Wikipedia
GTP' (GTP prime) is an IP based protocol used within GSM and UMTS networks. It can be used with UDP or TCP. GTP' uses the same message structure as GTP (GTP-C, GTP-U), but it is largely a separate protocol. GTP' uses registered UDP/TCP port 3386.
GTP' can be used for carrying charging data from the "Charging Data Function" (CDF) of the GSM or UMTS network to the "Charging Gateway Function" (CGF). In most cases, this should mean from many individual network elements such as the GGSNs to a centralised computer which then delivers the charging data more conveniently to the network operator's billing center.
GTP' is used on the Ga interface within the 3GPP GPRS Core Network definition.
GTP' reuses aspects of GTP although to quote 3GPP TS 32.295 "only the signalling plane of GTP is partly reused". GTP' defines a different header, additional messages, field values, as well as a synchronisation protocol to avoid losing or duplicating CDRs on CGF or SGSN/ GGSN failure. Transferred CDRs, if following 3GPP standards, are encoded in ASN.1.