CBQ(8)                  (8 December 2001)                  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 ] [ 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.

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

          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 throt-
          tled 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

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          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 consid-
          ers recent packets to be exponentially more important than
          past ones. The Unix loadaverage is calculated in the same

          The calculated idle time is subtracted from the EWMA mea-
          sured one, the resulting number is called 'avgidle'. A per-
          fectly 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 clas-
          sifying 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 setsockopt.  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.

               The defmap of a class, as set with the split & defmap

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               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

          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

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

               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 ref-
               erence 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 prior-
               ity yielded a class, enter the fallback algorithm.

          The fallback algorithm resides outside of the loop and is as

          (i)  Consult the defmap of the class at which the jump to
               fallback occurred. If the defmap contains a class for

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               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 prior-
               ity. If found, choose it, and terminate.

               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 mid-
          dle 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 (low-
          est 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 priori-


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

          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 zero.


          The root qdisc of a CBQ class tree has the following parame-

          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.

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          handle major:
               Like all other qdiscs, the CBQ can be assigned a han-
               dle. 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. Manda-

          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.
               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 opera-

          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.

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          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 dur-
               ing each round of the process. This parameter is
               weighted using the renormalized class weight described

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

          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 max-
               burst or minburst. Mandatory if specifying maxburst or

               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.

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               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

          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 Signifi-
               cant Bit corresponds to priority zero. The split param-
               eter 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

          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

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               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, <kuznet@ms2.inr.ac.ru>. This manpage
          maintained by bert hubert <ahu@ds9a.nl>

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