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uTorrent Transport Protocol library
C++ Python C
Branch: master
Pull request Compare This branch is 48 commits behind bittorrent:master.

Merge pull request #29 from cit/master

Just a clean target for the Makefile for utp_file
latest commit 2c678a26e5
@ghazel ghazel authored
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pyutp pass sendto userdata to incoming proc as well
utp_config_lib handy stdout logging
utp_test remove use of -ansi, since c++98 doesn't define snprintf. closes #20
Makefile remove use of -ansi, since c++98 doesn't define snprintf. closes #20
StdAfx.h initial import
templates.h PACKED_ATTRIBUTE to work around GCC accepting but not supporting prag…
utp.def tests
utp.h Merge branch 'master' of
utp_config_example.h initial import
utp_utils.cpp Fix compilation of utp_utils.cpp on Windows.
utp_utils.h fix compiler warnings about signedness and 64bit types
utypes.h fixes for using headers from ANSI C
win32_inet_ntop.h initial import

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 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 the utp_test and utp_file directories for examples.


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.

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