[Simple] Presence Data Model: Identifying services

[Markus.Isomaki at nokia.com]

Hi all,

I have a comment on Presence Data Model draft about the identification/characterization of services. (I know this has been discussed for a while already, but I still think the current way is somewhat unadequate.)

The current Data Model draft says:

   In this model, services are not explicitly enumerated.  That is,
   there is no "service" attribute with values of "ptt" or "telephony".
   Rather, the service is identified in one of two ways.  In many cases,
   the URI scheme is a clear indicator of service.  An "sms" URI clearly
   indicates SMS, and a "tel" URI clearly indicates telephony.  For some
   URIs, there may be many services available, for example, SIP.  For
   those services, each service has a set of characteristics, each of
   which has a well-defined meaning, such that a system can
   unequivocally determine whether or not the service has that
   characteristic.  This is discussed in more detail in [5]. 

I agree that the contact URI scheme is the most important piece of information to distinguish what is the "service". (There are some gaps here too, since e.g. the URI schemes for SMS or MMS do not even exist, and there may be other non-IETF protocols that face the same problem.) However, I'm not convinced that listing the signaling/media characteristics of the end-point or service really gives enough information to the watcher to really determine if it can communicate with the advertised service/application. 

The refenced draft [5] has a following example:

5.6 Walkie-talkie

   The walkie-talkie service allows real-time voice communication
   between participants.  Only one participant can speak at a time; that
   is, communication is half-duplex.  Typically, participants press a
   button to indicate that they are ready to speak, although other
   mechanism (e.g.  voice activation) are occasionally used.

   Support for the Walkie-Talkie service can be deduced by observing the
   presence of a contact URI with a scheme of "sip:", associated with
   the following minimal set of capabilities: <audio>true</audio>
   <duplex>half</duplex> <methods>INVITE,ACK,BYE,CANCEL</methods>

Presumably this is the same as the service sometimes called Push-to-Talk (PTT, PoC). The concrete problem is that there are several _non-interworking_ variants of this service, that still all would match the characteristics listed above, as the _SIP_ part might still be standard, at least for some methods etc. But still the communication would fail even if the tuple looks OK. (Also if I saw the characteristics above, I could as well determine that they describe normal VoIP application running in (very) some old PC with half-duplex audio drivers, so I claim the service description is hard to make unambiguous in the first place.)  
  
I think the main point of a service tuple is to give the watcher enough information to know whether he can really communicate & interoperate with the advertised service. Given that many services taht are using SIP also have propritary or non-SIP features, I don't think the current approach is enough. 

Of course each of these proprietary services are allowed to define additional status or tuple-level extensions to PIDF. However, I would like to have some kind of "service identifier" as part of the basic framework so that this could be done in a consistent manner. I think it would help especially in making of authorization and composition rules more simple. The current "class" attribute is way too loosely defined.

So the concrete proposal is to include in RPID a "service id" element that would have a vendor-specific namespace, similar to e.g. vendor-specific XCAP AUIDs. So for instance the SIP-based PTT app from vendor xyz.com would have, e.g. 

<service-id>com.xyz.ptt</service-id>

while the PTT app compliant with OMA would have, e.g.

<service-id>com.openmobilealliance.ptt</service-id>

in _addition_ to describing sip:, half-duplex audio, and the supported methods. Now:
1.) Watcher having one of those apps could see right away whether it is possible to communicate
2.) Composer getting PUBLISHes from 2 sources could immediately know that they are from a similar app
3.) The app could set its authorization rules using the unique id.

The main downside I can see in this approach that if there are two different proprietary apps that indeed could communicate at least partially, the watcher might make a conclusion that communication is not possible baed on different service-id. However, in this case the watcher still would have the characteristics visible and could determine that some interworking could be achieved. For this kind of reasons there should be careful text about when to use service-id in the first place, and what can be determined from it.

Does someone think that this is an absolutely bad idea? If so, how would you envision solving the issues discussed above?

Regards,
	Markus 


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