Simply put, a qdisc is a scheduler (Section 3.2, “Scheduling”). Every output interface needs a scheduler of some kind, and the default scheduler is a FIFO. Other qdiscs available under Linux will rearrange the packets entering the scheduler's queue in accordance with that scheduler's rules.

The qdisc is the major building block on which all of Linux traffic control is built, and is also called a queuing discipline.

The classful qdiscs can contain classes, and provide a handle to which to attach filters. There is no prohibition on using a classful qdisc without child classes, although this will usually consume cycles and other system resources for no benefit.

The classless qdiscs can contain no classes, nor is it possible to attach filter to a classless qdisc. Because a classless qdisc contains no children of any kind, there is no utility to classifying. This means that no filter can be attached to a classless qdisc.

A source of terminology confusion is the usage of the terms root qdisc and ingress qdisc. These are not really queuing disciplines, but rather locations onto which traffic control structures can be attached for egress (outbound traffic) and ingress (inbound traffic).

Each interface contains both. The primary and more common is the egress qdisc, known as the root qdisc. It can contain any of the queuing disciplines (qdiscs) with potential classes and class structures. The overwhelming majority of documentation applies to the root qdisc and its children. Traffic transmitted on an interface traverses the egress or root qdisc.

For traffic accepted on an interface, the ingress qdisc is traversed. With its limited utility, it allows no child class to be created, and only exists as an object onto which a filter can be attached. For practical purposes, the ingress qdisc is merely a convenient object onto which to attach a policer to limit the amount of traffic accepted on a network interface.

In short, you can do much more with an egress qdisc because it contains a real qdisc and the full power of the traffic control system. An ingress qdisc can only support a policer. The remainder of the documentation will concern itself with traffic control structures attached to the root qdisc unless otherwise specified.

Classes only exist inside a classful qdisc (e.g., HTB and CBQ). Classes are immensely flexible and can always contain either multiple children classes or a single child qdisc ^[5]. There is no prohibition against a class containing a classful qdisc itself, which facilitates tremendously complex traffic control scenarios.

Any class can also have an arbitrary number of filters attached to it, which allows the selection of a child class or the use of a filter to reclassify or drop traffic entering a particular class.

A leaf class is a terminal class in a qdisc. It contains a qdisc (default FIFO) and will never contain a child class. Any class which contains a child class is an inner class (or root class) and not a leaf class.

The filter is the most complex component in the Linux traffic control system. The filter provides a convenient mechanism for gluing together several of the key elements of traffic control. The simplest and most obvious role of the filter is to classify (see Section 3.3, “Classifying”) packets. Linux filters allow the user to classify packets into an output queue with either several different filters or a single filter.

Filters can be attached either to classful qdiscs or to classes, however the enqueued packet always enters the root qdisc first. After the filter attached to the root qdisc has been traversed, the packet may be directed to any subclasses (which can have their own filters) where the packet may undergo further classification.

Filter objects, which can be manipulated using tc, can use several different classifying mechanisms, the most common of which is the u32 classifier. The u32 classifier allows the user to select packets based on attributes of the packet.

The classifiers are tools which can be used as part of a filter to identify characteristics of a packet or a packet's metadata. The Linux classfier object is a direct analogue to the basic operation and elemental mechanism of traffic control classifying.

This elemental mechanism is only used in Linux traffic control as part of a filter. A policer calls one action above and another action below the specified rate. Clever use of policers can simulate a three-color meter. See also Section 10, “Diagram”.

Although both policing and shaping are basic elements of traffic control for limiting bandwidth usage a policer will never delay traffic. It can only perform an action based on specified criteria. See also Example 5, “tc filter”.

This basic traffic control mechanism is only used in Linux traffic control as part of a policer. Any policer attached to any filter could have a drop action.

	Note
	The only place in the Linux traffic control system where a packet can be explicitly dropped is a policer. A policer can limit packets enqueued at a specific rate, or it can be configured to drop all traffic matching a particular pattern [6].

There are, however, places within the traffic control system where a packet may be dropped as a side effect. For example, a packet will be dropped if the scheduler employed uses this method to control flows as the GRED does.

Also, a shaper or scheduler which runs out of its allocated buffer space may have to drop a packet during a particularly bursty or overloaded period.

Every class and classful qdisc (see also Section 7, “Classful Queuing Disciplines (qdiscs)”) requires a unique identifier within the traffic control structure. This unique identifier is known as a handle and has two constituent members, a major number and a minor number. These numbers can be assigned arbitrarily by the user in accordance with the following rules ^[7].

The special handle ffff:0 is reserved for the ingress qdisc.

The handle is used as the target in classid and flowid phrases of tc filter statements. These handles are external identifiers for the objects, usable by userland applications. The kernel maintains internal identifiers for each object.