GTP (GPRS Tunnelling Protocol)
Updated
GTP is the family of protocols that mobile core networks use to carry data sessions between network nodes: GTP-U carries the user’s packets inside tunnels, GTP-C sets those tunnels up. GTPv2-C, used in 4G, is specified in 3GPP TS 29.274.
In practice
GTP tunnels are what carry a roaming device’s traffic from the visited network back to its home gateway.