I also want to point out that we really need to work on our messaging and perception that URI extension relation type are in any way inferior or "less cool" than the short registered ones. The sooner protocol authors accept this as a perfectly valid choice for their protocol needs, the better the link framework will be.
I couldn't have said it better myself. Basically what you are saying by requesting a short relation is that you actively intend to share the relation with others with a view to improving interoperability. By using a URI you control you're saying you want it to have a specific, concrete meaning (while retaining the option of sharing). The more I hear the more it sounds like PubSubHubbub wants the latter and with that in mind I would suggest that http://pubsubhubbub.org/
is as good an identifier as any.
Imagine a feed advertising both rssCloud and PuSH - without clarification of the "hub" relation (for example by way of dedicated content types) clients will have to use protocol discovery or trial and error to work out what to use. It doesn't help that it's not so much a content type we're talking about but a protocol, in which case a [registered] service name would be a better fit (and even then registered service names are generally used in conjunction with well-known port assignments rather than "everything over HTTP").
I have a similar problem with OCCI
whereby I want to advertise console access to virtual machines via ssh, vnc-server or ms-wbt-server - I guess we'll have to assign a separate link relation (like http://purl.org/occi/console#ssh
) for each protocol, which is a bit of a hack when e.g. a "service=ssh" attribute would accomplish the same. Conversely content types work fine for advertising console screenshots in e.g. PNG or JPEG.
Mark: perhaps you could consider including some wording like Eran's and/or mine above into the Web Linking draft so we can point at it later? I would also suggest that the "broadly useful" test needs some refinement - the implication in rejecting applications is that the applicant's work is not "broadly useful" while what we mean by this is that it is useful for multiple/many standards.