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When one has limited usage, like say "only for usage in RTP" I think there are need to consider some wider interpretation of what "to some extent readable" means. We had an example in the 3GPP timed text discussion, where the RTP payload format consists of embedded UTF-8 or UTF-16 strings in an otherwise binary format. However for RTP that is the common case. Should such any clarification on the performance of such consideration be written into this spec or do it belong to a further registration document in relation do a specific domain of usage?

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I would say it belongs in the registration document.

does such a clarification belong under "Encoding considerations" or under "Restrictions on usage"? Until now most payload formats included a sentece like "This type is only defined for transfer via RTP." under encoding considerations. In this case, is it not more appropriate to do this under "Restrictions on usage"?

Encoding considerations is really intended to be a single value chosen from a list. As such, this really belongs under restrictions on usage.

This is how it would look like in our case:

"Encoding considerations:

RTP payloads conforming to this payload format do not comply with the strict rules for registration under the 'text' top-level MIME type, as outlined in RFC 2026 and its revision RFCYYYY [].

Wrong RFC # here,, I believe. Additionally, I don't see this as a case of noncompliance with the revised rules. A better approach would be to say that this type is incompatible with use of text media types in other protocols.

The reason for this is that the RTP client must, at least, understand the payload format fields (in binary) in order to decode the timed text contents (partly text strings, partly binary). This binary information is not considered 'directly readable' without 'special software' as required for 'text' subtypes (in contrast to text strings). Around the 60th IETF it was decided to relax these rules for RTP payload formats. In particular, it was allowed to register RTP payload formats under the 'text' top-level type (if appropriate) as long as the domain of applicability of the format is clearly specified (e.g. only for transmission using RTP). This payload format complies with these 'relaxed' rules.

I don't think having this sort of historical note in a registration is appropriate either. Ned