shorewall_code/Shorewall-docs2/traffic_shaping.xml

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  <articleinfo>
    <title>Traffic Shaping/Control</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

        <surname>Eastep</surname>
      </author>

      <author>
        <firstname>Arne</firstname>

        <surname>Bernin</surname>
      </author>
    </authorgroup>

    <pubdate>2005-11-21</pubdate>

    <copyright>
      <year>2001-2004</year>

      <holder>Thomas M. Eastep</holder>
    </copyright>

    <copyright>
      <year>2005</year>

      <holder>Arne Bernin &amp; 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>Starting with Version 2.5.5, Shorewall has builtin support for
    traffic shaping and control. Before this version, the support was quite
    limited. You were able to use your own tcstart script (and you still are),
    but besides the tcrules file it was not possible to define classes or
    queueing discplines inside the Shorewall config files.</para>

    <para>The support for traffic shaping and control still does not cover all
    options available (and especially all algorithms that can be used to queue
    traffic) in the Linux kernel but it should fit most needs. If you are
    using your own script for traffic control and you still want to use it in
    the future, you will find information on how to do this, <link
    linkend="owntcstart"> later in this document</link>. But for this to work,
    you will also need to enable traffic shaping in the kernel and Shorewall
    as covered by the next sections.</para>
  </section>

  <section>
    <title>Linux traffic shaping and control</title>

    <para>This section gives a brief introduction of how controlling traffic
    with the linux kernel works. Although this might be enough for configuring
    it in the Shorewall configuration files, it still might be a good idea to
    take a deeper look into the <ulink url="http://lartc.org/howto/">Linux
    Advanced Routing and Shaping HOWTO</ulink>. At the time of writing this,
    the current version is 1.0.0.</para>

    <para>Since kernel 2.2 Linux has extensive support for controlling
    traffic. You can define different algorithms that are used to queue the
    traffic before it leaves an interface. The standard one is called pfifo
    and is (as the name suggests) of the type First In First out. This means,
    that it does not shape anything, if you have a connection that eats up all
    your bandwidth, this qeueing algorithm will not stop it from doing
    so.</para>

    <para>For Shorewall traffic shaping we use two algorithms, one is called
    HTB (Hierarchical Token Bucket) and SFQ (Stochastic Fairness Queuing). SFQ
    is easy to explain: it just tries to track your connections (tcp or udp
    streams) and balances the traffic between them. This normally works well.
    HTB allows you to define a set of classes, and you can put the traffic you
    want into these classes. You can define minimum and maximum bandwitdh
    settings for those classes and order them hierachically (the less
    priorized classes only get bandwitdth if the more important have what they
    need). Shorewall builtin traffic shaping allows you to define these
    classes (and their bandwidth limits), and it uses SFQ inside these classes
    to make sure, that different data streams are handled equally.</para>

    <para>You can only shape outgoing traffic. The reason for this is simple,
    the packets were already received by your network card before you can
    decide what to do with them. So the only choice would be to drop them
    which normally makes no sense (since you received the packet already, it
    went through the possible bottleneck (the incoming connection); dropping
    packets <emphasis>does</emphasis> have an advantage when you are connected
    to the internet using a Cable Modem or DSL because it can prevent large
    queues of download traffic from forming at your ISP and thus can improve
    response time (see the definition of the IN-BANDWIDTH column of the
    <filename>/etc/shorewall/tcdevices</filename> file below). The next
    possible bottleneck might come if the packet leaves on another interface,
    so this will be the place where queuing might occur. So, defining queues
    for incoming packages is not very useful, you just want to have it
    forwarded to the outgoing interface as fast as possible.</para>

    <para>There is one exception, though. Limiting incoming traffic to a value
    a bit slower than your actual line speed will avoid queueing on the other
    end of that connection. This is mostly useful if you don't have access to
    traffic control on the other side and if this other side has a faster
    network connection than you do (the line speed between the systems is the
    bottleneck, e.g. a DSL connection to you providers router, the router
    itself is normally connected to a much faster backbone). So, if you drop
    packages that are coming in too fast, the underlaying protocol might
    recognize this and slow down the connection. TCP has a builtin mechanism
    for this, UDP has not (but the protocol over UDP might recognize it , if
    there is any).</para>

    <para>The reason why queing is bad in these cases is, that you might have
    packets which need to be priorized over others, e.g. VoIP or ssh. For this
    type of connections it is important that packets arrive in a certain
    amount of time. For others like http downloads, it does not really matter
    if it takes a few seconds more.</para>

    <para>If you have a large queue on the other side and the router there
    does not care about QoS or the QoS bits are not set properly, your
    important packets will go into the same queue as your less timecritical
    download packets which will result in a large delay.</para>

    <para>You shape and control outgoing traffic by assigning the traffic to
    <firstterm>classes</firstterm>. Each class is associated with exactly one
    network interface and has a number of attributes:</para>

    <orderedlist>
      <listitem>
        <para>PRIORITY - Used to give preference to one class over another
        when selecting a packet to send. The priority is a numeric value with
        1 being the highest priority, 2 being the next highest, and so
        on.</para>
      </listitem>

      <listitem>
        <para>RATE - The minimum bandwidth this class should get, when the
        traffic load rises. Classes with a higher priority (lower PRIORITY
        value) are served even if there are others that have a guaranteed
        bandwith but have a lower priority (higher PRIORITY value).</para>
      </listitem>

      <listitem>
        <para>CEIL - The maximum bandwidth the class is allowed to use when
        the link is idle.</para>
      </listitem>

      <listitem>
        <para>MARK - Netfilter has a facility for
        <firstterm>marking</firstterm> packets. Packet marks have a numberic
        value which is limited in Shorewall to the values 1-255. You assign
        packet marks to different types of traffic using entries in the
        <filename>/etc/shorewall/tcrules</filename> file.</para>
      </listitem>
    </orderedlist>

    <para>One class for each interface must be designated as the
    <firstterm>default class</firstterm>. This is the class to which unmarked
    traffic (packets to which you have not assigned a mark value in
    <filename>/etc/shorewall/tcrules</filename>) is assigned.</para>
  </section>

  <section>
    <title>Linux Kernel Configuration</title>

    <para>You will need at least kernel 2.4.18 for this to work, please take a
    look at the following screenshot for what settings you need to enable. For
    builtin support, you need the HTB scheduler, the PRIO pseudoscheduler and
    SFQ queue. The other scheduler or queue algorithms are not needed.</para>

    <para>This screen shot show how I've configured QoS in my Kernel:<graphic
    align="center" fileref="images/QoS.png" /></para>
  </section>

  <section>
    <title>Enable TC support in Shorewall</title>

    <para>You need this support whether you use the builtin support or whether
    you provide your own tcstart script.</para>

    <para>To enable the builtin traffic shaping and control in Shorewall, you
    have to do the following:</para>

    <itemizedlist>
      <listitem>
        <para>Set <emphasis role="bold">TC_ENABLED</emphasis> to "Internal" in
        /etc/shorewall/shorewall.conf. Setting TC_ENABLED=Yes causes Shorewall
        to look for an external tcstart file (See <link linkend="tcstart">a
        later section</link> for details).</para>
      </listitem>

      <listitem>
        <para>Setting <emphasis role="bold">CLEAR_TC</emphasis> parameter in
        /etc/shorewall/shorewall.conf to Yes will clear the traffic shaping
        configuration during Shorewall [re]start and Shorewall stop. This is
        normally what you want when using the builtin support (and also if you
        use your own tcstart script)</para>
      </listitem>

      <listitem>
        <para>The other steps that follow depend on whether you use your own
        script or the builtin solution. They will be explained in the
        following sections.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Using builtin traffic shaping/control</title>

    <para>For defining bandwidths (for either devices or classes) please use
    kbit or kbps(for Kilobytes per second) and make sure there is <emphasis
    role="bold">NO</emphasis> space between the number and the unit (it is
    100kbit <emphasis role="bold">not</emphasis> 100 kbit). Using mbit, mbps
    or a raw number (which means bytes) could be used, but note that only
    integer numbers are supported (0.5 is <emphasis role="bold">not
    valid</emphasis>).</para>

    <para>To properly configure the settings for your devices you might need
    to find out, the real up- and downstream rates you have. This is
    especially the case, if you are using a DSL connection or one of another
    type that do not have a guaranteed bandwidth.Don't trust the values your
    provider tells you for this, especially measuring the real download speed
    is important! There are several online tools that help you find out,
    search for "dsl speed test" on google (For Germany you can use <ulink
    url="http://www.speedcheck.arcor.de/cgi-bin/speedcheck.cgi">arcor speed
    check</ulink>). Be sure to choose a test located near you.</para>

    <section>
      <title>/etc/shorewall/tcrules</title>

      <para>The fwmark classifier provides a convenient way to classify
      packets for traffic shaping. The <quote>/etc/shorewall/tcrules</quote>
      file is used for specifying these marks in a tabular fashion.</para>

      <para>Normally, packet marking occurs in the PREROUTING chain before any
      address rewriting takes place. This makes it impossible to mark inbound
      packets based on their destination address when SNAT or Masquerading are
      being used. You can cause packet marking to occur in the FORWARD chain
      by using the MARK_IN_FORWARD_CHAIN option in shorewall.conf.</para>

      <para>Columns in the file are as follows:</para>

      <itemizedlist>
        <listitem>
          <para>MARK - Specifies the mark value is to be assigned in case of a
          match. This is an integer in the range 1-255. This value may be
          optionally followed by <quote>:</quote> and either <quote>F</quote>
          or <quote>P</quote> to designate that the marking will occur in the
          FORWARD or PREROUTING chains respectively. If this additional
          specification is omitted, the chain used to mark packets will be
          determined by the setting of the MARK_IN_FORWARD_CHAIN option in
          shorewall.conf.</para>
        </listitem>

        <listitem>
          <para>SOURCE - The source of the packet. If the packet originates on
          the firewall, place <quote>fw</quote> in this column. Otherwise,
          this is a comma-separated list of interface names, IP addresses, MAC
          addresses in Shorewall Format and/or Subnets.</para>

          <para>Examples <programlisting>    eth0     192.168.2.4,192.168.1.0/24</programlisting></para>
        </listitem>

        <listitem>
          <para>DEST -- Destination of the packet. Comma-separated list of IP
          addresses and/or subnets.</para>
        </listitem>

        <listitem>
          <para>PROTO - Protocol - Must be the name of a protocol from
          /etc/protocol, a number or <quote>all</quote></para>
        </listitem>

        <listitem>
          <para>PORT(S) - Destination Ports. A comma-separated list of Port
          names (from /etc/services), port numbers or port ranges (e.g.,
          21:22); if the protocol is <quote>icmp</quote>, this column is
          interpreted as the destination icmp type(s).</para>
        </listitem>

        <listitem>
          <para>CLIENT PORT(S) - (Optional) Port(s) used by the client. If
          omitted, any source port is acceptable. Specified as a
          comma-separate list of port names, port numbers or port
          ranges.</para>
        </listitem>

        <listitem>
          <para>USER/GROUP (Added in Shorewall version 1.4.10) - (Optional)
          This column may only be non-empty if the SOURCE is the firewall
          itself. When this column is non-empty, the rule applies only if the
          program generating the output is running under the effective user
          and/or group. It may contain :</para>

          <para>[!][&lt;user name or number&gt;]:[&lt;group name or
          number&gt;][+&lt;program name&gt;]</para>

          <para>The colon is optionnal when specifying only a user.</para>

          <para>Examples:</para>

          <programlisting>joe     #program must be run by joe
:kids   #program must be run by a member of the 'kids' group
!:kids  #program must not be run by a member of the 'kids' group
+upnpd  #program named upnpd (This feature was removed from Netfilter in kernel version 2.6.14).</programlisting>
        </listitem>
      </itemizedlist>

      <example>
        <title></title>

        <para>All packets arriving on eth1 should be marked with 1. All
        packets arriving on eth2 and eth3 should be marked with 2. All packets
        originating on the firewall itself should be marked with 3.</para>

        <programlisting>#MARK   SOURCE    DESTINATION    PROTOCOL   USER/GROUP
1       eth1      0.0.0.0/0      all
2       eth2      0.0.0.0/0      all
2       eth3      0.0.0.0/0      all
3       fw        0.0.0.0/0      all</programlisting>
      </example>

      <example>
        <title></title>

        <para>All GRE (protocol 47) packets not originating on the firewall
        and destined for 155.186.235.151 should be marked with 12.</para>

        <programlisting>#MARK   SOURCE    DESTINATION     PROTOCOL   USER/GROUP
12      0.0.0.0/0 155.182.235.151 47</programlisting>
      </example>

      <example>
        <title></title>

        <para>All SSH packets originating in 192.168.1.0/24 and destined for
        155.186.235.151 should be marked with 22.</para>

        <programlisting>#MARK   SOURCE         DESTINATION     PROTOCOL   USER/GROUP
22      192.168.1.0/24 155.182.235.151 tcp        22</programlisting>
      </example>
    </section>

    <section>
      <title>/etc/shorewall/tcdevices</title>

      <para>This file allows you to define the incoming and outgoing bandwidth
      for the devices you want traffic shaping to be enabled. That means, if
      you want to use traffic shaping for a device, you have to define it
      here.</para>

      <para>Columns in the file are as follows:</para>

      <itemizedlist>
        <listitem>
          <para>INTERFACE - Name of interface. Each interface may be listed
          only once in this file. You may NOT specify the name of an alias
          (e.g., eth0:0) here; see <ulink url="FAQ.htm#faq18">FAQ #18</ulink>.
          You man NOT specify wildcards here, e.g. if you have multiple ppp
          interfaces, you need to put them all in here!</para>
        </listitem>

        <listitem>
          <para>IN-BANDWIDTH - The incoming Bandwidth of that interface.
          Please note that you are not able to do traffic shaping on incoming
          traffic, as the traffic is already received before you could do so.
          This Column allows you to define the maximum traffic allowed for
          this interface in total, if the rate is exceeded, the packets are
          dropped. You want this mainly if you have a DSL or Cable Connection
          to avoid queuing at your providers side. If you don't want any
          traffic to be dropped set this to a value faster than your interface
          maximum rate.</para>
        </listitem>

        <listitem>
          <para>OUT-BANDWIDTH - Specifiy the outgoing bandwidth of that
          interface. This is the maximum speed your connection can handle. It
          is also the speed you can refer as "full" if you define the tc
          classes. Outgoing traffic above this rate will be dropped.</para>
        </listitem>
      </itemizedlist>

      <example>
        <title></title>

        <para>Suppose you are using PPP over Ethernet (DSL) and ppp0 is the
        interface for this. The device has an outgoing bandwidth of 500kbit
        and an incoming bandwidth of 6000kbit</para>

        <programlisting>#INTERFACE    IN-BANDWITH      OUT-BANDWIDTH
ppp0           6000kbit         500kbit</programlisting>
      </example>
    </section>

    <section>
      <title>/etc/shorewall/tcclasses</title>

      <para>This file allows you to define the actual classes that are used to
      split the outgoing traffic.</para>

      <itemizedlist>
        <listitem>
          <para>INTERFACE - Name of interface. Each interface may be listed
          only once in this file. You may NOT specify the name of an alias
          (e.g., eth0:0) here; see <ulink url="FAQ.htm#faq18">FAQ #18</ulink>.
          You man NOT specify wildcards here, e.g. if you have multiple ppp
          interfaces, you need to put them all in here!</para>
        </listitem>

        <listitem>
          <para>MARK - The mark value which is an integer in the range 1-255.
          You define these marks in the tcrules file, marking the traffic you
          want to go into the queueing classes defined in here. You can use
          the same marks for different Interfaces.</para>
        </listitem>

        <listitem>
          <para>RATE - The minimum bandwidth this class should get, when the
          traffic load rises. Please note that first the classes which equal
          or a lesser priority value are served even if there are others that
          have a guaranteed bandwith but a lower priority.</para>
        </listitem>

        <listitem>
          <para>CEIL - The maximum bandwidth this class is allowed to use when
          the link is idle. Useful if you have traffic which can get full
          speed when more important services (e.g. interactive like ssh) are
          not used. You can use the value "full" in here for setting the
          maximum bandwidth to the defined output bandwidth of that
          interface.</para>
        </listitem>

        <listitem>
          <para>PRIORITY - you have to define a priority for the class.
          packets in a class with a higher priority (=lesser value) are
          handled before less priorized onces. You can just define the mark
          value here also, if you are increasing the mark values with lesser
          priority.</para>
        </listitem>

        <listitem>
          <para>OPTIONS - A comma-separated list of options including the
          following:</para>

          <itemizedlist>
            <listitem>
              <para>default - this is the default class for that interface
              where all traffic should go, that is not classified
              otherwise.</para>

              <note>
                <para>defining default for exactly <emphasis
                role="bold">one</emphasis> class per interface is
                mandatory!</para>
              </note>
            </listitem>

            <listitem>
              <para>tos-&lt;tosname&gt; - this lets you define a filter for
              the given &lt;tosname&gt; which lets you define a value of the
              Type Of Service bits in the ip package which causes the package
              to go in this class. Please note, that this filter overrides all
              mark settings, so if you define a tos filter for a class all
              traffic having that mark will go in it regardless of the mark on
              the package. You can use the following for this option:
              tos-minimize-delay (16) tos-maximize-throughput (8)
              tos-maximize-reliability (4) tos-minimize-cost (2)
              tos-normal-service (0)</para>

              <note>
                <para>Each of this options is only valid for <emphasis
                role="bold">one</emphasis> class per interface.</para>
              </note>
            </listitem>

            <listitem>
              <para>tcp-ack - if defined causes an tc filter to be created
              that puts all tcp ack packets on that interface that have an
              size of &lt;=64 Bytes to go in this class. This is useful for
              speeding up downloads. Please note that the size of the ack
              packages is limited to 64 bytes as some applications (p2p for
              example) use to make every package an ack package which would
              cause them all into here. We want only packages WITHOUT payload
              to match, so the size limit. Bigger packets just take their
              normal way into the classes.</para>

              <note>
                <para>This option is only valid for <emphasis
                role="bold">class</emphasis> per interface.</para>
              </note>
            </listitem>
          </itemizedlist>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>ppp devices</title>

      <para>If you use ppp/pppoe/pppoa) to connect to your internet provider
      and you use traffic shaping you need to restart shorewall traffic
      shaping. The reason for this is, that if the ppp connection gets
      restartet (and it usally does this at least daily), all
      <quote>tc</quote> filters/qdiscs related to that interface are
      deleted.</para>

      <para>The easiest way to achieve this, is just to restart shorewall once
      the link is up. To achieve this add a small executable script
      to<quote>/etc/ppp/ip-up.d</quote>.</para>

      <programlisting>#! /bin/sh

/sbin/shorewall restart</programlisting>
    </section>

    <section>
      <title>Real life examples</title>

      <section>
        <title>Configuration to replace Wondershaper</title>

        <para>You are able to fully replace the wondershaper script by using
        the buitin traffic control.You can find example configuration files at
        <ulink
        url="http://www1.shorewall.net/pub/shorewall/Samples/tc4shorewall/">"http://www1.shorewall.net/pub/shorewall/Samples/tc4shorewall/</ulink>.
        Please note that they are just examples and need to be adjusted to
        work for you. In this examples it is assumed that your interface for
        you internet connection is ppp0 (for DSL) , if you use another
        connection type, you have to change it. You also need to change the
        settings in the tcdevices.wondershaper file to reflect your line
        speed. The relevant lines of the config files follow here. Please note
        that this is just an 1:1 replacement doing exactly what wondershaper
        should do. You are free to change it...</para>

        <section>
          <title>tcdevices file</title>

          <programlisting>#INTERFACE    IN-BANDWITH      OUT-BANDWIDTH
ppp0            5000kbit        500kbit</programlisting>
        </section>

        <section>
          <title>tcclasses file</title>

          <programlisting>#INTERFACE      MARK    RATE            CEIL        PRIORITY    OPTIONS
ppp0            1       full            full            1       tcp-ack,tos-minimize-delay
ppp0            2       9*full/10       9*full/10       2       default
ppp0            3       8*full/10       8*full/10       2</programlisting>
        </section>

        <section>
          <title>tcrules file</title>

          <programlisting>#MARK           SOURCE          DEST            PROTO   PORT(S) CLIENT   USER
#                                                              PORT(S)
1:P             0.0.0.0/0       0.0.0.0/0       icmp    echo-request
1:P             0.0.0.0/0       0.0.0.0/0       icmp    echo-reply
# mark traffic which should have a lower priority with a 3:
# mldonkey
3               0.0.0.0/0       0.0.0.0/0       udp     -        4666</programlisting>

          <para>Wondershaper allows you to define a set of hosts and/or ports
          you want to classify as low priority. To achieve this , you have to
          add these hosts to tcrules and set the mark to 3 (true if you use
          the example configuration files).</para>
        </section>

        <section>
          <title>Setting hosts to low priority</title>

          <para>lets assume the following settings from your old wondershaper
          script (don't assume these example values are really useful, they
          are only used for demonstrating ;-):</para>

          <programlisting>
# low priority OUTGOING traffic - you can leave this blank if you want
# low priority source netmasks
NOPRIOHOSTSRC="192.168.1.128/25 192.168.3.28"

# low priority destination netmasks
NOPRIOHOSTDST=60.0.0.0/24

# low priority source ports
NOPRIOPORTSRC="6662 6663"

# low priority destination ports
NOPRIOPORTDST="6662 6663"  </programlisting>

          <para>This would result in the following additional settings to the
          tcrules file:</para>

          <programlisting>3               192.168.1.128/25 0.0.0.0/0      all
3               192.168.3.28     0.0.0.0/0      all
3               0.0.0.0/0        60.0.0.0/24    all
3               0.0.0.0/0        0.0.0.0/0      udp     6662,6663
3               0.0.0.0/0        0.0.0.0/0      udp     -         6662,6663
3               0.0.0.0/0        0.0.0.0/0      tcp     6662,6663
3               0.0.0.0/0        0.0.0.0/0      tcp     -         6662,6663</programlisting>
        </section>
      </section>

      <section>
        <title>A simple setup</title>

        <para>This is a simple setup for people sharing an internet connection
        and using different computers for this. It just basically shapes
        between 2 hosts which have the ip addresses 192.168.2.23 and
        192.168.2.42</para>

        <section>
          <title>tcdevices file</title>

          <programlisting>#INTERFACE    IN-BANDWITH      OUT-BANDWIDTH
ppp0            6000kbit        700kbit</programlisting>

          <para>We have 6mbit down and 700kbit upstream.</para>
        </section>

        <section>
          <title>tcclasses file</title>

          <programlisting>#INTERFACE      MARK    RATE            CEIL            PRIORITY    OPTIONS
ppp0            1       10kbit          50kbit          1           tcp-ack
ppp0            2       300kbit         full            2
ppp0            3       300kbit         full            2
ppp0            4       90kbit          200kbit         3           default</programlisting>

          <para>We add a class for tcp ack packets with highest priority, so
          that downloads are fast. The following 2 classes share most of the
          bandwidth between the 2 hosts, if the connection is idle, they may
          use full speed. As the hosts should be treated equally they have the
          same priority. The last class is for the remaining traffic.</para>
        </section>

        <section>
          <title>tcrules file</title>

          <programlisting>#MARK           SOURCE          DEST            PROTO   PORT(S)      CLIENT   USER
#                                                                    PORT(S)
1:P             0.0.0.0/0       0.0.0.0/0       icmp    echo-request
1:P             0.0.0.0/0       0.0.0.0/0       icmp    echo-reply
2:P             192.168.2.23    0.0.0.0/0       all
3:P             192.168.2.42    0.0.0.0/0       all</programlisting>

          <para>We mark icmp ping and replies so they will go into the fast
          interactive class and set a mark for each host.</para>
        </section>
      </section>
    </section>
  </section>

  <section>
    <title id="tcstart">Using your own tc script</title>

    <section id="owntcstart">
      <title>Replacing builtin tcstart file</title>

      <para>If you prefer your own tcstart file, just install it in
      /etc/shorewall/tcstart.</para>

      <para>In your tcstart script, when you want to run the <quote>tc</quote>
      utility, use the run_tc function supplied by Shorewall if you want tc
      errors to stop the firewall.</para>

      <orderedlist>
        <listitem>
          <para>Set TC_ENABLED=Yes and CLEAR_TC=Yes</para>
        </listitem>

        <listitem>
          <para>Supply an /etc/shorewall/tcstart script to configure your
          traffic shaping rules.</para>
        </listitem>

        <listitem>
          <para>Optionally supply an /etc/shorewall/tcclear script to stop
          traffic shaping. That is usually unnecessary.</para>
        </listitem>

        <listitem>
          <para>If your tcstart script uses the <quote>fwmark</quote>
          classifier, you can mark packets using entries in
          /etc/shorewall/tcrules.</para>
        </listitem>
      </orderedlist>
    </section>

    <section>
      <title>Traffic control outside Shorewall</title>

      <para>To start traffic shaping when you bring up your network
      interfaces, you will have to arrange for your traffic shaping
      configuration script to be run at that time. How you do that is
      distribution dependent and will not be covered here. You then
      should:</para>

      <orderedlist>
        <listitem>
          <para>Set TC_ENABLED=No and CLEAR_TC=No</para>
        </listitem>

        <listitem>
          <para>If your script uses the <quote>fwmark</quote> classifier, you
          can mark packets using entries in /etc/shorewall/tcrules.</para>
        </listitem>
      </orderedlist>
    </section>
  </section>
</article>