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           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  effi-
           ciency of pipes to userspace - which not defined.
           During operations, the effective idletime is measured using an exponen-
           tial 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


           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 meth-
           ods 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 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  parame-
                  ters.  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 clas-
                  sid, 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
           (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
                  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  algo-
           rithm  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  con-
           tinue 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.
           TAIN AGAIN.


           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 exist-
                  ing 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
                  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.
                  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 min-
                  burst 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 sud-
                  den  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  ver-
                  dict,  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 corre-
                  sponds 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.


           o      Sally Floyd and Van Jacobson, "Link-sharing and Resource Manage-
                  ment  Models for Packet Networks", IEEE/ACM Transactions on Net-
                  working, Vol.3, No.4, 1995
           o      Sally Floyd, "Notes on CBQ and Guarantee Service", 1995
           o      Sally Floyd, "Notes on  Class-Based  Queueing:  Setting  Parame-
                  ters", 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)


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