Software-Defined Networking (SDN) enables the separation of data and control planes. Today, it is in use in several practical networks. Research on SDN is yet to focus on Transport Networks in a significant way. To provide efficient services at low cost for future technologies, such as 5G and beyond, the Transport Network will need to support network functions; adapt the network to different applications' traffic; provide high bandwidth at low latencies; integrate with other networks; etc. By fulfilling these requirements, a Transport Software-Defined Network (T-SDN) can become a fundamental part of the telecommunication infrastructure. However, most current SDN solutions have been developed for Layer 3, and cannot be directly applied to a T-SDN without proper adaptations, as Transport Networks have several characteristics that are different from those of Layer 3 networks. In particular, the design of a T-SDN control plane must address heterogeneity in terms of protocols and administrative network areas; as well as high reliability. A hierarchical control plane is suitable to support these characteristics. Accordingly, in this study, we analyze how to design a robust hierarchical control plane for T-SDNs. We discuss how resiliency against random failures can be provided through redundancy; and how survivability against correlated failures (such as disasters) can be achieved by effectively choosing network nodes where to place control-plane elements, and deciding how to route control-plane traffic. We formulate an Integer Linear Program to design a hierarchical, failure- and disaster-resilient T-SDN control plane. We also propose a heuristic for post-failure switch-controller reassignment. We compare our model with a disaster-unaware control-plane design whose objective is to reduce network-resource utilization. Our results show that we can achieve much higher disaster and failure resiliency, at the cost of slightly larger network-resource utilization.
