In order for the server to participate in the monitoring of user behavior, the NAS will periodically send an accounting of user activity to the server. This accounting is necessarily a summary of the users activities, and may include such information as how long the user has been online, and how much data they have transferred in their network session.

Since accounting data is a summary of user activity, it does not include detailed information such as web sites visited, or even how many bytes were transferred using what protocol (SMTP, HTTP, etc.) That detailed information is only available to the NAS, which is limited in its ability to perform accounting.

If detailed information about user activity is necessary, it can often be obtained through other protocols such as sFlow, or NetFlow. However, those protocols are not integrated into RADIUS systems, and it is usually difficult to tie the pieces together to get a more comprehensive understanding of user activity.

In general, the accounting data sent to a server by a NAS is used for purposes related to billing. ISP's are a large consumer of accounting data, as each user has to be billed for every minute of network access. Corporations using RADIUS did not historically depend on accounting information, as the belief was since that known employees were allowed on the network without being billed for their access, accounting was not that important.

As people become more aware of the importance of ongoing network maintenance, storing and processing accounting information is becoming important for almost every organization that depends on RADIUS. That maintenance brings us to the fourth member of the three-member AAA family, auditing.