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1、Chapter 8 Communication Networks and ServicesApplication Layer Protocols & IP Utilities1、DNS: queries examplecomabcxyzabcxyI root name serverlocal name authoritative name server“root”edu.edu root name serverylocal name authoritative name local name server authoritative name server IP 地址是什么？ 2
2、2、 The Web some jargonWeb page consists of a base HTML-file which includes several referenced objectsObject can be HTML file, JPEG image, Java applet, audio file,Each object is addressable by a URLExample URL:http:/www.someschool.edu/someDept/pic.gifhost namePath + File namelUser agent for Web: a br
3、owser, e.g.,lMS Internet ExplorerlNetscape CommunicatorlServer for Web is called Web server, e.g.,lApache (public domain)lMS Internet Information ServerAccess type3URLs Uniform Resource LocatorsWhat is the page called? Where is the page located? How can the page be accessed? URL is the solution chos
4、en identifies pages in a way that solve all three problems at once.:/:/ftp FTP 文件传送协议http HTTP 超文本传送协议 News USENET 新闻4Some common URLs are listed as:53、Hypertext Transfer ProtocolRFC 1945 (HTTP 1.0), RFC 2616 (HTTP 1.1)HTTP provides communications between web browsers & web serversWeb: framework for
5、 accessing documents & resources through the InternetHypertext documents: text, graphics, images, hyperlinksDocuments prepared using Hypertext Markup Language (HTML)6HTTP ProtocolHTTP servers use well-known port 80Client request / Server replyStateless: server does not keep any information about cli
6、ent7HTTP overview (continued)因特网S链接到URL的超链HTTP 使用此 TCP 连接browser programServer programHTTPclient清华大学院系设置 TCP Connection SetupTCP Connection ReleaseHTTP response 响应文档HTTP request 请求文档8HTTP Proxy Server & CachingWeb users generate large traffic volumesTraffic causes congestion & delayCan improve delay
7、 performance and reduce traffic in Internet by moving content to servers closer to the userWeb proxy servers cache web informationDeployed by ISPsCustomer browsers configured to first access ISPs proxy serversProxy replies immediately when it has requested object or retrieves the object if it does n
8、ot9Proxy server: network cachinguser sets browser: Web accesses via cachebrowser sends all HTTP requests to cacheif object in cache: returns object immediatelyelse cache requests object from origin server, then returns object to clientGoal: satisfy client request without involving origin serverclien
9、tProxyserverclientHTTP requestHTTP requestHTTP responseHTTP responseHTTP requestHTTP responseorigin serverorigin server104、File Transfer Protocol (RFC 959)Provides for transfer of file from one machine to another machineDesigned to hide variations in file storageFTP parameter commands specify file i
10、nfo File Type: ASCII, EBCDIC, image, local.Data Structure: file, record, or pageTransmission Mode: stream, block, compressedOther FTP commandsAccess Control: USER, PASS, CWD, QUIT, Service: RETR, STOR, PWD, LIST, 11FTP: separate control, data connectionsUser PIData Trans. Proc.User InterfaceServer P
11、IData Trans. Proc.ClientServerINTERNETTCP Control Connection port 21TCP Data Connectionport 2012Two TCP ConnectionsControl connectionSet up using Telnet protocol on well-known port 21FTP commands & replies between protocol interpretersPIs control the data transfer processUser requests close of contr
12、ol connection; server performs the closeData connectionTo perform file transfer, obtain lists of files, directoriesEach transfer requires new data connectionPassive open by user PI with ephemeral port #Port # sent over control connection Active open by server using port 20135、Email ProtocolSenderEma
13、il Buffer ReceiverEmail ServerClient AgentSMTPSMTPPOP3 Sender Email ServerClient Agent用户邮箱ReceiverClient Agent用户代理 EmailServer EmailServerSMTPSMTPPOP3(Sender Email)(Access Email )(读取邮件)(TCP Connetion )(TCP 连接)(TCP Connection)Internet(Sender Email)(Sender Email)(Sender Email)144、Telnet (RFC 854)Provides general bi-directional byte-oriented TCP-based communications facility (Network Virtual Terminal) Initiating machine treated as local to the remote hostUsed to connect to port # of other servers and to interact with them using command lineNVTNVTServer process15TCP/IP PROTOCOL SUITE16
