Diameter is a new #Triple A (AAA) protocol standardised by RFC3588 and RFC4005. It is derived from and aim to replace Remote Access Dial-In User Service (RADIUS) (also provides the transition support) with enhancements such as error handling and message delivery reliability. Instead of relying on a central AAA server, it employs a peer-to-peer architecture, which means that every host or node that implements Diameter could act as either a server or client, with additional support of peer discovery via static configuration or dynamic lookup. The protocols in used by Diameter are connection-oriented protocols such as #Transmission Control Protocol (TCP) and Stream Control Transmission Protocol (SCTP) with a maximum attribute data size of 16,777,215 octets (8 bit). It supports both Link Encryption# and End-to-end Encryption (e2e)#.
A Diameter agent could have agent support of Relay, Proxy, Redirect, or Translation. The server can initiate messages in time of, for example, re-authentication or session termination, and is capable of handling protocol and application errors. The client can negotiate supported applications and security level with the server using Diameter.
Comparing to Remote Access Dial-In User Service (RADIUS), it supports transport layer security such as IP Security (IPsec) and Transport Layer Security (TLS). Furthermore, it has larger address space for Attribute Value Pairs (AVP) and identifiers (32-bit instead of 8-bit). Moreover, it supports application layer acknowledgements and has error notification, basic support for user-sessions and accounting, and better roaming support than RADIUS. It is also more easily extended. It also supports both stateful and stateless models.