Short overview of AAA and the RADIUS protocol.
The term AAA (say triple A) subsumes the functions used in network access to allow a user or a computer to access a network and use its resources.
AAA stands for Authentication (is the user authentic?), Authorization (what is the user allowed to do?) and Accounting (track resource usage by the user).
AAA is typically employed at network ingress points to control user's access to the network and resources.
The most prominent protocol for AAA is RADIUS (Remote Authentication Dial In User Service) which defines messages for opening and closing a network session and counting network usage (packet and byte count).
RADIUS usually works in conjunction with an LDAP server that stores the policies and user authorizations in a central repository.