tc-cbq-details(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 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.


       Shaping is done using link idle time calculations, and actions
       taken if these calculations deviate from set limits.

       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.

       From the kernel's perspective, this is hard to measure, so CBQ
       instead derives the idle time from the number of microseconds (in
       fact, jiffies) that elapse between  requests from the device
       driver for more data. Combined with the  knowledge of packet
       sizes, this is used to approximate how full or empty the link is.

       This is rather circumspect and doesn't always arrive at proper
       results. For example, what is the actual link speed of an
       interface that is not really able to transmit the full 100mbit/s
       of data, perhaps because of a badly implemented driver? A PCMCIA
       network card will also never achieve 100mbit/s because of the way
       the bus is designed - again, how do we calculate the idle time?

       The physical link bandwidth may be ill defined in case of not-
       quite-real network devices like PPP over Ethernet or PPTP over
       TCP/IP. The effective bandwidth in that case is probably
       determined by the efficiency of pipes to userspace - which not

       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. If a
       verdict is reached, this process is repeated for the recipient
       class which might have further means of classifying traffic to
       its children, if any.

       CBQ has the following methods available to classify a packet to
       any child classes.

       (i)    skb->priority class encoding.  Can be set from userspace
              by an application with the SO_PRIORITY setsockopt.  The
              skb->priority class encoding only applies if the
              skb->priority holds a major:minor handle of an existing
              class within  this qdisc.

       (ii)   tc filters attached to the class.

       (iii)  The defmap of a class, as set with the split & defmap
              parameters. The defmap may contain instructions for each
              possible Linux packet priority.

       Each class also has a level.  Leaf nodes, attached to the bottom
       of the class hierarchy, have a level of 0.


       Classification is a loop, which terminates when a leaf class is
       found. At any point the loop may jump to the fallback algorithm.

       The loop consists of the following steps:

       (i)    If the packet is generated locally and has a valid classid
              encoded within its skb->priority, choose it and terminate.

       (ii)   Consult the tc filters, if any, attached to this child. If
              these return a class which is not a leaf class, restart
              loop from the class returned.  If it is a leaf, choose it
              and terminate.

       (iii)  If the tc filters did not return a class, but did return a
              classid, try to find a class with that id within this
              qdisc.  Check if the found class is of a lower level than
              the current class. If so, and the returned class is not a
              leaf node, restart the loop at the found class. If it is a
              leaf node, terminate.  If we found an upward reference to
              a higher level, enter the fallback algorithm.

       (iv)   If the tc filters did not return a class, nor a valid
              reference to one, consider the minor number of the
              reference to be the priority. Retrieve a class from the
              defmap of this class for the priority. If this did not
              contain a class, consult the defmap of this class for the
              BEST_EFFORT class. If this is an upward reference, or no
              BEST_EFFORT class was defined, enter the fallback
              algorithm. If a valid class was found, and it is not a
              leaf node, restart the loop at this class. If it is a
              leaf, choose it and terminate. If neither the priority
              distilled from the classid, nor the BEST_EFFORT priority
              yielded a class, enter the fallback algorithm.

       The fallback algorithm resides outside of the loop and is as

       (i)    Consult the defmap of the class at which the jump to
              fallback occurred. If the defmap contains a class for the
              priority of the class (which is related to the TOS field),
              choose this class and terminate.

       (ii)   Consult the map for a class for the BEST_EFFORT priority.
              If found, choose it, and terminate.

       (iii)  Choose the class at which break out to the fallback
              algorithm occurred. Terminate.

       The packet is enqueued to the class which was chosen when either
       algorithm terminated. It is therefore possible for a packet to be
       enqueued *not* at a leaf node, but in the middle of the


       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.


       Each class is not allowed to send at length though - they can
       only dequeue a configurable amount of data during each round.

       If a class is about to go overlimit, and it is not bounded it
       will try to borrow avgidle from siblings that are not isolated.
       This process is repeated from the bottom upwards. If a class is
       unable to borrow enough avgidle to send a packet, it is throttled
       and not asked for a packet for enough time for the avgidle to
       increase above zero.


QDISC         top

       The root qdisc of a CBQ 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.

       avpkt bytes
              For calculations, the average packet size must be known.
              It is silently capped at a minimum of 2/3 of the interface
              MTU. 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. Defaults to 5.
              Lower values imply greater sensitivity. Must be between 0
              and 31.

       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.

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

       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.

              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.

              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.

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 Guarantee 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

       This page is part of the iproute2 (utilities for controlling
       TCP/IP networking and traffic) project.  Information about the
       project can be found at 
       If you have a bug report for this manual page, send it to,  This page was
       obtained from the project's upstream Git repository
       ⟨⟩ on
       2022-12-17.  (At that time, the date of the most recent commit
       that was found in the repository was 2022-12-14.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to

iproute2                     8 December 2001                      CBQ(8)

Pages that refer to this page: tc-cbq(8)