Diameter

Diameter is an AAA protocol: it carries authentication, authorisation and accounting messages between network elements and central servers. Its base protocol is RFC 6733, which replaced the earlier RFC 3588. Study sources commonly describe it as the successor to RADIUS, and the name is commonly explained as a play on that, since a diameter is twice a radius. It was designed to address limits RADIUS showed as networks grew: it runs over reliable transports (TCP or SCTP) rather than UDP, is extended through defined applications for new services, and relies on TLS, DTLS or IPsec to protect messages between peers.

In practice Diameter is best known inside mobile operator networks, where the 4G LTE core uses it for subscriber authentication, policy and charging. Enterprise network access still commonly runs on RADIUS, and device administration on TACACS+. Diameter is not directly compatible with RADIUS, although the two can interwork through translation agents. See authentication, authorisation and accounting for the functions all three protocols serve.

Exam relevance: questions in this area tend to compare RADIUS, TACACS+ and Diameter. Candidates are expected to recognise Diameter as the newer AAA protocol commonly presented as RADIUS’s successor, running over a reliable transport, and to know that RADIUS remains the common choice for enterprise network access.