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<meta name="author" content="Arvid Norberg, arvid@rasterbar.com Ludvig Strigeus, ludde@utorrent.com" />
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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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<tr><th class="docinfo-name">Author:</th>
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<td>Arvid Norberg, <a class="reference" href="mailto:arvid@rasterbar.com">arvid@rasterbar.com</a>
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Ludvig Strigeus, <a class="last reference" href="mailto:ludde@utorrent.com">ludde@utorrent.com</a></td></tr>
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<h1><a id="extension-protocol-for-bittorrent" name="extension-protocol-for-bittorrent">extension protocol for bittorrent</a></h1>
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<p>The intention of this protocol is to provide a simple and thin transport
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for extensions to the bittorrent protocol. Supporting this protocol makes
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it easy to add new extensions without interfering with the standard
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bittorrent protocol or clients that don't support this extension or the
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one you want to add.</p>
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<p>To advertise to other clients that you support, one bit from the reserved
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<p>The bit selected for the extension protocol is bit 20 from the right (counting
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starts at 0). So (reserved_byte[5] & 0x10) is the expression to use for checking
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if the client supports extended messaging.</p>
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<p>Once support for the protocol is established, the client is supposed to
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support 1 new message:</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr><th class="head">name</th>
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<th class="head">id</th>
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<tr><td><tt class="docutils literal"><span class="pre">extended</span></tt></td>
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<p>This message is sent as any other bittorrent message, with a 4 byte length
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prefix and a single byte identifying the message (the single byte being 20
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in this case). At the start of the payload of the message, is a single byte
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message identifier. This identifier can refer to different extension messages
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and only one ID is specified, 0. If the ID is 0, the message is a handshake
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message which is described below. The layout of a general <tt class="docutils literal"><span class="pre">extended</span></tt> message
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follows (including the message headers used by the bittorrent protocol):</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr><th class="head">size</th>
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<th class="head">description</th>
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<td>length prefix. Specifies the number of bytes for the
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entire message. (Big endian)</td>
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<td>bittorrent message ID, = 20</td>
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<td>extended message ID. 0 = handshake, >0 = extended
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message as specified by the handshake.</td>
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<h2><a id="handshake-message" name="handshake-message">handshake message</a></h2>
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<p>The payload of the handshake message is a bencoded dictionary. All items
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in the dictionary are optional. Any unknown names should be ignored
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by the client. All parts of the dictionary are case sensitive.
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This is the defined item in the dictionary:</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr><th class="head">name</th>
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<th class="head">description</th>
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<td><p class="first">Dictionary of supported extension messages which maps
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names of extensions to an extended message ID for each
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extension message. The only requirement on these IDs
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is that no extension message share the same one. Setting
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an extension number to zero means that the extension is
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not supported/disabled. The client should ignore any
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extension names it doesn't recognize.</p>
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<p class="last">The extension message IDs are the IDs used to send the
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extension messages to the peer sending this handshake.
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i.e. The IDs are local to this particular peer.</p>
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<p>Here are some other items that an implementation may choose to support:</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr><th class="head">name</th>
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<th class="head">description</th>
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<td>Local TCP listen port. Allows each side to learn about
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the TCP port number of the other side. Note that there is
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no need for the receiving side of the connection to send
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this extension message, since its port number is already
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<td>Client name and version (as a utf-8 string).
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This is a much more reliable way of identifying the
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client than relying on the peer id encoding.</td>
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<td>A string containing the compact representation of the ip
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address this peer sees you as. i.e. this is the
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receiver's external ip address (no port is included).
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This may be either an IPv4 (4 bytes) or an IPv6
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(16 bytes) address.</td>
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<td>If this peer has an IPv6 interface, this is the compact
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representation of that address (16 bytes). The client may
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prefer to connect back via the IPv6 address.</td>
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<td>If this peer has an IPv4 interface, this is the compact
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representation of that address (4 bytes). The client may
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prefer to connect back via this interface.</td>
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<td>An integer, the number of outstanding request messages
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this client supports without dropping any. The default in
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in libtorrent is 250.</td>
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<p>The handshake dictionary could also include extended handshake
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information, such as support for encrypted headers or anything
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<p>An example of what the payload of a handshake message could look like:</p>
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<table border="1" class="docutils">
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<thead valign="bottom">
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<tr><th class="head" colspan="2">Dictionary</th>
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<tr><td><tt class="docutils literal"><span class="pre">m</span></tt></td>
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<td><table border="1" class="first last docutils">
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<thead valign="bottom">
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<tr><th class="head" colspan="2">Dictionary</th>
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<tr><td><tt class="docutils literal"><span class="pre">LT_metadata</span></tt></td>
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<tr><td><tt class="docutils literal"><span class="pre">ut_pex</span></tt></td>
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<tr><td><tt class="docutils literal"><span class="pre">p</span></tt></td>
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<tr><td><tt class="docutils literal"><span class="pre">v</span></tt></td>
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<td>"µTorrent 1.2"</td>
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<p>and in the encoded form:</p>
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<p><tt class="docutils literal"><span class="pre">d1:md11:LT_metadatai1e6:µT_PEXi2ee1:pi6881e1:v13:\xc2\xb5Torrent</span> <span class="pre">1.2e</span></tt></p>
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<p>To make sure the extension names do not collide by mistake, they should be
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prefixed with the two (or one) character code that is used to identify the
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client that introduced the extension. This applies for both the names of
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extension messages, and for any additional information put inside the
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top-level dictionary. All one and two byte identifiers are invalid to use
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unless defined by this specification.</p>
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<p>This message should be sent immediately after the standard bittorrent handshake
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to any peer that supports this extension protocol. It is valid to send the
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handshake message more than once during the lifetime of a connection,
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the sending client should not be disconnected. An implementation may choose
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to ignore the subsequent handshake messages (or parts of them).</p>
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<p>Subsequent handshake messages can be used to enable/disable extensions
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without restarting the connection. If a peer supports changing extensions
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at run time, it should note that the <tt class="docutils literal"><span class="pre">m</span></tt> dictionary is additive.
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It's enough that it contains the actual <em>CHANGES</em> to the extension list.
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To disable the support for <tt class="docutils literal"><span class="pre">LT_metadata</span></tt> at run-time, without affecting
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any other extensions, this message should be sent:
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<tt class="docutils literal"><span class="pre">d11:LT_metadatai0ee</span></tt>.
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As specified above, the value 0 is used to turn off an extension.</p>
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<p>The extension IDs must be stored for every peer, becuase every peer may have
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different IDs for the same extension.</p>
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<p>This specification, deliberately, does not specify any extensions such as
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peer-exchange or metadata exchange. This protocol is merely a transport
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for the actual extensions to the bittorrent protocol and the extensions
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named in the example above (such as <tt class="docutils literal"><span class="pre">p</span></tt>) are just examples of possible
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<div class="section">
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<h2><a id="rationale" name="rationale">rationale</a></h2>
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<p>The reason why the extension messages' IDs would be defined in the handshake
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is to avoid having a global registry of message IDs. Instead the names of the
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extension messages requires unique names, which is much easier to do without
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a global registry. The convention is to use a two letter prefix on the
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extension message names, the prefix would identify the client first
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implementing the extension message. e.g. <tt class="docutils literal"><span class="pre">LT_metadata</span></tt> is implemented by
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libtorrent, and hence it has the <tt class="docutils literal"><span class="pre">LT</span></tt> prefix.</p>
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<p>If the client supporting the extensions can decide which numbers the messages
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it receives will have, it means they are constants within that client. i.e.
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they can be used in <tt class="docutils literal"><span class="pre">switch</span></tt> statements. It's easy for the other end to
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store an array with the ID's we expect for each message and use that for
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lookups each time it sends an extension message.</p>
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<p>The reason for having a dictionary instead of having an array (using
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implicitly assigned index numbers to the extensions) is that if a client
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want to disable some extensions, the ID numbers would change, and it wouldn't
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be able to use constants (and hence, not use them in a <tt class="docutils literal"><span class="pre">switch</span></tt>). If the
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messages IDs would map directly to bittorrent message IDs, It would also make
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it possible to map extensions in the handshake to existing extensions with
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fixed message IDs.</p>
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<p>The reasoning behind having a single byte as extended message identifier is
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to follow the the bittorrent spec. with its single byte message identifiers.
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It is also considered to be enough. It won't limit the total number of
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extensions, only the number of extensions used simultaneously.</p>
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<p>The reason for using single byte identifiers for the standardized handshake
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identifiers is 1) The mainline DHT uses single byte identifiers. 2) Saves
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bandwidth. The only advantage of longer messages is that it makes the
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protocol more readable for a human, but the BT protocol wasn't designed to
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be a human readable protocol, so why bother.</p>
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