An apparatus of a RAN node in which flexible feedback for user data transferred between RAN nodes is described. The DL PDCP PDUs to be delivered to a UE is transmitted over an Xn or X2 interface between RAN and NG-RAN nodes, respectively. Feedback received allows transmitting RAN node to control downlink user data flow. The feedback contains a DDDS frame that includes a highest transmitted and delivered PDCP SN indicator parameter that respectively indicates whether a highest transmitted and successfully delivered PDCP PDU is present in the DDDS message and a highest transmitted and successfully delivered PDCP SN PDU parameter that provides feedback respectively about a transmitted status of the PDU sequence and an in-sequence delivery status of the PDUs to the UE. Buffered PDUs reported by the DDDS message are removed from memory as transmitted or successfully delivered PDUs.