OSI Model - Session Layer

Layer 3 - Session Layer

The session layer establishes a one-to-one communication session between sending and receiving computers. Data conversion can be established, controlled and terminated in this layer. It enables application running at two workstations to coordinates their communication into a single session. The request and responses between the application is synchronized with the help for protocol that are added in this layer. Only after a completion of conversation, the data or request will be sent to the presentation layer.

The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check pointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

Some of the protocols that are implemented in the session layer are

• Network File Transfer (NFS) : It is enveloped by Sun Microsystems and used with TCP/IP and unix workstations to allow transport access to remote resources.
• Structured Query Language (SQL) : It is developed by IBM to provide users with a simpler way to define their information requirement on the both local and remote system.
• Remote Procedure Call (RPC) : Its procedures are created on client and performed on servers
• X Window : It is used by intelligent terminals for communicating with remote Unix computer allowing them to operate even they have locally attached monitor.
• AppleTalk Session Protocol (ASP) : Another client/server mechanism which establishes and maintains session between AppleTalk client and server machine.
• Digital Network Architecture Session Control Protocol (DNA SCP) : A DECnet session layer protocol.