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       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 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 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


       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

       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 ’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
              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.

       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  smoothes
              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
              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 siblings

       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.


       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.


       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.




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