Re: meaning of tel URI in presence data model, was: Re: [Simple] Presence Data Model: Identifying services

[Henning Schulzrinne <hgs at cs.columbia.edu>]

> Yes, but the scope of that indirection is bounded with the SIP URI. At 
> least it applies to a multimedia communications service of some sort, 
> and we can include attributes that say something meaningful about what 
> it means. We can describe methods and extensions and all of the prescaps 
> stuff that help us say what we mean.

Nothing prevents a SIP redirect server from returning a mailto Contact, 
for example, so there doesn't seem a fundamental difference except of 
how the indirection occurs, whether by DNS or SIP. Even the SDP could 
contain some kind of chess application sharing protocol, which is hardly 
more meaningful than mailto to somebody that only expects audio.


> I can't do any of that for tel if you use it as a name. It could be a 
> mailto URL. Could be http. Could be sip. Could be nntp. How do you 
> define useful attributes in that context?

Couldn't you do this with prescaps in the presence document?

> My point is, those attributes are going to be wildly different for the 
> http URL that you get out of enum, compared to the sip URL. How can you 
> characterize the service associated with a name, when the name can 
> resolve to any service??

I don't see the difference. Both URLs can be described using prescaps. 
For both, you can get redirected to any resource whatsoever, including 
HTTP and email. In both cases, what's advertised in prescaps may or may 
not correspond to what's actually available.


> The nice thing about prescaps is that I can say things about SIP 
> capabilities. What do service independent capabilities mean? Can you 
> give an exmaple?

Are you suggesting that prescaps only be used for SIP URIs? Why couldn't 
they be used for tel and h323 URIs, to give two obvious examples? 
(Clearly, you couldn't use them there to restrict service, but only to 
describe service. That's a somewhat different problem, which has more to 
do with the limitations of H.323 than URI schemes.)

Henning

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