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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 timecon-
       stant ]

       Class Based Queueing is a classful qdisc that implements a rich linksharing hierar-
       chy  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  throt-
       tles and is then 'overlimit'.

       Conversely,  an  idle link might amass a huge avgidle, which would then allow infi-
       nite bandwidths after a few hours of silence. To prevent this, avgidle is capped at

       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 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 methods to deter-
       mine 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 unwilling-
       ness 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 determin-
              ing 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. Mandatory.

       cell   The cell size determines he granularity of packet transmission time calcula-
              tions. 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  Expo-
              nentially  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. Manda-

       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  min-

              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 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 avail-
              able, 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  Queue-
              ing", 1998, not published.


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

iproute2                       16 December 2001                         CBQ(8)

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