Section: 5.3.1.2.2 Allocation, renewal and release of the IPv6 default prefix via IPv6 stateless address autoconfiguration
Content: When the PLMN allocates an IPv6 prefix, it is the PDN GW responsibility to allocate and release the IPv6 prefix. The PDN GW may use an internal IPv6 prefix pool in this case. The PDN GW allocates a globally unique /64 IPv6 prefix via Router Advertisement to a given UE. When an IPv6 prefix is allocated from an external PDN, it is the PDN GW responsibility to obtain the IPv6 prefix from the external PDN and to allocate, renew and release the IPv6 prefix. The PDN GW may use DHCPv6 to obtain the IPv6 prefix from the external PDN. In this case, the PDN GW functions as a DHCPv6 client. If RADIUS or Diameter is used towards the external PDN as described in TS 29.061[ Interworking between the Public Land Mobile Network (PLMN) supporting packet based services and Packet Data Networks (PDN) ]  [38], the IPv6 prefix can be obtained, renewed and released as part of these procedures. If RADIUS is used, the PDN GW functions as the RADIUS Client. If Diameter is used, the PDN GW functions as the Diameter Client. The procedure of stateless IPv6 address autoconfiguration is the following: After default bearer establishment the UE may send a Router Solicitation message to the PDN GW to solicit a Router Advertisement message. The PDN GW sends a Router Advertisement message (solicited or unsolicited) to the UE. The Router Advertisement messages shall contain the same IPv6 prefix as the one provided during default bearer establishment. If the UE receives an IPv6 prefix from a SGSN during the PDP Context activation procedure, it shall ignore it. After the UE has received the Router Advertisement message, it constructs a full IPv6 address via IPv6 Stateless Address autoconfiguration in accordance with RFC 4862 [18]. To ensure that the link-local address generated by the UE does not collide with the link-local address of the PDN GW, the PDN GW shall provide an interface identifier (see RFC 4862 [18]) to the UE and the UE shall use this interface identifier to configure its link-local address. For stateless address autoconfiguration however, the UE can choose any interface identifier to generate IPv6 addresses, other than link-local, without involving the network. However, the UE shall not use any identifiers defined in TS 23.003[ Numbering, addressing and identification ]  [9] as the basis for generating the interface identifier. For privacy, the UE may change the interface identifier used to generate full IPv6 address, as defined in TS 23.221[ Architectural requirements ]  [27] without involving the network. Any prefix that the PDN GW advertises to the UE is globally unique. The PDN GW shall also record the relationship between the UE's identity (IMSI) and the allocated IPv6 prefix. Because any prefix that the PDN GW advertises to the UE is globally unique, there is no need for the UE to perform Duplicate Address Detection for any IPv6 address configured from the allocated IPv6 prefix. Even if the UE does not need to use Neighbor Solicitation messages for Duplicate Address Detection, the UE may, for example, use them to perform Neighbor Unreachability Detection towards the PDN GW, as defined in RFC 4861 [32]. Therefore, the PDN GW shall respond with a Neighbor Advertisement upon receiving a Neighbor Solicitation message from the UE. In order to renew the allocated IPv6 prefix, the PDN GW sends a Router Advertisement (solicited or unsolicited) to the UE with the same prefix and new non-zero values in preferred and valid lifetime fields. In order to release the allocated IPv6 prefix, the PDN GW shall initiate the PDN connection release procedure. Upon release of the PDN connection, the UE shall implicitly release the prefix for the corresponding PDN connection. NOTE 2: If the PDN type is IPv4v6, when the PDN Connection is released, the IPv4 address is also released. After releasing the IPv6 prefix, the PDN GW should not assign that IPv6 prefix to other user immediately.