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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article>
<!--$Id$-->
<articleinfo>
<title>Simple Traffic Shaping/Control</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2009</year>
<year>2010</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<legalnotice>
<para>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section>
<title>Introduction</title>
<para>Traffic shaping and control was originally introduced into Shorewall
in version 2.2.5. That facility was based on Arne Bernin's
<firstterm>tc4shorewall</firstterm> and is generally felt to be complex
and difficult to use.</para>
<para>In Shorewall 4.4.6, a second traffic shaping facility that is simple
to understand and to configure was introduced. This newer facility is
described in this document while the original facility is documented in
<ulink url="traffic_shaping.htm">Complex Traffic
Shaping/Control</ulink>.</para>
<para>In the absense of any traffic shaping, interfaces are configured
automatically with the pfifo_fast <firstterm>queuing
discipline</firstterm> (qdisc). From tc-pfifo_fast (8):</para>
<blockquote>
<para> The algorithm is very similar to that of the classful tc-prio(8)
qdisc. pfifo_fast is like three tc-pfifo(8) queues side by side, where
packets can be enqueued in any of the three bands based on their Type of
Service bits or assigned priority.</para>
<para>Not all three bands are dequeued simultaneously - as long as lower
bands have traffic, higher bands are never dequeued. This can be used to
prioritize interactive traffic or penalize lowest cost traffic.</para>
<para>Each band can be txqueuelen packets long, as configured with
ifconfig(8) or ip(8). Additional packets coming in are not enqueued but
are instead dropped.</para>
<para>See tc-prio(8) for complete details on how TOS bits are translated
into bands. </para>
</blockquote>
<para>In other words, if all you want is strict priority queuing, then do
nothing.</para>
<para>Shorewall's Simple Traffic Shaping configures the prio
qdisc(rx-prio(8)) on the designated interface then adds a
<firstterm>Stochastic Fair Queuing</firstterm> sfq (tc-sfq (8)) qdisc to
each of the classes that are implicitly created for the prio qdisc. The
sfq qdisc ensures fairness among packets queued in each of the classes
such that each <firstterm>flow</firstterm> (session) gets its turn to send
packets. The definition of flows can be altered to include all traffic
being sent <emphasis>by</emphasis> a given IP address (normally defined
for an external interface) or all traffic being sent
<emphasis>to</emphasis> a given IP address (internal interface).</para>
<para>Finally, Simple Traffic Shaping allows you to set a limit on the
total bandwidth allowed out of an interface. It does this by inserting a
Token Bucket Filter (tbf) qdisc ahead of the prio qdisc. Note that this
can have the effect of defeating the priority queuing provided by the prio
qdisc but seems to provide a benefit when the actual link output
temporarily drops below the limit imposed by tbf or when tbf allows a
burst of traffic to be released.</para>
</section>
<section>
<title>Enabling Simple Traffic Shaping</title>
<para>Simple traffic shaping is enabled by setting TC_ENABLED=Simple in
<ulink url="manpages/shorewall.conf.html">shorewall.conf</ulink>(5). You
then add an entry for your external interface to <ulink
url="manpages/shorewall-tcinterfaces.html">shorewall-tcinterfaces</ulink>(5)
(<filename>/etc/shorewall/tcinterfaces</filename>).</para>
<para>Assuming that your external interface is eth0:</para>
<programlisting>#INTERFACE TYPE IN-BANDWIDTH OUT-BANDWIDTH
eth0 External</programlisting>
<note>
<para>If you experience an error such as the following during
<command>shorewall start</command> or <command>shorewall
restart</command>, your kernel and iproute do not support the <emphasis
role="bold">flow</emphasis> classifier. In that case, you must leave the
TYPE column empty (or specify '-').</para>
<programlisting>Unknown filter "flow", hence option "hash" is unparsable
ERROR: Command "tc filter add dev eth0 protocol all prio 1 parent 11: handle 11 flow hash keys nfct-src divisor 1024" Failed</programlisting>
<para>RHEL5-based systems such as <trademark>CentOS</trademark> 5 and
<trademark>Foobar</trademark> 5 are known to experience this
error.</para>
<para><emphasis role="bold">Update</emphasis>: Beginning with Shorewall
4.4.7, Shorewall can determine that some environments, such as RHEL5 and
derivatives, are incapable of using the TYPE parameter and simply ignore
it.</para>
</note>
<para>With this simple configuration, packets to be sent through interface
eth0 will be assigned to a priority band based on the value of their TOS
field:</para>
<programlisting>TOS Bits Means Linux Priority BAND
------------------------------------------------------------
0x0 0 Normal Service 0 Best Effort 2
0x2 1 Minimize Monetary Cost 1 Filler 3
0x4 2 Maximize Reliability 0 Best Effort 2
0x6 3 mmc+mr 0 Best Effort 2
0x8 4 Maximize Throughput 2 Bulk 3
0xa 5 mmc+mt 2 Bulk 3
0xc 6 mr+mt 2 Bulk 3
0xe 7 mmc+mr+mt 2 Bulk 3
0x10 8 Minimize Delay 6 Interactive 1
0x12 9 mmc+md 6 Interactive 1
0x14 10 mr+md 6 Interactive 1
0x16 11 mmc+mr+md 6 Interactive 1
0x18 12 mt+md 4 Int. Bulk 2
0x1a 13 mmc+mt+md 4 Int. Bulk 2
0x1c 14 mr+mt+md 4 Int. Bulk 2
0x1e 15 mmc+mr+mt+md 4 Int. Bulk 2</programlisting>
<para>When dequeueing, band 1 is tried first and only if it did not
deliver a packet does the system try band 2, and so onwards. Maximum
reliability packets should therefore go to band 1, minimum delay to band 2
and the rest to band 3.</para>
<note>
<para>If you run both an IPv4 and an IPv6 firewall on your system, you
should define each interface in only one of the two
configurations.</para>
</note>
</section>
<section>
<title>Customizing Simple Traffic Shaping</title>
<para>The default mapping of TOS to bands can be changed using the
TC_PRIOMAP setting in <ulink
url="manpages/shorewall.conf.html">shorewall.conf</ulink>(5). The default
setting of this option is:</para>
<programlisting>TC_PRIOMAP="2 3 3 3 2 3 1 1 2 2 2 2 2 2 2 2"</programlisting>
<para>These entries map Linux Priority to priority BAND. So only entries
0, 1, 2, 4 and 6 in the map are relevant to TOS-&gt;BAND mapping.</para>
<para>Further customizations can be defined in <ulink
url="manpages/shorewall-tcpri.html">shorewall-tcpri</ulink>(5)
(<filename>/etc/shorewall/tcpri</filename>). Using that file, you
can:</para>
<orderedlist>
<listitem>
<para>Assign traffic entering the firewall on a particular interface
to a specific priority band:</para>
<programlisting>#BAND PROTO PORT(S) ADDRESS INTERFACE HELPER
2 - - - eth1</programlisting>
<para>In this example, traffic from eth1 will be assigned to priority
band 2.</para>
<note>
<para>When an INTERFACE is specified, the PROTO, PORT(S) and ADDRESS
column must contain '-'.</para>
</note>
</listitem>
<listitem>
<para>Assign traffic from a particular IP address to a specific
priority band:</para>
<programlisting>#BAND PROTO PORT(S) ADDRESS INTERFACE HELPER
1 - - 192.168.1.44</programlisting>
<para>In this example, traffic from 192.168.1.44 will be assigned to
priority band 1.</para>
<note>
<para>When an ADDRESS is specified, the PROTO, PORT(S) and INTERFACE
columns must be empty.</para>
</note>
</listitem>
<listitem>
<para>Assign traffic to/from a particular application to a specific
priority band:</para>
<programlisting>#BAND PROTO PORT(S) ADDRESS INTERFACE HELPER
1 udp 1194</programlisting>
<para>In that example, OpenVPN traffic is assigned to priority band
1.</para>
</listitem>
<listitem>
<para>Assign traffic that uses a particular Netfilter helper to a
particular priority band:</para>
<programlisting>#BAND PROTO PORT(S) ADDRESS INTERFACE HELPER
1 - - - - sip</programlisting>
<para>In this example, SIP and associated RTP traffic will be assigned
to priority band 1 (assuming that the nf_conntrack_sip helper is
loaded).</para>
</listitem>
</orderedlist>
<para>It is suggested that entries specifying an INTERFACE be placed at
the top of the file. That way, the band assigned to a particular packet
will be the <emphasis role="bold">last</emphasis> entry matched by the
packet. Packets which match no entry in <ulink
url="manpages/shorewall-tcpri.html">shorewall-tcpri</ulink>(5) are
assigned to priority bands using their TOS field as previously
described.</para>
<para>One cause of high latency on interactive traffic can be that queues
are building up at your ISP's gateway router. If you suspect that is
happening in your case, you can try to eliminate the problem by using the
IN-BANDWIDTH setting in <ulink
url="manpages/shorewall-tcinterfaces.html">shorewall-tcinterfaces</ulink>(5).
The contents of the column are a <replaceable>rate</replaceable>. For
defining the rate, use <emphasis role="bold">kbit</emphasis> or <emphasis
role="bold">kbps</emphasis> (for Kilobytes per second) and make sure there
is NO space between the number and the unit (it is 100kbit not 100 kbit).
<emphasis role="bold">mbit</emphasis>, <emphasis
role="bold">mbps</emphasis> or a raw number (which means bytes) can be
used, but note that before Shorewall 4.4.13 only integer numbers were
supported (0.5 was not valid). To pick an appropriate setting, we
recommend that you start by setting IN-BANDWIDTH significantly below your
measured download bandwidth (20% or so). While downloading, measure the
ping response time from the firewall to the upstream router as you
gradually increase the setting. The optimal setting is at the point beyond
which the ping time increases sharply as you increase the setting.</para>
<para>Simple Traffic Shaping is only appropriate on interfaces where
output queuing occurs. As a consequence, you usually only use it on
extermal interfaces. There are cases where you may need to use it on an
internal interface (a VPN interface, for example). If so, just add an
entry to <ulink
url="manpages/shorewall-tcinterfaces.html">shorewall-tcinterfaces</ulink>(5):</para>
<programlisting>#INTERFACE TYPE IN-BANDWIDTH
tun0 Internal</programlisting>
<para>For fast lines, the actual download rate may be significantly less
than the specified IN-BANDWIDTH. Beginning with Shoreall 4.4.13, you can
specify an optional burst</para>
<para>Also beginning with Shorewall 4.4.13, an OUT-BANDWIDTH column is
available in <ulink
url="manpages/shorewall-tcpri.html">shorewall-tcpri</ulink>(5). Limiting
to outgoing bandwidth can have a positive effect on latency for
applications like VOIP. We recommend that you begin with a setting that is
at least 20% less than your measured upload rate and then gradually
increase it until latency becomes unacceptable. Then reduce it back to the
point where latency is acceptable.</para>
</section>
<section>
<title>Additional Reading</title>
<para>The PRIO(8) (tc-prio) manpage has additional information on the
facility that Shorewall Simple Traffic Shaping is based on.</para>
<caution>
<para>Please note that Shorewall numbers the bands 1-3 whereas PRIO(8)
refers to them as bands 0-2.</para>
</caution>
</section>
</article>