  The concept of SDN has emerged as a way to address numerous challenges with
traditional network architectures by decoupling network control and forwarding.
So far, the use cases for SDN mainly targeted data-center applications. This
article considers SDN for network carriers, facing operation of large-scale
networks with millions of customers, multiple technologies, and high
availability demands. With specific carrier-grade features such as scalability,
reliability, flexibility, modularity, and virtualization in mind, the SPARC EU
project has developed the SPARC SplitArchitecture concept. The
SplitArchitecture control plane allows hierarchical layering of several control
plane layers which can be flexibly mapped to data plane layers. Between control
layers open interfaces are used. Furthermore, SplitArchitecture proposes an
additional split of forwarding and processing functions in data path elements,
enabling switch based OAM functionality and handling of tunneling techniques.
The SplitArchitecture concept is evaluated in a prototype demonstrating an SDN
version of BRAS: the floating BRAS. The floating BRAS allows creation of
residential Internet access services with dynamically managed BRAS instances.
The demo is based on a controller enabling protected MPLS transport connections
spanning SDN-controlled aggregation and IP/MPLS-based core domains. The
prototype showcases how SplitArchitecture enables virtualization of service
nodes in an SDN-controlled network, leading to increased flexibility in
configuration and placement of service creation functions. Overall, the results
show that it is technically and economically beneficial to apply SDN, and
specifically the SplitArchitecture concept, to the carrier domain.
