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libutp - The uTorrent Transport Protocol library.

Copyright (c) 2010 BitTorrent, Inc.

uTP is a TCP-like implementation of LEDBAT documented as a BitTorrent extension in BEP-29. uTP provides reliable, ordered delivery while maintaining minimum extra delay. It is implemented on top of UDP to be cross-platform and functional today. As a result, uTP is the primary transport for uTorrent peer-to-peer connections.

uTP is written in C++, but the external interface is strictly C (ANSI C89).

The Interface

The uTP socket interface is a bit different from the Berkeley socket API to avoid the need for our own select() implementation, and to make it easier to write event-based code with minimal buffering.

When you create a uTP socket, you register a set of callbacks. Most notably, the on_read callback is a reactive callback which occurs when bytes arrive off the network. The write side of the socket is proactive, and you call UTP_Write to indicate the number of bytes you wish to write. As packets are created, the on_write callback is called for each packet, so you can fill the buffers with data.

The libutp interface is not thread-safe. It was designed for use in a single-threaded asyncronous context, although with proper synchronization it may be used from a multi-threaded environment as well.

See utp.h for more details and other API documentation.


See ucat.c. Build with:

make ucat


uTP has been known to build on Windows with MSVC and on linux and OS X with gcc. On Windows, use the MSVC project files (utp.sln, and friends). On other platforms, building the shared library is as simple as:


To build one of the examples, which will statically link in everything it needs from libutp:

cd utp_test && make

Packaging and API

The libutp API is considered unstable, and probably always will be. We encourage you to test with the version of libutp you have, and be mindful when upgrading. For this reason, it is probably also a good idea to bundle libutp with your application.


libutp is released under the MIT license.

Related Work

Research and analysis of congestion control mechanisms can be found here.