How can I keep the VPN connection alive?

I'm currently running a screen on my machine (Ubuntu 16.04.3) to connect to the VPN in background:

    screen -S vpn

To connect to my VPN, I run the following command:

    ./forticlientsslvpn_cli --server ip:port --vpnuser user

Then I'll get a prompt to confirm the connection because of the certificate:

The certificate for the SSLVPN server is invalid. You are connecting to an untrusted server. which could put your confidential information at risk. Would you like to connect to this server? (Y/N)

Once I confirm the connection, everything works properly.

    STATUS::Login succeed
    STATUS::Starting PPPd
    STATUS::Initializing tunnel
    STATUS::Connecting to server
    STATUS::Connected
    Press Ctrl-C to quit
    STATUS::Tunnel running

However, after a few hours of connection, I'll get the following error:

    SSLVPN down unexpectedly with error:6
    STATUS::Stopping tunnel
    Clean up...

How can I get my machine to reconnect to the VPN automatically upon disconnection or keep the connection alive?