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Class Based Queue

This section discusses Class Based Queues in detail. The terms commonly used in the CBQ context and the user-level syntax to set up these queues are discussed in this section.

Let us first define some basic terms in CBQ. In CBQ, every class has variables idle and avgidle and parameter maxidle used in computing the limit status for the class, and the parameter offtime used in determining how long to restrict throughput for overlimit classes.

1.
Idle: The variable idle is the difference between the desired time and the measured actual time between the most recent packet transmissions for the last two packets sent from this class. When the connection is sending more than its allocated bandwidth, then idle is negative. When the connection is sending perfectly at its alloted rate, then idle is zero.
2.
avgidle: The variable avgidle is the average of idle, and it computed using an exponential weigted moving average (EWMA). When the avgidle is zero or lower, then the class is overlimit (the class has been exceeding its allocated bandwidth in a recent short time interval).
3.
maxidle: The parameter maxidle gives an upper bound for avgidle. Thus maxidle limits the credit given to a class that has recently been under its allocation.
4.
offtime: The parameter offtime gives the time interval that a overlimit must wait before sending another packet. This parameter determines the steady-state burst size for a class when the class is running over its limit.
5.
minidle: The minidle parameter gives a (negative) lower bound for avgidle. Thus, a negative minidle lets the scheduler remember that a class has recently used more than its allocated bandwidth.

There are three types of classes, namely leaf classes (such as a video class) that have directly assigned connections; nonleaf classes used for link-sharing; and the root class that represents the entire output link.

The syntax to create a CBQ is shown below:

tc qdisc [ add | del | replace | change | get ] dev STRING  \
cbq bandwidth BPS [ avpkt BYTES ] [ mpu BYTES ] [ cell BYTES ] [ ewma LOG ]

The interpretation of the fields:

For e.g.
tc qdisc add dev eth0 root handle 1: cbq bandwidth 10Mbit allot 1514 cell 8 avpkt 1000 mpu 64

In the above example, a class based queue is created and attached to device eth0. The handle for the queue is 1: (that is, 1:0), where 1 represents the major number and 0 represents the minor number. The bandwidth available on the outgoing link is 10 Mbit. allot is a parameter that is used by the link sharing scheduler. A cell value of 8 indicates that the packet transmission time will be measured in terms of 8 bytes.

Let us now discuss the syntax for creating a class for a CBQ.

tc qdisc [add | del | replace | change] cbq bandwidth BPS rate BPS maxburst PKTS \
[ avpkt BYTES ] [ minburst PKTS ] [ bounded ] [ isolated ] [ allot BYTES ] \
[ mpu BYTES ] [ weight RATE ] [ prio NUMBER ] [ cell BYTES ] [ ewma LOG ] \
[ estimator INTERVAL TIME_CONSTANT ] [ split CLASSID ] [ defmap MASK/CHANGE ]

The interpretation of the fields:

For e.g.
tc class add dev eth1 parent 1:1 classid 1:2 cbq bandwidth 10Mbit rate 1Mbit allot 1514 cell 8 weight 100Kbit prio 3 maxburst 20 avpkt 1000 split 1:0 defmap c0

In this example, a CBQ class with handle 1:2 is created. Its parent is identified by the handle 1:1. The priority assigned to it is 3, the average packet size is 1000 bytes. The split node is 1:0, which represents the root of the link sharing structure. The defmap is c0, that is, packets with this TOS (for incoming packets) or SO_PRIORITY (for locally generated packets) that DO NOT classify under any class are considered to belong to the class with handle 1:2. This was an excellent implementation innovation by Alexey Kuznetsov.


next up previous
Next: Diff-Serv queues Up: QoS Support in Linux Previous: Filters
Saravanan Radhakrishnan
1999-09-30