Could anyone tell me if and how it is possible to
keep a vpn tunnel open permanently.
A remote access IPsec tunnel or a Site-to-Site IPsec tunnel?
Normally, the tunnels are torn down after an idle timeout (inactivity) for security purposes.
To keep a tunnel up, you can keep constant traffic passing through the tunnel (so it's never idle and therefore never down).
Just out of curiosity, why would you keep the tunnel up permanently (even if there's no interesting traffic going through the tunnel)?