Re: [Simple] Fwd: Evaluation: draft-ietf-simple-winfo-package - A Session Initiation Protocol (SIP)Event Template-Package for Watcher Information to Proposed Standard

[Jonathan Rosenberg <[e-mail]>]

Sorry for taking so long to respond. They are not obvious issues in 
fact, so I wanted to ponder them a bit and respond after thinking about it.

Anyway, Harald, thanks for the comments, I am glad to have them. 
Responses inline:

Patrik Fältström wrote:

>> The simple-presence document says:
>>
>>> 5 Usage of Presence URIs
>>>
>>>    A presentity is identified in the most general way through a presence
>>>    URI [3], which is of the form pres:user@domain. These URIs are
>>>    protocol independent. They are resolved to protocol specific URIs,
>>>    such as a SIP or SIPS URI, through domain-specific mapping policies.
>>>
>>>    If a subscriber is only aware of the protocol-independent pres URI
>>>    for a presentity, it follows the procedures defined in [5]. These
>>>    procedures will result in the placement of the pres URI in the
>>>    Request-URI of the SIP request, followed by the usage of the DNS
>>>    procedures defined in [5] to determine the host to send the SIP
>>>    request to. Of course, a local outbound proxy may alternatively be
>>>    used, as specified in RFC 3261 [1]. If the subscriber is aware of
>>>    both the protocol-independent pres URI and the SIP or SIPS URI for
>>>    the same presentity, it SHOULD use the SIP or SIPS URI.
>>>
>>>    SUBSCRIBE messages also contain logical identifiers that define the
>>>    originator and recipient of the subscription (the To and From header
>>>    fields). These SHOULD contain SIP or SIPS URIs whenever possible, but
>>>    MAY contain a pres URI if a SIP or SIPS URI is not known or
>>>    available.
>>
>>
>> [5] is D. Crocker et al.  , "Address resolution for instant messaging
>>   and presence," Internet Draft, Internet Engineering Task Force, Oct.
>>   2002.  Work in progress.
>>
>> I assume this is draft-ietf-impp-srv-01.txt (same title!).

Yes.

>> That draft seems to envision DNS entries of the form
>> "_pres._sip.example.com" to look up which host to contact when one 
>> wants to use SIP to contact a presentity named "pres:[e-mail]".
>> There's no mapping from one type of URI to another - in either direction.
>>
>> This has two problems:
>>
>> 1) If a client uses SIP, he presumably has SIP URIs for himself. So he 
>> will use that SIP URI in the From field of a SUBSCRIBE, even though he 
>> also has his own PRES URI.
>> If the SUBSCRIBE has to be gatewayed to a non-SIP domain, per CPIM, 
>> how is the gateway to find the corresponding PRES URI?

This is a good question.

Unfortunately, this is a case which is unavoidable. We are going to run 
into cases where a SIMPLE client has no pres URI at all, and so the From 
field has a SIP URI. Worse yet, there is a great deal of SIMPLE deployed 
today, and most of it (unsurprisingly) using the URI format native to 
SIMPLE, the SIP URI.

So, what would happen at the gateway? I would imagine that, like happens 
in email systems today I believe, that when receiving a SUBSCRIBE 
request with From field sip:[e-mail], the gateway at 
gw.example.com would create a pres URI like this:

pres:[e-mail]

Should someone reply to this, it would route to the gateway, which would 
extract the user part, un-escape it, and send it using the sip URI 
within. Indeed, gateways might do such conversions directly between the 
native URIs of the various systems (SIMPLE and Jabber, for example).

Well, perhaps we would like to avoid this case as much as possible, even 
if it cannot be prevented altogether. So, making it a SHOULD to use a 
pres URI when you know both covers some cases. However, there is a 
practical issue that many existing presence servers don't use the pres 
URI at this time, and so there will be potential interop problems or 
bugs that surface.  No doubt this is a transient problem, but it is very 
possible that it will come up.

Thus, I would ask you, in your experience with other protocols, do you 
feel that given, (1) we will still have the issue you are worried about 
even if we make it a SHOULD to use the pres URI when you know both, and 
(2) there may be short term interoperability and deployment problems, 
that the benefit of this outweighs the cost?


>>
>> 2) If a client starts with the PRES URI, and later learns the SIP URI 
>> of someone, and those two identities part way for some reason (perhaps 
>> the client stops using SIP, or the SIP URI was to a proxy service, or 
>> something) - how does the client learn that it's time to go back to 
>> the PRES URI?

Another good question.

Really, it comes down to how you learn these URI in the first place. I 
might send a SIP SUBSCRIBE to a pres URI, and the Contact header that 
comes back in the 200 OK will definitely be a SIP URI. It identifies the 
specific SIP user agent device I am in contact with. Perhaps you would 
then use this SIP URI instead for future SUBSCRIBE. However, RFC 3261 
intends for the Contact URI to be scoped (in terms of temporal validity) 
to the duration of the dialog (aka subscription). Combing over the text 
in RFC3261 its not fabulously clear on this point, but thats the intent. 
So, there is a built in expiration mechanism which would make it invalid 
for future use. The SIP redirect response could also provide a SIP URI 
for the presence URI. Redirect responses have a built in mechanism for 
declaring the lifetime of the mapping, through the Expires header field.

Alternatively, if you somehow got the SIP URI from DNS when looking up 
the pres URI (note that the SRV spec does NOT give you back the SIP URI, 
but let us assume it did), the DNS TTLs would tell you when it expires.

So, I would argue that the only real way a user can get a hold of both 
the pres and the SIP URI for the user, and not be sure for how long the 
SIP URI is valid,  is if they were handed to that person, on a business 
card, or on a web page. In such cases, I would argue that it is not the 
role of the IETF to dictate which is more authoritative. After all, we 
do not say that if given an H.323 and a SIP URI you must use the SIP 
URI. Or, given a tel URI and a SIP URI, you must use the tel. This 
latter case is actually closer to the pres/sip case, since the tel URI 
can be looked up in DNS to obtain a SIP URI (using ENUM, RFC 2916).

The reason that the draft states a preference here is that within SIP, 
there will be interoperability problems with placeing the pres URI into 
the request URI. In many cases, SIP user agents use an outbound proxy, 
similar to an http proxy. This proxy must understand the request URI 
format in order to forward the request. If I place the pres URI in the 
request URI, and this goes to the outbound proxy, and it doesnt 
understand how to process the pres URI, the subscription will fail. 
Ultimately, our aim is to use our own native addressing format within 
SIP networks, since that promotes maximum interoperability.

So, once more, I would ask - do you think that the benefits of using the 
pres URI in the request URI (which are, that it is possibly more 
authoritative, although I am not so sure), outweigh the drawback of 
potential interoperability issues?

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
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http://www.jdrosen.net                      PHONE: [phone]
http://www.dynamicsoft.com

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