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diff --git a/man/man8/tc-sfb.8 b/man/man8/tc-sfb.8 new file mode 100644 index 0000000..1f2b8c5 --- /dev/null +++ b/man/man8/tc-sfb.8 @@ -0,0 +1,213 @@ +.TH SFB 8 "August 2011" "iproute2" "Linux" +.SH NAME +sfb \- Stochastic Fair Blue +.SH SYNOPSIS +.B tc qdisc ... blue +.B rehash +milliseconds +.B db +milliseconds +.B limit +packets +.B max +packets +.B target +packets +.B increment +float +.B decrement +float +.B penalty_rate +packets per second +.B penalty_burst +packets + +.SH DESCRIPTION +Stochastic Fair Blue is a classless qdisc to manage congestion based on +packet loss and link utilization history while trying to prevent +non-responsive flows (i.e. flows that do not react to congestion marking +or dropped packets) from impacting performance of responsive flows. +Unlike RED, where the marking probability has to be configured, BLUE +tries to determine the ideal marking probability automatically. + +.SH ALGORITHM + +The +.B BLUE +algorithm maintains a probability which is used to mark or drop packets +that are to be queued. If the queue overflows, the mark/drop probability +is increased. If the queue becomes empty, the probability is decreased. The +.B Stochastic Fair Blue +(SFB) algorithm is designed to protect TCP flows against non-responsive flows. + +This SFB implementation maintains 8 levels of 16 bins each for accounting. +Each flow is mapped into a bin of each level using a per-level hash value. + +Every bin maintains a marking probability, which gets increased or decreased +based on bin occupancy. If the number of packets exceeds the size of that +bin, the marking probability is increased. If the number drops to zero, it +is decreased. + +The marking probability is based on the minimum value of all bins a flow is +mapped into, thus, when a flow does not respond to marking or gradual packet +drops, the marking probability quickly reaches one. + +In this case, the flow is rate-limited to +.B penalty_rate +packets per second. + +.SH LIMITATIONS + +Due to SFBs nature, it is possible for responsive flows to share all of its bins +with a non-responsive flow, causing the responsive flow to be misidentified as +being non-responsive. + +The probability of a responsive flow to be misidentified is dependent on +the number of non-responsive flows, M. It is (1 - (1 - (1 / 16.0)) ** M) **8, +so for example with 10 non-responsive flows approximately 0.2% of responsive flows +will be misidentified. + +To mitigate this, SFB performs periodic re-hashing to avoid +misclassification for prolonged periods of time. + +The default hashing method will use source and destination ip addresses and port numbers +if possible, and also supports tunneling protocols. +Alternatively, an external classifier can be configured, too. + +.SH PARAMETERS +.TP +rehash +Time interval in milliseconds when queue perturbation occurs to avoid erroneously +detecting unrelated, responsive flows as being part of a non-responsive flow for +prolonged periods of time. +Defaults to 10 minutes. +.TP +db +Double buffering warmup wait time, in milliseconds. +To avoid destroying the probability history when rehashing is performed, this +implementation maintains a second set of levels/bins as described in section +4.4 of the SFB reference. +While one set is used to manage the queue, a second set is warmed up: +Whenever a flow is then determined to be non-responsive, the marking +probabilities in the second set are updated. When the rehashing +happens, these bins will be used to manage the queue and all non-responsive +flows can be rate-limited immediately. +This value determines how much time has to pass before the 2nd set +will start to be warmed up. +Defaults to one minute, should be lower than +.B +rehash. +.TP +limit +Hard limit on the real (not average) total queue size in packets. +Further packets are dropped. Defaults to the transmit queue length of the +device the qdisc is attached to. +.TP +max +Maximum length of a buckets queue, in packets, before packets start being +dropped. Should be slightly larger than +.B target +, but should not be set to values exceeding 1.5 times that of +.B target . +Defaults to 25. +.TP +target +The desired average bin length. If the bin queue length reaches this value, +the marking probability is increased by +.B increment. +The default value depends on the +.B max +setting, with max set to 25 +.B target +will default to 20. +.TP +increment +A value used to increase the marking probability when the queue appears +to be over-used. Must be between 0 and 1.0. Defaults to 0.00050. +.TP +decrement +Value used to decrease the marking probability when the queue is found +to be empty. Must be between 0 and 1.0. +Defaults to 0.00005. +.TP +penalty_rate +The maximum number of packets belonging to flows identified as being +non-responsive that can be enqueued per second. Once this number has been +reached, further packets of such non-responsive flows are dropped. +Set this to a reasonable fraction of your uplink throughput; the +default value of 10 packets is probably too small. +.TP +penalty_burst +The number of packets a flow is permitted to exceed the penalty rate before packets +start being dropped. +Defaults to 20 packets. + +.SH STATISTICS + +This qdisc exposes additional statistics via 'tc -s qdisc' output. +These are: +.TP +earlydrop +The number of packets dropped before a per-flow queue was full. +.TP +ratedrop +The number of packets dropped because of rate-limiting. +If this value is high, there are many non-reactive flows being +sent through sfb. In such cases, it might be better to +embed sfb within a classful qdisc to better control such +flows using a different, shaping qdisc. +.TP +bucketdrop +The number of packets dropped because a per-flow queue was full. +High bucketdrop may point to a high number of aggressive, short-lived +flows. +.TP +queuedrop +The number of packets dropped due to reaching limit. This should normally be 0. +.TP +marked +The number of packets marked with ECN. +.TP +maxqlen +The length of the current longest per-flow (virtual) queue. +.TP +maxprob +The maximum per-flow drop probability. 1 means that some +flows have been detected as non-reactive. + +.SH NOTES + +SFB automatically enables use of Explicit Congestion Notification (ECN). +Also, this SFB implementation does not queue packets itself. +Rather, packets are enqueued to the inner qdisc (defaults to pfifo). +Because sfb maintains virtual queue states, the inner qdisc must not +drop a packet previously queued. +Furthermore, if a buckets queue has a very high marking rate, +this implementation will start dropping packets instead of +marking them, as such a situation points to either bad congestion, or an +unresponsive flow. + +.SH EXAMPLE & USAGE + +To attach to interface $DEV, using default options: +.P +# tc qdisc add dev $DEV handle 1: root sfb + +Only use destination ip addresses for assigning packets to bins, perturbing +hash results every 10 minutes: +.P +# tc filter add dev $DEV parent 1: handle 1 flow hash keys dst perturb 600 + +.SH SEE ALSO +.BR tc (8), +.BR tc-red (8), +.BR tc-sfq (8) +.SH SOURCES +.TP +o +W. Feng, D. Kandlur, D. Saha, K. Shin, BLUE: A New Class of Active Queue Management Algorithms, +U. Michigan CSE-TR-387-99, April 1999. + +.SH AUTHORS + +This SFB implementation was contributed by Juliusz Chroboczek and Eric Dumazet. |