IMS standard bodies have chosen the Diameter protocol to enable AAA (Authentication, Authorization and Accounting) capabilities. As the AAA is a key functionality for managing a communication infrastructure, protocols involved have to be secure, to make the scalability easier, to be flexible and to be able to evolve easily and quickly.
This paper will show that the Diameter protocol delivers the combination of scalability, flexibility and security required for the delivery of multimedia services over IP. IMS architectures and servers are used to show the Diameter capabilities. And a number of issues, such as the compatibility of multiple Diameter releases used in the same network, will be presented.
Diameter is a signaling protocol created to resolve the main issues that the RADIUS (Remote Access Dial-In User Service) protocol left open. Specified by IETF, this protocol has been adopted by 3GPP (3rd Generation Partnership Project) and 3GPP2 standardization bodies for AAA in IMS mobile systems and networks. It is also the AAA protocol selected by TISPAN (Telecoms & Internet converged Services & Protocols for Advanced Networks), the ETSI (European Telecommunication Standards Institute) committee in charge of FMC (Fix Mobile Convergence) standardization.
The Diameter protocol consists of a base protocol, a transport profile defined in [RFC3539] and applications (also called extension of the base protocol). The base protocol brings the common functionalities supported by all the services like mechanism for message delivery, error notification, session handling and capabilities negotiation. As a result, the base protocol must be supported by all applications. The transport profile defines the main properties and recommendations of the transport layer for AAA systems (the failover mechanism and related state machines). The Diameter protocol uses Transmission Control Protocol (TCP) or Stream Control Transmission protocol (SCTP) for transport. Various Diameter applications are defined for extending the base protocol: this capacity is the major concept of the protocol. Depending on the final usage of the service, a Diameter application can be defined to add semantic information and, as a result, to specialize the base protocol. For example, the Diameter Credit Control Application (CCA) is defined in the [RFC4006] document to add all the information necessary for the on line charging services.
