Disambiguating Unconstrained OPTIONAL Components in H.245 (Q52)

The information in this article applies to:

This note proposes modifications to H.245 to remove the ambiguity between the present-but-empty case and the missing case of unconstrained OPTIONAL SEQUENCE OF and SET OF components that cannot otherwise be fixed by adding constraints. (For editorial reasons, this proposal was not adopted, although it would be prudent for all implementers to abide by the proposed text, below.)

1. Introduction

H.245 version 1 ASN.1 syntax contains components that express a possibly empty SET OF or SEQUENCE OF as being OPTIONAL and having a minimum size constraint that is greater than zero. However, components added in version 2 express the same thing as being OPTIONAL and unconstrained. This result is ambiguity between the present-but-empty case and the missing case, assuming that the author did not intend a semantic difference between the two cases.

For example, from H.245 version 1:

    capabilityTableEntryNumbers SET
    SIZE(1..65535) OF capabilityTableEntryNumber OPTIONAL

and from Version 2:

    centralizedData SEQUENCE OF DataApplicationCapability OPTIONAL

(For purposes of this example, the differences between SET OF and SEQUENCE OF is not important.)

The same type of components have been added in H.245 version 3, but since version 3 is not Decided, the syntax can be modified by adding minimum-size constraints. However, since version 2 is already Decided, the ambiguity must be eliminated by a statement that defines the two cases as having equivalent semantics.

2. Text Modifications

Insert into the second paragraph of section 5/H.245 before the last sentence:

Unless otherwise stated, an OPTIONAL SEQUENCE OF or SET OF component whose minimum size is zero, either because it is unconstrained or explicitly constrained to zero, shall have the same semantics regardless of whether its value is missing or is present but empty.

3. Proposal

That the text modification described above be applied to H.245 version 3 to resolve the ambiguity between missing and present-but-empty OPTIONAL SEQUENCE OF and SET OF components whose minimum size is constrained to zero or simply unconstrained.