GTP-U
GPRS Tunneling Protocol - User Plane: the tunneling protocol that encapsulates user data packets between RAN and UPF (N3) and between UPFs (N9) using GTP-U tunnel endpoints (TEIDs).
GTP-U is the tunnel that carries a user's actual data packets across the mobile core. On N3 (gNB to UPF) and N9 (UPF to UPF), each packet gets wrapped in a GTP-U header carrying a Tunnel Endpoint Identifier (TEID) that tells the receiving end which session and bearer/flow the packet belongs to. The original IP packet rides inside as payload, untouched. It runs over UDP, and the TEID — not the inner IP address — is what the endpoints use to demultiplex traffic.
The natural question is why tunnel at all instead of just routing the user's IP packet. Mobility is the answer. A device's IP address has to stay stable as it moves between cells and gNBs, but the path through the network changes; the tunnel decouples the user's addressing from the transport routing, so the network can re-anchor or hand over the path without the device ever renumbering. It also keeps per-flow context (the TEID) that lets the network apply QoS and charging to the right traffic.
Frequently asked questions
- Why use GTP-U tunnels instead of plain IP routing?
- Because the user's IP address must stay constant while the device moves, but the network path underneath changes constantly with handovers and re-anchoring. Tunnelling separates the user's IP (the inner packet) from the transport network's routing (the outer header and TEID), so the core can move the path without the device having to change its address or drop its sessions. The TEID also tags each packet's session/flow so QoS and charging land on the right traffic.
Related terms
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