Automatic connection management / on demand mode

[SecuMan]

I have a question about connection establishment or more precise on how to automate the establishment and termination of connections.

Currently I use a private OpenVPN server on the Internet to which about 100 VPN clients connect on regular bases.
The connections are not open permanently, but it is opened and closed manually using the OpenVPN CLI.
I would like to automate this process as follows: as soon as VPN clients receive data for the VPN server or for a network that can be reached via the VPN server, the VPN client should open the connection dynamically and independently.
The connection should be closed automatically after a timeout.
It would be great if you could point me to a description how to configure this kind on demand mode.

Thank you very much in advance. /Steve