MAVLink -- Micro Air Vehicle Message Marshalling Library.
MAVLink is a very lightweight, header-only message library for communication between drones and/or ground control stations. It consists primarily of message-set specifications for different systems ("dialects") defined in XML files, and Python tools that convert these into appropriate source code for supported languages. There are additional Python scripts providing examples and utilities for working with MAVLink data.
Tip MAVLink is very well suited for applications with very limited communication bandwidth. Its reference implementation in C is highly optimized for resource-constrained systems with limited RAM and flash memory. It is field-proven and deployed in many products where it serves as interoperability interface between components of different manufacturers.