Re: [AVTCORE] Multi-party real-time text requiring new RTP payload type

Gunnar Hellström <> Fri, 24 April 2020 08:55 UTC

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To: "Dale R. Worley" <>
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Date: Fri, 24 Apr 2020 10:55:04 +0200
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Subject: Re: [AVTCORE] Multi-party real-time text requiring new RTP payload type
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Den 2020-04-24 kl. 03:22, skrev Dale R. Worley:
> Gunnar Hellstrom <> writes:
>> Thanks, a good question. It is the right time to consider which source
>> multiplexing and reliability method to select, now when we need to
>> introduce a new negotiation.
> My idea wasn't that I had any better idea, but rather that there were
> some obvious ideas, and clearly, people who had studied the matter found
> that the obvious ideas were not the best.  What I really meant was that
> it would be useful to give brief documentation why the obvious ideas
> aren't best.

Your question provided a good opportunity to review solution alternatives.

Since the effort to specify multi-party real-time text aims at additions 
to current implementations of bridges and endpoints I think one 
requirement should be to keep the solution as close to the current 
solutions for the other media handles by these bridges and endpoints.

NG9-1-1, NG 112 and 3GPP CONF services specify use of the RFC 4535 SIP 
conference framework. Adding new participants in that framework usually 
does not cause any new sdp interaction with the other participants, as 
long as no new media is added. Just a notification is sent. I assume 
that it can be good to keep that way of action also for adding 
participants with real-time text. Using the RTP mixer model adding just 
members in the CSRC lists is then the most simple model. So, this is yet 
another reason for why to keep to the RTP-mixer solution for the first 
specification in the draft-hellstrom-avtcore-multi-party-rtt-source 
draft. It might be followed by other selected cases in the 
draft-hellstrom-avtcore-multi-party-rtt-solutions draft, where WebRTC 
and use of sdp-bundle can be spcified, which are out of scope for the 
first case.

Even if different solutions are discussed in the 
draft-hellstrom-avtcore-multi-party-rtt-solutions draft, I accept your 
point and will include a brief motivation also in the 
draft-hellstrom-avtcore-multi-party-rtt-source draft.

Comments are very welcome about the reasoning.



> Dale

Gunnar Hellström