I would like to hear what other ietf-types participants think about this.
My opinion is that content negotiation for versioning capability using MIME type parameters is unworkable. The use of the MIME type is to describe the payload sufficiently so that you know HOW to process it, but the content-type label cannot (and thus should not) be extended in an attempt to use it to determine WHETHER a given processor knows enough to be able to process it. So whether version information should or should not be in a media type parameter depends pretty much on whether there is an embedded, easy-to-find version indicator in the data itself; if there isn't, and processors need to know the version to choose between different processing methods, then the version parameter should be mandatory. There is no strong use case for an optional version parameter, or in general for duplicating, in MIME parameters, information that is readily obtained from the content itself. If you want to do content negotiation, then consider using media features and media feature negotiation; with media features you can negotiate not only version information, but other parameters that might also be necessary to know in order to determine interpretability, e.g., availability of compression modes, codecs, fonts, color capabilities, buffer size limitations, etc. Larry -- http://larry.masinter.net