OSI/MHS Gateway Programmatic Interface (GPI) Programming Guide

Planning Your Program
OSI/MHS Gateway Programmatic Interface (GPI) Programming Guide—424822-001
4-8
Unsupported Extensions
As mentioned before, the two supported extensions that correspond to CCITT extension-
attributes do not have criticality. You should not specify them as values of the MH-T-
CRITICAL-FOR- attributes.
Unsupported Extensions
A GPI gateway can also handle unsupported extensions. Unsupported extensions are
not explicitly known to the GPI. Within the gateway, each unsupported extension is
represented by an object of the MH-C-EXTENSION class (Figure 4-2). A root object
that contains unsupported extensions may have one or more subobjects of the
MH-C-EXTENSION class (one subobject per extension). Attributes of
MH-C-EXTENSION indicate the type of the unsupported extension, its value, and its
criticality.
Within each MH-C-EXTENSION object, criticality is indicated by three single-valued
attributes. These attributes are “flags” that inform the application if the extension must
be processed. They are:
•
mh-t-ext-critical-for-delivery
•
mh-t-ext-critical-for-submission
•
mh-t-ext-critical-for-transfer
Note that the single-valued criticality attributes apply only to unsupported extensions;
that is, for each unsupported extension there is a separate MH-C-EXTENSION object
that contains a set of the three Boolean flags. It should be emphasized that the
single-valued criticality attributes are different from and should not be confused with the
multivalued criticality attributes. The multivalued criticality attributes apply only to
supported extensions.
For unsupported extensions that correspond to CCITT extension-attributes, the
single-valued criticality attributes are not used.
EDI Extensions
X.435 (EDI) recommendations also provide for future extensions. Within a GPI
gateway, several types of objects that represent EDI constructs (heading, security
elements, and so on) may in the future have extensions. Currently, however, no EDI
extensions are defined.
For EDI, the GPI represents each extension by an object of the EDI-C-EDI-
EXTENSION class. Attributes of EDI-C-EDI-EXTENSION indicate the type of the
extension, its value, and its criticality. (For EDI, there is only one GPI mechanism for
extensions, and it is analogous to the GPI mechanism for unsupported X.400
extensions.)
The meaning of criticality differs between X.400 and X.435. For X.435, criticality
indicates whether or not an extension is critical for acceptance of EDIM responsibility.
(An EDI message cannot leave the EDI messaging system unless responsibility for the
message has been accepted by some recipient.) An extension marked noncritical may be
ignored or discarded by the recipient. An extension marked critical must be processed
by a recipient before the recipient can accept responsibility for the EDIM.