accounting for vpn usage

[morpheus50 at qq.com]

Hello,
I would like to ask some hint about accounting for ocserv vpn usage by 
users. Trying to look on the log file I can see this for example:

Mar 22 23:43:00 vn1 ocserv[7402]: sec-mod: using 'plain' authentication 
to authenticate user (session: Ud+Vf2)
Mar 22 23:43:06 vn1 ocserv[7402]: sec-mod: initiating session for user 
'morpheus' (session: Ud+Vf2)
Mar 22 23:43:07 vn1 ocserv[7402]: sec-mod: temporarily closing session 
for morpheus (session: Ud+Vf2)
Mar 22 23:43:08 vn1 ocserv[7402]: sec-mod: initiating session for user 
'morpheus' (session: Ud+Vf2)
Mar 22 23:43:41 vn1 ocserv[7402]: sec-mod: invalidating session of user 
'morpheus' (session: Ud+Vf2)

I am testing with my own user.
First of all I do not understand why there is written "invalidating 
session", my VPN session is working until I disconnect.
However I have no info regarding when a session is closed.
How can I have info about session accounting and traffic usage?
Thanks a lot.

Rob