XML-RPC Documentation

Is there a document that specifies exactly how to send data to the wp XML-RPC routines? I would like to write a front-end client that sends data to the wp database. NOT post data necessarily but a home-brewed extension. I’m looking for a simple and wp dedicated spec.

I have trawled the Codex and followed the links. But I have not coded XML before – yet alone RPC’s and most of the links are fairly generic.

Any help/advice greatly appreciated…