Service Provider SDN – Openness From The Top Down Is Critical
Software-Defined Networking (SDN) has already begun to reshape the networking landscape, yet it is still unclear what SDN means in different segments. What is required in a data center, research and education network, enterprise campus, or service provider Wide Area Network (WAN) varies widely.
In the “role-of-software” debates currently taking place, the focus has largely been on control plane – whether it should be embedded or abstracted, distributed or centralized, etc. Perhaps more important to operators is a focus on making software open as well, because there is little point in adopting software that remains as closed and proprietary as hardware-based systems are today.
And for service providers that operate the world’s largest WANs and support many customers across multiple service frameworks, openness is critical.
SDN Layers And Interfaces Through The Service Provider Lens
The Open Networking Foundation’s (ONF) seminal and foundational SDN architecture white paper defines three layers – the application layer, the control layer and the infrastructure layer – connected by open APIs. However, there is no real consensus on what defines the application and control layers.
For example in the data center, the entire network may consist of a single network domain, operated by and for a single organization. For large organizations, the data center may only support a few applications or, in some cases, even one application. Typically, the focus is on overall cost reduction, with OpEx predominant. As a result, the network is architected to support server-to-server (i.e., east-west) transfers, and the SDN control layer and underlying hardware may be quite streamlined to support this heterogeneous environment.
In a service provider network, however, the SDN software model is very different. Carrier networks consist of many service-specific domains, spanning access-to-core, legacy-to-next-generation technologies, and employ multiple vendors’ equipment and software. To deliver the service breadth necessary, a carrier SDN architecture will leverage multi-layer domains, such as packet and transport (optical circuits and/or wavelengths), that span broad geographical areas. SDN also facilitates the design and deployment of highly scalable, multi-tenant networks (á la cloud), and the granular policy mechanisms necessary for tomorrow’s dynamics networks. Service provider networks support multiple service frameworks and types, as well as many customers, and must continually evolve in the pursuit of new revenues and to offer new services. The provider does not control customer behaviors except through policies—if and as enforced—and indirectly through pricing.
These realities suggest a quite different view of the control and application layers in the SDN architectural diagram.
In the service provider WAN context, “applications” mean customer-facing, revenue-generating service applications. Such applications use network connection services, but are only partly defined by them. SDN enables the software that instantiates such service applications to be portable from one network platform to another, and to remain stable in the face of continuing network platform evolutions.
Because the service provider WAN infrastructure layer is relatively more complex than in the data center, the control layer is in turn relatively complex and functionally rich. Logically, the control layer assumes responsibility for path and flow connection control, management and related functions, which abstracts network functions from the underlying details — including customer management, customer interface, billing and payment — that today reside in other parts of the back-office system environment.
Openness – Northbound
In the service provider environment, the applications layer encompasses revenue-generating service application software. Service innovation requires abstraction of the application layer from the control and infrastructure layers. The control and infrastructure layers should comprise a network platform from the application layer’s point of view – and application software should be fully portable from one such network platform to another.
To achieve application portability, the northbound API, which connects the application and control layers, must operate autonomously from specific technologies, architectures or implementations of either control or infrastructure layers. Abstraction frees business applications from the details in interacting with the network, specifying only what to connect, and how. The semantics are expressed in strictly parametric terms: desired or required bandwidth, latency, quality, availability; when, and – potentially – at what acceptable price or cost. The control layer’s role is to decompose and deliver network service requests, efficiently and from available physical network resources.
Openness – Southbound
In the service provider network, the SDN control layer is as important in differentiating business value as the application layer. Service providers seek flexible control of network features and functions to innovate new services, and streamline network operations to sustain a competitive advantage. This will not be viable unless the control layer is abstracted from the infrastructure layer through an open southbound API.
OpenFlow is currently being embraced by dozens of hardware vendors to deliver open network programmability at a packet level. Presently, the OpenFlow standard supports Layer 2 and Layer 3 networks. Recently, the ONF embarked upon an effort to extend OpenFlow to support optical transport networks, with wireless networks under investigation. This will in turn enable multi-layer packet-optical networks that may be optimized to deliver the lowest cost per bit across the traditional communications layers.
Multi-layer networks are, and will continue to be, a reality in the service provider world. Planning and operating multi-layer networks with increased optimized efficiency and for increased monetization impact, will be a big part of the service provider WAN SDN challenge.
Open transport control, when combined with open packet control, will afford new and powerful business-impacting, software-based innovations going forward.
SDN For Service Providers Moves To Center Stage
Despite the industry’s attention, how SDN will unfold and evolve remains unclear. There are considerable differences among networking segments, and so far, the data center space has adopted SDN first. However, service provider WAN SDN is now moving to center stage thanks to the ONF’s recent charter of an Optical Transport Working Group. As service providers begin to take practical and navigable steps toward deployment of SDN, openness from the top down is critical.