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.TH CBQ 8 "16 December 2001" "iproute2" "Linux"
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.SH NAME
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CBQ \- Class Based Queueing
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.SH SYNOPSIS
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.B tc qdisc ... dev
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dev
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.B ( parent
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classid
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.B | root) [ handle
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major:
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.B ] cbq [ allot
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bytes
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.B ] avpkt
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bytes
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.B bandwidth
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rate
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.B [ cell
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bytes
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.B ] [ ewma
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log
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.B ] [ mpu
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bytes
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.B ]
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.B tc class ... dev
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dev
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.B parent
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major:[minor]
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.B [ classid
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major:minor
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.B ] cbq allot
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bytes
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.B [ bandwidth
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rate
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.B ] [ rate
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rate
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.B ] prio
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priority
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.B [ weight
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weight
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.B ] [ minburst
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packets
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.B ] [ maxburst
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packets
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.B ] [ ewma
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log
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.B ] [ cell
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bytes
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.B ] avpkt
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bytes
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.B [ mpu
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bytes
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.B ] [ bounded isolated ] [ split
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handle
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.B & defmap
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defmap
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.B ] [ estimator
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interval timeconstant
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.B ]
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.SH DESCRIPTION
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Class Based Queueing is a classful qdisc that implements a rich
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linksharing hierarchy of classes. It contains shaping elements as
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well as prioritizing capabilities. Shaping is performed using link
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idle time calculations based on the timing of dequeue events and
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underlying link bandwidth.
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.SH SHAPING ALGORITHM
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When shaping a 10mbit/s connection to 1mbit/s, the link will
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be idle 90% of the time. If it isn't, it needs to be throttled so that it
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IS idle 90% of the time.
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During operations, the effective idletime is measured using an
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exponential weighted moving average (EWMA), which considers recent
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packets to be exponentially more important than past ones. The Unix
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loadaverage is calculated in the same way.
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The calculated idle time is subtracted from the EWMA measured one,
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the resulting number is called 'avgidle'. A perfectly loaded link has
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an avgidle of zero: packets arrive exactly at the calculated
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interval.
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An overloaded link has a negative avgidle and if it gets too negative,
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CBQ throttles and is then 'overlimit'.
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Conversely, an idle link might amass a huge avgidle, which would then
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allow infinite bandwidths after a few hours of silence. To prevent
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this, avgidle is capped at
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.B maxidle.
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If overlimit, in theory, the CBQ could throttle itself for exactly the
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amount of time that was calculated to pass between packets, and then
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pass one packet, and throttle again. Due to timer resolution constraints,
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this may not be feasible, see the
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.B minburst
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parameter below.
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.SH CLASSIFICATION
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Within the one CBQ instance many classes may exist. Each of these classes
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contains another qdisc, by default
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.BR tc-pfifo (8).
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When enqueueing a packet, CBQ starts at the root and uses various methods to
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determine which class should receive the data.
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In the absence of uncommon configuration options, the process is rather easy.
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At each node we look for an instruction, and then go to the class the
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instruction refers us to. If the class found is a barren leaf-node (without
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children), we enqueue the packet there. If it is not yet a leaf node, we do
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the whole thing over again starting from that node.
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The following actions are performed, in order at each node we visit, until one
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sends us to another node, or terminates the process.
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.TP
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(i)
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Consult filters attached to the class. If sent to a leafnode, we are done.
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Otherwise, restart.
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.TP
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(ii)
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Consult the defmap for the priority assigned to this packet, which depends
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on the TOS bits. Check if the referral is leafless, otherwise restart.
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.TP
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(iii)
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Ask the defmap for instructions for the 'best effort' priority. Check the
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answer for leafness, otherwise restart.
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.TP
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(iv)
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If none of the above returned with an instruction, enqueue at this node.
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.P
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This algorithm makes sure that a packet always ends up somewhere, even while
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you are busy building your configuration.
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For more details, see
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.BR tc-cbq-details(8).
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.SH LINK SHARING ALGORITHM
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When dequeuing for sending to the network device, CBQ decides which of its
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classes will be allowed to send. It does so with a Weighted Round Robin process
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in which each class with packets gets a chance to send in turn. The WRR process
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starts by asking the highest priority classes (lowest numerically -
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highest semantically) for packets, and will continue to do so until they
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have no more data to offer, in which case the process repeats for lower
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priorities.
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Classes by default borrow bandwidth from their siblings. A class can be
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prevented from doing so by declaring it 'bounded'. A class can also indicate
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its unwillingness to lend out bandwidth by being 'isolated'.
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.SH QDISC
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The root of a CBQ qdisc class tree has the following parameters:
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.TP
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parent major:minor | root
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This mandatory parameter determines the place of the CBQ instance, either at the
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.B root
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of an interface or within an existing class.
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.TP
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handle major:
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Like all other qdiscs, the CBQ can be assigned a handle. Should consist only
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of a major number, followed by a colon. Optional, but very useful if classes
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will be generated within this qdisc.
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.TP
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allot bytes
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This allotment is the 'chunkiness' of link sharing and is used for determining packet
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transmission time tables. The qdisc allot differs slightly from the class allot discussed
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below. Optional. Defaults to a reasonable value, related to avpkt.
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.TP
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avpkt bytes
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The average size of a packet is needed for calculating maxidle, and is also used
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for making sure 'allot' has a safe value. Mandatory.
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.TP
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bandwidth rate
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To determine the idle time, CBQ must know the bandwidth of your underlying
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physical interface, or parent qdisc. This is a vital parameter, more about it
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later. Mandatory.
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.TP
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cell
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The cell size determines he granularity of packet transmission time calculations. Has a sensible default.
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.TP
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mpu
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A zero sized packet may still take time to transmit. This value is the lower
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cap for packet transmission time calculations - packets smaller than this value
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are still deemed to have this size. Defaults to zero.
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.TP
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ewma log
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When CBQ needs to measure the average idle time, it does so using an
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Exponentially Weighted Moving Average which smooths out measurements into
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a moving average. The EWMA LOG determines how much smoothing occurs. Lower
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values imply greater sensitivity. Must be between 0 and 31. Defaults
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to 5.
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.P
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A CBQ qdisc does not shape out of its own accord. It only needs to know certain
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parameters about the underlying link. Actual shaping is done in classes.
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.SH CLASSES
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Classes have a host of parameters to configure their operation.
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.TP
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parent major:minor
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Place of this class within the hierarchy. If attached directly to a qdisc
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and not to another class, minor can be omitted. Mandatory.
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.TP
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classid major:minor
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Like qdiscs, classes can be named. The major number must be equal to the
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major number of the qdisc to which it belongs. Optional, but needed if this
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class is going to have children.
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.TP
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weight weight
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When dequeuing to the interface, classes are tried for traffic in a
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round-robin fashion. Classes with a higher configured qdisc will generally
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have more traffic to offer during each round, so it makes sense to allow
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it to dequeue more traffic. All weights under a class are normalized, so
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only the ratios matter. Defaults to the configured rate, unless the priority
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of this class is maximal, in which case it is set to 1.
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.TP
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allot bytes
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Allot specifies how many bytes a qdisc can dequeue
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during each round of the process. This parameter is weighted using the
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renormalized class weight described above. Silently capped at a minimum of
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3/2 avpkt. Mandatory.
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.TP
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prio priority
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In the round-robin process, classes with the lowest priority field are tried
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for packets first. Mandatory.
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.TP
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avpkt
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See the QDISC section.
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.TP
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rate rate
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Maximum rate this class and all its children combined can send at. Mandatory.
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.TP
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bandwidth rate
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This is different from the bandwidth specified when creating a CBQ disc! Only
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used to determine maxidle and offtime, which are only calculated when
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specifying maxburst or minburst. Mandatory if specifying maxburst or minburst.
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.TP
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maxburst
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This number of packets is used to calculate maxidle so that when
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avgidle is at maxidle, this number of average packets can be burst
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before avgidle drops to 0. Set it higher to be more tolerant of
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bursts. You can't set maxidle directly, only via this parameter.
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.TP
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minburst
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As mentioned before, CBQ needs to throttle in case of
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overlimit. The ideal solution is to do so for exactly the calculated
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idle time, and pass 1 packet. However, Unix kernels generally have a
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hard time scheduling events shorter than 10ms, so it is better to
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throttle for a longer period, and then pass minburst packets in one
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go, and then sleep minburst times longer.
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The time to wait is called the offtime. Higher values of minburst lead
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to more accurate shaping in the long term, but to bigger bursts at
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millisecond timescales. Optional.
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.TP
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minidle
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If avgidle is below 0, we are overlimits and need to wait until
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avgidle will be big enough to send one packet. To prevent a sudden
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burst from shutting down the link for a prolonged period of time,
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avgidle is reset to minidle if it gets too low.
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Minidle is specified in negative microseconds, so 10 means that
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avgidle is capped at -10us. Optional.
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.TP
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bounded
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Signifies that this class will not borrow bandwidth from its siblings.
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.TP
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isolated
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Means that this class will not borrow bandwidth to its siblings
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.TP
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split major:minor & defmap bitmap[/bitmap]
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If consulting filters attached to a class did not give a verdict,
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CBQ can also classify based on the packet's priority. There are 16
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priorities available, numbered from 0 to 15.
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The defmap specifies which priorities this class wants to receive,
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specified as a bitmap. The Least Significant Bit corresponds to priority
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zero. The
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.B split
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parameter tells CBQ at which class the decision must be made, which should
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be a (grand)parent of the class you are adding.
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As an example, 'tc class add ... classid 10:1 cbq .. split 10:0 defmap c0'
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configures class 10:0 to send packets with priorities 6 and 7 to 10:1.
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The complimentary configuration would then
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be: 'tc class add ... classid 10:2 cbq ... split 10:0 defmap 3f'
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Which would send all packets 0, 1, 2, 3, 4 and 5 to 10:1.
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.TP
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estimator interval timeconstant
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CBQ can measure how much bandwidth each class is using, which tc filters
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can use to classify packets with. In order to determine the bandwidth
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it uses a very simple estimator that measures once every
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.B interval
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microseconds how much traffic has passed. This again is a EWMA, for which
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the time constant can be specified, also in microseconds. The
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.B time constant
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corresponds to the sluggishness of the measurement or, conversely, to the
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sensitivity of the average to short bursts. Higher values mean less
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sensitivity.
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.SH BUGS
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The actual bandwidth of the underlying link may not be known, for example
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in the case of PPoE or PPTP connections which in fact may send over a
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pipe, instead of over a physical device. CBQ is quite resilient to major
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errors in the configured bandwidth, probably a the cost of coarser shaping.
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Default kernels rely on coarse timing information for making decisions. These
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may make shaping precise in the long term, but inaccurate on second long scales.
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See
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.BR tc-cbq-details(8)
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for hints on how to improve this.
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.SH SOURCES
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.TP
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o
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Sally Floyd and Van Jacobson, "Link-sharing and Resource
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Management Models for Packet Networks",
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IEEE/ACM Transactions on Networking, Vol.3, No.4, 1995
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.TP
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o
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Sally Floyd, "Notes on CBQ and Guaranteed Service", 1995
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.TP
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o
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Sally Floyd, "Notes on Class-Based Queueing: Setting
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Parameters", 1996
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.TP
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o
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Sally Floyd and Michael Speer, "Experimental Results
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for Class-Based Queueing", 1998, not published.
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.SH SEE ALSO
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.BR tc (8)
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.SH AUTHOR
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Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
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bert hubert <ahu@ds9a.nl>
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