{
    "mode": "man",
    "parameter": "tc-cbq-details",
    "section": "8",
    "url": "https://www.chedong.com/phpMan.php/man/tc-cbq-details/8/json",
    "generated": "2026-05-30T07:11:40Z",
    "synopsis": "tc  qdisc  ...  dev  dev ( parent classid | root) [ handle major: ] cbq avpkt bytes bandwidth\nrate [ cell bytes ] [ ewma log ] [ mpu bytes ]\ntc class ... dev dev parent major:[minor] [ classid major:minor ] cbq allot bytes [ bandwidth\nrate  ] [ rate rate ] prio priority [ weight weight ] [ minburst packets ] [ maxburst packets\n] [ ewma log ] [ cell bytes ] avpkt bytes [ mpu bytes ] [ bounded isolated ] [ split handle &\ndefmap defmap ] [ estimator interval timeconstant ]",
    "sections": {
        "NAME": {
            "content": "CBQ - Class Based Queueing\n",
            "subsections": []
        },
        "SYNOPSIS": {
            "content": "tc  qdisc  ...  dev  dev ( parent classid | root) [ handle major: ] cbq avpkt bytes bandwidth\nrate [ cell bytes ] [ ewma log ] [ mpu bytes ]\n\ntc class ... dev dev parent major:[minor] [ classid major:minor ] cbq allot bytes [ bandwidth\nrate  ] [ rate rate ] prio priority [ weight weight ] [ minburst packets ] [ maxburst packets\n] [ ewma log ] [ cell bytes ] avpkt bytes [ mpu bytes ] [ bounded isolated ] [ split handle &\ndefmap defmap ] [ estimator interval timeconstant ]\n\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "Class  Based  Queueing  is  a  classful qdisc that implements a rich linksharing hierarchy of\nclasses. It contains shaping elements as well as prioritizing capabilities. Shaping  is  per‐\nformed using link idle time calculations based on the timing of dequeue events and underlying\nlink bandwidth.\n\n",
            "subsections": []
        },
        "SHAPING ALGORITHM": {
            "content": "Shaping is done using link idle time calculations, and actions taken  if  these  calculations\ndeviate from set limits.\n\nWhen  shaping  a 10mbit/s connection to 1mbit/s, the link will be idle 90% of the time. If it\nisn't, it needs to be throttled so that it IS idle 90% of the time.\n\nFrom the kernel's perspective, this is hard to measure, so CBQ instead derives the idle  time\nfrom the number of microseconds (in fact, jiffies) that elapse between  requests from the de‐\nvice driver for more data. Combined with the  knowledge of packet sizes, this is used to  ap‐\nproximate how full or empty the link is.\n\nThis  is rather circumspect and doesn't always arrive at proper results. For example, what is\nthe actual link speed of an interface that is not really able to transmit the full  100mbit/s\nof data, perhaps because of a badly implemented driver? A PCMCIA network card will also never\nachieve 100mbit/s because of the way the bus is designed - again, how  do  we  calculate  the\nidle time?\n\nThe physical link bandwidth may be ill defined in case of not-quite-real network devices like\nPPP over Ethernet or PPTP over TCP/IP. The effective bandwidth in that case is  probably  de‐\ntermined by the efficiency of pipes to userspace - which not defined.\n\nDuring  operations,  the  effective idletime is measured using an exponential weighted moving\naverage (EWMA), which considers recent packets to be exponentially more important  than  past\nones. The Unix loadaverage is calculated in the same way.\n\nThe  calculated  idle  time is subtracted from the EWMA measured one, the resulting number is\ncalled 'avgidle'. A perfectly loaded link has an avgidle of zero: packets arrive  exactly  at\nthe calculated interval.\n\nAn  overloaded  link has a negative avgidle and if it gets too negative, CBQ throttles and is\nthen 'overlimit'.\n\nConversely, an idle link might amass a huge avgidle, which would then  allow  infinite  band‐\nwidths after a few hours of silence. To prevent this, avgidle is capped at maxidle.\n\nIf  overlimit,  in  theory, the CBQ could throttle itself for exactly the amount of time that\nwas calculated to pass between packets, and then pass one packet, and throttle again. Due  to\ntimer resolution constraints, this may not be feasible, see the minburst parameter below.\n\n",
            "subsections": []
        },
        "CLASSIFICATION": {
            "content": "Within  the  one  CBQ instance many classes may exist. Each of these classes contains another\nqdisc, by default tc-pfifo(8).\n\nWhen enqueueing a packet, CBQ starts at the root and uses various methods to determine  which\nclass  should receive the data. If a verdict is reached, this process is repeated for the re‐\ncipient class which might have further means of classifying traffic to its children, if any.\n\nCBQ has the following methods available to classify a packet to any child classes.\n\n(i)    skb->priority class encoding.  Can be set from userspace by an  application  with  the\nSOPRIORITY  setsockopt.   The  skb->priority  class  encoding  only  applies  if  the\nskb->priority holds a major:minor handle of an existing class within  this qdisc.\n\n(ii)   tc filters attached to the class.\n\n(iii)  The defmap of a class, as set with the split & defmap parameters. The defmap may  con‐\ntain instructions for each possible Linux packet priority.\n\n\nEach class also has a level.  Leaf nodes, attached to the bottom of the class hierarchy, have\na level of 0.\n",
            "subsections": []
        },
        "CLASSIFICATION ALGORITHM": {
            "content": "Classification is a loop, which terminates when a leaf class is found. At any point the  loop\nmay jump to the fallback algorithm.\n\nThe loop consists of the following steps:\n\n(i)    If  the  packet  is  generated  locally  and  has  a  valid classid encoded within its\nskb->priority, choose it and terminate.\n\n\n(ii)   Consult the tc filters, if any, attached to this child. If these return a class  which\nis not a leaf class, restart loop from the class returned.  If it is a leaf, choose it\nand terminate.\n\n(iii)  If the tc filters did not return a class, but did return a  classid,  try  to  find  a\nclass  with  that  id within this qdisc.  Check if the found class is of a lower level\nthan the current class. If so, and the returned class is not a leaf node, restart  the\nloop  at the found class. If it is a leaf node, terminate.  If we found an upward ref‐\nerence to a higher level, enter the fallback algorithm.\n\n(iv)   If the tc filters did not return a class, nor a valid reference to one,  consider  the\nminor  number of the reference to be the priority. Retrieve a class from the defmap of\nthis class for the priority. If this did not contain a class, consult  the  defmap  of\nthis  class  for the BESTEFFORT class. If this is an upward reference, or no BESTEF‐‐\nFORT class was defined, enter the fallback algorithm. If a valid class was found,  and\nit  is not a leaf node, restart the loop at this class. If it is a leaf, choose it and\nterminate. If neither the priority distilled from the  classid,  nor  the  BESTEFFORT\npriority yielded a class, enter the fallback algorithm.\n\nThe fallback algorithm resides outside of the loop and is as follows.\n\n(i)    Consult  the defmap of the class at which the jump to fallback occurred. If the defmap\ncontains a class for the priority of the class (which is related to  the  TOS  field),\nchoose this class and terminate.\n\n(ii)   Consult  the  map  for  a class for the BESTEFFORT priority. If found, choose it, and\nterminate.\n\n(iii)  Choose the class at which break out to the fallback algorithm occurred. Terminate.\n\nThe packet is enqueued to the class which was chosen when either algorithm terminated. It  is\ntherefore possible for a packet to be enqueued *not* at a leaf node, but in the middle of the\nhierarchy.\n\n",
            "subsections": []
        },
        "LINK SHARING ALGORITHM": {
            "content": "When dequeuing for sending to the network device, CBQ decides which of its  classes  will  be\nallowed  to  send.  It  does  so with a Weighted Round Robin process in which each class with\npackets gets a chance to send in turn. The WRR process starts by asking the highest  priority\nclasses  (lowest  numerically - highest semantically) for packets, and will continue to do so\nuntil they have no more data to offer, in which case the process repeats  for  lower  priori‐\nties.\n",
            "subsections": [
                {
                    "name": "CERTAINTY ENDS HERE, ANK PLEASE HELP",
                    "content": "Each  class  is  not  allowed to send at length though - they can only dequeue a configurable\namount of data during each round.\n\nIf a class is about to go overlimit, and it is not bounded it will try to borrow avgidle from\nsiblings that are not isolated.  This process is repeated from the bottom upwards. If a class\nis unable to borrow enough avgidle to send a packet, it is throttled  and  not  asked  for  a\npacket for enough time for the avgidle to increase above zero.\n"
                },
                {
                    "name": "I REALLY NEED HELP FIGURING THIS OUT. REST OF DOCUMENT IS PRETTY CERTAIN AGAIN.",
                    "content": ""
                }
            ]
        },
        "QDISC": {
            "content": "The root qdisc of a CBQ class tree has the following parameters:\n\n\nparent major:minor | root\nThis  mandatory parameter determines the place of the CBQ instance, either at the root\nof an interface or within an existing class.\n\nhandle major:\nLike all other qdiscs, the CBQ can be assigned a handle. Should consist only of a  ma‐\njor number, followed by a colon. Optional.\n\navpkt bytes\nFor  calculations,  the  average packet size must be known. It is silently capped at a\nminimum of 2/3 of the interface MTU. Mandatory.\n\nbandwidth rate\nTo determine the idle time, CBQ must know the bandwidth of  your  underlying  physical\ninterface, or parent qdisc. This is a vital parameter, more about it later. Mandatory.\n\ncell   The  cell size determines he granularity of packet transmission time calculations. Has\na sensible default.\n\nmpu    A zero sized packet may still take time to transmit. This value is the lower  cap  for\npacket  transmission  time  calculations  -  packets smaller than this value are still\ndeemed to have this size. Defaults to zero.\n\newma log\nWhen CBQ needs to measure the average idle time, it does  so  using  an  Exponentially\nWeighted Moving Average which smooths out measurements into a moving average. The EWMA\nLOG determines how much smoothing occurs. Defaults to 5. Lower  values  imply  greater\nsensitivity. Must be between 0 and 31.\n\nA  CBQ  qdisc  does not shape out of its own accord. It only needs to know certain parameters\nabout the underlying link. Actual shaping is done in classes.\n\n",
            "subsections": []
        },
        "CLASSES": {
            "content": "Classes have a host of parameters to configure their operation.\n\n\nparent major:minor\nPlace of this class within the hierarchy. If attached directly to a qdisc and  not  to\nanother class, minor can be omitted. Mandatory.\n\nclassid major:minor\nLike  qdiscs, classes can be named. The major number must be equal to the major number\nof the qdisc to which it belongs. Optional, but needed if this class is going to  have\nchildren.\n\nweight weight\nWhen  dequeuing to the interface, classes are tried for traffic in a round-robin fash‐\nion. Classes with a higher configured qdisc will generally have more traffic to  offer\nduring  each round, so it makes sense to allow it to dequeue more traffic. All weights\nunder a class are normalized, so only the ratios matter. Defaults  to  the  configured\nrate, unless the priority of this class is maximal, in which case it is set to 1.\n\nallot bytes\nAllot  specifies  how many bytes a qdisc can dequeue during each round of the process.\nThis parameter is weighted using the renormalized class weight described above.\n\n\npriority priority\nIn the round-robin process, classes with the lowest priority field are tried for pack‐\nets first. Mandatory.\n\n\nrate rate\nMaximum rate this class and all its children combined can send at. Mandatory.\n\n\nbandwidth rate\nThis  is different from the bandwidth specified when creating a CBQ disc. Only used to\ndetermine maxidle and offtime, which are only calculated when specifying  maxburst  or\nminburst. Mandatory if specifying maxburst or minburst.\n\n\nmaxburst\nThis  number  of packets is used to calculate maxidle so that when avgidle is at maxi‐\ndle, this number of average packets can be burst before avgidle drops  to  0.  Set  it\nhigher  to  be  more tolerant of bursts. You can't set maxidle directly, only via this\nparameter.\n\n\nminburst\nAs mentioned before, CBQ needs to throttle in case of overlimit. The ideal solution is\nto  do  so for exactly the calculated idle time, and pass 1 packet. However, Unix ker‐\nnels generally have a hard time scheduling events shorter than 10ms, so it  is  better\nto  throttle  for  a longer period, and then pass minburst packets in one go, and then\nsleep minburst times longer.\n\nThe time to wait is called the offtime. Higher values of minburst lead to  more  accu‐\nrate shaping in the long term, but to bigger bursts at millisecond timescales.\n\n\nminidle\nIf  avgidle  is  below 0, we are overlimits and need to wait until avgidle will be big\nenough to send one packet. To prevent a sudden burst from shutting down the link for a\nprolonged period of time, avgidle is reset to minidle if it gets too low.\n\nMinidle  is  specified in negative microseconds, so 10 means that avgidle is capped at\n-10us.\n\n\nbounded\nSignifies that this class will not borrow bandwidth from its siblings.\n\nisolated\nMeans that this class will not borrow bandwidth to its siblings\n\n\nsplit major:minor & defmap bitmap[/bitmap]\nIf consulting filters attached to a class did not give a verdict, CBQ can  also  clas‐\nsify  based on the packet's priority. There are 16 priorities available, numbered from\n0 to 15.\n\nThe defmap specifies which priorities this class wants to receive, specified as a bit‐\nmap. The Least Significant Bit corresponds to priority zero. The split parameter tells\nCBQ at which class the decision must be made, which should be a (grand)parent  of  the\nclass you are adding.\n\nAs  an example, 'tc class add ... classid 10:1 cbq .. split 10:0 defmap c0' configures\nclass 10:0 to send packets with priorities 6 and 7 to 10:1.\n\nThe complimentary configuration would then be: 'tc class add ... classid 10:2 cbq  ...\nsplit 10:0 defmap 3f' Which would send all packets 0, 1, 2, 3, 4 and 5 to 10:1.\n\nestimator interval timeconstant\nCBQ  can  measure  how much bandwidth each class is using, which tc filters can use to\nclassify packets with. In order to determine the bandwidth it uses a very simple esti‐\nmator that measures once every interval microseconds how much traffic has passed. This\nagain is a EWMA, for which the time constant can be specified, also  in  microseconds.\nThe  time  constant corresponds to the sluggishness of the measurement or, conversely,\nto the sensitivity of the average to short bursts. Higher values mean  less  sensitiv‐\nity.\n\n\n\n",
            "subsections": []
        },
        "SOURCES": {
            "content": "o      Sally  Floyd and Van Jacobson, \"Link-sharing and Resource Management Models for Packet\nNetworks\", IEEE/ACM Transactions on Networking, Vol.3, No.4, 1995\n\n\no      Sally Floyd, \"Notes on CBQ and Guarantee Service\", 1995\n\n\no      Sally Floyd, \"Notes on Class-Based Queueing: Setting Parameters\", 1996\n\n\no      Sally Floyd and Michael Speer, \"Experimental Results for Class-Based Queueing\",  1998,\nnot published.\n\n\n\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "tc(8)\n\n",
            "subsections": []
        },
        "AUTHOR": {
            "content": "Alexey   N.  Kuznetsov,  <kuznet@ms2.inr.ac.ru>.  This  manpage  maintained  by  bert  hubert\n<ahu@ds9a.nl>\n\n\n\niproute2                                   8 December 2001                                    CBQ(8)",
            "subsections": []
        }
    },
    "summary": "CBQ - Class Based Queueing",
    "flags": [],
    "examples": [],
    "see_also": [
        {
            "name": "tc",
            "section": "8",
            "url": "https://www.chedong.com/phpMan.php/man/tc/8/json"
        }
    ]
}