LDP/LDP/howto/docbook/Traffic-Control-HOWTO/elements.xml

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   The article was authored by Martin A. Brown <martin@linux-ip.net>
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<section id="elements">

  <title>Traditional Elements of Traffic Control</title>
  <para>
  </para>

  <section id="e-shaping">
    <title>Shaping</title>
    <para>
      Shapers delay packets to meet a desired rate.
    </para>
    <para>
      Shaping is the mechanism by which packets are delayed before
      transmission in an output queue to meet a desired output rate.  This is
      one of the most common desires of users seeking bandwidth control
      solutions.  The act of delaying a packet as part of a traffic control
      solution makes every shaping mechanism into a non-work-conserving
      mechanism, meaning roughly:  "Work is required in order to delay
      packets."
    </para>
    <para>
      Viewed in reverse, a non-work-conserving queuing mechanism is performing
      a shaping function.  A work-conserving queuing mechanism (see
      &link-sch_prio;) would not be capable of delaying a packet.
    </para>
    <para>
      Shapers attempt to limit or ration traffic to meet but not exceed a
      configured rate (frequently measured in packets per second or bits/bytes
      per second).  As a side effect, shapers can smooth out bursty traffic
      <footnote>
        <para>
          This smoothing effect is not always desirable, hence the HTB
          parameters burst and cburst.
        </para>
      </footnote>.
      One of the advantages of shaping bandwidth is the ability to control
      latency of packets.  The underlying mechanism for shaping to a rate is
      typically a token and bucket mechanism.  See also
      <xref linkend="o-tokens"/> for further detail on tokens and buckets.
    </para>
  </section>

  <section id="e-scheduling">
    <title>Scheduling</title>
    <para>
      Schedulers arrange and/or rearrange packets for output.
    </para>
    <para>
      Scheduling is the mechanism by which packets are arranged (or
      rearranged) between input and output of a particular queue.  The
      overwhelmingly most common scheduler is the FIFO (first-in first-out)
      scheduler.  From a larger perspective, any set of traffic control
      mechanisms on an output queue can be regarded as a scheduler, because
      packets are arranged for output.
    </para>
    <para>
      Other generic scheduling mechanisms attempt to compensate for various
      networking conditions.  A fair queuing algorithm (see &link-sch_sfq;)
      attempts to prevent any single client or flow from dominating the
      network usage.  A round-robin algorithm (see &link-sch_wrr;) gives each
      flow or client a turn to dequeue packets.  Other sophisticated
      scheduling algorithms attempt to prevent backbone overload (see
      &link-sch_gred;) or refine other scheduling mechanisms (see
      &link-sch_esfq;).
    </para>
  </section>

  <section id="e-classifying">
    <title>Classifying</title>
    <para>
      Classifiers sort or separate traffic into queues.
    </para>
    <para>
      Classifying is the mechanism by which packets are separated for
      different treatment, possibly different output queues.  During the
      process of accepting, routing and transmitting a packet, a networking
      device can classify the packet a number of different ways.
      Classification can include
      <link linkend="e-marking">marking</link> the packet, which usually
      happens on the boundary of a network under a single administrative
      control or classification can occur on each hop individually.
    </para>
    <para>
      The Linux model (see
      <xref linkend="c-filter"/>) allows for a packet to cascade across a
      series of classifiers in a traffic control structure and to be
      classified in conjunction with
      <link linkend="e-policing">policers</link> (see also
      <xref linkend="c-police"/>).
    </para>
  </section>

  <section id="e-policing">
    <title>Policing</title>
    <para>
      Policers measure and limit traffic in a particular queue.
    </para>
    <para>
      Policing, as an element of traffic control, is simply
      a mechanism by which traffic can be limited.  Policing is most
      frequently used on the network border to ensure that a peer is not
      consuming more than its allocated bandwidth.  A policer will accept
      traffic to a certain rate, and then perform an action on traffic
      exceeding this rate.  A rather harsh solution is to
      <link linkend="e-dropping">drop</link> the traffic, although the
      traffic could be
      <link linkend="e-classifying">reclassified</link> instead of being
      dropped.
    </para>
    <para>
      A policer is a yes/no question about the rate at which traffic is
      entering a queue.  If the packet is about to enter a queue below a given
      rate, take one action (allow the enqueuing).  If the packet is about to
      enter a queue above a given rate, take another action.  Although the
      policer uses a token bucket mechanism internally, it does not have the
      capability to delay a packet as a &elements-shaping; mechanism does.
    </para>
  </section>

  <section id="e-dropping">
    <title>Dropping</title>
    <para>
      Dropping discards an entire packet, flow or classification.
    </para>
    <para>
      Dropping a packet is a mechanism by which a packet is discarded.
    </para>
    <para>
    </para>
  </section>

  <section id="e-marking">
    <title>Marking</title>
    <para>
      Marking is a mechanism by which the packet is altered.
    </para>
    <note>
      <para>
      This is not &fwmark;.  The &iptables; target &ipt-mark; and the
      &ipchains; &ipc-mark; are used to modify packet metadata, not the packet
      itself.
      </para>
    </note>
    <para>
      Traffic control marking mechanisms install a DSCP on the packet
      itself, which is then used and respected by other routers inside an
      administrative domain (usually for DiffServ).
    </para>
  </section>

</section>

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