Introduction

Diameter is an open standard Authentication, Authorization, and Accounting (AAA) protocol defined by the Internet Engineering Task Force (IETF). Diameter's features fulfill multiple requirements of network operators. The definition of the Diameter protocol is given in the Diameter base specification, RFC 6733. Various AAA protocols, such as the Common Open Policy Service Protocol and Remote Authentication Dial In User Service, had been developed before work on the Diameter protocol started. Experience with these protocols provided the IETF community with requirements for a next‐generation AAA protocol. The design of Diameter incorporated the lessons learned from these various AAA protocols. As work continued on Diameter, the AAA working group of the IETF evaluated the available AAA protocols against the requirements given in RFC 2989. The authors use freeDiameter to illustrate various concepts of the Diameter protocol.