IP address allocation is performed as described in TS 23.501, clause 184.108.40.206
In addition to the IP address management features described in TS 23.501, clause 220.127.116.11
the 5GC network functions and RG support the following mechanisms:
IPv6 address allocation using DHCPv6 may be supported for allocating individual /128 IPv6 address(es) for a PDU Session. The details of IPv6 address allocation using DHCPv6 are described in clause 18.104.22.168.
The details of Prefix Delegation are described in clause 22.214.171.124.
IPv6 Prefix Delegation using DHCPv6 may be supported for allocating additional IPv6 prefixes for a PDU Session.
The mechanisms in a. and b. above are only applicable for IPv6 and IPv4v6 PDU Session types.
The requested IPv6 address or set of IPv6 Prefixes may be (as defined in TS 23.501, clause 126.96.36.199.1
allocated from a local pool in the SMF or
obtained from the UPF. In that case the SMF shall interact with the UPF via N4 procedures to obtain a suitable IP address/Prefix, or
obtained from an external server.
When obtaining the IP address from the UPF, the SMF provides the UPF with the necessary information allowing the UPF to derive the proper IP address (e.g. the network instance, IP version, size of the IP address or Prefix the UPF is to allocate).
In this clause the RG may correspond either to a 5G RG or to a FN RG;
Optionally, and instead of using Stateless IPv6 Address Autoconfiguration, individual 128-bit IPv6 address(es) may be assigned to a PDU Session.
In this case, after PDU Session Establishment, the SMF sends a Router Advertisement message (solicited or unsolicited) towards the RG. The SMF shall set the Managed Address Configuration Flag (M-flag) in the Router Advertisement messages to indicate towards the RG that IPv6 Address allocation using DHCPv6 is available, as described in RFC 4861 . In that case the IPv6 address of the RG is allocated using DHCPv6 Identity Association for Non-temporary Addresses (IA_NA) and mechanisms defined in RFC 3315 .
The SMF may receive a Router Solicitation message, soliciting a Router Advertisement message.
When using DHCPv6 address allocation, a prefix (e.g. /64) may be allocated for the PDU Session at PDU Session Establishment from which the /128 addresses are selected. The SMF determines the size of the prefix for a PDU Session to a specific DNN and S-NSSAI based on subscription data and local configuration. The individual /128 address(es) allocated to the RG as part of DHCP IA_NA procedure are then selected from the prefix allocated to the PDU Session. For statically assigned prefix, the subscription data in UDM for a DNN and S-NSSAI includes the prefix. Alternatively, individual 128-bit address(es) are allocated for the PDU Session without allocating a prefix to the PDU Session and provided to the RG as part of DHCP IA_NA procedure.
When a prefix is allocated to the PDU Session, the SMF provides the prefix to the PCF instead of each /128 address. When individual /128 address(es) are allocated without allocating a prefix to the PDU Session, the SMF provides the /128 bits address(es) to PCF. Whether the SMF allocates a prefix for the PDU Session or individual 128-bit addresses is transparent to the RG and W-5GAN.
If Prefix Delegation (as described in clause 188.8.131.52
) is also supported, a SMF may receive both DHCP options for IA_NA and IA_PD together in a single DHCPv6 message. An SMF may provide a reply to both IA_NA and IA_PD in the same message or alternatively process the DHCPv6 IA_NA before the DHCPv6 IA_PD.
The SMF may receive multiple different IA_NA related DHCP requests within the same PDU Session.
This is applicable if the RG acts as a DHCP relay for devices behind the RG.
When IPv6 Address Allocation using DHCPv6 is used, 5GC does not support IPv6 multi-homing for enabling SSC mode 3 and PDU Sessions with multiple PDU Session Anchors.
Optionally a single network prefix shorter than the default /64 prefix may be assigned to a PDU Session.
If IPv6 stateless autoconfiguration is used, the /64 default prefix used for IPv6 stateless autoconfiguration may 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 TS 23.501, clause 184.108.40.206.3
. In this case 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 using stateless IPv6 address allocation. In that case it is possible to aggregate the total IPv6 address space available for the PDU Session into one IPv6 prefix that will represent all IPv6 addresses that the RG may use.
The SMF determines the maximum size of the prefix that may be allocated for the PDU Session based on subscription data and local configuration.
If stateless IPv6 address autoconfiguration procedure is used by the RG to get the first Prefix of the PDU Session, DHCPv6 is used to request additional IPv6 prefixes (e.g. prefixes in addition to the default prefix) from the SMF after completing stateless IPv6 address autoconfiguration procedures. If IPv6 address allocation using DHCPv6 is used, the DHCPv6 message may include a request for a delegated prefix (IA_PD) together with a request for an IPv6 address (IA_NA). Alternatively, a delegated prefix may be requested after an IPv6 address has been assigned using IA_NA.
The RG acts as a "Requesting Router" as described in IETF RFC 3633  and inserts one or more IA_PD option(s) into a DHCPv6 Solicit message sent 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 3633 . The RG optionally includes the RAPID_COMMIT option in the DHCPv6 Solicit message to trigger two-message DHCPv6 procedure instead of the four-message DHCPv6 procedure.
In response to the DHCPv6 Solicit message, the SMF sends a DHCPv6 Reply message with one or more IA_PD prefix(es) for every IA_PD option that was received in the DHCPv6 Solicit message.
If the DHCPv6 request indicates support for prefix exclusion via the OPTION_PD_EXCLUDE option code in an OPTION_ORO option and if the SMF accepts this option, the SMF delegates a prefix excluding the default prefix with help of OPTION_PD_EXCLUDE. Prefix exclusion procedures shall follow IETF RFC 6603 .