elated interfaces. In theory, Table 4.3 GGSN and PGW support for user plane IPv6 Functionality Remarks IPv6-only Release-99 Release-8 Dual-stack Release-9 Release-8 DHCPv6 Client Release-99 Release-8 DHCPv6 Relay Release-99 Release-8 Relay function is 'possible'. DHCPv6 Server Release-99 Release-8 Stateless untill Release-10 (DHCPv6 PD). RADIUS Client Release-99 Release-8 Diameter Client Release-99 Release-8 Deploying IPv6 in 3GPP Networks all PCC and charging related interfaces have been IPv6 aware since their introduction in Release-7. The support for dual-stack PDN Type did not cause any specific change to PCC signaling interfaces. Generally with all Diameter-based interfaces, the support for IPv6 at the Attribute Value Pair (AVP) level has to be verified for each vendor's product. There have been several cases where the IPv6 support has been left unimplemented at the AVP handling level. For example, a specific AVP even exists, but if the content is IPv6 instead of IPv4, that leads to an error or undefined behavior. Finally, the (S)Gi interface provides connectivity to external PDNs. At minimum, interface where (S)Gi is 'located' must understand IPv6 and be connected to a network that provides transit to some IPv6-enabled PDNs, such as the Internet. The connectivity from the (S)Gi to the external PDN can also be tunneled, for example, if the physical (transit) network directly connected to the (S)Gi is IPv4-only. We will discuss external PDN connectivity in Section 4.2.4. As mentioned earlier, the PGW may have both a RADIUS client [60, 61] and Diameter client functions. The Authentication, Authorization, and Accounting (AAA) server located in the external PDN network is connected over the (S)Gi interface using either of the AAA protocols. Diameter is by design IPv6-aware also on the AVPs and their values level. RADIUS had IPv6 aware attributes added later [62, 63, 64]. Subscriber Management Systems, HLR and HSS The PDN Type is part of the subscription profile data stored in an HLR or an HSS. Strictly from 3GPP