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       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 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 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 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. 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

       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.

       (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

       (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  packets,  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 zero.



       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.

       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 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  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 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.

       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.

       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 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.

              Signifies  that  this  class  will not borrow bandwidth from its

              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.


       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 <>