M17 Project is a modern, digital radio protocol built by hams, for hams. This repository contains the specification describing it exhaustively, from top to bottom. It is still a work in progress, meaning that this repository is meant to be updated now and then. After a few years of development, almost all of the changes are just to supplement the specification with new functions or improve the document.
The specification can be conveniently browsed here.
If you need a pdf file suitable for printing and binding, see the comment at the first line of the M17_spec.txt file.