计算机网络英文版课件：ch7-application

1、7 Application LayerApplication LayerIntroduction Domain Name System (DNS) Hierarchical Namespace & Resource Records Name servers & Name Resolution File Transfer Protocol (FTP) Email System and Protocols System Components Message Format SMTP,POP3,IMAP World Wide Web WWW Architecture Hypertext Transfe

2、r Protocol (HTTP) Homework Introduction Duties and Features of Application LayerProviding various applications to end usersMaximum number of protocols and most complexEach protocol is specified to a type of application, no general-purpose protocolsConventional Internet Application Protocols Client-s

3、erver ParadigmThe tasks, capabilities and protocol machines are different at each side.Client-side requests service, Server-side provides service.Forms the basis of computer networksPeer-to-peer ParadigmThe protocol machines are the same at both sidesThe same state-flow diagramThe same set of tasks

4、and responsibilitiesThe same set of capabilitiesBitTorrent, eMule, DHT, KADDomain Name System (DNS)Hierarchical Namespace &Resource RecordsDNS: Domain Name SystemOut-dated “hosts.txt”A client-server application that identifies each host on the Internet with a unique user-friendly name(domain name),

5、e.g. instead of 30Providing support for other applicationsTranslating domain name (host name) to IP address C function: gethostbyname()Windows XP commands: nslookup, ipconfig/displaydns, ipconfig/flushdnsFeaturesUDP datagram, DNS ServerHierarchical namespace: Host have a composite names that are all

6、 hierarchically organizedDistributed databaseNaming follows organizational boundaries, not physical networks Hierarchical NamespaceWithin an organizationSubdivision possibleArbitrary levels possible (maximum depth =128) Not standardized, controlled locally by the organizationNaming policy by the pat

7、h upward from leaf to root, separated by “.” comedugovintmilnetorgaecnzwmitcsxxcsedubuptGeneric domainsCountry domainsUnnamed rootTop level domains2nd level domainsZonesA zone corresponds to an administrative authority that is responsible for that portion of the hierarchy e.g: bupt controls *.Resour

8、ce Records (RR)Each domain in the DNS has one or more Resource Records (RRs)Each RR has the following informationOwner: the domain nameType: specifies the type of the resource in this RRSOA Start Of Authority, identifies the zone A Host Address MX Mail ExchangerCNAME Canonical Name, aliasHINFO Host

9、InformationClass: specifies the protocol family to useIN the Internet systemTTL: specifies the Time To Live (in unit of second) of the cached RRsSample of DNS DatabaseType Value Domain nameTime-to-liveClass Name servers &Name Resolution DNS Client-Server InteractionClient: resolversoftware running o

10、n client, access at least one name serverMultiple DNS servers usedArranged in hierarchyEach server has authority over a portion of the hierarchyEach server contains all the records for the hosts in its zoneHow does a server know about other servers that are responsible for the other zones?Every serv

11、er knows the rootRoot server knows about all top-level domainsEvery server knows the servers further down the hierarchyHierarchy of Name ServersPrimary and Secondary ServersDNS uses backup serversPrimary server: creates, maintains, and updates information about its zoneSecondary server: gets its inf

12、ormation from a primary server; as a backup server in case of failureBoth servers have authority over their zoneISPs Offering DNS service to subscribersSmall organizations and individualsOnly need domain names for computers running serversContract with an ISP for domain serviceDNS LookupApplications

13、Becoming DNS clientSending request first to local name serverLocal name serverListening at port 53, normally using UDPIf answer known, returns responseIf answer unknownRequests top-level name serverFollows linksUntil reaching Authoritative Name Server, which performs name translation and returns res

14、ponse Name Resolution MethodsRecursive resolutionA query is made to a local name serverIf the queried server does not have the information, it must make a query to anotherIterative resolutionAn iterative query is made to a name server, which may then respond with the address of another serverthe loc

15、al name server (on behalf of resolver) then queries that server (which might respond with the address of another server, and so on)Commonly used by name servers on InternetExample of Recursive ResolutionExample of Iterative ResolutionDNS CachingServer always caches answersHost can cache answersWorks

16、 well because high locality of referenceTTLFile Transfer Protocol (FTP)FTPFile Transfer Protocol (RFC959)Used to transfer files between hostsUsed to manipulate remote filesAuthList directoriesDelete filesRename filesUsing client/server modelUsing TCP for reliable transfersFTP Model (1)File SystemSer

17、ver PIServer DTPClient PIClient DTPUser InterfaceFile SystemUserFTP serverFTP clientControl ConnectionData ConnectionFTP Model (2)PI (Protocol Interpreter): Implements FTP protocol, interpreting FTP commands and replies. DTP (Data Transfer Process): Establishes and manages the data connection. Contr

18、ol Connection: The communication path between the client-PI and server-PI for the exchange of FTP commands and repliesData Connection: A full duplex connection over which data is transferredThe data transferred may be a part of a file, an entire fileFTP Model (3)FTP ClientUsers interact with FTP Cli

19、ent directlyActive open of control connectionControl connection uses ASCII plain-textSends commands (over control connection)Receives replies (over control connection)Data connection used to transfer file dataFTP ServerSystem process“Listens” for connection on well-known port 21Receives commandsSend

20、s repliesFTP Control Connection & Data ConnectionControl connectionRemain alive as long as the user keeps FTP session activePassing commands and repliesUsed to coordinate the ports used for data connection and establish data connectionServer well-known port: 21Data connectionBe created dynamically w

21、hen needed One data connection persists for one file transferUsed for data transmissionServer well-known port: 20FTP connections: Example of Uploading FileRFC959标准FTP的其他特性Traditional FTP vs. Anonymous FTP主动与被动（从服务器角度PORT/PASSIVE）主动：服务器向客户端发起TCP数据连接被动：服务器等待客户端建立TCP数据连接会话实例第三方传输：FXP断点续传FTP的缺陷传输中止，存在半个

22、文件未能传输文件属性（最后一次修改时间，文件权限等）Email System and Protocols System ComponentsWhat is Email?Electronic Mail (email, e-mail)Provides a means to send electronic messages from one person to another asynchronouslyOne of the most popular applications on the Internet and one of the most important communication me

23、thods todayEmail service can be provided byISPs: , , , , Corporations and institutes: , , , Bundled with other services: , , Components Of Email SystemInternetUser AgentMail ServerMail ServerUser AgentSMTPSMTPPOP3SenderReceiverMail Server of Sender SideMail Server of Receiver SideSpoolingUser mailbo

24、xSMTPSMTPPOP3Components Of Email SystemUA (User Agent)end-user mail programInterface between the end users and the email servers E.g., outlook, foxmail, Mail ServerResponsible for transmitting/receiving emails and reporting status information about mail transferring to the mail senderBoth a client a

25、nd a serverEmail protocolsSMTP: used for sending an emailPOP3: used for receiving an emailBasic Functions of Email SystemComposition - creating messages and answersTransfer - moving messages from the originator to the recipientReporting - informing the originator what happened to the messageDisplayi

26、ng - showing the messagesMessage Format (RFC822,2822)Internet Message Format (RFC822/2822)Message (ASCII)Messages consist of (1)a primitive envelope(2) some number of header fields, (3) a blank line(4) and then the message bodyRFC 822 does not clearly distinguish the envelope fields from the header

27、fields Each header field consists of a single line of ASCII text:(1)field name(2)a colon(3)for most fields, a value. 40Message Field DefinitionsCategoriesHeaderMeaningOriginator fieldsFrom:Person or people who created the messageSender:Email address of the actual senderReply-to:Email address to whic

28、h replies should be sentDestination address fieldsTo:Email address(es) of primary recipient(s)Cc:Email address(es) of secondary recipient(s)Bcc:Email address(es) for blind carbon copiesThe origination date fieldDate:The date and time the message was sentIdentification fieldsMessage-Id:Unique number

29、for referencing this message laterReferences:Other relevant Message-IdsInformation fieldsSubject:Short summary of the message for the one-line displayKeywords:User chosen keywordTrace fieldsReceived:Line added by each transfer agent along the routeReturn-Path:Can be used to identify a path back to t

30、he senderExample of Mail MessageThe header is everything up to the blank line, and the body is everything after the blank lineFrom: John Doe Sender: Michael Jones To: Mary Smith Subject: Saying HelloDate: Fri, 21 Nov 1997 09:55:06 -0600Message-ID: This is a message just to say hello.So, Hello. Autho

31、r of the message, requiredMailbox responsible for the actual transmission of the message, optional Address(es) of the primary recipient(s) of the messageheadersbody Problem of RFC 822: Only 7 bit ASCII format can be useda blank linePostal Mail vs. EmailForouzanMIMEMultipurpose Internet Mail Extensio

32、nExtension for multipart & multimedia emailAdditional mail headers define content Type: text, image, audio, video, application and subtype within (eg text/html, image/gif)Encoding (ASCII , quoted printable, base64) to handle arbitrary binary data when email system can only handle normal ASCII charsS

33、upports multipart message content type each part has its own type and encodingMIME messages can be sent using the existing mail programs and protocolsWidely used nowMIME: New HeadersMIME: Content Types and Subtypes46MIME: Message Example (1)Date: Sat, 07 Dec 2002 16:37:32 +0800From: Adun Gaos X-Acce

34、pt-Language: zh-cnMIME-Version: 1.0To: adungaosSubject: MIME message!Content-Type: multipart/mixed; boundary=080202030206040206090704This is a multi-part message in MIME format.080202030206040206090704Content-Type: text/html; charset=us-asciiContent-Transfer-Encoding: 7bit This is a MIME message. He

35、re is body.080202030206040206090704Content-Type: application/x-gtar; name=binary.tgzContent-Transfer-Encoding: base64Content-Disposition: inline; filename=binary.tgzH4sIABmy8T0AA+3OsQ3CMBQEUI/iEb6dBM9jhIRogpSQgu1BQhQUiCpU7zVX3BV3vMx9uaddRYk4RKSIKG36yLcUbWrjMJQo9bmv41BTjn1vvWzrrS85p37a5nO/rt92v3oAAAA

36、AAAAAAAD4oweF/KCgACgAAA=080202030206040206090704-47MIME: Message Example (2)When you save the above as .eml file and open it with outlook, you can see:Example of Base64Base64: a specific MIME content transfer encoding. 6 bit-data is converted into an 8-bit ASCII characterABCDEFGHIJKLMNOPQRSTUVWXYZab

37、cdefghijklmnopqrstuvwxyz0123456789+- (pad/=)SMTP,POP3,IMAP(RFC821,1939,1730)SMTP (RFC821)Simple Mail Transfer ProtocolSMTP is a simple ASCII protocolSMTP server listens to port 25 SMTP client establishes a TCP connection to portUsed in transferring mail messages from UA to mail server, and between m

38、ail servers SMTP Basic ModelMTAMTASMTP clientSMTP serverFile SystemFile SystemUserSMTP commands/replies and mail MTA: Mail Transfer Agent, the mail server using SMTPMessage TransferTransferring a message from elinore to carolyn.POP: Basic ModelTransfer mail from mail server to UASMTP/POP Mail Server

39、User AgentFile SystemInternetPOPSMTPSMTP Server: always “on and listening” via TCP port 25 holds incoming mail for userPOP Mail Access Server: runs the POP3 service by listening on TCP port 110Clients: connect to server to manipulate mail using POP3POP: Features Essentially store Mail is stored on t

40、he server until the client connects and then is downloaded to the client. You MAY be able to leave a copy on the serverSimple protocol and widely usedSimilar to SMTP command/reply lockstep protocolUsed to retrieve mail for a single user, requires authenticationMany clients available such as Eudora,

41、foxmail, outlookCommon used version: RFC1939: POP3 (POP Version 3)For mobile users or users that use multiple machines during the daySolution: IMAP (Internet Message Access Protocol)Using POP3 to Fetch 3 MessagesIMAP: Internet Message Access Protocol For mobile users or users that use multiple machi

42、nes during the dayIMAP (Internet Message Access Protocol) RFC1730: IMAP4POP3 vs IMAPWorld Wide Web (WWW) WWW ArchitectureWWW: World Wide WebA distributed hyper-media system, including multiple media information such as text, graph, image, audio, even videoInformation is distributed in Internet, stor

43、ed in web servers and accessed by hyperlinksClient/Server mode, with browser program as clientClient and server interact with HTTP: Hyper-Text Transfer ProtocolWWW documents are located with URL: Uniform Resource LocatorWWW documents are written in HTML: HyperText Markup LanguageSearching engines fa

44、cilitate user to find information neededArchitectural Overview of WWWClient Side: BrowserAn application program, as users interface to WebFetching information (web page) from Web serverDisplaying information for usersAfter the user clicks on a hyperlink, BrowserDetermines URL, name of the linked web

45、 page, eg. /example.htmlAsks DNS for the IP address of Establishes a TCP connection to replied IP addressAsks a request of web page example.html (HTTP)Releases TCP connection upon receiving the requested web pageDisplays all the texts in example.html (HTML)Fetches and displays all images in the file

46、The Client Side(a) A browser plug-in. (b) A helper application.Web ServerStores set of Web documentsResponds to request from browser by sending copy of documentSteps of servers loopAccepts a TCP connection from a browserResolves name of the web page requiredFetches the requested page from diskReturn

47、s the reply to the clientReleases the TCP connectionWeb DocumentsAlso called a web page (HTML)TypesStatic web pageStored in fileUnchangingDynamic web pageCan change whenever a server receives a request Generated on server sideOutput from a program on server, usually a scriptActive web pageCan change

48、 after the document has been loaded into a browserGenerated on client sideConsists of a computer program, known as Java, running in browser (applet)HTML HyperText Markup Language(a) The HTML for a sample Web page. (b) The formatted page.(b)Static Web PageDynamic Web PageActive Web PageClient-Side Dynamic Web Page GenerationT