Software-defined networking (SDN) is a cornerstone of next-generation networks and has already led to numerous advantages for data-center networks and wide-area networks. However, SDN is not widely adopted in constrained networks, such as Wireless Sensor Networks (WSN), due to excessive control overhead, lossy medium, and in-band control channels. Therefore, a key challenge to enable Software-Defined Wireless Sensor Networks (SD-WSN) is to reduce the number of control messages required to configure the data plane. In this paper, we propose a cluster-based flow control approach in hybrid SDNs. Our approach is hybrid in the sense that it takes advantage of distributed legacy routing and centralized SDN routing. In addition, it makes a trade-off between the granularity of flow control and the communication overhead induced by the SDN controller. The approach partitions a network into clusters with minimum number of border nodes. Instead of handling the individual flows of each node, the SDN controller only manages incoming and outgoing traffic flows of clusters through border nodes, while the flows inside each cluster are controlled by a distributed legacy WSN routing algorithm. Our proof-of-concept implementations in both software and hardware show that our approach is efficient with respect to reducing the number of nodes that must be managed and the number of control messages. In comparison to benchmark solutions with and without clustering, our solution reduces communication costs for flow configuration in an SD-WSN at least by 27% and at most by 88% respectively, without degrading packet delay nor delivery rate.

软件定义网络（SDN）是下一代网络的基石，并且已经为数据中心网络和广域网带来了众多优势。但是，由于控制开销过大，介质损耗和带内控制信道过多，SDN在诸如无线传感器网络（WSN）之类的受限网络中并未得到广泛采用。因此，启用软件定义的无线传感器网络（SD-WSN）的关键挑战是减少配置数据平面所需的控制消息的数量。在本文中，我们提出了一种在混合SDN中基于集群的流控制方法。我们的方法是混合的，因为它利用了分布式旧式路由和集中式SDN路由的优势。另外，它在流控制的粒度与SDN控制器引起的通信开销之间进行了权衡。该方法将网络划分为具有最少边界节点数量的群集。SDN控制器仅管理通过边界节点的群集的传入和传出通信流，而不是处理每个节点的单独流，而每个群集内部的流由分布式旧式WSN路由算法控制。我们在软件和硬件上的概念验证实现表明，我们的方法在减少必须管理的节点数量和控制消息的数量方面是有效的。与具有和不具有群集的基准解决方案相比，我们的解决方案分别将SD-WSN中流配置的通信成本分别降低了27％和至多88％，而不会降低数据包延迟或传输速率。