In the Open Systems Interconnection (OSI) communications model, 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.
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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.