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31
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33<table class="rfc2822 docutils field-list" frame="void" rules="none">
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">3</td>
38</tr>
39<tr class="field"><th class="field-name">Title:</th><td class="field-body">The BitTorrent Protocol Specification</td>
40</tr>
41<tr class="field"><th class="field-name">Version:</th><td class="field-body">11031</td>
42</tr>
43<tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference external" href="http://svn.bittorrent.org/trac/browser/dotorg/trunk/html/beps/bep_0003.rst">2008-02-28 16:43:58 -0800 (Thu, 28 Feb 2008)</a></td>
44</tr>
45<tr class="field"><th class="field-name">Author:</th><td class="field-body">Bram Cohen &lt;bram&#32;&#97;t&#32;bittorrent.com&gt;</td>
46</tr>
47<tr class="field"><th class="field-name">Status:</th><td class="field-body">Final</td>
48</tr>
49<tr class="field"><th class="field-name">Type:</th><td class="field-body">Standard</td>
50</tr>
51<tr class="field"><th class="field-name">Created:</th><td class="field-body">10-Jan-2008</td>
52</tr>
53<tr class="field"><th class="field-name">Post-History:</th><td class="field-body"></td>
54</tr>
55</tbody>
56</table>
57<hr />
58<div class="contents topic" id="contents">
59<p class="topic-title first">Contents</p>
60<ul class="simple">
61<li><a class="reference internal" href="#a-bittorrent-file-distribution-consists-of-these-entities" id="id2">A BitTorrent file distribution consists of these entities:</a></li>
62<li><a class="reference internal" href="#to-start-serving-a-host-goes-through-the-following-steps" id="id3">To start serving, a host goes through the following steps:</a></li>
63<li><a class="reference internal" href="#to-start-downloading-a-user-does-the-following" id="id4">To start downloading, a user does the following:</a></li>
64<li><a class="reference internal" href="#the-connectivity-is-as-follows" id="id5">The connectivity is as follows:</a></li>
65<li><a class="reference internal" href="#metainfo-files-are-bencoded-dictionaries-with-the-following-keys" id="id6">Metainfo files are bencoded dictionaries with the following keys:</a></li>
66<li><a class="reference internal" href="#tracker-get-requests-have-the-following-keys" id="id7">Tracker GET requests have the following keys:</a></li>
67<li><a class="reference internal" href="#all-non-keepalive-messages-start-with-a-single-byte-which-gives-their-type" id="id8">All non-keepalive messages start with a single byte which gives their type.</a></li>
68<li><a class="reference internal" href="#the-possible-values-are" id="id9">The possible values are:</a></li>
69<li><a class="reference internal" href="#copyright" id="id10">Copyright</a></li>
70</ul>
71</div>
72<p>BitTorrent is a protocol for distributing files. It identifies content
73by URL and is designed to integrate seamlessly with the web. Its
74advantage over plain HTTP is that when multiple downloads of the same
75file happen concurrently, the downloaders upload to each other, making
76it possible for the file source to support very large numbers of
77downloaders with only a modest increase in its load.</p>
78<div class="section" id="a-bittorrent-file-distribution-consists-of-these-entities">
79<h1>A BitTorrent file distribution consists of these entities:</h1>
80<ul class="simple">
81<li>An ordinary web server</li>
82<li>A static 'metainfo' file</li>
83<li>A BitTorrent tracker</li>
84<li>An 'original' downloader</li>
85<li>The end user web browsers</li>
86<li>The end user downloaders</li>
87</ul>
88<p>There are ideally many end users for a single file.</p>
89</div>
90<div class="section" id="to-start-serving-a-host-goes-through-the-following-steps">
91<h1>To start serving, a host goes through the following steps:</h1>
92<ol class="arabic simple">
93<li>Start running a tracker (or, more likely, have one running already).</li>
94<li>Start running an ordinary web server, such as apache, or have one already.</li>
95<li>Associate the extension .torrent with mimetype application/x-bittorrent on their web server (or have done so already).</li>
96<li>Generate a metainfo (.torrent) file using the complete file to be served and the URL of the tracker.</li>
97<li>Put the metainfo file on the web server.</li>
98<li>Link to the metainfo (.torrent) file from some other web page.</li>
99<li>Start a downloader which already has the complete file (the 'origin').</li>
100</ol>
101</div>
102<div class="section" id="to-start-downloading-a-user-does-the-following">
103<h1>To start downloading, a user does the following:</h1>
104<ol class="arabic simple">
105<li>Install BitTorrent (or have done so already).</li>
106<li>Surf the web.</li>
107<li>Click on a link to a .torrent file.</li>
108<li>Select where to save the file locally, or select a partial download to resume.</li>
109<li>Wait for download to complete.</li>
110<li>Tell downloader to exit (it keeps uploading until this happens).</li>
111</ol>
112</div>
113<div class="section" id="the-connectivity-is-as-follows">
114<h1>The connectivity is as follows:</h1>
115<ul class="simple">
116<li>Strings are length-prefixed base ten followed by a colon and the string. For example <tt class="docutils literal"><span class="pre">4:spam</span></tt> corresponds to 'spam'.</li>
117<li>Integers are represented by an 'i' followed by the number in base 10
118followed by an 'e'. For example <tt class="docutils literal"><span class="pre">i3e</span></tt> corresponds to 3 and
119<tt class="docutils literal"><span class="pre">i-3e</span></tt> corresponds to -3. Integers have no size
120limitation. <tt class="docutils literal"><span class="pre">i-0e</span></tt> is invalid. All encodings with a leading
121zero, such as <tt class="docutils literal"><span class="pre">i03e</span></tt>, are invalid, other than
122<tt class="docutils literal"><span class="pre">i0e</span></tt>, which of course corresponds to 0.</li>
123<li>Lists are encoded as an 'l' followed by their elements (also
124bencoded) followed by an 'e'. For example <tt class="docutils literal"><span class="pre">l4:spam4:eggse</span></tt>
125corresponds to ['spam', 'eggs'].</li>
126<li>Dictionaries are encoded as a 'd' followed by a list of alternating
127keys and their corresponding values followed by an 'e'. For example,
128<tt class="docutils literal"><span class="pre">d3:cow3:moo4:spam4:eggse</span></tt> corresponds to {'cow': 'moo',
129'spam': 'eggs'} and <tt class="docutils literal"><span class="pre">d4:spaml1:a1:bee</span></tt> corresponds to
130{'spam': ['a', 'b']}. Keys must be strings and appear in sorted order
131(sorted as raw strings, not alphanumerics).</li>
132</ul>
133</div>
134<div class="section" id="metainfo-files-are-bencoded-dictionaries-with-the-following-keys">
135<h1>Metainfo files are bencoded dictionaries with the following keys:</h1>
136<dl class="docutils">
137<dt>announce</dt>
138<dd>The URL of the tracker.</dd>
139<dt>info</dt>
140<dd><p class="first">This maps to a dictionary, with keys described below.</p>
141<p>The <tt class="docutils literal"><span class="pre">name</span></tt> key maps to a string which is the suggested name
142to save the file (or directory) as. It is purely advisory.</p>
143<p><tt class="docutils literal"><span class="pre">piece</span> <span class="pre">length</span></tt> maps to the number of bytes in each piece
144the file is split into. For the purposes of transfer, files are
145split into fixed-size pieces which are all the same length except for
146possibly the last one which may be truncated. <tt class="docutils literal"><span class="pre">piece</span>
147<span class="pre">length</span></tt> is almost always a power of two, most commonly 2 18 =
148256 K (BitTorrent prior to version 3.2 uses 2 20 = 1 M as
149default).</p>
150<p><tt class="docutils literal"><span class="pre">pieces</span></tt> maps to a string whose length is a multiple of
15120. It is to be subdivided into strings of length 20, each of which is
152the SHA1 hash of the piece at the corresponding index.</p>
153<p>There is also a key <tt class="docutils literal"><span class="pre">length</span></tt> or a key <tt class="docutils literal"><span class="pre">files</span></tt>,
154but not both or neither. If <tt class="docutils literal"><span class="pre">length</span></tt> is present then the
155download represents a single file, otherwise it represents a set of
156files which go in a directory structure.</p>
157<p>In the single file case, <tt class="docutils literal"><span class="pre">length</span></tt> maps to the length of
158the file in bytes.</p>
159<p>For the purposes of the other keys, the multi-file case is treated as
160only having a single file by concatenating the files in the order they
161appear in the files list. The files list is the value
162<tt class="docutils literal"><span class="pre">files</span></tt> maps to, and is a list of dictionaries containing
163the following keys:</p>
164<p><tt class="docutils literal"><span class="pre">length</span></tt> - The length of the file, in bytes.</p>
165<p><tt class="docutils literal"><span class="pre">path</span></tt> - A list of strings corresponding to subdirectory
166names, the last of which is the actual file name (a zero length list
167is an error case).</p>
168<p class="last">In the single file case, the name key is the name of a file, in the
169muliple file case, it's the name of a directory.</p>
170</dd>
171</dl>
172</div>
173<div class="section" id="tracker-get-requests-have-the-following-keys">
174<h1>Tracker GET requests have the following keys:</h1>
175<dl class="docutils">
176<dt>info_hash</dt>
177<dd>The 20 byte sha1 hash of the bencoded form of the info value from the
178metainfo file. Note that this is a substring of the metainfo
179file. This value will almost certainly have to be escaped.</dd>
180<dt>peer_id</dt>
181<dd>A string of length 20 which this downloader uses as its id. Each
182downloader generates its own id at random at the start of a new
183download. This value will also almost certainly have to be escaped.</dd>
184<dt>ip</dt>
185<dd>An optional parameter giving the IP (or dns name) which this peer is
186at. Generally used for the origin if it's on the same machine as the
187tracker.</dd>
188<dt>port</dt>
189<dd>The port number this peer is listening on. Common behavior is for a
190downloader to try to listen on port 6881 and if that port is taken try
1916882, then 6883, etc. and give up after 6889.</dd>
192<dt>uploaded</dt>
193<dd>The total amount uploaded so far, encoded in base ten ascii.</dd>
194<dt>downloaded</dt>
195<dd>The total amount downloaded so far, encoded in base ten ascii.</dd>
196<dt>left</dt>
197<dd>The number of bytes this peer still has to download, encoded in
198base ten ascii. Note that this can't be computed from downloaded and
199the file length since it might be a resume, and there's a chance that
200some of the downloaded data failed an integrity check and had to be
201re-downloaded.</dd>
202<dt>event</dt>
203<dd>This is an optional key which maps to <tt class="docutils literal"><span class="pre">started</span></tt>,
204<tt class="docutils literal"><span class="pre">completed</span></tt>, or <tt class="docutils literal"><span class="pre">stopped</span></tt> (or
205<tt class="docutils literal"><span class="pre">empty</span></tt>, which is the same as not being present). If not
206present, this is one of the announcements done at regular
207intervals. An announcement using <tt class="docutils literal"><span class="pre">started</span></tt> is sent when a
208download first begins, and one using <tt class="docutils literal"><span class="pre">completed</span></tt> is sent
209when the download is complete. No <tt class="docutils literal"><span class="pre">completed</span></tt> is sent if
210the file was complete when started. Downloaders send an announcement
211using <tt class="docutils literal"><span class="pre">stopped</span></tt> when they cease downloading.</dd>
212</dl>
213<p>Tracker responses are bencoded dictionaries. If a tracker response
214has a key <tt class="docutils literal"><span class="pre">failure</span> <span class="pre">reason</span></tt>, then that maps to a human
215readable string which explains why the query failed, and no other keys
216are required. Otherwise, it must have two keys: <tt class="docutils literal"><span class="pre">interval</span></tt>,
217which maps to the number of seconds the downloader should wait between
218regular rerequests, and <tt class="docutils literal"><span class="pre">peers</span></tt>. <tt class="docutils literal"><span class="pre">peers</span></tt> maps to
219a list of dictionaries corresponding to <tt class="docutils literal"><span class="pre">peers</span></tt>, each of
220which contains the keys <tt class="docutils literal"><span class="pre">peer</span> <span class="pre">id</span></tt>, <tt class="docutils literal"><span class="pre">ip</span></tt>, and
221<tt class="docutils literal"><span class="pre">port</span></tt>, which map to the peer's self-selected ID, IP
222address or dns name as a string, and port number, respectively. Note
223that downloaders may rerequest on nonscheduled times if an event
224happens or they need more peers.</p>
225<p>If you want to make any extensions to metainfo files or tracker
226queries, please coordinate with Bram Cohen to make sure that all
227extensions are done compatibly.</p>
228<p>BitTorrent's peer protocol operates over TCP. It performs efficiently
229without setting any socket options.</p>
230<p>Peer connections are symmetrical. Messages sent in both directions
231look the same, and data can flow in either direction.</p>
232<p>The peer protocol refers to pieces of the file by index as
233described in the metainfo file, starting at zero. When a peer finishes
234downloading a piece and checks that the hash matches, it announces
235that it has that piece to all of its peers.</p>
236<p>Connections contain two bits of state on either end: choked or not,
237and interested or not. Choking is a notification that no data will be
238sent until unchoking happens. The reasoning and common techniques
239behind choking are explained later in this document.</p>
240<p>Data transfer takes place whenever one side is interested and the
241other side is not choking. Interest state must be kept up to date at
242all times - whenever a downloader doesn't have something they
243currently would ask a peer for in unchoked, they must express lack of
244interest, despite being choked. Implementing this properly is tricky,
245but makes it possible for downloaders to know which peers will start
246downloading immediately if unchoked.</p>
247<p>Connections start out choked and not interested.</p>
248<p>When data is being transferred, downloaders should keep several
249piece requests queued up at once in order to get good TCP performance
250(this is called 'pipelining'.) On the other side, requests which can't
251be written out to the TCP buffer immediately should be queued up in
252memory rather than kept in an application-level network buffer, so
253they can all be thrown out when a choke happens.</p>
254<p>The peer wire protocol consists of a handshake followed by a
255never-ending stream of length-prefixed messages. The handshake starts
256with character ninteen (decimal) followed by the string 'BitTorrent
257protocol'. The leading character is a length prefix, put there in the
258hope that other new protocols may do the same and thus be trivially
259distinguishable from each other.</p>
260<p>All later integers sent in the protocol are encoded as four bytes
261big-endian.</p>
262<p>After the fixed headers come eight reserved bytes, which are all
263zero in all current implementations. If you wish to extend the
264protocol using these bytes, please coordinate with Bram Cohen to make
265sure all extensions are done compatibly.</p>
266<p>Next comes the 20 byte sha1 hash of the bencoded form of the info
267value from the metainfo file. (This is the same value which is
268announced as <tt class="docutils literal"><span class="pre">info_hash</span></tt> to the tracker, only here it's raw
269instead of quoted here). If both sides don't send the same value, they
270sever the connection. The one possible exception is if a downloader
271wants to do multiple downloads over a single port, they may wait for
272incoming connections to give a download hash first, and respond with
273the same one if it's in their list.</p>
274<p>After the download hash comes the 20-byte peer id which is reported
275in tracker requests and contained in peer lists in tracker
276responses. If the receiving side's peer id doesn't match the one the
277initiating side expects, it severs the connection.</p>
278<p>That's it for handshaking, next comes an alternating stream of
279length prefixes and messages. Messages of length zero are keepalives,
280and ignored. Keepalives are generally sent once every two minutes, but
281note that timeouts can be done much more quickly when data is
282expected.</p>
283</div>
284<div class="section" id="all-non-keepalive-messages-start-with-a-single-byte-which-gives-their-type">
285<h1>All non-keepalive messages start with a single byte which gives their type.</h1>
286</div>
287<div class="section" id="the-possible-values-are">
288<h1>The possible values are:</h1>
289<ul class="simple">
290<li>0 - choke</li>
291<li>1 - unchoke</li>
292<li>2 - interested</li>
293<li>3 - not interested</li>
294<li>4 - have</li>
295<li>5 - bitfield</li>
296<li>6 - request</li>
297<li>7 - piece</li>
298<li>8 - cancel</li>
299</ul>
300<p>'choke', 'unchoke', 'interested', and 'not interested' have no payload.</p>
301<p>'bitfield' is only ever sent as the first message. Its payload is a
302bitfield with each index that downloader has sent set to one and the
303rest set to zero. Downloaders which don't have anything yet may skip
304the 'bitfield' message. The first byte of the bitfield corresponds to
305indices 0 - 7 from high bit to low bit, respectively. The next one
3068-15, etc. Spare bits at the end are set to zero.</p>
307<p>The 'have' message's payload is a single number, the index which
308that downloader just completed and checked the hash of.</p>
309<p>'request' messages contain an index, begin, and length. The last
310two are byte offsets. Length is generally a power of two unless it
311gets truncated by the end of the file. All current implementations use
3122 15 , and close connections which request an amount greater than 2
31317.</p>
314<p>'cancel' messages have the same payload as request messages. They
315are generally only sent towards the end of a download, during what's
316called 'endgame mode'. When a download is almost complete, there's a
317tendency for the last few pieces to all be downloaded off a single
318hosed modem line, taking a very long time. To make sure the last few
319pieces come in quickly, once requests for all pieces a given
320downloader doesn't have yet are currently pending, it sends requests
321for everything to everyone it's downloading from. To keep this from
322becoming horribly inefficient, it sends cancels to everyone else every
323time a piece arrives.</p>
324<p>'piece' messages contain an index, begin, and piece. Note that they
325are correlated with request messages implicitly. It's possible for an
326unexpected piece to arrive if choke and unchoke messages are sent in
327quick succession and/or transfer is going very slowly.</p>
328<p>Downloaders generally download pieces in random order, which does a
329reasonably good job of keeping them from having a strict subset or
330superset of the pieces of any of their peers.</p>
331<p>Choking is done for several reasons. TCP congestion control behaves
332very poorly when sending over many connections at once. Also, choking
333lets each peer use a tit-for-tat-ish algorithm to ensure that they get
334a consistent download rate.</p>
335<p>The choking algorithm described below is the currently deployed
336one. It is very important that all new algorithms work well both in a
337network consisting entirely of themselves and in a network consisting
338mostly of this one.</p>
339<p>There are several criteria a good choking algorithm should meet. It
340should cap the number of simultaneous uploads for good TCP
341performance. It should avoid choking and unchoking quickly, known as
342'fibrillation'. It should reciprocate to peers who let it
343download. Finally, it should try out unused connections once in a
344while to find out if they might be better than the currently used
345ones, known as optimistic unchoking.</p>
346<p>The currently deployed choking algorithm avoids fibrillation by
347only changing who's choked once every ten seconds. It does
348reciprocation and number of uploads capping by unchoking the four
349peers which it has the best download rates from and are
350interested. Peers which have a better upload rate but aren't
351interested get unchoked and if they become interested the worst
352uploader gets choked. If a downloader has a complete file, it uses its
353upload rate rather than its download rate to decide who to
354unchoke.</p>
355<p>For optimistic unchoking, at any one time there is a single peer
356which is unchoked regardless of it's upload rate (if interested, it
357counts as one of the four allowed downloaders.) Which peer is
358optimistically unchoked rotates every 30 seconds. To give them a
359decent chance of getting a complete piece to upload, new connections
360are three times as likely to start as the current optimistic unchoke
361as anywhere else in the rotation.</p>
362</div>
363<div class="section" id="copyright">
364<h1>Copyright</h1>
365<p>This document has been placed in the public domain.</p>
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