The Session Layer

   Establishing, Conducting, and
   Ending a Session

A session controls the way information flows between entities over a connection that has been established by the Transport Layer. There are two possible relationships between sessions and connections:

One or more sessions can take place during a single connection. Several sessions can take place without reestablishing the connection over and over. This is analogous to passing a telephone around during a family call. Multiple conversation sessions begin and end during a single call connection. (However, the Session Layer does not combine several sessions onto a single connection. That is the job of the Transport Layer.)

A session can require several connections to complete. In other words, a connection can be interrupted and reestablished without disturbing the session. This is equivalent to a telephone connection that is disconnected before you are finished talking. Because your conversation was not complete when the call disconnected, you must reestablish the connection (call the person back) to finish the session.

An important component of Session Layer services is that of "orderly release" of the connection. The lower layers support only an abrupt termination of the connection. However, in a conversation, it is more polite and efficient to confirm that the other party is finished talking before you hang up the telephone. In other words, we end the conversation (session) before we hang up (end the connection). The Session Layer takes care of this for dialogs between nodes.