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NAME | SYNOPSIS | DESCRIPTION | SHAPING ALGORITHM | CLASSIFICATION | LINK SHARING ALGORITHM | QDISC | CLASSES | BUGS | SOURCES | SEE ALSO | AUTHOR | COLOPHON |
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CBQ(8) Linux CBQ(8)
CBQ - Class Based Queueing
tc qdisc ... dev dev ( parent classid | root) [ handle major: ]
cbq [ allot bytes ] 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.
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 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 parameter below.
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 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 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 '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 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, 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. Mandatory.
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.
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.
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. 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.
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.
avpkt 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.
maxburst
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.
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
minburst lead to more accurate shaping in the long term,
but to bigger bursts at millisecond timescales. Optional.
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 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.
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
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
Parameters", 1996
o Sally Floyd and Michael Speer, "Experimental Results for
Class-Based Queueing", 1998, not published.
tc(8)
Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage
maintained by bert hubert <ahu@ds9a.nl>
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2023-06-23. (At that time, the date of the most recent commit
that was found in the repository was 2023-06-09.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
iproute2 16 December 2001 CBQ(8)
Pages that refer to this page: tc(8)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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