Subject: Re: [Sip] UA Loose Routing Issue #1: Backwards compatibility

From: [email-masked]
Date: Wed, 8 Aug 2007 14:33:17 -0400

This is delayed, but I haven't heard of this point being addressed:

   From: Jonathan Rosenberg <[email-masked]>

   > On a more practical level, it only deals with one level of address
   > mapping, and it's not usable by a UA that doesn't implement the change
   > -- whereas I would expect that the optimal mechanism is one that
   > tweaks the behavior of the proxy doing the mapping, and requires no
   > change from any other system.

   If the purpose of this is to communicate the targeted URI from the 
   terminating proxy to the UA, I don't see how any mechanism can work 
   unless it is supported by that proxy and that UA. Any of the mechanisms 
   mentioned or discussed (P-Called-Party-ID, To, History-Info) have this 
   property.

The property I'm looking for is "If the proxy applies the mechanism,
but the UA doesn't understand it, the UA automatically processes the
request as if the mechanism was not employed."  If we carry the
"retargeted AOR" in a URI parameter or header and modify the
request-URI as now, we obtain this property.  If we don't modify the
request-URI, the proxy has to have accurate knowledge of which targets
support UA Loose Routing and which don't.

Dale

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From: Paul Kyzivat <[email-masked]>
Date: Wed, 08 Aug 2007 15:21:50 -0400

Dale,

What problem are you trying to solve that Jonathan's proposal didn't solve?

As proposed by him, the proxy wouldn't use loose routing in this case 
unless it knew that the UA supported it. (It learns based on the ;lr in 
the contact supplied with REGISTER.)

The functional difference between that and P-Called-Party is that when 
the proxy uses P-Called-Party it supplies the information even to UAs 
that don't support it. While that doesn't cause any harm, it also 
doesn't provide any benefit.

	Paul

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From: [email-masked]
Date: Thu, 9 Aug 2007 14:34:55 -0400

From: Paul Kyzivat <[email-masked]>

   What problem are you trying to solve that Jonathan's proposal
   didn't solve?

It's to avoid complexifying SIP by adding variants that need to be
negotiated between agents.  We've already got a set of rules that let
agents add information into messages so that it will be ignored by
recipients that don't understand it.  I think it is less desirable to
add a tweak that is negotiated between two agents, such that properly
interpreting and processing messages requires that you know whether
"UA loose routing" is in effect or not.  At the least, you have to
beware of this alternative whenever you propose any further addendum
to SIP.  When the next such tweak comes along, you'll get a total of 4
combinations, etc.

Dale