<html><head><title>BitTorrent Protocol Documentation</title></head><body><center><table border="1" cellpadding="10"><tbody><tr>
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</tr></tbody></table></center><p> BitTorrent is a protocol for distributing
files. It identifies content by url and is designed to integrate seamlessly
with the web. Its advantage over plain http is that when multiple downloads
of the same file happen concurrently, the downloaders upload to each other,
making it possible for the file source to support very large numbers of downloaders
with only a modest increase in its load.</p><p>
The life cycle of a BitTorrent file distribution.</p><p>
A BitTorrent file distribution consists of these entities -</p><p>
</p><ul>
<li>An ordinary web server
</li><li>A static 'metainfo' file
</li><li>A BitTorrent tracker
</li><li>An 'origin' downloader
</li><li>The end user web browsers
</li><li>The end user downloaders
</li></ul><p>
There are ideally many end users for a single file.</p><p>
To start serving, a host goes through the following steps -</p><p>
</p><ol>
<li>Start running a tracker (or, more likely, have one running already).
</li><li>Start running an ordinary web server, such as apache, or have one already.
</li><li>Associate the extension .torrent with mimetype application/x-bittorrent on their web server (or have done so already).
</li><li>Generate a metainfo file using the complete file to be served and the url of the tracker.
</li><li>Put the metainfo file on the web server.
</li><li>Link to the metainfo file from some other web page.
</li><li>Start a downloader which already has the complete file (the 'origin').
</li></ol><p>
To start downloading, a user does the following -</p><p>
</p><ol>
<li>Run a BitTorrent installer (or have done so already).
</li><li>Web surf.
</li><li>Click on a link to a .torrent file.
</li><li>Select where to save the file locally, or select a partial download to resume.
</li><li>Wait for download to complete.
</li><li>Tell downloader to exit (it keeps uploading until this happens).
</li></ol><p>
The connectivity is as follows -</p><p>
</p><ul>
<li>The web site is serving up static files as normal, but kicking off the BitTorrent helper app on the clients.
</li><li>The tracker is receiving information from all downloaders and giving
them random lists of peers. This is done over http or https. </li><li>Downloaders are periodically checking in with the tracker to keep
it informed of their progress, and are uploading to and downloading from
each other via direct connections. These connections use the BitTorrent peer
protocol, which operates over TCP. </li><li>The origin is uploading but not downloading at all, since it has
the entire file. The origin is necessary to get the entire file into the
network. Often for popular downloads the origin can be taken down after a
while since several downloads may have completed and been left running indefinitely.
</li></ul><p> Metainfo file and tracker responses are both sent in a simple,
efficient, and extensible format called bencoding (pronounced 'bee encoding').
Bencoded messages are nested dictionaries and lists, which can contain strings
and integers. Extensibility is supported by ignoring unexpected dictionary
keys, so additional optional ones can be added later.</p><p>
Bencoding is done as follows -</p><p>
</p><ul>
<li>Strings are length-prefixed base ten followed by a colon and the string. For example '4:spam' corresponds to 'spam'.
</li><li>Integers are represented by an 'i' followed by the number in base
10 followed by an 'e'. For example 'i3e' corresponds to 3 and 'i-3e' corresponds
to -3. Integers have no size limitation. 'i-0e' is invalid. All encodings
with a leading zero, such as 'i03e', are invalid, other than 'i0e', which
of course corresponds to 0. </li><li>Lists are encoded as an 'l' followed by their elements (also bencoded)
followed by an 'e'. For example 'l4:spam4:eggse' corresponds to ['spam',
'eggs']. </li><li>Dictionaries are encoded as a 'd' followed by a list of alternating
keys and their corresponding values followed by an 'e'. For example, 'd3:cow3:moo4:spam4:eggse'
corresponds to {'cow': 'moo', 'spam': 'eggs'} and 'd4:spaml1:a1:bee' corresponds
to {'spam': ['a', 'b']} . Keys must be strings and appear in sorted order
(sorted as raw strings, not alphanumerics). </li></ul><p>
Metainfo files are bencoded dictionaries with the following keys -</p><p>
</p><dl><dt>'announce'</dt><dd>
The url of the tracker.<p>
</p></dd><dt>'info'</dt><dd>
This maps to a dictionary, with keys described below.<p>
The 'name' key maps to a string which is the suggested name to save the file (or directory) as. It is purely advisory. </p><p>
'piece length' maps to the number of bytes in each piece the file is split
into. For the purposes of transfer, files are split into fixed-size pieces
which are all the same length except for possibly the last one which may
be truncated. Piece length is almost always a power of two, most commonly
2<sup>20</sup>. </p><p>
'pieces' maps to a string whose length is a multiple of 20. It is to be
subdivided into strings of length 20, each of which is the sha1 hash of the
piece at the corresponding index.</p><p>
There is also a key 'length' or a key 'files', but not both or neither.
If 'length' is present then the download represents a single file, otherwise
it represents a set of files which go in a directory structure.</p><p>
In the single file case, 'length' maps to the length of the file in bytes.</p><p>
For the purposes of the other keys, the multi-file case is treated as only
having a single file by concatenating the files in the order they appear
in the files list. The files list is the value 'files' maps to, and is a
list of dictionaries containing the following keys -</p><p>
</p></dd><dl><dt>'length'
</dt><dd>The length of the file, in bytes.
</dd><dt>'path'
</dt><dd>A list of strings corresponding to subdirectory names, the last
of which is the actual file name (a zero length list is an error case). </dd></dl><p>
In the single file case, the 'name' key is the name of a file, in the muliple file case, it's the name of a directory.</p><p>
</p></dl> Tracker queries are two way. The tracker receives information
via GET parameters and returns a bencoded message. Note that although the
current tracker implementation has its own web server, the tracker could
run very nicely as, for example, an apache module.<p>
Tracker GET requests have the following keys -</p><p>
</p><dl><dt>'info_hash'</dt><dd> The 20 byte sha1 hash of the bencoded form
of the 'info' value from the metainfo file. Note that this is a substring
of the metainfo file. This value will almost certainly have to be escaped.<p>
</p></dd><dt>'peer_id'</dt><dd> A string of length 20 which this downloader
uses as its id. Each downloader generates its own id at random at the start
of a new download. This value will also almost certainly have to be escaped.<p>
</p></dd><dt>'ip'</dt><dd> An optional parameter giving the ip (or dns name)
which this peer is at. Generally used for the origin if it's on the same
machine as the tracker.<p>
</p></dd><dt>'port'</dt><dd> The port number this peer is listening on. Common
behavior is for a downloader to try to listen on port 6881 and if that port
is taken try 6882, then 6883, etc. and give up after 6889.<p>
</p></dd><dt>'uploaded'</dt><dd>
The total amount uploaded so far, encoded in base ten ascii.<p>
</p></dd><dt>'downloaded'</dt><dd>
The total amount downloaded so far, encoded in base ten ascii.<p>
</p></dd><dt>'left'</dt><dd> The number of bytes this peer still has to download,
encoded in base ten ascii. Note that this can't be computed from downloaded
and the file length since it might be a resume, and there's a chance that
some of the downloaded data failed an integrity check and had to be re-downloaded.<p>
</p></dd><dt>'event'</dt><dd> This is an optional key which maps to 'started',
'completed', or 'stopped' (or '', which is the same as not being present).
If not present, this is one of the announcements done at regular intervals.
An announcement using 'started' is sent when a download first begins, and
one using 'completed' is sent when the download is complete. No 'completed'
is sent if the file was complete when started. Downloaders send an announcement
using 'stopped' when they cease downloading.<p>
</p></dd></dl> Tracker responses are bencoded dictionaries. If a tracker
response has a key 'failure reason', then that maps to a human readable string
which explains why the query failed, and no other keys are required. Otherwise,
it must have two keys - 'interval', which maps to the number of seconds the
downloader should wait between regular rerequests, and 'peers'. 'peers' maps
to a list of dictionaries corresponding to peers, each of which contains
the keys 'peer id', 'ip', and 'port', which map to the peer's self-selected
id, ip address or dns name as a string, and port number, respectively. Note
that downloaders may rerequest on nonscheduled times if an event happens
or they need more peers.<p>
If you want to make any extensions to metainfo files or tracker queries, please coordinate with <a href="mailto:bram@bitconjurer.org">Bram Cohen</a> to make sure that all extensions are done compatibly.</p><p>
BitTorrent's peer protocol operates over TCP. It performs efficiently without setting any socket options.</p><p>
Peer connections are symmetrical. Messages sent in both directions look the same, and data can flow in either direction.</p><p>
The peer protocol refers to pieces of the file by index as described in
the metainfo file, starting at zero. When a peer finishes downloading a piece
and checks that the hash matches, it announces that it has that piece to
all of its peers.</p><p>
Connections contain two bits of state on either end - choked or not, and
interested or not. Choking is a notification that no data will be sent until
unchoking happens. The reasoning and common techniques behind choking are
explained later in this document.</p><p>
Data transfer takes place whenever one side is interested and the other
side is not choking. Interest state must be kept up to date at all times
- whenever a downloader doesn't have something they currently would ask a
peer for in unchoked, they must express lack of interest, despite being choked.
Implementing this properly is tricky, but makes it possible for downloaders
to know which peers will start downloading immediately if unchoked.</p><p>
Connections start out choked and not interested.</p><p>
When data is being transferred, downloaders should keep several piece requests
queued up at once in order to get good TCP performance (this is called 'pipelining'.)
On the other side, requests which can't be written out to the TCP buffer
immediately should be queued up in memory rather than kept in an application-level
network buffer, so they can all be thrown out when a choke happens.</p><p>
The peer wire protocol consists of a handshake followed by a never-ending
stream of length-prefixed messages. The handshake starts with character ninteen
followed by the string 'BitTorrent protocol'. The leading character is a
length prefix, put there in the hope that other new protocols may do the
same and thus be trivially distinguishable from each other.</p><p>
All later integers sent in the protocol are encoded as four bytes big-endian.</p><p>
After the fixed headers come eight reserved bytes, which are all zero in
all current implementations. If you wish to extend the protocol using these
bytes, please coordinate with <a href="mailto:bram@bitconjurer.org">Bram Cohen</a> to make sure all extensions are done compatibly.</p><p>
Next comes the 20 byte sha1 hash of the bencoded form of the 'info' value
from the metainfo file. (This is the same value which is announced as info_hash
to the tracker, only here it's raw instead of quoted here). If both sides
don't send the same value, they sever the connection. The one possible exception
is if a downloader wants to do multiple downloads over a single port, they
may wait for incoming connections to give a download hash first, and respond
with the same one if it's in their list.</p><p>
After the download hash comes the 20-byte peer id which is reported in tracker
requests and contained in peer lists in tracker responses. If the receiving
side's peer id doesn't match the one the initiating side expects, it severs
the connection.</p><p>
That's it for handshaking, next comes an alternating stream of length prefixes
and messages. Messages of length zero are keepalives, and ignored. Keepalives
are generally sent once every two minutes, but note that timeouts can be
done much more quickly when data is expected.</p><p>
All non-keepalive messages start with a single byte which gives their type. The possible values are -</p><p>
</p><ul>
<li>0 - choke
</li><li>1 - unchoke
</li><li>2 - interested
</li><li>3 - not interested
</li><li>4 - have
</li><li>5 - bitfield
</li><li>6 - request
</li><li>7 - piece
</li><li>8 - cancel
</li></ul><p>
Choke, unchoke, interested, and not interested have no payload.</p><p>
Bitfield is only ever sent as the first message. Its payload is a bitfield
with each index that downloader has sent set to one and the rest set to zero.
Downloaders which don't have anything yet may skip the bitfield message.
The first byte of the bitfield corresponds to indices 0-7 from high bit to
low bit, respectively. The next one 8-15, etc. Spare bits at the end are
set to zero.</p><p>
The have message's payload is a single number, the index which that downloader just completed and checked the hash of.</p><p>
Request messages contain an index, begin, and length. The last two are byte
offsets. Length is generally a power of two unless it gets truncated by the
end of the file. All current implementations use 2<sup>15</sup>, and close connections which request an amount greater than 2<sup>17</sup>.</p><p>
Cancel messages have the same payload as request messages. They are generally
only sent towards the end of a download, during what's called 'endgame mode'.
When a download is almost complete, there's a tendency for the last few pieces
to all be downloaded off a single hosed modem line, taking a very long time.
To make sure the last few pieces come in quickly, once requests for all pieces
a given downloader doesn't have yet are currently pending, it sends requests
for everything to everyone it's downloading from. To keep this from becoming
horribly inefficient, it sends cancels to everyone else every time a piece
arrives.</p><p>
Piece messages contain an index, begin, and piece. Note that they are correlated
with request messages implicitly. It's possible for an unexpected piece to
arrive if choke and unchoke messages are sent in quick succession and/or
transfer is going very slowly.</p><p>
Downloaders generally download pieces in random order, which does a reasonably
good job of keeping them from having a strict subset or superset of the pieces
of any of their peers.</p><p>
Choking is done for several reasons. TCP congestion control behaves very
poorly when sending over many connections at once. Also, choking lets each
peer use a tit-for-tat-ish algorithm to ensure that they get a consistent
download rate.</p><p>
The choking algorithm described below is the currently deployed one. It
is very important that all new algorithms work well both in a network consisting
entirely of themselves and in a network consisting mostly of this one.</p><p>
There are several criteria a good choking algorithm should meet. It should
cap the number of simultaneous uploads for good TCP performance. It should
avoid choking and unchoking quickly, known as fibrillation. It should reciprocate
to peers who let it download. Finally, it should try out unused connections
once in a while to find out if they might be better than the currently used
ones, known as optimistic unchoking.</p><p>
The currently deployed choking algorithm avoids fibrillation by only changing
who's choked once every ten seconds. It does reciprocation and number of
uploads capping by unchoking the four peers which it has the best download
rates from and are interested. Peers which have a better upload rate but
aren't interested get unchoked and if they become interested the worst uploader
gets choked. If a downloader has a complete file, it uses its upload rate
rather than its download rate to decide who to unchoke.</p><p>
For optimistic unchoking, at any one time there is a single peer which is
unchoked regardless of it's upload rate (if interested, it counts as one
of the four allowed downloaders.) Which peer is optimistically unchoked rotates
every 30 seconds. To give them a decent chance of getting a complete piece
to upload, new connections are three times as likely to start as the current
optimistic unchoke as anywhere else in the rotation.</p><p></p></body></html>
