ing stateful address allocation using DHCPv6, except for requesting delegated prefixes. The 3GPP specifications [25] solve this using the combined DHCPv6 server+client mode in a PGW for PDN interworking. Furthermore, the DHCPv6 protocol itself is not entirely clear how to handle both PD (with IA_PD Option) and address assignment (with IA_NA or IA_TA Options) simultaneously. There is a good discussion of the known shortcomings and gray areas in [107, 118]. Using an external DHCPv6 server for the delegated prefix management falls into vendor specific deployment solutions as for 3GPP Release-11. Using external AAA servers over the (S)Gi-Interface has, on the other hand, been well defined in [25] for both RADIUS and Diameter. The Delegated-IPv6-Prefix IPv6 in 3GPP Networks Attribute/AVP [63] can be present in the RADIUS Access-Request or in the Diameter Authentication and Authorization Request message if the PGW assigned an IPv6 prefix from its internal pool. This is used mainly for authorization purposes. Similarly, the Delegated-IPv6-Prefix Attribute/AVP [63] can be present in the RADIUS Access-Accept or in the Diameter Authentication and Authorization Answer message when the external AAA server is used for the prefix assignment and management. 4.4.7 NAS Protocol Signaling and PCO Options Section 4.4.5 covered the details of the NAS signaling and PCOs when related to IPv6 address configuration in UEs. The NAS protocol signaling takes place between a UE and an SGSN for GPRS [12] and between a UE and an MME for EPS [13]. The signaling is used for the call control of Circuit Switched (CS) connections, session management and mobility management for GPRS/EPS services, and radio resource management for CS and GPRS/EPS services. The NAS protocol is also used for both PDP Context and EPS Bearer handling, and specifically for the control of the user plane, which is our concern within this section. The discussion and examples are limited to parts that involve the user plane IP configuration. The NAS protocol procedures take 