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# Rasterbar Software

## Software developement and consulting

# libtorrent manual

Author: | Arvid Norberg, arvid@rasterbar.com

Table of contents

- introduction
- features
- portability
- license

# introduction

libtorrent is a C++ library that aims to be a good alternative to all the other bittorrent implementations around. It is a library and not a full featured client, although it comes with a working example client.

The main goals of libtorrent are:

- to be cpu efficient
- to be memory efficient
- to be very easy to use

# features

libtorrent is still being developed, however it is stable. It is an ongoing project (including this documentation). The current state includes the following features:

- trackerless torrents (using the Mainline kademlia DHT protocol) with some DHT extensions.
- support for IPv6
- NAT-PMP and UPnP support (automatic port mapping on routers that supports it)
- uses a separate disk I/O thread to not have the disk ever block on network or client interaction. (see threads).
- supports the bittorrent extension protocol. See extensions.
- supports the uTorrent metadata transfer protocol (i.e. magnet links).
- supports the uTorrent peer exchange protocol (PEX).
- supports local peer discovery (multicasts for peers on the same local network)
- adjusts the length of the request queue depending on download rate.
- has an adjustable read and write disk cache for improved disk throughput.
- multitracker extension support (supports both the specification by John Hoffman and the uTorrent interpretation).
- tracker scrapes
- supports both sparse files and compact file allocation (where pieces are kept consolidated on disk)
- supports files > 2 gigabytes.
- serves multiple torrents on a single port and in a single thread
- fast resume support, a way to get rid of the costly piece check at the start of a resumed torrent. Saves the storage state, piece_picker state as well as all local peers in a separate fast-resume file.
- HTTP seeding, as specified by Michael Burford of GetRight.
- piece picking on block-level (as opposed to piece-level). This means it can download parts of the same piece from different peers. It will also prefer to download whole pieces from single peers if the download speed is high enough from that particular peer.
- supports the udp-tracker protocol by Olaf van der Spek.
- queues torrents for file check, instead of checking all of them in parallel.
- supports http proxies and basic proxy authentication
- gzipped tracker-responses
- can limit the upload and download bandwidth usage and the maximum number of unchoked peers
- implements fair trade. User settable trade-ratio, must at least be 1:1, but one can choose to trade 1 for 2 or any other ratio that isn't unfair to the other party.
- supports the `no_peer_id=1` extension that will ease the load off trackers.
- possibility to limit the number of connections.
- delays have messages if there's no other outgoing traffic to the peer, and doesn't send have messages to peers that already has the piece. This saves bandwidth.
- does not have any requirements on the piece order in a torrent that it resumes. This means it can resume a torrent downloaded by any client.
- supports the `compact=1` tracker parameter.
- selective downloading. The ability to select which parts of a torrent you want to download.
- ip filter to disallow ip addresses and ip ranges from connecting and being connected

# portability

libtorrent is portable at least among Windows, MacOS X and other UNIX-systems. It uses Boost.Thread, Boost.Filesystem, Boost.Date_time and various other boost libraries as well as zlib (shipped) and asio (shipped). At least version 1.33.1 of boost is required.

Since libtorrent uses asio, it will take full advantage of high performance network APIs on the most popular platforms. I/O completion ports on windows, epoll on linux and kqueue on MacOS X and BSD.

libtorrent has been successfully compiled and tested on:

- Windows 2000 vc7.1, vc8
- Linux x86 GCC 3.3, GCC 3.4.2
- MacOS X (darwin), (Apple's) GCC 3.3, (Apple's) GCC 4.0
- SunOS 5.8 GCC 3.1
- Cygwin GCC 3.3.3

Fails on:

- GCC 2.95.4
- msvc6

# license

libtorrent is released under the BSD-license.

This means that you can use the library in your project without having to release its source code. The only requirement is that you give credit to the author of the library by including the libtorrent license in your software or documentation.

Here's a list of some projects that uses libtorrent.