Session layer

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OSI Model
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer

The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer.

The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either full duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures.

The Session layer is typically completely unused, but it does have a few places where it is useful. The idea is to allow information on different streams, perhaps originating from different sources, to be properly combined. In particular, it deals with synchronization issues, and ensuring nobody ever sees inconsistent versions of data, and similar things.

One application which is fairly intuitively clear is web conferencing. Here, we want to make sure that the streams of audio and video match up - or in other words, that we do not have lipsync problems. We may also want to do "floor control" - ensuring that the person displayed on screen and whose words are relayed is the one selected by the speaker, or by some other criteria.

Another big application is in live TV programs, where streams of audio and video need to be seamlessly merged from one to the other so that we do not have half a second of blank airtime, or half a second when we transmit two pictures simultaneously.

In brief: the session layer establishes, manages and terminates connections (sessions) among cooperating applications. It also adds traffic flow information.

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