Section: Linux (8)
Updated: August 2011
tc qdisc ... blue rehash milliseconds db milliseconds limit packets max packets target packets increment float decrement float penalty_rate packets per second penalty_burst packets
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.
The 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 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 penalty_rate packets per second.
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.
This qdisc exposes additional statistics via 'tc -s qdisc' output. These are:
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.
To attach to interface $DEV, using default options:
Only use destination ip addresses for assigning packets to bins, perturbing hash results every 10 minutes:
This SFB implementation was contributed by Juliusz Chroboczek and Eric Dumazet.
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