GTPv1-U is used to tunnel user plane data between different network nodes e.g. on the Gn, Gp, S1-U and S5 interfaces. GTPv1-U is used with UDP (User Datagram Protocol) and IPv4/IPv6 as supporting layers. In order to multiplex multiple users onto the same transport bearer (port/IP address), TEIDs (Tunnel Endpoint Identifier) are used to differentiate one traffic stream from another (the TEID is simply a numerical identity which has to be unique for a given traffic stream). The receiving end side of a GTP tunnel locally assigns the TEID value that the transmitting side must use and vice versa, assuming bidirectional transmission. TEIDs will be negotiated during the establishment of the GTP tunnel, which is achieved using GTP-C (GPRS Tunnelling Protocol – Control).