A classful qdisc contains multiple classes. Some of these classes contains a further qdisc, which may again be classful, but need not be. According to the strict definition, pfifo_fast is classful, because it contains three bands which are, in fact, classes. However, from the user's configuration perspective, it is classless as the classes can't be touched with the tc tool.
Classes
A classful qdisc may have many classes, each of which is internal to the qdisc. A class, in turn, may have several classes added to it. So a class can have a qdisc as parent or an other class. A leaf class is a class with no child classes. This class has 1 qdisc attached to it. This qdisc is responsible to send the data from that class. When you create a class, a fifo qdisc is attached to it. When you add a child class, this qdisc is removed. For a leaf class, this fifo qdisc can be replaced with an other more suitable qdisc. You can even replace this fifo qdisc with a classful qdisc so you can add extra classes.
Classifier
Each classful qdisc needs to determine to which class it needs to send a packet. This is done using the classifier.
Filter
Classification can be performed using filters. A filter contains a number of conditions which if matched, make the filter match.
Scheduling
A qdisc may, with the help of a classifier, decide that some packets need to go out earlier than others. This process is called Scheduling, and is performed for example by the pfifo_fast qdisc mentioned earlier. Scheduling is also called 'reordering', but this is confusing.
Shaping
The process of delaying packets before they go out to make traffic confirm to a configured maximum rate. Shaping is performed on egress. Colloquially, dropping packets to slow traffic down is also often called Shaping.
Policing
Delaying or dropping packets in order to make traffic stay below a configured bandwidth. In Linux, policing can only drop a packet and not delay it - there is no 'ingress queue'.
Work-Conserving
A work-conserving qdisc always delivers a packet if one is available. In other words, it never delays a packet if the network adaptor is ready to send one (in the case of an egress qdisc).
non-Work-Conserving
Some queues, like for example the Token Bucket Filter, may need to hold on to a packet for a certain time in order to limit the bandwidth. This means that they sometimes refuse to pass a packet, even though they have one available.
1.3.2 Linux TC详解首先需要注意的是：Linux tc只对egress方向实现了良好的控制，而对ingress方向控制有限，简而言之，控发不控收。
下面看实现中的几个重要概念：
队列。队列是流控的基础概念。通过使用队列和其他机制，可以进行整流，调度等工作。
令牌桶。这是个非常重要的因素。为了控制出队的速率，一种方式就是直接统计队列中出队的报文或字节数，但是为了保证精确性就需要复杂的计算。在流量控制中广泛应用的另一种方式就是令牌桶，令牌桶以一定的速率产生令牌，报文或字节出队时从令牌桶中取令牌，只有取到令牌后才能出队。
我们可以打一个比方，一群人正排队等待乘坐游乐场的游览车。让我们想象现在有一条固定的道路，游览车以固定的速度抵达，每个人都必须等待游览车到达后才能乘坐。游览车和游客就可以类比为令牌和报文，这种机制就是速率限制或流量整形，在一个固定的时间段内只有一部分人能乘坐游览车。
继续上面的比方，设想有大量的游览车正停在车站等待游客乘坐，但现在没有一个游客。如果现在有一大群游客同时过来了，那么他们都可以马上乘上游览车。在这里，我们就可以将车站类比为桶，一个桶中包含一定数量的令牌，桶中的令牌可以一次性被使用完而不管数据包到达的时间。
让我们来完成这个比方，游览车以固定的速率抵达车站，如果没人乘坐就会停满车站，即令牌以一定的速率进入桶中，如果令牌一直没被使用那么桶就可以被装满，而如果令牌不断的被使用那么桶就不会满。令牌桶是处理会产生流量突发应用（比如HTTP）的关键思想。
使用令牌桶过滤器的排队规则（TBF qdisc，Token Bucket Filter）是流量整形的一个经典例子（在TBF小节中有一个图表，通过该图表可以形象化的帮助读者理解令牌桶）。TBF以给定的速度产生令牌，当桶中有令牌时才发送数据，令牌是整流的基本思想。
Linux tc中主要的组件是qdisc, class, filter。
qdisc包含classful qdisc和classless disc。两者的区别是glassful qdisc可以包含多个分类，可以更加精细的控制流量。
常见的classless qdisc有：choke, codel, p/bfifo,fq, fq_codel, gred, hhf, ingress,mqprio, multiq, netem, pfifo_fast, pie, red, rr, sfb, sfq, tbf。linux默认使用的就是fifo_fast。
常见的classful qdisc有：ATM, CBQ, DRR, DSMARK, HFSC, HTB, PRIO, QFQ