Introduction to AAA and RADIUS
AAA is a security architecture model.
RADIUS is a specific implementation of AAA.
FreeRADIUS is a practical application of RADIUS.
Thus we have AAA → RADIUS → FreeRADIUS.
RADIUS is all about central control and is the de facto standard supported by NAS vendors.
RADIUS is a client/server protocol. It uses UDP and listens on port 1812 for authentication and port 1813 for accounting requests.
RADIUS data packets have a code field, which specifies the type of RADIUS packet.
RADIUS data packets have zero or more AVPs, which contain the data used in RADIUS.
FreeRADIUS implements the RADIUS protocol along with its various extensions as specified in RFCs.
FreeRADIUS is a very popular, widely used, and very flexible RADIUS server.