how to inform pppd Session-Timeout value using radius

Hi,

I am trying to set up a vpn service using radius+mysql+pptpd+pppd, so
far everything is right that user in table radcheck can login using the
config'ed password. Now I want to control the behavior of a ppp
connection from the server side, e.g. forcing disconnecting a user.

pptpd is not a decent NAS that supports CoA (correct me?). So I can't
use radclient to send a CoA disconnect message.

Then people say using Acct-Interim-Interval and Session-Timeout in
radius accounting reply message to inform the NAS to timeout a
connection. As a start, I tried to set the Acct-Interim-Interval value
to 5 secs in auth reply message. I can see from the log that the
Acct-Interim-Interval message is successfully sent but seems pppd-radius
plugin doesn't respect it.

Am I doing right?

I failed to find the whole list of attributes that pppd-radius supports.
Can anyone please point me to?

Or if pptpd/pppd/pppd-radius combination is incapable to do simple CoA,
what alternative NAS should I turn to?

Thank you!