http://www.xs4all.nl/~freeswan/
http://wwwmez.med.uoeh-u.ac.jp/~linux/vpn/freeswan/freeswan.html
http://www.atmarkit.co.jp/flinux/special/ipsec2/ipsec02a.html
http://slashdot.jp/article.pl?sid=04/03/02/0730208&topic=28&mode=nested
http://www.tatsuyababa.com/MasteringIPsec/
http://www.yamdas.org/column/technique/ipsec2.html