This TS defines the GPRS Tunnelling Protocol (GTP), i.e. the protocol between GSN nodes in the GPRS backbone network. It includes both the GTP signalling and data transfer procedures. It also lists the messages and information elements used by the GTP based charging protocol GTP', which is described in TS 12.15
GTP is defined both for the Gn interface, i.e. the interface between GSNs within a PLMN, and the Gp interface between GSNs in different PLMNs. GTP' is defined for the interface between CDR generating functional network elements and Charging Gateway(s) within a PLMN. Charging Gateway(s) and GTP' protocol are optional, as the Charging Gateway Functionalities may either be located in separate network elements (Charging Gateways), or alternatively be embedded into the CDR generating network elements (GSNs) when the GSN-CGF interface is not necessarily visible outside the network element. These interfaces relevant to GTP are between the grey boxes shown in the figure below.