Asio wrapper over the uTorrent's (MIT licensed) uTP library

Features

Similar API to the TCP sockets in Asio. In particular, the utp::socket implements the AsyncReadStream and AsyncWriteStream requirements. Making it readily usable with Asio's free functions or classes utilizing those requirements. Such as async_read, async_write, ssl::stream, ...

Also similar to Asio's TCP sockets, utp::socket's async API supports callbacks, futures and coroutines as completion tokens.

The asio_utp::udp_multiplexer can be used to perform non uTP sending and receiving of UDP datagrams.

Advantages of uTP over TCP

Multiple uTP connections over one UDP port implies
- better options to do NAT hole-punching
- free hole-punching on certain types of NATs
- fewer open file descriptors
Low latency
Yields to TCP

Clone

asio-utp git repository contains libutp (the uTorrents uTP library written in C) as a submodule. Thus one has to clone it recursively:

git clone --recursive git@github.com:inetic/asio-utp.git

Build

cd asio-utp
mkdir build
cd build
cmake .. -DBOOST_ROOT=<PATH_TO_BOOST_DIRECTORY>
make -j$(nproc)

For more detailed instructions, have a look at the .circleci/config.yml file.

Caveats

An accepting socket may only start sending after it received some data from the connecting socket (likely a consequence of this)
One has to implement their own timeouts and keep-alive packets because otherwise if the FIN UDP packet gets dropped by the network then the remaining socket won't get destroyed. Note that there is a mention of keep-alive packets in libutp/utp_internals.c, but those seem to be only used for preserving holes in NATs (not to indicate whether the other end is still alive).
The call to socket::async_connect simply executes the underlying libutp/utp_connect function. The latter sends one SYN packet but does not implement any timeout nor resending of that packet. This needs to be done explicitly by closing the socket after some timeout and then re-starting the call to socket::async_connect.

Architecture

    asio_utp::socket --- asio_utp::socket_impl ---+
                                                   \
    asio_utp::socket --- asio_utp::socket_impl -----+--- asio_utp::context
                                                   /             \
    asio_utp::socket --- asio_utp::socket_impl ---+               \
                                                                   \
                                          +----------- asio_utp::udp_multiplexer_impl
    asio_utp::udp_multiplexer ---+       /                              \
                                  \     /                                \
    asio_utp::udp_multiplexer -----+---+                          asio::udp::socket
                                  /
    asio_utp::udp_multiplexer ---+

TODO

Handle ICMP messages
Thread safety