CAP ASN further info

["Jacob Westfall" <jake@jpw.biz>]

As part of developing the overview of where CAP ASN.1 stands right now, here is some further info on the issue.

- The CAP 1.1 Standard was sent to ITU; ITU requested ASN.1 schema to be added; CAP 1.1 errata created to add ASN.1 resulted in X.1303.

- There was an error in the ASN.1 for CAP 1.1; as that is normative, an errata could not correct, a new version would be required.

- CAP 1.2 updates the XML schema but the addition of the "avoid" response type is a substantive change to ASN.1

- CAP 1.2 also clarifies the use of Digital Signatures in the schema, which are not supported by ASN.1

- The ASN.1 schema for CAP does not have a method of denoting the version, so users cannot easily differentiate between 1.1 and 1.2

- There is no backward compatibility between CAP 1.1 ASN.1 and CAP 1.2 ASN.1  More details:

The X.694 standard, which was used to create the ASN.1 schemas in CAP 1.1
and CAP 1.2, specifies a rigorous mapping from XML Schema to ASN.1.  The
mapping rules are so rigorous that they ensure that one can seamlessly
translate between an XML encoding and a PER encoding just by applying the
standard encoding rules (PER and EXER) to the ASN.1 schema.  This makes it
easy to build an application that can handle both encodings, so long as the
application uses conforming ASN.1 encoder/decoders.

The downside of following strict mapping rules from XML Schema to ASN.1 is
that when the original XML schema is modified, the new ASN.1 schema may not
be backward-compatible with the old ASN.1 schema.  This can happen even in
case of seemingly minor changes such as a new item being added to an
enumeration, or a required attribute or child element becoming optional, or
vice versa.  In the case of CAP 1.2 vs. 1.1, the changes made to the XML
schema cause the PER encodings to change significantly.

- A liaison statement from ITU-T SG 17 regarding updating their approved version of CAP to X.1303bis was received by OASIS Admin in Mar 2014 but due to the changes requested, was not acted upon.