Re: [AVTCORE] I-D Action: draft-ietf-avtcore-multi-party-rtt-mix-00.txt

Gunnar Hellström <> Fri, 08 May 2020 21:18 UTC

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From: Gunnar Hellström <>
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Date: Fri, 08 May 2020 23:18:09 +0200
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Subject: Re: [AVTCORE] I-D Action: draft-ietf-avtcore-multi-party-rtt-mix-00.txt
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Thanks for the support,

draft-ietf-avtcore-multi-party-rtt-mix-00 has the same contents as 

I have notes of some issues that I want to act on now.

Some are related to questions that need rapid responses from the 
different areas where this draft is intended to be feasible to be 
applied:  NG9-1-1, NG112, 3GPP IMS MTSI, IETF RUM, and plain SIP 
multimedia calls.

1. Consider rapidly if there is any more reliable transport that is 
feasible to move to.
(e.g. Comedia RFC 4145 and RFC 4572, or the recently approved WebRTC 
t140 data channel draft-ietf-mmusic-t140-data-channel-usage, or use of 
SAVPF with NAK and retransmission RFC 4588)

The answer on issue 1 influences many of the following issues, currently 
written with the assumption that we continue with RTP transport.

2. Add sdp examples

3. Add SRTP hints and OSRTP preference (RFC 8643).

4. Add a brief motivation for the solution and why not rtp-translator is 
used. (or review rapidly the decision to go with the RTP-mixer model, 
three commenters have wondered why we do not use the rtp-translator model.)

5 We have indications  that the format with the strict use of the CSRC 
list to indicate sources also of the redundant parts might not be called 
text/red. A new format (e.g. text/rex) may then need to be registered, 
and sdp negotiation made between text/red and text/rex for 
interoperability reasons. Shall we accept that and move on with text/rex 
and the CSRC list that gives switching performance for 5 participants 
within 500 milliseconds delay (as compared to 3 participants within 600 
milliseconds for the text/red format.

6. If we agree to change format to text/rex, then also consider to move 
to the format proposed by James Hamlin, with primary text from up to 16 
participants in each packet. Provides good switching performance 
increase to 16 simultaneous sources.

7. Describe limitations in the multi-party-unaware case that 
unrecoverable packet loss can hit the label at source switch and 
therefore give the impression that text is coming from another source 
than it is.

8. Make explicit wording about what is to be regarded as update to RFC 
4103. It is important from regulatory points of view in Europe and USA 
that the result can be seen as an update of RFC 4103.

9. Include brief gateway considerations to WebRTC t140 data channel.

Comments are welcome.



Den 2020-05-08 kl. 17:28, skrev
> A New Internet-Draft is available from the on-line Internet-Drafts directories.
> This draft is a work item of the Audio/Video Transport Core Maintenance WG of the IETF.
>          Title           : RTP-mixer formatting of multi-party Real-time text
>          Author          : Gunnar Hellstrom
> 	Filename        : draft-ietf-avtcore-multi-party-rtt-mix-00.txt
> 	Pages           : 25
> 	Date            : 2020-05-07
> Abstract:
>     Real-time text mixers for multi-party sessions need to identify the
>     source of each transmitted group of text so that the text can be
>     presented by endpoints in suitable grouping with other text from the
>     same source.
>     Regional regulatory requirements specify provision of real-time text
>     in multi-party calls.  RFC 4103 mixer implementations can use
>     traditional RTP functions for source identification, but the mixer
>     source switching performance is limited when using the default
>     transmission with redundancy.
>     An enhancement for RFC 4103 real-time text mixing is provided in the
>     present specification, suitable for a centralized conference model
>     that enables source identification and efficient source switching.
>     The intended use is for real-time text mixers and multi-party-aware
>     participant endpoints.  The mechanism builds on use of the CSRC list
>     in the RTP packet.
>     A capability exchange is specified so that it can be verified that a
>     participant can handle the multi-party coded real-time text stream.
>     The capability is indicated by an sdp media attribute "rtt-mix".
>     A brief description about how a mixer can format text for the case
>     when the endpoint is not multi-party aware is also provided.
> The IETF datatracker status page for this draft is:
> There are also htmlized versions available at:
> Please note that it may take a couple of minutes from the time of submission
> until the htmlized version and diff are available at
> Internet-Drafts are also available by anonymous FTP at:
> _______________________________________________
> Audio/Video Transport Core Maintenance

Gunnar Hellström