On the Definition of Survivability

The computer systems that provide the information underpinnings for critical infrastructure applica-tions, such as banking systems and telecommunications networks, have become essential to the operation of those applications. Failure of the information systems will often cause a major loss of service, and so their dependability has become a major concern. Current facets of dependability, such as reliability and avail-ability, do not address the needs of critical information systems because they do not include the notion of degraded service as an explicit requirement. What is needed is a precise notion of what forms of degraded service are acceptable to the application, under what circumstances each form is most useful, and the frac-tion of time degraded service levels are acceptable. This concept is termed survivability and in this paper we show that it is a necessary new facet of dependability. We present a formal definition of survivability and present an example of its use