Re: [rtcweb] Draft proposal for updating Multiparty topologies in draft-ietf-rtcweb-rtp-usage

Harald Alvestrand <> Fri, 02 May 2014 09:03 UTC

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Date: Fri, 02 May 2014 11:03:06 +0200
From: Harald Alvestrand <>
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Subject: Re: [rtcweb] Draft proposal for updating Multiparty topologies in draft-ietf-rtcweb-rtp-usage
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On 04/30/2014 11:56 PM, Cullen Jennings (fluffy) wrote:
> On Apr 15, 2014, at 9:14 AM, Magnus Westerlund <> wrote:
>> This limitation means that
>>    some of the RTP middlebox-based topologies are not suitable for use
>>    in the WebRTC environment.  Specifically:
>>    o  Video switching MCUs (Topo-Video-switch-MCU) SHOULD NOT be used,
>>       since they make the use of RTCP for congestion control and quality
>>       of service reports problematic (see Section 3.8 of
>>       [I-D.ietf-avtcore-rtp-topologies-update]).
> I think this is deserving of some WG discussion as people may not be up to speed of what this is allowing or not allowing. My understanding was several companies at the last WebRTC Expo conference were demonstrating system that used this type of MCU.
> If SRTP were more flexible and there was a way to a mixer work without giving it the keys to the decrypt the media, I think people would be keener on mixers but right it seems like the pro / cons invovle a trade off between significant security functionally loss and possible loss of some RTCP data which many systems totally ignore.  Anyway, not taking any opinion other than this seems like a significant enough change to have some discussion on it.
Seems the discussion of what the effects are belongs to the threads on 

RTCWEB implementations that ignore RTCP are going to break all our 
attempts at congestion control anyway, so I'm inclined to leave them out 
of the spec. They'll break on their own.