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NAME | SYNOPSIS | DESCRIPTION | SHAPING ALGORITHM | CLASSIFICATION | CLASSIFICATION ALGORITHM | LINK SHARING ALGORITHM | QDISC | CLASSES | 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 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.
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 efficiency of pipes to userspace - which not
defined.
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. 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
parameters. 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 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
classid, 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 priority. If this did not
contain a class, consult the defmap of this class for the
BEST_EFFORT class. If this is an upward reference, or no
BEST_EFFORT class was defined, enter the fallback
algorithm. If a valid class was found, and it is not a
leaf node, restart the loop at this class. If it is a
leaf, choose it and terminate. If neither the priority
distilled from the classid, nor the BEST_EFFORT priority
yielded a class, enter the fallback algorithm.
The fallback algorithm resides outside of the loop and is as
follows.
(i) Consult the defmap of the class at which the jump to
fallback occurred. 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
algorithm terminated. It is therefore possible for a packet to be
enqueued *not* at a leaf node, but in the middle of the
hierarchy.
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.
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
CERTAIN AGAIN.
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
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.
avpkt bytes
For calculations, the average packet size must be known.
It is silently capped at a minimum of 2/3 of the interface
MTU. 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. 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 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.
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.
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.
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.
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 Guarantee 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 8 December 2001 CBQ(8)
Pages that refer to this page: tc-cbq(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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