What is Diameter? Role of Diameter in IMS and LTE

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Diameter is an authentication, authorization, and accounting protocol used in LTE, LTE-advanced and Voice over LTE (VoLTE). The Diameter protocol was derived from the RADIUS protocol playing the role of AAA (Authentication, Authorization, and Accounting)  protocol.

The Diameter protocol applied to LTE was inherited from IMS (IP Multimedia Subsystem) architecture for IMS entities to exchange AAA-related information. 

Diameter nodes and agents

Diameter is  a Peer-To-Peer architecture using a client-server architecture. There are four kinds of Diameter agents: Relay Agent, Proxy Agent, Redirect Agent and Translation Agent

A Relay Agent forwards a message to the appropriate destination, depending on the message. A Proxy Agent can forward and modify messages providing value-added services, policy enforcement  or perform administrative tasks for a specific realm. A Redirect Agent acts as a centralized configuration repository for other Diameter nodes. Translation Agent is a special agent in charge of  message translations.

Diameter Base Protocol by IETF is defined in RFC3588

• RFC3588; The Diameter Base Protocol.