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Content for  TS 23.304  Word version:  18.4.0

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5.4 5G ProSe UE-to-Network Relay  5.4.1 5G ProSe Layer-3 UE-to-Network Relay  5.4.1.1 General  5.4.1.2 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support  5.4.1.3 Policy control and session binding to support 5G ProSe Layer-3 UE-to-Network Relay without N3IWF  5.4.2 5G ProSe Layer-2 UE-to-Network Relay  5.4.3 Mobility Restrictions for 5G ProSe UE-to-Network Relaying  5.4.4 Support of emergency service from 5G ProSe Remote UE via 5G ProSe UE-to-Network Relay  5.4.4.1 General  5.4.4.2 Emergency service from 5G ProSe Remote UE via via 5G ProSe Layer-2 UE-to-Network Relay  5.4.4.3 Emergency service from 5G ProSe Remote UE via 5G ProSe Layer-3 UE-to-Network Relay 
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5.4  5G ProSe UE-to-Network Relay  p. 45

5.4.1  5G ProSe Layer-3 UE-to-Network Relay  p. 45

5.4.1.1  General  p. 45

The 5G ProSe Layer-3 UE-to-Network Relay shall provide generic function that can relay any IP, Ethernet or Unstructured traffic:
  • For IP traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay uses IP type PDU Session towards 5GC.
  • For Ethernet traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay can use Ethernet type PDU Session or IP type PDU Session towards 5GC.
  • For Unstructured traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay can use Unstructured type PDU Session or IP type PDU Session (i.e. IP encapsulation/de-capsulation by 5G ProSe Layer-3 UE-to-Network Relay) towards 5GC.
The type of traffic supported over PC5 reference point is indicated by the 5G ProSe Layer-3 UE-to-Network Relay e.g. using the corresponding RSC. The 5G ProSe Layer-3 UE-to-Network Relay determines the PDU Session Type based on configuration of the mapping between PDU Session parameters and RSC, as specified in clause 5.1.4.1.
IP type PDU Session and Ethernet type PDU Session can be used to support more than one 5G ProSe Layer-3 Remote UEs while Unstructured type PDU Session can be used to support only one 5G ProSe Layer-3 Remote UE.
The 5G ProSe Layer-3 UE-to-Network Relay provides the functionality to support indirect connectivity to the 5GS for 5G ProSe Layer-3 Remote UEs. A 5G ProSe Layer-3 Remote UE can be located within NG-RAN coverage or outside of NG-RAN coverage.
The 5G ProSe Layer-3 Remote UE and 5G ProSe Layer-3 UE-to-Network Relay may use the PC5 DRX mechanism to perform 5G ProSe UE-to-Network Relay Communications over PC5 reference point as specified in clause 5.13.
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5.4.1.2  5G ProSe Layer-3 UE-to-Network Relay with N3IWF support  p. 45

To support 5G ProSe Layer-3 Remote UE services with end-to-end confidentiality and IP address preservation requirements, the 5G ProSe Layer-3 UE-to-Network Relay with N3IWF shall provide access to the 5GC for the 5G ProSe Layer-3 Remote UE via N3IWF using the features defined in clause 4.2.8 of TS 23.501.
5G ProSe Layer-3 UE-to-Network Relay is provisioned with RSC(s) and the corresponding PDU session parameters (e.g. S-NSSAI) to support N3IWF access as part of 5G ProSe Layer-3 UE-to-Network Relay Policy/parameters. When a 5G ProSe Layer-3 Remote UE connects with the corresponding RSC, the 5G ProSe Layer-3 UE-to-Network Relay determines the corresponding PDU session parameters based on the requested RSC.
The 5G ProSe Layer-3 Remote UE selects N3IWF as specified in clause 6.5.1.2.2. The selection of N3IWF follows the regulatory rules of the country where it is located and when required by the regulations the 5G ProSe Layer-3 Remote UE only selects a N3IWF within the local country. QoS differentiation can be provided on per-IPsec Child Security Association basis and the details are provided in clause 5.6.2.2.
The 5GC to which the 5G ProSe Layer-3 UE-to-Network Relay registers and the 5GC to which the 5G ProSe Layer-3 Remote UE registers may be in the same PLMN or different PLMN.
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5.4.1.3  Policy control and session binding to support 5G ProSe Layer-3 UE-to-Network Relay without N3IWF  p. 46

To enable support for policy control for 5G ProSe Layer-3 Remote UEs accessing 5GC via a 5G ProSe Layer-3 UE-to-Network Relay without involving N3IWF, the policy control functionality specified in TS 23.503 is applied with the following functionalities:
  • For non-emergency service over the 5G ProSe Layer-3 UE-to-Network Relay, the SMF and PCF shall be configured with DNN(s) dedicated for UE-to-Network Relay connectivity, as specified in clause 5.1.4.1.
  • The AF discovers the PCF serving the 5G ProSe Layer-3 UE-to-Network Relay PDU Session as specified in clause 6.1.1.2 of TS 23.503.
  • The PCF may validate any 5G ProSe Layer-3 Remote UE related service information from the AF based on roaming agreement and the DNN dedicated for UE-to-Network Relay connectivity for non-emergency service from the 5G ProSe Layer-3 Remote UE or emergency DNN for emergency service from the 5G ProSe Layer-3 Remote UE.
  • The 5G ProSe Layer-3 Remote UEs may be assigned a /64 IPv6 Prefix from a shorter IPv6 prefix by the 5G ProSe Layer-3 UE-to-Network Relay.
  • For a PDU Session to the DNN dedicated for 5G ProSe Layer-3 UE-to-Network Relay connectivity and using IPv6 prefix delegation (i.e. the assigned IPv6 network prefix is shorter than 64), the PCF shall perform session binding based on the IPv6 network prefix only. A successful session binding occurs whenever a longer prefix received from an AF matches the prefix value of the PDU Session. PCF shall not use the UE Identity for session binding for this PDU Session.
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5.4.2  5G ProSe Layer-2 UE-to-Network Relay  p. 46

The 5G ProSe Layer-2 UE-to-Network Relay provides forwarding functionality that can relay any type of traffic over the PC5 link.
The 5G ProSe Layer-2 UE-to-Network Relay provides the functionality to support connectivity to the 5GS for 5G ProSe Layer-2 Remote UEs. A UE is considered to be a 5G ProSe Layer-2 Remote UE if it has successfully established a PC5 link to the 5G ProSe Layer-2 UE-to-Network Relay. A 5G ProSe Layer-2 Remote UE can be located within NG-RAN coverage or outside of NG-RAN coverage.
For PLMN selection and relay selection in the 5G ProSe Layer-2 Remote UE:
  • The 5G ProSe Layer-2 Remote UE checks whether the PLMN(s) within the RRC Container (see clause 5.8.3.3) obtained from the 5G ProSe Layer-2 UE-to-Network Relay(s) during 5G ProSe UE-to-Network Relay Discovery in clause 6.3.2.3 are authorized to be connected to via a 5G ProSe Layer-2 UE-to-Network Relay(s) and only the authorized PLMN(s) are then available PLMNs for NAS PLMN selection;
  • The 5G ProSe Layer-2 Remote UE selects the 5G ProSe Layer-2 UE-to-Network Relay considering the selected PLMN by NAS layer.
The 5G ProSe Layer-2 Remote UE and 5G ProSe Layer-2 UE-to-Network Relay may use the PC5 DRX mechanism to perform 5G ProSe UE-to-Network Relay Communications over PC5 reference point as specified in clause 5.13.
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5.4.3  Mobility Restrictions for 5G ProSe UE-to-Network Relaying  p. 47

The handling of Mobility Restrictions for 5G ProSe enabled UE follows the principles as specified in clause 5.3.4.1 of TS 23.501 with the following additions and clarifications:
  • Forbidden Area:
    • In a Forbidden Area, a 5G ProSe enabled UE is not allowed to perform the Relay operation. If the 5G ProSe UE-to-Network Relay operates in a Forbidden Area of the 5G ProSe Remote UE, the 5G ProSe Remote UE is not allowed to access the network via this 5G ProSe UE-to-Network Relay unless the 5G ProSe Remote UE is attempting for emergency service.
  • Service Area Restriction:
    • Service Area Restriction is not applicable to 5G ProSe Layer-3 Remote UE.
    • Service Area Restriction is not applicable to 5G ProSe enabled MCX-subscribed UEs as defined in clause 5.3.4.1 of TS 23.501.
    • In a Non-Allowed Area, a 5G ProSe enabled UE follows the principles of a UE in limited service state as specified in clause 5.9. It cannot perform the Relay operation as 5G ProSe Layer-2 UE-to-Network Relay or 5G ProSe Layer-3 UE-to-Network Relay based on the conditions described in clause 5.9 except for emergency service from the 5G ProSe Remote UE as specified in clause 5.4.4.
    • In a Non-Allowed Area, a 5G ProSe Layer-2 Remote UE follows the principles as specified in clause 5.3.4.1 of TS 23.501, for communication with the network via the 5G ProSe Layer-2 UE-to-Network Relay.
  • Core Network type restriction, RAT type restriction:
    In these cases, a 5G ProSe enabled UE is not able to register in the network for normal service and stays in limited service state and the principles in clause 5.9 shall apply. The 5G ProSe Layer-2 Remote UE is allowed to register to the network via the 5G ProSe UE-to-Network Relay for emergency service.
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5.4.4  Support of emergency service from 5G ProSe Remote UE via 5G ProSe UE-to-Network Relay |R18|  p. 47

5.4.4.1  General  p. 47

When a 5G ProSe enabled UE does not have direct connection to the network for emergency service, the UE may attempt to obtain emergency service via 5G ProSe Layer-2 or Layer-3 UE-to-Network Relay.
A 5G ProSe enabled UE acting as 5G ProSe UE-to-Network Relay shall have a normal registration (including also normal registration for a 5G ProSe Relay enabled UE in Non-Allowed Area). A 5G ProSe enabled UE in limited-service state shall not act as 5G ProSe UE-to-Network Relay. Mobility Restrictions that are overruled for UE requesting direct emergency service are overruled also for 5G ProSe UE-to-Network Relay that is relaying emergency service as specified in clause 5.4.3.
A 5G ProSe enabled UE acting as 5G ProSe UE-to-Network Relay shall only advertise its support for relaying emergency services as defined in clause 5.4.4.2 and clause 5.4.4.3 during 5G ProSe UE-to-Network Relay Discovery.
Dedicated RSC(s) for emergency service needs to be provisioned in the 5G ProSe enabled UEs with capability of 5G ProSe UE-to-Network Relay and/or 5G ProSe Remote UE as specified in clause 5.1.4.
The dedicated RSC(s) for emergency service are used by the 5G ProSe UE-to-Network Relay and 5G ProSe Remote UE during 5G ProSe UE-to-Network Relay Discovery and PC5 link establishment.
A PC5 link associated with a dedicated emergency RSC shall be used for emergency service and for emergency service only.
If the 5G ProSe UE-to-Network Relay needs to establish RRC Connection when the 5G ProSe Remote UE has requested emergency service over the PC5 link, the 5G ProSe UE-to-Network Relay shall use RRC establishment cause "emergency".
The existing positioning function as applicable is reused for the 5G ProSe Remote UE. If no other information is available, the location of the 5G ProSe UE-to-Network Relay can be used as Remote UE location estimate.
For more information on UE specific behaviour and location handling, refer to TS 23.167.
PC5 link for emergency services over 5G ProSe UE-to-Network Relay can be established with or without PC5 security; how to establish PC5 security link for emergency service is specified in TS 33.503.
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5.4.4.2  Emergency service from 5G ProSe Remote UE via via 5G ProSe Layer-2 UE-to-Network Relay  p. 48

For a 5G ProSe Layer-2 UE-to-Network Relay to participate in the 5G ProSe UE-to-Network Relay Discovery procedure for emergency service (i.e. sending the UE-to-Network Relay Discovery Announcement message or UE-to-Network Relay Discovery Response message), the serving NG-RAN support of emergency services is required, indicated by it broadcasting ims-EmergencySupport in system information as specified in TS 38.300 and TS 38.331. The 5G ProSe Layer-2 Remote UE may select a different PLMN from the 5G ProSe Layer-2 UE-to-Network Relay. The 5G ProSe Layer-2 Remote UE may select any PLMN within the RRC Container (see clause 5.8.3.3) obtained from any 5G ProSe Layer-2 UE-to-Network Relay(s) which has announced the RSC dedicated for emergency service during 5G ProSe UE-to-Network Relay Discovery in clause 6.3.2.3.
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5.4.4.3  Emergency service from 5G ProSe Remote UE via 5G ProSe Layer-3 UE-to-Network Relay  p. 48

A 5G ProSe Layer-3 UE-to-Network Relay participates in the 5G ProSe UE-to-Network Relay Discovery procedure (i.e., sending the UE-to-Network Relay Discovery Announcement message or UE-to-Network Relay Discovery Response message) for emergency service only when it has received the Emergency Service Support indicator in its latest Registration Accept from the AMF.
If PC5 connection is requested using dedicated RSC for emergency service, then the 5G ProSe Layer-3 UE-to-Network Relay sets the RRC Establishment cause to "emergency" when establishing an RRC connection.
For 5G ProSe Layer-3 UE-to-Network Relay without N3IWF:
  • the 5G ProSe Layer-3 UE-to-Network Relay sets up an emergency PDU Session to support the 5G ProSe Remote UE's emergency service.
  • A 5G ProSe Layer-3 UE-to-Network Relay shall only serve emergency services for either itself or for one 5G ProSe Layer-3 Remote UE but not for more than one UE at the same time. In possible conflict case, the prioritisation between the emergency calls is determined by the local regulations, if available. In the absence of local regulations, prioritisation between the emergency calls is 5G ProSe Layer-3 UE-to-Network Relay implementation specific.
    • If a 5G ProSe Layer-3 UE-to-Network Relay is not able to handle relayed emergency services due to having an active emergency service (of its own or for a 5G ProSe Layer-3 Remote UE emergency service), the 5G ProSe Layer-3 UE-to-Network Relay shall not advertise its support of emergency service and shall not respond to any new 5G ProSe Layer-3 Remote UE's requests for relaying emergency services.
When a 5G ProSe Layer-3 Remote UE needs to initiate emergency service, it should attempt to use 5G ProSe Communication for emergency service via 5G ProSe Layer-3 UE-to-Network Relay without N3IWF procedures before attempting to establish an emergency PDU Session via 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support.
For information on the handling of emergency number, P-CSCF address and access type setting, refer to TS 23.167.
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