Reliability and Availability in Virtualized Networks: A Survey on Standards, Modeling Approaches, and Research Challenges

Virtualized networks are built on the principle of replacing bulky and rather static hardware-based functions with software-based, virtualized instances of those functions, enabling more agile and cost-effective communication infrastructures. However, this shift brings new challenges for ensuring reliability and availability due to increased dependencies among system components introduced by virtualization technologies. Reliability, i.e., the ability of a system to perform regularly under specified conditions, and availability, i.e., the probability of a system of being ready to use, are critical requirements that must be guaranteed to maintain seamless network operations. Accurate modeling of these aspects is crucial for designing robust, fault-tolerant virtualized systems that can withstand service disruptions, ensuring continuous user access. Accordingly, this survey focuses on reliability and availability attributes of virtualized networks from a modeling perspective. We first introduce the Network Function Virtualization (NFV) architecture and relevant definitions, followed by a review of the European Telecommunications Standards Institute (ETSI) standardization efforts. We then explore key modeling formalisms and illustrate their use in characterizing failure and repair behaviors. A survey of related literature and supporting software tools is provided, along with a discussion on lessons learned and open research challenges to guide future work in designing fault-tolerant NFV systems.