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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 node.

       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. 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.

       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.


       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.

       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. Optional.

              Signifies that this class will not borrow bandwidth from its siblings.

              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