Section: 5.8.2.2.3 The procedure of Stateless IPv6 Address Autoconfiguration
Content: If Stateless IPv6 Address Autoconfiguration is used for IPv6 address allocation to the UE, after PDU Session Establishment the UE may send a Router Solicitation message to the SMF to solicit a Router Advertisement message. The SMF sends a Router Advertisement message (solicited or unsolicited) to the UE. The Router Advertisement messages shall contain the IPv6 prefix. 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 [10]. To ensure that the link-local address generated by the UE does not collide with the link-local address of the UPF and the SMF, the SMF shall provide an interface identifier (see RFC 4862 [10]) 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 ]  [19] 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 ]  [23] without involving the network. Any prefix that the SMF advertises to the UE is globally unique. The SMF shall also record the relationship between the UE's identity (SUPI) and the allocated IPv6 prefix. Because any prefix that the SMF 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 SMF, as defined in RFC 4861 [54]. Therefore, the SMF shall respond with a Neighbor Advertisement upon receiving a Neighbor Solicitation message from the UE. In IPv6 multi-homing PDU session, SMF shall not allocate an interface identifier when a new IPv6 prefix allocated corresponding to the new PDU Session Anchor. The above IPv6 related messages (e.g. Router Solicitation, Router Advertisement, Neighbor Solicitation, Neighbor Advertisement) are transferred between the SMF and UE via the UPF(s). If the Control Plane CIoT 5GS Optimisation is enabled for a PDU session, the above IPv6 related messages are transferred between the SMF and UE via the AMF after PDU Session Establishment, see clauses 4.3.2.2.1 and 4.3.2.2.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], using the Mobile Terminated Data Transport in Control Plane CIoT 5GS Optimisation procedures.