OSI/AS Programming Manual

Overview of OSI/AS Session Layer Support
Introduction
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Overview of OSI/AS
Session Layer Support
As mentioned earlier, the procedures described in this manual provide the functions of
the Session Layer of the OSI Reference Model. The formal definitions of the session
service and protocol are given in the international standard documents ISO 8326 and
ISO 8327. OSI/AS supports both version 1 and version 2 of the session service and
protocol.
The information in the session standards is not duplicated here, except where it is
needed to provide a context for the discussion of the APS procedures and their use.
The remainder of this subsection briefly outlines the functions of the Session Layer and
the Session Layer support provided by OSI/AS.
What Are the Functions of
the Session Layer?
The Session Layer is Layer 5 of the OSI Reference Model. Unlike Layers 1 through 4, it
is application-oriented rather than network-oriented. It provides the means for the
organized and synchronized exchange of data between cooperating service users
(applications). Users of session services can perform the following tasks:
Establish a connection with another session service user, exchange data with that
user in a synchronized manner, and release the connection in an orderly manner
Establish synchronization points within the dialog and, in case of errors, resume
the dialog from an agreed synchronization point
Negotiate for the use of tokens to exchange data, to synchronize and release the
connection, and to arrange for data exchange to be half-duplex or duplex
Interrupt a dialog and resume it later at a prearranged point
The Session Layer consists of a series of functional units that group together related
services. When you establish a connection, you negotiate with the remote application
to decide which functional units will be used. Only functional units selected by both
your application and the remote application are available on the connection.
Which Functional Units Are
Supported?
The functional units supported by OSI/AS are listed below. Note that the kernel
functional unit, together with either the duplex functional unit or the half-duplex
functional unit, provides the basic functions needed to establish, use, and release a
session connection. For information about the combinations of functional units you
can select, see the descriptions of the APS_ASSOC_CONNECTREQ_ and
APS_ASSOC_CONNECTRSP_ procedures in Section 3. For a description of tokens,
see “Tokens” in Section 2.
The kernel functional unit allows you to establish and release connections and to
send and receive normal data.
The negotiated release functional unit allows you to negotiate with the remote
application to release the connection in an orderly manner. Without this
functional unit, orderly release is still possible, but a release request cannot be
refused.
The half-duplex functional unit allows you to operate in half-duplex mode over a
connection. In this mode, data can be transferred in both directions, but only in
one direction at a time, and the direction of data flow alternates.