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><H1
CLASS="section"
><A
NAME="intro"
></A
>1. Introduction</H1
><P
> This is a brief tutorial on using <B
CLASS="command"
>tcng</B
>
(<A
HREF="http://tcng.sourceforge.net"
TARGET="_top"
>Traffic Control Next
Generation</A
>) with HTB
(<A
HREF="http://luxik.cdi.cz/~devik/qos/htb/"
TARGET="_top"
>Hierarchical Token
Bucket</A
>) to perform traffic shaping on a Linux machine.
</P
><P
> This tutorial is intended for systems administrators who have
<P
></P
><UL
><LI
><P
> AT LEAST, a basic understanding of traffic control
</P
></LI
><LI
><P
> EITHER the capability to compile iproute2 and tcng from source
</P
><P
> OR the capability of building RPMS from provided SRPMs
</P
></LI
><LI
><P
> EITHER a modular kernel with support for htb and dsmark
</P
><P
> OR capability to compile a kernel with support for htb and dsmark
</P
></LI
></UL
>
<DIV
CLASS="note"
><P
></P
><TABLE
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><P
>This article is neither comprehensive nor authoritative. The
author solicits positive and negative feedback at <TT
CLASS="email"
><<A
HREF="mailto:martin@linux-ip.net"
>martin@linux-ip.net</A
>></TT
>. Corrections,
additions, and further examples are always welcome.</P
></TD
></TR
></TABLE
></DIV
>
</P
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="intro-tc"
></A
>1.1. What is traffic control and how does it work?</H2
><P
> 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.
</P
><P
> 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.
</P
><P
> 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).
</P
><P
> See
<A
HREF="intro.html#intro-htb"
>Section 1.2</A
> for an example of buckets in a linux traffic
control system.
</P
><P
> Under linux, traffic control has historically been a complex
endeavor. The <B
CLASS="command"
>tc</B
> 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 <B
CLASS="command"
>tcng</B
> project provides a much
friendlier interface to the human by layering a language on top of the
powerful <B
CLASS="command"
>tc</B
> command line tool. By writing traffic
control configurations in <B
CLASS="command"
>tcng</B
> they become easily
maintainable, less arcane, and importantly also more portable.
</P
><P
> </P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="intro-htb"
></A
>1.2. What is htb?</H2
><P
> <A
HREF="http://luxik.cdi.cz/~devik/qos/htb/"
TARGET="_top"
>Hierarchichal Token
Bucket</A
> 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
<A
HREF="http://www.docum.org/"
TARGET="_top"
>Stef Coene's website</A
> about
HTB and its uses. Below is a very brief sketch of the HTB system.
</P
><P
> 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 <TT
CLASS="constant"
>root</TT
> qdisc.
</P
><P
> The <TT
CLASS="constant"
>root</TT
> qdisc will contain one class (complex
scenarios could have multiple classes attached to the
<TT
CLASS="constant"
>root</TT
> qdisc). This single HTB class will be set
with two parameters, a <TT
CLASS="constant"
>rate</TT
> and a
<TT
CLASS="constant"
>ceil</TT
>. These values should be the same for the
top-level class, and will represent the total
available bandwidth on the link.
</P
><P
> In HTB, <TT
CLASS="constant"
>rate</TT
> means the guaranteed bandwidth
available for a given class and <TT
CLASS="constant"
>ceil</TT
> is short for
ceiling, which indicates the maximum bandwidth that class is allowed to
consume. Any bandwidth used between <TT
CLASS="constant"
>rate</TT
> and
<TT
CLASS="constant"
>ceil</TT
> is borrowed from a parent class, hence the
suggestion that <TT
CLASS="constant"
>rate</TT
> and <TT
CLASS="constant"
>ceil</TT
>
be the same in the top-level class.
</P
><P
> 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 <TT
CLASS="constant"
>rate</TT
> and
<TT
CLASS="constant"
>ceil</TT
> 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.
</P
><P
> Hierarchical Token Bucket implements a classful queuing mechanism for
the linux traffic control system, and provides <TT
CLASS="constant"
>rate</TT
>
and <TT
CLASS="constant"
>ceil</TT
> 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 <TT
CLASS="constant"
>ceil</TT
>).
</P
><P
> 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.
</P
><P
> 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.
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="intro-tcng"
></A
>1.3. What is <B
CLASS="command"
>tcng</B
>?</H2
><P
> <A
HREF="http://tcng.sourceforge.net/"
TARGET="_top"
>Traffic Control Next
Generation (tcng)</A
> is a project by Werner Almesberger to provide
a powerful, abstract, and uniform language in which to describe traffic
control structures. The <B
CLASS="command"
>tcc</B
> parser in the
<B
CLASS="command"
>tcng</B
> distribution transforms tcng the language into a
number of output formats. By default, <B
CLASS="command"
>tcc</B
> will read
a file (specified as an argument or as STDIN) and print to STDOUT the
series of <B
CLASS="command"
>tc</B
> commands (see
<B
CLASS="command"
>iproute2</B
> below) required to create the desired traffic
control structure in the kernel.
</P
><P
> Consult the
<A
HREF="http://linux-ip.net/gl/tcng/node159.html"
TARGET="_top"
>parameter
reference for <B
CLASS="command"
>tcng</B
></A
> to see the supported
queuing disciplines. Jacob Teplitsky, active on the
<A
HREF="http://lartc.org/#mailinglist"
TARGET="_top"
>LARTC mailing list</A
>
and a contributor to the tcng project,
wrote the htb support for <B
CLASS="command"
>tcng</B
>.
</P
><P
> The <B
CLASS="command"
>tcc</B
> tool can produce a number of different types
of output, but this document will only consider the conventional and
default output. Consult the
<A
HREF="http://linux-ip.net/gl/tcng/"
TARGET="_top"
>TCNG manual</A
> for
more detailed information about the use of <B
CLASS="command"
>tcng</B
>.
</P
><P
> The
<B
CLASS="command"
>tcsim</B
> 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 <B
CLASS="command"
>tcsim</B
>
is a significant portion of the <B
CLASS="command"
>tcng</B
> project,
<B
CLASS="command"
>tcsim</B
> will not be covered here at all.
</P
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