Software-Defined Networks as Databases

In software-defined networks (SDN), the separation of the control and data-plane moves the concurrency control from the data-plane to a separate, now logically centralized controller program. As a result, despite its intention to simplify programming, the separation forces the programmer to deal with a spectrum of concurrent events (e.g. execution of controller programs, in-flight packets), a task that is notoriously challenging and error-prone. It is not even clear what concurrency problems the programmer shall account for. Although early stage works propose specific correctness conditions and point solutions, a comprehensive study is still lacking.

Most existing work focuses on the concurrency problem we call atomicity, which concerns one single network-wide update transaction. We use network-wide transaction (or transaction) to refer to a logical network operation that consists of potentially multiple switch-level updates. An atomicity failure scenario is shown in the figure (the red transaction spanning over switches 1; 2; 4) when in-flight packets during the transaction are processed by a mixture of switches with rules before or after the transaction. In addition to atomicity, we identify concurrency problems arising from multiple transactions, which we call the consistency and isolation. To the best of our knowledge, they are not addressed in existing works. Section 3 will connect atomicity, consistency, and isolation to the well-studied ACID transactional semantics in databases literature.