The Diameter effort started 20 years ago. Its roots were in the limitations of other technologies for user authentication in access networks.

The need for backend authentication servers to assist access networks had grown in the preceding era of modem pools and dial‐in access servers. Tools for the simple authentication task existed, but mobile networks in particular needed tools that were capable of growing beyond this task.

The Diameter protocol was born in the IETF. It was designed to be a general‐purpose Authentication, Authorization, and Accounting (AAA) protocol for many uses.

Like many other pieces of Internet technology, AAA is not really visible to the end user, but it is crucial for the functioning of the Internet, in particular for the various access networks that provide connectivity for users. When your phone connects to a mobile network, the mobile network's control functions that are needed in the background are built on Diameter.

Mobile networks are the prime area where Diameter is used; simpler other tools also continue their existence and are today equally broadly used, in wireless local area network authentication for instance. And the technology continues to evolve, with most recent designs starting to employ web‐based protocols.

The authors, Jean, Jouni, Sébastien, and Hannes, have all worked tirelessly for many years – or even decades – on Diameter, AAA, and other critical Internet technologies. They have written and reviewed specifications, worked on improvements, chaired working groups, built open source implementations, and helped the industry use this technology.

I am happy to see this book on Diameter come out. It focuses on the protocol itself, of course, but also covers open source systems. This is important, as we need both specifications and code to achieve something. It is code that ultimately provides a function, while specifications are needed to ensure that different systems interoperate. The authors approach this book as they have approached their work at the IETF and open source communities, by meticulous attention to detail while keeping the big picture and practical use also up front. Thank you!