tc-cbq-details


SYNOPSIS
       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 ]


DESCRIPTION
       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 ALGORITHM
       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
       avgidle of zero: packets arrive exactly at the calculated interval.


CLASSIFICATION
       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 ALGORITHM
       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
              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
              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  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.


LINK SHARING ALGORITHM
       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.

       CERTAINTY ENDS HERE, ANK PLEASE HELP

       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.

       I  REALLY  NEED HELP FIGURING THIS OUT. REST OF DOCUMENT IS PRETTY CER-
       TAIN AGAIN.


QDISC
       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
              For  calculations,  the average packet size must be known. It is
              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.


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 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.

              ant  of  bursts.  You  can't set maxidle directly, only via this
              parameter.


       minburst
              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.


       minidle
              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.


       bounded
              Signifies  that  this  class  will not borrow bandwidth from its
              siblings.

       isolated
              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.



SOURCES
       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.




SEE ALSO
       tc(8)


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





iproute2                        8 December 2001                         CBQ(8)
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