19.1. Introduction
Diameter is an Authentication, Authorization and Accounting (AAA) protocol developed by the Internet Engineering Task Force (IETF). Diameter is used to provide AAA services for a range of access technologies. Instead of building the protocol from scratch, Diameter is loosely based on the Remote Authentication Dial In User Service (RADIUS)[] [RFC2865], which has previously been used to provide AAA services, at least for dial-up and terminal server access environments. As the basis for Diameter work, the AAA Working Group first gathered requirements for AAA services as they apply to network access from different interest groups:
[] The name is derived from geometry, as Diameter = 2 * Radius.
IP Routing for Wireless/Mobile Hosts WG (MOBILEIP) [RFC2977].
Network Access Server Requirements WG (NASREQ) [RFC3169].
Roaming Operations WG (ROAMOPS) [RFC2477].
Telecommunications Industry Association (TIA).
The final Diameter protocol is actually split into two parts: Diameter Base Protocol and Diameter applications. The base protocol is needed for delivering Diameter data units, negotiating capabilities, handling errors and providing for extensibility. A Diameter application defines application-specific functions and data units. Each Diameter application is specified separately. Currently, in addition to the base protocol [RFC3588], a few Diameter applications have been defined and some are in the process of being defined: namely, Mobile IP [Draft-ietf-aaa-diameter-mobileip], ...
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