RADIUS accounting can be used to track wireless users sessions for information such as number of packets transferred, and the amount of time the user session was active. Typically when the user logs onto the wireless network and the users connection is activated the Access Point will send an Accounting Start message to the RADIUS server, and when the session ends an Accounting-Stop message is sent.
Two potential issues here are:
- Due to network issues or the AP or the RADIUS server rebooting the Accounting Stop message could be lost. If that happens, all the users session information would be lost, and potential billing/charges lost if this was a paid wifi scenario.
- If the server is relying on the information to send a CoA or Disconnect Message to teh AP it has no way until the sesion ends to know how the session is doing.
Interim accounting is a solution to these problems: the AP will periodically send updates to the RADIUS server about the users session. This way if the Accounting-Stop is lost for some reason, only part of the users session information is lost. The interim accounting packets would have provided atleast some information to the server. Also for CoA this keeps the server updated.
While Interim accounting provides additional and up-to-date information to the RADIUS server, it does this at the cost of increased messages and load on the server, so the frequency of updates needs to be calibrated to the conncetion between the AP and the server, and the number of APs and clients.
Interim accounting can be configured on the E400 and the E500 access points in the RADIUS section of the WLAN configuration page. Two steps to enabling it are to set the accounting-mode to Start-Interim-Stop, and to set the Interim-Accoutning-interval to the value in seconds for the interval at which these updates should be sent.