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Content for  TS 23.304  Word version:  17.4.0

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5.5  IP address allocationp. 44

5.5.1  Generalp. 44

5.5.1.1  IP address allocation for unicast mode of 5G ProSe direct communicationp. 44

For unicast mode of 5G ProSe direct communication, the following mechanism for IP address/prefix allocation may be used:
  1. DHCP-based IPv4 address allocation with one of the two UEs acting as a DHCP server.
  2. IPv6 Stateless Address auto configuration specified in RFC 4862 for assignment of IPv6 prefix, with one of the two UEs acting as IPv6 default router.
  3. IPv6 link-local addresses as defined in RFC 4862 are formed by UEs locally. The IPv6 link-local addresses are exchanged during the establishment of a secure layer-2 link over PC5. The UEs shall disable duplicate address detection after the layer-2 link is established.
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5.5.1.2  IP address allocation for broadcast and groupcast modes of 5G ProSe direct communicationp. 44

For broadcast and groupcast modes of 5G ProSe direct communication, the following source IP address management applies:
  1. the UE configures a link local IPv4 address to be used as the source IP address, as defined in clause 4.5.3 of TS 23.303. If it is not configured with an address, it uses Dynamic Configuration of IPv4 Link-Local Addresses RFC 3927.
  2. the UE configures a link local IPv6 address to be used as the source IP address, as defined in clause 4.5.3 of TS 23.303. The UE may use this IP address for direct communication without sending Neighbour Solicitation and Neighbour Advertisement message for Duplicate Address Detection.
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5.5.1.3  IP address allocation for communication with a 5G ProSe Layer-3 ProSe UE-to-Network Relayp. 45

For communication with a 5G ProSe Layer-3 UE-to-Network Relay, the following mechanism for IP address/prefix allocation applies:
  • The PDU Session Type used for the relay traffic shall support the IP version used by the 5G ProSe Layer-3 Remote UE. If the 5G ProSe Layer-3 Remote UE initiates an allocation of IPv4 address or an IPv6 prefix when the requested IP version is not supported in the corresponding PDU Session then IP address/prefix allocation fails.
    1. When the 5G ProSe Layer-3 Remote UE uses IPv4 to access the external DN:
      1. The IPv4 address allocation and IPv4 parameter configuration via DHCPv4 are performed according to RFC 2131 and RFC 4039 procedures. The IPv4 address provided to the 5G ProSe Layer-3 Remote UE from the 5G ProSe Layer-3 UE-to-Network Relay by DHCPv4 procedure shall correspond to a local IPv4 address range configured in the 5G ProSe Layer-3 UE-to-Network Relay.
      2. The DHCPv4 request from the 5G ProSe Layer-3 Remote UE is always sent subsequent to the establishment of the one-to-one 5G ProSe Direct Communication between the 5G ProSe Layer-3 Remote UE and the 5G ProSe Layer-3 UE-to-Network Relay, see details for the IPv4 address allocation in clause 5.4.4.3 of TS 23.303 with the following difference:
        • The ProSe Relay UE ID of the ProSe UE-to-Network Relay is replaced by the source Layer-2 ID of the 5G ProSe UE-to-Network Relay for PC5 unicast communication.
    2. When the 5G ProSe Layer-3 Remote UE uses IPv6 to access the external DN:
      1. IPv6 network prefix allocation via IPv6 Stateless Address auto-configuration. Router solicitation from the 5G ProSe Layer-3 Remote UE is always sent subsequent to the establishment of the one-to-one ProSe Direct Communication between the 5G ProSe Layer-3 Remote UE and the 5G ProSe Layer-3 UE-to-Network Relay, see details for IPv6 prefix allocation in clause 5.4.4.2 of TS 23.303 with the following differences:
        • The 5G ProSe Layer-3 UE-to-Network Relay shall obtain the IPv6 prefix assigned to the 5G ProSe Layer-3 Remote UE via prefix delegation function from the network as defined in clause 5.5.2.
        • The ProSe Relay UE ID of the ProSe UE-to-Network Relay is replaced by the source Layer-2 ID of the 5G ProSe UE-to-Network Relay for PC5 unicast communication.
        • PDN connection is replaced by PDU Session.
      2. IPv6 parameter configuration via Stateless DHCPv6: The UE may use stateless DHCPv6 for additional parameter configuration.
      3. The 5G ProSe Layer-3 UE-to-Network Relay assigns IPv6 prefixes from IPv6 prefix range that have been assigned to the PDU Session used for the relay traffic via IPv6 prefix delegation.
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5.5.2  IPv6 Prefix Delegation via DHCPv6 for 5G ProSe Layer-3 UE-to-Network Relayp. 45

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 of TS 23.501. When PLMN based parameter configuration is used, the SMF provides the requested IPv6 prefix from a locally provisioned pool. When external DN based IPv6 prefix allocation is used, the SMF obtains the prefix 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 of TS 23.501. 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 RFC 6603.
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 RFC 8415 and inserts one or more IA_PD option(s) into a DHCPv6 Solicit message sent from the UE to the SMF. The SMF acts as the DHCP server and fulfils the role of a "Delegating Router" according to RFC 8415. 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 RFC 6603.
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