[AVTCORE] Re: BUNDLE question - relationship to PR-Answer?

[Christer Holmberg <[e-mail]>]

Hi,

I am sorry for the late response. I don't actively monitor the AVTCORE list, so....

First, it is important to keep in mind that BUNDLE is based on SDP offer/answer. As others have pointed out, it means that there is one offer and one associated answer (or multiple SIP dialogs, with one answer in each dialog). 

There is no "PR-ANSWER" concept in SDP offer/answer, or in BUNDLE.  Section 7.3.2 is about moving an m= section out of a BUNDLE group in such single SDP answer to an offer - it is NOT about first sending an answer with BUNDLE, and then another answer (to the same offer, as part of the same SIP dialog) where the BUNDLE group is removed.

Also note that, as described in Section 7.3.2, even if there is only a single answer, there are cases where you can not remove the BUNDLE group with the answer, without having to reject the answer.

Regards,

Christer



> -----Original Message-----
> From: Harald Alvestrand <[e-mail]>
> Sent: Tuesday, 28 April 2026 11.47
> To: [e-mail]
> Subject: [AVTCORE] Re: BUNDLE question - relationship to PR-Answer?
> 
> On 4/22/26 16:34, Paul Kyzivat wrote:
> > Roman,
> >
> > On 4/21/26 11:14 PM, Roman Shpount wrote:
> >> Paul,
> >>
> >> I think I can provide some context here. WebRTC acts similarly to a
> >> physical SIP UA when placing a call. When multiple parallel dialogs
> >> are created for a single outbound call, it is hard to represent this
> >> to the user. At the same time, if you get several dialogs that occur
> >> one after the other, it should be possible for a normal SIP UA to
> >> handle this gracefully, discard the dialog created by the previous
> >> provisional answer, and re-run the offer/answer for the new dialog,
> >> and present the user with what appears to be a single call.
> >>
> >> So, in case Harald is asking about, the case
> >>
> >> Offer: BUNDLE(a, b)
> >> PR-Answer: BUNDLE(a, b)
> >> Answer: No bundle
> >>
> >> should be handled the same as
> >>
> >> Offer: BUNDLE(a, b)
> >> Answer: No bundle
> >>
> >> The use case is that the call connects to a user's phone and
> >> establishes an early media session with bundle to play ringback. The
> >> call to the end user device times out, and the call is connected to
> >> an answering service that does not support bundle to record the message.
> >
> > The original question didn't say whether the PR-Answer and Answer were
> > in the same fork or different forks.
> >
> > *If* they are in different forks then then I agree.
> 
> That's the problem in discussing this question ... the concept of "forks" does
> not exist in WebRTC, but PR-Answer was invented to be able to support some
> types of forks (as part of enabling SIP support).
> 
> We're likely to have to update the WebRTC specs to reflect the answer to this
> question.
> 
> 
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