Session (computer science)

In <a href="/facts/Computer_science/lN3pEyPz">computer science</a> and <a href="/facts/Computer_network/3w5RM99p">networking</a> in particular, a session is a time-delimited two-way link, a practical (relatively high) layer in the <a href="/facts/Internet_protocol_suite/KNkxtSpw">TCP/IP protocol</a> enabling interactive expression and information exchange between two or more communication devices or ends – be they computers, <a href="/facts/Automation/3YlF2K5q">automated systems</a>, or live active users (see <a href="/facts/Login_session/plmwnSNU">login session</a>). A session is established at a certain point in time, and then ‘torn down’ - brought to an end - at some later point. An established communication session may involve more than one message in each direction. A session is typically <a href="/facts/Stateful/4Yu8xXHS">stateful</a>, meaning that at least one of the communicating parties needs to hold current state information and save information about the session history to be able to communicate, as opposed to <a href="/facts/Stateless_server/TgJncyPa">stateless</a> communication, where the communication consists of independent <a href="/facts/Request-response/2TV4bxJl">requests</a> with responses.
An established session is the basic requirement to perform a <a href="/facts/Connection-oriented_communication/2yAhgueh">connection-oriented communication</a>. A session also is the basic step to transmit in <a href="/facts/Connectionless_communication/MfMys8t7">connectionless communication</a> modes. However, any unidirectional transmission does not define a session.
Communication Transport may be implemented as part of protocols and services at the <a href="/facts/Application_layer/FAFhLruA">application layer</a>, at the <a href="/facts/Session_layer/Yxo8uqLr">session layer</a> or at the <a href="/facts/Transport_layer/UmIGUCeK">transport layer</a> in the <a href="/facts/OSI_model/oxN9Xhjz">OSI model</a>.

<ul><li>Application layer examples:
<ul><li><a href="/facts/HTTP/TWtvf7JD">HTTP sessions</a>, which allow associating information with individual visitors</li>
<li>A <a href="/facts/Telnet/hxT0e85h">telnet</a> remote login session</li></ul></li>
<li>Session layer example:
<ul><li>A <a href="/facts/Session_Initiation_Protocol/cbjUoNqP">Session Initiation Protocol</a> (SIP) based <a href="/facts/Internet_phone/5QGBbXYg">Internet phone</a> call</li></ul></li>
<li>Transport layer example:
<ul><li>A <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP</a> session, which is synonymous to a TCP <a href="/facts/Virtual_circuit/8HqpnTxl">virtual circuit</a>, a TCP connection, or an established TCP <a href="/facts/Internet_socket/r1ldfkb5">socket</a>.</li></ul></li></ul>
In the case of transport protocols that do not implement a formal session layer (e.g., <a href="/facts/User_Datagram_Protocol/dxLU0D6q">UDP</a>) 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 <a href="/facts/HTTP_cookie/o1laXyng">HTTP cookie</a> which identifies state, such as a unique <a href="/facts/Session_ID/v7h7vIVX">session ID</a>, information about the user's preferences or authorization level.
<a href="/facts/HTTP%2f1.0/TWtvf7JD">HTTP/1.0</a> was thought to only allow a single request and response during one Web/HTTP Session. Protocol version <a href="/facts/HTTP%2f1.1/TWtvf7JD">HTTP/1.1</a> improved this by completing the <a href="/facts/Common_Gateway_Interface/zWsLFXGJ">Common Gateway Interface</a> (CGI), making it easier to maintain the Web Session and supporting <a href="/facts/HTTP_cookie/o1laXyng">HTTP cookies</a> 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 requires a <a href="/facts/Session_ID/v7h7vIVX">session ID</a>. The <a href="/facts/Session_ID/v7h7vIVX">session ID</a> is embedded within the <A HREF> or <FORM> links of <a href="/facts/Dynamic_web_page/psjqGKBx">dynamic web pages</a> so that it is passed back to the CGI. CGI then uses the <a href="/facts/Session_ID/v7h7vIVX">session ID</a> to ensure session continuity between transaction phases. One advantage of one connection-per-phase is that it works well over low bandwidth (modem) connections.

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Session (computer science)