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Chapter 2: Application Layer

Our goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm. learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS

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Chapter 2: Application Layer

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  1. Our goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS programming network applications socket API Chapter 2: Application Layer 2: Application Layer

  2. Application: communicating, distributed processes e.g., e-mail, Web, P2P file sharing, instant messaging running in end systems (hosts) exchange messages to implement application Application-layer protocols one “piece” of an app define messages exchanged by apps and actions taken use communication services provided by lower layer protocols (TCP, UDP) application transport network data link physical application transport network data link physical application transport network data link physical Applications and application-layer protocols 2: Application Layer

  3. Process: program running within a host. within same host, two processes communicate using interprocess communication (IPC). processes running in different hosts communicate with an application-layer protocol user agent: software process, interfacing with user “above” and network “below”. implements application-level protocol Web: browser E-mail: mail reader streaming audio/video: media player Network applications: some jargon 2: Application Layer

  4. Typical network app has two pieces: client and server request reply application transport network data link physical application transport network data link physical Client-server paradigm Client: • initiates contact with server (“speaks first”) • typically requests service from server, • Web: client implemented in browser; e-mail: in mail reader Server: • provides requested service to client • e.g., Web server sends requested Web page, mail server delivers e-mail 2: Application Layer

  5. API: Application Programming Interface defines interface between application and transport layers socket: Internet API two processes communicate by sending data into socket, reading data out of socket Q: how does a process “identify” the other process with which it wants to communicate? IP address of host running other process “port number” - allows receiving host to determine to which local process the message should be delivered Processes communicating across network … lots more on this later. 2: Application Layer

  6. 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” What transport service does an app need? Bandwidth • some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” • other apps (“elastic apps”) make use of whatever bandwidth they get 2: Application Layer

  7. Transport service requirements of common apps Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games instant messaging Bandwidth elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss 2: Application Layer

  8. TCP 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 providing: timing, minimum bandwidth guarantees UDP service: connectionless: no connection setup between sending and receiving process does not provide: reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? Internet transport protocols services 2: Application Layer

  9. Internet apps: application, transport protocols Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad) Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony 2: Application Layer

  10. www.someschool.edu/someDept/pic.gif path name host name Web and HTTP First 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 (universal resource locator) • Example URL: 2: Application Layer

  11. HTTP: 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 HTTP overview HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator 2: Application Layer

  12. 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 closed HTTP is “stateless” server maintains no information about past client requests HTTP overview (continued) aside Protocols that maintain “state” are complex! • past history (state) must be maintained • if server/client crashes, their views of “state” may be inconsistent, must be reconciled 2: Application Layer

  13. Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 HTTP Example (contains text, references to 10 jpeg images) 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time 2: Application Layer

  14. 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects HTTP Example (cont.) 4. HTTP server closes TCP connection. time 6.Steps 1-5 repeated for each of 10 jpeg objects 2: Application Layer

  15. Non-persistent HTTP/1.0: server parses request, responds, closes TCP connection 2 RTTs to fetch object TCP connection object request/transfer each transfer suffers from TCP’s initially slow sending rate many browsers open multiple parallel connections Persistent default for HTTP/1.1 on same TCP connection: server, parses request, responds, parses new request, ... client sends requests for all referenced objects as soon as it receives base HTML. fewer RTTs, less slow start. Non-persistent, persistent connections 2: Application Layer

  16. initiate TCP connection RTT request file time to transmit file RTT file received time time Response time modeling Definition of RRT: time to send a small packet to travel from client to server and back. Response time: • one RTT to initiate TCP connection • one RTT for HTTP request and first few bytes of HTTP response to return • file transmission time total = 2RTT+transmit time 2: Application Layer

  17. HTTP request message • two types of HTTP messages: request, response • HTTP request message: • ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message 2: Application Layer

  18. HTTP request message: general format 2: Application Layer

  19. HTTP response message status line (protocol status code status phrase) HTTP/1.1 200 OK Connection close 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: text/html data data data data data ... header lines data, e.g., requested HTML file 2: Application Layer

  20. 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported HTTP response status codes In first line in server->client response message. A few sample codes: 2: Application Layer

  21. 1. Telnet to your favorite Web server: Trying out HTTP (client side) for yourself Opens TCP connection to port 80 (default HTTP server port) at www.eurecom.fr. Anything typed in sent to port 80 at www.eurecom.fr telnet www.eurecom.fr 80 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET /~ross/index.html HTTP/1.0 3. Look at response message sent by HTTP server! 2: Application Layer

  22. Authorization : control access to server content authorization credentials: typically name, password stateless: client must present authorization in each request authorization: header line in each request if no authorization: header, server refuses access, sends WWW authenticate: header line in response usual http request msg + Authorization: <cred> usual http request msg + Authorization: <cred> usual http response msg usual http response msg time User-server interaction: authorization server client usual http request msg 401: authorization req. WWW authenticate: 2: Application Layer

  23. server-generated # , server-remembered #, client-stored #, server sends “cookie” to client in response msg Set-cookie: 1678453 client presents cookie in later requests cookie: 1678453 usual http request msg cookie: # usual http request msg cookie: # usual http response msg usual http response msg time Cookies: keeping “state” client server usual http request msg usual http response + Set-cookie: # cookie- spectific action 2 weeks later cookie- spectific action 2: Application Layer

  24. What cookies can bring: authorization user preferences shopping carts recommendations user session state (Web e-mail) Cookies (continued) aside Cookies and privacy: • cookies permit sites to learn a lot about you • you may supply name and e-mail to sites • search engines use redirection & cookies to learn yet more • advertising companies obtain info across sites 2: Application Layer

  25. Goal: don’t send object if client has up-to-date cached version client: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP response HTTP/1.0 304 Not Modified Conditional GET: client-side caching server client HTTP request msg If-modified-since: <date> object not modified HTTP request msg If-modified-since: <date> object modified HTTP response HTTP/1.0 200 OK <data> 2: Application Layer

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