The Application Layer  application and application requirements  sample network applications and protocols  SMTP, POP3  WWW: http1.1  teleconferencing  reading: text sections 7.4, 7.6, 7.7 Copyright All Rights Reserved, J. Kurose, D. Towsley

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

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?

Application Requirements

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

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

Application Example:  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 standards that don’t interoperate  mail gateways convert from one format to another

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

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

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”

SMTP sender-receiver commands

Example SMTP Exchange

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

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

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

 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

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

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

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

Example http Request Message Suppose you access sample GET command plus header: GET /cs /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 */

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

Example http reply Message a sample http server reply: HTTP/ 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>

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

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

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!