 Fortunately, lack of dual-stack charging information does not imply that the PDN Connection activation would fail, only that the charging information received by the operator lacks information. In the case of inter-operator roaming, the lack of charging information can, of course, be a commercial-level barrier not to deploy IPv6. In addition to 3GPP defined charging interfaces and CDR formats, the GSM Association (GSMA) has also defined its own inter-operator account record format called Deploying IPv6 in 3GPP Networks TAP3 [87] for roaming billing. The TAP3 is still widely used and was updated years ago to support IPv6 and dual-stack. However, whether operators have deployed recent enough TAP3 in their roaming billing systems has to be verified case by case (most roaming contracts today are bilateral). 4.2.4 External PDN Access and the (S)Gi Interface Access to an external PDN over the (S)Gi-interface (may) involve functions starting from a subscriber authentication and authorization, IP address allocation either from PLMN's address space or from the external PDN's address space, providing secure access to the external PDN network, and SO forth. 3GPP defines two ways of accessing the external PDN such as the Internet, some corporate intranet or Internet Service Provider (ISP) network [25]. These two access methods are: Transparent access to the Internet - IP addressing and all subscriber management belongs to the PLMN. The PGW does to need to take any part in the subscriber authen- tication/authorization towards the external PDN. The transparent access provides basic ISP services. Non-transparent access to intranets or ISP networks - the UE is assigned an IP address from the corporate intranet or ISP addressing space. Furthermore, the PGW has to requests subscriber authentication/authorization and IP configuration information from servers (such as AAA, Dynamic Host Configuration Protocol (DHCP), and SO forth) belonging to the corporate intranet or the ISP backend. The non-transparent case is also frequently dep