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16<h1><a href="../index.html">BitTorrent<span>.org</span></a></h1>
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29
30
31
32<table class="rfc2822 docutils field-list" frame="void" rules="none">
33<col class="field-name" />
34<col class="field-body" />
35<tbody valign="top">
36<tr class="field"><th class="field-name">BEP:</th><td class="field-body">6</td>
37</tr>
38<tr class="field"><th class="field-name">Title:</th><td class="field-body">Fast Extension</td>
39</tr>
40<tr class="field"><th class="field-name">Version:</th><td class="field-body">10528</td>
41</tr>
42<tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference external" href="https://svn.bittorrent.com/trac.cgi/browser/dotorg/trunk/html/beps/bep_0006.rst">2008-02-04 16:10:30 -0800 (Mon, 04 Feb 2008)</a></td>
43</tr>
44<tr class="field"><th class="field-name">Author:</th><td class="field-body">David Harrison &lt;dave&#32;&#97;t&#32;bittorrent.com&gt;, Bram Cohen &lt;bram&#32;&#97;t&#32;bittorrent.com&gt;</td>
45</tr>
46<tr class="field"><th class="field-name">Status:</th><td class="field-body">Draft</td>
47</tr>
48<tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards Track</td>
49</tr>
50<tr class="field"><th class="field-name">Created:</th><td class="field-body">01-Feb-2008</td>
51</tr>
52<tr class="field"><th class="field-name">Post-History:</th><td class="field-body"></td>
53</tr>
54</tbody>
55</table>
56<hr />
57<div class="contents topic" id="contents">
58<p class="topic-title first">Contents</p>
59<ul class="simple">
60<li><a class="reference internal" href="#modifications-to-semantics-of-existing-messages" id="id6">Modifications to Semantics of Existing Messages</a></li>
61<li><a class="reference internal" href="#have-all-have-none" id="id7">Have All/Have None</a></li>
62<li><a class="reference internal" href="#suggest-piece" id="id8">Suggest Piece</a></li>
63<li><a class="reference internal" href="#reject-request" id="id9">Reject Request</a></li>
64<li><a class="reference internal" href="#allowed-fast" id="id10">Allowed Fast</a></li>
65<li><a class="reference internal" href="#allowed-fast-set-generation" id="id11">Allowed Fast Set Generation</a></li>
66<li><a class="reference internal" href="#example-implementation" id="id12">Example Implementation</a></li>
67<li><a class="reference internal" href="#id1" id="id13">References</a></li>
68</ul>
69</div>
70<p>The Fast Extension packages several extensions: <em>Have None/Have All</em>,
71<em>Reject Requests</em>, <em>Suggestions</em> and <em>Allowed Fast.</em>
72These are enabled by setting the third least significant bit of the
73last reserved byte in the BitTorrent handshake:</p>
74<pre class="literal-block">
75reserved[7] |= 0x04
76</pre>
77<p>The extension is enabled only if both ends of the connection set this bit.</p>
78<p>The following proposed messages adhere to the syntax of messages found
79in v1.0 of the BitTorrent protocol.  All integers are four bytes
80big-endian.  All messages start with an integer message length.  All
81messages but the Keep-Alive follow the message length with a single
82byte opcode and zero or more opcode-dependant arguments.</p>
83<p>The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL
84NOT&quot;, &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,  &quot;MAY&quot;, and
85&quot;OPTIONAL&quot; in this document are to be interpreted as described in
86IETF <a class="reference external" href="http://www.ietf.org/rfc/rfc2119.txt">RFC 2119</a> <a class="footnote-reference" href="#id2" id="id3">[1]</a>.</p>
87<div class="section" id="modifications-to-semantics-of-existing-messages">
88<h1>Modifications to Semantics of Existing Messages</h1>
89<p>The Fast Extension modifies the semantics of the
90<em>Request</em>, <em>Choke</em>, <em>Unchoke</em>, and <em>Cancel</em>
91messages, and adds a <em>Reject Request.</em>  Now, every request
92is guaranteed to result in EXACTLY ONE response
93which is either the corresponding reject or corresponding piece
94message.  Even when a request is cancelled, the peer receiving
95the cancel should respond with either the corresponding reject or
96the corresponding piece: requests that are being processed are
97allowed to complete.</p>
98<p>Choke no longer implicitly rejects all pending requests,
99thus eliminating some race conditions which could cause pieces
100to be needlessly requested multiple times.</p>
101<p>Additionally, if a peer receives a piece that was never requested,
102the peer MUST close the connection.</p>
103</div>
104<div class="section" id="have-all-have-none">
105<h1>Have All/Have None</h1>
106<pre class="literal-block">
107*Have All*: &lt;len=0x0001&gt; &lt;op=0x0E&gt;
108</pre>
109<pre class="literal-block">
110*Have None*: &lt;len=0x0001&gt;&lt;op=0x0F&gt;
111</pre>
112<p><em>Have All</em> and <em>Have None</em> specify that the message sender
113has all or none of the pieces respectively.  When present, <em>Have All</em>
114or <em>Have None</em> replace the <em>Have Bitfield.</em>  Exactly one of <em>Have All</em>,
115<em>Have None</em>, or <em>Have Bitfield</em> MUST appear and only immediately after
116the handshake.  The reason for these messages is to save bandwidth.  Also
117slightly to remove the idiosyncrasy of sending no message when a peer
118has no pieces.</p>
119<p>When the fast extension is disabled, if a peer receives <em>Have All</em> or
120<em>Have None</em> then the peer MUST close the connection.</p>
121</div>
122<div class="section" id="suggest-piece">
123<h1>Suggest Piece</h1>
124<pre class="literal-block">
125*Suggest Piece*: &lt;len=0x0005&gt;&lt;op=0x0D&gt;&lt;index&gt;
126</pre>
127<p><em>Suggest Piece</em> is an advisory message meaning &quot;you might like to
128download this piece.&quot;  The intended usage is for 'super-seeding'
129without throughput reduction, to avoid redundant downloads, and so that
130a seed which is disk I/O bound can upload continguous or identical
131pieces to avoid excessive disk seeks.  In all cases, the seed SHOULD
132operate to maintain a roughly equal number of copies of each piece in
133the network.  A peer MAY send more than one <em>suggest piece</em> message at
134any given time.  A peer receiving multiple <em>suggest piece</em> messages
135MAY interpret this as meaning that all of the suggested pieces
136are equally appropriate.</p>
137<p>When the fast extension is disabled, if a peer receives a
138<em>Suggest Piece</em> message, the peer MUST close the connection.</p>
139</div>
140<div class="section" id="reject-request">
141<h1>Reject Request</h1>
142<pre class="literal-block">
143*Reject Request*: &lt;len=0x000D&gt;&lt;op=0x10&gt;&lt;index&gt;&lt;begin&gt;&lt;offset&gt;
144</pre>
145<p><em>Reject Request</em> notifies a requesting peer that its request will not be satisfied.</p>
146<p>If the fast extension is disabled and a peer receives a reject
147request then the peer MUST close the connection.</p>
148<p>When the fast extension is enabled:</p>
149<ul class="simple">
150<li>If a peer receives a reject for a request that was never sent then
151the peer SHOULD close the connection.</li>
152<li>If a peer sends a choke, it MUST reject all requests from the peer
153to whom the choke was sent except it SHOULD NOT reject requests for
154pieces that are in the <em>allowed fast set.</em> A peer SHOULD choke first
155and then reject requests so that the peer receiving the choke does not
156re-request the pieces.</li>
157<li>If a peer receives a request from a peer its choking, the peer
158receiving the request SHOULD send a reject unless the piece is in the
159<em>allowed fast set.</em></li>
160<li>If a peer receives an excessive number of requests from a peer it is
161choking, the peer receiving the requests MAY close the connection
162rather than reject the request.  However, consider that it can take
163several seconds for buffers to drain and messages to propagate once a
164peer is choked.</li>
165</ul>
166</div>
167<div class="section" id="allowed-fast">
168<h1>Allowed Fast</h1>
169<pre class="literal-block">
170* Allowed Fast: &lt;len=0x0005&gt;&lt;op=0x11&gt;&lt;index&gt;*
171</pre>
172<p>With the BitTorrent protocol specified in <a class="reference external" href="http://www.bittorrent.org/beps/bep_0002.html">BEP 0002</a> <a class="footnote-reference" href="#id4" id="id5">[2]</a>, new peers take
173several minutes to ramp up before they can effectively engage in
174BitTorrent's tit-for-tat. The reason is simple: starting peers have
175few pieces to trade.</p>
176<p><em>Allowed Fast</em> is an advisory message which means &quot;if you ask for this
177piece, I'll give it to you even if you're choked.&quot; <em>Allowed Fast</em> thus
178shortens the awkward stage during which the peer obtains occasional
179optimistic unchokes but cannot sufficiently reciprocate to remain
180unchoked.</p>
181<p>The pieces that can be downloaded when choked constitute a peer's
182<em>allowed fast set.</em> The set is generated using a canonical algorithm
183that produces piece indices unique to the message receiver so that if
184two peers offer <em>k</em> pieces fast it will be the same <em>k</em>, and if one
185offers <em>k+1</em> it will be the same <em>k</em> plus one more. <em>k</em> should be
186small enough to avoid abuse, but large enough to ramp up
187tit-for-tat. We currently set <em>k</em> to 10, but peers are free to change
188this number, e.g., to suit load.</p>
189<p>The message sender MAY list pieces that the message sender does not
190have. The receiver MUST NOT interpret an Allowed Fast message as
191meaning that the message sender has the piece. This allows peers to
192generate and communicate allowed fast sets at the beginning of a
193connection. However, a peer MAY send Allowed Fast messages at any
194time.</p>
195<p>A peer SHOULD send Allowed Fast messages to any starting peer unless
196the local peer lacks sufficient resources. A peer MAY reject requests
197for already Allowed Fast pieces if the local peer lacks sufficient
198resources, if the requested piece has already been sent to the
199requesting peer, or if the requesting peer is not a starting peer. Our
200current implementation rejects requests for Allowed Fast messages
201whenever the requesting peer has more than * k * pieces.</p>
202<blockquote>
203When the fast extension is disabled, if a peer recieves an Allowed
204Fast message then the peer MUST close the connection.</blockquote>
205</div>
206<div class="section" id="allowed-fast-set-generation">
207<h1>Allowed Fast Set Generation</h1>
208<p>The canonical algorithm for computing a peer <em>P'*s *allowed fast set</em>
209follows.  All integers in this pseudocode are four bytes represented
210in network (big-endian) byte order.  <em>[a:b]</em> denotes the sequence of
211consecutive bytes from <em>a</em> to <em>b</em> excluding <em>b</em>, i.e., <em>(a, a+1,
212a+2,..., b-1)</em>. <em>x[a:b]</em> denotes a subsequence of elements in an array
213<em>x</em> starting from index <em>a</em> to but not including index <em>b</em>.</p>
214<p>Let <em>ip</em> denote <em>P'*s IPv4 address.  We currently have no
215provisions for IPv6. If a peer is behind a Network Address Translator
216(NAT) then *ip</em> should be the externally facing IP address of the
217NAT.  Since the node sending the <em>Allowed Fast</em> messages computes
218the set, the correct <em>ip</em> is usually the <em>ip</em> address on the other
219end of the connection.  The host computing the set MAY use the <em>ip</em>
220address on the other end of the connection regardless</p>
221<p>Let <em>sz</em> denote the number of pieces in the torrent.</p>
222<p>Let <em>a</em> denote the allowed fast set.</p>
223<p>Let <em>k</em> denote the final number of pieces in the allowed fast set.</p>
224<pre class="literal-block">
225x = 0xFFFFFF00 &amp; ip                           (1)
226x.append(infohash)                            (2)
227while |a| &lt; k:
228  x = SHA1(x)                                 (3)
229  for i in [0:5] and |a| &lt; k:                 (4)
230    j = i*4                                   (5)
231    y = x[j:j+4]                              (6)
232    index = y % sz                            (7)
233    if index not in a:                        (8)
234      add index to a                          (9)
235</pre>
236<p>Step (1) selects the most significant octets in peer <em>P'*s
237ip address.  We do this to prevent a user that obtains more than one
238IP address on the same network from obtaining more than one
239*allowed fast set.</em>  Use of three bytes is heuristic and
240historical.</p>
241<p>Step (3) generates a 20-byte random number on each call.  By
242performing a SHA-1 hash on the previous iteration's hash, we can
243generate an arbitrarily long pseudorandom sequence.</p>
244<p>Steps (4) through (9) partition the 20-byte hash into piece indices
245and add them to the allowed fast set.</p>
246</div>
247<div class="section" id="example-implementation">
248<h1>Example Implementation</h1>
249<p>The following C++ implementation was provided by CacheLogic:</p>
250<pre class="literal-block">
251void generate_fast_set(
252  uint32 k,     // number of pieces in set
253  uint32 sz,    // number of pieces in torrent
254  const char infohash[20], // infohash of torrent
255  uint32 ip, // in host byte order, ie localhost is 0x7f000001
256  std::
257vector&lt;uint32&gt; &amp;a) // generated set of piece indices
258{
259   a.clear();
260   std::string x;
261   char buf[4];
262   *(uint32*)buf = htonl(ip &amp; 0xffffff00);
263   x.assign(buf, 4);
264   x.append(infohash, 20); // (3)
265   while (a.size()&lt;k) {
266     x = SHA1(x); // (4)
267     for ( int i=0&amp;nbsp;; i&lt;5 &amp;&amp; a.size()&lt;k; i++ ) { // (5)
268       int j = i*4; // (6)
269       uint32 y = ntohl(*(uint32*)(x.data()+j)); // (7)
270       uint32 index = y % sz; // (8)
271       if (std::find(a.begin(), a.end(), index)==a.end()) { // (9)
272         a.push_back(index); // (10)
273       }
274     }
275   }
276}
277</pre>
278<p>Example results generated by this function:</p>
279<pre class="literal-block">
2807 piece allowed fast set for torrent with 1313 pieces and hex infohash
281aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa for node with IP 80.4.4.200:
282  1059,431,808,1217,287,376,1188
2839 piece allowed fast set for torrent with 1313 pieces and hex infohash
284aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa for node with IP 80.4.4.200:
285  1059,431,808,1217,287,376,1188,353,508
286</pre>
287<!-- Local Variables:
288mode: indented-text
289indent-tabs-mode: nil
290sentence-end-double-space: t
291fill-column: 70
292coding: utf-8
293End: -->
294</div>
295<div class="section" id="id1">
296<h1>References</h1>
297<table class="docutils footnote" frame="void" id="id2" rules="none">
298<colgroup><col class="label" /><col /></colgroup>
299<tbody valign="top">
300<tr><td class="label"><a class="fn-backref" href="#id3">[1]</a></td><td><a class="reference external" href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a></td></tr>
301</tbody>
302</table>
303<table class="docutils footnote" frame="void" id="id4" rules="none">
304<colgroup><col class="label" /><col /></colgroup>
305<tbody valign="top">
306<tr><td class="label"><a class="fn-backref" href="#id5">[2]</a></td><td><a class="reference external" href="http://www.bittorrent.org/beps/bep_0002.html">http://www.bittorrent.org/beps/bep_0002.html</a></td></tr>
307</tbody>
308</table>
309</div>
310
311
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