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