Re: [sipcore] Draft new: draft-holmberg-sipcore-proxy-feature [was: Feature-tags in the Path header field]

"Andrew Allen" <> Fri, 15 October 2010 13:58 UTC

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Date: Fri, 15 Oct 2010 08:59:58 -0500
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Thread-Topic: [sipcore] Draft new: draft-holmberg-sipcore-proxy-feature [was: Feature-tags in the Path header field]
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From: "Andrew Allen" <>
To: "Hadriel Kaplan" <>, "Peter Musgrave" <>
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Subject: Re: [sipcore] Draft new: draft-holmberg-sipcore-proxy-feature [was: Feature-tags in the Path header field]
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And there I was thinking that SBCs were these amazingly flexible devices
that could remove, modify or add any SIP primitive to any message based
on the service providers desires - but now it turns out that in reality
that they can't even add a feature tag to a routing header!


-----Original Message-----
From: [] On
Behalf Of Hadriel Kaplan
Sent: Tuesday, September 28, 2010 9:29 AM
To: Peter Musgrave
Subject: Re: [sipcore] Draft new: draft-holmberg-sipcore-proxy-feature
[was: Feature-tags in the Path header field]

On Sep 27, 2010, at 7:14 PM, Peter Musgrave wrote:

> Hi,
> I read the draft and I think the semantics are ok (although John's
> point about multiple proxies with these tags is a good one).
> One use which occurred to me - but which is no doubt controversial  -
> is whether a proxy/B2B which is an SBC could provide a feature tag
> indicating it is an SBC - so that UAs which register and call through
> it could skip adding ICE candidates. (My point is merely that I
> suspect other uses for this capability will be found - even if you
> think this  particular idea is mis-guided).

Right, that particular idea is mis-guided. :)

What you'd really do for that is indicate whether or not the b2bua
supports ICE (since some SBC's do, and some don't, indicating you're an
SBC doesn't help).  

There already is a feature tag for ICE (""), but such a thing
would break immediately - since ICE is a new thing, legacy SBC's
wouldn't know to say they don't do it, and there is no "!(feature-tag)"
anyway; and the UA doesn't know whether it's talking through SBC's or
plain proxies to begin with, and if they're plain proxies they wouldn't
say anything about ice ever... so the UA can never guess this right.  If
it gets back "" then it knows for that one hop that said so it's
ok, but it knows nothing about the rest.  


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