The first step toward establishing an association between an initiator and a responder is taken when an application program invokes the service A-ASSOCIATE. In the parameters of A-ASSOCIATE, a user proposes a set of desired characteristics, the context, for the association. These characteristics include the environment in which this application will run; specifically, what application service elements will be active in the cooperating systems. This is the only application related parameter that is required to establish the association. Other required parameters identify the Presentation Service Access Point (PSAP) of both ends of the association and define the Presentation Context.
A PSAP is a
concatenation of the network address of the host and selectors for
entities at the transport, session, and presentation layers. The PSAP
on the initiating system is
obtained by a call on a system service and is not something a user or an
application developer needs to construct. The PSAP of the responding
system must be obtained from a database of applications and their
locations. Commonly used remote applications may be listed in a local
database. To establish an association with a remote process not listed
locally, an application process may query a global database using a
service such as the Directory discussed in
Chapter 
.
The Presentation Context allows the Presentation Layer in the initiator system to inform its peer in the responder system what type of information to expect over this connection and how it will be encoded. To do this, the Presentation Layer needs to know what information forms the Application Entity will use as well as any special requirements for encoding information in transit. Thus, the AE might specify to the Presentation Layer that it will use the abstract syntax known as ``PersonnelData'' and wishes the information to be conveyed in encrypted form. This is done by specifying a simple identifier, a small integer, that selects this choice of abstract syntax and transfer mode from a collection of available combinations stored locally.
For example, suppose that the local set of abstract syntaxes and
associated transfer modes is the following:
{1{PersonnelData,Encrypted},2{AnnualReport,Compressed},3{AnnualReport,Basic}}
The numbers serve as identifiers for the various combinations of
information and transfer modes. To request communication of the annual
report using basic encoding, the AE would specify the Presentation
Context Identifier 2 in the Presentation Context Definition List of the
ACSE parameters.
Among the optional parameters of A-ASSOCIATE.request is user data. User
data in this first communication between the cooperating applications
allows data delivery to occur even before the association is
established. A reasonable use of this option is to carry authentication
information, such as login name and password.
Finally, the parameters of A-ASSOCIATE include an opportunity for the
user to specify the quality of service (QOS) expected in this session.
Quality of service was described in Chapter .
An example invocation of A-ASSOCIATE.request appears in the example code
of
Section . This code sample uses the ISODE
implementation of the OSI protocols. Other implementations may have
different interfaces.