The Internet is not a safe place. Unsecured hosts can expect to be compromised within minutes of connecting to the Internet. However, while such threats to host systems are widely understood, it is less well appreciated that the network infrastructure itself is subject to constant attack as well. Indeed, through combinations of social engineering and exploitation of weak passwords, attackers have seized control over of thousands of routers. By compromising a router, an attacker may interpose on the traffic stream and manipulate it maliciously to attack others: selectively dropping, modifying or re-routing packets. This work is an important step in being able to tolerate such attacks on key network infrastructure components. This book is based on the author''s doctoral research, which is recognized with the William C. Carter Award, in 2005: "The award is presented annually since 1997 to recognize an individual who has made a significant contribution to the field of dependable computing through his or her graduate dissertation research. The award is sponsored by the IEEE Technical Committee on Fault-Tolerant Computing and the IFIP WG-10.4 on Dependable Computing and Fault Tolerance."