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69 lines
2.4 KiB
69 lines
2.4 KiB
11 years ago
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:Author: Arvid Norberg, arvid@rasterbar.com
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Mainline DHT extensions
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=======================
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libtorrent implements a few extensions to the Mainline DHT protocol.
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get_peers response
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------------------
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libtorrent always responds with ``nodes`` to a get_peers request. If it has
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peers for the specified info-hash, it will return ``values`` as well. This is
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because just because some peer announced to us, doesn't mean that we are
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among the 8 closest nodes of the info hash. libtorrent also keeps traversing
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nodes using get_peers until it has found the 8 closest ones, and then announces
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to those nodes.
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forward compatibility
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---------------------
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In order to support future DHT messages, any message which is not recognized
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but has either an ``info_hash`` or ``target`` argument is interpreted as
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find node for that target. i.e. it returns nodes. This allows future messages
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to be properly forwarded by clients that don't understand them instead of
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being blocked.
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client identification
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---------------------
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In each DHT packet, an extra key is inserted named "v". This is a string
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describing the client and version used. This can help alot when debugging
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and finding errors in client implementations. The string is encoded as four
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characters, two characters describing the client and two characters interpreted
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as a binary number describing the client version.
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Currently known clients:
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+---------------+--------+
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| uTorrent | ``UT`` |
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+---------------+--------+
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| libtorrent | ``LT`` |
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+---------------+--------+
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| MooPolice | ``MP`` |
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+---------------+--------+
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| GetRight | ``GR`` |
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+---------------+--------+
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IPv6 support
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------------
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**This extension is superseeded by** `BEP 32`_.
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.. _`BEP 32`: http://bittorrent.org/beps/bep_0032.html
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The DHT messages that don't support IPv6 are the ``nodes`` replies.
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They encode all the contacts as 6 bytes packed together in sequence in a
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string. The problem is that IPv6 endpoints cannot be encoded as 6 bytes, but
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needs 18 bytes. The extension libtorrent applies is to add another key, called
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``nodes2``.
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``nodes2`` may be present in replies that contains a ``nodes`` key. It is encoded
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as a list of strings. Each string represents one contact and is encoded as 20
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bytes node-id and then a variable length encoded IP address (6 bytes in IPv4 case
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and 18 bytes in IPv6 case).
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As an optimization, libtorrent does not include the extra key in case there are
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only IPv4 nodes present.
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