifso,how?是的，只需要在UDP的协议上添加验证的数据，比如给每个包传输时加个头，并且设置返回值，还有很多其他方法在此不一一列举了。7.SupposeaprocessinHostChasaUDPsocketwithportnumber6789.SupposebothHostAandHostBeachsendsaUDPsegmenttoHostCwithdestination

4、portnumber6789.WillbothofthesesegmentsbedirectedtothesamesocketatHostC?Ifso,howwilltheprocessatHostCknowthatthesetwosegmentsoriginatedfromtwodifferenthosts?正解，两个部分将针对同一接口。在套接字接口，对于每个收到的片段，操作系统将提供过程与IP地址确定各段的起源。8.SupposethataWebserverrunsinHostConport80.SupposethisWe

5、bserverusespersistentconnections,andiscurrentlyreceivingrequestsfromtwodifferentHosts,AandB.ArealloftherequestsbeingsentthroughthesamesocketatHostC?Iftheyarebeingpassedthroughdifferentsockets,dobothofthesocketshaveport80?Discussandexplain.对于每个坚持的连接，web服务器会创建一个连接套接字，

6、每个连接套接字由4个数组定义。当主机C收到一个请求的时候，它会检查数据报和段中的4个区域，以此决定用哪个区域来接收TCP段。因此，A和B传输的套接字是不一样的。每一个套接字的标识符都使用80端口作为目的的，但是不同的资源有不同的IP地址。和UDP不同，当传输层传输一个TCP段至应用层的时候，它并不需要明确指定IP地址，因为套接字标识符会帮它指定。14.Trueorfalse?(F)a.HostAissendingHostBalargefileoveraTCPconnection.AssumeHostBhasnodatatose

7、ndHostA.HostBwillnotsendacknowledgmentstoHostAbecauseHostBcannotpiggybacktheacknowledgmentsondata.(F)b.ThesizeoftheTCPrwndneverchangesthroughouttheduration(持续时间)oftheconnection.(T)c.SupposeHostAissendingHostBalargefileoveraTCPconnection.Thenumberofunacknowledgedbyte

8、sthatAsendscannotexceedthesizeofthereceivebuffer.(F)d.SupposeHostAissendingalargefiletoHostBoveraTCPconnection.Ifthesequencenumberforasegm