Session Layer

The session layer is also known as a network dialog controller, it creates, maintains, synchronizes the interaction between communicating applications.

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.

• The session layer tracks the dialogs between systems, which are also called sessions. This layer manages a session by initiating the opening and closing of sessions between end-user application processes.
• It also controls single or multiple connections for each end-user application and directly communicates with both the presentation and the transport layers. The services provided by the session layer are generally implemented in the application environment using remote procedure calls (RPCs).
• In the 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.
• A protocol such as Zone Information Protocol, AppleTalk Protocol, and Session Control Protocol are used to implement sessions on Web browsers.
• Through check pointing and recovery session management and restoration are possible using these protocols.

Functionalities of the Session layer

Specific functionalities of the transport layer are as follows:

1. Dialog Control

• The session layer behaves as a dialog controller.
• It allows two communication machines to enter into a dialog.
• It permits to communicate in either half-duplex (one way at a time) or full-duplex (two ways at a time) mode of communication.

For Example, A dialog between a terminal connected to the mainframe can be half-duplex.

2. Synchronization

• This layer permitted a process to add checkpoints which are referred to as synchronization points into the stream of data.

Example: If a system is sending a file of 2500 pages, It is advisable to add checkpoints after every 100 pages to ensure that a 100-page unit is successfully received and acknowledged independently. In this case, if a crash happens during transmission of page number 824; then retransmission begins on page 801. There is no need to retransmit pages 1 to 800 pages.

3. Token Management

This layer is also responsible for managing tokens. Through this, it prevents the two users to simultaneously attempt access of the same critical operation.

 

Design Issues with Session Layer:

1. Establish sessions between machines –
   The establishment of session between machines is an important service provided by session layer. This session is responsible for creating a dialog between connected machines. The Session Layer provides mechanism for opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue. This session consists of requests and responses that occur between applications.
2. Enhanced Services –
   Certain services such as checkpoints and management of tokens are the key features of session layer and thus it becomes necessary to keep enhancing these features during the layer’s design.
3. To help in Token management and Synchronization –
   The session layer plays an important role in preventing collision of several critical operation as well as ensuring better data transfer over network by establishing synchronization points at specific intervals. Thus it becomes highly important to ensure proper execution of these services.

 

Some of the famous session layer's protocols are:

• Remote procedure call protocol (RPC)
• Point-to-Point Tunneling Protocol (PPTP)
• Session Control Protocol (SCP)
• Session Description Protocol (SDP) etc.