Session Layer in OSI Model

This article will help you in getting a better understanding of the Session layer in the OSI model.

The session layer is the fifth layer from the bottom in the OSI model. The session Layer helps in establishing communication sessions between users on discrete machines. It terminates the connection between network applications after the completion of their communication. Some types of connections will last long for the transfer of messages in a single direction. This is called a half-duplex transmission. The transmission in which the message is transferred in two directions is called duplex transmission.

The Session layer is also capable of handling the failures of transport layer from upper-layer protocols. Handling Failures may include handling errors and error correction in endpoint transmission. Session layers also help in grouping various transport streams which are related to a particular application. There are various protocols involved in the session layer a few of them are mentioned below

AppleTalk, Zone Information Protocol(ZIP), X.225, etc. The main duty/job of establishing, Maintaining, synchronizing, and terminating the communication sessions between various applications

Working of Session layer

The Session layer considers the work of transport layer(which passes its all services to the session layer) as input and establishes, maintains and synchronizes the communication sessions between various end-user applications. Before establishing the sessions, it follows various criteria and things.

Firstly, it maps the session address with the shipping address. After that, it selects the necessary transport quality service parameters. Now it transfers user data limitedly. The session layer properly monitors the transfer of data. The session layer is capable of carrying out the transfer of large data.

Functionalities of Session layer

The main functionalities of the Session Layer are mentioned below.

1. Dialog Control:

Session layer acts as a dialog controller i.e., it can easily identify and choose the mode of communication for a specific session. It allows two communication machines to enter into a single dialog.

There two modes of communication that are allowed by the session layer they are half-duplex and duplex. Half-duplex is a mode of communication in which the transfer of message will occur in a single direction whereas, in duplex communication mode, the transfer of message will occur in a double direction.

2. Synchronization

Synchronization is a methodology of marking up the checkpoints while working  of Session layer so that whenever there is a pause in the working of the session layer the work can be resumed from the checking rather than again starting the process from the initial point.

For example: If a System is transferring a file consisting of 5000 pages so adding a checkpoint after the transfer of every 200 pages is suggestable so that 200 pages unit of the file is successfully transferred and acknowledged. If there is a crash or a failure while transferring page number 755 then simply the transfer will again from 701 rather than starting from page number 1 to 700.

3. Token Management

This layer also provides the service of managing tokens. So, the situation in which multiple users try to access the same critical operation simultaneously can be prevented.

All the functionalities of the session layer are implemented in the application environment with the help of remote procedure calls(RPC). The session layer also synchronizes the data coming from various sources.

It efficiently controls the single or multiple connections for every end-user application and it can directly communicate with transport and presentation layers.

Protocols of Session layer

There are multiple protocols involved in session layer of OSI model they are:

• AppleTalk Data Stream Protocol(ADSP): This protocol is designed and developed by Apple Inc in 1985. it provides various features like it allows Local area networks to get connected without any setup.
  Again, this ADSP consists of 2 protocols they are AppleTalk Address resolution protocol(AARP) and name binding protocol(NBP).

• Password Authentication Protocol(PAP): it is a password-centric protocol which is accessed by point-to-point protocol(PPP)for validating the users whether they are authorized or not. It validates the identity of the client using two-way handshake property. Here, client transfers the data to the server and server receives the data and acknowledges it by setting ACK flag to 1.

• Point to point Tunnelling protocol(PPTP): PPTP protocol is used for implementing virtual private networks(VPN).  It provides high security levels and remote access levelsto VPNs.

• Real-time Transport Control Protocol(RTCP): Thiss provides all the statistics and detailed control information of a specific RTP(Real-time transport protocol) session.
  Its main function or purpose is to facilitate the feedback on service quality in media distribution by sending the statistical data at regular interval of time. Here the statistical data can be data packet count or packet loss to the attenders in the streaming session of multimedia.