ppp：http://en.wikipedia.org/wiki/Point-to-Point_Tunneling_Protocol

l2tp: http://en.wikipedia.org/wiki/Layer_2_Tunneling_Protocol

l2tp: https://baike.baidu.com/item/L2TP/609253?fr=aladdin

pptp：http://en.wikipedia.org/wiki/Point-to-Point_Tunneling_Protocol

ipsec: http://en.wikipedia.org/wiki/IPsec

ipsec: http://bbs.51cto.com/viewthread.php?tid=1119459

ipsec： https://baike.baidu.com/item/ipsec/2472311?fr=aladdin

strongSwan： https://baike.baidu.com/item/strongSwan/2873029?fr=aladdin

An Illustrated Guide to IPsec: http://www.unixwiz.net/techtips/iguide-ipsec.html

gre：http://en.wikipedia.org/wiki/Generic_Routing_Encapsulation

ms-chap-v2破解：https://www.cloudcracker.com/blog/2012/07/29/cracking-ms-chap-v2/

strongswan: https://www.strongswan.org/docs/OSTD_2013.pdf

ISAKMP、IKE、IKEv1、IKEv2、NAT-Tranversal：http://security.hsr.ch/lectures/Information_Security_2/Vorlesungsunterlagen/04.6-IKE_Notes.pdf