session layer

The fifth layer of the OSI Reference Model is the session layer. The session layer is
responsible for initiating the setup and teardown of connections. To perform these
functions, the session layer must determine whether data stays local to a computer
alternatively, must be obtained or sent to a remote networking component. In the latter case,
the session layer initiates the connection.
The session layer is also responsible for differentiating among multiple network connections, ensuring that data is sent across the correct connection as well as taking data from a connection and forwarding it to the correct local application. The actual mechanics of this process, however, are implemented at the transport layer. To set up connections or tear down connections,
the session layer communicates with the transport layer. Remote Procedure Calls
(RPCs) are an example of a TCP/IP session protocol; the Network File System
(NFS), which uses RPCs, is another example of a protocol at this layer. The
session layer is also responsible for error reporting of any issues at the application,
presentation, and session layers and for implementing any type of class of service
(CoS) to give preference to some types of traffic or connections over others.