I'm aiming for making a(n) RTS game with networking so instead of doing tedious stuff like networking I wonder if there is a library/framework that will save me the effort. I intend to make that game in Unity if it is of any interest.
I don't believe you will find a specialized RTS network library. You can find an open source RTS engine, such as OpenRTS, or a general purpose network library, like the ones Valmond recommended.
If you will not use an RTS engine, then I recommend reading this awesome series of posts on how to design your network layer: Synchronous RTS Engine and a tale of desyncs. Or the Gamasutra article 1500 Archers on a 28.8: Network Programming in Age of Empires and Beyond
Check out Raknet, it is not a RTS-specialized library but a quite good one.
Boost::Asio is "better" but less a library 'for games', you might want to check out what it offers though.
Usually you use a lockstep approach for RTS games and I know of no 'open' libraries handling those (you might want to search for it though).
Worst case, you can make a game running on a LAN (no latency etc) and move further later (as long as you have a fully functionnal multiplayer game up and running, switching from LAN to say the internet should be quite possible).