211 lines
9.7 KiB
HTML
211 lines
9.7 KiB
HTML
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<div class="document" id="libtorrent-hacking">
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<div id="headerNav">
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<li class="first"><a href="/">Home</a></li>
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</div>
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<div id="main">
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<h1 class="title">libtorrent hacking</h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid@rasterbar.com">arvid@rasterbar.com</a></td></tr>
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<tr><th class="docinfo-name">Version:</th>
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<td>1.0.0</td></tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first">Table of contents</p>
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<ul class="simple">
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<li><a class="reference internal" href="#terminology" id="id1">terminology</a></li>
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<li><a class="reference internal" href="#structure" id="id2">structure</a><ul>
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<li><a class="reference internal" href="#session-impl" id="id3">session_impl</a></li>
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<li><a class="reference internal" href="#session" id="id4">session</a></li>
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<li><a class="reference internal" href="#torrent-handle" id="id5">torrent_handle</a></li>
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<li><a class="reference internal" href="#torrent" id="id6">torrent</a></li>
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<li><a class="reference internal" href="#peer-connection" id="id7">peer_connection</a></li>
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<li><a class="reference internal" href="#policy" id="id8">policy</a></li>
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<li><a class="reference internal" href="#piece-picker" id="id9">piece_picker</a></li>
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<li><a class="reference internal" href="#torrent-info" id="id10">torrent_info</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#threads" id="id11">threads</a></li>
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</ul>
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</div>
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<p>This describe some of the internals of libtorrent. If you're looking for
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something to contribute, please take a look at the <a class="reference external" href="todo.html">todo list</a>.</p>
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<div class="section" id="terminology">
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<h1>terminology</h1>
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<p>This section describes some of the terminology used throughout the
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libtorrent source. Having a good understanding of some of these keywords
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helps understanding what's going on.</p>
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<p>A <em>piece</em> is a part of the data of a torrent that has a SHA-1 hash in
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the .torrent file. Pieces are almost always a power of two in size, but not
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necessarily. Each piece is plit up in <em>blocks</em>, which is a 16 kiB. A block
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never spans two pieces. If a piece is smaller than 16 kiB or not divisible
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by 16 kiB, there are blocks smaller than that.</p>
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<p>16 kiB is a de-facto standard of the largest transfer unit in the bittorrent
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protocol. Clients typically reject any request for larger pieces than this.</p>
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<p>The <em>piece picker</em> is the part of a bittorrent client that is responsible for
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the logic to determine which requests to send to peers. It doesn't actually
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pick full pieces, but blocks (from pieces).</p>
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<p>The file layout of a torrent is represented by <em>file storage</em> objects. This
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class contains a list of all files in the torrent (in a well defined order),
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the size of the pieces and implicitly the total size of the whole torrent and
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number of pieces. The file storage determines the mapping from <em>pieces</em>
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to <em>files</em>. This representation may be quite complex in order to keep it extremely
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compact. This is useful to load very large torrents without exploding in memory
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usage.</p>
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<p>A <em>torrent</em> object represents all the state of swarm download. This includes
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a piece picker, a list of peer connections, file storage (torrent file). One
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important distiction is between a connected peer (<em>peer_connection</em>) and a peer
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we just know about, and may have been connected to, and may connect to in the
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future (<em>policy::peer</em>). The list of (not connected) peers may grow very large
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if not limited (through tracker responses, DHT and peer exchange). This list
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is typically limited to a few thousand peers.</p>
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<p>The <em>policy</em> in libtorrent is somewhat poorly named. It was initially intended
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to be a customization point where a client could define peer selection behavior
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and unchoke logic. It didn't end up being though, and a more accurate name would
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be peer_list. It really just maintains a potentially large list of known peers
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for a swarm (not necessarily connected).</p>
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</div>
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<div class="section" id="structure">
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<h1>structure</h1>
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<p>This is the high level structure of libtorrent. Bold types are part of the public
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interface:</p>
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<pre class="literal-block">
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+=========+ pimpl +-------------------+
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| <strong>session</strong> | ---------> | aux::session_impl |
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+=========+ +-------------------+
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m_torrents[] | |
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+================+ | |
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| <strong>torrent_handle</strong> | ------+ | |
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+================+ | | |
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| | | m_connections[]
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| | |
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| | +---------------------+
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m_picker v v |
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+--------------+ +---------+---------+-- . . |
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| piece_picker | <--+-| torrent | torrent | to |
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+--------------+ | +---------+---------+-- . . |
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m_torrent_file | | m_connections[] |
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+==============+ | | |
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| <strong>torrent_info</strong> | <--+ v v
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+==============+ | +-----------------+-----------------+-- . .
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m_policy | | peer_connection | peer_connection | pe
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+--------+ | +-----------------+-----------------+-- . .
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| policy | <--------+ | | m_socket
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+--------+ | |
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| m_peers[] | v
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| | +-----------------------+
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| | | socket_type (variant) |
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v | +-----------------------+
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+--------------+ |
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| policy::peer | |
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+--------------+ |
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| policy::peer | |
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+--------------+ m_peer_info|
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| policy::peer | <----------+
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+--------------+
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. .
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+ - - - - - - -+
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</pre>
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<div class="section" id="session-impl">
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<h2>session_impl</h2>
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<p>This is the session state object, containing all session global information, such as:</p>
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<blockquote>
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<ul class="simple">
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<li>the list of all torrents <tt class="docutils literal">m_torrent</tt>.</li>
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<li>the list of all peer connections <tt class="docutils literal">m_connections</tt>.</li>
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<li>the global rate limits <tt class="docutils literal">m_settings</tt>.</li>
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<li>the DHT state <tt class="docutils literal">m_dht</tt>.</li>
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<li>the port mapping state, <tt class="docutils literal">m_upnp</tt> and <tt class="docutils literal">m_natpmp</tt>.</li>
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</ul>
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</blockquote>
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</div>
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<div class="section" id="session">
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<h2>session</h2>
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<p>This is the public interface to the session. It implements pimpl (pointer to implementation)
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in order to hide the internal representation of the <tt class="docutils literal">session_impl</tt> object from the user and
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make binary compatibility simpler to maintain.</p>
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</div>
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<div class="section" id="torrent-handle">
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<h2>torrent_handle</h2>
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<p>This is the public interface to a <tt class="docutils literal">torrent</tt>. It holds a weak reference to the internal
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<tt class="docutils literal">torrent</tt> object and manipulates it by sending messages to the network thread.</p>
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</div>
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<div class="section" id="torrent">
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<h2>torrent</h2>
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</div>
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<div class="section" id="peer-connection">
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<h2>peer_connection</h2>
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</div>
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<div class="section" id="policy">
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<h2>policy</h2>
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</div>
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<div class="section" id="piece-picker">
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<h2>piece_picker</h2>
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</div>
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<div class="section" id="torrent-info">
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<h2>torrent_info</h2>
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</div>
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</div>
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<div class="section" id="threads">
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<h1>threads</h1>
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<p>libtorrent starts 2 or 3 threads.</p>
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<blockquote>
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<ul class="simple">
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<li>The first thread is the main thread that will sit
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idle in a <tt class="docutils literal">kqueue()</tt> or <tt class="docutils literal">epoll</tt> call most of the time.
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This thread runs the main loop that will send and receive
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data on all connections.</li>
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<li>The second thread is the disk I/O thread. All disk read and write operations
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are passed to this thread and messages are passed back to the main thread when
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the operation completes. The disk thread also verifies the piece hashes.</li>
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<li>The third and forth threads are spawned by asio on systems that don't support
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non-blocking host name resolution to simulate non-blocking getaddrinfo().</li>
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</ul>
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</blockquote>
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</div>
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</div>
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</body>
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</html>
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