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+.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 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 sightly 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.
@@ -372,6 +372,7 @@ was written by Alexey N. Kuznetsov and added in Linux 2.2.
.BR tc-htb (8),
.BR tc-hfsc (8),
.BR tc-hfsc (7),
+.BR tc-sfb (8),
.BR tc-sfq (8),
.BR tc-red (8),
.BR tc-tbf (8),