  As an effective approach to thwarting advanced attacks, moving target defense
(MTD) has been applied to various domains. Previous works on MTD, however,
mainly focus on deciding the sequence of system configurations to be used and
have largely ignored the equally important timing problem. Given that both the
migration cost and attack time vary over system configurations, it is crucial
to jointly optimize the spatial and temporal decisions in MTD to better protect
the system from persistent threats. In this work, we propose a Stackelberg game
model for MTD where the defender commits to a joint migration and timing
strategy to cope with configuration-dependent migration cost and attack time
distribution. The defender's problem is formulated as a semi-Markovian decision
process and a nearly optimal MTD strategy is derived by exploiting the unique
structure of the game.
