The rapid growth of the Internet has meant that many services are regarded as critical – such as web applications, including email and e-commerce, and real-time applications, such as video conferencing and Voice over IP (VoIP). These rely on the Internet infrastructure to provide them with reliable, efficient and secure communications. However, the routing protocols that the Internet is based on were originally designed to operate in a completely trusted and open environment, assuming no malicious nodes or behaviour. The routing infrastructure was not constructed with security in mind. 1 , 2 As a result, routers are subject to malicious attacks targeting not only a single subnet or individual users, but also the overall network performance. 3 Therefore the need to secure the Internet has become a significant issue.