In computer science, in particular networking, a session is a semi-permanent interactive information exchange, 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). A session is set up or established at a certain point in time, and torn down at a later point in time. 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 need to save information about the session history in order to be able to communicate, as opposed to stateless communication, where the communication consists of independent requests with responses.
Communication sessions 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:
o HTTP sessions, which may allow dynamic web pages, i.e. interactive web pages, as opposed to static web pages.
o A telnet remote login session
* Session layer example:
o A Session Initiation Protocol (SIP) based Internet phone call
* Transport layer example:
o 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 which do not implement a formal session layer (e.g., UDP) or where sessions at the session 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.
Protocol version HTTP/1.1 makes it possible to reuse the same TCP session for a sequence of service requests and responses (a sequence of file transfers) in view to reduce the session establishment time, while HTTP/1.0 only allows a single request and response during one TCP session. However, this transport layer session mechanism should not be confused with a so called HTTP session, since it is not lasting sufficiently long time, and does not provide application level interactive services such as dynamic web pages.
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