CBQ(8)                               Linux                              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 ] [ split 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

       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 exponen-
       tial 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 con-
       straints, 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 meth-
       ods 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

       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  leaf-
              less, 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  con-
       tinue 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'.

       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 exist-
              ing 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.

       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 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 toler-
              ant of bursts. You can't set maxidle  directly,  only  via  this

              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  min-
              burst  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 sud-
              den 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 siblings

       split major:minor & defmap bitmap[/bitmap]
              If  consulting  filters  attached to a class did not give a ver-
              dict, 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 corre-
              sponds  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 slug-
              gishness  of  the measurement or, conversely, to the sensitivity
              of the average to short bursts. Higher values mean  less  sensi-

       The actual bandwidth of the underlying link may not be known, for exam-
       ple 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 sec-
       ond long scales.

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

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

       o      Sally Floyd, "Notes on CBQ and Guaranteed Service", 1995

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

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


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

iproute2                       16 December 2001                         CBQ(8)
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