Section: Linux (8)
Updated: 24 January 2012
tc qdisc ... [ divisor hashtablesize ] [ limit packets ] [ perturb seconds ] [ quantum bytes ] [ flows number ] [ depth number ] [ headdrop ] [ redflowlimit bytes ] [ min bytes ] [ max bytes ] [ avpkt bytes ] [ burst packets ] [ probability P ] [ ecn ] [ harddrop ]
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.
On enqueueing, each packet is assigned to a hash bucket, based on the packets hash value. This hash value is either obtained from an external flow classifier (use tc filter to set them), or a default internal classifier if no external classifier has been configured.
When the internal classifier is used, sfq uses
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 source. 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, sfqs internal hashing algorithm may be perturbed at configurable intervals so that the unfairness lasts only for a short while. Perturbation may however cause some inadvertent packet reordering to occur. After linux-3.3, there is no packet reordering problem, but possible packet drops if rehashing hits one limit (number of flows or packets per flow)
When dequeuing, each hashbucket with data is queried in a round robin fashion.
Before linux-3.3, 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.
After linux-3.3, maximum length of SFQ is 65535 packets, and divisor limit is 65536. In case of overflow, tail-drop is performed on the fullest bucket, unless headdrop was requested.
redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.:
Default value is : (2 * min + max) / (3 * avpkt)
To attach to device ppp0:
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.
It is possible to use external classifiers with sfq, for example to hash traffic based only on source/destination ip addresses:
Note that the given divisor should match the one used by sfq. If you have changed the sfq default of 1024, use the same value for the flow hash filter, too.
Example of sfq with optional RED mode :
redflowlimit 100000 min 8000 max 60000 probability 0.20 ecn headdrop
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