a) client and server that only use the WebRTC data connection for low latency UDP based communications
b) a p2p example for a group of clients using WebRTC data connection
This website seems to cover b) but not a)
It's a minimal WebRTC server implementation (SDP, STUN, SCTP, DTLS and everything else) contained in a single library. Note that it only implements a subset of the specs to get it working and probably still is buggy (experimental!) as I'm developing it while working on another project that uses it.
The example in my blog post isn't really minimal because it assumes that you're writing your server in C++ for performance reasons. You can cut away a lot of the setup and boilerplate if you're fine with your server running on node and delegating all WebRTC tasks to a background electron process (see electron-webrtc).
Also, recently Mozilla has released their own WebRTC abstraction (https://hacks.mozilla.org/2017/06/introducing-humblenet-a-cr...) which seems to be easy to use on the server. I personally haven't tried it yet, but I've heard good things.