GPRS Tunnelling Protocol is employed on the Gn interface in order to tunnel user data between different GSN (GPRS Support Node). Version zero of the protocol supports both signalling and user data under one generic header. It can be used with UDP (User Datagram Protocol) or TCP (Transmission Control Protocol) on the registered port 3386. GTP version one is used only on UDP. The control plane protocol GTP-C (Control) using registered port 2123 and the user plane protocol GTP-U (User) using registered port 2152.