Functions of Session Layer (original) (raw)

Last Updated : 11 Feb, 2026

The Session Layer (OSI Layer 5) establishes, manages, and terminates communication sessions between applications, providing dialog control and synchronization so data exchange stays organized and recoverable.

• Acts as a dialog controller by managing who communicates when (sequencing).
• Maintains session state and supports synchronization points (checkpoints) for recovery.
• Receives data from the Presentation Layer and coordinates with the Transport Layer for end-to-end delivery.
• Ensures sessions can be resumed or restarted cleanly after interruptions.

1. Session Establishment

The most important role of the Session Layer is to establish and manage communication sessions between devices. These sessions can be connection-oriented or connectionless. Sessions can be mapped to transport connections in three ways:

• **One-to-One Mapping: One session uses a single transport connection.
• **Many-to-One Mapping: Multiple sessions share a single transport connection.
• **One-to-Many Mapping: A single session uses multiple transport connections.

**Note: Once a session is released, the associated transport connection is also released.

2. Communication Synchronization

The Session Layer ensures proper synchronization during data transfer. To achieve this, it uses synchronization bits and checkpoints:

• Checkpoints are inserted into the data stream to allow recovery in case of failure.
• If an error occurs, retransmission begins from the last checkpoint rather than restarting the entire communication.

**Note: This functionality is crucial in long and complex communications, as the Transport Layer can only handle communication errors, not application-level synchronization.

3. Activity Management

The Session Layer divides a continuous stream of communication into logical units called activities.

• Each activity is treated independently, which allows better management of operations performed during a session.
• This makes it easier to differentiate between tasks within a single communication stream.

4. Dialog Management

The Session Layer manages dialogues (conversations) between communicating devices. This involves deciding who will send data and when. Two modes of communication are supported:

• **Half-duplex Mode: Only one party can transmit at a time. A token mechanism is used, where the device holding the token has the right to send data.
• **Full-duplex Mode: Both devices can transmit data simultaneously, eliminating the need for tokens.

**Note: This ensures efficiency and prevents data collisions during communication.

5. Data Transfer

Data transfer is a fundamental function of the Session Layer. It manages the exchange of information between two systems by maintaining the mode of communication:

• **Half-duplex: One device transmits while the other listens.
• **Full-duplex: Both devices can send and receive data at the same time.

**Note: By organizing data flow, the Session Layer ensures reliable and orderly communication.

6. Resynchronization

Resynchronization (or backward synchronization) allows a session to be restored to a previously defined state after an error or failure. There are three options for resynchronization:

• **Set: Assigns a new synchronization point serial number chosen by the user.
• **Abandon: Discards the current synchronization point serial number and sets it to an unused value.
• **Restart: Assigns a synchronization point serial number greater than the last acknowledged major synchronization point.

**Note: This process ensures that sessions can recover from disruptions without restarting entirely.