A few pptpd/ppp questions (Or: How to make sure the VPN-Tunnel is always up?)

Goodday,

I got a question relating to PPTPd and PPPd on two Linux 2.4 (SuSE
Linux 9) boxes.

Our setup is as follows:

------------------ ----------------
| Box 1 | <--- internet ---> | Box 2 |
| 192.168.168.0/24 | < pptp-tunnel > | 192.168.0.0/24 |
------------------ ----------------

The tunnel itself works just fine, no problems anywhere. We are doing
a NAT from network to network on a virtual ppp interface, which gets
registered as ppp1 (ppp0 is taken by the dsl connection) when the
tunnel is up.

The problem is that the tunnel (but not the dsl connection) gets
closed down after some time of inactivity. Is there a way to keep it
from disconnecting and closing the tunnel?

Is there a directive I can set in the ppp configuration that does keep
the connection alive?

Thanks for tips,
Basti