CBQ(8)                 (16 December 2001)                  CBQ(8)

          CBQ - Class Based Queueing

          tc qdisc ... dev dev ( parent classid | root) [ handle
          major: ] cbq [ allot bytes ] avpkt bytes bandwidth rate [
          cell bytes ] [ ewma log ] [ mpu bytes ]

          tc class ... dev dev parent major:[minor] [ classid
          major:minor ] cbq allot bytes [ bandwidth rate ] [ rate rate
          ] prio priority [ weight weight ] [ minburst packets ] [
          maxburst packets ] [ ewma log ] [ cell bytes ] avpkt bytes [
          mpu bytes ] [ bounded isolated ] [ handle & defmap defmap ]
          [ estimator interval timeconstant ]

          Class Based Queueing is a classful qdisc that implements a
          rich linksharing hierarchy of classes. It contains shaping
          elements as well as prioritizing capabilities. Shaping is
          performed using link idle time calculations based on the
          timing of dequeue events and underlying link bandwidth.

          When shaping a 10mbit/s connection to 1mbit/s, the link will
          be idle 90% of the time. If it isn't, it needs to be throt-
          tled so that it IS idle 90% of the time.

          During operations, the effective idletime is measured using
          an exponential weighted moving average (EWMA), which consid-
          ers recent packets to be exponentially more important than
          past ones. The Unix loadaverage is calculated in the same

          The calculated idle time is subtracted from the EWMA mea-
          sured one, the resulting number is called 'avgidle'. A per-
          fectly loaded link has an avgidle of zero: packets arrive
          exactly at the calculated interval.

          An overloaded link has a negative avgidle and if it gets too
          negative, CBQ throttles and is then 'overlimit'.

          Conversely, an idle link might amass a huge avgidle, which
          would then allow infinite bandwidths after a few hours of
          silence. To prevent this, avgidle is capped at maxidle.

          If overlimit, in theory, the CBQ could throttle itself for
          exactly the amount of time that was calculated to pass
          between packets, and then pass one packet, and throttle

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          again. Due to timer resolution constraints, this may not be
          feasible, see the minburst parameter below.

          Within the one CBQ instance many classes may exist. Each of
          these classes contains another qdisc, by default

          When enqueueing a packet, CBQ starts at the root and uses
          various methods to determine which class should receive the

          In the absence of uncommon configuration options, the pro-
          cess is rather easy.  At each node we look for an instruc-
          tion, and then go to the class the instruction refers us to.
          If the class found is a barren leaf-node (without children),
          we enqueue the packet there. If it is not yet a leaf node,
          we do the whole thing over again starting from that node.

          The following actions are performed, in order at each node
          we visit, until one sends us to another node, or terminates
          the process.

          (i)  Consult filters attached to the class. If sent to a
               leafnode, we are done.  Otherwise, restart.

          (ii) Consult the defmap for the priority assigned to this
               packet, which depends on the TOS bits. Check if the
               referral is leafless, otherwise restart.

               Ask the defmap for instructions for the 'best effort'
               priority. Check the answer for leafness, otherwise res-

          (iv) If none of the above returned with an instruction,
               enqueue at this node.

          This algorithm makes sure that a packet always ends up some-
          where, even while you are busy building your configuration.

          For more details, see tc-cbq-details(8).

          When dequeuing for sending to the network device, CBQ
          decides which of its classes will be allowed to send. It
          does so with a Weighted Round Robin process in which each
          class with packets gets a chance to send in turn. The WRR
          process starts by asking the highest priority classes (low-
          est numerically - highest semantically) for packets, and

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          will continue to do so until they have no more data to
          offer, in which case the process repeats for lower priori-

          Classes by default borrow bandwidth from their siblings. A
          class can be prevented from doing so by declaring it
          'bounded'. A class can also indicate its unwillingness to
          lend out bandwidth by being 'isolated'.

          The root of a CBQ qdisc class tree has the following parame-

          parent major:minor | root
               This mandatory parameter determines the place of the
               CBQ instance, either at the root of an interface or
               within an existing class.

          handle major:
               Like all other qdiscs, the CBQ can be assigned a han-
               dle. Should consist only of a major number, followed by
               a colon. Optional, but very useful if classes will be
               generated within this qdisc.

          allot bytes
               This allotment is the 'chunkiness' of link sharing and
               is used for determining packet transmission time
               tables. The qdisc allot differs slightly from the class
               allot discussed below. Optional. Defaults to a reason-
               able value, related to avpkt.

          avpkt bytes
               The average size of a packet is needed for calculating
               maxidle, and is also used for making sure 'allot' has a
               safe value. Mandatory.

          bandwidth rate
               To determine the idle time, CBQ must know the bandwidth
               of your underlying physical interface, or parent qdisc.
               This is a vital parameter, more about it later. Manda-

          cell The cell size determines he granularity of packet
               transmission time calculations. Has a sensible default.

          mpu  A zero sized packet may still take time to transmit.
               This value is the lower cap for packet transmission
               time calculations - packets smaller than this value are
               still deemed to have this size. Defaults to zero.

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          ewma log
               When CBQ needs to measure the average idle time, it
               does so using an Exponentially Weighted Moving Average
               which smooths out measurements into a moving average.
               The EWMA LOG determines how much smoothing occurs.
               Lower values imply greater sensitivity. Must be between
               0 and 31. Defaults to 5.

          A CBQ qdisc does not shape out of its own accord. It only
          needs to know certain parameters about the underlying link.
          Actual shaping is done in classes.

          Classes have a host of parameters to configure their opera-

          parent major:minor
               Place of this class within the hierarchy. If attached
               directly to a qdisc and not to another class, minor can
               be omitted. Mandatory.

          classid major:minor
               Like qdiscs, classes can be named. The major number
               must be equal to the major number of the qdisc to which
               it belongs. Optional, but needed if this class is going
               to have children.

          weight weight
               When dequeuing to the interface, classes are tried for
               traffic in a round-robin fashion. Classes with a higher
               configured qdisc will generally have more traffic to
               offer during each round, so it makes sense to allow it
               to dequeue more traffic. All weights under a class are
               normalized, so only the ratios matter. Defaults to the
               configured rate, unless the priority of this class is
               maximal, in which case it is set to 1.

          allot bytes
               Allot specifies how many bytes a qdisc can dequeue dur-
               ing each round of the process. This parameter is
               weighted using the renormalized class weight described
               above. Silently capped at a minimum of 3/2 avpkt.

          prio priority
               In the round-robin process, classes with the lowest
               priority field are tried for packets first. Mandatory.

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               See the QDISC section.

          rate rate
               Maximum rate this class and all its children combined
               can send at. Mandatory.

          bandwidth rate
               This is different from the bandwidth specified when
               creating a CBQ disc! Only used to determine maxidle and
               offtime, which are only calculated when specifying max-
               burst or minburst. Mandatory if specifying maxburst or

               This number of packets is used to calculate maxidle so
               that when avgidle is at maxidle, this number of average
               packets can be burst before avgidle drops to 0. Set it
               higher to be more tolerant of bursts. You can't set
               maxidle directly, only via this parameter.

               As mentioned before, CBQ needs to throttle in case of
               overlimit. The ideal solution is to do so for exactly
               the calculated idle time, and pass 1 packet. However,
               Unix kernels generally have a hard time scheduling
               events shorter than 10ms, so it is better to throttle
               for a longer period, and then pass minburst packets in
               one go, and then sleep minburst times longer.

               The time to wait is called the offtime. Higher values
               of minburst lead to more accurate shaping in the long
               term, but to bigger bursts at millisecond timescales.

               If avgidle is below 0, we are overlimits and need to
               wait until avgidle will be big enough to send one
               packet. To prevent a sudden burst from shutting down
               the link for a prolonged period of time, avgidle is
               reset to minidle if it gets too low.

               Minidle is specified in negative microseconds, so 10
               means that avgidle is capped at -10us. Optional.


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               Signifies that this class will not borrow bandwidth
               from its siblings.

               Means that this class will not borrow bandwidth to its

          split major:minor & defmap bitmap[/bitmap]
               If consulting filters attached to a class did not give
               a verdict, CBQ can also classify based on the packet's
               priority. There are 16 priorities available, numbered
               from 0 to 15.

               The defmap specifies which priorities this class wants
               to receive, specified as a bitmap. The Least Signifi-
               cant Bit corresponds to priority zero. The split param-
               eter tells CBQ at which class the decision must be
               made, which should be a (grand)parent of the class you
               are adding.

               As an example, 'tc class add ... classid 10:1 cbq ..
               split 10:0 defmap c0' configures class 10:0 to send
               packets with priorities 6 and 7 to 10:1.

               The complimentary configuration would then be: 'tc
               class add ... classid 10:2 cbq ... split 10:0 defmap
               3f' Which would send all packets 0, 1, 2, 3, 4 and 5 to

          estimator interval timeconstant
               CBQ can measure how much bandwidth each class is using,
               which tc filters can use to classify packets with. In
               order to determine the bandwidth it uses a very simple
               estimator that measures once every interval microsec-
               onds how much traffic has passed. This again is a EWMA,
               for which the time constant can be specified, also in
               microseconds. The time constant corresponds to the
               sluggishness of the measurement or, conversely, to the
               sensitivity of the average to short bursts. Higher val-
               ues mean less sensitivity.

          The actual bandwidth of the underlying link may not be
          known, for example in the case of PPoE or PPTP connections
          which in fact may send over a pipe, instead of over a physi-
          cal device. CBQ is quite resilient to major errors in the
          configured bandwidth, probably a the cost of coarser shap-

          Default kernels rely on coarse timing information for making

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          decisions. These may make shaping precise in the long term,
          but inaccurate on second long scales.

          See tc-cbq-details(8) for hints on how to improve this.

          o    Sally Floyd and Van Jacobson, "Link-sharing and
               Resource Management Models for Packet Networks",
               IEEE/ACM Transactions on Networking, Vol.3, No.4, 1995

          o    Sally Floyd, "Notes on CBQ and Guaranteed Service",

          o    Sally Floyd, "Notes on Class-Based Queueing: Setting
               Parameters", 1996

          o    Sally Floyd and Michael Speer, "Experimental Results
               for Class-Based Queueing", 1998, not published.


          Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage
          maintained by bert hubert <ahu@ds9a.nl>

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