What is Session Layer ?

The services provided by the first three layers (physical, data link, and network) are
not sufficient for some processes. The session layer is the network dialog controller.
It establishes, maintains, and synchronizes the interaction among communicating
systems.
          The session layer is responsible for dialog control and synchronization. Specific responsibilities of the session layer include the following:
Dialog control.
The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode.
Synchronization. 
          The session layer allows a process to add checkpoints, or synChronization points, to a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent. Figure  illustrates the relationship of the session layer to the transport and presentation layers.