It is one of the parts of the upper layers. The Application Layer, Presentation Layer, and Session Layers form the OSI reference model's upper layers. They provide user-oriented services. The session layer provides a defined set of services to the presentation layer
Presentation Layer uses session layer protocols and transport services. The session layer service’s actual user is the application layer through which the presentation layer lies between them. It has relatively few features as compared to lower layers. It controls structures and the interaction between the application programme.
It synchronizes the interaction between elements and controls the direction of information flow, but it is not taken. It has nothing to do with error detection and correction functions. It provides services to the presentation layer. The service definition and protocol specifications are defined in ISO 8326 and ISO 8327.
The session layer is the thinnest layer with the most negligible numbers of protocols in the OSI model. The session layer objective is to create, maintain and synchronize dialogs between transmitting upper layers. Communication can take place between either users or applications.
The session layer helps coordinate connection and release of dialogs connections between the communicating applications. It communicates with the transport layer. The communication may be one to one, many to one and one to many. In one to one, one session layer connection establishes for each transport layer connection.
In many to one, multiple session layer connections are shared with the services of one transport layer connection. The one to many connection communication is set up when one session layer connection calls for many transport layer connections to handle the service.
The session layer aims to decide whose turn it is to talk. Some of the applications operate in half-duplex mode. The half-duplex provides two sides alternate communication between sending and receiving messages and never sending data simultaneously.
The dialog management is implemented using a data token transmitted back and forth to provide a user with a right to transmit only when it possesses the token.
The session layer enables the user to delimit data into logical units called activities. Each activity is treated as a separate activity and independent from the preceding and following activities to that activity.
Activities are used to delimit files of a multi-file transfer. Activities are used for quarantining, collecting all the data of a multi-message exchange together before processing them. The receiving application begins processing data only after all the data arrives. This ensures that all or none of a set of operations is performed.
For example, a bank transaction may involve locking a record, updating a value, and unlocking the record. When an application processes the first operation but could not receive the remaining operations due to the client or network failures. The record will remain locked forever. Quarantining solves this problem.
It is a general-purpose mechanism for reporting errors.