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The session layer establishes conversations — sessions — between networked devices. A session is an exchange of connection-oriented transmissions between two network devices. Each transmission is handled by the transport layer protocol. The session itself is managed by the session layer protocol.

A single session can include many exchanges of data between the two computers involved in the session. After a session between two computers has been established, it's maintained until the computers agree to terminate the session.

The session layer allows three types of transmission modes:

 Simplex: Data flows in only one direction.

 Half-duplex: Data flows in both directions, but only in one direction at a time.

 Full-duplex: Data flows in both directions at the same time.

In actual practice, the distinctions in the session, presentation, and application layers are often blurred, and some commonly used protocols actually span all three layers. For example, SMB — the protocol that is the basis of file sharing in Windows networks — functions at all three layers.