Re: [rtcweb] Working Group Last Call for draft-uberti-rtcweb-rfc8829bis-01.txt

[Christer Holmberg <[e-mail]>]

Hi,

What about something like this:

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OLD:

“The Session Initiation Protocol (SIP) [RFC3261] allows a User Agent Client (UAC) to send a re-INVITE request without an SDP body (sometimes referred to as an empty re-INVITE).
In such cases, the User Agent Server (UAS) will include an SDP Offer in the associated 200 OK response. This is typically used for 3rd Party Call Control (3PCC) scenarios.
From a BUNDLE perspective, such SDP Offer SHOULD be generated using the procedures defined in Section 7.2.”

NEW:

“The Session Initiation Protocol (SIP) [RFC3261] allows a User Agent Client (UAC) to send a re-INVITE request without an SDP body (sometimes referred to as an empty re-INVITE).
In such cases, the User Agent Server (UAS) will include an SDP offer in the associated 200 OK response. This is typically used for 3rd Party Call Control (3PCC) scenarios.

In some 3PCC scenarios the UAS will be part of an ongoing session, and will therefore include a subsequent offer in the 200 OK responses. The offer will be
received by a 3PCC controller (UAC) and then forwarded as an initial offer to another User Agent (UA) that is currently not part of a session.

When the BUNDLE mechanism is used, as an initial BUNDLE offer look different than a subsequent BUNDLE offer, it cannot be assumed that a UA that expects an initial offer
will be able to properly process a subsequent offer. Therefore, the 3PCC controller needs to act as a Back-To-Back User Agent (B2BUA), and when it receives the subsequent
offer it needs to rewrite it into an initial offer before it is forwarded to such UA.”

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Regards,

Christer








From: Roman Shpount <[e-mail]>
Sent: tiistai 2. marraskuuta 2021 10.41
To: Justin Uberti <[e-mail]>
Cc: Christer Holmberg <[e-mail]>; Justin Uberti <[e-mail]>; RTCWeb IETF <[e-mail]>
Subject: Re: [rtcweb] Working Group Last Call for draft-uberti-rtcweb-rfc8829bis-01.txt

On Mon, Nov 1, 2021 at 2:52 PM Justin Uberti <[e-mail]<mailto:[e-mail]>> wrote:
The PROBLEM is that we have two endpoints, where one sends a subsequent offer, and the other one expects an initial offer. What do you normally do when you have that kind of problem? You use an SBC/B2BUA. In this case that SBC/B2BUA would be the 3PCC controller.

So, my suggestion would be to remove the SHOULD text from 8843bis, and simply add a note somewhere (in 8843bis and/or 8829bis) which describes the issue and says that the 3GPP controller needs to modify the offer accordingly.

Roman, thoughts on this? If the 3PCC is going to rewrite the offer SDP anyway then maybe adding a=bundle-only isn't the end of the world.

I am not opposed to this idea. 3PCC typically knows that the subsequent offer is going to be used as initial, and should be able to rewrite the offer to make it valid. We can change SIP Considerations section in 8843bis (https://www.ietf.org/archive/id/draft-ietf-mmusic-rfc8843bis-05.html#name-sip-consideration), remove the SHOULD, and specify that 3PCC controller should fix the offer. We can then reference this note from 8829bis or restate the same guidance.
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Roman Shpount