[rmcat] Doubt regarding REMB and SSRCs

Iñaki Baz Castillo <ibc@aliax.net> Wed, 01 March 2017 13:49 UTC

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Date: Wed, 1 Mar 2017 14:49:31 +0100
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Subject: [rmcat] Doubt regarding REMB and SSRCs
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It's not clear for me what the "SSRC feedback" entries in the REMB
message mean. According to the draft [*]:

  Num SSRC (8 bits):  Number of SSRCs in this message.

  SSRC feedback (32 bits): Consists of one or more SSRC entries which
  this feedback message applies to.

At the same time, the draft says:

   This feedback message is used to notify a sender of multiple media
   streams over the same RTP session of the total estimated available
   bit rate on the path to the receiving side of this RTP session.

The meaning of "RTP session" is hard to explain nowadays, so I'll
assume it means "a media transport" (which in the context of WebRTC
points to an ICE established "connection" which can carry multiple
SSRC streams).

So, if for example Alice is receiving audio (SSRC 1111) and video
(SSRC 2222) over a BUNDLEd transport, and assuming Alice can just
receive 500 kbits/s, how should REMB messages generated by Alice look

Should these REMB messages contain both SSRC 1111 and SSRC 2222 and
indicate 500000 as media bit rate?

What would happen if Alice sends a REMB just including the SSRC 1111
and 500000 as media bit rate? Should Bob limit its audio bitrate up to
500kbit/s and send whatever he wants regarding its video stream?

Thanks a lot.

[*] https://tools.ietf.org/html/draft-alvestrand-rmcat-remb-03

Iñaki Baz Castillo