[media-types] Media subtypes containing "+"

["Murray S. Kucherawy" <[e-mail]>]

Before I joined the IESG, we (the media type reviewers) processed a request
for a media type that included a "+" character.  Specifically, the request
was to register "application/ld+json", and an implied media type suffix of
"+ld+json".  This was a revision to an existing media type.

It included an implicit media type suffix registration, i.e., prose that
said:

"Other specifications MAY create further structured subtypes by using +ld+json
as a suffix for a new base subtype, as in application/example+ld+json.
Unless defined otherwise, such subtypes use the same fragment identifier
behavior as application/ld+json."

We had the applicant remove this text and instead suggest that they need to
register "+ld+json" independently as a media type suffix, but that such a
registration would likely be denied until such time as the community has
had a chance to discuss its implications.

The question is around nested suffixes like this.  There's already a
"+json", of course, but we should probably be clear someplace (maybe an
"updates" document to 6838, maybe a revision) about how to process these if
we're going to allow them.  One might think that "+json+zstd", for example,
would do the "obvious" thing, but that's not the kind of thing that should
be left to interpretation.

More generally, if I were to see application/foobar+a+b+c, should I be
looking for one suffix called "+a+b+c", or three individually named
suffixes, or other combinations, and in what order would they be applied,
etc.?

-MSK