OSI/AS Programming Manual

OSI Upper-Layer Concepts
APS Procedures Programming Concepts
056783 Tandem Computers Incorporated 2–3
Although you will find numeric values for error and status codes in this manual, we
strongly recommend that you always use symbolic names. When creating data names
for your own fields, we recommend that you avoid names that start with the letter Z
(uppercase or lowercase). Such names could conflict with names defined in Tandem
software.
Detailed descriptions of the DDL structures appear in Appendix A, which also
contains definitions of many data values.
OSI Upper-Layer
Concepts
The following subsections describe some concepts that have special significance for the
Session Layer, the Presentation Layer, and ACSE. The subsections also point out the
APS procedure calls used to implement the OSI upper-layer features described.
Primitives Communication between applications and a particular OSI layer is accomplished
using primitives. There are four types of primitives: request, indication, response,
and confirm. You send information to a service provider by invoking request or
response primitives. You receive information from a remote application or from the
service provider in the form of indication or confirm primitives.
Some APS procedures send primitives to the session, presentation, or ACSE service
provider. Other APS procedures retrieve primitives from the service provider.
The four types of primitives are used as follows:
A request primitive initiates a session, presentation, or ACSE operation of some
kind.
An indication primitive tells you that the remote application or service provider
has initiated an operation.
A response primitive carries your response to an indication primitive. Note,
however, that not all indications require a response.
A confirm primitive completes the cycle by giving you the remote application’s
response to the request you sent. Note that despite their name, confirm primitives
can sometimes carry a negative reply.