We will devote a chapter to the services provided by the Session Layer. Here we restrict our attention to the particular requirements of reliable bulk transfer. Applications such as simple file transfer, message handling, directory queries, remote video screening, etc. might use such a facility.
Given the availability of reliable machine to machine transport, what must a reliable transfer service add? The reliable transfer service must deal with possible application failures. In the example of a failed disk access, the application needs to pause while a new disk is inserted, then restart. In the restart, the application must resend data that was lost on the bad disk. The service must provide a means to note when the application has finished with some data so the sender can clear it from its buffers. It must also provide a means for the receiver to request retransmission of information that was lost or corrupted in the application.
In the file transfer example, data flows in one direction only. Some applications require two way communication. A reliable transfer service should provide a mechanism to coordinate sending and receiving and to prevent one side from closing the association before the other has finished sending data.
The Reliable Transfer Service Element(RTSE) provides for dependable transfer of application protocol data units (APDUs) of substantial length. RTSE invokes the services of the Session Layer on behalf of the application process, providing a simpler interface to those services.