Session layer

In the seven-layer OSI model of computer networking, the session layer is layer 5.

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses that occur between applications.Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs).

An example of a session-layer protocol is the session-layer protocol, also known as X.225 or ISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it. It provides for either full duplex or half-duplex operation and provides synchronization points in the stream of exchanged messages.

Other examples of session layer implementations include (ZIP) – the AppleTalk protocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet Phase IV session-layer protocol.

Within the service layering semantics of the OSI network architecture, the session layer responds to service requests from the presentation layer and issues service requests to the transport layer.

The session layer of the OSI model is responsible for session checkpointing and recovery. It allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized.

An example usage of the session layer is session beans, which are only active as long as the session is active, and are deleted when the session is disconnected. Java developers can use them to store information about the user during a web session.

An example application is web conferencing, in which the streams of audio and video must be synchronous to avoid so-called lip synch problems. Flow control ensures that the person displayed on screen is the current speaker.

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