This Recommendation defines quality of service (QoS) assurance mechanisms for Quantum Key Distribution Networks (QKDNs) based on Software-Defined Networking (SDN). By separating the control and data planes, SDN enables centralized network control, programmability, and flexible resource management, providing an efficient foundation for end-to-end QKDN operation. Building on the SDN architecture specified in ITU-T Y.3805 and ITU-T Y.3820, this Recommendation specifies QoS assurance mechanisms that leverage SDN control capabilities to improve the performance, reliability, and service quality of QKDNs.