1. A method of operating a data communication system to determine Software Defined Network (SDN) Quality-of-Service (QoS) for a data communication service, the method comprising: SDN computer circuitry processing SDN applications and responsively transferring SDN controller Application Programming Interface (API) calls and receiving SDN controller API responses; the SDN computer circuitry processing an SDN controller and responsively receiving the SDN controller API calls, transferring the SDN controller API responses, transferring SDN data machine API calls, and receiving SDN data machine API responses; SDN data machines receiving the SDN data machine API calls, performing SDN actions on user data responsive to the SDN data machine API calls, and transferring the SDN data machine API responses; the SDN computer circuitry processing the SDN applications and responsively measuring SDN application Key Performance Indicators (KPIs) and transferring SDN application KPI data, wherein the SDN application KPI data indicates SDN data machine Flow Descriptor Table (FDT) update times based on the SDN controller API calls for FDT updates and the SDN controller API responses for the FDT updates; the SDN computer circuitry processing the SDN controller and responsively measuring SDN controller KPIs and transferring SDN controller KPI data, wherein the SDN controller KPI data indicates additional SDN data machine FDT update times based on the SDN data machine API calls for the FDT updates and the SDN data machine API responses for the FDT updates; the SDN data machines measuring SDN data machine KPIs and transferring SDN data machine KPI data; and an SDN QoS server receiving and processing the SDN application KPI data including the SDN data machine FDT update times, the SDN controller KPI data including the additional SDN data machine FDT update times, and the SDN data machine KPI data to generate an SDN QoS score.