Re: [aqm] AQM schemes: Queue length vs. delay based

[Naeem Khademi <[e-mail]>]

> Delay-based ARED behaves similar to tail drop at max threshold.
>>
>
I think I now understand what you mean by this sentence ;-) and therefore
please ignore the paragraph responding to this specific point in the
previous email (sorry about that, I got it mixed with max_target). Indeed
ARED drops every packet when the average queue length grows beyond th_max
as drop probability jumps to 1 (when it's not in gentle mode). For this to
happen, the "average queue length" has to grow beyond th_max and depending
on how the averaging is done, that can correspond to certain burst
allowance at AQM. Moreover, this is done over 500 ms interval which means
the average queue length has to stay above th_max for mostly few rounds of
RTTs (assuming the typical distribution of RTTs on the Internet to have a
mean/average somewhere below that value e.g. 100~200 ms). This allows burst
that don't take the average queue size to above th_max over 500 ms period
to pass and clear themselves up the queue.

 > This is unacceptable of an AQM even for max thresholds around 50ms.

 Whether this is a good thing or bad thing is a subjective matter in my
opinion, as most AQMs at some point will drop every packet whether it be
the maximum queue length or at some thresholds.

Cheers,
Naeem