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testing/5-11-2019

@olgaalb olgaalb tagged this 04 Nov 08:24
DualPI2 provides L4S-type low latency & loss to traffic that uses a
scalable congestion controller (e.g. TCP-Prague, DCTCP) without
degrading the performance of 'classic' traffic (e.g. Reno,
Cubic etc.). It is intended to be the reference implementation of the
IETF's DualQ Coupled AQM.

The qdisc provides two queues called low latency and classic. It
classifies packets based on the ECN field in the IP headers. By
default it directs non-ECN and ECT(0) into the classic queue and
ECT(1) and CE into the low latency queue, as per the IETF spec.

Each queue runs its own AQM:
* The classic AQM is called PI2, which is similar to the PIE AQM but
   more responsive and simpler. Classic traffic requires a decent
   target queue (default 15ms for Internet deployment) to fully
   utilize the link and to avoid high drop rates.
* The low latency AQM is, by default, a very shallow ECN marking
   threshold (1ms) similar to that used for DCTCP.

The DualQ isolates the low queuing delay of the Low Latency queue
from the larger delay of the 'Classic' queue. However, from a
bandwidth perspective, flows in either queue will share out the link
capacity as if there was just a single queue. This bandwidth pooling
effect is achieved by coupling together the drop and ECN-marking
probabilities of the two AQMs.

The PI2 AQM has two main parameters in addition to its target delay.
All the defaults are suitable for any Internet setting, but it can
be reconfigured for a Data Centre setting. The integral gain factor
alpha is used to slowly correct any persistent standing queue error
from the target delay, while the proportional gain factor beta is
used to quickly compensate for queue changes (growth or shrinkage).
Either alpha and beta are given as a parameter, or they can be
calculated by tc from alternative typical and maximum RTT parameters.

Internally, the output of a linear Proportional Integral (PI)
controller is used for both queues. This output is squared to
calculate the drop or ECN-marking probability of the classic queue.
This counterbalances the square-root rate equation of Reno/Cubic,
which is the trick that balances flow rates across the queues. For
the ECN-marking probability of the low latency queue, the output of
the base AQM is multiplied by a coupling factor. This determines the
balance between the flow rates in each queue. The default setting
makes the flow rates roughly equal, which should be generally
applicable.

If DUALPI2 AQM has detected overload (due to excessive non-responsive
traffic in either queue), it will switch to signaling congestion
solely using drop, irrespective of the ECN field. Alternatively, it
can be configured to limit the drop probability and let the queue
grow and eventually overflow (like tail-drop).

Additional details can be found in the draft:
https://www.ietf.org/id/draft-ietf-tsvwg-aqm-dualq-coupled

Signed-off-by: Olga Albisser <olga@albisser.org>
Signed-off-by: Koen De Schepper <koen.de_schepper@nokia-bell-labs.com>
Signed-off-by: Olivier Tilmans <olivier.tilmans@nokia-bell-labs.com>
Signed-off-by: Bob Briscoe <research@bobbriscoe.net>
Signed-off-by: Henrik Steen <henrist@henrist.net>
Assets 2