  In this paper we introduce a notion of fault-tolerance distance between
labeled transition systems. Intuitively, this notion of distance measures the
degree of fault-tolerance exhibited by a candidate system. In practice, there
are different kinds of fault-tolerance, here we restrict ourselves to the
analysis of masking fault-tolerance because it is often a highly desirable goal
for critical systems. Roughly speaking, a system is masking fault-tolerant when
it is able to completely mask the faults, not allowing these faults to have any
observable consequences for the users. We capture masking fault-tolerance via a
simulation relation, which is accompanied by a corresponding game
characterization. We enrich the resulting games with quantitative objectives to
define the notion of masking fault-tolerance distance. Furthermore, we
investigate the basic properties of this notion of masking distance, and we
prove that it is a directed pseudo metric. We have implemented our approach in
a prototype tool that automatically compute the masking distance between a
nominal system and a fault-tolerant version of it. We have used this tool to
measure the masking tolerance of multiple instances of several case studies
