ce The Xn-U uses the same transport and protocol stack (3GPP TS 38.424) as NG-U, i.e. GTP-U/UDP/IP-based protocol stack as shown in Figure 3.3.6. The Xn-U supports three different types of payloads - PDCP Service Data Units (SDUs) (e.g. in case of DRB-level data forwarding upon handover), Service Data Adaptation Protocol (SDAP) SDUs (e.g. in the case of PDU session-level data forwarding upon handover), or PDCP PDUs (e.g. in the case of dual connectivity). For dual connectivity, the Xn-U uses an NR user plane (NR-U)¹0 protocol (3GPP TS 38.425); that is, the content of the GTP-U container it uses and the functionality it supports are different to that of NG-U. As mentioned above, this user-plane protocol is common to Xn and F1 (described in Section 4.2). Unlike the PDU session user plane protocol (3GPP TS 38.415) defined for NG-U, in the NR-U protocol GTP-U tunnels are mapped to NG-RAN bearers (corresponding to the DRBs), not PDU sessions. This is because even though the 5GC stopped using the notion of bearers, these are still present between UE and NG-RAN. Furthermore, one additional reason for defining user plane enhancements for Xn and F1 network interfaces is that these interfaces are considered somewhat less reliable, compared with core network interfaces. Moreover, some NG-RAN network nodes may have smaller buffers. All this may lead to congestion on network interfaces and packet loss, which is why flow control and in-sequence delivery mechanisms have to be defined, which also helps when packet loss over the air interface occurs. Primary NR-U functions are: Data transfer: transfer of data between NG-RAN nodes to support dual connectivity or CU/DU split deployment. Flow control: enabling a NG-RAN node to provide feedback information associated with the data flow received from a second NG-RAN node. Retransmissions: allowing retransmissions through the same node the data were forwarded to or through a different node. Transfer of assistance information: transfer of radio-related assistance information when the ra