Retransmission rate limiter logics

[brown zhang]

After investigating the code logic related to packet retransmission in WebRTC, I found that the bandwidth limitation for retransmission on the sender side is completely independent. Additionally, the maximum bandwidth for retransmission is also equal to the target rate (which is the same as the REMB bandwidth reported by Homer). This means that theoretically, the maximum actual bandwidth sent by the sender could be twice the target rate.

Since our server (SFU) uses REMB to report the bandwidth to the sender, this poses a challenge for evaluating the REMB bandwidth. To provide a more accurate estimation, it may be necessary to evaluate the bandwidth used by retransmitted packets (lost packets) on the server side and subtract it from the reported REMB bandwidth given to the sender. Another option is to consider stopping the server from sending NACK (Negative Acknowledgment) when there is a significant change in network bandwidth.

Do you have any better suggestions for this issue? Thank you.

[spr...@webrtc.org]

Note that the BitrateAllocator class will subtract the protection bitrate when allocating target media rates in order to compensate for retransmission/fec etc. There's some lag in that feedback loop - but on average the target rates shouldn't overshot.

That said, doing smarter handling of NACK requests at the server isn't a bad idea :)

[Harald Alvestrand]

For realtime video, if the server notices packet loss so great that getting the retransmissions would consume a significant part of the bandwidth, it is likely that the right solution is to reduce the bandwidth estimate considerably and send a FIR (keyframe request); all those retransmitted packets are unlikely to get there in a timely manner.

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