On the Availability Property and Its Metric for NPP IACS

The article considers some vital cybersecurity issues regarding industrial control. Proposing more realistic confidentiality, integrity, and availability (CIA) triad, the article advocates availability as the significant property for industrial automation and control systems (IACS) and the associated problem of availability assessment to be the most urgent. The challenges arising during the IACS availability assessment are highlighted. The results of the analysis of availability interpretations in international regulatory documents are presented; the IEC 62443 definition is proved to be the most appropriate for the quantitative availability assessment. The availability reference model with such domains as Time-dependent parameters, Delay, Platform, System, Function, and Availability metric is proposed. Using this model, the availability metric based on the time of fulfillment of IACS functions, or delay, as a measure is derived, and the technique for its calculation according to the system domain is discussed as well.