Abstract
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It is well known that authentication, integrity, and confidentiality are the most important principles of network security. However, recent reports about a number of prominent Internet service providers that broke down because of malicious attacks [2, 3, 32,32] urge people to realize that all security principles must be based on service availability. Availability in this context refers to a service that can be accessed within a reasonable amount of waiting time after a legitimate client sends a request. © 2010 Springer Science+Business Media, LLC.