[R-C] Feedback mechanisms

[John Leslie]

Stefan Holmer <stefan at webrtc.org> wrote:
> 
> Worth noting that we have the option of only sending frequent feedback
> (e.g. once per packet) when we experience congestion. The rest of the time
> we can simply trigger the feedback periodically.

   This, alas, is optimizing the wrong thing.

   We can, indeed, send very-short feedback when no congestion is evident
to the receiver. But in the absence of a heartbeat, we're at the mercy
of Murphy's law whether the signal of congestion detected reaches the
sender. (Indeed, one of the likely sources of congestion is a wireless
link which is likely to show congestion in both directions.)

   Thus, I recommend continuous feedback on a heartbeat schedule, so that
the absence of feedback could start a backing-off of send rate. We're
working in unreliable IP packets, not reliable "circuits".

   The feedback in the absence of congestion could be essentially zero
length -- and I hope we will consider folding it into a reverse path
media packet (where the "receiver" detecting congestion is "sender" of
a corresponding media stream in the other direction). This _will_ be
a common case, and quite possibly the most-common case.

--
John Leslie <john at jlc.net>