A method of operating a data communication system (100, 300) to determine Software Defined Network, SDN, Quality-of-Service, QoS, for a data communication service, the method comprising SDN applications (111-113, 351) executing on a computer system to deliver the data communication service by transferring SDN controller Application Programming Interface, API, calls and receiving SDN controller API responses, the method comprising: an SDN controller (121, 352) executing on the computer system to deliver the data communication service by 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 (131-133, 331-333) delivering the data communication service by 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 method characterized by: the SDN applications (111-113, 351) executing on the computer system to measure SDN QoS by measuring SDN application Key Performance Indicators, KPIs, and transferring SDN application KPI data, wherein the SDN applications (111-113, 351) measure SDN data machine Flow Descriptor Table, FDT, update times based on the SDN controller API calls and the SDN controller API responses; the SDN controller (121, 352) executing on the computer system to measure the SDN QoS by measuring SDN controller KPIs and transferring SDN controller KPI data, wherein the SDN controller (121, 352) measures additional SDN data machine FDT update times based on the SDN data machine API calls and the SDN data machine API responses; the SDN data machines (131-133, 331-333) measuring the SDN QoS by measuring SDN data machine KPIs and transferring SDN data machine KPI data; and an SDN QoS server (103, 361) to measure the SDN QoS by receiving and processing the SDN application KPI data including the SDN data machine FDT update times measured by the SDN applications, the SDN controller KPI data including the additional SDN data machine FDT update times measured by the SDN controller, and the SDN data machine KPI data to generate an SDN QoS score. an SDN controller (121, 352) executing on the computer system to deliver the data communication service by 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 (131-133, 331-333) delivering the data communication service by 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 method characterized by: the SDN applications (111-113, 351) executing on the computer system to measure SDN QoS by measuring SDN application Key Performance Indicators, KPIs, and transferring SDN application KPI data, wherein the SDN applications (111-113, 351) measure SDN data machine Flow Descriptor Table, FDT, update times based on the SDN controller API calls and the SDN controller API responses; the SDN controller (121, 352) executing on the computer system to measure the SDN QoS by measuring SDN controller KPIs and transferring SDN controller KPI data, wherein the SDN controller (121, 352) measures additional SDN data machine FDT update times based on the SDN data machine API calls and the SDN data machine API responses; the SDN data machines (131-133, 331-333) measuring the SDN QoS by measuring SDN data machine KPIs and transferring SDN data machine KPI data; and an SDN QoS server (103, 361) to measure the SDN QoS by receiving and processing the SDN application KPI data including the SDN data machine FDT update times measured by the SDN applications, the SDN controller KPI data including the additional SDN data machine FDT update times measured by the SDN controller, and the SDN data machine KPI data to generate an SDN QoS score. the SDN applications (111-113, 351) executing on the computer system to measure SDN QoS by measuring SDN application Key Performance Indicators, KPIs, and transferring SDN application KPI data, wherein the SDN applications (111-113, 351) measure SDN data machine Flow Descriptor Table, FDT, update times based on the SDN controller API calls and the SDN controller API responses; the SDN controller (121, 352) executing on the computer system to measure the SDN QoS by measuring SDN controller KPIs and transferring SDN controller KPI data, wherein the SDN controller (121, 352) measures additional SDN data machine FDT update times based on the SDN data machine API calls and the SDN data machine API responses; the SDN data machines (131-133, 331-333) measuring the SDN QoS by measuring SDN data machine KPIs and transferring SDN data machine KPI data; and an SDN QoS server (103, 361) to measure the SDN QoS by receiving and processing the SDN application KPI data including the SDN data machine FDT update times measured by the SDN applications, the SDN controller KPI data including the additional SDN data machine FDT update times measured by the SDN controller, and the SDN data machine KPI data to generate an SDN QoS score.