Which OSI layer is responsible for establishing, maintaining, and terminating sessions?

• A. Session Layer
• B. Presentation Layer
• C. Application Layer
• D. Transport Layer

Answer: (A)

The main idea is session management: establishing, maintaining, and terminating conversations between two application endpoints. The Session Layer handles the lifecycle of a communication session, including dialog control and synchronization (such as inserting checkpoints so a long transfer can resume after a failure) and then cleanly ending the session when the exchange is finished. It sits above the Transport Layer, which is responsible for delivering data end-to-end and providing reliability, while the Presentation and Application Layers deal with data formats, encryption, and user services. In practice, some of these tasks are merged in other models, but in the OSI framework this responsibility belongs to the Session Layer.