Smart Queue Management

“Smart Queue Management”, or “SQM” is shorthand for an integrated network system that performs better per-packet/per flow network scheduling, active queue length management (AQM), traffic shaping/rate limiting, and QoS (prioritization).

“Classic” QoS does prioritization only.

“Classic” AQM manages queue lengths only.

“Classic” packet scheduling does some form of fair queuing only.

“Classic” traffic shaping and policing sets hard limits on queue lengths and transfer rates

“Classic” rate limiting sets hard limits on network speeds.

It has become apparent that in order to ensure a good internet experience all of these techniques need to be combined and used as an integrated whole, and also represented as such to end-users. After years of debate on the bloat and AQM mailing lists, no name for the idea has been agreed on. A lot of people object to “smart queue management” as being too general a phrase (“you can do anything and call it smart”), but we hope that by defining it as we have above to limit the mis-appropriations. A trademarked name has been suggested as well…

Probably the first widely deployed fully integrated “smart queue management” system was the venerated wondershaper, which emerged in the early 2000s as the linux based shaper of choice. It was widely deployed in internet cafes around the world, and in Linux users’ homes and workplaces. Although for the time it was a breakthrough, it has since been obsoleted by events and bugs in its design. See Wondershaper Must Die.

Much work on AQM (active queue length management) technologies like RED and BLUE took place in the period 1990-2002. Many variants of RED appeared between 2002 and 2013 - FRED, ARED, LRED, etc, but work mostly stagnated under variants of the same set of ideas, until the creation of the bufferbloat effort in 2011, which begat Codel and PIE. Feedback from the effects of these algorithms have led to improvements in various TCP implementations as well.

Packet scheduling has a longer and more successful history, starting with the first research into “fair queueing” in the mid-1980s and continuing to the present day with ideas like DRR, SFQ, QFQ and SQF. Along the way it was realized that strictly “fair” queueing was not desirable, which led to optimizations like WFQ (weighted Fair Queueing), SQF (shortest queue first), and sparse stream optimizations like those in fq_codel. Lacking a better phrase we try to distinguish between old-style “fair” queuing and new-style “flow aware queueing”, but the common understanding of the abbreviation “FQ” = Fair Queueing is the source of much confusion.

These techniques ( shaping, prioritization, packet scheduling, and AQM) are often used in serial, rather than parallel.

It is possible to make things worse by applying only a few of these techniques, a classic example of this is in NetGear’s current QoS system which allows you to rate limit but holds the fifo queue lengths constant, and does not apply either packet scheduling or AQM, leading to exorbitant delays. SFQ, used alone at higher bandwidths gets the bursty tail drop problem an AQM can solve. An AQM, used alone, has trouble managing bursts. QoS, used alone, only works on what packet types can be classified easily. And, policing, set incorrectly, can seriously damage downloads.

WRED was probably the most successful of the packet scheduling/QoS/AQM hybrids. fq_codel (the principal candidate for a successor to WRED) combines smarter packet scheduling with a innovative AQM design into a single algorithm also, but does not have native support for QoS packet markings. It is usually combined with something else to do that.

Most QoS systems as shipped today are terrifically elaborate and let you prioritize certain packet types to your heart’s content, but this is of no help in a world that consists mostly of bursty web traffic on ports 80 and 443.

Rate limiting is presently stuck with token bucket designs descended from the original CBQ, like HTB. Software rate limiting is far more abusive of CPU than any of the packet scheduling or AQM algorithms discussed above, or their hybrids.

Traffic shaping (“policing”) was a not very good idea hard limiting ingress speeds that become common because of the ease of implementation. It is far saner to apply Smart Queue Management to “police” traffic at today’s higher speeds.

Examples of deployed Smart Queue Management systems include CeroWrt’s SQM implementation, OpenWrt’s qos-scripts, IPFire, the Gargoyle router project, and Streamboost . WRED is deployed in many locations. France Telecom deploys SFQ. Free.fr has the first known large-scale fq_codel deployment, using three bands of fq_codel for tiers of QoS. (it is also the largest ECN enabled end-user deployment). The Streamboost product (now coming available in multiple 802.11ac routers) combines a bandwidth sensor, with a packet classification engine, with a multi-band fq_codel implementation.

We are beginning to characterise these in an upcoming internet draft on comprehensive queue management in home routers

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