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      sfq-implementation.xml on Ticket #179 – Attachment
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        <h1><a href="/tools/xml2rfc/trac/ticket/179">Ticket #179</a>: sfq-implementation.xml</h1>
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              <th scope="col">File sfq-implementation.xml,
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  <table class="code"><thead><tr><th class="lineno" title="Line numbers">Line</th><th class="content"> </th></tr></thead><tbody><tr><th id="L1"><a href="#L1">1</a></th><td><span class="cp">&lt;?xml version="1.0" encoding="US-ASCII"?&gt;</span></td></tr><tr><th id="L2"><a href="#L2">2</a></th><td><span class="cp">&lt;!DOCTYPE rfc SYSTEM "rfc2629.dtd"&gt;</span></td></tr><tr><th id="L3"><a href="#L3">3</a></th><td><span class="cp">&lt;?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?&gt;</span></td></tr><tr><th id="L4"><a href="#L4">4</a></th><td><span class="c">&lt;!-- Some of the more generally applicable PIs that most I-Ds might want to use --&gt;</span></td></tr><tr><th id="L5"><a href="#L5">5</a></th><td><span class="c">&lt;!-- Try to enforce the ID-nits conventions and DTD validity --&gt;</span></td></tr><tr><th id="L6"><a href="#L6">6</a></th><td><span class="cp">&lt;?rfc strict="yes" ?&gt;</span></td></tr><tr><th id="L7"><a href="#L7">7</a></th><td><span class="c">&lt;!-- Items used when reviewing the document --&gt;</span></td></tr><tr><th id="L8"><a href="#L8">8</a></th><td><span class="cp">&lt;?rfc comments="no" ?&gt;</span></td></tr><tr><th id="L9"><a href="#L9">9</a></th><td><span class="c">&lt;!-- Controls display of &lt;cref&gt; elements --&gt;</span></td></tr><tr><th id="L10"><a href="#L10">10</a></th><td><span class="cp">&lt;?rfc inline="no" ?&gt;</span></td></tr><tr><th id="L11"><a href="#L11">11</a></th><td><span class="c">&lt;!-- When no, put comments at end in comments section,</span></td></tr><tr><th id="L12"><a href="#L12">12</a></th><td><span class="c">                                 otherwise, put inline --&gt;</span></td></tr><tr><th id="L13"><a href="#L13">13</a></th><td><span class="cp">&lt;?rfc editing="no" ?&gt;</span></td></tr><tr><th id="L14"><a href="#L14">14</a></th><td><span class="c">&lt;!-- When yes, insert editing marks: editing marks consist of a </span></td></tr><tr><th id="L15"><a href="#L15">15</a></th><td><span class="c">                                 string such as &lt;29&gt; printed in the blank line at the </span></td></tr><tr><th id="L16"><a href="#L16">16</a></th><td><span class="c">                                 beginning of each paragraph of text. --&gt;</span></td></tr><tr><th id="L17"><a href="#L17">17</a></th><td><span class="c">&lt;!-- Create Table of Contents (ToC) and set some options for it.  </span></td></tr><tr><th id="L18"><a href="#L18">18</a></th><td><span class="c">         Note the ToC may be omitted for very short documents,but idnits insists on a ToC </span></td></tr><tr><th id="L19"><a href="#L19">19</a></th><td><span class="c">         if the document has more than 15 pages. --&gt;</span></td></tr><tr><th id="L20"><a href="#L20">20</a></th><td><span class="cp">&lt;?rfc toc="yes"?&gt;</span></td></tr><tr><th id="L21"><a href="#L21">21</a></th><td><span class="cp">&lt;?rfc tocompact="yes"?&gt;</span></td></tr><tr><th id="L22"><a href="#L22">22</a></th><td><span class="c">&lt;!-- If "yes" eliminates blank lines before main section entries. --&gt;</span></td></tr><tr><th id="L23"><a href="#L23">23</a></th><td><span class="cp">&lt;?rfc tocdepth="3"?&gt;</span></td></tr><tr><th id="L24"><a href="#L24">24</a></th><td><span class="c">&lt;!-- Sets the number of levels of sections/subsections... in ToC --&gt;</span></td></tr><tr><th id="L25"><a href="#L25">25</a></th><td><span class="c">&lt;!-- Choose the options for the references. </span></td></tr><tr><th id="L26"><a href="#L26">26</a></th><td><span class="c">         Some like symbolic tags in the references (and citations) and others prefer </span></td></tr><tr><th id="L27"><a href="#L27">27</a></th><td><span class="c">         numbers. The RFC Editor always uses symbolic tags.</span></td></tr><tr><th id="L28"><a href="#L28">28</a></th><td><span class="c">         The tags used are the anchor attributes of the references. --&gt;</span></td></tr><tr><th id="L29"><a href="#L29">29</a></th><td><span class="cp">&lt;?rfc symrefs="yes"?&gt;</span></td></tr><tr><th id="L30"><a href="#L30">30</a></th><td><span class="cp">&lt;?rfc sortrefs="yes" ?&gt;</span></td></tr><tr><th id="L31"><a href="#L31">31</a></th><td><span class="c">&lt;!-- If "yes", causes the references to be sorted in order of tags.</span></td></tr><tr><th id="L32"><a href="#L32">32</a></th><td><span class="c">                                 This doesn't have any effect unless symrefs is "yes" also. --&gt;</span></td></tr><tr><th id="L33"><a href="#L33">33</a></th><td><span class="c">&lt;!-- These two save paper: Just setting compact to "yes" makes savings by not starting each </span></td></tr><tr><th id="L34"><a href="#L34">34</a></th><td><span class="c">         main section on a new page but does not omit the blank lines between list items. </span></td></tr><tr><th id="L35"><a href="#L35">35</a></th><td><span class="c">         If subcompact is also "yes" the blank lines between list items are also omitted. --&gt;</span></td></tr><tr><th id="L36"><a href="#L36">36</a></th><td><span class="cp">&lt;?rfc compact="yes" ?&gt;</span></td></tr><tr><th id="L37"><a href="#L37">37</a></th><td><span class="cp">&lt;?rfc subcompact="no" ?&gt;</span></td></tr><tr><th id="L38"><a href="#L38">38</a></th><td><span class="c">&lt;!-- end of list of popular I-D processing instructions --&gt;</span></td></tr><tr><th id="L39"><a href="#L39">39</a></th><td><span class="c">&lt;!-- end of list of processing instructions --&gt;</span></td></tr><tr><th id="L40"><a href="#L40">40</a></th><td><span class="nt">&lt;rfc</span> <span class="na">category=</span><span class="s">"info"</span> <span class="na">docName=</span><span class="s">"draft-baker-aqm-sfq-implementation-00"</span></td></tr><tr><th id="L41"><a href="#L41">41</a></th><td>     <span class="na">ipr=</span><span class="s">"trust200902"</span><span class="nt">&gt;</span></td></tr><tr><th id="L42"><a href="#L42">42</a></th><td>  <span class="nt">&lt;front&gt;</span></td></tr><tr><th id="L43"><a href="#L43">43</a></th><td>    <span class="nt">&lt;title</span> <span class="na">abbrev=</span><span class="s">""</span><span class="nt">&gt;</span>On Queuing, Marking, and Dropping<span class="nt">&lt;/title&gt;</span></td></tr><tr><th id="L44"><a href="#L44">44</a></th><td>    <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Fred Baker"</span> <span class="na">initials=</span><span class="s">"F.J."</span> <span class="na">surname=</span><span class="s">"Baker"</span><span class="nt">&gt;</span></td></tr><tr><th id="L45"><a href="#L45">45</a></th><td>      <span class="nt">&lt;organization&gt;</span>Cisco Systems<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L46"><a href="#L46">46</a></th><td>      <span class="nt">&lt;address&gt;</span></td></tr><tr><th id="L47"><a href="#L47">47</a></th><td>        <span class="nt">&lt;postal&gt;</span></td></tr><tr><th id="L48"><a href="#L48">48</a></th><td>          <span class="nt">&lt;street&gt;&lt;/street&gt;</span></td></tr><tr><th id="L49"><a href="#L49">49</a></th><td>          <span class="nt">&lt;city&gt;</span>Santa Barbara<span class="nt">&lt;/city&gt;</span></td></tr><tr><th id="L50"><a href="#L50">50</a></th><td>          <span class="nt">&lt;code&gt;</span>93117<span class="nt">&lt;/code&gt;</span></td></tr><tr><th id="L51"><a href="#L51">51</a></th><td>          <span class="nt">&lt;region&gt;</span>California<span class="nt">&lt;/region&gt;</span></td></tr><tr><th id="L52"><a href="#L52">52</a></th><td>          <span class="nt">&lt;country&gt;</span>USA<span class="nt">&lt;/country&gt;</span></td></tr><tr><th id="L53"><a href="#L53">53</a></th><td>        <span class="nt">&lt;/postal&gt;</span></td></tr><tr><th id="L54"><a href="#L54">54</a></th><td>        <span class="nt">&lt;email&gt;</span>fred@cisco.com<span class="nt">&lt;/email&gt;</span></td></tr><tr><th id="L55"><a href="#L55">55</a></th><td>      <span class="nt">&lt;/address&gt;</span></td></tr><tr><th id="L56"><a href="#L56">56</a></th><td>    <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L57"><a href="#L57">57</a></th><td>    <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Rong Pan"</span> <span class="na">initials=</span><span class="s">"R."</span> <span class="na">surname=</span><span class="s">"Pan"</span><span class="nt">&gt;</span></td></tr><tr><th id="L58"><a href="#L58">58</a></th><td>      <span class="nt">&lt;organization&gt;</span>Cisco Systems<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L59"><a href="#L59">59</a></th><td>      <span class="nt">&lt;address&gt;</span></td></tr><tr><th id="L60"><a href="#L60">60</a></th><td>        <span class="nt">&lt;postal&gt;</span></td></tr><tr><th id="L61"><a href="#L61">61</a></th><td>          <span class="nt">&lt;street&gt;&lt;/street&gt;</span></td></tr><tr><th id="L62"><a href="#L62">62</a></th><td>          <span class="nt">&lt;city&gt;</span>Milpitas<span class="nt">&lt;/city&gt;</span></td></tr><tr><th id="L63"><a href="#L63">63</a></th><td>          <span class="nt">&lt;code&gt;</span>95035<span class="nt">&lt;/code&gt;</span></td></tr><tr><th id="L64"><a href="#L64">64</a></th><td>          <span class="nt">&lt;region&gt;</span>California<span class="nt">&lt;/region&gt;</span></td></tr><tr><th id="L65"><a href="#L65">65</a></th><td>          <span class="nt">&lt;country&gt;</span>USA<span class="nt">&lt;/country&gt;</span></td></tr><tr><th id="L66"><a href="#L66">66</a></th><td>        <span class="nt">&lt;/postal&gt;</span></td></tr><tr><th id="L67"><a href="#L67">67</a></th><td>        <span class="nt">&lt;email&gt;</span>ropan@cisco.com<span class="nt">&lt;/email&gt;</span></td></tr><tr><th id="L68"><a href="#L68">68</a></th><td>      <span class="nt">&lt;/address&gt;</span></td></tr><tr><th id="L69"><a href="#L69">69</a></th><td>    <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L70"><a href="#L70">70</a></th><td>    <span class="nt">&lt;date</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L71"><a href="#L71">71</a></th><td>    <span class="nt">&lt;area&gt;</span>Transport<span class="nt">&lt;/area&gt;</span></td></tr><tr><th id="L72"><a href="#L72">72</a></th><td>    <span class="nt">&lt;workgroup&gt;</span>Active Queue Management<span class="nt">&lt;/workgroup&gt;</span></td></tr><tr><th id="L73"><a href="#L73">73</a></th><td>    <span class="nt">&lt;abstract&gt;</span></td></tr><tr><th id="L74"><a href="#L74">74</a></th><td>      <span class="nt">&lt;t&gt;</span>This note discusses implementation strategies for coupled queuing and</td></tr><tr><th id="L75"><a href="#L75">75</a></th><td>      mark/drop algorithms.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L76"><a href="#L76">76</a></th><td>    <span class="nt">&lt;/abstract&gt;</span></td></tr><tr><th id="L77"><a href="#L77">77</a></th><td>    <span class="c">&lt;!--                </span></td></tr><tr><th id="L78"><a href="#L78">78</a></th><td><span class="c">                &lt;note title="Foreword"&gt;</span></td></tr><tr><th id="L79"><a href="#L79">79</a></th><td><span class="c">                &lt;/note&gt;</span></td></tr><tr><th id="L80"><a href="#L80">80</a></th><td><span class="c">                --&gt;</span></td></tr><tr><th id="L81"><a href="#L81">81</a></th><td>    <span class="c">&lt;!--</span></td></tr><tr><th id="L82"><a href="#L82">82</a></th><td><span class="c">        &lt;texttable anchor="table_example" title="A Very Simple Table"&gt;</span></td></tr><tr><th id="L83"><a href="#L83">83</a></th><td><span class="c">&lt;preamble&gt;Tables use ttcol to define column headers and widths.</span></td></tr><tr><th id="L84"><a href="#L84">84</a></th><td><span class="c">      Every cell then has a &amp;quot;c&amp;quot; element for its content.&lt;/preamble&gt;</span></td></tr><tr><th id="L85"><a href="#L85">85</a></th><td><span class="c">        &lt;ttcol align="center"&gt;ttcol #1&lt;/ttcol&gt;</span></td></tr><tr><th id="L86"><a href="#L86">86</a></th><td><span class="c">        &lt;ttcol align="center"&gt;ttcol #2&lt;/ttcol&gt;</span></td></tr><tr><th id="L87"><a href="#L87">87</a></th><td><span class="c">&lt;c&gt;c #1&lt;/c&gt;             &lt;c&gt;c #2&lt;/c&gt;</span></td></tr><tr><th id="L88"><a href="#L88">88</a></th><td><span class="c">&lt;c&gt;c #3&lt;/c&gt;             &lt;c&gt;c #4&lt;/c&gt;</span></td></tr><tr><th id="L89"><a href="#L89">89</a></th><td><span class="c">&lt;c&gt;c #5&lt;/c&gt;             &lt;c&gt;c #6&lt;/c&gt;</span></td></tr><tr><th id="L90"><a href="#L90">90</a></th><td><span class="c">&lt;postamble&gt;which is a very simple example.&lt;/postamble&gt;</span></td></tr><tr><th id="L91"><a href="#L91">91</a></th><td><span class="c">&lt;/texttable&gt;</span></td></tr><tr><th id="L92"><a href="#L92">92</a></th><td><span class="c">    --&gt;</span></td></tr><tr><th id="L93"><a href="#L93">93</a></th><td>  <span class="nt">&lt;/front&gt;</span></td></tr><tr><th id="L94"><a href="#L94">94</a></th><td>  <span class="nt">&lt;middle&gt;</span></td></tr><tr><th id="L95"><a href="#L95">95</a></th><td>    <span class="c">&lt;!--                </span></td></tr><tr><th id="L96"><a href="#L96">96</a></th><td><span class="c">        &lt;t&gt;There are multiple list styles: "symbols", "letters", "numbers",</span></td></tr><tr><th id="L97"><a href="#L97">97</a></th><td><span class="c">"hanging", "format", etc.&lt;/t&gt;</span></td></tr><tr><th id="L98"><a href="#L98">98</a></th><td><span class="c">        &lt;t&gt;</span></td></tr><tr><th id="L99"><a href="#L99">99</a></th><td><span class="c">        &lt;list style="symbols"&gt;</span></td></tr><tr><th id="L100"><a href="#L100">100</a></th><td><span class="c">            &lt;t&gt;First bullet&lt;/t&gt;</span></td></tr><tr><th id="L101"><a href="#L101">101</a></th><td><span class="c">            &lt;t&gt;Second bullet&lt;/t&gt;</span></td></tr><tr><th id="L102"><a href="#L102">102</a></th><td><span class="c">        &lt;/list&gt;</span></td></tr><tr><th id="L103"><a href="#L103">103</a></th><td><span class="c">&lt;/t&gt;</span></td></tr><tr><th id="L104"><a href="#L104">104</a></th><td><span class="c">--&gt;</span></td></tr><tr><th id="L105"><a href="#L105">105</a></th><td>    <span class="c">&lt;!--</span></td></tr><tr><th id="L106"><a href="#L106">106</a></th><td><span class="c">&lt;figure anchor="reference" title="Figure"&gt;</span></td></tr><tr><th id="L107"><a href="#L107">107</a></th><td><span class="c">&lt;artwork align="center"&gt;</span></td></tr><tr><th id="L108"><a href="#L108">108</a></th><td><span class="c">&lt;![CDATA[</span></td></tr><tr><th id="L109"><a href="#L109">109</a></th><td><span class="c">        ASCII artwork goes here... </span></td></tr><tr><th id="L110"><a href="#L110">110</a></th><td><span class="c">]]&gt;</span></td></tr><tr><th id="L111"><a href="#L111">111</a></th><td><span class="c">&lt;/artwork&gt;</span></td></tr><tr><th id="L112"><a href="#L112">112</a></th><td><span class="c">&lt;/figure&gt;</span></td></tr><tr><th id="L113"><a href="#L113">113</a></th><td><span class="c">--&gt;</span></td></tr><tr><th id="L114"><a href="#L114">114</a></th><td>    <span class="nt">&lt;section</span> <span class="na">title=</span><span class="s">"Introduction"</span><span class="nt">&gt;</span></td></tr><tr><th id="L115"><a href="#L115">115</a></th><td>      <span class="nt">&lt;t&gt;</span>In the discussion of Active Queue Management, there has been</td></tr><tr><th id="L116"><a href="#L116">116</a></th><td>      discussion of the coupling of queue management algorithms such as <span class="nt">&lt;xref</span></td></tr><tr><th id="L117"><a href="#L117">117</a></th><td>      <span class="na">target=</span><span class="s">"SFQ"</span><span class="nt">&gt;</span>Stochastic Fairness Queuing<span class="nt">&lt;/xref&gt;</span> with mark/drop</td></tr><tr><th id="L118"><a href="#L118">118</a></th><td>      algorithms such as <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"I-D.nichols-tsvwg-codel"</span><span class="nt">&gt;</span>CoDel<span class="nt">&lt;/xref&gt;</span></td></tr><tr><th id="L119"><a href="#L119">119</a></th><td>      or <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"I-D.pan-tsvwg-pie"</span><span class="nt">&gt;</span>PIE<span class="nt">&lt;/xref&gt;</span>. In the interest of</td></tr><tr><th id="L120"><a href="#L120">120</a></th><td>      clarifying the discussion, we document possible implementation</td></tr><tr><th id="L121"><a href="#L121">121</a></th><td>      approaches to that, and analyze the possible effects and side-effects.</td></tr><tr><th id="L122"><a href="#L122">122</a></th><td>      The language and model derive from the <span class="nt">&lt;xref</span></td></tr><tr><th id="L123"><a href="#L123">123</a></th><td>      <span class="na">target=</span><span class="s">"RFC2475"</span><span class="nt">&gt;</span>Architecture for Differentiated Services<span class="nt">&lt;/xref&gt;</span>.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L124"><a href="#L124">124</a></th><td>      <span class="c">&lt;!--</span></td></tr><tr><th id="L125"><a href="#L125">125</a></th><td><span class="c">&lt;section title="Requirements Language"&gt;</span></td></tr><tr><th id="L126"><a href="#L126">126</a></th><td><span class="c">        &lt;t&gt;The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",</span></td></tr><tr><th id="L127"><a href="#L127">127</a></th><td><span class="c">      "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this</span></td></tr><tr><th id="L128"><a href="#L128">128</a></th><td><span class="c">      document are to be interpreted as described in &lt;xref</span></td></tr><tr><th id="L129"><a href="#L129">129</a></th><td><span class="c">      target="RFC2119"&gt;&lt;/xref&gt;.&lt;/t&gt;</span></td></tr><tr><th id="L130"><a href="#L130">130</a></th><td><span class="c">&lt;/section&gt;</span></td></tr><tr><th id="L131"><a href="#L131">131</a></th><td><span class="c">    --&gt;</span></td></tr><tr><th id="L132"><a href="#L132">132</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L133"><a href="#L133">133</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"fq"</span> <span class="na">title=</span><span class="s">"Fair Queuing: Algorithms and History"</span><span class="nt">&gt;</span></td></tr><tr><th id="L134"><a href="#L134">134</a></th><td>      <span class="nt">&lt;t&gt;</span>There has been quite a bit of history in the set of algorithms</td></tr><tr><th id="L135"><a href="#L135">135</a></th><td>      collectively referred to as "Fair Queuing". The model was initially</td></tr><tr><th id="L136"><a href="#L136">136</a></th><td>      discuss in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"RFC0970"</span><span class="nt">&gt;&lt;/xref&gt;</span>, which proposed it</td></tr><tr><th id="L137"><a href="#L137">137</a></th><td>      hypothetically, as a solution to the TCP Silly Window Syndrome issue in</td></tr><tr><th id="L138"><a href="#L138">138</a></th><td>      BSD 4.1. The problem was that, due to a TCP implementation bug, some</td></tr><tr><th id="L139"><a href="#L139">139</a></th><td>      senders would settle into sending a long stream of very short segments,</td></tr><tr><th id="L140"><a href="#L140">140</a></th><td>      which unnecessarily consumed bandwidth on TCP and IP headers and</td></tr><tr><th id="L141"><a href="#L141">141</a></th><td>      occupied short packet buffers, thereby disrupting competing sessions.</td></tr><tr><th id="L142"><a href="#L142">142</a></th><td>      Nagle suggested that if packet streams were sorted by their source</td></tr><tr><th id="L143"><a href="#L143">143</a></th><td>      address and the sources treated in a round robin fashion, a sender's</td></tr><tr><th id="L144"><a href="#L144">144</a></th><td>      effect on end-to-end latency and increased loss rate would primarily</td></tr><tr><th id="L145"><a href="#L145">145</a></th><td>      affect only itself. This touched off perhaps a decade of work by various</td></tr><tr><th id="L146"><a href="#L146">146</a></th><td>      researchers on what was and in termed "Fair Queuing," philosophical</td></tr><tr><th id="L147"><a href="#L147">147</a></th><td>      discussions of the meaning of the word "fair," operational reasons that</td></tr><tr><th id="L148"><a href="#L148">148</a></th><td>      one might want a "weighted" or "predictably unfair" queuing algorithm,</td></tr><tr><th id="L149"><a href="#L149">149</a></th><td>      and so on.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L150"><a href="#L150">150</a></th><td>      <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"gps"</span> <span class="na">title=</span><span class="s">"Generalized Processor Sharing"</span><span class="nt">&gt;</span></td></tr><tr><th id="L151"><a href="#L151">151</a></th><td>        <span class="nt">&lt;t&gt;</span>Conceptually, any Fair Queuing algorithm attempts to implement some</td></tr><tr><th id="L152"><a href="#L152">152</a></th><td>        approximation to the <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"GPS"</span><span class="nt">&gt;</span>Generalized Processor</td></tr><tr><th id="L153"><a href="#L153">153</a></th><td>        Sharing<span class="nt">&lt;/xref&gt;</span> model.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L154"><a href="#L154">154</a></th><td>        <span class="nt">&lt;t&gt;</span>The GPS model, in its essence, presumes that a set of identified</td></tr><tr><th id="L155"><a href="#L155">155</a></th><td>        data streams, called "flows", pass through an interface. Each flow has</td></tr><tr><th id="L156"><a href="#L156">156</a></th><td>        a rate when measured over a period of time; A voice session might, for</td></tr><tr><th id="L157"><a href="#L157">157</a></th><td>        example, require 64 KBPS plus whatever overhead is necessary to</td></tr><tr><th id="L158"><a href="#L158">158</a></th><td>        deliver it. The premise is that on all time scales, the flow occupies</td></tr><tr><th id="L159"><a href="#L159">159</a></th><td>        the same bit rate, so that if there is enough bandwidth for the flow</td></tr><tr><th id="L160"><a href="#L160">160</a></th><td>        in the long term, it also lacks nothing in the short term. "All time</td></tr><tr><th id="L161"><a href="#L161">161</a></th><td>        scales" is obviously untenable in a packet network - and even in a</td></tr><tr><th id="L162"><a href="#L162">162</a></th><td>        traditional TDM circuit switch network. But it provides an ideal for</td></tr><tr><th id="L163"><a href="#L163">163</a></th><td>        other models to be compared against.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L164"><a href="#L164">164</a></th><td>        <span class="nt">&lt;t&gt;</span>There are a number of attributes of approximations to the GPS model</td></tr><tr><th id="L165"><a href="#L165">165</a></th><td>        that bear operational consideration, including at least the</td></tr><tr><th id="L166"><a href="#L166">166</a></th><td>        transmission quanta, the definition of a "flow", the unit of</td></tr><tr><th id="L167"><a href="#L167">167</a></th><td>        measurement. Implementation algorithms have different practical</td></tr><tr><th id="L168"><a href="#L168">168</a></th><td>        impacts as well.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L169"><a href="#L169">169</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"gps-quanta"</span></td></tr><tr><th id="L170"><a href="#L170">170</a></th><td>                 <span class="na">title=</span><span class="s">"GPS Comparisons: transmission quanta"</span><span class="nt">&gt;</span></td></tr><tr><th id="L171"><a href="#L171">171</a></th><td>          <span class="nt">&lt;t&gt;</span>The most obvious comparison between the GPS model and common</td></tr><tr><th id="L172"><a href="#L172">172</a></th><td>          approximations to it is that real world data is not delivered</td></tr><tr><th id="L173"><a href="#L173">173</a></th><td>          uniformly, but in some quantum. The smallest quantum, in a packet</td></tr><tr><th id="L174"><a href="#L174">174</a></th><td>          network, is a packet. But quanta can be larger; for example, in</td></tr><tr><th id="L175"><a href="#L175">175</a></th><td>          video applications it is common to describe data flow in frames per</td></tr><tr><th id="L176"><a href="#L176">176</a></th><td>          second, where a frame describes a picture on a screen or the changes</td></tr><tr><th id="L177"><a href="#L177">177</a></th><td>          made from a previous one. A single video frame is commonly on the</td></tr><tr><th id="L178"><a href="#L178">178</a></th><td>          order of tens of packets. If a codec is delivering thirty frames per</td></tr><tr><th id="L179"><a href="#L179">179</a></th><td>          second, it is conceivable that the packets comprising a frame might</td></tr><tr><th id="L180"><a href="#L180">180</a></th><td>          be sent as thirty bursts per second, with each burst sent at the</td></tr><tr><th id="L181"><a href="#L181">181</a></th><td>          interface rate of the camera or other sender. Similarly, TCP</td></tr><tr><th id="L182"><a href="#L182">182</a></th><td>          exchanges have an initial window, which might be any number of</td></tr><tr><th id="L183"><a href="#L183">183</a></th><td>          packets; common values are 1, 2, 3, 4, and perhaps 10. After that</td></tr><tr><th id="L184"><a href="#L184">184</a></th><td>          initial burst, TCP senders commonly send pairs of packets, but may</td></tr><tr><th id="L185"><a href="#L185">185</a></th><td>          send either smaller or larger bursts, and the rate at which they</td></tr><tr><th id="L186"><a href="#L186">186</a></th><td>          send is governed by the arrival rate of acknowledgements from the</td></tr><tr><th id="L187"><a href="#L187">187</a></th><td>          receiver.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L188"><a href="#L188">188</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L189"><a href="#L189">189</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"gps-flow"</span> <span class="na">title=</span><span class="s">"GPS Comparisons: flow definition"</span><span class="nt">&gt;</span></td></tr><tr><th id="L190"><a href="#L190">190</a></th><td>          <span class="nt">&lt;t&gt;</span>An important engineering trade-off relevant to GPS is the</td></tr><tr><th id="L191"><a href="#L191">191</a></th><td>          definition of a "flow". A flow is, by definition, a defined data</td></tr><tr><th id="L192"><a href="#L192">192</a></th><td>          stream. Common definitions include: <span class="nt">&lt;list</span> <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L193"><a href="#L193">193</a></th><td>              <span class="nt">&lt;t&gt;</span>Packets in a single transport layer session ("microflow"),</td></tr><tr><th id="L194"><a href="#L194">194</a></th><td>              identified by a five-tuple <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"RFC2990"</span><span class="nt">&gt;&lt;/xref&gt;</span>,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L195"><a href="#L195">195</a></th><td>              <span class="nt">&lt;t&gt;</span>Packets between a single pair of addresses, identified by a</td></tr><tr><th id="L196"><a href="#L196">196</a></th><td>              source and destination address or prefix,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L197"><a href="#L197">197</a></th><td>              <span class="nt">&lt;t&gt;</span>Packets from a single source address or prefix <span class="nt">&lt;xref</span></td></tr><tr><th id="L198"><a href="#L198">198</a></th><td>              <span class="na">target=</span><span class="s">"RFC0970"</span><span class="nt">&gt;&lt;/xref&gt;</span>,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L199"><a href="#L199">199</a></th><td>              <span class="nt">&lt;t&gt;</span>Packets to a single destination address or prefix,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L200"><a href="#L200">200</a></th><td>              <span class="nt">&lt;t&gt;</span>Packets to or from a single subscriber, customer, or peer</td></tr><tr><th id="L201"><a href="#L201">201</a></th><td>              <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"RFC6057"</span><span class="nt">&gt;&lt;/xref&gt;</span>. In Service Provider operations,</td></tr><tr><th id="L202"><a href="#L202">202</a></th><td>              this might be a neighboring Autonomous System; in broadband, a</td></tr><tr><th id="L203"><a href="#L203">203</a></th><td>              residential customer.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L204"><a href="#L204">204</a></th><td>            <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L205"><a href="#L205">205</a></th><td>          <span class="nt">&lt;t&gt;</span>The difference should be apparent. Consider a comparison between</td></tr><tr><th id="L206"><a href="#L206">206</a></th><td>          sorting by source address or destination address, to pick two</td></tr><tr><th id="L207"><a href="#L207">207</a></th><td>          examples, in the case that a given router interface has N</td></tr><tr><th id="L208"><a href="#L208">208</a></th><td>          application sessions going through it between N/2 local destinations</td></tr><tr><th id="L209"><a href="#L209">209</a></th><td>          and N remote sources. Sorting by source, or in this case by</td></tr><tr><th id="L210"><a href="#L210">210</a></th><td>          source/destination pair, would give each remote peer an upper bound</td></tr><tr><th id="L211"><a href="#L211">211</a></th><td>          guarantee of 1/N of the available capacity, which might be</td></tr><tr><th id="L212"><a href="#L212">212</a></th><td>          distributed very unevenly among the local destinations. Sorting by</td></tr><tr><th id="L213"><a href="#L213">213</a></th><td>          destination would give each local destination an upper bound</td></tr><tr><th id="L214"><a href="#L214">214</a></th><td>          guarantee of 2/N of the available capacity, which might be</td></tr><tr><th id="L215"><a href="#L215">215</a></th><td>          distributed very unevenly among the remote systems and correlated</td></tr><tr><th id="L216"><a href="#L216">216</a></th><td>          sessions. Who is one fair to? In both cases, they deliver equal</td></tr><tr><th id="L217"><a href="#L217">217</a></th><td>          service by their definition, but that might not be someone else's</td></tr><tr><th id="L218"><a href="#L218">218</a></th><td>          definition.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L219"><a href="#L219">219</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L220"><a href="#L220">220</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"gps-unit"</span></td></tr><tr><th id="L221"><a href="#L221">221</a></th><td>                 <span class="na">title=</span><span class="s">"GPS Comparisons: unit of measurement"</span><span class="nt">&gt;</span></td></tr><tr><th id="L222"><a href="#L222">222</a></th><td>          <span class="nt">&lt;t&gt;</span>And finally, there is the question of what is measured for rate.</td></tr><tr><th id="L223"><a href="#L223">223</a></th><td>          If the sole objective is to force packet streams to not dominate</td></tr><tr><th id="L224"><a href="#L224">224</a></th><td>          each other, it is sufficient to count packets. However, if the issue</td></tr><tr><th id="L225"><a href="#L225">225</a></th><td>          is the bit rate of an SLA, one must consider the sizes of the</td></tr><tr><th id="L226"><a href="#L226">226</a></th><td>          packets (the aggregate throughput of a flow, measured in bits or</td></tr><tr><th id="L227"><a href="#L227">227</a></th><td>          bytes). And if predictable unfairness is a consideration, the value</td></tr><tr><th id="L228"><a href="#L228">228</a></th><td>          must be weighted accordingly.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L229"><a href="#L229">229</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L230"><a href="#L230">230</a></th><td>      <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L231"><a href="#L231">231</a></th><td>      <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"approx"</span> <span class="na">title=</span><span class="s">"GPS Approximations"</span><span class="nt">&gt;</span></td></tr><tr><th id="L232"><a href="#L232">232</a></th><td>        <span class="nt">&lt;t&gt;</span>Carrying the matter further, a queuing algorithm may also be termed</td></tr><tr><th id="L233"><a href="#L233">233</a></th><td>        "Work Conserving" or "Non Work Conserving". A "work conserving"</td></tr><tr><th id="L234"><a href="#L234">234</a></th><td>        algorithm, by definition, is either empty, in which case no attempt is</td></tr><tr><th id="L235"><a href="#L235">235</a></th><td>        being made to dequeue data from it, or contains something, in which</td></tr><tr><th id="L236"><a href="#L236">236</a></th><td>        case it continuously tries to empty the queue. A work conserving queue</td></tr><tr><th id="L237"><a href="#L237">237</a></th><td>        that contains queued data, at an interface with a given rate, will</td></tr><tr><th id="L238"><a href="#L238">238</a></th><td>        deliver data at that rate until it empties. A non-work-conserving</td></tr><tr><th id="L239"><a href="#L239">239</a></th><td>        queue might stop delivering even through it still contains data. A</td></tr><tr><th id="L240"><a href="#L240">240</a></th><td>        common reason for doing this is to impose an artificial upper bound on</td></tr><tr><th id="L241"><a href="#L241">241</a></th><td>        a class of traffic that is lower than the rate of the underlying</td></tr><tr><th id="L242"><a href="#L242">242</a></th><td>        physical interface.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L243"><a href="#L243">243</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"queue-algorithm"</span></td></tr><tr><th id="L244"><a href="#L244">244</a></th><td>                 <span class="na">title=</span><span class="s">"Definition of a queuing algorithm"</span><span class="nt">&gt;</span></td></tr><tr><th id="L245"><a href="#L245">245</a></th><td>          <span class="nt">&lt;t&gt;</span>In the discussion following, we assume a basic definition of a</td></tr><tr><th id="L246"><a href="#L246">246</a></th><td>          queuing algorithm. A queuing algorithm has, at minimum: <span class="nt">&lt;list</span></td></tr><tr><th id="L247"><a href="#L247">247</a></th><td>              <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L248"><a href="#L248">248</a></th><td>              <span class="nt">&lt;t&gt;</span>Some form of internal storage for the elements kept in the</td></tr><tr><th id="L249"><a href="#L249">249</a></th><td>              queue,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L250"><a href="#L250">250</a></th><td>              <span class="nt">&lt;t&gt;</span>If it has multiple internal classifications, <span class="nt">&lt;list</span></td></tr><tr><th id="L251"><a href="#L251">251</a></th><td>                  <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L252"><a href="#L252">252</a></th><td>                  <span class="nt">&lt;t&gt;</span>a method for classifying elements,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L253"><a href="#L253">253</a></th><td>                  <span class="nt">&lt;t&gt;</span>additional storage for the classifier and implied</td></tr><tr><th id="L254"><a href="#L254">254</a></th><td>                  classes,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L255"><a href="#L255">255</a></th><td>                <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L256"><a href="#L256">256</a></th><td>              <span class="nt">&lt;t&gt;</span>a method for creating the queue,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L257"><a href="#L257">257</a></th><td>              <span class="nt">&lt;t&gt;</span>a method for destroying the queue,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L258"><a href="#L258">258</a></th><td>              <span class="nt">&lt;t&gt;</span>a method, called "enqueue", for placing packets into the</td></tr><tr><th id="L259"><a href="#L259">259</a></th><td>              queue or queuing system<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L260"><a href="#L260">260</a></th><td>              <span class="nt">&lt;t&gt;</span>a method, called "dequeue", for removing packets from the</td></tr><tr><th id="L261"><a href="#L261">261</a></th><td>              queue or queuing system<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L262"><a href="#L262">262</a></th><td>            <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L263"><a href="#L263">263</a></th><td>          <span class="nt">&lt;t&gt;</span>There may also be other information or methods, such as the</td></tr><tr><th id="L264"><a href="#L264">264</a></th><td>          ability to inspect the queue. It also often has inspectable external</td></tr><tr><th id="L265"><a href="#L265">265</a></th><td>          attributes, such as the total volume of packets or bytes in queue,</td></tr><tr><th id="L266"><a href="#L266">266</a></th><td>          and may have limit thresholds, such as a maximum number of packets</td></tr><tr><th id="L267"><a href="#L267">267</a></th><td>          or bytes the queue might hold.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L268"><a href="#L268">268</a></th><td>          <span class="nt">&lt;t&gt;</span>For example, a simple FIFO queue has a linear data structure,</td></tr><tr><th id="L269"><a href="#L269">269</a></th><td>          enqueues packets at the tail, and dequeues packets from the head. It</td></tr><tr><th id="L270"><a href="#L270">270</a></th><td>          might have a maximum queue depth and a current queue depth,</td></tr><tr><th id="L271"><a href="#L271">271</a></th><td>          maintained in packets or bytes.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L272"><a href="#L272">272</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L273"><a href="#L273">273</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"DRR"</span> <span class="na">title=</span><span class="s">"Round Robin Models"</span><span class="nt">&gt;</span></td></tr><tr><th id="L274"><a href="#L274">274</a></th><td>          <span class="nt">&lt;t&gt;</span>One class of implementation approaches, generically referred to</td></tr><tr><th id="L275"><a href="#L275">275</a></th><td>          as "Weighted Round Robin", implements the structure of the queue as</td></tr><tr><th id="L276"><a href="#L276">276</a></th><td>          an array or ring of queues associated with flows, for whatever</td></tr><tr><th id="L277"><a href="#L277">277</a></th><td>          definition of a flow is important.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L278"><a href="#L278">278</a></th><td>          <span class="nt">&lt;t&gt;</span>On enqueue, the enqueue function classifies a packet and places</td></tr><tr><th id="L279"><a href="#L279">279</a></th><td>          it into a simple FIFO sub-queue.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L280"><a href="#L280">280</a></th><td>          <span class="nt">&lt;t&gt;</span>On dequeue, the sub-queues are searched in round-robin order, and</td></tr><tr><th id="L281"><a href="#L281">281</a></th><td>          when a sub-queue is identified that contains data, removes a</td></tr><tr><th id="L282"><a href="#L282">282</a></th><td>          specified quantum of data from it.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L283"><a href="#L283">283</a></th><td>          <span class="nt">&lt;t&gt;</span>If a hash is used as a classifier, the modulus of the hash might</td></tr><tr><th id="L284"><a href="#L284">284</a></th><td>          be used as an array index, selecting the sub-queue that the packet</td></tr><tr><th id="L285"><a href="#L285">285</a></th><td>          will go into. One can imagine other classifiers, such as using a</td></tr><tr><th id="L286"><a href="#L286">286</a></th><td>          DSCP value as an index into an array containing the queue number for</td></tr><tr><th id="L287"><a href="#L287">287</a></th><td>          a flow, or more complex access list implementations.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L288"><a href="#L288">288</a></th><td>    <span class="cp">&lt;?rfc needLines="20"?&gt;</span></td></tr><tr><th id="L289"><a href="#L289">289</a></th><td>          <span class="nt">&lt;figure</span> <span class="na">anchor=</span><span class="s">"roundrobin"</span> <span class="na">title=</span><span class="s">"Round Robin Queues"</span><span class="nt">&gt;</span></td></tr><tr><th id="L290"><a href="#L290">290</a></th><td>            <span class="nt">&lt;artwork</span> <span class="na">align=</span><span class="s">"center"</span><span class="nt">&gt;</span><span class="cp">&lt;![CDATA[</span></td></tr><tr><th id="L291"><a href="#L291">291</a></th><td><span class="cp">      +-+</span></td></tr><tr><th id="L292"><a href="#L292">292</a></th><td><span class="cp">    +&gt;|1|     </span></td></tr><tr><th id="L293"><a href="#L293">293</a></th><td><span class="cp">    | +-+</span></td></tr><tr><th id="L294"><a href="#L294">294</a></th><td><span class="cp">    |  |</span></td></tr><tr><th id="L295"><a href="#L295">295</a></th><td><span class="cp">    | +-+               +-+</span></td></tr><tr><th id="L296"><a href="#L296">296</a></th><td><span class="cp">    | |1|             +&gt;|3|  </span></td></tr><tr><th id="L297"><a href="#L297">297</a></th><td><span class="cp">    | +-+             | +-+</span></td></tr><tr><th id="L298"><a href="#L298">298</a></th><td><span class="cp">    |  |              |  |</span></td></tr><tr><th id="L299"><a href="#L299">299</a></th><td><span class="cp">    | +-+      +-+    | +-+</span></td></tr><tr><th id="L300"><a href="#L300">300</a></th><td><span class="cp">    | |1|    +&gt;|2|    | |3|  </span></td></tr><tr><th id="L301"><a href="#L301">301</a></th><td><span class="cp">    | +-+    | +-+    | +-+</span></td></tr><tr><th id="L302"><a href="#L302">302</a></th><td><span class="cp">    |  A     |  A     |  A</span></td></tr><tr><th id="L303"><a href="#L303">303</a></th><td><span class="cp">    |  |     |  |     |  |</span></td></tr><tr><th id="L304"><a href="#L304">304</a></th><td><span class="cp">   ++--++   ++--++   ++--++</span></td></tr><tr><th id="L305"><a href="#L305">305</a></th><td><span class="cp">+-&gt;| Q  |--&gt;| Q  |--&gt;| Q  |--+</span></td></tr><tr><th id="L306"><a href="#L306">306</a></th><td><span class="cp">|  +----+   +----+   +----+  |</span></td></tr><tr><th id="L307"><a href="#L307">307</a></th><td><span class="cp">+----------------------------+</span></td></tr><tr><th id="L308"><a href="#L308">308</a></th><td><span class="cp">]]&gt;</span><span class="nt">&lt;/artwork&gt;</span></td></tr><tr><th id="L309"><a href="#L309">309</a></th><td>          <span class="nt">&lt;/figure&gt;</span></td></tr><tr><th id="L310"><a href="#L310">310</a></th><td>          <span class="nt">&lt;t&gt;</span>In any event, a sub-queue contains the traffic for a flow, and</td></tr><tr><th id="L311"><a href="#L311">311</a></th><td>          data is sent from each sub-queue in succession.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L312"><a href="#L312">312</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L313"><a href="#L313">313</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"WFQ"</span> <span class="na">title=</span><span class="s">"Calendar Queue Models"</span><span class="nt">&gt;</span></td></tr><tr><th id="L314"><a href="#L314">314</a></th><td>          <span class="nt">&lt;t&gt;</span>Another class of implementation approaches, generically referred</td></tr><tr><th id="L315"><a href="#L315">315</a></th><td>          to as "Weighted Fair Queues" or "Calendar Queue Implementations",</td></tr><tr><th id="L316"><a href="#L316">316</a></th><td>          implements the structure of the queue as an array or ring of queues</td></tr><tr><th id="L317"><a href="#L317">317</a></th><td>          associated with time or sequence; each queue contains the set of</td></tr><tr><th id="L318"><a href="#L318">318</a></th><td>          packets, which may be null, intended to be sent at a certain time or</td></tr><tr><th id="L319"><a href="#L319">319</a></th><td>          following the emptying of the previous queue. It has a look-aside</td></tr><tr><th id="L320"><a href="#L320">320</a></th><td>          table that indicates the current depth of any given class of</td></tr><tr><th id="L321"><a href="#L321">321</a></th><td>          traffic, which might similarly be identified using a hash, a DSCP,</td></tr><tr><th id="L322"><a href="#L322">322</a></th><td>          an access list, or any other classifier. Conceptually, the queues</td></tr><tr><th id="L323"><a href="#L323">323</a></th><td>          each contain zero or more packets from each class of traffic. One is</td></tr><tr><th id="L324"><a href="#L324">324</a></th><td>          the queue being emptied "now"; the rest are associated with some</td></tr><tr><th id="L325"><a href="#L325">325</a></th><td>          time or sequence in the future.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L326"><a href="#L326">326</a></th><td>          <span class="nt">&lt;t&gt;</span>On enqueue, the enqueue function classifies a packet and</td></tr><tr><th id="L327"><a href="#L327">327</a></th><td>          determines the current depth of that class, with a view to</td></tr><tr><th id="L328"><a href="#L328">328</a></th><td>          scheduling it for transmission at some time in the future. If the</td></tr><tr><th id="L329"><a href="#L329">329</a></th><td>          unit of scheduling is a packet and the queuing quantum is one packet</td></tr><tr><th id="L330"><a href="#L330">330</a></th><td>          per sub-queue, a burst of packets arrives in a given flow, and at</td></tr><tr><th id="L331"><a href="#L331">331</a></th><td>          the start the flow has no queued data, the first packet goes into</td></tr><tr><th id="L332"><a href="#L332">332</a></th><td>          the "next" queue, the second into its successor, and so on. If the</td></tr><tr><th id="L333"><a href="#L333">333</a></th><td>          unit of scheduling is time, the explanation in <span class="nt">&lt;xref</span></td></tr><tr><th id="L334"><a href="#L334">334</a></th><td>          <span class="na">target=</span><span class="s">"VC"</span><span class="nt">&gt;&lt;/xref&gt;</span> might be simplest to follow. A necessary</td></tr><tr><th id="L335"><a href="#L335">335</a></th><td>          side-effect, memory being finite, is that there is a finite number</td></tr><tr><th id="L336"><a href="#L336">336</a></th><td>          of "future" sub-queues. If enough traffic arrives to cause a</td></tr><tr><th id="L337"><a href="#L337">337</a></th><td>          sub-queue to wrap, one is forced to drop something (tail-drop).<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L338"><a href="#L338">338</a></th><td>          <span class="nt">&lt;t&gt;</span>On dequeue, the sub-queues are searched at their stated times or</td></tr><tr><th id="L339"><a href="#L339">339</a></th><td>          in their stated sequence, and when a sub-queue is identified that</td></tr><tr><th id="L340"><a href="#L340">340</a></th><td>          contains data, removes a specified quantum of data from it.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L341"><a href="#L341">341</a></th><td>          <span class="nt">&lt;figure</span> <span class="na">anchor=</span><span class="s">"wfq"</span> <span class="na">title=</span><span class="s">"Calendar Queue"</span><span class="nt">&gt;</span></td></tr><tr><th id="L342"><a href="#L342">342</a></th><td>    <span class="cp">&lt;?rfc needLines="25"?&gt;</span></td></tr><tr><th id="L343"><a href="#L343">343</a></th><td>            <span class="nt">&lt;artwork</span> <span class="na">align=</span><span class="s">"center"</span><span class="nt">&gt;</span><span class="cp">&lt;![CDATA[</span></td></tr><tr><th id="L344"><a href="#L344">344</a></th><td><span class="cp">           +-+</span></td></tr><tr><th id="L345"><a href="#L345">345</a></th><td><span class="cp">         +&gt;|1|   </span></td></tr><tr><th id="L346"><a href="#L346">346</a></th><td><span class="cp">         | +-+</span></td></tr><tr><th id="L347"><a href="#L347">347</a></th><td><span class="cp">         |  |</span></td></tr><tr><th id="L348"><a href="#L348">348</a></th><td><span class="cp">         | +-+      +-+</span></td></tr><tr><th id="L349"><a href="#L349">349</a></th><td><span class="cp">         | |2|    +&gt;|2|</span></td></tr><tr><th id="L350"><a href="#L350">350</a></th><td><span class="cp">         | +-     +    | +-+</span></td></tr><tr><th id="L351"><a href="#L351">351</a></th><td><span class="cp">         |  |     |  |</span></td></tr><tr><th id="L352"><a href="#L352">352</a></th><td><span class="cp">         | +-+    | +-+      +-+</span></td></tr><tr><th id="L353"><a href="#L353">353</a></th><td><span class="cp">         | |3|    | |1|    +&gt;|1|</span></td></tr><tr><th id="L354"><a href="#L354">354</a></th><td><span class="cp">         | +-+    | +-+    | +-+</span></td></tr><tr><th id="L355"><a href="#L355">355</a></th><td><span class="cp">         |  A     |  A     |  A</span></td></tr><tr><th id="L356"><a href="#L356">356</a></th><td><span class="cp">         |  |     |  |     |  |</span></td></tr><tr><th id="L357"><a href="#L357">357</a></th><td><span class="cp">        ++--++   ++--++   ++--++</span></td></tr><tr><th id="L358"><a href="#L358">358</a></th><td><span class="cp">"now"+-&gt;| Q  |--&gt;| Q  |--&gt;| Q  |--&gt;...</span></td></tr><tr><th id="L359"><a href="#L359">359</a></th><td><span class="cp">        +----+   +----+   +----+</span></td></tr><tr><th id="L360"><a href="#L360">360</a></th><td><span class="cp">           A       A         A</span></td></tr><tr><th id="L361"><a href="#L361">361</a></th><td><span class="cp">           |3      |2        |1</span></td></tr><tr><th id="L362"><a href="#L362">362</a></th><td><span class="cp">        +++++++++++++++++++++++</span></td></tr><tr><th id="L363"><a href="#L363">363</a></th><td><span class="cp">        ||||     Flow      ||||</span></td></tr><tr><th id="L364"><a href="#L364">364</a></th><td><span class="cp">        +++++++++++++++++++++++</span></td></tr><tr><th id="L365"><a href="#L365">365</a></th><td><span class="cp">]]&gt;</span><span class="nt">&lt;/artwork&gt;</span></td></tr><tr><th id="L366"><a href="#L366">366</a></th><td>          <span class="nt">&lt;/figure&gt;</span></td></tr><tr><th id="L367"><a href="#L367">367</a></th><td>          <span class="nt">&lt;t&gt;</span>In any event, a sub-queue contains the traffic for a point in</td></tr><tr><th id="L368"><a href="#L368">368</a></th><td>          time or a point in sequence, and data is sent from each sub-queue in</td></tr><tr><th id="L369"><a href="#L369">369</a></th><td>          succession. If sub-queues are associated with time, an interesting</td></tr><tr><th id="L370"><a href="#L370">370</a></th><td>          end case develops: If the system is draining a given sub-queue, and</td></tr><tr><th id="L371"><a href="#L371">371</a></th><td>          the time of the next sub-queue arrives, what should the system do?</td></tr><tr><th id="L372"><a href="#L372">372</a></th><td>          One potentially valid line of reasoning would have it continue</td></tr><tr><th id="L373"><a href="#L373">373</a></th><td>          delivering the data in the present queue, on the assumption that it</td></tr><tr><th id="L374"><a href="#L374">374</a></th><td>          will likely trade off for time in the next. Another potentially</td></tr><tr><th id="L375"><a href="#L375">375</a></th><td>          valid line of reasoning would have it discard any waiting data in</td></tr><tr><th id="L376"><a href="#L376">376</a></th><td>          the present queue and move to the next.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L377"><a href="#L377">377</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L378"><a href="#L378">378</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"sfq"</span></td></tr><tr><th id="L379"><a href="#L379">379</a></th><td>                 <span class="na">title=</span><span class="s">"Work Conserving Models and Stochastic Fairness Queuing"</span><span class="nt">&gt;</span></td></tr><tr><th id="L380"><a href="#L380">380</a></th><td>          <span class="nt">&lt;t&gt;</span>McKenney's <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"SFQ"</span><span class="nt">&gt;</span>Stochastic Fairness Queuing<span class="nt">&lt;/xref&gt;</span></td></tr><tr><th id="L381"><a href="#L381">381</a></th><td>          is an example of a work conserving algorithm, published in Infocomm</td></tr><tr><th id="L382"><a href="#L382">382</a></th><td>          1990. In this algorithm, McKenney measured packets, and considered a</td></tr><tr><th id="L383"><a href="#L383">383</a></th><td>          "flow" to be an equivalence class of traffic defined by a hashing</td></tr><tr><th id="L384"><a href="#L384">384</a></th><td>          algorithm. As packets arrive, the enqueue function performs the</td></tr><tr><th id="L385"><a href="#L385">385</a></th><td>          indicated hash (in his case, of the address pair), and places the</td></tr><tr><th id="L386"><a href="#L386">386</a></th><td>          packet into the indicated sub-queue. The dequeue function operates</td></tr><tr><th id="L387"><a href="#L387">387</a></th><td>          as described in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"DRR"</span><span class="nt">&gt;&lt;/xref&gt;</span>; sub-queues are inspected</td></tr><tr><th id="L388"><a href="#L388">388</a></th><td>          in round-robin sequence, and if they contain one or more packets, a</td></tr><tr><th id="L389"><a href="#L389">389</a></th><td>          packet is removed.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L390"><a href="#L390">390</a></th><td>          <span class="nt">&lt;t&gt;</span>This could be implemented as described in <span class="nt">&lt;xref</span></td></tr><tr><th id="L391"><a href="#L391">391</a></th><td>          <span class="na">target=</span><span class="s">"WFQ"</span><span class="nt">&gt;&lt;/xref&gt;</span>. The weakness of a WRR approach is the search</td></tr><tr><th id="L392"><a href="#L392">392</a></th><td>          time expended when the queuing system is relatively empty, which the</td></tr><tr><th id="L393"><a href="#L393">393</a></th><td>          calendar queue model obviates.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L394"><a href="#L394">394</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L395"><a href="#L395">395</a></th><td>        <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"VC"</span></td></tr><tr><th id="L396"><a href="#L396">396</a></th><td>                 <span class="na">title=</span><span class="s">"Non Work Conserving Models and Virtual Clock"</span><span class="nt">&gt;</span></td></tr><tr><th id="L397"><a href="#L397">397</a></th><td>          <span class="nt">&lt;t&gt;</span>Zhang's <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"VirtualClock"</span><span class="nt">&gt;</span>Virtual Clock<span class="nt">&lt;/xref&gt;</span> is an</td></tr><tr><th id="L398"><a href="#L398">398</a></th><td>          example of a non-work-conserving algorithm. It is trivially</td></tr><tr><th id="L399"><a href="#L399">399</a></th><td>          implemented as described in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"WFQ"</span><span class="nt">&gt;&lt;/xref&gt;</span>. It</td></tr><tr><th id="L400"><a href="#L400">400</a></th><td>          associates sub-queues with intervals in time, with durations on the</td></tr><tr><th id="L401"><a href="#L401">401</a></th><td>          order of microseconds to tens of milliseconds. Each flow is assigned</td></tr><tr><th id="L402"><a href="#L402">402</a></th><td>          a rate in bytes/interval. The flow entry maintains a point in time</td></tr><tr><th id="L403"><a href="#L403">403</a></th><td>          the "next" packet in the flow should be scheduled.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L404"><a href="#L404">404</a></th><td>          <span class="nt">&lt;t&gt;</span>On enqueue, the method determines whether that time is "in the</td></tr><tr><th id="L405"><a href="#L405">405</a></th><td>          past"; if so, the packet is scheduled "now", and if not, the packet</td></tr><tr><th id="L406"><a href="#L406">406</a></th><td>          is scheduled at that time. It then calculates the time of next</td></tr><tr><th id="L407"><a href="#L407">407</a></th><td>          transmission plus the length of the packet divided by the rate; if</td></tr><tr><th id="L408"><a href="#L408">408</a></th><td>          that time is in the past, the time of next transmission is set to</td></tr><tr><th id="L409"><a href="#L409">409</a></th><td>          "now", and otherwise to the calculated time. As noted in <span class="nt">&lt;xref</span></td></tr><tr><th id="L410"><a href="#L410">410</a></th><td>          <span class="na">target=</span><span class="s">"WFQ"</span><span class="nt">&gt;&lt;/xref&gt;</span>, there is an interesting point regarding "too</td></tr><tr><th id="L411"><a href="#L411">411</a></th><td>          much time in the future"; if a packet is scheduled too far into the</td></tr><tr><th id="L412"><a href="#L412">412</a></th><td>          future, it may be marked or dropped.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L413"><a href="#L413">413</a></th><td>          <span class="nt">&lt;t&gt;</span>On dequeue, the sub-queue associated with the time "now" is</td></tr><tr><th id="L414"><a href="#L414">414</a></th><td>          inspected. If it contains a packet, the packet is dequeued and</td></tr><tr><th id="L415"><a href="#L415">415</a></th><td>          transmitted. If the queue is empty and the time for the next</td></tr><tr><th id="L416"><a href="#L416">416</a></th><td>          sub-queue has not arrived, the system waits, even if there is a</td></tr><tr><th id="L417"><a href="#L417">417</a></th><td>          packet in the next sub-queue. As noted in <span class="nt">&lt;xref</span></td></tr><tr><th id="L418"><a href="#L418">418</a></th><td>          <span class="na">target=</span><span class="s">"WFQ"</span><span class="nt">&gt;&lt;/xref&gt;</span>, there is an interesting point regarding the</td></tr><tr><th id="L419"><a href="#L419">419</a></th><td>          queue associated with "now". If a subsequent sub-queue would be</td></tr><tr><th id="L420"><a href="#L420">420</a></th><td>          delayed by the transmission of a packet, one could imagine marking</td></tr><tr><th id="L421"><a href="#L421">421</a></th><td>          the packet <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"RFC3168"</span><span class="nt">&gt;</span>ECN CE<span class="nt">&lt;/xref&gt;</span> or dropping the</td></tr><tr><th id="L422"><a href="#L422">422</a></th><td>          packet.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L423"><a href="#L423">423</a></th><td>        <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L424"><a href="#L424">424</a></th><td>      <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L425"><a href="#L425">425</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L426"><a href="#L426">426</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"integration"</span> <span class="na">title=</span><span class="s">"Queuing, Marking, and Dropping"</span><span class="nt">&gt;</span></td></tr><tr><th id="L427"><a href="#L427">427</a></th><td>      <span class="nt">&lt;t&gt;</span>Queuing, marking, and dropping are integrated in any system that has</td></tr><tr><th id="L428"><a href="#L428">428</a></th><td>      a queue. If nothing else, as memory is finite, a system has to drop as</td></tr><tr><th id="L429"><a href="#L429">429</a></th><td>      discussed in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"WFQ"</span><span class="nt">&gt;&lt;/xref&gt;</span> and <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"VC"</span><span class="nt">&gt;&lt;/xref&gt;</span> in</td></tr><tr><th id="L430"><a href="#L430">430</a></th><td>      order to protect itself. However, host transports interpret drops as</td></tr><tr><th id="L431"><a href="#L431">431</a></th><td>      signals, so AQM algorithms use that as a mechanism to signal.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L432"><a href="#L432">432</a></th><td>      <span class="nt">&lt;t&gt;</span>It is useful to think of the effects of queuing as a signal as well.</td></tr><tr><th id="L433"><a href="#L433">433</a></th><td>      In TCP, SCTP, and protocols like them, delay experienced by a packet can</td></tr><tr><th id="L434"><a href="#L434">434</a></th><td>      be used to guess the rate available at a given time on a path even</td></tr><tr><th id="L435"><a href="#L435">435</a></th><td>      though the characteristics of the path and competing traffic remain</td></tr><tr><th id="L436"><a href="#L436">436</a></th><td>      unknown <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"PacketPair"</span><span class="nt">&gt;&lt;/xref&gt;</span>. The mathematical side of that</td></tr><tr><th id="L437"><a href="#L437">437</a></th><td>      is that if two packets were sent at the same time, the ratio of the size</td></tr><tr><th id="L438"><a href="#L438">438</a></th><td>      of the second packet divided by the difference in arrival times of the</td></tr><tr><th id="L439"><a href="#L439">439</a></th><td>      two packets cannot exceed the capacity of the link (although it may well</td></tr><tr><th id="L440"><a href="#L440">440</a></th><td>      be lower). From an engineering perspective, the receiver sends</td></tr><tr><th id="L441"><a href="#L441">441</a></th><td>      acknowledgements as data is received, so the arrival of acknowledgements</td></tr><tr><th id="L442"><a href="#L442">442</a></th><td>      at the sender paces the sender at approximately the average rate it is</td></tr><tr><th id="L443"><a href="#L443">443</a></th><td>      able to achieve through the network. This is true even if the sender</td></tr><tr><th id="L444"><a href="#L444">444</a></th><td>      keeps an arbitrarily large amount of data stored in network queues, and</td></tr><tr><th id="L445"><a href="#L445">445</a></th><td>      is the basis for delay-based congestion control algorithms. So, delaying</td></tr><tr><th id="L446"><a href="#L446">446</a></th><td>      a packet momentarily in order to permit another session to improve its</td></tr><tr><th id="L447"><a href="#L447">447</a></th><td>      operation has the effect of signaling a slightly lower capacity to the</td></tr><tr><th id="L448"><a href="#L448">448</a></th><td>      sender.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L449"><a href="#L449">449</a></th><td>      <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"integration-fifo"</span> <span class="na">title=</span><span class="s">"Queuing with Tail Mark/Drop"</span><span class="nt">&gt;</span></td></tr><tr><th id="L450"><a href="#L450">450</a></th><td>        <span class="nt">&lt;t&gt;</span>In the default case, in which a FIFO queue is used with defensive</td></tr><tr><th id="L451"><a href="#L451">451</a></th><td>        tail-drop only, the effect is therefore to signal to the sender in two</td></tr><tr><th id="L452"><a href="#L452">452</a></th><td>        ways: <span class="nt">&lt;list</span> <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L453"><a href="#L453">453</a></th><td>            <span class="nt">&lt;t&gt;</span>Ack Clocking, pacing the sender to send at approximately the</td></tr><tr><th id="L454"><a href="#L454">454</a></th><td>            rate it can deliver data to the receiver, and<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L455"><a href="#L455">455</a></th><td>            <span class="nt">&lt;t&gt;</span>Defensive loss, when a sender sends faster than available</td></tr><tr><th id="L456"><a href="#L456">456</a></th><td>            capacity (such as by probing network capacity when fully utilizing</td></tr><tr><th id="L457"><a href="#L457">457</a></th><td>            that capacity) and overburdens a queue.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L458"><a href="#L458">458</a></th><td>          <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L459"><a href="#L459">459</a></th><td>      <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L460"><a href="#L460">460</a></th><td>      <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"integration-codel"</span> <span class="na">title=</span><span class="s">"Queuing with CoDel Mark/Drop"</span><span class="nt">&gt;</span></td></tr><tr><th id="L461"><a href="#L461">461</a></th><td>        <span class="nt">&lt;t&gt;</span>In any case wherein a queuing algorithm is used along with <span class="nt">&lt;xref</span></td></tr><tr><th id="L462"><a href="#L462">462</a></th><td>        <span class="na">target=</span><span class="s">"I-D.nichols-tsvwg-codel"</span><span class="nt">&gt;</span>CoDel<span class="nt">&lt;/xref&gt;</span>, the sequence of events</td></tr><tr><th id="L463"><a href="#L463">463</a></th><td>        is that a packet is time-stamped, enqueued, dequeued, compared to a</td></tr><tr><th id="L464"><a href="#L464">464</a></th><td>        subsequent reading of the clock, and then acted on, whether by</td></tr><tr><th id="L465"><a href="#L465">465</a></th><td>        dropping it, marking and forwarding it, or simply forwarding it. This</td></tr><tr><th id="L466"><a href="#L466">466</a></th><td>        is to say that the only drop algorithm inherent in queuing is the</td></tr><tr><th id="L467"><a href="#L467">467</a></th><td>        defensive drop when the queue's resources are overrun. However, the</td></tr><tr><th id="L468"><a href="#L468">468</a></th><td>        intention of marking or dropping is to signal to the sender much</td></tr><tr><th id="L469"><a href="#L469">469</a></th><td>        earlier, when a certain amount of delay has been observed, and the</td></tr><tr><th id="L470"><a href="#L470">470</a></th><td>        CoDel algorithm is completely separate from the queuing algorithm.</td></tr><tr><th id="L471"><a href="#L471">471</a></th><td>        Hence, in a FIFO+CoDel, SFQ+CoDel, or Virtual Clock+CoDel</td></tr><tr><th id="L472"><a href="#L472">472</a></th><td>        implementation, the queuing algorithm is completely separate from the</td></tr><tr><th id="L473"><a href="#L473">473</a></th><td>        AQM algorithm. Using them in series results in four signals to the</td></tr><tr><th id="L474"><a href="#L474">474</a></th><td>        sender: <span class="nt">&lt;list</span> <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L475"><a href="#L475">475</a></th><td>            <span class="nt">&lt;t&gt;</span>Ack Clocking, pacing the sender to send at approximately the</td></tr><tr><th id="L476"><a href="#L476">476</a></th><td>            rate it can deliver data to the receiver through a queue that</td></tr><tr><th id="L477"><a href="#L477">477</a></th><td>            intentionally ensures that large data flows do not unnecessarily</td></tr><tr><th id="L478"><a href="#L478">478</a></th><td>            dominate smaller flows,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L479"><a href="#L479">479</a></th><td>            <span class="nt">&lt;t&gt;</span>Lossless signaling that a certain delay threshold has been</td></tr><tr><th id="L480"><a href="#L480">480</a></th><td>            reached, if ECN is in used,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L481"><a href="#L481">481</a></th><td>            <span class="nt">&lt;t&gt;</span>Intentional signaling via loss that a certain delay threshold</td></tr><tr><th id="L482"><a href="#L482">482</a></th><td>            has been reached, if ECN is not in use, and<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L483"><a href="#L483">483</a></th><td>            <span class="nt">&lt;t&gt;</span>Defensive loss, when a sender sends faster than available</td></tr><tr><th id="L484"><a href="#L484">484</a></th><td>            capacity (such as by probing network capacity when fully utilizing</td></tr><tr><th id="L485"><a href="#L485">485</a></th><td>            that capacity) and overburdens a queue.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L486"><a href="#L486">486</a></th><td>          <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L487"><a href="#L487">487</a></th><td>      <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L488"><a href="#L488">488</a></th><td>      <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"integration-pie"</span> <span class="na">title=</span><span class="s">"Queuing with PIE Mark/Drop"</span><span class="nt">&gt;</span></td></tr><tr><th id="L489"><a href="#L489">489</a></th><td>        <span class="nt">&lt;t&gt;</span>In any case wherein a queuing algorithm is used along with <span class="nt">&lt;xref</span></td></tr><tr><th id="L490"><a href="#L490">490</a></th><td>        <span class="na">target=</span><span class="s">"I-D.pan-tsvwg-pie"</span><span class="nt">&gt;</span>PIE<span class="nt">&lt;/xref&gt;</span>, RED, or other such algorithms,</td></tr><tr><th id="L491"><a href="#L491">491</a></th><td>        the sequence of events is that a queue is inspected, a packet is</td></tr><tr><th id="L492"><a href="#L492">492</a></th><td>        dropped, marked, or left unchanged, enqueued, dequeued, compared to a</td></tr><tr><th id="L493"><a href="#L493">493</a></th><td>        subsequent reading of the clock, and then forwarded on. This is to say</td></tr><tr><th id="L494"><a href="#L494">494</a></th><td>        that the AQM Drop Algorithm precedes enqueue; if it has not been</td></tr><tr><th id="L495"><a href="#L495">495</a></th><td>        effective and as a result the queue is out of resources anyway, the</td></tr><tr><th id="L496"><a href="#L496">496</a></th><td>        defensive drop algorithm steps in, and failing that, the queue</td></tr><tr><th id="L497"><a href="#L497">497</a></th><td>        operates in whatever way it does. Hence, in a FIFO+PIE, SFQ+PIE, or</td></tr><tr><th id="L498"><a href="#L498">498</a></th><td>        Virtual Clock+PIE implementation, the queuing algorithm is again</td></tr><tr><th id="L499"><a href="#L499">499</a></th><td>        completely separate from the AQM algorithm. Using them in series</td></tr><tr><th id="L500"><a href="#L500">500</a></th><td>        results in four signals to the sender: <span class="nt">&lt;list</span> <span class="na">style=</span><span class="s">"symbols"</span><span class="nt">&gt;</span></td></tr><tr><th id="L501"><a href="#L501">501</a></th><td>            <span class="nt">&lt;t&gt;</span>Ack Clocking, pacing the sender to send at approximately the</td></tr><tr><th id="L502"><a href="#L502">502</a></th><td>            rate it can deliver data to the receiver through a queue that</td></tr><tr><th id="L503"><a href="#L503">503</a></th><td>            intentionally ensures that large data flows do not unnecessarily</td></tr><tr><th id="L504"><a href="#L504">504</a></th><td>            dominate smaller flows,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L505"><a href="#L505">505</a></th><td>            <span class="nt">&lt;t&gt;</span>Lossless signaling that a queue depth that corresponds to a</td></tr><tr><th id="L506"><a href="#L506">506</a></th><td>            certain delay threshold has been reached, if ECN is in used,<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L507"><a href="#L507">507</a></th><td>            <span class="nt">&lt;t&gt;</span>Intentional signaling via loss that a queue depth that</td></tr><tr><th id="L508"><a href="#L508">508</a></th><td>            corresponds to a certain delay threshold has been reached, if ECN</td></tr><tr><th id="L509"><a href="#L509">509</a></th><td>            is not in use, and<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L510"><a href="#L510">510</a></th><td>            <span class="nt">&lt;t&gt;</span>Defensive loss, when a sender sends faster than available</td></tr><tr><th id="L511"><a href="#L511">511</a></th><td>            capacity (such as by probing network capacity when fully utilizing</td></tr><tr><th id="L512"><a href="#L512">512</a></th><td>            that capacity) and overburdens a queue.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L513"><a href="#L513">513</a></th><td>          <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L514"><a href="#L514">514</a></th><td>      <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L515"><a href="#L515">515</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L516"><a href="#L516">516</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"conclusion"</span> <span class="na">title=</span><span class="s">"Conclusion"</span><span class="nt">&gt;</span></td></tr><tr><th id="L517"><a href="#L517">517</a></th><td>      <span class="nt">&lt;t&gt;</span>To summarize, in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"fq"</span><span class="nt">&gt;&lt;/xref&gt;</span>, implementation approaches</td></tr><tr><th id="L518"><a href="#L518">518</a></th><td>      for several classes of queueing algorithms were explored. Queuing</td></tr><tr><th id="L519"><a href="#L519">519</a></th><td>      algorithms such as SFQ and Virtual Clock have value in the network, in</td></tr><tr><th id="L520"><a href="#L520">520</a></th><td>      that they delay packets to enforce a rate upper bound or to permit</td></tr><tr><th id="L521"><a href="#L521">521</a></th><td>      competing flows to compete more effectively. ECN Marking and loss are</td></tr><tr><th id="L522"><a href="#L522">522</a></th><td>      also useful signals if used in a manner that enhances TCP/SCTP operation</td></tr><tr><th id="L523"><a href="#L523">523</a></th><td>      or restrains unmanaged UDP data flows.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L524"><a href="#L524">524</a></th><td>      <span class="nt">&lt;t&gt;</span>It is, however, incorrect to discuss a queuing and a mark/drop</td></tr><tr><th id="L525"><a href="#L525">525</a></th><td>      algorithm working together as a single algorithm. They operate in</td></tr><tr><th id="L526"><a href="#L526">526</a></th><td>      series, as discussed in <span class="nt">&lt;xref</span> <span class="na">target=</span><span class="s">"integration"</span><span class="nt">&gt;&lt;/xref&gt;</span>.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L527"><a href="#L527">527</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L528"><a href="#L528">528</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"IANA"</span> <span class="na">title=</span><span class="s">"IANA Considerations"</span><span class="nt">&gt;</span></td></tr><tr><th id="L529"><a href="#L529">529</a></th><td>      <span class="nt">&lt;t&gt;</span>This memo asks the IANA for no new parameters.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L530"><a href="#L530">530</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L531"><a href="#L531">531</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"Security"</span> <span class="na">title=</span><span class="s">"Security Considerations"</span><span class="nt">&gt;</span></td></tr><tr><th id="L532"><a href="#L532">532</a></th><td>      <span class="nt">&lt;t&gt;</span>This memo adds no new security issues; it observes on implementation</td></tr><tr><th id="L533"><a href="#L533">533</a></th><td>      strategies for Diffserv implementation.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L534"><a href="#L534">534</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L535"><a href="#L535">535</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"Acknowledgements"</span> <span class="na">title=</span><span class="s">"Acknowledgements"</span><span class="nt">&gt;</span></td></tr><tr><th id="L536"><a href="#L536">536</a></th><td>      <span class="nt">&lt;t&gt;</span>This note grew out of, and is in response to, mailing list</td></tr><tr><th id="L537"><a href="#L537">537</a></th><td>      discussions in AQM, in which some have pushed an algorithm the compare</td></tr><tr><th id="L538"><a href="#L538">538</a></th><td>      to AQM marking and dropping algorithms, but which includes SFQ. The</td></tr><tr><th id="L539"><a href="#L539">539</a></th><td>      authors think highly of queuing algorithms that can ensure certain</td></tr><tr><th id="L540"><a href="#L540">540</a></th><td>      behaviors, but in this context believe that coupling queuing and marking</td></tr><tr><th id="L541"><a href="#L541">541</a></th><td>      or dropping is unwarranted and masks issues with the mark/drop algorithm</td></tr><tr><th id="L542"><a href="#L542">542</a></th><td>      in question.<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L543"><a href="#L543">543</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L544"><a href="#L544">544</a></th><td>  <span class="nt">&lt;/middle&gt;</span></td></tr><tr><th id="L545"><a href="#L545">545</a></th><td>  <span class="nt">&lt;back&gt;</span></td></tr><tr><th id="L546"><a href="#L546">546</a></th><td>    <span class="c">&lt;!-- references split to informative and normative --&gt;</span></td></tr><tr><th id="L547"><a href="#L547">547</a></th><td>    <span class="nt">&lt;references</span> <span class="na">title=</span><span class="s">"Normative References"</span><span class="nt">&gt;</span></td></tr><tr><th id="L548"><a href="#L548">548</a></th><td>      <span class="cp">&lt;?rfc include="reference.RFC.2475" ?&gt;</span></td></tr><tr><th id="L549"><a href="#L549">549</a></th><td>    <span class="nt">&lt;/references&gt;</span></td></tr><tr><th id="L550"><a href="#L550">550</a></th><td>    <span class="nt">&lt;references</span> <span class="na">title=</span><span class="s">"Informative References"</span><span class="nt">&gt;</span></td></tr><tr><th id="L551"><a href="#L551">551</a></th><td>      <span class="cp">&lt;?rfc include="reference.I-D.nichols-tsvwg-codel" ?&gt;</span></td></tr><tr><th id="L552"><a href="#L552">552</a></th><td>      <span class="cp">&lt;?rfc include="reference.I-D.pan-tsvwg-pie" ?&gt;</span></td></tr><tr><th id="L553"><a href="#L553">553</a></th><td>      <span class="cp">&lt;?rfc include="reference.RFC.0970" ?&gt;</span></td></tr><tr><th id="L554"><a href="#L554">554</a></th><td>      <span class="cp">&lt;?rfc include="reference.RFC.2990" ?&gt;</span></td></tr><tr><th id="L555"><a href="#L555">555</a></th><td>      <span class="cp">&lt;?rfc include="reference.RFC.3168"?&gt;</span></td></tr><tr><th id="L556"><a href="#L556">556</a></th><td>      <span class="cp">&lt;?rfc include="reference.RFC.6057" ?&gt;</span></td></tr><tr><th id="L557"><a href="#L557">557</a></th><td>      <span class="nt">&lt;reference</span> <span class="na">anchor=</span><span class="s">"VirtualClock"</span></td></tr><tr><th id="L558"><a href="#L558">558</a></th><td>                 <span class="na">target=</span><span class="s">"http://www.cs.ucla.edu/~lixia/papers/90sigcomm.pdf"</span><span class="nt">&gt;</span></td></tr><tr><th id="L559"><a href="#L559">559</a></th><td>        <span class="nt">&lt;front&gt;</span></td></tr><tr><th id="L560"><a href="#L560">560</a></th><td>          <span class="nt">&lt;title&gt;</span>Virtual Clock<span class="nt">&lt;/title&gt;</span></td></tr><tr><th id="L561"><a href="#L561">561</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Lixia Zhang"</span><span class="nt">&gt;</span></td></tr><tr><th id="L562"><a href="#L562">562</a></th><td>            <span class="nt">&lt;organization&gt;</span>Xerox PARC<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L563"><a href="#L563">563</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L564"><a href="#L564">564</a></th><td>          <span class="nt">&lt;date</span> <span class="na">month=</span><span class="s">"September"</span> <span class="na">year=</span><span class="s">"1990"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L565"><a href="#L565">565</a></th><td>        <span class="nt">&lt;/front&gt;</span></td></tr><tr><th id="L566"><a href="#L566">566</a></th><td>        <span class="nt">&lt;seriesInfo</span> <span class="na">name=</span><span class="s">"ACM SIGCOMM"</span> <span class="na">value=</span><span class="s">"1990"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L567"><a href="#L567">567</a></th><td>      <span class="nt">&lt;/reference&gt;</span></td></tr><tr><th id="L568"><a href="#L568">568</a></th><td>      <span class="nt">&lt;reference</span> <span class="na">anchor=</span><span class="s">"PacketPair"</span></td></tr><tr><th id="L569"><a href="#L569">569</a></th><td>                 <span class="na">target=</span><span class="s">"http://blizzard.cs.uwaterloo.ca/keshav/home/Papers/data/91/ch4.pdf"</span><span class="nt">&gt;</span></td></tr><tr><th id="L570"><a href="#L570">570</a></th><td>        <span class="nt">&lt;front&gt;</span></td></tr><tr><th id="L571"><a href="#L571">571</a></th><td>          <span class="nt">&lt;title&gt;</span>Congestion Control in Computer Networks<span class="nt">&lt;/title&gt;</span></td></tr><tr><th id="L572"><a href="#L572">572</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Keshav"</span><span class="nt">&gt;</span></td></tr><tr><th id="L573"><a href="#L573">573</a></th><td>            <span class="nt">&lt;organization&gt;</span>University of California Berkeley<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L574"><a href="#L574">574</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L575"><a href="#L575">575</a></th><td>          <span class="nt">&lt;date</span> <span class="na">month=</span><span class="s">"September"</span> <span class="na">year=</span><span class="s">"1991"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L576"><a href="#L576">576</a></th><td>        <span class="nt">&lt;/front&gt;</span></td></tr><tr><th id="L577"><a href="#L577">577</a></th><td>        <span class="nt">&lt;seriesInfo</span> <span class="na">name=</span><span class="s">"UC Berkeley TR-654"</span> <span class="na">value=</span><span class="s">"1991"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L578"><a href="#L578">578</a></th><td>      <span class="nt">&lt;/reference&gt;</span></td></tr><tr><th id="L579"><a href="#L579">579</a></th><td>      <span class="nt">&lt;reference</span> <span class="na">anchor=</span><span class="s">"GPS"</span></td></tr><tr><th id="L580"><a href="#L580">580</a></th><td>                 <span class="na">target=</span><span class="s">"http://blizzard.cs.uwaterloo.ca/keshav/home/Papers/data/89/fq.pdf"</span><span class="nt">&gt;</span></td></tr><tr><th id="L581"><a href="#L581">581</a></th><td>        <span class="nt">&lt;front&gt;</span></td></tr><tr><th id="L582"><a href="#L582">582</a></th><td>          <span class="nt">&lt;title&gt;</span>Analysis and simulation of a fair queueing algorithm<span class="nt">&lt;/title&gt;</span></td></tr><tr><th id="L583"><a href="#L583">583</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Demers"</span><span class="nt">&gt;</span></td></tr><tr><th id="L584"><a href="#L584">584</a></th><td>            <span class="nt">&lt;organization&gt;</span>Xerox PARC<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L585"><a href="#L585">585</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L586"><a href="#L586">586</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Keshav"</span><span class="nt">&gt;</span></td></tr><tr><th id="L587"><a href="#L587">587</a></th><td>            <span class="nt">&lt;organization&gt;</span>University of California, Berkeley<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L588"><a href="#L588">588</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L589"><a href="#L589">589</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Scott Shenker"</span><span class="nt">&gt;</span></td></tr><tr><th id="L590"><a href="#L590">590</a></th><td>            <span class="nt">&lt;organization&gt;</span>Xerox PARC<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L591"><a href="#L591">591</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L592"><a href="#L592">592</a></th><td>          <span class="nt">&lt;date</span> <span class="na">month=</span><span class="s">"September"</span> <span class="na">year=</span><span class="s">"1989"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L593"><a href="#L593">593</a></th><td>        <span class="nt">&lt;/front&gt;</span></td></tr><tr><th id="L594"><a href="#L594">594</a></th><td>        <span class="nt">&lt;seriesInfo</span> <span class="na">name=</span><span class="s">"ACM SIGCOMM"</span> <span class="na">value=</span><span class="s">"1989"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L595"><a href="#L595">595</a></th><td>      <span class="nt">&lt;/reference&gt;</span></td></tr><tr><th id="L596"><a href="#L596">596</a></th><td>      <span class="nt">&lt;reference</span> <span class="na">anchor=</span><span class="s">"SFQ"</span></td></tr><tr><th id="L597"><a href="#L597">597</a></th><td>                 <span class="na">target=</span><span class="s">"http://www2.rdrop.com/~paulmck/scalability/paper/sfq.2002.06.04.pdf"</span><span class="nt">&gt;</span></td></tr><tr><th id="L598"><a href="#L598">598</a></th><td>        <span class="nt">&lt;front&gt;</span></td></tr><tr><th id="L599"><a href="#L599">599</a></th><td>          <span class="nt">&lt;title&gt;</span>Stochastic Fairness Queuing<span class="nt">&lt;/title&gt;</span></td></tr><tr><th id="L600"><a href="#L600">600</a></th><td>          <span class="nt">&lt;author</span> <span class="na">fullname=</span><span class="s">"Paul E. Mckenney"</span><span class="nt">&gt;</span></td></tr><tr><th id="L601"><a href="#L601">601</a></th><td>            <span class="nt">&lt;organization&gt;</span>SRI International<span class="nt">&lt;/organization&gt;</span></td></tr><tr><th id="L602"><a href="#L602">602</a></th><td>          <span class="nt">&lt;/author&gt;</span></td></tr><tr><th id="L603"><a href="#L603">603</a></th><td>          <span class="nt">&lt;date</span> <span class="na">month=</span><span class="s">"June"</span> <span class="na">year=</span><span class="s">"1990"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L604"><a href="#L604">604</a></th><td>        <span class="nt">&lt;/front&gt;</span></td></tr><tr><th id="L605"><a href="#L605">605</a></th><td>        <span class="nt">&lt;seriesInfo</span> <span class="na">name=</span><span class="s">"IEEE Infocom"</span> <span class="na">value=</span><span class="s">"1990"</span> <span class="nt">/&gt;</span></td></tr><tr><th id="L606"><a href="#L606">606</a></th><td>      <span class="nt">&lt;/reference&gt;</span></td></tr><tr><th id="L607"><a href="#L607">607</a></th><td>    <span class="nt">&lt;/references&gt;</span></td></tr><tr><th id="L608"><a href="#L608">608</a></th><td>    <span class="nt">&lt;section</span> <span class="na">anchor=</span><span class="s">"log"</span> <span class="na">title=</span><span class="s">"Change Log"</span><span class="nt">&gt;</span></td></tr><tr><th id="L609"><a href="#L609">609</a></th><td>      <span class="nt">&lt;t&gt;&lt;list</span> <span class="na">style=</span><span class="s">"hanging"</span><span class="nt">&gt;</span></td></tr><tr><th id="L610"><a href="#L610">610</a></th><td>          <span class="nt">&lt;t</span> <span class="na">hangText=</span><span class="s">"Initial Version:"</span><span class="nt">&gt;</span>June 2013<span class="nt">&lt;/t&gt;</span></td></tr><tr><th id="L611"><a href="#L611">611</a></th><td>        <span class="nt">&lt;/list&gt;&lt;/t&gt;</span></td></tr><tr><th id="L612"><a href="#L612">612</a></th><td>    <span class="nt">&lt;/section&gt;</span></td></tr><tr><th id="L613"><a href="#L613">613</a></th><td>  <span class="nt">&lt;/back&gt;</span></td></tr><tr><th id="L614"><a href="#L614">614</a></th><td><span class="nt">&lt;/rfc&gt;</span></td></tr></tbody></table>

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