PPP over L2TP

modulename: pppox.ko

configname: CONFIG_PPPOL2TP

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>PPP over L2TP
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Support for PPP-over-L2TP socket family. L2TP is a protocol
used by ISPs and enterprises to tunnel PPP traffic over UDP
tunnels. L2TP is replacing PPTP for VPN uses.

source code:
drivers/net/ppp/pppox.c