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Manual Reference Pages  -  NET::BITTORRENT::PROTOCOL (3)

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



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

    # Tell them what we want...
    my $handshake = build_handshake(
        pack(C*, split(, 00000000)),
        pack(H*, ddaa46b1ddbfd3564fca526d1b68420b6cd54201),

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


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.

Exporting from Net::BitTorrent::Protocol

There are three tags available for import. To get them all in one go, use the :all tag.
:types Packet types

For more on what these packets actually mean, see the BitTorrent Spec. This is a list of the currently supported packet types:

:build These create packets ready-to-send to remote peers. See Building Functions.
:parse These are used to parse unknown data into sensible packets.

    Building Functions

build_handshake ( RESERVED, INFOHASH, PEERID ) Creates an initial handshake packet. All parameters must conform to the BitTorrent spec:
RESERVED the 8 byte string used to represent a client’s capabilities for extensions to the protocol.
INFOHASH the 20 byte SHA1 hash of the bencoded info from the metainfo file.
PEERID 20 bytes.
build_keepalive ( ) Creates a keep-alive packet. The keep-alive packet is 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 packets (keep-alive or any other packet) for a certain period of time, so a keep- alive packet 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 packet. The choke packet is fixed-length and has no payload.

See Also: - Choking and Optimistic Unchoking

build_unchoke ( ) Creates an unchoke packet. The unchoke packet is fixed-length and has no payload.

See Also: - Choking and Optimistic Unchoking

build_interested ( ) Creates an interested packet. The interested packet is fixed-length and has no payload.
build_not_interested ( ) Creates a not interested packet. The not interested packet is fixed-length and has no payload.
build_have ( INDEX ) Creates a have packet. The have packet 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 suppression will result in a 50% reduction in the number of HAVE packets, this translates to around a 25-35% reduction in protocol overhead. At the same time, it may be worthwhile to send a HAVE packet 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 packet. The bitfield packet 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 packet may only be sent immediately after the handshaking sequence is completed, and before any other packets are sent. It is optional, and need not be sent if a client has no pieces or uses one of the Fast Extension packets: have all or have none.

build_request ( INDEX, OFFSET, LENGTH ) Creates a request packet. The request packet is fixed length, and is used to request a block. The payload contains the following information:
INDEX an integer specifying the zero-based piece index.
OFFSET an integer specifying the zero-based byte offset within the piece.
LENGTH an integer specifying the requested length.

See Also: build_cancel

build_piece ( INDEX, OFFSET, DATA ) Creates a piece packet. The piece packet is variable length, where X is the length of the DATA. The payload contains the following information:
INDEX an integer specifying the zero-based piece index.
OFFSET an integer specifying the zero-based byte offset within the piece.
DATA the block of data, which is a subset of the piece specified by INDEX.

Before sending pieces to remote peers, the client should verify that the piece matches the SHA1 hash related to it in the .torrent metainfo.

build_cancel ( INDEX, OFFSET, LENGTH ) Creates a cancel packet. The cancel packet is fixed length, and is used to cancel block requests. The payload is identical to that of the request packet. It is typically used during ’End Game.’

See Also: - End Game

build_extended ( DATA ) Creates an extended protocol packet.
Legacy Packets

The following packets are either part of the base protocol or one of the common extensions but have either been superseded 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 packet.

See also: - The BitTorrent Protocol Specification

build_allowed_fast ( INDEX ) Creates an Allowed Fast packet.

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

See also: - Fast Extension

build_suggest ( INDEX ) Creates a Suggest Piece packet.

Super seeding is not supported by Net::BitTorrent. Yet.

See also: - Fast Extension

build_reject ( INDEX, OFFSET, LENGTH ) Creates a Reject Request packet.

See also: - Fast Extension

build_have_all ( ) Creates a Have All packet.

See also: - Fast Extension

build_have_none ( ) Creates a Have None packet.

See also: - Fast Extension

    Parsing Function(s)

parse_packet( DATA ) Attempts to parse any known packet from the data (a scalar ref) passed to it. On success, the payload and type are returned and the packet is removed from the incoming data ref. undef is returned on failure.

See Also - The BitTorrent Protocol Specification - Fast Extension - Extension Protocol - An annotated guide to the BitTorrent protocol

Net::BitTorrent::PeerPacket - by Joshua McAdams


Sanko Robinson <> -


License and Legal

Copyright (C) 2008-2009 by Sanko Robinson <>

This program is free software; you can redistribute it and/or modify it under the terms of The Artistic License 2.0. See the LICENSE file included with this distribution or For clarification, see

When separated from the distribution, all POD documentation is covered by the Creative Commons Attribution-Share Alike 3.0 License. See For clarification, see

Neither this module nor the Author is affiliated with BitTorrent, Inc.

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