Mission-critical information systems are commonly used in critical infrastructure assets such as the electric power grid, telecommunications networks, healthcare systems, water management systems and national defense. Damage or disruption of these systems could result in the loss of services and potentially serious societal consequences. Therefore, it is important to ensure that the essential services provided by these systems are reliable and dependable. This paper presents a modeling framework for service migration and relocation, which can dynamically transfer critical services from a compromised platform to other healthy platforms. The mechanism guarantees that vital services are continuously available despite malicious attacks and system failures. When the compromised platform has recovered, the services can be moved back to the platform. The modeling framework provides a means for studying the important factors that impact service migration and relocation, and how an assured service migration mechanism can be designed to increase confidence about the reliability of mission-critical information systems.