L2TP

L2TP server:
http://linuxexplore.com/how-tos/l2tp-vpn-using-xl2tpd/

L2TP client:
http://www.softether.org/4-docs/2-howto/9.L2TPIPsec_Setup_Guide_for_SoftEther_VPN_Server/4.Windows_L2TP_Client_Setup
https://pandapow.co/how-to/win-l2tp-setup/