NGAP
Next Generation Application Protocol: the control-plane protocol between the (R)AN and AMF over the N2 interface, handling UE context, handover, and paging procedures.
NGAP is the control-plane protocol on N2, the link between the gNB and the AMF. It does two distinct kinds of work. Some NGAP procedures are about a specific UE — setting up and modifying its context in the RAN, managing PDU session resources on the radio side, handovers, and paging. Others are about the interface itself — NG Setup when a gNB first connects, configuration updates, overload control. So it carries both per-UE signalling and the housekeeping that keeps the N2 association healthy.
A point worth holding onto: NGAP also ferries NAS messages between the UE and the AMF. The NAS dialogue is logically UE-to-core, but over N2 it's wrapped inside NGAP transport messages, so on the wire you see NAS riding within NGAP. NGAP runs over SCTP for reliable, ordered delivery. It's the 5G equivalent of S1AP in LTE — same role, between RAN and the mobility core — and if you're tracing attach or handover problems on the core-facing side of the gNB, NGAP is the protocol you're reading.
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