Re: [sipcore] draft-holmberg-sipcore-proxy-feature- why not just use Supported

Hadriel Kaplan <> Wed, 10 November 2010 06:38 UTC

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From: Hadriel Kaplan <>
To: Cullen Jennings <>
Date: Wed, 10 Nov 2010 01:37:58 -0500
Thread-Topic: [sipcore] draft-holmberg-sipcore-proxy-feature- why not just use Supported
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Subject: Re: [sipcore] draft-holmberg-sipcore-proxy-feature- why not just use Supported
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Not to contradict myself, but hey it's a new week. :)
A rationale for it could be the cases in which a proxy is a proxy, but switches to b2bua behavior in certain cases. (Paul's regular reminder that being a proxy vs. a b2bua is a dynamic role not a permanent system type)  For example a proxy doing privacy service, or 3pcc for certain call cases, or a generating BYE due to rfc4028 timeouts.  So in the Register/Invite such a proxy would be a proxy, while during certain session/events it may behave as a b2bua?

But it would be good to see/discuss/explain/flesh-out example use-cases. (i.e., "the problem")


On Nov 9, 2010, at 3:42 PM, Cullen Jennings wrote:

> So, in my week of agreeing with Hadriel, I think he got it exactly right when he said there is pretty much no feature a "plain" proxy would need for this. If we are talking about things that would be a strict technical read be considered B2BUA even if they are implemented a lot like a proxy, then I can why something provided something like this functionality would be needed. 
> But given this would be used by a B2BUA, I don't see why you need anything more than just normal option tags. Say a SIP message comes from A to B to C and now the response is coming back. C says it supports feature c1, c2, and c3. B knows that it can transparently forward on whatever is needed for c1, c2, but not c3 and it knows that in additional to this it can do features b4 and b5. It modifies the Supported header to have c1,c2,b4, and b5. and sends that back to A. 
> If the real uses cases have to do with caller pref features, then B can modify those in the same way. 
> Anyways, I think we are way way ahead of ourselves discussing mechanism before we even understand what the use cases are we are trying to solve. I'd like to see some specific real world use cases and so we can work out the real requirements. I'm expect the uses cases will contain B2BUA in the call flows - that's reality. 
> On Sep 24, 2010, at 5:04 AM, Christer Holmberg wrote:
>> Hi,
>> I've submitted a draft, which extends the rr-param rule, allowing proxies to indicate supported features using feature tags in Path, Record-Route etc.
>> The draft can also be found at:
>> As I indicated earlier on the list, and as you can read in the draft, there is a 3GPP use-case where we believe the mechanism could be used. But, there is nothing 3GPP specific about the mechanism as such.
>> Regards,
>> Christer
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