The Application Layer

1. The Application Layer • application and application requirements • sample network applications and protocols • email: SMTP, POP3 • WWW: http1.1 • teleconferencing • reading: Tannenbaum 7.4, 7.6, 7.7 Ross, Kurose 2.1 - 2.5, 2.9

2. Network Applications our goal: understand • service requirements applications place on network infrastructure • protocols distributed applications use to implement application two views: • applications drive technology • build it and they will come

3. Application Requirements • bandwidth: how many bits/sec required? Smooth or bursty traffic stream? • protocol processing: how many MIPs required for application software and underlying protocols? • holding time: how long does application run? • data-level reliability: reliable (in-order, no loss) delivery needed? • performance:constraints on maximum application-to-application delay, tail of delay distribution? • QOS: quality of service guarantees required? • communication structure: 1-1, 1-many, many-many? • security: authentication, encryption required?

4. Application Requirements

5. Application Structure network application distributed in nature • set of communicating application-level processes (usually) on different hosts provide/implement

6. Application Structure client server model: • server offers service via well defined interface, client request service • client: how to locate/request service • server how/whether to provide service • example: WWW client (browser), server peer/peer model: • symmetric: each process an equal • example: teleconferencing both require transport of request/replies, sharing of data

7. Application Example: email • generic issues: • addressing: identities of receiver/sender, format of address info • privacy/security • notification of receipt, reading, disposition • different types of media: text, audio, video, document types • competing email standards that don’t interoperate • mail gateways convert from one format to another

8. Internet Mail: SMTP SMTP: simple mail transfer protocol • RFC 821, RFC 822 define protocol activities and message structure for SMTP • RFC: request for comments are Internet draft, standards, information documents. • locations: ftp://nic.ddn.mil, http://www.cis.ohio-state.edu/hypertext/information/rfc.html • uses underlying reliable transport service (TCP)

9. SMTP Sending mail: • essentially a SMTP-mediated file transfer: sender SMTP protocol entity sends to receiver SMTP protocol entity • sender learns receivers host address (how?) • sender contacts receiver at well known “port” number (25) at that host

10. SMTP sender-receiver commands SMTP sender and receiver exchange typical of peer-peer interactions: • control commands, replies, data • three basic steps: “greeting”, data exchange, “goodbye”

11. SMTP sender-receiver commands

12. Example SMTP Exchange

13. Another Example gaia 3% telnet gaia.cs.umass.edu 25 Trying 128.119.40.186… Connected to gaia. Escape character is '^]'. 220 gaia.cs.umass.edu ESMTP Sendmail 8.7.6/8.6.9 ready at Wed, 11 Feb 1998 21:13:58 -0500 (EST) HELO abc.com 250 gaia.cs.umass.edu Hello gaia [128.119.40.186], pleased to meet you MAIL FROM:<towsley@abc.com> 250 <towsley@abc.com>... Sender ok RCPT TO: <towsley@gaia.cs.umass.edu 553 <towsley@gaia.cs.umass.edu... Unbalanced '<’

14. Another Example (cont.) RCPT TO: <towsley@gaia.cs.umass.edu> 250 Recipient ok DATA 354 Enter mail, end with "." on a line by itself From:towsley@abc.com … Don . 250 VAA04032 Message accepted for delivery QUIT 221 gaia.cs.umass.edu closing connection

15. SMTP: Closing Comments • extensions to SMTP: • MIME (multipurpose Internet mail extensions): multiple body parts, multimedia mail, multiple fonts and character sets, RFC 1324 • PEM (privacy-enhanced mail): RFC 1422-1424

16. Post Office Protocol (POP3) • SMTP assumes there is an SMTP server at recipient • SMTP can also deliver to a Post Office (server) • client can retrieve mail remotely (e.g., over net) using Post Office Protocol (POP3) to interact with server. Three phases: • greeting • transactions (retrieve mail) • quit • mail stored at client • Interactive Mail Access Protocol (IMAP) • similar to POP3 except mail stored at server

17. Hypertext Transfer Protocol: http • WWW implemented using client/server paradigm: • client (browser) requests and displays received html documents • server receives requests, responds with requested html documents • http protocol defines format of packets exchanged between client and server, actions taken upon receipt • http is a stateless, transaction-oriented protocol: • client contacts server at port 80, using TCP • client makes request to server • reply from server to client • TCP connection closed

18. Http (cont.) • request/replies have headers and bodies similar to SMTP and MIME • RFC 2068 (long!) defines http

19. HTTP Request Message • sent from client to server • general format: • request line (method, identifier, version) • header (additional info) • body

20. Example http Request Message Suppose you access http://gaia.cs.umass.edu/cs653-1998/index.html: sample GET command plus header: GET /cs653-1998/index.html HTTP/1.0 User-Agent: Mozilla/2.01 (X11; I; IRIX 5.2 IP7) Accept: image/gif, image/x-bitmap, image/jpeg /* a blank line */

21. http Reply Message • sent from server to client • general format: • status line (status code, text phrase) • header (additional info) • body

22. Example http reply Message a sample http server reply: HTTP/1.0 200 Document follows MIME-Version: 1.0 Server: CERN/3.0 Date: Wednesday 10-Apr-96 03:59:47 GMT Content-type: text/html Content-length: 2168 Last-Modified: Friday 06-Oct-95 07:16:52 GMT /* a blank line */ <html> <head> : /* HTML text of the Web page */ </html>

23. Teleconferencing applications: requirements goal: interactive (real time) communication with multiple media components (audio, video, text (whiteboard). continuous media such as audio, video • require “smooth” playout at receiver • Internet does not provide jitter-free communication • jitter: change in network delay • receiver buffers packets and “plays out” periodically • delayed playout of first packet allows receiver buffer to “absorb” network jitter • late packets lost • can tolerate “some” lost packets

24. Audio Playout audio samples time Q:what kind of service, best effort or guaranteed with resource reservation?

25. Applications: Summary • diverse applications place widely varying service requirements on network bandwidth, reliability, real-time constraint, QoS guarantees, security • application-level protocols between distributed entities implement application using network services via network API • client-server paradigm • applications rule!