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

       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


       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 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.

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

       (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

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.

       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 minburst.

              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

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

COLOPHON         top

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iproute2                      16 December 2001                        CBQ(8)