How to charge based on accounting correctly

[George Chelidze]

Hello,

We have a system which sends radius accounting messages to our radius. 
Based on this accounting we charge subscribers. Interim accounting is 
enabled so for a normal session we get one Start, zero or more 
Interim-Update and one Stop packets. Every accounting packet except 
Start contains Acct-Session-Time which is "how many seconds the user has 
received service for" according to rfc2866. That means that it's always 
incrementing during a session. To charge a session in chunks we 
calculate a difference between the recent value and the previous one. In 
our case we get Interim-Update records every 15 minutes, so this 
difference between 2 sequential Interim-Update records is 900 secs +/- 
2-3 secs. What happens when for some reason one Interim-Update records 
is lost? In such case we get diff - 1800 secs and everything is fine. 
However recently we discovered that a system which is sending accounting 
records will resend unconfirmed packets after some time (I do not mean 
Retransmit-Interval/Retransmit-Count feature which exists on most if not 
all NAS-es). The later means that in some cases we can get an 
Interim-Update packet with Acct-Session-Timeout = 1800 calculate a 
difference against a Start record (that is 1800 secs) and charge it, 
later get Interim-Update with Acct-Session-Timeout = 900. So my 
questions can be stated like this:

1. Is it better to charge the whole difference between current and 
previous Session-Timeout values and later ignore any previous packets 
which arrive out of order, or it's better to charge last 15 minutes (I 
get Interim-Update records every 15 minutes as already stated above) and 
do not care about missing parts (if any) of a session?

2. Is it a correct behavior of a NAS to store accounting information on 
it's internal disk if it can't get acknowledgment for accounting 
request/s and resend it later?

Best Regards,

George Chelidze