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Software Defined Networking (SDN) is an emerging network control paradigm focused on logical centralization and programmability. At the same time, distributed routing protocols, most notably OSPF and IS-IS, are still prevalent in IP networks, as they provide shortest path routing, fast topological convergence after network failures, and, perhaps most importantly, the confidence based on decades of reliable operation. Therefore, a hybrid SDN/OSPF operation remains a desirable proposition. In this paper, we propose a new method of hybrid SDN/OSPF operation. Our method is different from other hybrid approaches, as it uses SDN nodes to partition an OSPF domain into sub-domains thereby achieving the traffic engineering capabilities comparable to full SDN operation. We place SDN-enabled routers as subdomain border nodes, while the operation of the OSPF protocol continues unaffected. In this way, the SDN controller can tune routing protocol updates for traffic engineering purposes before they are flooded into sub-domains. While local routing inside sub-domains remains stable at all times, inter-sub-domain routes can be optimized by determining the routes in each traversed sub-domain. As the majority of traffic in non-trivial topologies has to traverse multiple subdomains, our simulation results confirm that a few SDN nodes allow traffic engineering up to a degree that renders full SDN deployment unnecessary.

Hybrid IP networks that use both control paradigms -distributed and centralized -promise the best of two worlds: programmability of SDN, and reliability and fault tolerance of distributed routing protocols, like OSPF. Typically, a hybrid network deploys SDN to control prioritized traffic and OSPF to assure care-free operation of best effort traffic. We propose a new hybrid network architecture, called SDN Partitioning (SDNp), which establishes centralized control over the distributed routing protocol by partitioning the topology into sub-domains with SDN-enabled border nodes. In our approach, OSPF's routing updates have to traverse SDN border nodes to reach neighboring sub-domains. This allows the central controller to modify how sub-domains view one another, which in turn allows to steer inter-subdomain traffic. The degree of dynamic control against simplicity of OSPF can be traded off by adjusting the size of the sub-domains. This paper studies the technical requirements, presents a novel scheme for balanced topology partitioning, and provides the corresponding network management models for SDNp. Our performance evaluation shows that -already in its minimum configuration with two sub-domains -SDN Partitioning provides significant improvements in all respects compared to legacy routing protocols, whereas smaller sub-domains provide network control capabilities comparable to full SDN deployment.

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