6.5 5G ProSe UE-to-Network Relay Communication

23.3043GPPProximity based Services (ProSe) in the 5G System (5GS)Release 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.5.1 5G ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay

6.5.1.1 5G ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay without N3IWF

A 5G ProSe Layer-3 UE-to-Network Relay registers to the network (if not already registered). 5G ProSe Layer-3 UE-to-Network Relay establishes a PDU Session(s) or modifies an existing PDU Session(s) in order to provide relay traffic towards 5G ProSe Layer-3 Remote UE(s). PDU Session(s) supporting 5G ProSe Layer-3 UE-to-Network Relay shall only be used for 5G ProSe Layer-3 Remote UE(s) relay traffic.

The PLMN serving the 5G ProSe Layer-3 UE-to-Network Relay and the PLMN to which the 5G ProSe Layer-3 Remote UE registers can be the same PLMN or two different PLMNs.

Figure 6.5.1.1-1: 5G ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay without N3IWF

1. Service authorization and provisioning are performed for the 5G ProSe Layer-3 UE-to-Network Relay (step 1a) and 5G ProSe Layer-3 Remote UE (step 1b) as described in clause 6.2.

2. The 5G ProSe Layer-3 UE-to-Network Relay may establish a PDU Session for relaying. In the case of IPv6, the 5G ProSe Layer-3 UE-to-Network Relay obtains the IPv6 prefix via prefix delegation function from the network as defined in TS 23.501 [4].

NOTE 1: 5G ProSe Layer-3 UE-to-Network Relay can establish a PDU Session for any Relay Service Code it supports before the connection is established with the 5G ProSe Layer-3 Remote UE.

3. The 5G ProSe Layer-3 Remote UE performs discovery of a 5G ProSe Layer-3 UE-to-Network Relay as described in clause 6.3.2.3. As part of the discovery procedure the 5G ProSe Layer-3 Remote UE learns about the connectivity service the 5G ProSe Layer-3 UE-to-Network Relay provides.

4. The 5G ProSe Layer-3 Remote UE selects a 5G ProSe Layer-3 UE-to-Network Relay and establishes a connection for unicast mode communication as described in clause 6.4.3.6. If there is no PDU Session associated with the Relay Service Code or a new PDU Session for relaying is needed, the 5G ProSe Layer-3 UE-to-Network Relay initiates a new PDU Session establishment procedure for relaying before completing the PC5 connection establishment.

The network decides that the PDU session to be established is for relay traffic, and then generates the QoS rules and QoS Flow level QoS parameters to 5G ProSe Layer-3 UE-to-Network Relay with relay consideration and can initiate the setup of QoS flows as specified in clause 5.6.2.1. The Remote UE can also initiate the setup of QoS flows by providing PC5 QoS info and (optionally) PC5 QoS rule(s) to the 5G ProSe Layer-3 UE-to-Network Relay during connection setup, according to the procedure as specified in clause 5.6.2.1.

The 5G ProSe Layer-3 UE-to-Network Relay determines the PDU Session type for relaying as specified in clause 5.4.1.1.

According to the PDU Session Type for relaying, the 5G ProSe Layer-3 UE-to-Network Relay performs relaying function at the corresponding layer as follows:

– When the IP type PDU Session is used for IP traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay acts as an IP router. For IPv4, the 5G ProSe Layer-3 UE-to-Network Relay performs IPv4 NAT between IPv4 addresses assigned to the 5G ProSe Layer-3 Remote UE and the IPv4 address assigned to the PDU Session used for the relay traffic.

– When the Ethernet type PDU Session is used for Ethernet traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay acts as an Ethernet switch.

– When the Unstructured type PDU Session is used for Unstructured traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay performs traffic relaying based on a mapping between the PC5 Link Identifier and the PDU Session ID, and a mapping between PFI for PC5 Layer-2 link and the QFI for the PDU Session. These mappings are created when the Unstructured type PDU Session is established for the 5G ProSe Layer-3 Remote UE.

– When the IP type PDU Session is used for Ethernet or Unstructured traffic over PC5 reference point, the 5G ProSe Layer-3 UE-to-Network Relay uses IP tunneling. For this IP tunnelling, the 5G ProSe Layer-3 UE-to-Network Relay locally assigns an IP address/prefix for the 5G ProSe Layer-3 Remote UE and uses it on the Uu reference point to encapsulate and decapsulate the uplink and downlink traffic for the 5G ProSe Layer-3 Remote UE. The tunnelled traffic over Uu reference point is transported over the PC5 reference point as Ethernet or Unstructured traffic.

5. For IP PDU Session Type and IP traffic over PC5 reference point, IPv6 prefix or IPv4 address (including NAT case) is allocated for the 5G ProSe Layer-3 Remote UE as defined in clause 5.5.1.3.

6. The 5G ProSe Layer-3 Remote UE may provide PC5 QoS Info and PC5 QoS rule(s) to the 5G ProSe Layer-3 UE-to-Network Relay using Layer-2 link modification procedure as specified in clause 6.4.3.4. The 5G ProSe Layer-3 UE-to-Network Relay generates the Packet Filters used over Uu interface based on the received PC5 QoS Info and QoS Rule(s) as described in clause 5.6.2.1, and may perform the UE requested PDU Session Modification as defined in TS 23.502 [5] clause 4.3.3 to setup a new QoS Flow or bind the traffic to an existing QoS Flow.

From this point the uplink and downlink relaying can start. For downlink traffic forwarding, the PC5 QoS Rule is used to map the downlink packet to the PC5 QoS Flow. For uplink traffic forwarding, the 5G QoS Rule is used to map the uplink packet to the Uu QoS Flow.

7. The 5G ProSe Layer-3 UE-to-Network Relay shall send a Remote UE Report (Remote User ID, Remote UE info) message to the SMF for the PDU Session associated with the relay. The Remote User ID is an identity of the 5G ProSe Layer-3 Remote UE user that was successfully connected in step 4. The Remote UE info is used to assist identifying the 5G ProSe Layer-3 Remote UE in the 5GC. For IP PDU Session Type, the Remote UE info is Remote UE IP info. For Ethernet PDU Session Type, the Remote UE info is Remote UE MAC address which is detected by the 5G ProSe Layer-3 UE-to-Network Relay. For Unstructured PDU Session Type, the Remote UE info is not included. The SMF stores the Remote User IDs and the related Remote UE info in the 5G ProSe Layer-3 UE-to-Network Relay’s SM context for this PDU Session associated with the relay.

The Remote UE Report is N1 SM NAS message sent with the PDU Session ID to the AMF, in turn delivered to the SMF.

NOTE 2: The privacy protection for Remote User ID depends on SA WG3 design.

For IP info the following principles apply:

– for IPv4, the 5G ProSe Layer-3 UE-to-Network Relay shall report TCP/UDP port ranges assigned to individual 5G ProSe Layer-3 Remote UE(s) (along with the Remote User ID);

– for IPv6, the 5G ProSe Layer-3 UE-to-Network Relay shall report IPv6 prefix(es) assigned to individual 5G ProSe Layer-3 Remote UE(s) (along with the Remote User ID).

If the PDU Session for relaying is released by the UE-to-Network Relay or the network as described in clause 4.3.4 of TS 23.502 [5], the UE-to-Network Relay should initiate the release of the layer-2 links associated with the released PDU Session using the procedure defined in clause 6.4.3.3.

The PDU Session(s) used for relaying should be released as described in clause 4.3.4 of TS 23.502 [5] (e.g. by 5G ProSe Layer-3 UE-to-Network Relay), if the service authorization for acting as a 5G ProSe Layer-3 UE-to-Network Relay in the serving PLMN is revoked.

The 5G ProSe Layer-3 UE-to-Network Relay shall send the Remote UE Report message when the 5G ProSe Layer-3 Remote UE disconnects from the 5G ProSe Layer-3 UE-to-Network Relay (e.g. upon explicit layer-2 link release or based on the absence of keep alive messages over PC5) to inform the SMF that the 5G ProSe Layer-3 Remote UE(s) have left.

NOTE 3: In order for the SMF to have the 5G ProSe Layer-3 Remote UE(s) information, the HPLMN and the VPLMN where the 5G ProSe Layer-3 UE-to-Network Relay is authorised to operate, needs to support the transfer of the 5G ProSe Layer-3 Remote UE related parameters if the SMF is in the HPLMN.

It is up to 5G ProSe Layer-3 UE-to-Network Relay implementation how PDU Session(s) used for relaying are released or QoS Flow(s) used for relaying are removed by the 5G ProSe Layer-3 UE-to-Network Relay when 5G ProSe Layer-3 Remote UE(s) disconnect from the 5G ProSe Layer-3 UE-to-Network Relay.

6.5.1.2 5G ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support

6.5.1.2.1 Connection management via 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support

In order to relay 5G ProSe Layer-3 Remote UE’s traffic via N3IWF, the 5G ProSe Layer-3 UE-to-Network Relay needs suitable ProSe Policies configured for establishing a PDU Session associated with a UPF that conveys the traffic towards the N3IWF.5G ProSe Layer-3 UE-to-Network Relay registers to the network as specified in clause 6.5.1.1. Based on configuration and authorization, the 5G ProSe Layer-3 UE-to-Network Relay is provisioned with PDU Session parameters in the ProSe Policy allowing the access to the N3IWF. When the corresponding PDU Session is established, the 5GS, e.g. SMF, based on the parameters (i.e. DNN, S-NSSAI) selects the UPF that ensures the connection to the N3IWF. The UPF for the 5G ProSe UE-to-Network Relay and the N3IWF may be collocated.

A 5G ProSe Layer-3 UE-to-Network Relay with a PDU Session providing access via N3IWF may also have other PDU Sessions for supporting access from the 5G ProSe Layer-3 Remote UE without going through a N3IWF.

NOTE 1: Whether a different PDU Sessions need to be established to serve traffics for Layer-3 Remote UE with or without going through a N3IWF is determined by Layer-3 Relay UE per TS 23.503 [9].

Figure 6.5.1.2.1-1: Connection establishment over 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support

1. 5G ProSe Layer-3 UE-to-Network Relay performs Registration procedures and obtains the ProSe Policy that corresponds to the operation supporting the access to N3IWF. The ProSe Policy includes the RSC and PDU Session parameters allowing the access to the N3IWF.

The 5G ProSe Layer-3 Remote UE is configured with the corresponding ProSe Policy and URSP rules. The URSP policy indicates if a particular service needs to be accessed within a PDU Session and thus should use a 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support as described in clause 6.5.4.

2-4. A 5G ProSe Layer-3 UE-to-Network Relay and 5G ProSe Layer-3 Remote UE follow the procedures described in steps 3-5 in clause 6.5.1.1 using the RSC configured for making the 5G ProSe Layer-3 Remote UE access to 5GC via N3IWF.

NOTE 2: The services requiring the access via N3IWF can be configured with the RSC(s) that can be served by the same 5G ProSe UE-to-Network Relay.

5. The 5G ProSe Layer-3 Remote UE that connects to a 5G ProSe Layer-3 UE-to-Network Relay with N3IWF support selects an N3IWF and determines the N3IWF IP address. The 5G ProSe Layer-3 Remote UE follows the N3IWF selection procedure as described in clause 6.5.1.2.2.

6. The 5G ProSe Layer-3 Remote UE establishes a signalling IPsec tunnel using IKE procedures with a N3IWF and performs NAS Registration as shown in Figure 4.12.2.2-1 of TS 23.502 [5]. After the IPSec tunnel is established, the 5G ProSe Layer-3 Remote UE can perform any of the NAS procedures (incl. PDU Session establishment for the 5G ProSe Layer-3 UE-to-Network Relay PDU Sessions) as specified in clause 4.12 of TS 23.502 [5].

7. After the PDU session(s) and associated QoS Flows are established in the 5G ProSe Layer-3 Remote UE’s 5GC, the N3IWF determines the number of IPsec Child SA(s) that is needed and initiates the creation of the Child SA(s) as specified in clause 4.12.5 of TS 23.502 [5]. Once the Child SA(s) has been created the 5G ProSe Layer‑3 Remote UE will have the mapping between the DSCP markings for the IPsec Child SA(s), the corresponding QoS, and N3IWF IP address(es) and provides this information, if needed, to the 5G ProSe Layer-3 UE-to-Network Relay as specified in clause 5.6.2.2. If needed, the 5G ProSe Layer-3 UE-to-Network Relay performs the PDU Session Modification procedure to request QoS flow(s) configuration that maps to the 5G ProSe Layer-3 Remote UE’s Child SA(s).

IKE keep alive(s) between the 5G ProSe Layer-3 Remote UE and the N3IWF are used for detecting possible path failure. The 5G ProSe Layer-3 Remote UE may change 5G ProSe Layer-3 UE-to-Network Relay(s) while maintain the session with the N3IWF when the 5G ProSe Layer-3 Remote UE and the N3IWF support MOBIKE. This is negotiated between the 5G ProSe Layer-3 Remote UE and the N3IWF as specified in TS 23.502 [5], clause 4.12.2.2). When IKE keep alive(s) are used, the 5G ProSe Layer-3 Remote UE needs to keep the PC5 connection and 5G ProSe Layer-3 UE-to-Network Relay keeps the PDU Session.

When 5G ProSe Remote UE is in CM-CONNECTED state, the 5G ProSe Remote UE keeps 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.

6.5.1.2.2 N3IWF selection for 5G ProSe Layer-3 Remote UE procedure

When the 5G ProSe Layer-3 Remote UE relays traffic over 5G ProSe Layer-3 UE-to-Network Relay that supports N3IWF, the 5G ProSe Layer-3 Remote UE selects the N3IWF using the N3IWF selection procedure that is specified in clause 6.3.6.2 of TS 23.501 [4] for untrusted non-3GPP access with the following differences.

To support the N3IWF selection for 5G ProSe Layer-3 Remote UE, a 5G ProSe Layer-3 Remote UE is configured by HPLMN with N3IWF identifier configuration for 5G ProSe Layer-3 Remote UE and 5G ProSe Layer-3 UE-to-Network Relay access node selection information as described in clause 5.1.4.1.

When the 5G ProSe Layer-3 Remote UE decides to select an N3IWF in the HPLMN, the 5G ProSe Layer-3 Remote UE uses the N3IWF identifier configuration for 5G ProSe Layer-3 Remote UE, if configured, to find the IP address of the N3IWF in the HPLMN. Otherwise, 5G ProSe Layer-3 Remote UE constructs N3IWF FQDN based on either the Tracking Area Identity FQDN or on Operator Identifier FQDN of the 5G ProSe Layer-3 UE-to-Network Relay node selection information.

To assist the 5G ProSe Layer-3 Remote UE with N3IWF selection, the 5G ProSe Layer-3 UE-to-Network Relay supporting N3IWF access advertises the 5GS TAI corresponding to the serving cell, as defined in clause 5.8.3.2, in the 5G ProSe UE-to-Network Relay Discovery procedure as defined in clause 6.3.2.3.

A 5G ProSe Layer-3 Remote UE constructs the FQDN using either Tracking Area Identity FQDN or on Operator Identifier FQDN and selects the N3IWF using the procedures of N3IWF selection in clause 6.3.6.2 of TS 23.501 [4].

6.5.1.2.3 Mobility of 5G ProSe Layer-3 Remote UE between Direct and Indirect Network communication path

When 5G ProSe Layer-3 Remote UE changes from Direct Network Communication to Indirect Network Communication path, TS 23.502 [5] clause 4.9.2.2 applies after the 5G ProSe Layer-3 Remote UE establishes PC5 connection to the 5G Layer-3 UE-to-Network Relay.

When 5G ProSe Layer-3 Remote UE changes from Indirect Network Communication path to Direct Network Communication, the 5G ProSe Layer-3 Remote UE follows TS 23.502 [5] clause 4.9.2.1.

6.5.1.3 Additional parameters announcement procedure

Additional parameters announcement procedure outlined in figure 6.5.1.3-1 is used by a 5G ProSe Remote UE to request a 5G ProSe UE-to-Network Relay to announce additional parameters (for model A) as defined in clause 5.8.3.

Figure 6.5.1.3-1: Additional parameters announcement procedure

1. 5G ProSe Remote UE has discovered a 5G ProSe UE-to-Network Relay and requires additional parameters.

2. The 5G ProSe Remote UE sends to the 5G ProSe UE-to-Network Relay an Additional Parameters Announcement Request to obtain additional parameters.

3. The 5G ProSe UE-to-Network Relay acknowledges receipt of the request in step 2 with an Additional Parameters Announcement Response (Additional_Parameters_Announcement_Request_Refresh Timer). The Additional_Parameters_Announcement_Request_Refresh Timer (configurable in the 5G ProSe UE-to-Network Relay), is provided to the 5G ProSe Remote UE so that when this timer expires the 5G ProSe Remote UE repeats the Additional Parameters Announcement Request procedure if it still needs to obtain the additional parameters. If the 5G ProSe Remote UE does not initiate new Additional Parameters Announcement Request procedure when this Additional_Parameters_Announcement_Request_Refresh Timer expires and no other UE request additional parameters announcement before the Additional_Parameters_Announcement_Request_Refresh timer expires in the 5G ProSe UE-to-Network Relay, then the relay shall stop announcing the addtional parameters.

4. The 5G ProSe UE-to-Network Relay announces the additional parameters by sending Relay Discovery Additional Information message as defined in clause 5.8.3. This is repeated periodically with a configurable frequency (normally higher than the one related to the Additional_Parameters_Announcement_Request_Refresh Timer) until there is no UE requesting to announce the additional parameters as determined by the Additional_Parameters_Announcement_Request_Refresh Timer running in the 5G ProSe UE-to-Network Relay.

NOTE: Based on UE implementation, the 5G ProSe UE-to-Network Relay can send the Relay Discovery Additional Information message several times consecutively in step 4 if there are other 5G ProSe Remote UE(s) that have connected to the 5G ProSe UE-to-Network Relay but not yet requested any additional parameters. This ensures the other 5G ProSe Remote UE(s) obtain such additional parameters without invoking any new request(s).

5. The 5G ProSe UE-to-Network Relay detects new or updated additional parameters.

6. Detection of new or updated additional parameters in step 5 triggers the 5G ProSe UE-to-Network Relay to announce the additional parameters by sending a Relay Discovery Additional Information Message immediately and to repeat it periodically with a configurable frequency as in step 4 until there are no UEs requesting to announce the additional parameters, i.e. until the Additional_Parameters_Announcement_Request_Refresh Timer expires in the 5G ProSe UE-to-Network Relay.

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

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 [4] and TS 23.502 [5]. 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

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 [4] and TS 23.502 [5] 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.

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 [4] and TS 23.502 [5], and the paging message delivery from NG-RAN to 5G ProSe Layer-2 Remote UE is specified in TS 38.300 [12].

6.5.2.2 Connection establishment

Figure 6.5.2.2-1: Connection Establishment for 5G ProSe Layer-2 UE-to-Network Relay

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 [5].

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 in coverage, 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.

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.

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.

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 [12].

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 [5].

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 [12]. 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 [12].

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 [5].

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 [12].

6.5.3 5G ProSe UE-to-Network Relay reselection

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.

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

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 [9]. The URSP rules defined in clause 6.6.2.1 of TS 23.503 [9] applies for the 5G ProSe Remote UE, with RSD enhanced to include:

– a new "5G ProSe Layer-3 UE-to-Network Relay Offload indication".

– 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.

– If the selected RSD contains "5G 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 " 5G 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 [9].

– If the selected RSD does not contain the "5G 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 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 [9] 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.