mirror of https://github.com/aria2/aria2
Added technical notes
It contains control file format and dht.dat file format.pull/25/merge
Tatsuhiro Tsujikawa
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@ -13,3 +13,4 @@ Contents:
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aria2c
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README
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technical-notes
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@ -0,0 +1,183 @@
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Technical Notes
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===============
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This document describes additional technical information of aria2. The
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expected audience is developers.
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Control File (*.aria2) Format
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-----------------------------
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The control file uses a binary format to store progress information of
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a download. Here is the diagram for each field::
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+---+-------+-------+-------------------------------------------+
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|VER| EXT |INFO |INFO HASH ... |
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|(2)| (4) |HASH | (INFO HASH LENGTH) |
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| | |LENGTH | |
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| | | (4) | |
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+---+---+---+-------+---+---------------+-------+---------------+
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|PIECE |TOTAL LENGTH |UPLOAD LENGTH |BIT- |BITFIELD ... |
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|LENGTH | (8) | (8) |FIELD | (BITFIELD |
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| (4) | | |LENGTH | LENGTH) |
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| | | | (4) | |
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+-------+-------+-------+-------+-------+-------+---------------+
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|NUM |INDEX |LENGTH |PIECE |PIECE BITFIELD ... |
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|IN- | (4) | (4) |BIT- | (PIECE BITFIELD LENGTH) |
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|FLIGHT | | |FIELD | |
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|PIECE | | |LENGTH | |
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| (4) | | | (4) | |
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+-------+-------+-------+-------+-------------------------------+
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^ ^
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+-------------------------------------------------------+
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Repeated in (NUM IN-FLIGHT) PIECE times
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``VER`` (VERSION): 2 bytes
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Should be either version 0(0x0000) or version 1(0x0001). In
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version 1, all multi-byte integers are saved in network byte
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order(big endian). In version 0, all multi-byte integers are saved
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in host byte order. aria2 1.4.1 can read both versions and only
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writes a control file in version 1 format. version 0 support will
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be disappear in the future version.
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``EXT`` (EXTENSION): 4 bytes
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If LSB is 1(i.e. ``EXT[3]&1 == 1``), aria2 checks whether the saved
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!InfoHash and current downloading one are the same. If they are not
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the same, an exception is thrown. This is called "infoHashCheck"
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extension.
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``INFO HASH LENGTH``: 4 bytes
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The length of InfoHash that is located after this field. If
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"infoHashCheck" extension is enabled, if this value is 0, then an
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exception is thrown. For http/ftp downloads, this value should be
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0.
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``INFO HASH``: ``(INFO HASH LENGTH)`` bytes
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BitTorrent InfoHash.
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``PIECE LENGTH``: 4 bytes
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The length of the piece.
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``TOTAL LENGTH``: 8 bytes
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The total length of the download.
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``UPLOAD LENGTH``: 8 bytes
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The uploaded length in this download.
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``BITFIELD LENGTH``: 4 bytes
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The length of bitfield.
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``BITFIELD``: ``(BITFIELD LENGTH)`` bytes
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This is the bitfield which represents current download progress.
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``NUM IN-FLIGHT PIECE``: 4 bytes
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The number of in-flight pieces. These piece is not marked
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'downloaded' in the bitfield, but it has at least one downloaded
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chunk.
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The following 4 fields are repeated in ``(NUM IN-FLIGHT PIECE)``
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times.
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``INDEX``: 4 bytes
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The index of the piece.
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``LENGTH``: 4 bytes
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The length of the piece.
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``PIECE BITFIELD LENGTH``: 4 bytes
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The length of bitfield of this piece.
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``PIECE BITFIELD``: ``(PIECE BITFIELD LENGTH)`` bytes
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The bitfield of this piece. The each bit represents 16KiB chunk.
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DHT routing table file format
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-----------------------------
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aria2 saves IPv4 DHT routing table in ``${HOME}/.aria2/dht.dat`` and
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IPv6 DHT routing table in ``${HOME}/.aria2/dht6.dat`` by default.
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``dht.dat`` and ``dht6.dat`` files use same binary encoding and have
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following fields. All multi byte integers are in network byte
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order. ``RSV`` (RESERVED) fields are reserved for future use. For now
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they should be all zeros::
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+---+-+---+-----+---------------+---------------+---------------+
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|MGC|F|VER| RSV | MTIME | RSV |LOCAL NODE ID :
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|(2)|M|(2)| (3) | (8) | (8) | (20) :
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| |T| | | | | :
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+---+-+---+-----+-------+-------+-------+-------+---------------+
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:LOCAL NODE ID | RSV | NUM | RSV |
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: (continued) | (4) | NODE | (4) |
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: | | (4) | |
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+-+-------------+-------+-------+-+-----+-------+---------------+
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|P| RSV |COMPACT PEER INFO| RSV | <-+
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|L| (7) | (PLEN) | (24 - PLEN) | |
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|E| | | | |
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|N| | | | |
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+-+-------------+-----------------+-----+-------+---------------+ |
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| NODE ID | RSV | |
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| (20) | (4) | <-----------------+
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+---------------------------------------+-------+ Repeated in
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(NUM NODE) times.
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``MGC`` (MAGIC): 2 bytes
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It must be ``0xa1 0xa2``.
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``FMT`` (FORMAT ID): 1 byte
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The format ID should be ``0x02``.
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``VER`` (VERSION): 2 bytes
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The version number should be ``0x00 0x03``.
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``MTIME``: 8 bytes
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This is the time when aria2 saved the file. The value is the time
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since the Epoch(1970/1/1 00:00:00) in 64 bits integer.
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``LOCALNODE ID``: 20 bytes
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Node ID of the client.
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``NUM NODE``: 4 bytes
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The number of nodes the routing table has. ``NUM NODE`` node
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information follows.
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The data of ``NUM NODE`` node will follow. The node information are
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stored in the following fields. They are repeated in ``NUM NODE``
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times.
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``PLEN`` (COMPACT PEER INFO LENGTH): 1 byte
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The length of compact peer info. For IPv4 DHT, it must be 6. For
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IPv6 DHT, it must be 18.
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``COMPACT PEER INFO``: ``(PLEN)`` bytes
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The address and port of peer in compact peer format.
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``NODE ID``: 20 bytes
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The node ID of this node.
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