In the discussion of CO and CL communications services above we noted that
it was sometimes desirable that two (maybe more) application entities
should form a long-term association. Most of the OSI applications defined
to date assume that this is desirable. The Association Control Service
Elements (ACSE) are designed to manage these associations. There are four
elements, shown in table #tbacse#1496>.
Table: ACSE Service Primatives
The last three are concerned with breaking associations by agreement, by
fiat from one end, and as a result of some inadequacy of the service
The most important parameters of an A.ASSOCIATE.req are:
In practice, A.ASSOCIATE does little more than collect together the
parameters to be placed in a P.CONNECT.req. In future it may do more; it
would be an excellent place to implement Application Entity mutual
authentication for example.
The P-SAP Address to which the remote Application entity is attached
The Application Context Name, which specifies the activity in which we
are about to engage (File Transfer, Electronic Mail etc.)
Presentation Context information, which is concerned with how
information is to be represented in transit. This is considered in the
User data. Since A.ASSOCIATE is used to set up an association on
behalf of some application, there will usually be some
application-specific initialisation data to be transferred.