--- /dev/null
+.TH CBQ 8 "8 December 2001" "iproute2" "Linux"
+.SH NAME
+CBQ \- Class Based Queueing
+.SH SYNOPSIS
+.B tc qdisc ... dev
+dev
+.B ( parent
+classid
+.B | root) [ handle
+major:
+.B ] cbq avpkt
+bytes
+.B bandwidth
+rate
+.B [ cell
+bytes
+.B ] [ ewma
+log
+.B ] [ mpu
+bytes
+.B ]
+
+.B tc class ... dev
+dev
+.B parent
+major:[minor]
+.B [ classid
+major:minor
+.B ] cbq allot
+bytes
+.B [ bandwidth
+rate
+.B ] [ rate
+rate
+.B ] prio
+priority
+.B [ weight
+weight
+.B ] [ minburst
+packets
+.B ] [ maxburst
+packets
+.B ] [ ewma
+log
+.B ] [ cell
+bytes
+.B ] avpkt
+bytes
+.B [ mpu
+bytes
+.B ] [ bounded isolated ] [ split
+handle
+.B & defmap
+defmap
+.B ] [ estimator
+interval timeconstant
+.B ]
+
+.SH DESCRIPTION
+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.
+
+.SH SHAPING ALGORITHM
+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
+.B 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
+.B minburst
+parameter below.
+
+.SH CLASSIFICATION
+Within the one CBQ instance many classes may exist. Each of these classes
+contains another qdisc, by default
+.BR 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.
+.TP
+(i)
+.B skb->priority class encoding.
+Can be set from userspace by an application with the
+.B SO_PRIORITY
+setsockopt.
+The
+.B skb->priority class encoding
+only applies if the skb->priority holds a major:minor handle of an existing
+class within this qdisc.
+.TP
+(ii)
+tc filters attached to the class.
+.TP
+(iii)
+The defmap of a class, as set with the
+.B split & defmap
+parameters. The defmap may contain instructions for each possible Linux packet
+priority.
+
+.P
+Each class also has a
+.B level.
+Leaf nodes, attached to the bottom of the class hierarchy, have a level of 0.
+.SH CLASSIFICATION ALGORITHM
+
+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:
+.TP
+(i)
+If the packet is generated locally and has a valid classid encoded within its
+.B skb->priority,
+choose it and terminate.
+
+.TP
+(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.
+.TP
+(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
+.B 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.
+.TP
+(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
+.B BEST_EFFORT
+class. If this is an upward reference, or no
+.B 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
+.B BEST_EFFORT
+priority yielded a class, enter the fallback algorithm.
+.P
+The fallback algorithm resides outside of the loop and is as follows.
+.TP
+(i)
+Consult the defmap of the class at which the jump to fallback occured. If
+the defmap contains a class for the
+.B
+priority
+of the class (which is related to the TOS field), choose this class and
+terminate.
+.TP
+(ii)
+Consult the map for a class for the
+.B BEST_EFFORT
+priority. If found, choose it, and terminate.
+.TP
+(iii)
+Choose the class at which break out to the fallback algorithm occured. Terminate.
+.P
+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.
+
+.SH LINK SHARING ALGORITHM
+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.
+
+.B 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
+.B bounded
+it will try to borrow avgidle from siblings that are not
+.B 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.
+
+.B I REALLY NEED HELP FIGURING THIS OUT. REST OF DOCUMENT IS PRETTY CERTAIN
+.B AGAIN.
+
+.SH QDISC
+The root qdisc of a CBQ class tree has the following parameters:
+
+.TP
+parent major:minor | root
+This mandatory parameter determines the place of the CBQ instance, either at the
+.B root
+of an interface or within an existing class.
+.TP
+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.
+.TP
+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.
+.TP
+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.
+.TP
+cell
+The cell size determines he granularity of packet transmission time calculations. Has a sensible default.
+.TP
+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.
+.TP
+ewma log
+When CBQ needs to measure the average idle time, it does so using an
+Exponentially Weighted Moving Average which smoothes 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.
+.P
+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.
+
+.SH CLASSES
+Classes have a host of parameters to configure their operation.
+
+.TP
+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.
+.TP
+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.
+.TP
+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.
+.TP
+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.
+
+.TP
+priority priority
+In the round-robin process, classes with the lowest priority field are tried
+for packets first. Mandatory.
+
+.TP
+rate rate
+Maximum rate this class and all its children combined can send at. Mandatory.
+
+.TP
+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.
+
+.TP
+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.
+
+.TP
+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.
+
+.TP
+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.
+
+.TP
+bounded
+Signifies that this class will not borrow bandwidth from its siblings.
+.TP
+isolated
+Means that this class will not borrow bandwidth to its siblings
+
+.TP
+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
+.B 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.
+.TP
+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
+.B interval
+microseconds how much traffic has passed. This again is a EWMA, for which
+the time constant can be specified, also in microseconds. The
+.B 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.
+
+
+
+.SH SOURCES
+.TP
+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
+
+.TP
+o
+Sally Floyd, "Notes on CBQ and Guarantee Service", 1995
+
+.TP
+o
+Sally Floyd, "Notes on Class-Based Queueing: Setting
+Parameters", 1996
+
+.TP
+o
+Sally Floyd and Michael Speer, "Experimental Results
+for Class-Based Queueing", 1998, not published.
+
+
+
+.SH SEE ALSO
+.BR tc (8)
+
+.SH AUTHOR
+Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
+bert hubert <ahu@ds9a.nl>
+
+
git://git.onelab.eu / iproute2.git / blobdiff