Previous Work Most of previous work on firewalls focuses on improving the performance of firewalls in the area of packet classification [Singh et al. (2003); Spitznagel et al. (2003); Woo (2000); Qiu et al. (2001); Baboescu and Varghese (2001); Baboescu et al. (2003); Srinivasan et al. (1999, 1998)]. Because the central theme of this book concerns about the correctness of firewalls, below we mainly survey related work in this respect.