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    Command:

    tc-cbq

    
    
    

    SYNOPSIS

           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

           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 ALGORITHM

           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.
    
    
    

    CLASSIFICATION

           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-
    
           (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).
    
    
    

    LINK SHARING ALGORITHM

           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'.
    
    
    

    QDISC

           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
                  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 smoothes
                  out measurements into a moving average. The EWMA LOG  determines
                  how  much smoothing occurs. Lower values imply greater sensitiv-
                  ity. 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

           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.
    
                  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
                  parameter.
    
           minburst
                  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.
    
           minidle
                  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.
    
           bounded
                  Signifies  that  this  class  will not borrow bandwidth from its
                  siblings.
    
           isolated
                  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.
                  tivity.
    
    
    

    BUGS

           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.
    
    
    

    SOURCES

           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.
    
    
    

    SEE ALSO

           tc(8)
    
    
    

    AUTHOR

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

    iproute2 16 December 2001 CBQ(8)

    
    
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