• SDN
• Reliability
• OpenFlow
• Cluster
• Atomix
• ONOS
• scalability

Distributed systems play an increasingly important role in Software Defined Networks (SDN), where replicating network state information is essential to balance the controller load and provide resilience in case of failures. Such an example, if the traffic requests on the network exceed expected levels for a period, it is important that the SDN system continues to work properly seamlessly serving the incoming requests. For guaranteeing the required level of scalability and reliability the open-source ONOS SDN controller adopted a logically centralized, but physically distributed architecture. Using this solution, a number of controller instances exploit the Java-based Atomix framework to share and synchronize the network state information. However, previous work demonstrated that under traffic requests overload the ONOS controller may show critical behavior resulting in the loss of requests to the network, and with the entire SDN network eventually crashing. This work first provides a literature overview on the topic of multi-instance SDN control of OpenFlow networks. Then, an accurate SDN environment is deployed with an emulated data plane based on OpenFlow virtual switches and a multi-instance ONOS controller. In this thesis some measurements have been performed to evaluate the scalability and the reliability provided by the system.