Session Layer - OSI Reference Model

The Session Layer allows users on different machines to establish active communication sessions between them.

It's main aim is to establish, maintain and synchronize the interaction between communicating systems. Session layer manages and synchronize the conversation between two different applications. In Session layer, streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

Functions of Session Layer

1. Dialog Control : This layer allows two systems to start communication with each other in half-duplex or full-duplex.
2. Token Management: This layer prevents two parties from attempting the same critical operation at the same time.
3. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Design Issues with Session Layer

• To allow machines to establish sessions between them in a seamless fashion.
• Provide enhanced services to the user.
• To manage dialog control.
• To provide services such as Token management and Synchronization.