root/dotorg/trunk/html/beps/bep_0003.rst @ 11162

BEP: 3
Title: The BitTorrent Protocol Specification
Version: $Revision$
Last-Modified: $Date$
Author:  Bram Cohen <bram@bittorrent.com>
Status:  Final
Type:    Standard
Created: 10-Jan-2008
Post-History: 24-Jun-2009, clarified the encoding of strings in torrent files

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.

A BitTorrent file distribution consists of these entities:
----------------------------------------------------------

- An ordinary web server
- A static 'metainfo' file
- A BitTorrent tracker
- An 'original' downloader
- The end user web browsers
- The end user downloaders

There are ideally many end users for a single file.

To start serving, a host goes through the following steps:
----------------------------------------------------------

#. Start running a tracker (or, more likely, have one running already).
#. Start running an ordinary web server, such as apache, or have one already.
#. Associate the extension .torrent with mimetype application/x-bittorrent on their web server (or have done so already).
#. Generate a metainfo (.torrent) file using the complete file to be served and the URL of the tracker.
#. Put the metainfo file on the web server.
#. Link to the metainfo (.torrent) file from some other web page.
#. Start a downloader which already has the complete file (the 'origin').

To start downloading, a user does the following:
------------------------------------------------

#. Install BitTorrent (or have done so already).
#. Surf the web.
#. Click on a link to a .torrent file.
#. Select where to save the file locally, or select a partial download to resume.
#. Wait for download to complete.
#. Tell downloader to exit (it keeps uploading until this happens).

The connectivity is as follows:
-------------------------------

- Strings are length-prefixed base ten followed by a colon and the string. For example ``4:spam`` corresponds to 'spam'.

- 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.

- 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'].

- 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).


Metainfo files are bencoded dictionaries with the following keys:
-----------------------------------------------------------------

announce
  The URL of the tracker.

info
  This maps to a dictionary, with keys described below.

  The ``name`` key maps to a UTF-8 encoded string which is the
  suggested name to save the file (or directory) as. It is purely advisory.

  ``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 18 =
  256 K (BitTorrent prior to version 3.2 uses 2 20 = 1 M as
  default).

  ``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.

  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.

  In the single file case, ``length`` maps to the length of
  the file in bytes.

  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:
  
  ``length`` - The length of the file, in bytes.

  ``path`` - A list of UTF-8 encoded strings corresponding to subdirectory
  names, the last of which is the actual file name (a zero length list
  is an error case).

  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.

  All strings in a .torrent file that contains text must be UTF-8
  encoded.


Tracker GET requests have the following keys:
---------------------------------------------

info_hash
  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.

peer_id
  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.

ip
  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.

port
  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.

uploaded
  The total amount uploaded so far, encoded in base ten ascii.

downloaded
  The total amount downloaded so far, encoded in base ten ascii.

left
  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.

event
  This is an optional key which maps to ``started``,
  ``completed``, or ``stopped`` (or
  ``empty``, 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.

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.

If you want to make any extensions to metainfo files or tracker
queries, please coordinate with Bram Cohen to make sure that all
extensions are done compatibly.

BitTorrent's peer protocol operates over TCP. It performs efficiently
without setting any socket options.

Peer connections are symmetrical. Messages sent in both directions
look the same, and data can flow in either direction.

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.

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.

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.

Connections start out choked and not interested.

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.

The peer wire protocol consists of a handshake followed by a
never-ending stream of length-prefixed messages. The handshake starts
with character ninteen (decimal) 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.

All later integers sent in the protocol are encoded as four bytes
big-endian.

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 Bram Cohen to make
sure all extensions are done compatibly.

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.

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.

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.

All non-keepalive messages start with a single byte which gives their type.
---------------------------------------------------------------------------
The possible values are:
------------------------

- 0 - choke
- 1 - unchoke
- 2 - interested
- 3 - not interested
- 4 - have
- 5 - bitfield
- 6 - request
- 7 - piece
- 8 - cancel

'choke', 'unchoke', 'interested', and 'not interested' have no payload.

'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.

The 'have' message's payload is a single number, the index which
that downloader just completed and checked the hash of.

'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 15 , and close connections which request an amount greater than 2
17.

'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.

'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.

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.

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.

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.

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.

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.


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.

Copyright
---------

This document has been placed in the public domain.


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