6.4 5G ProSe Direct Communication

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

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

6.4.1 Broadcast mode 5G ProSe Direct Communication

To perform 5G ProSe direct communication over PC5 reference point in broadcast mode operation, the UE is configured with the related information as described in clause 5.1.3.

Figure 6.4.1-1 shows the procedure for broadcast mode of 5G ProSe direct communication over PC5 reference point.

Figure 6.4.1-1: Procedure for Broadcast mode of 5G ProSe direct communication over PC5 reference point

1. The ProSe layer of receiving UE(s) determines the following for the broadcast mode communication reception:

– the destination Layer-2 ID for broadcast reception as specified in clause 5.8.2.2;

– the PC5 QoS parameters for this broadcast ProSe service as specified in clause 5.6.1; and

– the NR Tx Profile based on the configuration as specified in clause 5.1.3.1.

The destination Layer-2 ID, the NR Tx Profile and the PC5 QoS parameters are passed down to the AS layer of receiving UE(s) for the reception.

The AS layer of receiving UE(s) determines the PC5 DRX parameter values as specified in clause 5.13.

2. The transmitting UE ProSe application layer provides data unit and may provide ProSe Application Requirements specified in clause 5.6.1 to ProSe layer.

3. The ProSe layer of transmitting UE determines the following for the broadcast mode communication transmission:

– the destination Layer-2 ID for broadcast as specified in clause 5.8.2.2;

– the PC5 QoS parameters for this broadcast ProSe service as specified in clauses 5.6.1; and

– the NR Tx Profile based on the configuration as specified in clause 5.1.3.1.

The transmitting UE self-assigns the source Layer-2 ID as specified in clause 5.8.2.2.

The source Layer-2 ID, the destination Layer-2 ID, the NR Tx Profile and the PC5 QoS parameters are passed down to the AS layer of transmitting UE for the transmission.

The AS layer of transmitting UE determines the PC5 DRX parameter values as specified in clause 5.13.

4. The transmitting UE sends the ProSe data using the source Layer-2 ID and the destination Layer-2 ID.

NOTE: In step 4, there is only one broadcast message from the transmitting UE.

6.4.2 Groupcast mode 5G ProSe Direct Communication

Figure 6.4.2-1 shows the procedure for groupcast mode of 5G ProSe Direct Communication.

Figure 6.4.2-1: Procedure for groupcast mode 5G ProSe Direct communication

Steps 1 to 3 are optional, e.g. applicable for Application Layer managed group. Groupcast mode ProSe Direct communication can be performed without steps 1 to 3 based on the provisioned parameters as defined in clause 5.1.3.1.

1. ProSe Group management is carried out at ProSe application layer. This may be performed in coordination with ProSe Application Server.

Group Member Discovery parameters as specified in clause 5.1.2.1 can be provisioned in ME from PCF or configured in UICC or provided from ProSe Application Server to the UE.

2. Each UE may perform a ProSe group member discovery (similar to the procedures in clause 6.3.2.2) that is restricted only to users sharing the same Application Layer Group ID obtained in step 1.

The Application layer discovery messages including the Application Layer Group ID for Application Layer managed group may be exchanged between UEs to discover each other sharing the same Application Layer Group ID. When the group is formed, the Application layer discovery message including group size and member ID may be provided by one UE to all other UEs. The Application layer discovery messages may be included as metadata in a PC5 direct discovery message.

3. The ProSe Application layer may provide group identifier information (i.e. the Application Layer Group ID discovered based on discovery messages exchanged in step 2) as specified in clause 5.8.2.3.

The ProSe application layer may also provide ProSe Application Requirements for this communication.

If the ProSe application layer does not provide ProSe Application Requirements, the ProSe layer determines the PC5 QoS parameters based on the mapping of ProSe service to PC5 QoS parameters as specified in clause 5.1.3.1.

The ProSe application layer may provide a group size and a member ID (optionally based on discovery messages exchanged in step 2) as specified in clause 5.3.3 for Application Layer managed group.

4. The ProSe layer of transmitting UE self-assigns a source Layer-2 ID and determines the following for the groupcast mode communication transmission:

– a destination Layer-2 ID as specified in clauses 5.8.2.1 and 5.8.2.3.

– The PC5 QoS parameters for this groupcast mode communication as specified in clause 5.6.1.

– the NR Tx Profile based on the configuration as specified in clause 5.1.3.1.

The source Layer-2 ID, destination Layer-2 ID, the NR Tx Profile and the PC5 QoS parameters are passed down to the AS layer of transmitting UE for the groupcast mode communication transmission.

The ProSe layer of receiving UE(s) determines the following for the groupcast mode communication reception:

– destination Layer-2 ID as specified in clauses 5.8.2.1 and 5.8.2.3;

– the PC5 QoS parameters for this groupcast mode communication as specified in clause 5.6.1; and

– the NR Tx Profile based on the configuration as specified in clause 5.1.3.1.

The destination Layer-2 ID, the NR Tx Profile and the PC5 QoS parameters are passed down to the AS layer of receiving UE(s) for the reception.

If the group size and the member ID for Application Layer managed group are provided by the ProSe application layer, the ProSe layer passes them to the AS layer as described in clause 5.3.3.

The AS layer of transmitting UE and the AS layer of receiving UE(s) determine the PC5 DRX parameter values as specified in clause 5.13.

5. Transmitting UE sends the ProSe data using the source Layer-2 ID and the destination Layer-2 ID.

NOTE: In step 5, there is only one groupcast message from the transmitting UE. That is, all receiving UEs can receive the groupcast message directly from the transmitting UE.

6.4.3 Unicast mode 5G ProSe Direct Communication

6.4.3.1 Layer-2 link establishment over PC5 reference point

To perform unicast mode of ProSe Direct communication over PC5 reference point, the UE is configured with the related information as described in clause 5.1.3.

Figure 6.4.3.1-1 shows the layer-2 link establishment procedure for the unicast mode of ProSe Direct communication over PC5 reference point.

Figure 6.4.3.1-1: Layer-2 link establishment procedure

1. The UE(s) determine the destination Layer-2 ID for signalling reception for PC5 unicast link establishment as specified in clause 5.8.2.4.

2. The ProSe application layer in UE-1 provides application information for PC5 unicast communication. The application information includes the ProSe Service Info, UE’s Application Layer ID. The target UE’s Application Layer ID may be included in the application information.

The ProSe application layer in UE-1 may provide ProSe Application Requirements for this unicast communication. UE-1 determines the PC5 QoS parameters and PFI as specified in clause 5.6.1.

If UE-1 decides to reuse the existing PC5 unicast link as specified in clause 5.3.4, the UE triggers the Layer-2 link modification procedure as specified in clause 6.4.3.4.

3. UE-1 sends a Direct Communication Request message to initiate the unicast layer-2 link establishment procedure. The Direct Communication Request message includes:

– Source User Info: the initiating UE’s Application Layer ID (i.e. UE-1’s Application Layer ID).

– If the ProSe application layer provided the target UE’s Application Layer ID in step 2, the following information is included:

– Target User Info: the target UE’s Application Layer ID (i.e. UE-2’s Application Layer ID).

– ProSe Service Info: the information about the ProSe identifier(s) requesting Layer-2 link establishment.

– Security Information: the information for the establishment of security.

NOTE 1: The Security Information and the necessary protection of the Source User Info and Target User Info are defined by SA WG3.

The source Layer-2 ID and destination Layer-2 ID used to send the Direct Communication Request message are determined as specified in clauses 5.8.2.1 and 5.8.2.4. The destination Layer-2 ID may be broadcast or unicast Layer-2 ID. When unicast Layer-2 ID is used, the Target User Info shall be included in the Direct Communication Request message.

UE-1 sends the Direct Communication Request message via PC5 broadcast or unicast using the source Layer-2 ID and the destination Layer-2 ID.

A default PC5 DRX configuration may be used for transmitting and receiving of this message (see TS 38.300 [12]).

4. Security with UE-1 is established as below:

4a. If the Target User Info is included in the Direct Communication Request message, the target UE, i.e. UE-2, responds by establishing the security with UE-1.

4b. If the Target User Info is not included in the Direct Communication Request message, the UEs that are interested in using the announced ProSe Service(s) over a PC5 unicast link with UE-1 responds by establishing the security with UE-1.

NOTE 2: The signalling for the Security Procedure is defined by SA WG3.

When the security protection is enabled, UE-1 sends the following information to the target UE:

– If IP communication is used:

– IP Address Configuration: For IP communication, IP address configuration is required for this link and indicates one of the following values:

– "DHCPv4 server" if only IPv4 address allocation mechanism is supported by the initiating UE, i.e., acting as a DHCPv4 server; or

– "IPv6 Router" if only IPv6 address allocation mechanism is supported by the initiating UE, i.e., acting as an IPv6 Router; or

– "DHCPv4 server & IPv6 Router" if both IPv4 and IPv6 address allocation mechanism are supported by the initiating UE; or

– "address allocation not supported" if neither IPv4 nor IPv6 address allocation mechanism is supported by the initiating UE.

– Link-Local IPv6 Address: a link-local IPv6 address formed locally based on RFC 4862 [17] if UE-1 does not support the IPv6 IP address allocation mechanism, i.e. the IP Address Configuration indicates" address allocation not supported".

– QoS Info: the information about PC5 QoS Flow(s). For each PC5 QoS Flow, the PFI and the corresponding PC5 QoS parameters (i.e. PQI and conditionally other parameters such as MFBR/GFBR, etc.) and optionally the associated ProSe identifier(s).

– Optional PC5 QoS Rule(s).

The source Layer-2 ID used for the security establishment procedure is determined as specified in clauses 5.8.2.1 and 5.8.2.4. The destination Layer-2 ID is set to the source Layer-2 ID of the received Direct Communication Request message.

Upon receiving the security establishment procedure messages, UE-1 obtains the peer UE’s Layer-2 ID for future communication, for signalling and data traffic for this unicast link.

5. A Direct Communication Accept message is sent to UE-1 by the target UE(s) that has successfully established security with UE-1:

5a. (UE oriented Layer-2 link establishment) If the Target User Info is included in the Direct Communication Request message, the target UE, i.e. UE-2 responds with a Direct Communication Accept message if the Application Layer ID for UE-2 matches.

5b. (ProSe Service oriented Layer-2 link establishment) If the Target User Info is not included in the Direct Communication Request message, the UEs that are interested in using the announced ProSe Service(s) respond to the request by sending a Direct Communication Accept message (UE-2 and UE-4 in Figure 6.4.3.1-1).

The Direct Communication Accept message includes:

– Source User Info: Application Layer ID of the UE sending the Direct Communication Accept message.

– QoS Info: the information about PC5 QoS Flow(s). For each PC5 QoS Flow, the PFI and the corresponding PC5 QoS parameters requested by UE-1 (i.e. PQI and conditionally other parameters such as MFBR/GFBR, etc.) and optionally the associated ProSe identifiers(s).

– Optional PC5 QoS Rule(s).

– If IP communication is used:

– IP Address Configuration: For IP communication, IP address configuration is required for this link and indicates one of the following values:

– " DHCPv4 server " if only IPv4 address allocation mechanism is supported by the target UE, i.e., acting as a DHCPv4 server; or

– "IPv6 Router" if only IPv6 address allocation mechanism is supported by the target UE, i.e., acting as an IPv6 Router; or

– "DHCPv4 server & IPv6 Router" if both IPv4 and IPv6 address allocation mechanism are supported by the target UE; or

– "address allocation not supported" if neither IPv4 nor IPv6 address allocation mechanism is supported by the target UE.

– Link-Local IPv6 Address: a link-local IPv6 address formed locally based on RFC 4862 [17] if the target UE does not support the IPv6 IP address allocation mechanism, i.e. the IP Address Configuration indicates "address allocation not supported", and UE-1 included a link-local IPv6 address in the Direct Communication Request message. The target UE shall include a non-conflicting link-local IPv6 address.

If both UEs (i.e. the initiating UE and the target UE) are selected to use link-local IPv6 address, they shall disable the duplicate address detection defined in RFC 4862 [17].

NOTE 3: When either the initiating UE or the target UE indicates the support of IPv6 routing, the corresponding address configuration procedure would be carried out after the establishment of the layer 2 link, and the link-local IPv6 addresses are ignored.

The ProSe layer of the UE that established PC5 unicast link passes the PC5 Link Identifier assigned for the unicast link and the PC5 unicast link related information down to the AS layer. The PC5 unicast link related information includes Layer-2 ID information (i.e. source Layer-2 ID and destination Layer-2 ID). This enables the AS layer to maintain the PC5 Link Identifier together with the PC5 unicast link related information.

Two UEs may negotiate the PC5 DRX configuration in the AS layer, and the PC5 DRX parameter values can be configured per pair of source and destination Layer-2 IDs in the AS layer.

6. ProSe data is transmitted over the established unicast link as below:

The PC5 Link Identifier and PFI are provided to the AS layer, together with the ProSe data.

Optionally in addition, the Layer-2 ID information (i.e. source Layer-2 ID and destination Layer-2 ID) is provided to the AS layer.

NOTE 4: It is up to UE implementation to provide the Layer-2 ID information to the AS layer.

UE-1 sends the ProSe data using the source Layer-2 ID (i.e. UE-1’s Layer-2 ID for this unicast link) and the destination Layer-2 ID (i.e. the peer UE’s Layer-2 ID for this unicast link).

NOTE 5: PC5 unicast link is bi-directional, therefore the peer UE of UE-1 can send the ProSe data to UE-1 over the unicast link with UE-1.

6.4.3.2 Link identifier update for a unicast link

Figure 6.4.3.2-1 shows the link identifier update procedure for a unicast link. When privacy requirements are configured for a ProSe Identifier associated with the unicast link, identifiers used for the unicast mode of 5G ProSe communication over PC5 reference point (e.g. Application Layer ID, Source Layer-2 ID, and IP address/prefix) shall be changed over time as specified in clauses 5.8.2.1 and 5.8.2.4. A UE may decide to change the identifiers for other reasons, e.g. application layer requirement. This procedure is used to update and exchange new identifiers between the source and the peer UEs for a unicast link before using the new identifiers, to prevent service interruptions. When there are privacy requirements as indicated above, this procedure is executed over a security protected unicast link.

If a UE has multiple unicast links using the same Application Layer IDs or Layer-2 IDs, the UE needs to perform the link identifier update procedure over each of the unicast links.

Figure 6.4.3.2-1: Link identifier update procedure

0. UE-1 and UE-2 have a unicast link established as described in clause 6.4.3.1.

1. UE-1 decides to change its identifier(s), e.g. due to the Application Layer ID change or upon expiry of a timer. UE-1 generates its new Layer-2 ID and sends a Link Identifier Update Request message to UE-2 using the old identifiers.

The Link Identifier Update Request message includes the new identifier(s) to use (including the new Layer-2 ID, Security Information, optionally the new Application Layer ID, and optionally new IP address/prefix if IP communication is used). The new identifier(s) shall be cyphered to protect privacy if security is configured for the unicast link. After sending the Link Identifier Update Request message, if the UE-1 has data to send, UE-1 keeps sending data traffic to UE-2 with the old identifiers until UE-1 sends the Link Identifier Update Ack message to UE-2.

NOTE 1: The timer is running on per Source Layer-2 ID.

NOTE 2: When one of the two UEs acts as IPv6 router as described in clause 5.5.1.1 and the IP address/prefix also needs to be changed, the corresponding address configuration procedure would be carried out after the Link Identifier update procedure.

2. Upon reception of the Link Identifier Update Request message, UE-2 changes its identifier(s). UE-2 responds with a Link Identifier Update Response message which includes the new identifier(s) to use (including the new Layer-2 ID, Security Information, optionally the new Application Layer ID, and optionally a new IP address/prefix if IP communication is used). The new identifier(s) shall be cyphered to protect privacy if security is configured for the unicast link. The Link Identifier Update Response message is sent using the old identifiers. UE-2 continues to receive traffic with the old Layer-2 ID from UE-1 until UE-2 receives traffic with the new Layer-2 ID from UE-1. After sending the Link Identifier Update Response message, UE-2 keeps sending data traffic to UE-1 with the old identifier, if UE-2 has data to send, until UE-2 receives the Link Identifier Update Ack message from UE-1.

3. Upon reception of the Link Identifier Update Response message, UE-1 responds with a Link Identifier Update Ack message. The Link Identifier Update Ack message includes the new identifier(s) from UE-2, as received on the Link Identifier Update Response message. The Link Identifier Update Ack message is sent using the old identifiers. UE-1 continues to receive traffic with the old Layer-2 ID from UE-2 until UE-1 receives traffic with the new Layer-2 ID from UE‑2.

4. The ProSe layer of UE-1 passes the PC5 Link Identifier for the unicast link and the updated Layer-2 IDs (i.e. new Layer-2 ID for UE-1 for the source and new Layer-2 ID of UE-2 for the destination) down to the AS layer. This enables the AS layer to update the provided Layer-2 IDs for the unicast link.

UE-1 starts using its new identifiers and UE-2’s new identifiers for this unicast link.

5. Upon reception of the Link Identifier Update Ack message, the ProSe layer of UE-2 passes the PC5 Link Identifier for the unicast link and the updated Layer-2 IDs (i.e. new Layer-2 ID of UE-2 for the source and new Layer-2 ID for UE-1 for the destination) down to the AS layer. This enables the AS layer to update the provided Layer-2 IDs for the unicast link.

UE-2 starts using its new identifiers and UE-1’s new identifiers for this unicast link.

NOTE 3: The Security Information in the above messages also needs to be updated at the same time as the Layer-2 IDs. This is defined in TS 33.503 [29].

6.4.3.3 Layer-2 link release over PC5 reference point

Figure 6.4.3.3-1 shows the layer-2 link release procedure over PC5 reference point.

Figure 6.4.3.3-1: Layer-2 link release procedure

0. UE-1 and UE-2 have a unicast link established as described in clause 6.4.3.1.

1. UE-1 sends a Disconnect Request message to UE-2 in order to release the layer-2 link and deletes all context data associated with the layer-2 link. The Disconnect Request message includes Security Information.

2. Upon reception of the Disconnect Request message, UE-2 shall respond with a Disconnect Response message and deletes all context data associated with the layer-2 link. The Disconnect Response message includes Security Information.

The ProSe layer of each UE informs the AS layer that the unicast link has been released. The ProSe layer uses PC5 Link Identifier to indicate the released unicast link. This enables the AS layer to delete the context related to the released unicast link.

NOTE: The Security Information in the above messages is defined in TS 33.503 [29].

6.4.3.4 Layer-2 link modification for a unicast link

Figure 6.4.3.4-1 shows the layer-2 link modification procedure for a unicast link. This procedure is used to:

– add new PC5 QoS Flow(s) in the existing PC5 unicast link.

– This covers the case for adding new PC5 QoS Flow(s) to the existing ProSe service(s) as well as the case for adding new PC5 QoS Flow(s) to new ProSe service(s).

– modify existing PC5 QoS Flow(s) in the existing PC5 unicast link.

– This covers the case for modifying the PC5 QoS parameters for existing PC5 QoS Flow(s).

– This also covers the case for removing the associated ProSe service(s) from existing PC5 QoS Flow(s) as well as the case for associating new ProSe service(s) with existing PC5 QoS Flow(s).

– remove existing PC5 QoS Flow(s) in the existing PC5 unicast link.

Figure 6.4.3.4-1: Layer-2 link modification procedure

0. UE-1 and UE-2 have a unicast link established as described in clause 6.4.3.1.

1. The ProSe application layer in UE-1 provides application information for PC5 unicast communication. The application information includes the ProSe Service Info and the initiating UE’s Application Layer ID. The target UE’s Application Layer ID may be included in the application information. If UE-1 decides to reuse the existing PC5 unicast link as specified in clause 5.3.4, so decides to modify the unicast link established with UE-2, UE-1 sends a Link Modification Request to UE-2.

The Link Modification Request message includes:

a) To add new PC5 QoS Flow(s) in the existing PC5 unicast link:

– QoS Info: the information about PC5 QoS Flow(s) to be added. For each PC5 QoS Flow, the PFI, the corresponding PC5 QoS parameters (i.e. PQI and conditionally other parameters such as MFBR/GFBR, etc.) and optionally the associated ProSe identifier(s).

– Optional PC5 QoS Rule(s).

b) To modify PC5 QoS Flow(s) in the existing PC5 unicast link:

– QoS Info: the information about PC5 QoS Flow(s) to be modified. For each PC5 QoS Flow, the PFI, the corresponding PC5 QoS parameters (i.e. PQI and conditionally other parameters such as MFBR/GFBR, etc.) and optionally the associated ProSe identifier(s).

– Optional PC5 QoS Rule(s).

c) To remove PC5 QoS Flow(s) in the existing PC5 unicast link:

– PFIs.

2. UE-2 responds with a Link Modification Accept message.

The Link Modification Accept message includes:

– For case a) and case b) described in step 1:

– QoS Info: the information about PC5 QoS Flow(s) requested by UE-1. For each PC5 QoS Flow, the PFI, the corresponding PC5 QoS parameters (i.e. PQI and conditionally other parameters such as MFBR/GFBR, etc.) and optionally the associated ProSe identifier(s).

– Optional PC5 QoS Rule(s).

The ProSe layer of each UE provides information about the unicast link modification to the AS layer. This enables the AS layer to update the context related to the modified unicast link.

6.4.3.5 Layer-2 link maintenance over PC5 reference point

The PC5 Signalling Protocol shall support keep-alive functionality that is used to detect if a particular PC5 unicast link is still valid. Either side of the PC5 unicast link can initiate the layer-2 link maintenance procedure (i.e. keep-alive procedure), based on for example triggers from the AS layer or internal timers. The UEs shall minimize the keep-alive signalling, e.g. cancel the procedure if data are successfully received over the PC5 unicast link.

Figure 6.4.3.5-1: Layer-2 link maintenance procedure

0. UE-1 and UE-2 have a unicast link established as described in clause 6.4.3.1.

1. Based on trigger conditions, UE-1 sends a Keep-alive message to UE-2 in order to determine the status of the PC5 unicast link.

NOTE 1: It is left to Stage 3 to determine the exact triggers for the keep-alive messages. For example, the trigger can be based on a timer associated with the Layer-2 link. The timer can be reset with a successful reception event defined by TS 38.300 [12].

2. Upon reception of the Keep-alive message, UE-2 responds with a Keep-alive Ack message.

The UE initiating the keep-alive procedure shall determine the follow-up actions based on the result of the signalling, e.g. proceed with implicit layer-2 link release.

NOTE 2: It is left to Stage 3 to determine the follow-up actions. For example, a successful reception event can also cancel the layer-2 link release if received in time.

6.4.3.6 Layer-2 link management over PC5 reference point for 5G ProSe UE-to-Network Relay

The Layer-2 link procedures over PC5 reference point for unicast mode 5G ProSe Direct Communication as depicted from clause 6.4.3.1 to clause 6.4.3.5 can be used for the PC5 reference point between 5G ProSe Remote UE and 5G ProSe UE-to-Network Relay, with the following differences and clarifications:

– The Layer-2 link modification procedure is applicable to ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay, other procedures are applicable to both ProSe Communication via 5G ProSe Layer-2 UE-to-Network Relay and ProSe Communication via 5G ProSe Layer-3 UE-to-Network Relay.

– The UE oriented Layer-2 link establishment is used with UE-1 representing the 5G ProSe Remote UE and UE-2 representing the 5G ProSe UE-to-Network Relay. For other procedures either UE-1 represents the 5G ProSe Remote UE and UE-2 represents the 5G ProSe UE-to-Network Relay, or UE-1 represents the 5G ProSe UE-to-Network Relay and UE-2 represents the 5G ProSe Remote UE. I.e. the Layer-2 link establishment is initiated by the 5G ProSe Remote UE, while other procedures may be initiated either by the 5G ProSe Remote UE or by the 5G ProSe UE-to-Network Relay.

For the UE oriented Layer-2 link establishment as described in the clause 6.4.3.1,

– In step 1, the 5G ProSe Remote UE determines the destination Layer-2 ID for PC5 unicast link establishment based on the unicast source Layer-2 ID of the selected 5G ProSe UE-to-Network Relay (as specified in clause 5.8.3) during UE-to-Network Relay discovery as specified in clause 6.3.2.3.

– In step 2, 5G ProSe Remote UE (UE-1) determines the Relay Service Code to be used. The Relay Service Code to be used is selected from the received Relay Service Code(s) during UE-to-Network Relay discovery as specified in clause 6.3.2.3.

– In step 3, 5G ProSe Remote UE (UE-1) sends a unicast Direct Communication Request message to the selected 5G ProSe UE-to-Network Relay. The destination Layer-2 ID used to send the Direct Communication Request message shall be unicast Layer-2 ID as determined in step 1. The Direct Communication Request message includes:

– Source User Info: the identity of the Remote UE requesting relay operation (i.e. User Info ID).

NOTE 1: The details of which additional identity/identities of the Remote UE to be included during Layer-2 link establishment will be decided by SA WG3.

– Target User Info: the identity of the UE-to-Network Relay provided to the 5G ProSe Remote UE during UE-to-Network Relay Discovery procedure (i.e. User Info ID).

– Relay Service Code: indicates the connectivity service provided by the 5G ProSe UE-to-Network Relay as requested by the 5G ProSe Remote UE.

– Security Information: the information for the establishment of security.

– In step 4 and step 5, step 4a and step 5a are performed if the 5G ProSe UE-to-Network Relay’s identity matches the Target User Info and the Relay Service Code is one of the Relay Service Codes included during UE-to-Network Relay discovery as specified in clause 6.3.2.3. The Source User Info in the Direct Communication Accept message is the identity of the UE-to-Network Relay (i.e. User Info ID). In the case of 5G ProSe Layer-2 UE-to-Network Relay, the Remote UE does not send the IP Address Configuration, Link-Local IPv6 Address and QoS Info to the 5G ProSe Layer-2 UE-to-Network Relay, and the Direct Communication Accept message does not include IP Address Configuration, Link-Local IPv6 Address and QoS Info. In the case of 5G ProSe Layer-3 UE-to-Network Relay, the Direct Communication Accept message does not include the IP Address Configuration indicating the value "address allocation not supported".

– In the case of 5G ProSe Layer-2 UE-to-Network Relay, step 6 is not performed.

For the link identifier update as described in the clause 6.4.3.2,

– Application Layer ID is replaced by User Info ID.

– In the case of 5G ProSe Layer-2 UE-to-Network Relay, the changed identifiers do not include IP address/prefix.

For the Layer-2 link release as described in the clause 6.4.3.3,

– In step1, if the Layer-2 link release procedure is initiated by the 5G ProSe UE-to-Network Relay, the Disconnect Request message may indicate the 5G ProSe UE-to-Network Relay is temporarily not available as described in clause 5.12.

NOTE 2: The form of the temporarily not available indication will be determined by stage 3.

– If the service authorization for acting as a 5G ProSe Remote UE or as a 5G ProSe UE-to-Network Relay is revoked, the 5G ProSe Remote UE or the 5G ProSe UE-to-Network Relay should initiate the release of the layer-2 link that the revoked authorization affects.

– A 5G ProSe Layer-2 Remote UE or a 5G ProSe Layer-2 UE-to-Network Relay initiates the release of the layer-2 link upon receiving an indication from its AS layer to trigger PC5 unicast link release as specified in TS 38.331 [16].

NOTE 3: The timing to initiates layer-2 link release is up to UE implementation.

A 5G ProSe Remote UE and a 5G ProSe UE-to-Network Relay shall set up a separate PC5 unicast links if an existing unicast link(s) was established with a different Relay Service Code or without a Relay Service Code.

NOTE 4: A single PC5 unicast link is established between a 5G ProSe Layer-2 UE-to-Network Relay and a 5G ProSe Layer-2 Remote UE, as specified in TS 38.300 [12], for supporting PDU sessions of the 5G ProSe Layer-2 Remote UE.

Each PC5 unicast link for 5G ProSe UE-to-Network Relay is associated with a Unicast Link Profile, which includes:

– User Info ID and Layer-2 ID of 5G ProSe Remote UE; and

– User Info ID and Layer-2 ID of 5G ProSe UE-to-Network Relay; and

– Relay Service Code; and

– In the case of 5G ProSe Layer-3 UE-to-Network Relay, the network layer protocol and the information about PC5 QoS Flow(s).

The Unicast Link Profile shall be updated accordingly after a Layer-2 link modification or Layer-2 link identifier update.