Layer 5: session layer
The session layer
controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application.
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It provides for full
‐
duplex, half
‐
duplex, or simplex
operation, and establishes checkpointing, adjournment, termination, and restart procedures.
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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 checkpointing and recovery, which is not usually used in the Internet Protocol Suite.