Using OpenFlow to extend software-defined networking (SDN) to the optical layer is a compelling prospect for enterprises seeking to achieve joint orchestration of information technology (IT) and network resources for cloud services, to virtualize the network and to more simply manage interconnections of distributed data centers that require synchronization.

Today's fragmented, specialized management and control approaches are fraught with proprietary protocols and management systems, limited scalability and configuration complexities. With an OpenFlow-enabled transport network, an enterprise could instead engage in a kind of "one-stop shopping" for control of cloud computing, storage and networking resources - all via one, unified application programming interface (API). The benefits could include significantly simplified configuration, management and scaling of large-scale enterprise infrastructures through integration and automation.

That's a new role for OpenFlow, demanding strategic tailoring of the protocol for the optical transport domain. Demonstration and development of the capability are closely watched by enterprises that are under incessant pressure to cost-effectively meet ever-increasing demand for bandwidth and services.

Virtualization's New Frontier
Servers and storage have been virtualized in the enterprise; the next great frontier for virtualization is the network.

Because of the substantial cost savings and performance benefits that it can deliver, SDN-based virtualization is of prime interest to enterprises for a wide range of applications. OpenFlow has emerged as one of the most popular SDN protocols. Web 2.0 network operators and national research and education network (NREN) operators, especially, like OpenFlow.

With OpenFlow, an abstraction of the network's packet switches can be generated and flow-forwarding behavior can be specified across an infrastructure via an external controller. Operations can be substantially automated and streamlined by breaking up the monolithically integrated control and forwarding paradigm of today's switches.

Using OpenFlow, could SDN be extended across layers and create a scenario in which - with a single instruction - the controller could jointly create virtual machines and enable enterprise network administrators to reserve computing, networking and storage resources in one stroke?

It is an obviously compelling notion for enterprise network staffs who desperately need to simplify operations. However, the problem is OpenFlow deployment and development has largely been limited to the electrical packet layer, whereas the interconnection beyond the data center is typically comprised of optical transport technology. Furthermore, the optical domain is where things get hazy for many enterprise network administrators. Their comfort zone tends to be packets - not wavelengths and optics.

The result is that cloud computing is currently decoupled from the transport networking control and operation. The network exists as a static, separated entity in today's cloud implementations. There is no interaction between cloud computing processes and the statically configured network. The two are not in any way interoperable; they speak different languages.

Converging cloud computing and networking requires a more dynamic mode of control and operation, but enterprises largely have judged integrating management of the optical network into the data-center environment to be too complex.

To extend OpenFlow from its established role in the electrical packet domain to the optical layer (and, thereby, extend SDN across multiple network layers), a range of optical-specific concerns must be tackled.

Crafting and Experimenting
Within the European Commission's FP7 ICT Work Programme is a collaborative project, "OpenFlow in Europe - Linking Infrastructure and Applications" (OFELIA), that provides researchers with a test bed in which to experiment with SDN applications and virtual multi-layer networks over shared network infrastructure.

Via standardized, secure interfaces through GÉANT, a high-bandwidth interconnection of European R&E networks, researchers develop, run and control experiments using packet switches and application servers at the University of Essex and seven other test-bed facilities throughout Europe.

OFELIA hosts a prototype implementation of dynamic control of wavelength-switched optical networks via OpenFlow. Bandwidth, latency and power consumption can be adjusted to meet the specific requirements of specific applications.

To make it happen, key OpenFlow additions had to be engineered in order for the protocol to effectively control the optical domain. Optical-specific considerations were required to adapt OpenFlow from the packet world. A packet can travel from any ingress to any egress port in an electrical switch or from any time slot in a time-division multiplexing (TDM) device. The optical domain, however, introduces strict switching constraints, with regard to wavelength continuity, optical impairments, optical power leveling on the line side, etc.

Augmenting OpenFlow to address those optical-specific concerns has resulted in an OFELIA prototype that demonstrates a truly transparent, wavelength-switched optical network. The research community is able to experiment with the capability via a flexible, Web-services approach; commercial enterprises, too, are interested in trialing the capability for their specific applications and environments.

OpenFlow is not sufficient in itself to enable the complete transformation that enterprise network administrators envision, to SDN-enable virtualization across all layers of their infrastructures. The additions to OpenFlow that were engineered for the OFELIA test bed provide only the bridge between the optical layer and packet layer and allow integration into a cloud operating system such as OpenStack.

But that is one very important bridge, and the promise for enterprise network administrators is considerable. The OpenFlow innovation could seamlessly integrate the optical transport network under a common management umbrella with an enterprise's routers and switches - all via one familiar interface. Management of the optical domain could become as simple as the management of Ethernet boxes - using an encapsulation of virtual resources that enterprise network administrators could manage via typical and familiar infrastructure. That's a significant breakthrough. With many enterprises already considering usage of an OpenFlow-based control for their packet networks, extending the framework to the wavelength-switched optical layer would be a natural migration.

Virtualization has developed over phases in enterprise networking. First, resource virtualization inside data centers delivered economic savings through enhanced utilization, scalability and redundancy. Data-center virtualization conveyed greater infrastructure flexibility, higher availability and better workload balancing. The next frontier, network virtualization, promises true platform agility and, with it, a host of long-sought-after enterprise capabilities: capacity on-demand, adaptive infrastructure and dynamic service automation, among them. Adapting OpenFlow and extending SDN to the optical transport domain comprise an important step toward that vision.