In addition to the organization and control of communications, the synchronization of data exchange is another essential service provided by the session layer. This takes on its full importance when a data transfer is unexpectedly and accidentally interrupted at one of the first four levels.
To deal with any eventuality, the OSI session layer therefore creates special synchronization points. If communication is interrupted, the transfer can therefore resume at one of these points, which avoids having to start all over again. This service can be particularly useful in the context of communications where connections are slow and unstable, or whose files are particularly large.
The synchronization points provided by the session layer can fall into two broad categories.
- THE major synchronization points allow the data to be transferred to be split into individual units. Explicit confirmation is required for these synchronization points.
- THE minor synchronization points, meanwhile, guarantee the logical and practical aspect within these units; their confirmation is optional.
