ity synchronization, the HFN is maintained and the source gNB provides the target gNB one reference HFN for the uplink and one for the downlink, that is, HFN and corresponding SN. In both UE and target gNB, a window-based mechanism is used for duplication detection and reordering. The occur- rence of duplicates over the air interface in the target gNB is minimized by means of PDCP-SN-based reporting at the target gNB by the UE. In the uplink, the reporting is optionally configured on as per DRB basis by the gNB and the UE initially starts by trans- mitting those reports when granted resources are in the target gNB. In the downlink, the gNB can decide when and for which bearers a report is sent, and the UE does not wait for the report to resume UL transmission [8]. The target gNB retransmits and prioritizes all downlink data forwarded by the source gNB excluding the PDCP SDUs for which the reception was acknowledged through PDCP-SN- based reporting by the UE, that is, the target gNB should initially send all forwarded PDCP SDUs with PDCP SNs, then all forwarded downlink PDCP SDUs without SNs before send- ing new data from 5GC. Lossless delivery, when a QoS flow is mapped to a different DRB at handover, requires that the old DRB to be configured in the target cell. For in-order delivery in the downlink, the target gNB should first transmit the forwarded PDCP SDUs on the old DRB before transmitting new data from 5G CN on the new DRB. In the uplink, the target gNB should not deliver data of the QoS flow from the new DRB to 5G CN before receiving the end-marker on the old DRB from the UE [8]. The UE retransmits in the target gNB all uplink PDCP SDUs starting from the oldest PDCP SDU that has not been acknowledged at RLC sublayer in the source, excluding PDCP SDUs for which the reception was acknowledged through PDCP-SN-based reporting by the target. For RLC-UM bearers the PDCP SN and HFN are reset in the target gNB; no PDCP SDUs are retransmitted in the target gNB; and the target gNB prioritizes all downlink SDAP SDUs