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Content for  TS 23.304  Word version:  18.4.0

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6.5.2 5G ProSe Communication via 5G ProSe Layer-2 UE-to-Network Relay  6.5.2.1 Registration and Connection Management  6.5.2.1.1 Registration Management  6.5.2.1.2 Connection Management  6.5.2.2 Connection establishment  6.5.2.3 Path switching between direct network communication path and indirect network communication path via 5G ProSe Layer-2 UE-to-Network Relay  6.5.3 5G ProSe UE-to-Network Relay reselection  6.5.4 5G ProSe Remote UE traffic handling for 5G ProSe UE-to-Network Relay support  6.5.5 Procedures for Communication Path Switching between Two UE-to-Network Relays  6.5.5.1 General  6.5.5.2 Procedures for Communication Path Switching between UE-to-Network Relays 
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6.5.2  5G ProSe Communication via 5G ProSe Layer-2 UE-to-Network Relay  p. 114

6.5.2.1  Registration and Connection Management  p. 114

6.5.2.1.1  Registration Management  p. 114
Registration Management for the 5G ProSe Layer-2 Remote UE and the 5G ProSe Layer-2 UE-to-Network Relay follows the principles and procedures defined in TS 23.501 and TS 23.502. The 5G ProSe Layer-2 Remote UE and the 5G ProSe Layer-2 UE-to-Network Relay may be served by the same AMF or different AMFs.
6.5.2.1.2  Connection Management  p. 114
Connection Management for the 5G ProSe Layer-2 Remote UE and the 5G ProSe Layer-2 UE-to-Network Relay follows the principles and procedures defined in TS 23.501 and TS 23.502 with the following modifications.
The 5G ProSe Layer-2 UE-to-Network Relay may only relay data/signalling for the 5G ProSe Layer-2 Remote UE(s) when the 5G ProSe Layer-2 UE-to-Network Relay is in CM-CONNECTED state. If the 5G ProSe Layer-2 UE-to-Network Relay is in CM_IDLE state and receives a connection request from the 5G ProSe Layer-2 Remote UE for relaying, the 5G ProSe Layer-2 UE-to-Network Relay shall trigger Service Request procedure to enter CM_CONNECTED state before relaying the 5G ProSe Layer-2 Remote UEs traffic. If the 5G ProSe Layer-2 UE-to-Network Relay in RRC_IDLE receives a connection request from the 5G ProSe Layer-2 Remote UE using emergency RSC, then 5G ProSe Layer-2 UE-to-Network Relay sets the establishment cause to "emergency".
The state of 5G ProSe UE-to-Network Relay is controlled by NG-RAN with the following:
  • If any 5G ProSe Layer-2 Remote UE connected to the 5G ProSe Layer-2 UE-to-Network Relay is in CM-CONNECTED with RRC Connected state, the 5G ProSe Layer-2 UE-to-Network Relay shall remain CM-CONNECTED state with RRC Connected state unless the network needs to release the connection.
  • If all 5G ProSe Layer-2 Remote UEs connected to the 5G ProSe Layer-2 UE-to-Network Relay enter CM-IDLE or CM-CONNECTED with RRC Inactive state, the 5G ProSe Layer-2 UE-to-Network Relay may enter CM-IDLE state or CM-CONNECTED with RRC Inactive state, or may remain CM-CONNECTED with RRC Connected state.
When 5G ProSe Layer-2 Remote UE is in CM-CONNECTED state, the 5G ProSe Layer-2 UE-to-Network Relay and 5G ProSe Layer-2 Remote UE keep the PC5 link. When the 5G ProSe Remote UE is in CM-IDLE state, it may either release the PC5 link for relaying or not.
For paging a 5G ProSe Layer-2 Remote UE, it follows the principles and procedures defined in TS 23.501 and TS 23.502 and the paging message delivery from NG-RAN to 5G ProSe Layer-2 Remote UE is specified in TS 38.300.
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6.5.2.2  Connection establishment  p. 115

Copy of original 3GPP image for 3GPP TS 23.304, Fig. 6.5.2.2-1: Connection Establishment for 5G ProSe Layer-2 UE-to-Network Relay
Figure 6.5.2.2-1: Connection Establishment for 5G ProSe Layer-2 UE-to-Network Relay
(⇒ copy of original 3GPP image)
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Step 0.
If in coverage, the 5G ProSe Layer-2 Remote UE and 5G ProSe Layer-2 UE-to-Network Relay may independently perform the initial registration to the network according to registration procedures in TS 23.502.
Step 1.
If in coverage, the 5G ProSe Layer-2 Remote UE and 5G ProSe Layer-2 UE-to-Network Relay independently get the service authorization for 5G ProSe Layer-2 UE-to-Network Relay operation from the network. Service authorization and parameters provisioning for 5G ProSe Layer-2 UE-to-Network Relay operation are performed for the 5G ProSe Layer-2 UE-to-Network Relay and 5G ProSe Layer-2 Remote UE as specified in clause 5.1.4.
If the 5G ProSe Layer-2 Remote UE is "not served by NG-RAN", the pre-configured parameters are used and the service authorization and parameters may be updated after step 6.
If the 5G ProSe Layer-2 Remote UE has not performed Initial Registration, the 5G ProSe Layer-2 Remote UE may perform the Initial Registration in step 6.
Step 2.
The 5G ProSe Layer-2 Remote UE and 5G ProSe Layer-2 UE-to-Network Relay perform 5G ProSe UE-to-Network Relay Discovery and selection, as specified in clause 6.3.2.3.
Step 3.
The 5G ProSe Layer-2 Remote UE initiates a one-to-one communication connection with the selected 5G ProSe Layer-2 UE-to-Network Relay over PC5 using the procedure as described in clause 6.4.3.
Step 4.
The 5G ProSe Layer-2 Remote UE establishes an RRC Connection with the same NG-RAN serving the selected 5G ProSe Layer-2 UE-to-Network Relay, specified in TS 38.300. The 5G ProSe Layer-2 Remote UE sets the RRC Establishment Cause as defined in TS 38.331.
Step 5.
During step 4, if the 5G ProSe Layer-2 UE-to-Network Relay is in CM_IDLE state and receives a trigger from the AS layer to enter CM-CONNECTED state due to Remote UE's AS layer connection set up with the NG-RAN, the 5G ProSe Layer-2 UE-to-Network Relay performs Service Request procedure in the clause 4.2.3.2 of TS 23.502.
Step 6.
The 5G ProSe Layer-2 Remote UE sends a NAS message to the serving AMF. The NAS message is encapsulated in an Uu RRC message that is sent over PC5 to the 5G ProSe Layer-2 UE-to-Network Relay and the 5G ProSe Layer-2 UE-to-Network Relay forwards the Uu RRC message to the NG-RAN specified in TS 38.300. NG-RAN selects the 5G ProSe Layer-2 Remote UE's serving AMF and forwards the NAS message to this AMF.
If 5G ProSe Layer-2 Remote UE has not performed the initial registration, the NAS message is an initial Registration message. Otherwise, the NAS message is either a service request message, or a mobility or periodic Registration message taking into account the TAI in the RRC container received from the 5G ProSe Layer-2 UE-to-Network Relay during Relay Discovery (see clause 5.8.3.3) or PC5-RRC message, as specified in TS 38.300.
Step 7.
The 5G ProSe Layer-2 Remote UE may trigger the PDU Session Establishment procedure as defined in clause 4.3.2.2 of TS 23.502.
Step 8.
The data is transferred between the 5G ProSe Layer-2 Remote UE and UPF via the 5G ProSe Layer-2 UE-to-Network Relay and NG-RAN. The 5G ProSe Layer-2 UE-to-Network Relay forwards all the data messages between the 5G ProSe Layer-2 Remote UE and NG-RAN, as specified in TS 38.300.
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6.5.2.3  Path switching between direct network communication path and indirect network communication path via 5G ProSe Layer-2 UE-to-Network Relay |R18|  p. 116

Xn based (as defined in clause 4.9.1.2 of TS 23.502) and N2 based (as defined in clause 4.9.1.3 of TS 23.502) Handover procedure is used for inter-gNB indirect-to-direct and inter-gNB direct-to-indirect path switching for 5G ProSe Layer-2 Remote UE in CM-CONNECTED with RRC_CONNECTED state.
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6.5.3  5G ProSe UE-to-Network Relay reselection  p. 116

After being connected to the 5G ProSe UE-to-Network Relay, the 5G ProSe Remote UE keeps performing the measurement of the signal strength of PC5 unicast link with the 5G ProSe UE-to-Network Relay for relay reselection.
The 5G ProSe UE-to-Network Relay discovery procedures defined in clause 6.3.2.3 are used to discover available 5G ProSe UE-to-Network Relays for 5G ProSe UE-to-Network Relay reselection.

6.5.4  5G ProSe Remote UE traffic handling for 5G ProSe UE-to-Network Relay support  p. 116

For the 5G ProSe Remote UE to access the service via 5G ProSe UE-to-Network Relay, the following apply:
  • The application traffic on the 5G ProSe Remote UE is managed by URSP rules (with consideration of local configurations), following the procedure defined in clauses 6.1.2.2.1 and 6.6.2.3 of TS 23.503. The URSP rules defined in clause 6.6.2.1 of TS 23.503 applies for the 5G ProSe Remote UE, with RSD enhanced to include:
    • a "ProSe Layer-3 UE-to-Network Relay Offload indication"; or
    • a "ProSe Multi-path Preference".
  • If an application or application traffic matches a URSP rule, corresponding RSDs shall be used to evaluate the existing PDU sessions, or establish a new PDU session, or determine to offload outside of a PDU session, or multi-path communication via 5G ProSe Layer-3 UE-to-Network Relay outside of a PDU session and a PDU Session over Uu reference point.
  • If the selected RSD contains "ProSe Layer-3 UE-to-Network Relay Offload indication":
    • The 5G ProSe Remote UE will route the traffic to the 5G ProSe Layer-3 UE-to-Network Relay connection without establishing a PDU session, when such connection is available.
      This may trigger the 5G ProSe Remote UE to start 5G ProSe UE-to-Network Relay discovery if it is not yet started. The discovery and establishment of the connection with the 5G ProSe Layer-3 UE-to-Network Relay is controlled by the ProSe Policy (pre-) configured on the 5G ProSe Remote UE.
    • If the matched URSP rule contains both a RSD with "Non-Seamless Offload indication" and a RSD with "ProSe Layer-3 UE-to-Network Relay Offload indication", whether to offload the traffic to non-3GPP access or the 5G ProSe Layer-3 UE-to-Network Relay connection depends on the priority of the RSDs and the availability of the connections, as specified in the clause 6.6.2.3 of TS 23.503.
    • If the selected RSD does not contain the "ProSe Layer-3 UE-to-Network Relay Offload indication" or, "Non-Seamless Offload indication", the 5G ProSe Remote UE shall use a PDU session to route the corresponding application traffic.
      If configured in the ProSe Policy, the 5G ProSe Remote UE may attempt the discovery of a Relay Service Code corresponding to a 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support in the discovery procedure, when the non-3GPP access type is preferred in the selected RSD. The 5G ProSe Remote UE may attempt the discovery of a Relay Service Code corresponding to a 5G ProSe Layer-2 UE-to-Network Relay in the discovery procedure, when the 3GPP access type is preferred in the selected RSD.
    • If the selected RSD contains "ProSe Multi-path Preference":
      • The 5G ProSe Remote UE is preferred to route the traffic over a PC5 connection with a 5G ProSe Layer-3 UE-to-Network Relay and a PDU Session matched to other components (e.g., Network Slice Selection) in the selected RSD. If either connection is not available then the traffic may be routed via a PC5 connection with a 5G ProSe Layer-3 UE-to-Network Relay or a PDU Session matched to other components.
      • If the PC5 connection with 5G ProSe Layer-3 UE-to-Network Relay associated with the desired RSC is not available, this may trigger the 5G ProSe Remote UE to start 5G ProSe UE-to-Network Relay discovery and connection establishment, controlled by the ProSe Policy configured on the 5G ProSe Remote UE. If the PC5 connection with 5G ProSe Layer-3 UE-to-Network Relay associated with the desired RSC is available, this may trigger Layer-2 link modification to associate the corresponding ProSe service into the PC5 connection. Determination on the desired RSC is based on the Policy/Parameters specified in clause 5.1.4.1.
      • If the PDU Session matched to other components in the selected RSD is not available, this may trigger the establishment of a new PDU Session over Uu reference point using the values specified by the selected RSD.
  • If the 5G ProSe Remote UE has an indirect connection via a 5G ProSe Layer-2 UE-to-Network Relay connection available, it will be treated as the "3GPP" access type. If the 5G ProSe Remote UE has an indirect connection via a 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support available, it will be treated as the "non-3GPP" access type. The URSP handling as defined in TS 23.503 applies.
For the 5G ProSe Layer-3 UE-to-Network Relay and 5G ProSe Layer-2 UE-to-Network Relay, the URSP handling does not apply to the relayed traffic from the 5G ProSe Remote UE.
For the 5G ProSe Layer-3 UE-to-Network Relay, the PDU session established for relaying the 5G ProSe Remote UE's traffic is controlled by the ProSe Policy.
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6.5.5  Procedures for Communication Path Switching between Two UE-to-Network Relays |R18|  p. 117

6.5.5.1  General  p. 117

The communication path switching between two UE-to-Network Relays includes the following scenarios:
  • Layer-3 UE-to-Network Relay with N3IWF switching from/to Layer-3 UE-to-Network Relay with N3IWF.
  • Layer-3 UE-to-Network Relay without N3IWF switching from/to Layer-3 UE-to-Network Relay without N3IWF.
  • Layer-3 UE-to-Network Relay without N3IWF switching from/to Layer-3 UE-to-Network Relay with N3IWF.
  • Layer-2 UE-to-Network Relay switching from/to Layer-2 UE-to-Network Relay.
  • Layer-2 UE-to-Network Relay switching from/to Layer-3 UE-to-Network Relay without N3IWF.
  • Layer-2 UE-to-Network Relay switching from/to Layer-3 UE-to-Network Relay with N3IWF.
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6.5.5.2  Procedures for Communication Path Switching between UE-to-Network Relays  p. 118

This clause specifies the communication path switching procedures between two UE-to-Network Relays.
When performing the path switching between two 5G ProSe Layer-3 UE-to-Network Relay without N3IWF, or between 5G ProSe Layer-3 UE-to-Network Relay without N3IWF and 5G ProSe Layer-2 UE-to-Network Relay, or between 5G ProSe Layer-3 UE-to-Network Relay without N3IWF and 5G ProSe Layer-3 UE-to-Network Relay with N3IWF, the application layer procedures are used for service continuity support which is out of scope of this specification.
Path switching between two 5G ProSe Layer-3 UE-to-Network Relay with N3IWF is performed using the mechanism specified for Local mobility anchor within untrusted non-3GPP access networks using MOBIKE in clause 6.2.9 of TS 23.501.
When a 5G ProSe-enabled UE as a 5G ProSe Layer-2 Remote UE in CM-CONNECTED state switches the communication path to Layer-3 UE-to-Network Relay with N3IWF, 5G ProSe-enabled UE's PDU session(s) can handover to the indirect path via Layer-3 UE-to-Network Relay with N3IWF by reusing the procedures specified in clause 4.9.2 of TS 23.502.
When a 5G ProSe-enabled UE as a 5G ProSe Layer-3 Remote UE switches the communication path from Layer-3 UE-to-Network Relay with N3IWF to an indirect path via Layer-2 UE-to-Network Relay, 5G ProSe-enabled UE's PDU session(s) can handover to the indirect path via Layer-2 UE-to-Network Relay by reusing the procedures specified in clause 4.9.2 of TS 23.502.
Procedure on Path switching between two 5G ProSe Layer-2 UE-to-Network Relays is specified in TS 38.300.
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