View source for Session (computer science)

In [[computer science]], in particular [[computer network|networking]], a '''session''' is a semi-permanent interactive information interchange, also known as a dialogue, a conversation or a meeting, between two or more communicating devices, or between a computer and user (see [[Login session]]).

== Description ==

A session is set up or established at a certain point in time, and then torn down at some later point. An established communication session may involve more than one message in each direction. A session is typically, but not always, [[stateful]], meaning that at least one of the communicating parts needs to save information about the session history in order to be able to communicate, as opposed to [[stateless server|stateless]] communication, where the communication consists of independent requests with responses.

An established session is the basic requirement to perform a [[connection-oriented communication]]. A session also is the basic step to transmit in [[connectionless communication]] modes. However any unidirectional transmission does not define a session.

'''Communication Transport''' may be implemented as part of protocols and services at the [[application layer]], at the [[session layer]] or at the [[transport layer]] in the [[OSI model]].
* Application layer examples:
** [[HTTP#HTTP session|HTTP sessions]], which allow associating information with individual visitors
** A [[telnet]] remote login session
* Session layer example:
** A [[Session Initiation Protocol]] (SIP) based [[Internet phone]] call
* Transport layer example:
** A [[Transmission Control Protocol|TCP]] session, which is synonymous to a TCP [[virtual circuit]], a TCP connection, or an established TCP [[Internet socket|socket]].

In the case of transport protocols that do not implement a formal session layer (e.g., [[User Datagram Protocol|UDP]]) or where sessions at the application layer are generally very short-lived (e.g., HTTP), sessions are maintained by a higher level program using a method defined in the data being exchanged.  For example, an HTTP exchange between a browser and a remote host may include an [[HTTP cookie]] which identifies state, such as a unique [[session ID]], information about the user's preferences or authorization level.

[[HTTP/1.0]] was thought to only allow a single request and response during one Web/HTTP Session. Protocol version [[HTTP/1.1]] improved this by completing the [[Common Gateway Interface]] (CGI), making it easier to maintain the Web Session and supporting [[HTTP cookie]]s and file uploads.

Most client-server sessions are maintained by the transport layer - a single connection for a single session. However each transaction phase of a Web/HTTP session creates a separate connection. Maintaining session continuity between phases required a [[session ID]]. The [[session ID]] is embedded within the &lt;A HREF&gt; or &lt;FORM&gt; links of [[dynamic web page]]s so that it is passed back to the CGI. CGI then uses the [[session ID]] to ensure session continuity between transaction phases. One advantage of one connection-per-phase is that it works well over low bandwidth (modem) connections.

== See also ==

* [[Client-server model]]
* [[Computer science]]
* [[Computing]]
* [[Login session]]
* [[Session token]]
* [[Stateless protocol]]

== External links == 

* [https://en.wikipedia.org/wiki/Session_(computer_science) Session (computer science)] @ Wikipedia

[[Category:Computer networks]]
[[Category:Computer science]]
[[Category:Web design and development]]