unction virtualization (NFV) will enable the launch of network slices to meet the requirements of various 5G use cases in a more cost efficient way [5]. There are already prototype implementations of RAN slicing in wireless mobile networks, which enable the placement of virtual CN functions including virtual PDN gateway, serving gateway, mobility management entity [6-8], and RAN network functions including virtual base station (vBS) and base band units (BBUs) [9-11] in the framework of SDN. However, the reliability problems in SDN/NFV supported RAN slices have been poorly addressed [12]. We assume that a slice is constituted by a set of VNFs and virtual links. Leveraging NFV technology, multiple types of VNFs (e.g. vBSs and vSGWs) of a slice are run as instances on servers in data centers or network nodes [13, 14], in which failures may occur occasionally due to servers being powered down for maintenance, software faults (e.g. firmware errors), or misconfiguration of servers [15-17]. Therefore, it is imperative to develop effective mechanisms for link/node monitoring and Radio Access Network Slicing and Virtualization for 5G Vertical Industries, First Edition. Edited by Lei Zhang, Arman Farhang, Gang Feng, and Oluwakayode Onireti. © 2021 John Wiley & Sons Ltd. Published 2021 by John Wiley & Sons Ltd. 10 Robust RAN Slicing slice recovery to relieve RAN slices from substrate link and node failures, especially for the use cases of ultra-reliable machine-type communications [5]. Besides link and network function failures in SDN/NFV supported RAN slices, which will trigger a RAN slice recovery process, the uncertain traffic demands of users in a RAN slice may also trigger the reconfiguration of a RAN slice, as the link resources allocated to a RAN slice are assumed to be deterministic parameters and shared among the users within the same RAN slice. However, variation in the traffic demands of these users results in uncertain (stochastic) aggregated demands of the link resources of a RAN slice, which may in turn violat