Denial or disruption of service attacks affect the availability of data, services, and network elements. In the past, computer intruders have crashed or disrupted signal transfer points, traffic switches, and OAM& systems, reportedly planted destructive ``time bomb'' programs designed to shut down major switching hubs, and disrupted E911 services throughout the eastern seaboard [19]. For the most part, service disruptions caused by computer intruders have been brought about by accidental actions. Unintentional disruptions caused by computer intruders are much more common than malicious disruptions.
The ``Report of the Network Security Task Force'' presents the following conclusion [5]:
A motivated and resourceful adversary, in one concerted manipulation of network software, could degrade at least portions of the PSN and monitor or disrupt the telecommunications serving NS/EP users.