Session Layer

In this tutorial, we will be covering the session layer that is layer-5 of the OSI reference model in Computer networks.

It is layer-5 of the OSI reference model. The session layer is the network Dialog Controller

This layer mainly allows the users on different machines to establish active communication sessions between them.

The main aim of this layer is to establish, maintain and synchronize the interaction between communicating systems. The session layer manages and synchronizes the conversation between two different applications. 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.

This layer is one of the important components of the OSI model and is used by many computers and applications on the regular basis.

• This layer mainly defines how to start, control, and end conversations between different applications.

• The session layer responds to the service requests from the presentation layer and then issue service requests to the Transport layer.

• This layer is mainly responsible for synchronization and dialog control.

• This layer provides the following services:

  • full-duplex,

  • half-duplex,

  • the simplex operations,

  • establishes check-pointing,

  • adjournment procedures,

  • termination procedures,

  • and restart procedures.

• The implementation of this layer is done explicitly in the application environments by using remote procedure calls.

• This layer is also responsible for finding and sending data across the correct connection.

• In order to set up and tear down the connection the session layer mainly communicates with the transport layer.

Protocols in the Session layer

Given below are some of the protocols used in the session layer:

• RPC(Remote Procedure Call) protocol

• SCP(Session control protocol)

• PPTP(Point-to-Point tunneling protocol)

• PAP(password authentication protocol)

• RTCP(Real-time transport control protocol)

Responsibilities 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 the stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages are recommended. This ensures that the 50-page unit is successfully received and acknowledged. This is beneficial at the time of the crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Functions of the Session layer

Given below are some of the main functions of the Session layer:

• This layer is used to establish the session between the session entities.

• This layer is responsible for the data transfer phase.

• This layer is used to select the required transport Quality of service(QoS) parameters.

• This layer is used to identify each session.

• This layer is used to transmit a limited amount of transparent user data.

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.