Hierarchical Software-Defined Networks for Wide-Area Air Traffic Management Networks

With new data hungry application flooding the air traffic management (ATM) world, driven by system-wide information management (SWIM), advanced aeronautical information management (AIM) data exchange, and the latest remote virtual tower (RVT) video applications, the ATM industry is looking at modern, safe, and secure network technologies such as Software Defined Networks (SDN). Just a few years ago, only forward-leaning organizations embraced this compelling new concept to programmatically control their network resources.Today, SDN is acknowledged as a major building block of next generation ATM networks. The SDN concept separates the routing decisions, the `control plane', from the actual data forwarding hardware, the `data plane'. In SDN the routing decisions are performed in a separate SDN controller. Depending on the network architecture, SDN controllers can be deployed in a distributed or centralized fashion.In the distributed approach, SDN controllers are deployed at various key locations within the network, each responsible for a sub-set of network devices with no communication to other SDN controllers. Although this concept allows for lean deployments at each site it may lead to unstable and/or asymmetric routing because of sub-optimal routing decisions by each individual controller due to the lack of information of the complete network.To overcome this, in the centralized approach, clusters of synchronized SDN controllers are deployed at central locations within the network, each being able to take over the whole set of network devices. This allows for end-to-end optimized routing decisions as each cluster has all the network information available and the cluster members operate upon a synchronized information base.A fault case analysis shows that the centralized approach may fail if the network becomes partitioned. Partitioning occurs if the SDN cluster loses connectivity to a portion of the network. These network elements are then considered orphaned. To ensure continuity of service, a hierarchical SDN deployment approach provides the best solution, where the central, synchronized SDN controllers are augmented by dedicated, distributed SDN controllers being able to take over orphaned devices.This paper describes and analyzes a hierarchical SDN architecture where a centralized SDN controller cluster manages the network during regular operation, and making use of a distributed SDN approach to ensure business continuity during degraded operation. Finally, network performance advantages of the hierarchical approach compared to conventional central SDN cluster deployments are highlighted.