ontroller performs network-resource allocation by relying on a path computation entity that could be integrated with the SDN controller. The data center contains both an optical steering domain and a packet steering domain. The optical steering domain conducts traffic steering for large aggregated flows. The entry point is an ROADM, which either forwards a wavelength flow to the optical domain or sends it to the packet domain for fine-grained processing. After a wavelength flow has gone through the needed VNFs, it is steered back to the ROADM. The flow is controlled to route either back to the packet domain for fine-grained processing, or forward to the optical domain for high-capacity optical processing. SDN controller Cloud manager Operator's Internet Optical steering domain access/metro Pod 2 Pod n Optical steering domain Operator's DC/cloud WSS2 WSS3 Out Figure 5.11 Optical service function chaining framework. 5.2 Service Function Chaining 5.2.3.1 Service Placement in Optical NFV As shown above, optical switching and packet switching can be jointly employed to simultaneously address the efficiency and the flexibility requirements of VNF chaining. Optical switching serves as the "back- bone" of a DC network to steer large flows to pods in the form of wave- length. Upon arrival to the pod, the flow is converted to packet traffic, and steered across the servers where the needed VNFs are hosted. The flow will need to be converted back to wavelength after it has been processed by all the needed VNFs in that given pod. For each pod the traffic of a given VNF chain visits, an O/E/O conversion is needed between the pod and the optical steering domain. Note that for small flows, it could be more efficient to go through their service chain in a pure packet form, for example, a parallel packet steering domain that handles small flows. The data plane of the referenced architecture consists of an opera- tor's access/metro networks, and an operator's DC/cloud. Traffic flows destined to the same set of core VNFs are aggrega