Re: [dispatch] IANA Registry for parameters for the MIME header field "Content-Type" ?

[[e-mail]]

John C Klensin <[e-mail]> writes:
> (1) There was, at least as I recall, never a plan for parameters
> to be shared, using the same keywords, value set, and semantics
> across content-types, especially top-level content types.
> Unless we intent to change that, it is not clear what value a
> registry might have.

Perhaps this is just my ignorance, but by default, I would expect a
registry for such parameters to be little more than a mapping from
parameter names to the RFCs that define their usage.

I would say that such a registry would be useful, unless that mapping is
"straightforward", by which I mean that one can *always* find the
definition of a parameter relative to a media type by looking at:

   1. The definition of the top-level type, pointed to by the text at
   the top of
   https://www.iana.org/assignments/media-types/media-types.xhtml

   2. The definition of the subtype involved, pointed to by the
   subtype's entry in media-types.shtml.

   3. Whatever parameters are defined as "parameters of the Content-Type
   header field" in RFC 2045 and its updates rather than "parameters of
   the media type".  There appear to be none of these, per sec. 5:

       There are NO globally-meaningful parameters that apply to all media
       types.

OK, and looking at the list I just wrote, it does appear that the mapping
*is* straightforward.

Which leads me to pop up a level and note that the Subject in this
thread is "IANA Registry for parameters for the MIME header field
'Content-Type'".  But in fact, the parameters aren't defined as parts of
the "Content-Type" header field, they're parts of the media type, which
is the value part of the header field.

Then again, there is the fact that a registry would be reasonably
complete, and if one wanted a complete list, one would not have to scan
the 800+ non-vnd. registered media types.  But is there a use for such a
complete list?  In a MUA, one could not implement a parameter for a
media type without understanding the specific semantics for that
parameter for that media type, which requires examining all relevant
media type registrations.

Dale