Ravel: Orchestrating Software-Defined Networks

Software-defined networking (SDN) use cases commonly involve multiple applications that collectively drive the network. The use cases are growing bigger and more complex, raising the need to break them into more manageable and reusable small pieces, as the design principle of separation of concerns suggests. Simultaneously, new business models [2] are driving the use cases beyond the control of one single administrator. For example, providers that lease cloud computing and network functions virtualization (NFV) are infrastructure services that host multiple user applications. The first SDN App store, recently announced by HP, is promoting SDN innovations that span over multiple applications contributed by different parties. These applications can form complex behaviors due to interactions and interleavings that depend on the runtime dynamics. For example, the action of blacklisting a path by an access control application should invoke proper response by the routing application, such that the newly prohibited path is avoided. In addition to coordination, conflict occurs when applications disagree on what states are allowable: a lightly used path favored by the load balancer application could be rejected as unsafe by access control. An operator could manually construct a master program — composition of the component applications — that statically specifies the inter-application interactions [3]. This method requires the control of a single administrator, and requires the components to commit to a fixed interface. However, the SDN ecosystem today has to accommodate use cases built from programs of disparate sources. Likewise, privacy and trust concerns hinder the naive composition approach. Besides, the master program is tightly coupled. Every time a new component is introduced, or an existing one is modified, the master program, as well as all relevant components need to be revised and tested. Programming interoperating software, in general, is expensive and challenging. We propose a complementary solution — data-centric orchestration — that coordinates runtime interleaving by coordinating data access. The data-centric solution is based on a database-centered SDN design, inspired by a transition in online commercial data management in the 1980s. At that time operating system and programming language techniques were proven to fall short, and gave rise to database systems [1], which later developed into the solution for coordinating data access and mediating between multiple users. Likewise, we shift to a database-centered design of SDN that uses a standard SQL database. While the relational data representation leads to a clean orchestration semantics, the SQL language, together with the extensions of rule and trigger, enables powerful interoperability with existing management techniques and tools. Specifically, we present Ravel, a database-centric solution to orchestrating SDN applications. Ravel takes the entire SDN network under the hood of a standard SQL database, including network configurations as stored (base) tables in the data-store, and network controls as virtual views that are derived from the store. In Ravel, applications are programs that interact with the network via its dedicated virtual view. A view is a specification that is queried on the fly: a SQL query produces the view from other views and/or the base. For example, the access control application is a program that reads and writes following view: