 YANG MODEL What services can be invoked from North Bound Service Ordering system? SERVICE ABSTRACTION SERVICE YANG MODEL What does the customer want? (e.g., VPN with 2 sites, Internet Access, QoS) COMMON NETWORK ABSTRACTION NETWORK YANG MODELS DEVICE MODELS What needs to be built in the network? UCPE-1 UCPE- VCE-1 VCE-2 vPE-1 (e.g., L2-VPN, QoS, BGP) SDN CONTROLLER MODEL INSTANTIATION DEVICE YANG MODELS What Vendor-specific configuration is needed? UCPE-1 vCE-1 vPE-1 uCPE-2 (e.g., uCPE from vendor 1, vCE from vendor 2) uCPE: Universal CPE, Vendor 1,2 vCE: Virtual CE. Vendor 1 vPE: Virtual PE, Vendor 1 FIGURE 3.7 YANG abstraction layers. Network Functions Virtualization The process of disaggregating a VNF is complex and a detailed discussion of all the engineering considerations and possible tradeoffs is beyond the scope of this chapter. At a high level, the process involves Identifying and classifying the external interfaces and protocols by workload. Separating the functions of the VNF around those interfaces and protocols. Identifying and separating any storage or management functions that are candidates for disaggregation. Identifying candidate functions for disaggregation for reasons other than separation of workloads, (e.g., separation of front-end processing from data storage for better fault han- dling, component reuse, reducing individual VNF complexity, ...). Identifying suitable interfaces between candidate disaggregated functions. Evaluating whether the benefits of disaggregating each function outweigh the potential costs, considering factors such as Efficiency tradeoff Fault domains and fault handling Management and orchestration complexity 3.3.2 SERVICE CHAINING A network service might be composed of a source, a destination, and a set of intermediate intercon- nected VNFs that process the traffic from the source to the destination. This is commonly referred to as service chaining (Central). This chaining of VNFs might occur in a number of scenarios, for example, Chaining based on a network service d