L2TP/IPsec VPN Failure

Hello all, I've just got a SM-G920F, Android 5.1.1 (kernel 3.10.61-5012324), and I am experiencing a problem with VPN connect to a Linux box that is running libreswan. This box works wonders with an iphone 5 and a Nexus 7. With the S6 i can track the pluto activity on the server and phase 1 and 2 correctly done, but after the IP assignment the l2tpd never gets called, and after a while the phone signals connection failed. Any idea about this beahviour? Suggestions about what to analyze in logcat ?

regards
Mat

How can connect to L2TP/IPSEC VPN Server?

I have a vpn account from a L2TP/IPSEC VPN Server and use four attributes to connect it from Android or Windows devices: IP Adress, User Name, User Password and PreShared Key. I can connect with these details successfully manually. I also want to connect it programmatically in Android 4.x.

I looked into OrbotVPN and ToyVPN projects but couldn't see any details about connecting to existing VPN server. And I think there must be some configuration(server side code) at the remote point (VPN server) to response requests from clients in these projects. But in default; a L2TP/IPSEC VPN server configured already and ready for connections.

Also googled it, all of them says "VpnService is designed for creating custom-protocol VPN applications" but there is no an example of IPSEC implementation.

Is there any example of IPSEC implementation?