libtorrent — High-Performance C++ BitTorrent Library

Introduction

libtorrent (libtorrent-rasterbar) is a C++ library implementing the BitTorrent protocol with a focus on performance and standards compliance. It serves as the engine behind popular torrent clients and is used in both desktop and embedded applications.

What libtorrent Does

• Implements the full BitTorrent protocol including DHT, PEX, and magnet links
• Manages concurrent downloads with disk caching, piece prioritization, and rate limiting
• Supports protocol extensions including uTP (micro transport protocol) for low-overhead transfers
• Provides Python bindings for scripted torrent management and automation
• Handles web seeding (HTTP/FTP sources) alongside peer-to-peer transfers

Architecture Overview

libtorrent uses an async I/O model built on Boost.Asio. A session object manages all torrents, coordinating peer connections, disk I/O, and bandwidth allocation. The disk subsystem uses memory-mapped files and an LRU cache to minimize system calls. Each torrent runs its own piece picker and choking algorithm.

Self-Hosting & Configuration

• Link against libtorrent-rasterbar via CMake with find_package(LibtorrentRasterbar)
• Create a session with settings_pack to configure ports, rate limits, and connection caps
• Add torrents via .torrent files, magnet URIs, or in-memory resume data
• Tune disk cache size and hashing threads for your storage hardware
• Enable encryption, DHT, and local peer discovery in settings

Key Features

• Powers qBittorrent, Deluge, and other widely used torrent clients
• uTP transport reduces network congestion while maintaining throughput
• Full DHT implementation with BEP-44 mutable items for decentralized data storage
• Efficient memory-mapped disk I/O with configurable caching strategies
• Supports seeding, super-seeding, and web seeding from HTTP sources

Comparison with Similar Tools

• libtorrent (Rakshasa) — Rakshasa's libtorrent powers rTorrent; libtorrent-rasterbar offers broader protocol support and Python bindings
• Transmission — Transmission is a full client; libtorrent is an embeddable library for building custom clients
• aria2 — aria2 is a multi-protocol downloader; libtorrent specializes in BitTorrent with deeper protocol features
• WebTorrent — WebTorrent runs in browsers via WebRTC; libtorrent targets native applications with system-level performance

FAQ

Q: What is the difference between libtorrent and libtorrent-rasterbar? A: libtorrent-rasterbar (by Arvid Norberg) is the widely used library powering qBittorrent and Deluge. A separate, older project by Rakshasa powers rTorrent.

Q: Does libtorrent support magnet links? A: Yes. libtorrent fully supports magnet links, downloading metadata from the DHT or peers before starting the transfer.

Q: Can libtorrent be used in commercial software? A: Yes. libtorrent uses a BSD license, allowing unrestricted commercial use.

Q: What platforms does libtorrent support? A: Linux, macOS, Windows, FreeBSD, and Android. Any platform with a C++17 compiler and Boost.

Sources

• https://github.com/arvidn/libtorrent
• https://libtorrent.org/