Diameter is used to provide the Authentication, Authorization, and Accounting (AAA) functionality required by IMS. We can characterize the main components of AAA as follows:
Authentication ◾ is the process of verifying that the user is who he or she claims to be. In the context of IMS, the client device (e.g., wireless handset) also verifies that it is registering to the right network-rather than a rogue network masquerading as such-in this step. Authorization ◾ is the mechanism for determining what a user is allowed to do. That is, authorization mechanisms are used to answer questions such as:
What services will be accessible to this user? − What information will be accessible to this user? −
Accounting ◾ is the mechanism for recording consumption of network resources by users. Among other things, accounting functionality is a prerequisite to usage-based billing. Accounting functionality is also important because it enables creation of audit trails.