5.8.2.2.4 IPv6 Prefix Delegation via DHCPv6

Optionally, a single network prefix shorter than the default /64 prefix may be assigned to a PDU Session. In this case, the /64 default prefix used for IPv6 stateless autoconfiguration will be allocated from this network prefix; the remaining address space from the network prefix can be delegated to the PDU Session using prefix delegation after the PDU Session establishment and IPv6 prefix allocation via IPv6 stateless address autoconfiguration as defined in clause 5.8.2.2.3.

Depending on configuration, the SMF may obtain the prefix from a locally provisioned pool, from the PSA UPF or from the external DN.

The address space provided is maintained as an IPv6 address space pool available to the PDU Session for DHCPv6 IPv6 prefix requests with the exclusion of the IPv6 prefix that is allocated to the PDU Session during PDU Session establishment as defined in clause 5.8.2.2.3. The total IPv6 address space available for the PDU Session (UE PDU Session prefix and UE PDU Session IPv6 address space pool) shall be possible to aggregate into one IPv6 prefix that will represent all IPv6 addresses that the UE may use.

If the UE had indicated that it supports prefix exclusion and the prefix to be delegated to the UE includes the /64 prefix that was allocated to the PDU Session, the SMF shall utilise the prefix exclusion feature as specified for DHCPv6 Prefix Delegation in IETF RFC 6603 [162].

NOTE: Support of the IPv6 prefix delegation in the SMF is assumed to be ensured by the operator e.g. by configuring specific DNN/S-NSSAI for PDU Sessions that are used by UEs that utilize IPv6 prefix delegation.

The UE uses DHCPv6 to request additional IPv6 prefixes (i.e. prefixes in addition to the default prefix) from the SMF after completing stateless IPv6 address autoconfiguration procedures. The UE acts as a "Requesting Router" as described in IETF RFC 8415 [163] and inserts one or more IA_PD option(s) into a DHCPv6 Solicit message sent from the UE to the SMF via the user plane and the UPF. The SMF acts as the DHCP server and fulfils the role of a "Delegating Router" according to IETF RFC 8415 [163]. The UE optionally includes the RAPID_COMMIT option in the DHCPv6 Solicit message to trigger two-message DHCPv6 procedure instead of the four-message DHCPv6 procedure. The UE shall include OPTION_PD_EXCLUDE option code in an OPTION_ORO option to indicate support for prefix exclusion. In response to the DHCPv6 Solicit message, the UE receives a DHCPv6 Reply message with one or more IA_PD prefix(es) for every IA_PD option that it sent in the DHCPv6 Solicit message. The SMF delegates a prefix excluding the default prefix with help of OPTION_PD_EXCLUDE. Prefix exclusion procedures shall follow IETF RFC 6603 [162].

For scenarios with RG connecting to 5GC, additional feature for IPv6 Prefix Delegation via DHCPv6 is defined in TS 23.316 [84].