Subject: [rmcat] Doubt regarding REMB and SSRCs

From: Iñaki Baz Castillo <[email-masked]>
Date: Wed, 1 Mar 2017 14:49:31 +0100

Hi,

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
like?

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
<[email-masked]>

---

From: Iñaki Baz Castillo <[email-masked]>
Date: Wed, 1 Mar 2017 15:16:38 +0100

> message mean. According to the draft [*]:

I've just found a related thread:

https://mailarchive.ietf.org/arch/msg/rmcat/t71fzDtaQatyi2yA9wa0yRfhXHI

-------------------------------------------------
>> the maximum bitrate, is it defined as the maximum bitrate for  a particular stream, or the maximum bitrate for the whole "Session"?

> It depends on how many and which SSRCs are reported in the feedback message. As per the draft, it is up to the implementation (or the congestion control algorithm) to choose which SSRCs, i.e., it could report just the SSRCs of audio or video or a combination.
-------------------------------------------------

So it seems that it's up to the sender to re-distribute the estimated
bit rate across all the indicated SSRCs (if many in a single REMB),
right?

-- 
Iñaki Baz Castillo
<[email-masked]>