Diameter is an authentication, authorization, and accounting protocol for computer networks. It evolved from the earlier RADIUS protocol. It belongs to the application layer protocols in the internet protocol suite. Diameter Applications extend the base protocol by adding new commands and/or attributes, such as those for use with the Extensible Authentication Protocol (EAP). Comparison with RADIUS The name is a play on words, derived from the RADIUS protocol, which is the predecessor (a diameter is twice the radius). Diameter is not directly backward compatible but provides an upgrade path for RADIUS. The main features provided by Diameter but lacking in RADIUS are: Support for SCTP Capability negotiation Application layer acknowledgements; Diameter defines failover methods and state machines (RFC 3539) Extensibility; new commands can be defined Aligned on 32 bit boundaries Also: Like RADIUS, it is intended to work in both local and roaming AAA situations. It uses TCP or SCTP, unlike RADIUS which uses UDP. Unlike RADIUS it includes no encryption but can be protected by transport-level security (IPSEC or TLS). The base size of the AV identifier is 32 bit unlike RADIUS which uses 8 bit as the base AV identifier size. Like RADIUS, it supports stateless as well as stateful modes. Like RADIUS, it supports application-layer acknowledgment and defines failover. Diameter is used for many different interfaces defined by the 3GPP standards, with each interface typically defining new commands and attributes. Applications A Diameter Application is not a software application but is a protocol based on the Diameter base protocol defined in RFC 6733 and RFC 7075 (Obsoletes: RFC 3588). Each application is defined by an application identifier and can add new command codes and/or new mandatory AVPs (Attribute-Value Pair). Adding a new optional AVP does not require a new application. Examples of Diameter applications: Diameter Mobile IPv4 Application (MobileIP, RFC 4004) Diameter Network Access Server Application (NASREQ, RFC 7155)(Obsoletes: RFC 4005) Diameter Extensible Authentication Protocol Application (RFC 4072) Diameter Credit-Control Application (DCCA, RFC 8506])(Obsoletes: RFC 4006) Diameter Session Initiation Protocol Application (RFC 4740) Various applications in the 3GPP IP Multimedia Subsystem Both the HSS and the SLF communicate using the Diameter protocol. (Generic Bootstrapping Architecture): Bootstrapping Server Function History The Diameter protocol was initially developed by Pat R. Calhoun, Glen Zorn, and Ping Pan in 1998 to provide a framework for authentication, authorization, and accounting (AAA) that could overcome the limitations of RADIUS. RADIUS had issues with