The design of accounting in RADIUS is based upon three major characteristics:

Accounting will be based on a client/server model.: The RADIUS accounting machine is the server to the RADIUS client gear, which acts as the client. The client passes the usage data to the RADIUS server for processing. The RADIUS server acknowledges successful receipt of the data. It is also possible for the RADIUS server to act as an accounting proxy, much like the similar capability in the authentication and authorization realms.
Communications between devices will be secure.: All data is passed to and from the RADIUS server and the client gear through the use of a shared secret, which is never transmitted across the wire.
RADIUS accounting will be extensible.: The format of the accounting attributes is much like those of the authentication and authorization attributes, in that most of the services offered by the implementations can be defined and qualified using AVPs. AVPs can be added and modified to an existing implementation without disrupting the functionality already in use.