[5] Cheswick, William R., “The Design of a Secure Internet Gateway,” USENIX Summer Technical Conference Proceedings, Anaheim, California, USA, June 1990. pp233-238
[6] Cheswick, William R., Bellovin, Steven M., Rubin, Aviel D., “Firewalls and Internet Security: Repelling the Wily Hacker,” 2003, Addison-Wesley Professional.