Background and Motivation Firewalls are crucial elements in network security, and have been widely deployed in most businesses and institutions for securing private networks. A firewall is a security guard placed at the point of entry between a private network and the outside Internet such that all incoming and outgoing packets have to pass through it. A packet can be viewed as a tuple with a finite number of fields such as source IP address, destination IP address, source port number, destination port number, and protocol type. By examining the values of these fields for each incoming and outgoing packet, a firewall accepts legitimate packets and discards illegitimate ones according to its configuration.