Achieving dependability in software-defined networking — A perspective

In this paper we take a closer look at the operation of software defined networking (SDN) in intra-domain networks. The focus is on the dependability issues related to interworking of SDN controllers, network OS (NOS), and forwarding in the data plane. Both the separation of the control and data planes, and the (virtually) centralized control processes, are challenging from a dependability perspective. In particular, consistency in operation and information is a challenge, both between the control and data planes, but also within a given plane. To ensure the necessary level of consistency there could be a conflict with the strict realtime requirements given by the per-flow operation of the SDN controller. A principle system model is introduced to discuss the consistency challenge, and to point out undesirable cyclic dependencies between functions that are necessary to configure and operate SDN. The separation of control processing and forwarding do also introduce structural vulnerabilities, which are exemplified.