Re: [aqm] PIE vs. RED

[Roland Bless <[e-mail]>]

Hi Jonathan,

Am 13.08.2015 um 19:35 schrieb Jonathan Morton:
> In the real world, the hardware buffer size is rarely matched to the
> real BDP.  There are several reasons for this, but a couple of
> fundamental ones are:
> 
> - BDP varies with RTT, which is in general different for flows
> simultaneously using the same link/queue to reach different remote
> hosts, and therefore cannot be accurately predicted by a hardware vendor.

Yep, sure. My point was not to promote setting the buffers according to
"the BDP", but rather arguing that one should use comparable target
settings when comparing AQMs, see below...

> - Frequently, the queue size is tuned for the maximum capability of the
> device and a pessimistic value for RTT, but the same hardware is more
> often used (at least initially) at lower link speeds and thebqueue size
> is not adjusted to compensate.  Eg. DOCSIS 2 cable but DOCSIS 3 modem,
> Ethernet NIC or switch capable of 1000Mbps but operating at 100 or even
> 10, 802.11ac wifi struggling with a marginal 802.11g link...
> 
> Thus substantially oversized raw buffers are quite normal.  It is AQM's
> job to keep the *actual* queue occupancy low; with a properly
> functioning AQM, the effects of an oversized raw queue are nil.

That's correct. However, IMHO if one compares AQMs one should set/tune
the individual parameters of the AQMs so that they achieve a similar
target value (and not more than an order of magnitude apart).
This is probably relevant for the aqm eval guidelines, but
I'll come up with a detailed review for the draft within the next days...

Regards,
 Roland