[R-C] Timely reaction time (Re: Comments on draft-alvestrand-rtcweb-congestion-01)

[Harald Alvestrand]

On 04/07/2012 07:37 PM, Michael Welzl wrote:
>>
>> What's the RTO in this case, since we're talking UDP media streams?  
>> TCP RTO?
>
> Something in the order of that is what I had in mind. An RTT is the 
> control interval that we can and should act upon, and of course you'd 
> rather have an estimate that is averaged, and you really want to avoid 
> having false positives from outliers, so you want to give it a 
> reasonable safety margin. The TCP RTO has all that. 

Forgive me for being dense, but what is the metric by which the RTT is 
the right interval to act upon?

I know that it's the shortest interval we CAN react upon, because it's 
impossible for signals to travel source -> destination -> source in less 
time than the RTT, but what's the logic that says it's the interval we 
MUST act upon?

In particular, for the "bad" case of an unidirectional media stream that 
would normally use the AVP profile, with RTCP packets going back every 5 
seconds, there's a real engineering cost in requiring that reactions 
happens on the RTT timescale, especially if the algorithm is required to 
react to the absence of feedback signals.

I would like to have at least some idea of the benefit we're gaining 
from a short reaction time in order to evaluate what the cost/benefit 
tradeoff is.

                                Harald