University of the Western Cape Chapter 13: - The Session Layer The session layer (Layer 5) establishes, manages, and terminates sessions between applications. This includes starting, stopping, and re- synchronizing communications between applications on two computers.

University of the Western Cape Dialogue control Communication between two computers involves many mini-conversations. One requirement of these mini-conversations is that each host plays dual roles. They are requesting service, like a client, and replying with service, like a server. Determining which role they are playing at any given moment is called dialogue control. The session layer decides whether to use two-way simultaneous conversation or two-way alternate communication. This decision is referred to as dialogue control. If two-way simultaneous (TWS) communication is allowed, then the communication can go in both directions at the same time. It is possible to have session layer collisions. At this level, collisions can only occur as two messages pass each other, and cause confusion in either, or both, communicating hosts. Two-way alternate (TWA) communication involves the use of a session layer data token that allows each host to take turns.

University of the Western Cape Dialogue separation Dialogue separation is the orderly initiation, termination, and managing of communication. The session layer provides a way to insert checkpoints into the data stream, so if the data flow is interrupted, only the data after the last checkpoint has to be repeated. The graphic illustrates a minor synchronization. At the "Time Axis, t = checkpoint", the host A session layer sends a synchronization message to host B, at which time both hosts perform the following routine: back up the particular files save the network settings save the clock settings make note of the end point in the conversation A major synchronization would involve more back-and-forth steps and conversation than is shown in this diagram.

University of the Western Cape The Session Layer Protocols Session layer has a number of important protocols. Examples of Layer 5 protocols are: Structured Query Language (SQL) Remote Procedure Call (RPC) X-Window System AppleTalk Session Protocol (ASP) Digital Network Architecture Session Control Protocol (DNA SCP)

University of the Western Cape The End