二章節(jié)應(yīng)用層ApplicationLayer課件

1、第二章應(yīng)用層 (Application Layer)Ch.2 Application-Layer1(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用層-簡介本章主要介紹在應(yīng)用層所常用的協(xié)定HTTP (section 2.2)FTP (section 2.3)E-mail (section 2.4)DNS (section 2.5)並介紹用戶端（client）和伺服器端（server）的關(guān)係Service modelSocket programming (section 2.6)Ch.2 Application-Layer2(C) All

2、rights reserved by Professor Wen-Tsuen Chen用戶端（client）和伺服器端（server）用戶端和伺服器端是一種互相依持的關(guān)係（一個用戶端，一個伺服器端）或（一個用戶端，數(shù)個伺服器端）或（數(shù)個用戶端，一個伺服器端）或（數(shù)個用戶端，數(shù)個伺服器端）在執(zhí)行一般的網(wǎng)路應(yīng)用程式情況下，一臺機(jī)器 是用戶端亦是伺服器端Ch.2 Application-Layer3(C) All rights reserved by Professor Wen-Tsuen Chen用戶端（client）和伺服器端（server(cont.)用戶端通常為送出要求(request)的一

3、方或最初建立連線(connection)要求的一方向伺服器端要求提供服務(wù)(service)的一方例如：使用者端的瀏覽器(browser)伺服器端通常為送出回應(yīng)(reply)的一方提供服務(wù)的一方例如：網(wǎng)頁伺服器(web server)Ch.2 Application-Layer4(C) All rights reserved by Professor Wen-Tsuen Chen用戶端（client）和伺服器端（server(cont.)Ch.2 Application-Layer6(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用層(Ap

4、plication Layer)應(yīng)用層是位於OSI七層中的最上層為什麼我們需要應(yīng)用層呢？因?yàn)閼?yīng)用層提供了一個平臺，可以讓許多的網(wǎng)路應(yīng)用程式在應(yīng)用層執(zhí)行這一些應(yīng)用程式都是在對等的應(yīng)用層做溝通Ch.2 Application-Layer7(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用層協(xié)定(Application-layer Protocol)屬於應(yīng)用程式的一部份協(xié)定(protocol)中定義使用此應(yīng)用程式所需要的流程所交換的訊息格式訊息所表示的意義與下層協(xié)定溝通的方法例如：我們?yōu)g覽網(wǎng)頁需要HTTP協(xié)定Ch.2 Application-L

5、ayer9(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用層協(xié)定(Application-layer Protocol)(cont.)Q:網(wǎng)路應(yīng)用程式跟應(yīng)用層的協(xié)定有什麼關(guān)係？應(yīng)用層的協(xié)定只是網(wǎng)路應(yīng)用程式的一部份而已例如：HTTP和網(wǎng)頁應(yīng)用程式的關(guān)係Ch.2 Application-Layer10(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用層協(xié)定(Application-layer Protocol)(cont.)應(yīng)用程式介面（API, Application Program

6、mers Interface）API就像是進(jìn)出應(yīng)用程式的一扇門應(yīng)用程式透過API與下層協(xié)定溝通在這裡網(wǎng)路程式所指的API就是常見的SocketSocket在後面有專節(jié)介紹定址(Addressing)IP (ch. 4)或是主機(jī)名稱(需搭配DNS)指定應(yīng)用程式(Socket port)Socket port就像是門的鑰匙Ch.2 Application-Layer11(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用程式所需要的條件資料流失(data loss)在資料傳輸?shù)倪^程難免會有資料流失，我們要注意應(yīng)用程式所能夠接受的範(fàn)圍，並加以改善

7、網(wǎng)路頻寬(Network Bandwidth)我們需要考慮每一個應(yīng)用程式所需要的頻寬需求時間的影響(Time delay)我們需要考慮時間對於應(yīng)用程式的影響例如：播放影片或是聲音，就不允許資料延遲過久Ch.2 Application-Layer12(C) All rights reserved by Professor Wen-Tsuen Chen應(yīng)用程式所需要的條件(cont.)ApplicationData lossBandwidthTime SensitiveFile transferNo losselasticNoE-mailNo losselasticNoWeb documentsL

8、oss-tolerantelasticNoReal-time audio/videoLoss-tolerantAudio: 5kb-1MbVideo: 10kb-5MbYes, 100s msecStored audio/videoLoss-tolerantSame as aboveYes, few secsInteractive gamesLoss-tolerantFew Kbps upYes, 100s msecFinancial appsNo losselasticYes and noCh.2 Application-Layer13(C) All rights reserved by P

9、rofessor Wen-Tsuen Chen網(wǎng)路傳輸層(Transport layer) - overview傳輸層屬於OSI第四層在應(yīng)用層的下層傳輸層(ch. 3)提供了兩種不同的服務(wù)協(xié)定TCP (Transmission Control Protocol )UDP (User Datagram Protocol)在應(yīng)用層所執(zhí)行的網(wǎng)路應(yīng)用程式都是透過這兩種協(xié)定來與網(wǎng)路溝通Ch.2 Application-Layer14(C) All rights reserved by Professor Wen-Tsuen ChenTCPThree way handshakeCh.2 Applicati

10、on-Layer16(C) All rights reserved by Professor Wen-Tsuen Chen常見的網(wǎng)路應(yīng)用程式所使用的service應(yīng)用程式應(yīng)用層協(xié)定傳輸層協(xié)定電子郵件SmtpRFC 821TCP遠(yuǎn)端裝置存取telnetRFC 854TCP網(wǎng)頁httpRFC2068TCP檔案傳輸ftpRFC959TCP多媒體串流(stream)ProprietaryTCP or UDP遠(yuǎn)端檔案伺服器NSFTCP or UDP網(wǎng)路電話ProprietaryTypically UDPCh.2 Application-Layer17(C) All rights reserved by

11、Professor Wen-Tsuen ChenURL: Uniform Resource LocatorRFC編號：1738, 1808.A URL is compact representation of the location and access method for a resource available via the Internet.一個resource 是一個可以透過網(wǎng)路存取的任何物件, 包括檔案目錄、檔案、文件聲音和影像等URL的一般表示法:Ch.2 Application-Layer19(C) All rights reserved by Professor Wen-

12、Tsuen ChenRFC 1738為了下列存取機(jī)制所定義之URL formatsftp File Transfer Protocolhttp Hypertext Transfer Protocolgopher The Gopher Protocolmailto Electronic mail addressnews USNET newsnntp USNET news using NNTP accesstelnet Reference to interactive sessionswais Wide-Area Information Serversfile Host-specific file

13、 namesprospero Prospero Directory ServiceCh.2 Application-Layer20(C) All rights reserved by Professor Wen-Tsuen ChenCh.2 Application-Layer21(C) All rights reserved by Professor Wen-Tsuen ChenURI: Universal Resource Identifier (URI)RFC編號1630.是一種將用在網(wǎng)際網(wǎng)路中的物件的名稱和位址的表示一致的語法URL是URI的一種。 Decouple resource的名

14、稱從它所在的位置或是從它所存取的方法Ch.2 Application-Layer22(C) All rights reserved by Professor Wen-Tsuen ChenHTTPHTTP: Hyper Text Transfer ProtocolWeb的應(yīng)用層協(xié)定採用用戶端和伺服器端的服務(wù)架構(gòu)用戶端：要求、接收並顯示所接收的web pages伺服器端：接受要求並回應(yīng)所要求的web pagesCh.2 Application-Layer23(C) All rights reserved by Professor Wen-Tsuen ChenHTTPWeb page所包含的原件有O

15、bjectsHTML file, jpeg image, java appletBase HTML fileHTML是用來描述web page的一種語言URL：是用來表示一個page或是object.tw/index-c.html (hostname + path name)目前HTTP有兩種標(biāo)準(zhǔn)http 1.0 (RFC 1945)http 1.1 (RFC 2068)目前所使用Ch.2 Application-Layer24(C) All rights reserved by Professor Wen-Tsuen ChenHTTPHTTP下層協(xié)定使用TCP傳送服務(wù)由用戶端初始化連線(se

16、tup TCP connection)，使用socket port 80與伺服器端溝通伺服器端回應(yīng)後，兩端開始交換訊息直到結(jié)束(TCP connection close)HTTP是一個stateless的協(xié)定Stateless意指伺服器端不需要保有任何有關(guān)過去用戶端的要求訊息(request message)通常協(xié)定要保有過去的state，就會變的很複雜Ch.2 Application-Layer26(C) All rights reserved by Professor Wen-Tsuen ChenNon-persistent和persistent連線的不同Non-persistentHTT

17、P/1.0每一個TCP連線都會在server送完物件後結(jié)束所以每一個連線都只會有一個request和一個respondRespond time = 2 RTTs + Ttrans每一個所傳送的物件，都會受到slow start影響PersistentDefault for HTTP/1.1在同一個用戶端會一直使用同一個TCP連線，直到time out才關(guān)掉有較少的RTT和slow start有分成pipeline和non-pipelineCh.2 Application-Layer27(C) All rights reserved by Professor Wen-Tsuen ChenHTTP

18、的要求訊息格式- 一般式Ch.2 Application-Layer29(C) All rights reserved by Professor Wen-Tsuen ChenHTTP的訊息格式- responsestatus line(protocolstatus codestatus phrase)HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 . Content-Length: 6821 Content-Type:

19、text/html data data data data data . data, e.g., requestedhtml fileheader linesCh.2 Application-Layer30(C) All rights reserved by Professor Wen-Tsuen Chen常見的回應(yīng)訊息200 OK要求成功, 所要求的物件會夾帶在接下來的訊息301 Moved Permanently所要求的物件已經(jīng)一到別處，新的位置將會出現(xiàn)在下一個訊息中 (Location:)400 Bad Request所要求的訊息，伺服器無法判讀404 Not Found所要求的物件並不

20、存在伺服器裡505 HTTP Version Not Supported所要求使用的協(xié)定，伺服器端並不支援Ch.2 Application-Layer31(C) All rights reserved by Professor Wen-Tsuen ChenJava Language (爪窪語言)在1995年，由昇揚(yáng)公司(Sun Microsystems Inc.)所制訂For waiting interactive web pages.A small Java program, called an applet, is associated with a web page and downlo

21、aded to the client machine to interpret the web page.Ch.2 Application-Layer32(C) All rights reserved by Professor Wen-Tsuen ChenJava Language (cont.)Ch.2 Application-Layer33(C) All rights reserved by Professor Wen-Tsuen Chen認(rèn)證(Authentication)目的：為了要控制存取伺服器資料的權(quán)力因?yàn)槭莝tateless所以用戶端每一次request都要認(rèn)證一次認(rèn)證通常使用n

22、ame和password為避免需要重覆輸入認(rèn)證資料，瀏覽器會自行讀取catch的資料認(rèn)證步驟如右serverusual http request msg401: authorization req.WWW authenticate:usual http request msg+ Authorization:lineusual http response msgusual http request msg+ Authorization:lineusual http response msgtimeclientCh.2 Application-Layer34(C) All rights reser

23、ved by Professor Wen-Tsuen Chencookies設(shè)定cookie是為了方便在下一次存取網(wǎng)站時，可以不需要再輸入一些重複性的資料Cookie是由伺服器端要求用戶端建立，並且存在用戶端的機(jī)器上Cookie大多用來認(rèn)證使用者的訊息建立步驟如右clientserverusual http request msgusual http response +Set-cookie: #usual http request msgcookie: #usual http response msgusual http request msgcookie: #usual http resp

24、onse msgcookie-spectificactioncookie-spectificactionCh.2 Application-Layer35(C) All rights reserved by Professor Wen-Tsuen ChenWeb快?。╬roxy伺服器）目的：為了可以使用戶端送出的request不用直接到原來的伺服器，如此一來便可以加速網(wǎng)頁的取得及瀏覽所以大部分的用戶端都會透過proxy server來存取網(wǎng)路資源，而proxy server會保有一些web的快取資料，以利存取如果現(xiàn)在的網(wǎng)頁資料不是最新的，則proxy會去原始的網(wǎng)站去更新目前保有的資料client

25、Proxyserverclienthttp requesthttp requesthttp responsehttp responsehttp requesthttp responsehttp requesthttp responseorigin serverorigin serverCh.2 Application-Layer36(C) All rights reserved by Professor Wen-Tsuen Chen為什麼使用web catching因?yàn)閜roxy一般會離用戶端較近，會使的存取的速度較快可以減少一些較長距離的traffic同一筆資料可以統(tǒng)一由proxy來維持更新

26、右圖為示意圖1.5 Mbps access link是一個bottleneckoriginserverspublic Internetinstitutionalnetwork10 Mbps LAN1.5 Mbps access linkCh.2 Application-Layer37(C) All rights reserved by Professor Wen-Tsuen ChenFTP: File Transfer ProtocolFTP是現(xiàn)在較普遍也較常用的一個檔案傳輸?shù)膮f(xié)定採用用戶端與伺服器端模式溝通RFC編號：959所使用的port:21file transferFTPserverF

27、TPuserinterfaceFTPclientlocal filesystemremote filesystemuser at hostCh.2 Application-Layer38(C) All rights reserved by Professor Wen-Tsuen ChenFTP: File Transfer Protocol (cont.)FTP使用TCP作為傳輸層所使用的協(xié)定在FTP的連結(jié)中有分為兩種Control：在client和server之間交換commands和response，屬於”out of band control”Data：在client和server之間的

28、data交換FTP和HTTP不同，他會maintain state目前所在目錄登錄所做的認(rèn)證FTPclientFTPserverTCP control connectionport 21TCP data connectionport 20Ch.2 Application-Layer39(C) All rights reserved by Professor Wen-Tsuen ChenFTP: File Transfer Protocol (cont.)CommandsOpen:連接FTP serverUser: 使用者帳號Pass: 密碼RETR filename: 取得檔案STOR fil

29、ename: 上傳檔案Bye: 結(jié)束連線回應(yīng)訊息331 帳號 OK,需要密碼125 data connection 已經(jīng)開啟; 傳送開始425 無法開啟 data connection452 檔案寫入錯誤如何連結(jié)上FTP server?ftp xxx.xxx.xxx.xxx port get file.xxx (取得檔案)put file.xxx (上傳檔案)Ch.2 Application-Layer40(C) All rights reserved by Professor Wen-Tsuen ChenElectronic-mail (E-mail, 電子郵件)包含三個部分使用者代理人(u

30、ser agent)郵件伺服器(mail server)SMTP協(xié)定mailserveruseragentuseragentuseragentmailserveruseragentuseragentmailserveruseragentSMTPSMTPSMTPuser mailboxoutgoing message queueCh.2 Application-Layer41(C) All rights reserved by Professor Wen-Tsuen ChenE-mail user agent有時亦稱為郵件讀取者(mail reader)它可以讓user讀取、回復(fù)、轉(zhuǎn)發(fā)、儲存和建

31、構(gòu)訊息(message, mail)使用者需要讀取信息時，user agent會去跟郵件伺服器溝通，取回信息，或是送出信息User agent常見的有：Microsoft Outlook or Outlook ExpressNetscape MessengerCh.2 Application-Layer42(C) All rights reserved by Professor Wen-Tsuen ChenE-mail Mail server每一個user都會有一個信箱(mail box)來存放尚未讀取的信件尚未傳送出去的信件會暫時存在message queueSMTP扮演了兩種角色Clien

32、t：當(dāng)作傳送信件的serverServer：當(dāng)作接收信件的serverClient和server這兩種服務(wù)都會執(zhí)行在每一臺郵件伺服器上Ch.2 Application-Layer43(C) All rights reserved by Professor Wen-Tsuen ChenE-mail SMTPSMTP: Simple Mail Transfer ProtocolRFC編號: 821使用TCP作為下層溝通的協(xié)定Port: 25傳送三部曲HandshakingTransfer of messagesclosure所有信息資訊必須是7-bit ASCIICh.2 Application-

33、Layer44(C) All rights reserved by Professor Wen-Tsuen ChenSMTP和HTTP的比較相同處使用TCP為傳輸層協(xié)定Client and server model使用persistent connections不同處http是pull protocol而SMTP是push protocolSMTP的message限定在7 bit ASCII文件組成方式不同Ch.2 Application-Layer45(C) All rights reserved by Professor Wen-Tsuen ChenSMTP溝通範(fàn)例 S: 220 cs.n

34、.tw C: HELO .tw S: 250 Hello .tw, pleased to meet you C: MAIL FROM: S: 250 alice.tw. Sender ok C: RCPT TO: S: 250 .tw . Recipient ok C: DATA S: 354 Enter mail, end with . on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery

35、C: QUIT S: 221 .tw closing connectionCh.2 Application-Layer46(C) All rights reserved by Professor Wen-Tsuen Chen信息格式RFC 822為text message exchange標(biāo)準(zhǔn)Header linesTo:From:Subject:BodyThe message, ASCII character onlyheaderbodyblanklineCh.2 Application-Layer47(C) All rights reserved by Profess

36、or Wen-Tsuen Chen信息格式 (cont.)Ch.2 Application-Layer48(C) All rights reserved by Professor Wen-Tsuen Chen信息格式 (cont.)Ch.2 Application-Layer49(C) All rights reserved by Professor Wen-Tsuen ChenMIME extension for Non-ASCII dataMIME: multimedia mail extensionRFC 2045, 2056目的：為了要能夠支援非ASCII的data在標(biāo)頭檔定義MIME

37、的格式From: alicecrepes.fr To: bob Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data 多媒體資料的種類及副檔名(type, subtype)壓縮資料的方法(encode data)MIME version(版本)壓縮的資料(encoded data)Ch.2 Application-Layer50(C) All righ

38、ts reserved by Professor Wen-Tsuen ChenMIME (cont.)RFC 822 MIME所增加的標(biāo)頭檔(headers)HeaderMeaningMIME-version定義MIME的版本Content-Description描述信息內(nèi)容Content-ID唯一的識別碼Content-Transfer-Encoding在傳送所壓縮的方法Content-Type信息的種類Ch.2 Application-Layer51(C) All rights reserved by Professor Wen-Tsuen ChenCh.2 Application-Lay

39、er52(C) All rights reserved by Professor Wen-Tsuen ChenCh.2 Application-Layer53(C) All rights reserved by Professor Wen-Tsuen ChenMIME 的種類(types)Content-Type: 種類/副檔名; 參數(shù) (type/subtype; parameters)文字(Text)例如：副檔名為 plain, html圖片(Image)例如：副檔名為 jpeg, gif聲音(Audio)例如：副檔名為 basic (8-bit mu-law encoded), 32ka

40、dpcm (32 kbps coding)影像(Video)例如：副檔名為 mpeg, quicktime應(yīng)用程式(Application)other data that must be processed by reader before “viewable”例如：副檔名為 msword, octet-stream Ch.2 Application-Layer54(C) All rights reserved by Professor Wen-Tsuen Chen郵件存取協(xié)定(mail access protocol)SMTP負(fù)責(zé)接收端server的傳送及儲存信息Mail access pr

41、otocol:從server取回信件POP: Post Office ProtocolRFC 1939提供認(rèn)證及下載服務(wù)IMAPRFC 2060可以遠(yuǎn)端操作信件，不需下載信件至機(jī)器上HTTP (web mail)Hot mail, mail2000, yahooCh.2 Application-Layer55(C) All rights reserved by Professor Wen-Tsuen ChenPOP3協(xié)定authorization phaseclient commands: user: declare usernamepass: passwordserver responses

42、+OK-ERRtransaction phase, client:list: list message numbersretr: retrieve message by numberdele: deletequit C: list S: 1 498 S: 2 912 S: . C: retr 1 S: S: . C: dele 1 C: retr 2 S: S: . C: dele 2 C: quit S: +OK POP3 server signing offS: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK

43、 user successfully logged onCh.2 Application-Layer56(C) All rights reserved by Professor Wen-Tsuen ChenEmail Security (電子郵件的安全性)PGP: Pretty Good Privacy, by Phil Zimmermaun in 1995.Support text compression, secrecy and digital signatures.Ch.2 Application-Layer57(C) All rights reserved by Professor W

44、en-Tsuen ChenPGP message format(訊息格式)Ch.2 Application-Layer58(C) All rights reserved by Professor Wen-Tsuen ChenPEM: Privacy Enhanced MailAn official Internet standard described in RFC 1421-1424.Support privacy and authentication for RFC 822 based email systems.The message together with its message

45、digest is encrypted using DES with a one-time key that is enclosed along with the message.The key can be protected with RSA and certified by certification authorities.Ch.2 Application-Layer59(C) All rights reserved by Professor Wen-Tsuen ChenPGP和PEM的比較Ch.2 Application-Layer60(C) All rights reserved

46、by Professor Wen-Tsuen ChenDNS (Domain Name System)- Overview名稱我們可以拿來做什麼？確認(rèn)、識別一個物件(object)幫助我們放置(allocate)一個物件定義在一個團(tuán)體中的關(guān)係 明確定義一個角色(role)傳送一個機(jī)密訊息 Name space(名稱空間)定義一組可用的名稱 由一組名稱與數(shù)值的配對所組成 Ch.2 Application-Layer61(C) All rights reserved by Professor Wen-Tsuen ChenDNS (Domain Name System) (cont.)為什麼我們要使

47、用DNS？可以幫助我們對應(yīng)IP位址和主機(jī)名字(hostname)無須記憶IP位址，只需記憶名字DNS是一個分散式的資料庫將許多name servers變成階層式架構(gòu)DNS是屬於應(yīng)用層的協(xié)定Host, routers, name server透過溝通來達(dá)成解析名稱的目的(address/name translation)Ch.2 Application-Layer62(C) All rights reserved by Professor Wen-Tsuen ChenDNS (Domain Name System) (cont.)HierarchyNameCh.2 Application-Lay

48、er63(C) All rights reserved by Professor Wen-Tsuen ChenDNS (Domain Name System)- ExampleHosts 77 80:23:A8:33:5B:9F Files/usr/llp/tmp/foo (server, fileid)Users Larry Peterson llp Ch.2 Application-Layer64(C) All rights reserved by Professor Wen-Tsuen ChenDNS (Domain Name System)- Example (cont.)Mailbo

49、xesNameserverMailprogramUserTCPIP23user 145Ch.2 Application-Layer65(C) All rights reserved by Professor Wen-Tsuen ChenDNS name server為什麼我們不採取集中式DNS？如果單一DNS server壞了，會影響很大Traffic volume (一臺server不可能處理全部的位址解析)Server所放置的位置過遠(yuǎn)維護(hù)不易DNS server可分為當(dāng)?shù)孛Q伺服器(Local name server)根名稱伺服器(Root name server)認(rèn)證名稱伺服器(Auth

50、oritative name server)Ch.2 Application-Layer66(C) All rights reserved by Professor Wen-Tsuen ChenDNS name serverLocal name server負(fù)責(zé)當(dāng)?shù)?local)名稱解析，如解析不成，往上詢問root serverRoot name server負(fù)責(zé)解析當(dāng)?shù)谼NS無法解析之nameAuthoritative name serverHost都會來此主機(jī)註冊當(dāng)root server亦無法解析名稱時，就會詢問authoritative name serverCh.2 Applicat

51、ion-Layer67(C) All rights reserved by Professor Wen-Tsuen ChenFQDN (Fully Qualify Domain Name) 一個將名稱與位址對應(yīng)的機(jī)制為的避免名稱重複做流量控制一個DNS name = 一個FQDN一個FQDN名稱可以對應(yīng)到多個IP名稱解析的流程：Interactive Query以下為範(fàn)例：Search Ch.2 Application-Layer68(C) All rights reserved by Professor Wen-Tsuen Chen名稱解析範(fàn)例Ch.2 Application-Layer69

52、(C) All rights reserved by Professor Wen-Tsuen Chen名稱解析範(fàn)例(cont.)Step1：web. nthu.Step2：nthu. Root（所以Root需要很多臺）（ Q：com.的IP）（ com. DNS的IP）Step3：nthu. com.（ Q：.的IP）（ . DNS的IP）Step4：nthu. microsoft.（ Q：.的IP）（ . DNS的IP） 取得名稱解析的IP，此IP會存放在nthu的catch中，存放TTL的時間Step5：nthu. web. Ch.2 Application-Layer70(C) All

53、rights reserved by Professor Wen-Tsuen ChenZONEZONE：是一個特殊單位。一個ZONE一定要有一個DNS來管，但是一個DNS可以管理許多的ZONE，而每一個ZONE都有一個ZONE FileZONE FileSOA（Start-of-Authority）Version Number（判斷新舊，但clock要相同）Refresh timer（一個ZONE有兩個DNS Server, Secondary Server每隔一段時間去跟Master Server複製）Entry timerExpire timeTTL（問到機(jī)器的IP後可存放在catch中的時間。if TTL=0，多用於Proxy, DHCP）Ch.2 Application-Layer71(C) All rights reserved by Professor Wen-Tsuen ChenZONE (cont.)Administrator mailDomain nameNS（Name Server）MX（Mail Exchange Server）A（Host）：ZONE有多少臺機(jī)器，需要Dynamic updateCN（alias）：多IP對應(yīng)一臺主機(jī)PTR（Pointer）：有IP查名