IPSEC IOS Help

Hi

I am struggling with setting up a VPN connection from IOS devices to my internal network (Fortigate firewall 500D)

I started by using SSL VPN, but found the IOS app was next to useless, as it doesn't support tunneling and as soon as you close the app, you lose the VPN. 

I started looking into IPSEC via the IOS Dialup, but cannot get my head around it. My knowledge of IPSEC was that it requires a source IP and destination IP, however, with the mobile devices constantly on the road, near on impossible to lock down the devices IP. It seems IPSEC IOS doesnt require this, but i am lost to understand how it authenticates the tunnel between the devices? 

Any help to get my head around this would be much appreciated. 

Thanks

Paul