1. Introduction

1.1. What is traffic control and how does it work?

Traffic control is the term given to the entire packet queuing subsystem in a network or network device. Traffic control consists of several distinct operations. Classifying is a mechanism by which to identify packets and place them in individual flows or classes. Policing is a mechanism by which one limits the number of packets or bytes in a stream matching a particular classification. Scheduling is the decision-making process by which packets are ordered and re-ordered for transmission. Shaping is the process by which packets are delayed and transmitted to produce an even and predictable flow rate.

These many characteristics of a traffic control system can be combined in complex ways to reserve bandwidth for a particular flow (or application) or to limit the amount of bandwidth available to a particular flow or application.

One of the key concepts of traffic control is the concept of tokens. A policing or shaping implementation needs to calculate the number of bytes or packets which have passed at what rate. Each packet or byte (depending on the implementation), corresponds to a token, and the policing or shaping implementation will only transmit or pass the packet if it has a token available. A common metaphorical container in which an implementation keeps its token is the bucket. In short, a bucket represents the both the number of tokens which can be used instantaneously (the size of the bucket), and the rate at which the tokens are replenished (how fast the bucket gets refilled).

See Section 1.2 for an example of buckets in a linux traffic control system.

Under linux, traffic control has historically been a complex endeavor. The tc command line tool provides an interface to the kernel structures which perform the shaping, scheduling, policing and classifying. The syntax of this command is, however, arcane. The tcng project provides a much friendlier interface to the human by layering a language on top of the powerful tc command line tool. By writing traffic control configurations in tcng they become easily maintainable, less arcane, and importantly also more portable.

1.2. What is htb?

Hierarchichal Token Bucket is a classful qdisc written by Martin Devera with a simpler set of configuration parameters than CBQ. There is a great deal of documentation on the author's site and also on Stef Coene's website about HTB and its uses. Below is a very brief sketch of the HTB system.

Conceptually, HTB is an arbitrary number of token buckets arranged in a hierarchy (yes, you probably could have figured that out without my sentence). Let's consider the simplest scenario. The primary egress queuing discipline on any device is known as the root qdisc.

The root qdisc will contain one class (complex scenarios could have multiple classes attached to the root qdisc). This single HTB class will be set with two parameters, a rate and a ceil. These values should be the same for the top-level class, and will represent the total available bandwidth on the link.

In HTB, rate means the guaranteed bandwidth available for a given class and ceil is short for ceiling, which indicates the maximum bandwidth that class is allowed to consume. Any bandwidth used between rate and ceil is borrowed from a parent class, hence the suggestion that rate and ceil be the same in the top-level class.

A number of children classes can be made under this class, each of which can be allocated some amount of the available bandwidth from the parent class. In these children classes, the rate and ceil parameter values need not be the same as suggested for the parent class. This allows you to reserve a specified amount of bandwidth to a particular class. It also allows HTB to calculate the ratio of distribution of available bandwidth to the ratios of the classes themselves. This should be more apparent in the examples below.

Hierarchical Token Bucket implements a classful queuing mechanism for the linux traffic control system, and provides rate and ceil to allow the user to control the absolute bandwidth to particular classes of traffic as well as indicate the ratio of distribution of bandwidth when extra bandwidth becomes available (up to ceil).

Keep in mind when choosing the bandwidth for your top-level class that traffic shaping only helps if you are the bottleneck between your LAN and the Internet. Typically, this is the case in home and office network environments, where an entire LAN is serviced by a DSL or T1 connection.

In practice, this means that you should probably set the bandwidth for your top-level class to your available bandwidth minus a fraction of that bandwidth.

1.3. What is tcng?

Traffic Control Next Generation (tcng) is a project by Werner Almesberger to provide a powerful, abstract, and uniform language in which to describe traffic control structures. The tcc parser in the tcng distribution transforms tcng the language into a number of output formats. By default, tcc will read a file (specified as an argument or as STDIN) and print to STDOUT the series of tc commands (see iproute2 below) required to create the desired traffic control structure in the kernel.

Consult the parameter reference for tcng to see the supported queuing disciplines. Jacob Teplitsky, active on the LARTC mailing list and a contributor to the tcng project, wrote the htb support for tcng.

The tcc tool can produce a number of different types of output, but this document will only consider the conventional and default output. Consult the TCNG manual for more detailed information about the use of tcng.

The tcsim tool is a traffic control simulator which accepts tcng configuration files and reads a control language to simulate the behaviour of a kernel sending and receiving packets with the specified control structures. Although tcsim is a significant portion of the tcng project, tcsim will not be covered here at all.