.4.3 Stateful IPv6 Address Configuration The 3GPP architecture and the address allocation model does not, as of Release-11, support DHCPv6-based stateful IPv6 address allocation [26] for the PDP Context or the PDN Connection. There is, however, a stateful DHCPv6-based IPv6 prefix delegation defined in 3GPP specifications. On the other hand, DHCPv4-based stateful IPv4 address allocation is supported in the 3GPP architecture. 4.4.4 Deferred Address Allocation Although deferred address allocation is specifically meant for IPv4 address management, its use is interesting in the context of PDN Type IPv4v6 as a technique to conserve IPv4 number resources. The deferred address allocation may be built on top of DHCPv4 or DSMIPv6 (the S2c-interface). In the case of DHCPv4, the UE has to specifically indicate that it wants to configure its IPv4 address using DHCPv4. The indication is done at the NAS layer using a specific Protocol Configuration Option (PCO) option (see Section 4.4.7 for further details). As a 3GPP 'specification wise' note, the deferred address allocation as a term appears only for the non-3GPP access and DHCPv4 in stage 2 [16] (as of Release-11). The DHCPv4-based address allocation for UE is, on the other hand, possible also for 3GPP IPv6 in 3GPP Networks access. According to the 3GPP stage-3 specifications [25], the deferred address allocation applies also for 3GPP access. However, it is not entirely clear, since the deferred address allocation requires additional PDN Connection signaling between the SGW and the PGW after the initial PDN Connection has been established, and this signaling is currently absent from 3GPP access specifications [15]. In any case, the basic idea of deferred address allocation using DHCPv4 is the fol- lowing. When the PDN Connection of PDN Type IPv4v6 gets activated, the UE would indicate its intention to configure the IPv4 address using DHCPv4. During PDN Con- nection activation, the PGW does not assign any IPv4 address to the UE, only the IPv6 prefix. The PDN Connection would 