Diameter is a protocol designed to carry Authentication, Authorization and Accounting (AAA) payload. It is an evolution of the RADIUS protocol (as the name suggests).

freeDiameter is an implementation of the Diameter protocol.

This simple daemon parses the command line and initializes the freeDiameter framework. Use it for your Diameter server & agent components. In case of Diameter clients, you probably will prefer linking the libfdcore directly with your client application that must be made Diameter-aware.