The IETF has produced a considerable number of data models in the YANG modelling language. The majority of these models are used to model devices or monolithic functions and they allow access for configuration and to read operational status. A small number of YANG modules have been defined to model services (for example, the Layer Three Virtual Private Network Service Model produced by the L3SM working group). This document briefly sets out the scope of and purpose of an IETF service model, and it also shows where a service model might fit into a Software Defined Networking architecture. Note that service models do not make any assumption of how a service is actually engineered and delivered to a customer; details of how network protocols and devices are engineered to deliver a service are captured in other models that are not exposed through the Customer- Provider Interface.