tc-cbq(8) — Linux manual page


CBQ(8)                            Linux                           CBQ(8)

NAME         top

       CBQ - Class Based Queueing

SYNOPSIS         top

       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
       ] [ split handle & defmap defmap ] [ estimator interval
       timeconstant ]

DESCRIPTION         top

       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 throttled so
       that it IS idle 90% of the time.

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

       The calculated idle time is subtracted from the EWMA measured
       one, the resulting number is called 'avgidle'. A perfectly 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 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 tc-pfifo(8).

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

       In the absence of uncommon configuration options, the process is
       rather easy.  At each node we look for an instruction, 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

       (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.

       (iii)  Ask the defmap for instructions for the 'best effort'
              priority. Check the answer for leafness, otherwise

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

       This algorithm makes sure that a packet always ends up somewhere,
       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 (lowest numerically - highest
       semantically) for packets, and will continue to do so until they
       have no more data to offer, in which case the process repeats for
       lower priorities.

       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'.

QDISC         top

       The root of a CBQ qdisc class tree has the following parameters:

       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 handle.
              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 reasonable 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. Mandatory.

       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.

       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         top

       Classes have a host of parameters to configure their operation.

       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 during
              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. Mandatory.

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

       avpkt  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
              maxburst 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. Optional.

              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.

              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 Significant Bit
              corresponds to priority zero. The split parameter 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 10:1.

       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 microseconds
              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 values
              mean less sensitivity.

BUGS         top

       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 physical device. CBQ is quite
       resilient to major errors in the configured bandwidth, probably a
       the cost of coarser shaping.

       Default kernels rely on coarse timing information for making
       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.

SOURCES         top

       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", 1995

       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.

SEE ALSO         top


AUTHOR         top

       Alexey N. Kuznetsov, <>. This manpage
       maintained by bert hubert <>

COLOPHON         top

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iproute2                    16 December 2001                      CBQ(8)

Pages that refer to this page: tc(8)