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 defined.

       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. 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

       (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

       (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 follows.

       (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 hierarchy.


       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.

       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.

       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.

       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 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.

              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

              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

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iproute2                       8 December 2001                        CBQ(8)

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