In telecommunication, a session is a series of interactions
between two communication end points that occur during the span of a single
connection. Typically, one end point requests a connection with another
specified end point and if that end point replies agreeing to the connection,
the end points take turns exchanging commands and data ("talking
to each other"). The session begins when the connection is established
at both ends and terminates when the connection is ended.
In the standard industry communications reference model, Open Systems
Interconnection (OSI), the Session layer (sometimes called the "port
layer") manages the setting up and taking down of the association
between two communicating end points that is called a connection. A connection
is maintained while the two end points are communicating back and forth
in a conversation or session of some duration. Some connections and sessions
last only long enough to send a message in one direction. However, other
sessions may last longer, usually with one or both of the communicating
parties able to terminate it. For Internet applications, each session
is related to a particular port, a number that is associated with a particular
upper layer application. For example, the HTTP program or daemon always
has port number 80. The port numbers associated with the main Internet
applications are referred to as well-known port numbers. Most port numbers,
however, are available for dynamic assignment to other applications.
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