CBQ - Class Based Queueing
tc qdisc ... dev
dev ( parent
classid | root) [ handle
major: ] cbq [ allot
bytes ] avpkt
bytes ] [ ewma
log ] [ mpu
tc class ... dev
major:[minor] [ classid
major:minor ] cbq allot
bytes [ bandwidth
rate ] [ rate
rate ] prio
priority [ weight
weight ] [ minburst
] [ maxburst
packets ] [ ewma
log ] [ cell
bytes [ mpu
bytes ] [ bounded isolated ] [ split
handle & defmap
defmap ] [ estimator
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.
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
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
Within the one CBQ instance many classes may exist. Each of these classes
contains another qdisc, by default tc-pfifo
When enqueueing a packet, CBQ starts at the root and uses various methods to
determine which class should receive the data.
In the absence of uncommon configuration options, the process is rather easy. At
each node we look for an instruction, and then go to the class the instruction
refers us to. If the class found is a barren leaf-node (without children), we
enqueue the packet there. If it is not yet a leaf node, we do the whole thing
over again starting from that node.
The following actions are performed, in order at each node we visit, until one
sends us to another node, or terminates the process.
- Consult filters attached to the class. If sent to a
leafnode, we are done. Otherwise, restart.
- Consult the defmap for the priority assigned to this
packet, which depends on the TOS bits. Check if the referral is leafless,
- Ask the defmap for instructions for the 'best effort'
priority. Check the answer for leafness, otherwise restart.
- If none of the above returned with an instruction, enqueue
at this node.
This algorithm makes sure that a packet always ends up somewhere, even while you
are busy building your configuration.
For more details, see tc-cbq-details(8).
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
Classes by default borrow bandwidth from their siblings. A class can be
prevented from doing so by declaring it 'bounded'. A class can also indicate
its unwillingness to lend out bandwidth by being 'isolated'.
The root of a CBQ qdisc 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
- 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, but
very useful if classes will be generated within this qdisc.
- allot bytes
- This allotment is the 'chunkiness' of link sharing and is
used for determining packet transmission time tables. The qdisc allot
differs slightly from the class allot discussed below. Optional. Defaults
to a reasonable value, related to avpkt.
- avpkt bytes
- The average size of a packet is needed for calculating
maxidle, and is also used for making sure 'allot' has a safe value.
- 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.
- The cell size determines he granularity of packet
transmission time calculations. Has a sensible default.
- 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
- 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. Lower values imply greater sensitivity. Must be between
0 and 31. Defaults to 5.
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 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
- 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. Silently capped at a minimum of
3/2 avpkt. Mandatory.
- prio priority
- In the round-robin process, classes with the lowest
priority field are tried for packets first. Mandatory.
- See the QDISC section.
- 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. Optional.
- 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. Optional.
- Signifies that this class will not borrow bandwidth from
- Means that this class will not borrow bandwidth to its
- 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.
The actual bandwidth of the underlying link may not be known, for example in the
case of PPoE or PPTP connections which in fact may send over a pipe, instead
of over a physical device. CBQ is quite resilient to major errors in the
configured bandwidth, probably a the cost of coarser shaping.
Default kernels rely on coarse timing information for making decisions. These
may make shaping precise in the long term, but inaccurate on second long
for hints on how to improve this.
- Sally Floyd and Van Jacobson, "Link-sharing and
Resource Management Models for Packet Networks", IEEE/ACM
Transactions on Networking, Vol.3, No.4, 1995
- Sally Floyd, "Notes on CBQ and Guaranteed
- Sally Floyd, "Notes on Class-Based Queueing: Setting
- Sally Floyd and Michael Speer, "Experimental Results
for Class-Based Queueing", 1998, not published.
Alexey N. Kuznetsov, <email@example.com>. This manpage maintained by
bert hubert <firstname.lastname@example.org>