sessionlayer

The session layer is the fifth layer of the OSI model. It enables, manages, and terminates conversations, or sessions, between application processes located on different machines across a network. The layer provides dialogue control to determine which party may transmit at a given time, and it offers synchronization facilities to insert points in a data stream so that communication can resume from a known state after an interruption.

Its primary responsibilities include establishing, maintaining, and terminating sessions; handling the setup, control, and release of dialogue between end-user processes; providing tokens or mechanisms for controlling who can send data; and enabling full-duplex or half-duplex communication modes. It also supports synchronization and checkpointing, so that a session can be resumed from a known checkpoint after a failure, rather than starting over from the beginning.

In modern networks, the session layer’s functions are typically implemented by other layers rather than by a distinct, standalone protocol. The Internet protocol suite does not define a separate session-layer protocol; instead, session management tasks are commonly handled by the transport layer (for example, TCP) and by application-layer protocols that coordinate dialog and state. Historically, OSI defined specific session-layer services and some vendor systems provided session management features, but these have largely been absorbed into other layers in contemporary stacks.

Overall, the session layer remains a historical reference in the OSI model. In practice, its responsibilities are distributed across the transport and application layers rather than implemented as a separate protocol stack.