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CBQ(8)                               Linux                              CBQ(8)

       CBQ - Class Based Queueing

       tc qdisc ... dev dev ( parent classid | root) [ handle major: ] cbq 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.

       Shaping  is  done  using  link idle time calculations, and actions taken if these calculations deviate from set

       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.

       From the kernel's perspective, this is hard to measure, so CBQ instead derives the idle time from the number of
       microseconds (in fact, jiffies) that elapse between  requests from the device driver for  more  data.  Combined
       with the  knowledge of packet sizes, this is used to approximate how full or empty the link is.

       This  is  rather  circumspect and doesn't always arrive at proper results. For example, what is the actual link
       speed of an interface that is not really able to transmit the full 100mbit/s of  data,  perhaps  because  of  a
       badly implemented driver? A PCMCIA network card will also never achieve 100mbit/s because of the way the bus is
       designed - again, how do we calculate the idle time?

       The physical link bandwidth may be ill defined in case of not-quite-real network devices like PPP over Ethernet
       or  PPTP over TCP/IP. The effective bandwidth in that case is probably determined by the efficiency of pipes to
       userspace - which not defined.

       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. If a verdict is reached, this process is repeated for the recipient class  which  might  have
       further means of classifying traffic to its children, if any.

       CBQ has the following methods available to classify a packet to any child classes.

       (i)    skb->priority class encoding.  Can be set from userspace by an application with the SO_PRIORITY setsock-
              opt.  The skb->priority class encoding only applies if the skb->priority holds a major:minor  handle  of
              an existing class within  this qdisc.

       (ii)   tc filters attached to the class.

       (iii)  The  defmap  of  a class, as set with the split & defmap parameters. The defmap may contain instructions
              for each possible Linux packet priority.

       Each class also has a level.  Leaf nodes, attached to the bottom of the class hierarchy, have a level of 0.

       Classification is a loop, which terminates when a leaf class is found. At any point the loop may  jump  to  the
       fallback algorithm.

       The loop consists of the following steps:

       (i)    If  the  packet is generated locally and has a valid classid encoded within its skb->priority, choose it
              and terminate.

       (ii)   Consult the tc filters, if any, attached to this child. If these return a class  which  is  not  a  leaf
              class, restart loop from the class returned.  If it is a leaf, choose it and terminate.

       (iii)  If  the  tc  filters  did not return a class, but did return a classid, try to find a class with that id
              within this qdisc.  Check if the found class is of a lower level than the current class. If so, and  the
              returned class is not a leaf node, restart the loop at the found class. If it is a leaf node, terminate.
              If we found an upward reference to a higher level, enter the fallback algorithm.

       (iv)   If the tc filters did not return a class, nor a valid reference to one, consider the minor number of the
              reference  to  be the priority. Retrieve a class from the defmap of this class for the priority. If this
              did not contain a class, consult the defmap of this class for the  BEST_EFFORT  class.  If  this  is  an
              upward  reference,  or  no BEST_EFFORT class was defined, enter the fallback algorithm. If a valid class
              was found, and it is not a leaf node, restart the loop at this class. If it is a  leaf,  choose  it  and
              terminate.  If  neither  the priority distilled from the classid, nor the BEST_EFFORT priority yielded a
              class, enter the fallback algorithm.

       The fallback algorithm resides outside of the loop and is as follows.

       (i)    Consult the defmap of the class at which the jump to fallback occured. If the defmap  contains  a  class
              for the priority of the class (which is related to the TOS field), choose this class and terminate.

       (ii)   Consult the map for a class for the BEST_EFFORT priority. If found, choose it, and terminate.

       (iii)  Choose the class at which break out to the fallback algorithm occurred. Terminate.

       The packet is enqueued to the class which was chosen when either algorithm terminated. It is therefore possible
       for a packet to be enqueued *not* at a leaf node, but in the middle of the hierarchy.

       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.


       Each class is not allowed to send at length though - they can only dequeue a configurable amount of data during
       each round.

       If a class is about to go overlimit, and it is not bounded it will try to borrow avgidle from siblings that are
       not isolated.  This process is repeated from the bottom upwards. If a class is unable to borrow enough  avgidle
       to  send a packet, it is throttled and not asked for a packet for enough time for the avgidle to increase above


       The root qdisc of a CBQ 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.

       avpkt bytes
              For calculations, the average packet size must be known. It is silently capped at a minimum  of  2/3  of
              the interface MTU. 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. Defaults to 5. Lower values imply greater sensitivity. Must be between 0 and 31.

       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.

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

       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.

              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.

              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.

       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 Guarantee 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                        8 December 2001                         CBQ(8)