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34<col class="field-name" />
35<col class="field-body" />
36<tbody valign="top">
37<tr class="field"><th class="field-name">BEP:</th><td class="field-body">23</td>
38</tr>
39<tr class="field"><th class="field-name">Title:</th><td class="field-body">Tracker Returns Compact Peer Lists</td>
40</tr>
41<tr class="field"><th class="field-name">Version:</th><td class="field-body">11117</td>
42</tr>
43<tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference external" href="http://bittorrent.org/trac/browser/dotorg/trunk/html/beps/bep_0023.rst">2008-08-05 18:54:24 -0700 (Tue, 05 Aug 2008)</a></td>
44</tr>
45<tr class="field"><th class="field-name">Author:</th><td class="field-body">David Harrison &lt;dave&#32;&#97;t&#32;bittorrent.com&gt;</td>
46</tr>
47<tr class="field"><th class="field-name">Status:</th><td class="field-body">Accepted</td>
48</tr>
49<tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards track</td>
50</tr>
51<tr class="field"><th class="field-name">Content-Type:</th><td class="field-body"><a class="reference external" href="http://www.bittorrent.org/beps/bep-0012">text/x-rst</a></td>
52</tr>
53<tr class="field"><th class="field-name">Created:</th><td class="field-body">12-May-2008</td>
54</tr>
55<tr class="field"><th class="field-name">Post-History:</th><td class="field-body"></td>
56</tr>
57</tbody>
58</table>
59<hr />
60<div class="contents topic" id="contents">
61<p class="topic-title first">Contents</p>
62<ul class="simple">
63<li><a class="reference internal" href="#abstract" id="id5">Abstract</a></li>
64<li><a class="reference internal" href="#overview" id="id6">Overview</a></li>
65<li><a class="reference internal" href="#references" id="id7">References</a></li>
66<li><a class="reference internal" href="#copyright" id="id8">Copyright</a></li>
67</ul>
68</div>
69<div class="section" id="abstract">
70<h1>Abstract</h1>
71<p>To reduce the size of tracker responses and to reduce memory and
72computational requirements in trackers, trackers may return
73peers as a packed string rather than as a bencoded list.</p>
74<p>The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL
75NOT&quot;, &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,  &quot;MAY&quot;, and
76&quot;OPTIONAL&quot; in this document are to be interpreted as described in
77IETF <a class="reference external" href="http://www.faqs.org/rfcs/rfc2119.html">RFC 2119</a> <a class="footnote-reference" href="#rfc-2119" id="id1">[3]</a>.</p>
78</div>
79<div class="section" id="overview">
80<h1>Overview</h1>
81<p>In BitTorrent as described in BEP 3 <a class="footnote-reference" href="#bep-3" id="id2">[1]</a>, peers wishing to
82transfer a file contact a central tracker.  This tracker returns a
83list of peers that are currently transferring the file.  The list of
84peers is implemented as a list of bencoded dicts.  Each dict in the
85list contains three fields: <em>peer id</em>, <em>ip</em>, and <em>port</em>.  The <em>peer
86id</em> is 20 bytes plus 3 bytes bencoding overhead.  The <em>ip</em> is a string
87containing a domain name or an IP address, and an integer port number.
88The <em>ip</em> is variable length, but since in its longest form it is a
89domain name it cannot exceed 255 bytes <a class="footnote-reference" href="#rfc-1034" id="id3">[2]</a> plus 4 bytes
90bencoding overhead.  Bencoded integers are also variable length but
91since it is a port number, it cannot be more than 7 bytes including
92bencoding overhead.  Thus,</p>
93<pre class="literal-block">
94total peer list length in bytes &lt; n * ( 23 + 259 + 7 )
95</pre>
96<p>It is common now to use a compact format where each peer is represented
97using only 6 bytes.  The first 4 bytes contain the 32-bit ipv4 address.
98The remaining two bytes contain the port number.  Both address and port
99use network-byte order.</p>
100<p>When the client's announce URL contains <em>compact=1</em> the tracker MAY
101return the peers using compact format.  When <em>compact=0</em> or it is not
102specified then the tracker MUST return using the original format
103described in <a class="footnote-reference" href="#bep-3" id="id4">[1]</a>.  For example,</p>
104<pre class="literal-block">
105GET /announce?peer_id=aaaaaaaaaaaaaaaaaaaa&amp;info_hash=aaaaaaaaaaaaaaaaaaaa
106&amp;port=6881&amp;left=0&amp;downloaded=100&amp;uploaded=0&amp;compact=1
107</pre>
108<p>The compact format uses the same <em>peers</em> key in the bencoded tracker
109response, but the value is a bencoded string rather than a bencoded
110list.</p>
111<p>The peer id does not appear in the compact format.  The format has been
112in use for years and the lack of a peer id has posed no problems.</p>
113<p>This compact format is supported by BitTorrent mainline, Azureus,
114libtorrent, uTorrent, and probably most other clients.</p>
115</div>
116<div class="section" id="references">
117<h1>References</h1>
118<table class="docutils footnote" frame="void" id="bep-3" rules="none">
119<colgroup><col class="label" /><col /></colgroup>
120<tbody valign="top">
121<tr><td class="label">[1]</td><td><em>(<a class="fn-backref" href="#id2">1</a>, <a class="fn-backref" href="#id4">2</a>)</em> BEP_0003. The BitTorrent Protocol Specification. Cohen.
122(<a class="reference external" href="http://www.bittorrent.org/beps/bep_0003.html">http://www.bittorrent.org/beps/bep_0003.html</a>)</td></tr>
123</tbody>
124</table>
125<table class="docutils footnote" frame="void" id="rfc-1034" rules="none">
126<colgroup><col class="label" /><col /></colgroup>
127<tbody valign="top">
128<tr><td class="label"><a class="fn-backref" href="#id3">[2]</a></td><td><a class="reference external" href="http://www.faqs.org/rfcs/rfc1034.html">RFC-1034</a>. DOMAIN NAMES - CONCEPTS AND FACILITIES. Mockapetris,
129November 1987. (<a class="reference external" href="http://tools.ietf.org/html/rfc1034">http://tools.ietf.org/html/rfc1034</a>)</td></tr>
130</tbody>
131</table>
132<table class="docutils footnote" frame="void" id="rfc-2119" rules="none">
133<colgroup><col class="label" /><col /></colgroup>
134<tbody valign="top">
135<tr><td class="label"><a class="fn-backref" href="#id1">[3]</a></td><td><a class="reference external" href="http://www.faqs.org/rfcs/rfc2119.html">RFC-2119</a>. (<a class="reference external" href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>)</td></tr>
136</tbody>
137</table>
138</div>
139<div class="section" id="copyright">
140<h1>Copyright</h1>
141<p>This document has been placed in the public domain.</p>
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