n the Access- Accept/Authentication and Authorization Answer message. Whether the PGW actually allows the AAA server to supply the IID is then dependent on the PGW implementation. The PGW may also inform the AAA server about the IID it assigned to the UE. The Framed-Interface-Id in the Access-Request/Authentication and Authorization Request can Deploying IPv6 in 3GPP Networks be used for this purpose. The (S)Gi specification as of Release-11 [25] is not clear whether the AAA server is then allowed to override the IID selected by the PGW, thus it is safer to assume that this is not the case. Also, the specifications are not clear what to include in Framed-IPv6-* attributes/AVPs in the Access-Request/Authentication and Authorization Request when the address allocation is entirely delegated to the external AAA server that is, the PGW itself has no prefixes to propose to the AAA server. DHCP Infrastructure The PGW may act as a DHCPv6 client and request an IPv6 prefix for a UE from an external DHCPv6 server. Interestingly, the stateful DHCPv6 procedure as defined in RFC 3315 [26] is used and not the DHCPv6 PD procedure [27] as one might expect. The 3GPP specifications are unclear how to populate certain critical DHCPv6 parameters such as DHCP Unique IDentifier (DUID) and Identity Association IDentifier (IAID) (see Section 3.5.2). In the case of dynamic addressing, this is fortunately not that much of an issue. The DUID for the PGW should be the same for all DHCPv6 messages it exchanges. Regarding the point that DHCPv6 is used instead of DHCPv6 PD over the (S)Gi, the 3GPP specifications are silent as to whether the IID gets conveyed over to the UE in the case of DHCPv6. The IPv6 address received from the DHCPv6 server is a /128 IPv6 address after all. Furthermore, there is no guarantee that the UE would ever respect the IID it received from the PGW when it configures its IPv6 Global Unicast Address (GUA). The details of DHCPv6 functionality fall into a vendor-specific category and UE-implementation-specific behavior. 4