[Cheswick90] Cheswick, B. The Design of a Secure Internet Gateway, Proceedings of the Summer Usenix Conference, Anaheim, CA, June 1990. [Brief abstract (slight paraphrase from the original abstract): AT&T maintains a large internal Internet that needs to be protected from outside attacks, while providing useful services between the two. This paper describes AT&T's Internet gateway. This gateway passes mail and many of the common Internet services between AT&T internal machines and the Internet. This is accomplished without IP connectivity using a pair of machines: a trusted internal machine and an untrusted external gateway. These are connected by a private link. The internal machine provides a few carefully-guarded services to the external gateway. This configuration helps protect the internal internet even if the external machine is fully compromised.]