dio layer is not managed in the same node as the data control. Data transfer is also used during the mobility operation, though in this case data are trans- ferred without the NR-U extension, so all the NR-U functions are not available. 10 NR User Plane (NR-U) should not be confused with NR Unlicensed (NR-U), for which the same acronym is used in 3GPP specifications. 3 5G System Overview The NR-U functions are implemented as a set of additional messages carried inside the NR Container in a GTP-U extension header of a GTP-UPDU. The messages include: Downlink User Data Downlink Data Delivery Status (DDDS) Assistance Information Data. The purpose of the Downlink User Data message is to carry the NR-U sequence numbers, to trigger the receiving node (e.g. gNB-DU in the case of downlink) to report various information (see below) and to request the receiving node to discard certain packets that are buffered. The NR-U sequence number is assigned consecutively for each and every Downlink User Data message transmitted to detect possible loss over the radio interface. The receiving node may feed back the DDDS message carrying information on PDCP PDUs that have been successfully delivered to the UE, buffer status, lost NR-U sequence numbers, and some other information. The main purpose of this message is to prevent buffer underrun and overrun in the gNB-DU. Sending of the DDDS is up to the implemen- tation, but can be requested by the sending node using a specific flag in the Downlink User Data message. While the DDDS provides a coarse estimate of the UE downlink rate, the receiving node can also provide the Assistance Information Data message to report more precise radio-related information, such as: average channel quality indicator (CQI), average Hybrid ARQ (HARQ) retransmissions, etc. This information helps the sending node schedule downlink traffic for the UE. The definitions of the NR-U procedures in the specification (3GPP TS 38.425) use some- what obscure terms "the node hosting the NR PDCP entity" and "the correspon