GPRS Tunnelling Protocol User Plane (GTP-U)
GTP-U protocol is used over S1-U, X2, S4, S5 and S8 interfaces of the Evolved Packet System (EPS). GTP-U Tunnels are used to carry encapsulated T-PDUs and signalling messages between a given pair of GTP-U Tunnel Endpoints. The Tunnel Endpoint ID (TEID) which is present in the GTP header indicates which tunnel a particular T-PDU belongs to.
The transport bearer is identified by the GTP-U TEID and the IP address (source TEID, destination TEID, source IP address, destination IP address).
3GPP TS 29.281 - General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)