Day 05: Application layer – HTTP, FTP, Email, and DNSSlide 2World Wide Web (WWW)Processes communicating across networkAddressing processes:What transport service does an app need?Internet transport protocols servicesInternet apps: application, transport protocolsWeb and HTTPHTTP overviewHTTP overview (continued)HTTP connectionsNonpersistent HTTPNonpersistent HTTP (cont.)Response time modelingPersistent HTTPHTTP request messageHTTP request message: general formatUploading form inputMethod typesHTTP response messageHTTP response status codesTrying out HTTP (client side) for yourselfFTP: the File Transfer ProtocolFTP: separate control, data connectionsSample commandsWhy two connections?FTP, SFTPElectronic MailElectronic Mail: mail serversScenario: Alice sends message to BobElectronic Mail: SMTP [RFC 2821]Sample SMTP interactionSMTP: final wordsMail message formatSlide 38Slide 39Message format: multimedia extensionsMIME types Content-Type: type/subtype; parametersMultipart TypeMail access protocolsPOP3 protocolPOP3 (more) and IMAPWeb-based E-mailDNS: Domain Name SystemDNS name serversSlide 49DNS: Root name serversSimple DNS exampleDNS exampleDNS: iterated queriesDNS: caching and updating recordsDNS records2: Application Layer 1Day 05: Application layer – HTTP, FTP, Email, and DNS2: Application Layer 2Chapter 2Application LayerComputer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004. A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights Reserved2: Application Layer 3World Wide Web (WWW)2: Application Layer 4Processes communicating across networkprocess sends/receives messages to/from its socketsocket analogous to doorsending process shoves message out doorsending process assumes transport infrastructure on other side of door will deliver message to socket at receiving processprocessTCP withbuffers,variablessockethost orserverprocessTCP withbuffers,variablessockethost orserverInternetcontrolledby OScontrolled byapp developer2: Application Layer5Addressing processes:For a process to receive messages, it must have an identifierEvery host has a unique 32-bit IP addressQ: Does the IP address of the host on which the process runs suffice for identifying the process?Answer: No, many processes can be running on same hostIdentifier includes both the IP address and port numbers associated with the process on the host.Example port numbers:HTTP server: 80Mail server: 25More on this later2: Application Layer 6What transport service does an app need?Data losssome apps (e.g., audio) can tolerate some lossother apps (e.g., file transfer, telnet) require 100% reliable data transfer Timingsome apps (e.g., Internet telephony, interactive games) require low delay to be “effective”Bandwidthsome apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”other apps (“elastic apps”) make use of whatever bandwidth they get2: Application Layer 7Internet transport protocols servicesTCP service:connection-oriented: setup required between client and server processesreliable transport between sending and receiving processflow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloadeddoes not provide: timing, minimum bandwidth guaranteesUDP service:unreliable data transfer between sending and receiving processdoes not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee No setup required.2: Application Layer 8Internet apps: application, transport protocolsApplicatione-mailremote terminal accessWeb file transferstreaming multimediaInternet telephonyApplicationlayer protocolSMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]proprietary(e.g. Real Networks)proprietary(e.g., Net2Phone)Underlyingtransport protocolTCPTCPTCPTCPTCP or UDPtypically UDP2: Application Layer 9Web and HTTPFirst some jargonWeb page consists of objectsObject can be HTML file, JPEG image, Java applet, audio file,…Web page consists of base HTML-file which includes several referenced objectsEach object is addressable by a URLExample URL:www.someschool.edu/someDept/pic.gifhost namepath name2: Application Layer 10HTTP overviewHTTP: HyperText Transfer ProtocolWeb’s application layer protocolclient/server modelclient: browser that requests, receives, “displays” Web objectsserver: Web server sends objects in response to requestsHTTP 1.0: RFC 1945HTTP 1.1: RFC 2068*NetCraft's Web Server SurveyPC runningExplorerServer runningApache Webserver*Mac runningNavigatorHTTP requestHTTP requestHTTP responseHTTP response2: Application Layer 11HTTP overview (continued)Uses TCP:client initiates TCP connection (creates socket) to server, port 80server accepts TCP connection from clientHTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)TCP connection closedHTTP is “stateless”server maintains no information about past client requestsProtocols that maintain “state” are complex!past history (state) must be maintainedif server/client crashes, their views of “state” may be inconsistent, must be reconciledaside2: Application Layer 12HTTP connectionsNonpersistent HTTPAt most one object is sent over a TCP connection.HTTP/1.0 uses nonpersistent HTTPPersistent HTTPMultiple objects can be sent over single TCP connection between client and server.HTTP/1.1 uses persistent connections in default mode2: Application Layer 13Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index1a. HTTP client