on UDP port 2152. The GTPv1-U protocol is used to exchange user data over GTP tunnels across the Sx interfaces. An IP packet for a UE (user endpoint) is encapsulated in an GTPv1-U packet and tunnelled between the P-GW and the eNodeB for transmission with respect to a UE over S1-U and S5/S8 interfaces. GTP' - charging transfer The GTP' protocol is used to transfer charging data to the Charging Gateway Function. GTP' uses TCP/UDP port 3386. Within the GPRS core network GTP is the primary protocol used in the GPRS core network. It is the protocol which allows end users of a GSM or UMTS network to move from place to place whilst continuing to connect to the Internet as if from one location at the GGSN. It does this by carrying the subscriber's data from the subscriber's current SGSN to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network. GTP-U for transfer of user data in separated tunnels for each PDP context GTP-C for control reasons including: setup and deletion of PDP contexts verification of GSN reachability updates; e.g., as subscribers move from one SGSN to another. GTP' for transfer of charging data from GSNs to the charging function. GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152. This communication happens within a single network or may, in the case of international roaming, happen internationally, probably across a GPRS roaming exchange (GRX). The Charging Gateway Function (CGF) listens to GTP' messages sent from the GSNs on TCP/UDP port 3386. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. However, this communication which occurs within one network is less standardized and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system. Use on the IuPS interface GTP-U is used on the IuPS between the GPRS core network and the RAN, however the GTP-C protocol is not used. In this case, RANAP is used as a control protocol and establishes GTP-U tunnels between the SGSN and the radio network controller (RNC). Protocol Stack GTP can be used with UDP or TCP. GTP version 1 is used only on UDP. there are three versions