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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 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.
Examples
See the utp_test and utp_file directories for examples.
Building
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:
make
To build one of the examples, which will statically link in everything it needs from libutp:
cd utp_test && make
License
libutp is released under the MIT license.
Related Work
Research and analysis of congestion control mechanisms can be found here.