Clarify Common Encryption version

[cconcolato]

The current specification references a non-dated version of the Common Encryption specification. That specification has been amended in 2019 to define a new version of the 'senc' box. The AV1-ISOBMFF spec should either use a dated reference to CENC or clarify, possibly through CMAF, the version of the 'senc' box (and other boxes) that it uses (senc v0).

[cconcolato]

Using dated versions loses the benefit of picking up corrigendum items.
We could use a non-dated version but be explicit about the features we support (e.g. box versions).

[cconcolato]

We may want MPEG to clearly identify versions of its spec, for example with brands or at least make it easy for derived specs to identify changes, that they may or may not adopt.

[cconcolato]

We could clarify in our spec that we only support boxes, with versions and flags as tested in the conformance suite.

[cconcolato]

The current spec has 2 undated references: ISOBMFF and CMAF.

Regarding ISOBMFF, support for iso6 is required:

Parsers SHALL support the structures required by the iso6 brand and MAY support structures required by further ISOBMFF structural brands.

This means we could use an undated reference for ISOBMFF.

#158 deals with CMAF. We could also have an undated reference for CMAF.

[cconcolato]

The link to the ISOBMFF 2015 is no longer valid https://standards.iso.org/ittf/PubliclyAvailableStandards/c068960_ISO_IEC_14496-12_2015.zip

[cconcolato]

This issue needs 2 actions:

• explicitly list what support is required for CENC (boxes, versions, scheme identifiers, ...)
• update the ISOBMFF link to use a non dated reference

[cconcolato]

The group agrees to postpone the remaining item "explicitly list what support is required for CENC (boxes, versions, scheme identifiers, ...)" to the version after 1.3, as it is dependent on the work of MPEG to define the different dimensions of CENC that could be restricted in AV1-ISOBMFF.