OpenFlow: Why latency does matter

In the OpenFlow framework, packet forwarding (data plane) and routing decisions (control plane) run on different devices. OpenFlow switches are in charge of packet forwarding, whereas a Controller, which can be situated very far from a networking point of view from the switches its manages, sets up switch forwarding tables on a per-flow basis. The connection between a switch and its Controller is thus of primary importance for the performances of the network. In this paper, we study the impact of the latency between an OpenFlow switch and its Controller. We show that UDP and TCP have different effects depending on the available bandwidth on the control link. Bandwidth arbitrates how many flows the Controller can process, as well as the loss rate if the system is under heavy load, while latency drives the overall behavior of the network, that is the time to reach its full capacity. Finally, we propose solutions to mitigate the phenomenons we outline.