Sun, Mar 24, 2019 at 10:52:53PM CET, olga...@gmail.com wrote: >DUALPI2 provides extremely low latency & loss to traffic that uses a >scalable congestion controller (e.g. L4S, 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 their 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. > >There is an AQM in each queue. >* 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. >* The low latency AQM is, by default, a simple very shallow ECN marking >threshold similar to that used for DCTCP. > >The DualQ isolates the extremely 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). > >Internally, the output of a simple 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 parameter k . 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 (when excessive non-responsive >traffic is sent), it will switch to signalling congestion solely using >drop, irrespective of the ECN field, or 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 <o...@albisser.org> >Signed-off-by: Koen De Schepper <koen.de_schep...@nokia-bell-labs.com> >Signed-off-by: Oliver Tilmans <olivier.tilm...@nokia-bell-labs.com> >Signed-off-by: Bob Briscoe <resea...@bobbriscoe.net> >Signed-off-by: Henrik Steen <henr...@henrist.net> >---
For v2 and above, please provide changelog here. Thanks!
