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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 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 cal-
       culated 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 below.

       Within the one CBQ instance many classes may exist. Each of these classes contains another  qdisc,  by  default

       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

       (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 config-

       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 pack-
       ets, 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. Mandatory.

       cell   The cell size determines he granularity  of  packet  transmission  time  calculations.  Has  a  sensible

       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

       ewma log
              When CBQ needs to measure the average idle time, it does so using an Exponentially Weighted Moving Aver-
              age 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.

       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  mat-
              ter.  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.

       prio priority
              In the round-robin process, classes with the lowest priority field are tried for packets  first.  Manda-

       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 min-
              burst 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 available, numbered from 0 to 15.

              The defmap specifies which priorities this class wants to receive, specified as a bitmap. The Least Sig-
              nificant  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 connec-
       tions 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 <>

iproute2                       16 December 2001                         CBQ(8)