RP-L2TP is a user-space implementation of L2TP for Linux and other UNIX systems.
I used this application and really found it very useful. I didn't found its documentation so i wrote a How to for it. I would like to share it with all may be it can help you. Search "l2tp how to" in google you will find linuxexplore link, check that it is a good how to. Thanks Linux Explore Happy Exploring..