Net::BitTorrent::Notes - Annotated Guide to the Ins and Outs of Net::BitTorrent


This is a first draft attempt at defining a road map for future Net::BitTorrent development and a behavioral reference for third-party client developers. A few bits for users might slip in too.

Net::BitTorrent's Way-too-Obvious Class Hierarchy

                                            .---- N::B::T::T::UDP
                      .-------- N::B::T::Tracker
                     /                     \
                    /   .-- N::B::T::File   `--- N::B::T::T::HTTP
                   /   /
            .---- Net::BitTorrent::Torrent
          /   .--- Net::BitTorrent::DHT
         /   /
          `---- Net::BitTorrent::Peer

The following utility modules also come with the distribution and will be of great use if you decide to whip together your own BitTorrent module.


Correctly builds and parses all known BitTorrent packets.


Contains basic functions to parse and create .torrent metadata files, and compact peer lists from trackers.

To pick and choose which functions will be exported into your namespace from these modules, see their POD documentation for more information.


This distribution uses Module::Build for installation, so use the following procedure:

  perl Build.PL
  ./Build test
  ./Build install

See also: Automated Test Reports


Net::BitTorrent requires version and Digest::SHA. On Win32, we try to use Win32API::File and Encode to handle extended charset filenames[1]. As of perl 5.10, all of these modules are are CORE; they come bundled with the distribution.

I have listed these modules as prerequisites in Build.PL so, unless you answer 'no' when prompted, the CPAN shell should automagically install them for you.

We also use the internal utf8() functions which didn't appear until perl 5.8.1.

How Do I...

Parts that aren't handled internally are described here (eventually) with sample code to get you started.

Get basic info from a .torrent without adding it to a client

Net::BitTorrent::Torrent objects can be created directly without a parent client. While functionally limited (obvious things like an inability to serve data, etc.) basic information is available and some 'advanced' functions still work (hashchecking, for example). See Net::BitTorrent::Torrent for more.

Pause and Resume a .torrent Session

See Net::BitTorrent::Torrent::pause( ) and Net::BitTorrent::Torrent::start( )

Stop and Resume a .torrent Session

See See Net::BitTorrent::Torrent::stop( ) and Net::BitTorrent::Torrent::start( )

Quick Resume a .torrent Session Between Client Sessions

Note: This section describes resume functionality as of v0.045.

Net::BitTorrent is capable of restoring the state of single torrents between sessions. To store resume data, use the save_resume_data( )

To resume a single torrent, use a variation of the following to save the data...

 my $torrent = $bt->add_torrent( { Path=> 'some.torrent', Resume = '.resume' });

 # later...


...and unless Net::BitTorrent::Torrent decides the resume data is bad, you'll start right were you left off. I would suggest storing resume data on a regular basis while the client runs and again on exit.

To view a fully functioning example, see /tatoeba/

For more on what exactly you're saving and the structure of the data, see Resume API in the <Net::BitTorrent Internals|/"Net::BitTorrent Internals"> section.

Save and Restore Client-wide State and DHT Data

Unless you've hard coded everything, being able to restore client-wide state is a necessary feature. Besides, DHT can be very slow to boot without a good set of initial nodes and the spec states that the local nodeID should not change very often, so resume is a useful thing.

I would use a hash with the following keys:


This would be a SHA-1 hash of the bencoded data in the .t, dht, nodes, and torrents keys. On restore, I would use this to validate the data in case it was tampered with.




To avoid problems (API changes, etc.), I would use an API version number and ignore resume data created with a newer/incompatible version. This value would not be included in the SHA-1 digest used to prevent tampering.


A hash with the following keys:


The local node ID used in the DHT swarm. To obtain this, see node_id( ).


List of nodes in the DHT routing table.

To make life easy, each node would be a hash with the following keys:


IP or hostname of the remote node.


UDP port number the remote port has been contacted on.

A list of nodes with this format is obtained from nodes ( ). To reload these later, use the add_node ( ) method.


These would be any client-wide settings I allow users to change.


This would be a list of filenames, their current status, and some sort of verification that the .torrent file hasn't been replaced; the infohash would do.

I would save a bencoded version of this data in a file for later. For now, putting all of this into practice is an exercise for the reader.

Note: Reloading the data may require using otherwise private methods (specifically the private Net::BitTorrent::DHT->_set_node_id( ) method). Changes to private methods are not listed in the changelog found in this distribution but they are noted in the public git repository's log.

Set File Download Priorities

See Net::BitTorrent::Torrent::File.

Implement My Own Event Loop


Net::BitTorrent Internals

This section describes all the behind the scenes stuff that makes Net::BitTorrent work. Or not work. It depends.

Peer ID Specification

Please see Net::BitTorrent::Version.

Socket6 does not seem to work with Win32 so... no plans for IPv6 right now.

Implemented Extensions

The BitTorrent Community Forum coordinates the development of the BitTorrent protocol suite, its reference implementation, and BitTorrent Enhancement Proposals (BEPs). For more information, see BEP 0: Index of BitTorrent Enhancement Proposals

This is the list of extensions used by this release of Net::BitTorrent sorted by their progress toward standardization.

  • BEP 5: DHT Protocol -

  • BEP 6: Fast Extension -

    Note: the Fast Extension is only partially implemented in Net::BitTorrent.

  • BEP 10: Extension Protocol -

  • BEP 12: Multitracker Metadata Extension -

  • BEP 15: UDP Tracker Protocol -

  • BEP 27: Private Torrents -

  • BEP 32: Tracker Returns Compact Peer Lists -

Resume API

Net::BitTorrent::Torrent's resume data is bencoded and stored in a file. To restore this data, use the Resume parameter when calling Net::BitTorrent::Torrent->new( ) or Net::BitTorrent->add_torrent( ).

Note: The structure and data held in the resume data is subject to change in future versions.

Data Structure

Parsed resume data is a simple hash containing the following keys:


A string describing what sort of file this is. For now, it's value is Net::BitTorrent resume.


Timestamp when data was gathered.


API version used to gather data. To avoid problems (API changes, etc.), Net::BitTorrent::Torrent will not load resume data created with a higher version.


A bitvector representing the pieces we already have.


A list of hashes (one for each file in the .torrent) with the following keys:


The modified times for the files (or 0 if the file does not exist).


The file's download priority.


Compact list of peers we've found either through DHT or from a tracker.


List of hashes representing pieces we are currently downloading with the following keys: (Subject to change)


Number of blocks contained in the piece.


Size of blocks in piece.


Size of final block in piece.


Bitfield representing which blocks have been received and written to disk.


Boolean value dependent on endgame state when we began working on this piece.


Zero-based index of the piece.


Total size of the piece in bytes.


Priority based (partially) on the piece's containing file.


Boolean value dependent on how long ago we received a block contained within this piece.

Development Policy

  • All APIs are subject to change.

    Changes to documented or well established parts will be clearly listed and archived in the CHANGES file.

  • All undocumented functionality is subject to change without notice.

    Net::BitTorrent is just asploding with incomplete bits of stuff so I reserve the right to change or eliminate code at any time without warning unless functionality is defined in POD documentation. If you sift through the source and find something nifty that isn't described in full in POD, don't expect your client to work with future releases.

Compatibility Notes

This section lists recent major changes in API or behavior between stable releases. For older news see the Changes file included with this distribution. For detail see the commit logs.


Protocol Encryption (MSE) is now supported and enabled by default. It is still rather unstable, so outgoing connections use header-only encryption but incoming connections allow for full RC4 encrypted sessions.


Entire distribution was rewritten. Both the internals and the API have broken compatibility.

Giving back

If you're interested in assisting with Net::BitTorent's development but don't know where to begin, here are a few ideas.

Joining the Project

Net::BitTorrent is too large for just one person to hack on. If you're Perl adept and would like to help, you can start by forking the project on github: When ready, send me a pull request, I'll look over your changes and get back to you. Minor patches get your name in the changelog. Major patches get your name in the Acknowledgments section and an invitation to be a trusted collaborator. Oooo. Ahhh.


If you're interested in helping clear things out of the TODO list or if you have a suggestion and are willing to see it through to completion, contact me or, better yet, go ahead and fork the project on github:

In general, N::B could use a second or third pair of eyes. So, if you're interested in BitTorrent, p2p, or just Perl in general, let me know.

Bug Reporting

Found bugs should be reported through Net::BitTorrent's Issue Tracker. Before creating a new report through Net::BitTorrent's Issue Tracker, please review the following list:

  1. Make sure you are using the most recent stable release.

  2. Make sure the bug is reproducible.

  3. Please write in clear English.

  4. Provide "baby steps" to describe exactly how to replicate the bug. Sample code is welcome. The issue tracker also allows attachments so, if relevant, include the .torrent file.

  5. Search the list of open and resolved issues to make sure the flaw hasn't already been reported. If it has, star the issue to stay up to date.

  6. One bug is one bug report. Please do not include multiple, separate issues in one report unless they are explicitly related to each other.

  7. Star the issue so you'll be notified of any changes.

  8. Look over your report before submission to be sure you've included as much detail as possible. Seriously. Get up, have a drink of water, come back, read over it again to make sure you've included everything you intended, and then submit it.

Automated Test Reports

Becoming a CPAN Tester is an easy, automatic way to contribute to the quality of your favorite module and CPAN in general. If you would like to contribute automated test reports for Net::BitTorrent, install CPAN::Reporter from the CPAN shell first:

 $ cpan
 cpan> install CPAN::Reporter
 cpan> reload cpan
 cpan> o conf init test_report
   [...follow the CPAN::Reporter setup prompts...]
 cpan> o conf commit
 cpan> install Net::BitTorrent

For more on becoming a CPAN Tester and why this is useful, see the CPAN::Reporter documentation and

See Also

Website and Blog

The official website is and related blog entries are posted to

If you're looking for a feed with only on-topic articles, subscribe to

Live support

The official means of support for Net::BitTorrent is through #net-bittorrent on the p2p IRC network: irc://

Receive Commit and Issue Tracker Updates

These are posted to the public mailing list for which both ATOM 1.0 and RSS 2.0 feeds are available; see


To subscribe to the list or view the archive, visit Both ATOM 1.0 and RSS 2.0 feeds are provided; see for a list.

Issue Tracker

Use for bug tracking. Please include as much information as possible and review the list above.


I announce stable builds and occasionally complain on Twitter:

Other Recommend Open Source BitTorrent Clients

RFC 3986 (URI: Generic Syntax)

Section 2.3. "Unreserved Characters" (

PAUSE FAQ sub-section entitled "Developer Releases"



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.