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 time.
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
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.
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.
(i) Consult filters attached to the class. If sent to a leafnode,
we are done. Otherwise, restart.
(ii) Consult the defmap for the priority assigned to this packet,
which depends on the TOS bits. Check if the referral is
leafless, otherwise restart.
(iii) Ask the defmap for instructions for the 'best effort'
priority. Check the answer for leafness, otherwise restart.
(iv) If none of the above returned with an instruction, enqueue at
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 priorities.
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
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
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 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.
The average size of a packet is needed for calculating
maxidle, and is also used for making sure 'allot' has a safe
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.
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.
Place of this class within the hierarchy. If attached directly
to a qdisc and not to another class, minor can be omitted.
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.
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 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.
In the round-robin process, classes with the lowest priority
field are tried for packets first. Mandatory.
avpkt See the QDISC section.
Maximum rate this class and all its children combined can send
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
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
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 its
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 scales.
See tc-cbq-details(8) for hints on how to improve this.
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 Guaranteed Service", 1995
o Sally Floyd, "Notes on Class-Based Queueing: Setting
o Sally Floyd and Michael Speer, "Experimental Results for
Class-Based Queueing", 1998, not published.
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iproute2 16 December 2001 CBQ(8)