NAME

Net::BitTorrent::Protocol - Packet utilities for the BitTorrent protocol

Synopsis

use Net::BitTorrent::Protocol qw[:build parse_packet];

# Tell them what we want...
my $handshake = build_handshake(
    pack('C*', split('', '00000000')),
    pack('H*', 'ddaa46b1ddbfd3564fca526d1b68420b6cd54201'),
    'your-peer-id-in-here'
);

# And the inverse...
my ($reserved, $infohash, $peerid) = parse_packet( $handshake );

Description

What would BitTorrent be without packets? TCP noise, mostly.

For similar work and links to the specifications behind these packets, move on down to the See Also section.

Functions

There are three types of functions exported by this module:

Packet Types

These functions return the BitTorrent

HANDSHAKE
KEEPALIVE
CHOKE
UNCHOKE
INTERESTED
NOT_INTERESTED
HAVE
BITFIELD
REQUEST
PIECE
CANCEL
PORT
SUGGEST
HAVE_ALL
HAVE_NONE
REJECT
ALLOWED_FAST
EXTPROTOCOL

Building Functions

These create packets ready-to-send to remote peers. To import these in one go, use the :build tag.

Parsing Functions

These are used to parse unknown data into sensable packets. To import these in one go, use the :parse tag.

Building Functions

build_handshake ( RESERVED, INFOHASH, PEERID )

Creates an initial handshake packet. All parameters must conform to the BitTorrent spec:

RESEREVED: ...is the 8 byte string used to represent a client's capabilities for extentions to the protocol.
INFOHASH: ...is the 20 byte SHA1 hash of the bencoded info from the metainfo file.
PEERID: ...is 20 bytes.

build_keepalive ( )

Creates a keep-alive message. The keep-alive message is a message with zero bytes, specified with the length prefix set to zero. There is no message ID and no payload. Peers may close a connection if they receive no messages (keep-alive or any other message) for a certain period of time, so a keep-alive message must be sent to maintain the connection alive if no command have been sent for a given amount of time. This amount of time is generally two minutes.

build_choke ( )

Creates a choke message. The choke message is fixed-length and has no payload.

See Also: http://tinyurl.com/NB-docs-choking - Choking and Optimistic Unchoking

build_unchoke ( )

Creates an unchoke message. The unchoke message is fixed-length and has no payload.

See Also: http://tinyurl.com/NB-docs-choking - Choking and Optimistic Unchoking

build_interested ( )

Creates an interested message. The interested message is fixed-length and has no payload.

build_not_interested ( )

Creates a not interested message. The not interested message is fixed-length and has no payload.

build_have ( INDEX )

Creates a have message. The have message is fixed length. The payload is the zero-based INDEX of a piece that has just been successfully downloaded and verified via the hash.

That is the strict definition, in reality some games may be played. In particular because peers are extremely unlikely to download pieces that they already have, a peer may choose not to advertise having a piece to a peer that already has that piece. At a minimum "HAVE supression" will result in a 50% reduction in the number of HAVE messages, this translates to around a 25-35% reduction in protocol overhead. At the same time, it may be worthwhile to send a HAVE message to a peer that has that piece already since it will be useful in determining which piece is rare.

A malicious peer might also choose to advertise having pieces that it knows the peer will never download. Due to this attempting to model peers using this information is a bad idea.

build_bitfield ( BITFIELD )

Creates a bitfield message. The bitfield message is variable length, where X is the length of the BITFIELD. The payload is a BITFIELD representing the pieces that have been successfully downloaded. The high bit in the first byte corresponds to piece index 0. Bits that are cleared indicated a missing piece, and set bits indicate a valid and available piece. Spare bits at the end are set to zero.

A bitfield message may only be sent immediately after the handshaking sequence is completed, and before any other messages are sent. It is optional, and need not be sent if a client has no pieces or uses one of the Fast Extention packets: have all or have none.

build_request ( INDEX, OFFSET, LENGTH )

Creates a request message. The request message is fixed length, and is used to request a block. The payload contains the following information:

INDEX: ...is an integer specifying the zero-based piece index.
OFFSET: ...is an integer specifying the zero-based byte offset within the piece.
LENGTH: ...is an integer specifying the requested length.

Legacy Messages

The following messages are either part of the base protocol or one of the common extentions but have either been superceeded or simply removed from the majority of clients. I have provided them here only for legacy support; they will not be removed in the future.

build_port ( PORT )

Creates a port message.

See also: http://bittorrent.org/beps/bep_0003.html - The BitTorrent Protocol Specification

build_allowed_fast ( INDEX )

Creates an Allowed Fast message.

uTorrent never advertises a fast set... why should we?