libbt2/docs/protocol.txt

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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.

The life cycle of a BitTorrent file distribution.

A BitTorrent file distribution consists of these entities -

    * An ordinary web server
    * A static 'metainfo' file
    * A BitTorrent tracker
    * An 'origin' 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 -

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

To start downloading, a user does the following -

   1. Run a BitTorrent installer (or have done so already).
   2. Web surf.
   3. Click on a link to a .torrent file.
   4. Select where to save the file locally, or select a partial
      download to resume.
   5. Wait for download to complete.
   6. Tell downloader to exit (it keeps uploading until this happens). 

The connectivity is as follows -

    * The web site is serving up static files as normal, but kicking off
      the BitTorrent helper app on the clients.
    * The tracker is receiving information from all downloaders and
      giving them random lists of peers. This is done over http or https.
    * 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.
    * 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.

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.

Bencoding is done 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 -
*
*    md={
*      announce=>'url',
*      info=>{
*          name=>'top-level-file-or-directory-name',
*          piece length=>12345,
*          pieces=>'md5sums',
*
*          length=>12345,
*            -or-
*          files=>[
*              {
*                  length=>12345,
*                  path=>['sub','directory','path','and','filename']
*
*              }, ... {}
*          ]
*      }


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 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 2^20 .

    '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 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.

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.


* Tracker GET requests have the following keys urlencoded -
*   req = {
*      info_hash => 'hash'
*      peer_id => 'random-20-character-name'
*      ip => 'ip-address' -or- 'dns-name'
*      port => '12345'
*      uploaded => '12345'
*      downloaded => '12345'
*      left => '12345'
*      event => 'started', 'completed' -or- 'stopped'
*   }

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 '', 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.
*
*  resp = {
*     failure reason => 'error text'
*       - or -
*     interval => 12345
*     peers => {
*         peer id => 'identifier'
*         ip => 'ip-address' -or- 'hostname'
*         port => 12345
*     }
*  }


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 <mailto:bram@bitconjurer.org> to make
sure that all extensions are done compatibly.

* BitTorrent's peer protocol operates over TCP
*
*   peer = {
*   }
*
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.

*    Handshake:
*    \x13BitTorrent protocol\0\0\0\0\0\0\0\0<sha1 info hash><20byte peerid>
*
*    Keep alive:
*    \x0000000
*
*    0 - choke:
*    \x0000001\00
*    
*    1 - unchoke:
*    \x0000001\01
*
*    2 - interested:
*    \x0000001\02
* 
*    3 - not interested:
*    \x0000001\03
*
*    4 - have:
*    \x0000005\04<index>
*
*    5 - bitfield:
*    \x00000??\05<bitfield-data>
*
*    6 - request:
*    \x000000d\06<index><begin><length>
*    length must be less than 2^17, recommended 2^15, regardless of pieces size
*
*    7 - piece:
*    \x???????\07<index><begin><data>
*
*    8 - cancel:
*    \x000000d\08<index><begin><length>

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.

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
<mailto:bram@bitconjurer.org> 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.