In computer science and networking in particular, a session is a time-delimited two-way link, a practical (relatively high) layer in the TCP/IP protocol enabling interactive expression and information exchange between two or more communication devices or ends – be they computers, automated systems, or live active users (see login session). 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 stateful, 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 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 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 TCP session, which is synonymous to a TCP virtual circuit, a TCP connection, or an established TCP socket.
In the case of transport protocols that do not implement a formal session layer (e.g., 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 cookies 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 session ID. The session ID is embedded within the <A HREF> or <FORM> links of dynamic web pages 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.
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