还剩8页未读,继续阅读
本资源只提供10页预览,全部文档请下载后查看!喜欢就下载吧,查找使用更方便
文本内容:
Review QuestionsSection
2.11)List fivenon-proprietary(
三、专有的)Internet applicationsand theapplication-layerprotocols that they use.列举五种非专用的英特网应用以及它们所使用的应用层协议applications protocolse-mail SMTPWEBHTTPstreaming multimedia HTTP RTPffiletransfer FTPremoteterminal accessTelnet2)For acommunication session(会议)between twohosts,which hostis the client andwhichis the server对俩主机之间的通信会话,哪个主机是客户机,哪个主机是服务器?(发起)The processwhich initiatesthe communicationis the client;the processthat waitsto becontacted isthe server.3)What information is used by aprocess runningon one host toidentify aprocessrunning on another host运行在一台主机上的进程使用什么信息标识运行在另一台主机上的进程?The IP address of the destinationhost and the portnumber of the destinationsocket.4)List thevarious network-application useragents thatyou useonadaily basis.列举你每天都会使用的网络应用程序用户代理a Supposeyou runTCPCIient beforeyou runTCPServer.What happensWhyb Supposeyou runUDPCIient beforeyou runUDPServer.What happensWhycWhat happensif youuse differentport numbers for the client andserversides9Rewrite TCPServer.java so that itcan acceptmultiple connections.Hint:You willneedto usethreads.TCPServerJava,以便它能够接受多个连接提示您将需要使用线程DISCUSSION QUESTIONS1What is a CGIscript Giveexamples oftwo popularWeb sitesthat use CGI scripts.Explain howthese sitesuseCGI.Which languagesare CGIscripts typicallywrittenin什么是CGI脚本?给出了两个流行的网站,使用CGI脚本的例子说明这些网站如何使用CGIo哪些语言是CGI脚本通常写在?Abbreviation ofCommon GatewayInterface,a specificationfor transferringinformationbetween aWorld WideWebserver anda CGIprogram,w canyou configureyourbrowser forlocal cachingWhat kindsof optionsdo you have2Can youconfigure yourbrowser toopen multiplesimultaneous同时发生的connections toa Website What are theadvantages anddisadvantages ofhaving alargenumber ofsimultaneous TCP connections您可以配置您的浏览器打开多个同时连接到一个网站吗?什么是有大量并发的TCP连接的优势和劣势?3Discussion question:Consider SMTP,POP3and IMAP.Are thesestateless protocolsWhyor whynot讨论的问题考虑SMTP,POP3和IMAP这些无状态协议?为什么或为什么不呢?4We haveseen thatInternet TCPsockets treatthe databeing sentas abyte streambutUDP socketsrecognize messageboundaries.What isone advantageand onedisadvantageof byte-oriented APIversus havingthe APIexplicitly recognizeandpreserve application-defined messageboundaries我们已经看到,互联网TCP套接字治疗作为一个字节流,但UDP套接字发送的数据识别消息边界一个优势,而不是API的明确承认和保护应用程序定义的消息边界面向字节API的一个缺点是什么?6Would itbe possibleto implementa connection-oriented serviceeg,SMTP orHTTPon top of a connectionless serviceWhat wouldbe someof thedifficulties involvedindoing soand howcould thesebe overcomez怎样才有可能实现一个无连接的服务上的一个面向连接的服务(如SMTP或HTTP)什么会参与这样的一些困难,以及如何能加以克服?Web browserE-mail applicationuser agentFileTransfer useragentP2p useragentaudio/video useragent5Referring toFigure
2.1-2,we seethat notnone of applications listedin thetablerequire bothno data loss andtiming.Can youconceive ofan applicationthatrequires no dataloss and thatis alsohighly timesensitive参见图
2.1-2,我们可以看到表中所列举的应用没有一个既要求没有数据丢失又要求定时你能设想出一个应用,既既要求没有数据丢失又要求定时吗?There areno goodexamples ofan applicationthat requiresnodatalossandtiming.6What ismeant bya handshakingprotocol握手协议的意义?A protocoluses handshakingif thetwo communicatingentities firstexchangecontrol packetsbefore sendingdata toeach other.SMTPuses handshakingattheapplication layerwhereas HTTPdoes not.7Why doHTTP,FTP,SMTP,POP3and IMAPrun ontopofTCP ratherthan UDP为什么HTTP,FTP,SMTP,POP3和IMAP都运行在TCP协议而不是UDP协议上?The applicationsassociated withthose protocolsrequire thatallapplication databe receivedin thecorrect orderand withoutgaps.TCP providesthis servicewhereas UDPdoes not.8Consider an e-commerce sitethat wantsto keepa purchaserecord foreach ofitscustomers.Describe howthis can be done with HTTPauthentication.认证Describe howthiscanbedonewithcookies.考虑一个电子商务网站需要保留每一个客户的购买记录,描述如何使用HTTP认证;描述这是如何用cookies来实现的When the user firstvisits the site,thesitereturns acookie number.This cookienumber isstored on the usershost andis managedbythe browser.During eachsubsequent visitand purchase,thebrowser sendsthe cookienumber backto thesite.Thus thesiteknows whenthis usermore precisely,this browseris visitingthesite.9What isthe differencebetween persistent HTTP withpipelining持久HTTP流水线and persistentHTTP withoutpipelining没有流水线的持久HTTP连接Which of the twoisused byHTTP/
1.1In persistentHTTP withoutpipelining,the browserfirst waitsto receiveaHTTPresponse fromthe server beforeissuing a new HTTPrequest.In persistentHTTP withpipelining,the browserissuesrequests as soon asit has a needto doso withoutwaiting forresponse messages from thezserver.persistentHTTP with pipeliningisusedbyHTTP/
1.1O10Telnet intoa Web server andsend amuli-line request message.Include in therequest messagethe if-modified-sinceheader lineto forcea response message with the304Not Modifiedstatus code.用Telnet向web服务器发送一条多行的请求报文在该请求报文中包含工f-modified-since:首部行,迫使响应报文中出现304Not Modified状态码11Why isit saidthat FTP sends control information out of band1频带外的为什么说FTP在“带外”发送控制信息?平行的FTP usestwo parallelTCP connections,one connection forsending controlinformation such asarequest totransfer a fileand anotherconnectionfor actually transferringthe file.Becausethe controlinformationis not sentover the same connectionthatthe fileis sentover,FTPsendscontrolinformationoutofband.12Suppose Alicewith a Web-based e-mail accountsuchasYahoo!mail orHotmailsends amessage toBob,who accesseshis mailfrom hismail serverusing POP
3.Discuss howthe messagegets fromAlices hostto Bobshost.Be sureto listthe seriesofapplication-layer protocolsthat areused tomove the message betweenthe twohosts.假设Alice使用一个基于web的电子邮件账户向Bob发送邮件,而Bob使用POP3协议访问他的邮件服务器来获取邮件讨论信息是如何从Alice的主机到Bob的主机的一定要列出在两台主机间移动信息所用的应用层协议Message issent fromAlices hostto hermail serverover HTTP.Alices mailserverthen sendsthemessageto Bobsmail serverover SMTP.Bob thentransfers themessagefrom hismail serverto hishost overPOP
3.13Suppose thatyou sendane-mail messagewhose onlydata isa MicrosoftExcelattachment.What mightthe header lines includingMIME lineslook like假定你发送一封电子邮件,唯一的数据就是一个表格附件可能的首部行包含MIME行是什么样的?Subject:helloMIME-Version:
1.0Content-Transfer-Encoding:base64Content-Type:Application/MS-Excel14Print outthe headerofamessage thatyouhaverecently received.How manyRecievedheader linesare thereAnalyze eachoftheheaderlinesin themessage.打印出你最近收到的报文的首部,在Received中,首部行有多少行?分析该报文首部行的每一行15From a users perspective,what isthe differencebetween thedownload-and-delete modeandthedownload-and-keep modein POP3从用户的观点来看,POP3中“下载并删除”模式和“下载并保留”模式有什么区别?取回With download and delete,after auser retrievesits messagesfrom aPOP游动的server,the messages are deleted.This posesa problemfor thenomadic user,who maywant toaccess the messagesfrommany differentmachines officePC,homePC,etc..配置,In thedownload andkeep configurationmessagesarenot deletedafterthe userretrieves themessages.This canalso beinconvenient,as eachtimetheuserretrieves thestored messagesfrom anew machine,all ofnon-deletedmessages willbe transferredto thenew machineincluding veryold messages.16Redraw Figure
2.5-4for whenall queriesfrom the local nameserverare iterative.重绘时,从本地域名服务器所有查询迭代图2,5-4o17Each Internethost willhave atleast onelocal name server andone authoratativename server.What roledoes eachof theseservers havein DNS每一台英特网主机都至少有一个本地域名服务器和一个权威域名服务器这两种服务器在域名解析中起什么作用?A hostauthoritative nameserver providesthe officialtranslation ofahostname toan IP address amongother things.A hostlocalnameserverisa代理月艮务器proxy tothe DNSsystem:the hostfirst queriesits localnameserver for a mapping;the localnameserverthen obtainsthe mappingon the代表behalf ofthe queryinghost.18Is itpossible thatan organizationsWeb server and mail server haveexactly thesamealias另名forahostname e.g.fooWhat wouldbe thetype forthe RRthatzcontains thehostname ofthe mailserver一个机构的web服务器和邮件服务器可以有有完全相同的主机别名吗?包含了邮件服务器的主机名的RR有什么样的类型?Yes oAn organizationmailserverandWebservercan havethesamealias forahost name.The MXrecord isused tomap themailserverhost nameto itsIPaddress.19Use nslookupto findaWebserver thathas multiple IP addresses.Does theWebserver ofyour institutionschool,company,etc.have multipleIP addresses使用nslookup找到一个Web服务器有多个IP地址贵机构的Web服务器如学校,公司,等是否有多个IP地址?My insititutiondoesnt havemultipleIPaddress.20The UDP server described in Section
2.7only neededone socket,whereas the TCPserver describedin Section
2.6needed twosockets.Why If theTCP server weretosupport n simultaneous同时发生的connections,each froma differentclient host,howmany socketswould theTCP serverneed27节描述的UDP服务器仅需要一个套接字,而2・6节描述的TCP服务器则需要两个套接字为什么?如果TCP服务器要支持n个并行连接,且每个连接都来自不同的客户机主机,那么TCP服务器需要多少个套接字?With the UDPserver,there isno welcomingsocket,and alldata fromdifferentclients entersthe serverthrough thisone socketWith theTCPserver,there isa welcomingsocket,and eachtime a client发起initiates aconnection tothe server,anewsocket iscreated.Thus,to supportnsimultaneousconnections,the serverwouldneed sockets.n+121For the client-server applicationover TCPdescribedin Section
2.6,why musttheserver program be executed before theclient programFor theclient-server applicationoverUDP describedinSection
2.7,why maytheclientprogrambeexecutedbeforetheserver program在
2.6节所描述的运行于TCP上的客户机■服务器应用程序,为什么服务器程序要先于客户机程序运行?在
2.7节描述的运行于UDP上的客户机-服务器应用程序,为什么客户机程序要先于服务器程序运行?For theTCP application,assoonas theclient isexecuted,it attemptsto initiatea TCPconnectionwith theserver.IftheTCPserverisnotrunning,then theclient willfailto makeaconnection.For theUDP application,theclientdoes notinitiate connectionsorattempt tocommunicate withtheUDPserver immediatelyuponexecutionPROBLEMS1True or false.a Supposeauserrequests aWeb pagethat consistsof sometext andtwoimages.For thispage theclient willsend onerequestmessageand recievethreeresponse messages假设一个用户请求一个网页,包含一些文本和两个图像这个页面的客户端将发送一个请求消息和接收三个响应消息?Fb Trueorfalse.Two distinctWeb pagese.g.,mit.edu/research.html andmit.edu/students.html canbe sentover thesame persistentconnection真或假两种截然不同的网页例如,mit.edu/research.html和mit.edu/students.html可发送相同的持久连接?Tc Withnon-persistent connectionsbetween browserand originserver,it ispossibleforasingle TCPsegment tocarry twodistinct HTTPrequest messages浏览器和源服务器之间的非持久连接,它有可能为一个单一的TCP段进行两个不同的HTTP请求消息?Fd TheDateheader in the HTTPresponsemessageindicates whenthe objectinthe responsewas lastmodified日期在HTTP响应消息头指示的最后修订在响应对象时?2Read RFC959for FTP.List all oftheclient commandsthat aresupported bythe RFC.FTP阅读RFC959o列出所有的客户端支持的命令是由RFCAccess controlcommands:USER,PASS,ACT,CWD,CDUP,SMNT,REIN,QUIT.参数Transfer parametercommands:PORT,PASV,TYPE STRU,MODE.Service commands:RETR,STOR,STOU,APPE,ALLO,REST,RNFR,RNTO,ABOR,DELE,RMD,MRD,PWD,LIST,NLST,SITE,SYST,STAT,HELP,NOOP.3Read RFC
1700.Whatarethe well-known portnumbersforthe simplefile transferprotocolsftp Forthe networknews transfoerprotocol nntp阅读RFC1700中什么是‘、简单文件传输协议〃SFTP众所周知的端口号?对于“新闻联播transfoerW NNTP4Suppose within your webbrowser youclick ona linkto obtaina webpage.Suppose thatthe IPaddress forthe associatedURL isnot cached缓存inyourlocal host,sothata DNSlook upis necessaryto obtainthe IPaddress.Suppose thatn DNSserversare visitedbefore yourhost receives the IPaddress fromDNS;the successivevisitsincur产生a RTTof RTT工RTT-Further supposethat webpage associatedwiththelinkncontains exactlyone object,a smallamount ofHTML text.Let RTTodenote表示the RTTbetweenthelocalhost andtheservercontaining the object.Assuming zerotransmissiontime oftheobject,how muchtime elapses流逝from whenthe clientclickson thelink untiltheclientreceivestheobject.假定在你的浏览器中,你点击一个超链接来获得一个web页面假定相关的URL的工P地址没有缓存在你的本地主机上,因此必须进行DNS查询以获得IP地址假设你的主机从DNS获得IP地址之前,已经访问了n个DNS服务器,相继产生的RTT依次为RTT工,~RTTn进一步假设,与连接相关的web网页只含有一个对象,即少量的HTML文本让RTTO表示本地主机和包含对象的服务器之间的RTT假设对象传输时间为0,从客户点击连接到接收到对象需要多长时间?The totalamount oftime toget theIPaddressis】RTT+RTT+--+RTT.2nOnce theIPaddressis known,RTT elapses to setup theTCPconnectionandanother RTTelapsestorequest andreceive thesmall object.The totalresponse timeis4Referring toquestion4suppose thepage containsthree verysmall objects.zNeglecting transmissiontimes,how muchtime elapseswith anonpersistentHTTP withno parallelTCP connections,b nonpersistentHTTP withparallelconnections,c persistentHTTPwithpipelining.提到的问题4,假设该页面包含三个非常小的物体忽略的传输时间,间隔时间多长一非持久的HTTP没有并行的TCP连接,b与并行连接的非持久的HTTP,c与持久HTTP流水线;=8R77+RTT[+…+RTT”.b=4RTT+RTT+---+RTT.i inc6Two HTTPrequest methodsare GETand POST.Are thereany othermethods inHTTP/
1.0If so,what arethey usedfor Howabout HTTP/
1.1两个HTTP请求方法是GET和POSTo在HTTP/
1.0是否有任何其他的方法?如果是这样,它们是什么使用?关于HTTP/
1.1的怎么样?7Write asimple TCPprogram fora serverthat acceptslines ofinput fromaclientandprints thelines ontothe serversstandard output.You cando thisby modifyingtheTCPServer.java programinthetext.Compile andexecute yourprogram.On anyothermachine whichcontains aWeb browser,set theproxy serverinthebrowser tothemachine inwhich your server programis running;also configurethe portnumberappropriately.Your browsershould nowsend itsGET requestmessages toyourserver,and yourserver shoulddisplay themessages onits standardoutput.Use thisplatformto determinewhether yourbrowser generatesconditional GETmessages forobjectsthat arelocally cached.写一个简单的TCP服务器接受从客户端的输入线和输出到服务器的标准输出线方案您可以通过修改文本TCPServer.java方案编译和执行程序在任何其他机器,其中包含一个Web浏览器,在浏览器中设置代理服务器在您的服务器程序运行的机器,也适当配置的端口号您的浏览器现在应该GET请求消息发送到您的服务器,您的服务器上显示的消息,它的标准输出利用这一平台,以确定是否您的浏览器生成本地缓存的对象,有条件的GET消息8Read thePOP3RFC,RFC
1939.What isthe purposeofthe UIDLPOP3command阅读的POP3RFC,RFC1939中UIDL POP3命令的目的是什么?是的缩写UIDL abbreviates〜unique・ID listing”.When aPOP3client issuestheUIDLcommand,theserverresponds withthe uniquemessageID forallofthemessagespresent inthe usersmailbox.Thiscommand isuseful for“downloadandkeep”.By keepingafilethat liststhemessages retrievedin earliersessions,theclientcan usethe UIDLcommandto determinewhich messagesontheserver havealready beenseen.9Install andcompile theJava programsTCPCIient andUDPCIient ononehostandTCPServer andUDPServer onanother host.。