Finally, Figure 3 depicts emergent behavior requirements at the network level. These requirements are characterized as emergent because they are not associated with particular nodes, but rather emerge from the collective behavior of node services in communicating across the network. We envision survivable systems that are capable of adapting their behavior, function, and resource allocation in response to intrusions. For example, when necessary, functions and resources devoted to non-essential services could be reallocated to the delivery of essential services and to intrusion resistance, recognition, and recovery. Requirements for such systems must also specify how the system should adapt and reconfigure itself in response to intrusions.
Systems can exhibit large variations in survivability requirements.
Small local networks may require few or no essential services and recovery times measured in hours. Conversely, large-scale networks of networks may require a core set of essential services, automated intrusion detection, and recovery times measured in minutes. Embedded command and control systems may require essential services to be maintained in real time and recovery times measured in milliseconds. The attainment and maintenance of survivability consume resources in system development, operation, and evolution. The resources allocated to a system s survivability should be based on the costs and risks to an organization associated with the loss of essential services.