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


   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 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 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 <>


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