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.
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, otherwise restart.
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).
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.
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.
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 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.
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 its siblings.
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.
- 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 Service", 1995
- Sally Floyd, "Notes on Class-Based Queueing: Setting Parameters", 1996
- Sally Floyd and Michael Speer, "Experimental Results for Class-Based Queueing", 1998, not published.