Stochastic Fairness Queueing is a classless queueing discipline available for traffic control with the tc(8) command.
SFQ does not shape traffic but only schedules the transmission of packets, based on 'flows'. The goal is to ensure fairness so that each flow is able to send data in turn, thus preventing any single flow from drowning out the rest.
This may in fact have some effect in mitigating a Denial of Service attempt.
SFQ is work-conserving and therefore always delivers a packet if it has one available.
If these are available. SFQ knows about ipv4 and ipv6 and also UDP, TCP and ESP. Packets with other protocols are hashed based on the 32bits representation of their destination and the socket they belong to. A flow corresponds mostly to a TCP/IP connection.
Each of these buckets should represent a unique flow. Because multiple flows may get hashed to the same bucket, the hashing algorithm is perturbed at configurable intervals so that the unfairness lasts only for a short while. Perturbation may however cause some inadvertent packet reordering to occur.
When dequeuing, each hashbucket with data is queried in a round robin fashion.
The compile time maximum length of the SFQ is 128 packets, which can be spread over at most 128 buckets of 1024 available. In case of overflow, tail-drop is performed on the fullest bucket, thus maintaining fairness.
To attach to device ppp0:
# tc qdisc add dev ppp0 root sfq perturb 10
Please note that SFQ, like all non-shaping (work-conserving) qdiscs, is only useful if it owns the queue. This is the case when the link speed equals the actually available bandwidth. This holds for regular phone modems, ISDN connections and direct non-switched ethernet links.
Most often, cable modems and DSL devices do not fall into this category. The same holds for when connected to a switch and trying to send data to a congested segment also connected to the switch.
In this case, the effective queue does not reside within Linux and is therefore not available for scheduling.
Embed SFQ in a classful qdisc to make sure it owns the queue.