3GPP 38.300 v19.3.0 — the document's own text
4.3.1.1 NG User Plane
Taught in 4. What NR is — the shape of the 5G radio network (TSG RAN — the group that specifies the radio, in depth), 8. Where the radio network meets the core (TSG RAN — the group that specifies the radio, in depth), 2. The six working groups and the line between them (TSG RAN — the group that specifies the radio, overview), 4. What NR actually is, in the shape its own document gives it (TSG RAN — the group that specifies the radio, overview), 2. The six groups, and what each one worries about (TSG RAN — the group that specifies the radio, quick start), 3. What NR is, read off the document itself (TSG RAN — the group that specifies the radio, quick start).
The NG user plane interface (NG-U) is defined between the NG-RAN node and the UPF. The user plane protocol stack of the NG interface is shown on Figure 4.3.1.1-1. The transport network layer is built on IP transport and GTP-U is used on top of UDP/IP to carry the user plane PDUs between the NG-RAN node and the UPF.
NG-U provides non-guaranteed delivery of user plane PDUs between the NG-RAN node and the UPF.