Re: [aqm] PIE vs. RED

[Jonathan Morton <[e-mail]>]

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.

- 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.

- Jonathan Morton