  In order to automate actions, such as defences against network attacks, one
needs to quantify their efficiency. This can subsequently be used in
post-evaluation, learning, etc. In order to quantify the defence efficiency as
a function of the impact of the defence and its total cost, we present several
natural requirements from such a definition of efficiency and provide a natural
definition that complies with these requirements. Next, we precisely
characterize our definition of efficiency by the axiomatic approach; namely, we
strengthen the original requirements from such a definition and prove that the
given definition is the unique definition that satisfies those requirements.
Finally, we generalize the definition to the case of any number of input
variables in two natural ways, and compare these generalizations.
