The Session Layer
The Session Layer is the fifth layer of the OSI model. Once the Transport Layer successfully establishes a connection between two processes, the Session Layer takes over to manage the ongoing conversation.
Its primary role is to establish, manage, synchronize, and gracefully terminate communication sessions between applications on different devices.
A Note on Modern Networks
It is important to understand that in the modern TCP/IP model, the Session Layer does not exist as a standalone layer. Its critical functions, such as session release and dialogue control, have been absorbed into either the Transport Layer or the Application Layer.
Key Functions of the Session Layer
- Session Establishment: Initiates the session and negotiates communication parameters, such as user authentication and security credentials.
- Dialog Management: Controls whose turn it is to send or receive data, preventing communication collisions. It determines whether the communication will be half duplex or full duplex.
- Synchronization and Checkpointing: Inserts synchronization checkpoints into the data stream. For example, if a user is downloading a massive 1000 page document, the Session Layer might insert a checkpoint every 100 pages. If the network crashes at page 520, the download safely resumes from page 500 instead of restarting from page 1.
- Resynchronization and Recovery: Uses those exact checkpoints to gracefully recover from network failures without data duplication.
- Session Termination: Ensures that the communication session is cleanly terminated only after both sides confirm that all data has been successfully exchanged.
Important Session Layer Protocols
Several legacy and specialized protocols operate exclusively at this layer to maintain dialogue control.
- Real Time Transport Control Protocol (RTCP): Provides crucial quality of service feedback for real time multimedia sessions like voice calls.
- Point to Point Tunneling Protocol (PPTP): Enables the creation of secure Virtual Private Networks over standard IP infrastructure.
- Password Authentication Protocol (PAP): Manages simple password based user authentication during the initial session establishment phase.
- Remote Procedure Call Protocol (RPCP): Allows a software program to execute functions and procedures on a completely different remote server as if it were local.
- Sockets Direct Protocol (SDP): Supports high performance socket communication over specialized direct memory access networks.
Devices Operating at the Session Layer
- Firewalls: Continuously monitor the state of active sessions to block unauthorized access and malicious data injections.
- Proxy Servers: Act as intermediaries that safely manage and cache sessions between internal clients and external web servers.
- Session Border Controllers: Specially designed hardware that exclusively secures and manages live voice over IP sessions.
- Application Servers: Create, maintain, and eventually destroy user sessions for complex software applications.