Radius question?

[yick xie]

Hi
Recently I just deployed the freeradius to auth and account the users.
Occasionally it was found that the ocserv didn't send terminate signal
to freeradius server while actually the client already disconnected.
The radius system would treated it as an online user, just for the
acctstoptime is NULL;
sec-mod: temporarily closing session for xxx (session: OSYo5)
main: 198.2.254.144:54226 user disconnected
main: termination request received; waiting for children to die
radius-auth: sending session interim update

On the other hand, the normal log goes like this:
sec-mod: invalidating session of user xxx (session: PiJsw)
radius-auth: closing session

Such scenes quite easily reoccur no matter how connection quality with
servers is.
Can anybody do me some favor to find a possible solution?