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23/11/2001 : This page is not valid. Plz go to docum.org 

 

 

 

  • General

    • The traffic control mechanism in the linux kernel consists of the following components: Qdiscs are responsible of transmitting the data. Classes are attached to qdiscs and can hold a part of the traffic. Each class has allways 1 qdisc associated to it to transmit the classes and this qdisc holds all the traffic that flows in that class. Filters are attached to qdiscs and classes and split the traffic in different classes.

    Other info: "Linux Traffic Control - Implementation Overview" (04/02/01). The original paper was found on ftp://icaftp.epfl.ch/pub/people/almesber/junk/ and is called tc-<<DATE>>.

  • Root qdisc

    • Each NIC has one special queueing discipline associated with it: the root-qdisc. Normally this is a very basic one (a FIFO qdisc) that transmits the packets in the same order as they are entered in the queue and as fast as possible, but it's doing this not very fair. There are more complex queueing disciplines with classes wich can hold a part of the traffic.

    The root qdisc holds all the data that's transmitted. You can replace this root qdisc with an other qdisc. So when you want to replace it with a sfq qdisc, you can use this command: 

    tc qdisc add dev eth0 root handle 10: sfq

  • qdiscs-classes-filters

    •

    Classes are attached to qdiscs and hold a part of the traffic. Classes and qiscs are very tied to each other. Normally, classes are not responsible to handle the packets themself. They have a qdisc associated with them to do this and this is stadard a FIFO qdisc. Of course you can change this and attach a new qdisc with classes, etc ... . Each time you can split the traffic to the different classes with filters. So filters are allways attached to qdiscs.

    The qdiscs CBQ, DS_MARK, CSZ and p-FIFO can contain classes. All the traffic that's been put in a class by a filter, enters also the qdisc that's attached to the class.

    The classes attached to a qdisc can have a different priority and/or rate. So when one packet enters class A, 2 packets enter class B. So the rate of class B is twice the rate of class A.

    It's possible that a qdisc has a built-in filter (wrr for example uses the mac-address or ip-address to put the traffic in classes so each address has it's own class). But normally you will have to attach a filter to a qdisc to put the traffic in classes.

  • Numbering

    • Each class and qdisc has a unique number: "<major number:minor number>". For a qdisc is the minor number zero, or you give no number. The major number of a class is the same as the major number of the qdisc where the class belongs to. So, pairs x:y are class handles and x:0 or x: are qdisc handles.

    The major numbers are unique for each qdisc. The minor numbers are unique per major number. You can use hexadecimal numbers.

    From include/linux/pkt_sched.h: 

    /* "Handles"
   ---------
 
    All the traffic control objects have 32bit identifiers, or "handles".
 
    They can be considered as opaque numbers from user API viewpoint,
    but actually they always consist of two fields: major and
    minor numbers, which are interpreted by kernel specially,
    that may be used by applications, though not recommended.
 
    F.e. qdisc handles always have minor number equal to zero,
    classes (or flows) have major equal to parent qdisc major, and
    minor uniquely identifying class inside qdisc.
 */