RE: [Simple] Presence Data Model: Identifying services

[Markus.Isomaki at nokia.com]

Hi Stefan,

Comments back to you inline. 

(Yes, I absolutely agree with you on the usefulness of the Presence Data Model draft and that it should have been the starting point, at least right after PIDF.)

> -----Original Message-----
> From: simple-bounces at ietf.org 
> [mailto:simple-bounces at ietf.org]On Behalf
> Of ext Goeman Stefan
> Sent: 23 September, 2004 14:42
> To: 'simple at ietf.org'
> Subject: RE: [Simple] Presence Data Model: Identifying services
> 
> 
> Hello,
> 
> Comments inline
> 
> P.S.: In general, I find the Presence Data model draft a good 
> document. This
> is how things should have started in the first place.
> 
> Greetings,
> Stefan.
> 
> > -----Original Message-----
> > From: simple-bounces at ietf.org 
> > [mailto:simple-bounces at ietf.org] On Behalf Of 
> Markus.Isomaki at nokia.com
> > Sent: donderdag 23 september 2004 10:10
> > To: simple at ietf.org
> > Subject: [Simple] Presence Data Model: Identifying services
> > 
> > 
> > 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.
> I don't think I understand you here. I would expect that if 
> both end-point
> SIP US support exactly the same methods, the SIP UA should 
> also be able to
> handle those SIP method. Or, to phase it in another way, both 
> end-point
> should be able to communicate with each other only based on 
> using these
> capabilities.
>

To some extent that might be possible, but there might be differences e.g. in floor control etc., which make the service still practically unusable, which means that the presence document shouldn't give the impression that the communication will work. Obviously it is possible to list all those additional characteristics as well (and this might be a good idea too), but for instance if presence of a network game is published, it is easier to _explicitely_ tell what the game is by providing some sort of unique id which would be only meaningful to those similar game clients.  
 
> > (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 also have the impression that it will be difficult to determine the
> services merely based on the SIP methods (and ...) supported 
> by the SIP UA.
> It possibly might mean that the SIP-based ptt-service on my 
> mobile phone
> might be mapped onto some communication service on your PC. 
> Then, I wonder
> who will be responsible for such kind of mapping? Only the 
> end terminals, or
> is support from the network, presence server, necessary? And, 
> how will one
> service in one domain be mapped into a service in another 
> domain? I believe
> there are some open issues here. 
>

I think that it is fully OK to describe some rather simple things using characteristics, but in many cases applications are more complex.
    
> > 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. 
> > 
> Do you have an example of a service that uses SIP and also has non
> SIP-features? If your service also uses non-SIP features, you can not
> determine the type of service by only looking at the SIP capabilities
> supported by the SIP UA. Some indication of that non-SIP 
> feature should also
> be present in the presence document. 
> 

True, and since some of those non-SIP features might not be based on (IETF) standards, it might be good to have the possibility for the developers of such applications to have a short cut of saying what the app really is, rather than having to list characteristics. In the end, some applications are really not even meant to work with any other than the exactly same application, for instance some networking games. 

> > Of course each of these proprietary services are allowed to 
> > define additional status or tuple-level extensions to PIDF.
> Yes, you can always extend the PIDF. My comment here is that 
> this might lead
> to an "explosion" of applications or services. It will 
> certainly not help
> interoperability.
>  

And who would standardize such characteristic definitions. It has already taken a couple of years to standardize even "prescaps", which is just a starting point. 

> > 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 general, I could agree with that approach. However, I 
> don't see a reason
> why the vendor specific SIP-based PTT should be different 
> from the OMA based
> PTT. Simply define one PTT, based on the SIP UA capabilities.
> 

Well, for instance Nokia has a prorietary PTT application, which uses SIP INVITE etc., so users are essentially addressed with sip: URI. However, it has some floor control, media etc. features which make it basically non-interoperable with clients that are not specifically following that proprietary spec. 

OMA PTT is closer to standard IETF SIP. Unless there is some conversion, it does not work with Nokia PTT directly. There should be some way for distinguishing these two in a presence document, so that there won't be confusion. I think the current idea is to do some kind of proprietary PIDF extensions, but in my opinion a standard XML element in RPID to tell this kind of information would be better, also for the purposes of authorization rule making.

> > 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. 
> Personally, I don't think it is a good idea that an end-user 
> should make
> such decission. If I get a presence document that indicates 
> that another
> person is able to receive some type of communication, I 
> expect that that
> type of communication would actually work when I use it.
> 

Exactly right. If you are an application who does not necessarily aim for interoperability, but still uses sip:, INVITE etc., you should be able to explicitely say so.

> >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?
> > 
> I think more care should be taken in mapping SIP UA 
> capabilities into actual
> services. That is not so clear to me!
>

The idea is nice, but I don't see a problem in _addition_ defining the application with something like "com.openmobilealliance.ptt" or "com.nokia.ptt", if that kind of profile would have more meaning than the list of characteristics.
 
> 
> > Regards,
> > 	Markus 
> > 
> > 
> > _______________________________________________
> > Simple mailing list
> > Simple at ietf.org
> > https://www1.ietf.org/mailman/listinfo/simple
> > 
> 
> _______________________________________________
> Simple mailing list
> Simple at ietf.org
> https://www1.ietf.org/mailman/listinfo/simple
> 

Markus

_______________________________________________
Simple mailing list
Simple at ietf.org
https://www1.ietf.org/mailman/listinfo/simple