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Diameter is an authentication, authorization and accounting (AAA) protocol used by computer networks. It defines the minimum requirements that an AAA protocol must support and was built to surpass and replace the older RADIUS protocol that preceded it. It came about because of various developments made to address the limitations of the RADIUS gateway.
Diameter is a base foundation protocol with routing capabilities, negotiation capabilities, error handling and transmission of diameter messages. It serves to authenticate, authorize and account for the activities of a user before being allowed into the network to use an ISP's services.
AAA is simply a process that filters information before granting access to a client or user into a network in order to provide a secure and reliable output. One of the early standards used to implement AAA is the Remote Authentication Dial-In User Service (RADIUS) that Diameter replaced. RADIUS was quite popular, however it was very limited in security and reliability, so it was improved on by adding advanced processes and new operations such as attribute-value pairs and error notification. The base RADIUS gateway protocol plus the new added features became the Diameter protocol, which is just a pun on RADIUS since the diameter of a circle is equal to twice its radius.
The design of Diameter was started by the 3rd Generation Partnership Project (3GPP) for their IP Multimedia Subsystem (IMS). It supports interfaces such as Dx, Dh, Cx, Ro, Rh and Sh. It was not backwards compatible, so older applications utilizing RADIUS had to adapt to the new protocol.