GTP-C

GTP-C is the control-plane sibling of GTP-U. Where GTP-U carries the user's data through tunnels, GTP-C is the signalling that creates, modifies and deletes those tunnels and manages the sessions behind them. In EPC it runs on interfaces like S11 (MME to S-GW) and S5/S8 (S-GW to P-GW), handling bearer setup, updates during mobility, and teardown. It's how the control plane tells the gateways "build this user a tunnel with these QoS parameters."

In pure 5G SA the equivalent job is done differently — PFCP programs the user plane and the SBI handles session control — so native 5G doesn't use GTP-C between core functions. Where it stays relevant is interworking: when a device moves between 5G and 4G, or when an operator runs the two together, GTP-C appears on the EPC side and at the interworking interfaces (such as the S5/S8-based path used by the SMF+PGW-C and UPF+PGW-U combined nodes). So even in a 5G-centric shop, GTP-C is a protocol you keep around for the 4G estate and the seams between generations.