WO2024146771A1 - Ue-to-ue relay communication for non-ip traffic - Google Patents

Abstract

A relay UE comprising a receiver for receiving a control plane (CP) message transmitted by an end UE to the relay UE. The CP message comprises a data link layer (DLL) address (e.g., Ethernet MAC address) of the end UE. The relay UE further comprises processing circuitry configured to cause the relay UE to maintain an association between the DLL address of the end UE and a link established with the end UE.

Description

UE-TO-UE RELAY COMMUNICATION FOR NON-IP TRAFFIC

TECHNICAL FIELD

[001] Disclosed are embodiments related to a user equipment (UE) relay for relaying traffic (e.g.,

Ethernet protocol data units (PDUs) or Unstructured traffic) from a source UE to a target UE or from the source UE (a.k.a., remote UE) to a network node (e.g., application server)).

BACKGROUND

[002] The Third Generation Partnership Project (3GPP) Technical Report (TR) 23.700-33 V1.2.0 (“TR

23.700-33”) provides an evaluation of possible architecture enhancements to the fifth-generation (5G) Proximitybased Services (5G ProSe) system defined in 3GPP Technical Specification (TS) 23.304 V18.0.0 (“TS 23.304”). TR

23.700-33 describes UE-to-UE (a.k.a., U2U) relay, which is a UE that relays traffic from a source UE to a target UE. A UE is any communication device capable of wireless communication. A UE-to-Network (U2N) relay is a UE that relays traffic from a UE (a.k.a., "remote UE”) to a network node (e.g., an application server).

[003] When a source UE wants to communicate with a target UE, it will first try to find the target UE by either sending a Direct Communication Request (DCR) or a Solicitation message with the target UE info. If the source UE cannot reach the target UE directly, it will try to discover a UE-to-UE relay to reach the target UE which may also trigger the relay to discover the target UE.

[004] For IP traffic being relayed from the source UE to the target UE, the U2U relay acts as an IP router, the link between source UE and U2U relay can be shared by multiple target UEs, the link between the U2U relay and target UE can be shared by multiple source UEs.

SUMMARY

[005] Certain challenges presently exist.

[006] While it is agreed that 5G ProSe Layer-3 UE-to-UE Relay shall provide generic function that can relay any IP, Ethernet or Unstructured traffic, how to support Ethernet or Unstuctured traffic by Layer-3 U2U Relay is not captured in TS 23.304. Another challenge is that, when relaying data link layer (a.k.a., layer-2) traffic (e.g., Ethernet traffic), the UE relay (U2U relay or U2N relay) can act as a layer-2 switch (e.g., Ethernet switch), but, if the UE relay (or "relay” for short) acts as a switch, then the relay needs to maintain (e.g., create) an association between the data link layer (DLL) address (e.g., media access control (MAC) address) and the UEs's user info ID or the link between the relay and the source/target UE (e.g., the relay builds a mapping table that maps DLL addresses to PC5 links). Currently, however, there is no solution addressing how the relay obtains the MAC address from the source UE (and the target UE if the relay is a U2U relay) before an end UE (a source UE or a target UE) sends an Ethernet packet. Thus, for example, when the relay UE receives from a source UE an Ethernet packet addressed to a target UE which itself has not sent an Ethernet packet to the relay UE, the relay UE either drops the packet or sends it via all of its links except the link on which the relay received the Ethernet packet.

[007] Accordingly, in one aspect there is provided a relay UE that is configured to receive a control plane (CP) message transmitted by an end UE. The CP message comprises a data link layer (DLL) address of the end UE. The relay us is also configured to maintain an association between the DLL address of the end UE and a link established with the end UE.

[008] In another aspect there is provided an end UE (source UE or target UE). The end UE is configured to create a CP message, wherein the CP message comprises a DLL address of the end UE. The end UE is also configured to wirelessly transmit the CP message to a UE relay.

[009] In another aspect there is provided a relay UE that is configured to receive from a first UE a first

CP message comprising a DLL address of the first UE. The relay UE is also configured to transmit to a second UE a second CP message comprising the DLL address of the first UE, wherein the second CP message is not the first CP message.

[0010] In another aspect there is provided a method performed by a relay UE. The method includes receiving a control plane (CP) message transmitted by an end UE to the relay UE. The CP message comprising a DLL address of the end UE. The method also includes maintaining an association between the DLL address of the end UE and a link established with the end UE.

[0011] In another aspect there is provided a method performed by an end UE (source UE or target UE). The method includes creating a CP message, wherein the CP message comprises a DLL address of the end UE. The method also includes wirelessly transmitting the CP message to a UE relay.

[0012] In another aspect there is a provided method performed by a relay UE. The method includes receiving from a first UE a first CP message comprising a DLL address of the first UE. The method also includes transmitting to a second UE a second CP message comprising the DLL address of the first UE, wherein the second CP message is not the first CP message.

[0013] In another aspect there is provided a computer program comprising instructions which when executed by processing circuitry of a UE causes the UE to perform any of the methods disclosed herein. In one embodiment, there is provided a carrier containing the computer program wherein the carrier is one of an electronic signal, an optical signal, a radio signal, and a computer readable storage medium. [0014] In another aspect there is provided a UE that is configured to perform the methods disclosed herein. The UE may include memory and processing circuitry coupled to the memory.

[0015] An advantage of the embodiments disclosed herein is that they enable a relay UE to support nonIP traffic (e.g., Ethernet traffic).

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments.

[0017] FIG. 1A illustrates a system according to an embodiment.

[0018] FIG. 1 B illustrates protocol stacks according to an embodiment.

[0019] FIG. 10 illustrates protocol stacks according to an embodiment.

[0020] FIG. 2A is a message flow diagram according to an embodiment.

[0021] FIG. 2B is a message flow diagram according to an embodiment.

[0022] FIG. 3 is a message flow diagram according to an embodiment.

[0023] FIG. 4 is a flowchart illustrating a process according to an embodiment.

[0024] FIG. 5 is a flowchart illustrating a process according to an embodiment.

[0025] FIG. 6 is a flowchart illustrating a process according to an embodiment

[0026] FIG. 7 is a block diagram of a UE according to an embodiment.

DETAILED DESCRIPTION

[0027] FIG. 1 A illustrates a system 100 according to an embodiment. System 100 includes a source UE 101, a UE relay 102 (a.k.a., "relay UE 102”), a target UE 103, and a network node (NN) 104 (e.g., application server). Each UE 101, 102, and 103 may be an enhanced 5G ProSe Layer-3 UE. Relay 102 may be employed to relay data link layer (DLL) (e.g., Ethernet) traffic from source UE 101 to target UE 103 and/or NN 104. When relay 102 relays DLL traffic, relay 102 acts as a layer-2 switch (e.g., an Ethernet switch). As shown in FIG. 1, a network 110 enables communication between relay 102 and NN 104.

[0028] As noted above, if relay 102 acts as a switch, then the relay needs to create at least a first mapping between a data link layer address (e.g., Ethernet media access control (MAC) address) allocated to source UE 102 and source UE 102's user info ID or a link 112 between the relay and the source UE. If relay 102 relays DLL traffic between source UE 101 and target UE 102, then relay 102 also needs to obtain the DLL address of target UE 103 and associate target UE's DLL address with a link 114 between relay 102 and target UE 103.

[0029] FIG. 1B illustrates example user plane protocol stacks for source UE 101, relay 102, and target UE 103. In the example shown in FIG. 1 B, non-IP data, e.g., an Ethernet PDU (a.k.a., Ethernet frame), generated at UE

101 is processed by a Service Data Adaption Protocol (SDAP) layer (which is a layer-2 protocol) and the other layer- 2 protocol layers (i.e., Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer, Media Access Control (MAC) layer) before being transmitted over the air by the Physical (PHY) layer.

[0030] Relay 102 receives the over-the-air transmission and obtains the non-IP data that was transmitted by source UE 101. In this example, the non-IP data is an Ethernet frame addressed to target UE 103. After obtaining the Ethernet frame, relay 101 discovers the destination address included in the Ethernet frame and uses a mapping table (or the like) to map the destination address to a link (which in this case is link 114 between relay 102 and target UE 103). After determining the link to which the destination address is associated, the Ethernet frame is provided to and processed by the layer-2 layers (SDAP, PDCP, RLC, MAC) and then transmitted over-the-air to target UE 103. That is, relay 102 transmits to target UE 103 a layer-2 message (e.g., SDAP PDU) generated by relay 102, where the layer-2 message encapsulates the Ethernet frame received from source UE 101 . In this way, relay

102 relays non-IP data from the source UE to the target UE.

[0031] The same process occurs in the reverse direction. That is, relay 102 receives from target 103 a layer-2 message (DLL PDU) containing an Ethernet frame addressed to source UE 102, relay 101 discovers the destination address included in the Ethernet frame and uses a mapping table (or the like) to map the destination address to a link (which in this case is link 112 between relay 102 and source UE 101). After determining the link to which the destination address is associated, the Ethernet frame is provided to and processed by the layer-2 layers (SDAP, PDCP, RLC, MAC) and then transmitted over-the-air to source UE 1031. That is, relay 102 transmits to source UE 101 a layer-2 message (e.g., SDAP PDU) generated by relay 102, where the layer-2 message encapsulates the Ethernet frame received from target UE 103. In this way, relay 102 relays non-IP data from the target UE to the source UE.

[0032] To map the source UE's DLL address to link 112 and to map the target UE's DLL address to link 114, relay 102 must discover source UE's DLL address and the target UE's DLL address. In one embodiment, this is accomplished using control plane (CP) signaling between the source UE and the relay the target UE and the relay. FIG. 1C illustrate an example control plane protocol stack to allow source/target UE to transmit to, and receive from, relay 102 CP messages. In the example, shown each UE (source, target, relay) includes a CP layer (which in this example is a PC5 Signaling Protocol layer) that sits on top of one or more layer-2 layers (e.g., PDCP, RLC, and MAC). [0033] FIG. 2A and FIG. 2B illustrate embodiments that enable relay 102 to obtain the DLL (e.g., Ethernet MAC) address of source UE 101 and the DLL address of target UE 103 so that relay 102 can create the mappings.

[0034] Providing DLL Address in the PC5 Link Establishment Procedure

[0035] FIG. 2A is a message flow diagram illustrating a link establishment procedure. In 3GPP 5G terminology, the link is known as the "PC5 link” or "PC5” for short. The message flow diagram shown in FIG. 2A assumes that i) each UE has performed service authorization and provisioning, ii) the source UE 101 has discovered relay 102, and iii) relay 102 has discovered target UE 103. As described below, as part of the link establishment procedure, source UE 101 and target UE 103 provide their DLL address to relay 102.

[0036] As shown in FIG. 2A, source UE 101 transmits to relay 102 a CP message m201 (which in this case is a direct communication request (DCR) message) to initiate the layer-2 link establishment procedure with relay 102 (i.e., source UE 101 initiates the establishment of a PC5 link with relay 102). The DCR message m201 is transmitted using a source layer-2 ID and a destination layer-2 ID. The source layer-2 ID is self-assigned by source UE 101, and the destination layer-2 ID is set to a layer-2 ID that source UE 101 received from relay 102 (e.g., the destination layer-2 ID is set to the source layer-2 ID that was included in a discovery message transmitted by relay 102 to source UE 101). DCR m201 may include a relay service code (RSC) and/or a User Info ID. The RSC may indicate that the link to be established is for DLL traffic (or the DCR m201 may include another indicator to indicate that the link to be established is for DLL traffic).

[0037] If the User Info ID in DCR message m201 matches the relay's User Info ID and/or the RSC in the DCR message matches an RSC that relay 102 supports, then relay 102 responds by establishing security protection with source UE 101 . That is, relay 102 sends a security command message m202 to source UE 101 (e.g., a PROSE DIRECT LINK SECURITY MODE COMMAND message defined in 3GPP TS 24.554 V17.2.1 (“TS 24.554”)). When the security protection is enabled, source UE 101 sends IP Address Configuration, QoS Info to relay 102.

[0038] If the requested PC5 link is used for transferring DLL traffic (e.g., Ethernet traffic), then, in one embodiment, source UE 101 sends its DLL address (e.g,. Ethernet MAC address) to relay 102 after the security protection is enabled, and the relay UE will map the DLL address to the PC5 link between source UE 101 and relay 102. For example, source UE 101 may send its DLL address in a security complete message m203 (e.g., a PROSE DIRECT LINK SECURITY MODE COMPLETE message defined in TS 24.554). If the DLL address is already being used by another UE, then the relay will send a message to the source UE indicating there is a DLL address conflict. In another embodiment, source UE 101 may send its DLL address before security is established to enable early address conflict detection. For example, source UE 101 may include its DLL address in DCR message m201. In either case, relay 102 maps (i.e., associates) the DLL address of source UE 101 with the PC5 link between UE 101 and relay 102. [0039] After the Security Establishment procedure is completed, relay 102 transmits to target UE 103 a CP message m204 (which in this case is a direct communication request (DCR) message) to initiate the layer-2 link establishment procedure with target UE 103 (i.e., relay 102 initiates the establishment of a PC5 link with target UE 103). The DCR message m204 is transmitted using a source layer-2 ID and a destination layer-2 ID. The source layer-2 ID is self-assigned by relay 102, and the destination layer-2 ID is set to a layer-2 ID that relay 102 received from target UE 103 (e.g., the destination layer-2 ID is set to the source layer-2 ID that was included in a discovery message transmitted by target UE 103 to relay 102). DCR m204 may include a relay service code (RSC) and/or a User Info ID. Relay 102 shall choose different Source Layer-2 IDs for PC5 links of different types of traffic, i.e., IP traffic, Ethernet traffic, and Unstructured traffic. The RSC may indicate that the link to be established is for DLL traffic (or the DCR m204 may include another indicator to indicate that the link to be established is for DLL traffic).

[0040] If the User Info ID of target 5G ProSe Layer-3 End UE and RSC included in the DCR message m204 match the target UE's User Info ID and the RSC that the target UE supports, then target UE 103 responds by establishing the security protection with relay 102. That is, target UE 103 transmits to relay 102 a security message m205 (e.g., the PROSE DIRECT LINK SECURITY MODE COMMAND message defined in TS 24.554 or the PROSE DIRECT LINK SECURITY ESTABLISHMENT REQUEST message defined in 3GPP Technical Document (Tdoc) C1 -238124). When the security protection is enabled, relay 102 sends IP Address Configuration, QoS Info to target UE 103.

[0041] If the PC5 link is used for transferring DLL traffic, relay 102 may send the DLL address of source UE 101 to target UE 103. For example, relay 102 may include the DLL address of source UE 101 In a security message m206 (e.g., the PROSE DIRECT LINK SECURITY MODE COMPLETE message defined in TS 24.554 or the PROSE DIRECT LINK SECURITY ESTABLISHMENT ACCEPT message defined in C1-238124) that is transmitted by relay 102 to target 103. Alternatively, relay 102 may include the DLL address of UE 101 in DCR message m204.

[0042] The Source Layer-2 ID used for the security establishment procedure is self-assigned by target UE 103, and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message m204.

[0043] Upon receiving the security establishment procedure messages, relay 102 obtains target UE 103's Layer-2 ID for future communication, for signaling and data traffic for this unicast link.

[0044] After the security protection is established, target UE 103 sends a Direct Communication Accept (DCA) message m207 to relay 102. If the PC5 link is used for transferring DLL traffic, then, in one embodiment, target UE 103 sends its DLL address to relay 102 in the DCA message m207. If the DLL address is already being used by another UE, then the relay will send a message to the target UE indicating there is an address conflict. [0045] After receiving DCA message m207, relay 102 sends a DCA message m208 to source UE 101. The parameters included in the DCA message m208 are described in clause 6.4.3 of TS 23.304. If the PC5 link between source UE 101 and relay 102 is used for transferring DLL traffic, relay 102 may send the DLL address of target UE 103 to source UE 101 in DCA message m208.

[0046] For IP communication, relay 102 may store an association of User Info ID and the IP address of target UE 103 into its DNS entries, and relay 102 may act as a DNS server to other UEs.

[0047] After source UE 101 receives DCA message m208, source UE 101 may communicate with target UE 103 via relay 102. That is, relay 102 may relay DLL traffic from source UE 101 to target UE 102. For instance, when relay 102 receives a DLL frame transmitted by source UE 101, relay 102 routes the frame based on the destination DLL address included in the frame. Hence, if the destination DLL address included in the DLL frame matches the DLL address of target UE 103, then relay 102 forwards the frame to target UE 103 over the PC5 link that was previously established between relay 102 and target 103. That is, for example, relay 102 may maintain a table that maps a DLL destination address to a PC5 link so that the DLL frame can be routed to the correct target.

[0048] Providing DLL Address in the PC5 Link Modification Procedure

[0049] FIG. 2B is a message flow diagram illustrating a link modification procedure. The message flow diagram shown in FIG. 2B assumes that there is already a PC5 link for DLL traffic between source UE 101 and relay 102 and/or between relay 102 and target UE 103. The existing link will be used for transferring the DLL traffic between the source and the target UE via the relay.

[0050] As shown in FIG. 2B, source UE 101 sends a Link Modification Request (LMR) message m252 to relay 102. LMR message m252 contains User Info ID and Layer-2 ID of target UE 103, and end-to-end QoS to the target UE. Relay 102 already knows the DLL address of the source UE because the link is already established.

[0051] If there's an existing PC5 link between relay 102 and the target UE for the required RSC which is related to Ethernet type traffic, the relay sends a LMR message m254 to the target UE. LMR message m254 contains User Info ID of source UE, source UE's IP address if it's available in the relay, QoS part between relay and the target UE. The relay includes the DLL address of the source UE in the LMR message m254 sent to the target UE.

[0052] Target UE 103 then sends an LMA message m256 to the relay. The relay already knows the DLL of the target UE because the link is already established.

[0053] After receiving LMA message m256, the relay sends an LMA message m258 to the source UE. In one embodiment, the relay includes the DLL address of the target UE in LMA message 258 sent to the source UE.

[0054] The above example illustrated the case where relay 102 is a U2U relay, however, as noted previously, relay 102 may be a U2N relay. When source UE 101 sets up a PC5 link for Ethernet traffic with a relay 102 functioning as a U2N relay, the source UE sends its DLL address (e.g., Ethernet MAC address) to relay 102 as described above. For example, the source UE sends its Ethernet MAC address of the in the PROSE DIRECT LINK SECURITY MODE COMPLETE message.

[0055] Additional Embodiments

[0056] 3GPP has agreed that 5G ProSe Layer-3 UE-to-UE Relay (e.g., relay 102) shall provide a generic function that can relay any IP, Ethernet, or Unstructured traffic. However, how to support Ethernet or Unstuctured traffic by Layer-3 U2U Relay is not captured in TS 23.304. That is, according to3GPP, how to support Ethernet and Unstructured traffic is for further study (FFS). This disclosure describes that Ethernet and Unstructured traffic can be supported by having the Layer-3 U2U relay forwards the non-IP traffic by maintaining 1 :1 mapping between the link with source UE and the link with target UE.

[0057] FIG. 3 is a message flow diagram according to an embodiment. The message flow shown in FIG. 3 illustrates the procedure for 5G ProSe Communication via 5G ProSe Layer-3 UE-to-UE Relay. The twelve steps shown in FIG. 3 are described below.

[0058] 1. Service authorization and provisioning are performed for source 5G ProSe Layer-3 End UE, target 5G ProSe Layer-3 End UE and 5G ProSe Layer-3 UE-to-UE Relay as described in clause 6.2 of TS 23.304.

[0059] 2. The source 5G ProSe Layer-3 End UE performs discovery of a 5G ProSe Layer-3 UE-to-UE

Relay as described in clause 6.3.2.4 of TS 23.304.

[0060] 3. The source 5G ProSe Layer-3 End UE sends a Direct Communication Request message to initiate the unicast Layer-2 link establishment procedure with the 5G ProSe Layer-3 UE-to-UE Relay. The parameters included in the Direct Communication Request message are described in clause 6.4.3 of TS 23.304. The Source Layer-2 ID of the DCR message is self-assigned by the source 5G ProSe Layer-3 End UE, and the Destination Layer-2 ID is set to the Source Layer-2 ID of the discovery message of the 5G ProSe Layer-3 UE-to-UE Relay.

[0061] 4. If the User Info ID of 5G ProSe Layer-3 UE-to-UE Relay in the DCR message matches the relay's User Info ID and/or the RSC in the Direct Communication Request matches one RSC that the relay supports, the 5G ProSe Layer-3 UE-to-UE Relay responds by establishing the security with the source 5G ProSe Layer-3 End UE. When the security protection is enabled, the source 5G ProSe Layer-3 End UE sends IP Address Configuration, QoS Info (as described in clause 6.4.3) to the 5G ProSe Layer-3 UE-to-UE Relay.

[0062] If the PC5 link is used for transferring Ethernet traffic, the source 5G ProSe Layer-3 End UE sends its Ethernet MAC address to the 5G ProSe Layer-3 UE-to-UE Relay after the security protection is enabled. If the Ethernet MAC address already used by another 5G ProSe Layer-3 End UE, then the 5G ProSe Layer-3 UE-to-UE Relay may send a message to the source 5G ProSe Layer-3 End UE indicating there is Ethernet MAC address conflict.

[0063] The Source Layer-2 ID used for the security establishment procedure is self-assigned by the 5G ProSe Layer-3 UE-to-UE Relay, and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message.

[0064] The 5G ProSe Layer-3 UE-to-UE Relay shall choose different Source Layer-2 IDs for PC5 links of different types of traffic, i.e. , IP traffic, Ethenet traffic, and Unstructured traffic.

[0065] If the PC5 link is used for transferring Unstructured traffic, the 5G ProSe Layer-3 UE-to-UE Relay shall choose different Source Layer-2 IDs for different pair of source and target 5G ProSe Layer-3 End UEs.

[0066] Upon receiving the security establishment procedure messages, the source 5G ProSe Layer-3 End UE obtains the 5G ProSe Layer-3 UE-to-UE Relay's Layer-2 ID for future communication, for signalling and data traffic for this unicast link.

[0067] 5. After the Security Establishment procedure in step 4 is completed, the 5G ProSe Layer-3 UE-to-

UE Relay sends a Direct Communication Request message to initiate the unicast Layer-2 link establishment procedure with the target 5G ProSe Layer-3 End UE. The parameters included in the Direct Communication Request message are described in clause 6.4.3 of TS 23.304.

[0068] The Source Layer-2 ID of the DCR message is self-assigned by the 5G ProSe Layer-3 UE-to-UE Relay, and the Destination Layer-2 ID is the unicast Layer-2 ID of target 5G ProSe Layer-3 End UE associated with the User Info ID of target 5G ProSe Layer-3 End UE.

[0069] The 5G ProSe Layer-3 UE-to-UE Relay shall choose different Source Layer-2 IDs for PC5 links of different types of traffic, i.e., IP traffic, Ethenet traffic, and Unstructured traffic.

[0070] If the PC5 link is used for transferring Unstructured traffic, the 5G ProSe Layer-3 UE-to-UE Relay shall choose different Source Layer-2 IDs for different pair of source and target 5G ProSe Layer-3 End UEs.

[0071] The 5G ProSe Layer-3 UE-to-UE Relay may choose the same Source Layer-2 ID as step 4.

[0072] 6. If the User Info ID of target 5G ProSe Layer-3 End UE and RSC included in the Direct

Communication Request match the target UE's User Info ID and the RSC that the target UE supports, the target 5G ProSe Layer-3 End UE responds by establishing the security with the 5G ProSe Layer-3 UE-to-UE Relay. When the security protection is enabled, the 5G ProSe Layer-3 UE-to-UE Relay sends IP Address Configuration, QoS Info (as described in clause 6.4.3 of TS 23.304) to the target 5G ProSe Layer-3 End UE. [0073] If the PC5 link is used for transferring Ethernet traffic, the 5G ProSe Layer-3 UE-to-UE Relay sends the Ethernet MAC address of the source 5G ProSe Layer-3 End UE to the target 5G ProSe Layer-3 End UE after the security protection is enabled.

[0074] The Source Layer-2 ID used for the security establishment procedure is self-assigned by the target 5G ProSe Layer-3 End UE, and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message.

[0075] Upon receiving the security establishment procedure messages, the 5G ProSe Layer-3 UE-to-UE Relay obtains the target 5G ProSe Layer-3 End UE's Layer-2 ID for future communication, for signalling and data traffic for this unicast link.

[0076] 7. The target 5G ProSe Layer-3 End UE sends a Direct Communication Accept message to the 5G

ProSe Layer-3 UE-to-UE Relay that has successfully established security with. The parameters included in the Direct Communication Accept message are described in clause 6.4.3 of TS 23.304.

[0077] 8. For IP traffic, IPv6 prefix or IPv4 address is allocated for the target 5G ProSe Layer-3 End UE as defined in clause 5.5.1.4 of TS 23.304.

[0078] 9. After receiving the Direct Communication Accept message from the target 5G ProSe Layer-3

End UE, the 5G ProSe Layer-3 UE-to-UE Relay sends a Direct Communication Accept message to the source 5G ProSe Layer-3 End UE that has successfully established security with. The parameters included in the Direct Communication Accept message are described in clause 6.4.3 of TS 23.304.

[0079] 10. For IP traffic, IPv6 prefix or IPv4 address is allocated for the source 5G ProSe Layer-3 End UE as defined in clause 5.5.1.4 of TS 23.304.

[0080] 11 . For IP communication, the 5G ProSe Layer-3 UE-to-UE Relay may store an association of

User Info ID and the IP address of target 5G ProSe Layer-3 End UE into its DNS entries, and the 5G ProSe Layer-3 UE-to-UE Relay may act as a DNS server to other UEs. The source 5G ProSe Layer-3 End UE may send a DNS query to the 5G ProSe Layer-3 UE-to-UE Relay to request IP address of target 5G ProSe Layer-3 End UE after step 10 if the IP address of target 5G ProSe Layer-3 End UE in not received in step 9, and the 5G ProSe Layer-3 UE-to- UE Relay returns the IP address of the target 5G ProSe Layer-3 End UE to the source 5G ProSe Layer-3 End UE.

[0081] For Ethernet communication, the 5G ProSe Layer-3 UE-to-UE Relay is acting as an Ethernet switch by maintaining the association between PC5 links and Ethernet MAC addresses received from the 5G ProSe Layer-3 End UE. [0082] For Unstructured traffic communication, for each pair of source and target 5G ProSe Layer-3 End UEs, the 5G ProSe Layer-3 UE-to-UE Relay maintains the 1 :1 mapping between the PC5 link with source 5G ProSe Layer-3 End UE and the PC5 link with target 5G ProSe Layer-3 End UE.

[0083] 12. The source 5G ProSe Layer-3 End UE communicates with the target 5G ProSe Layer-3 End

UE via the 5G ProSe Layer-3 UE-to-UE Relay.

[0084] In the case of one source 5G ProSe Layer-3 End UE communicates with multiple target 5G ProSe Layer-3 End UEs, the PC5 link between the source 5G ProSe Layer-3 End UE and the 5G ProSe Layer-3 UE-to-UE Relay can be shared for multiple target 5G ProSe Layer-3 End UEs per RSC while the PC5 links may be established individually between the 5G ProSe Layer-3 UE-to-UE Relay and target 5G ProSe Layer-3 End UEs per RSC. For the shared PC5 link, the Layer-2 link modification procedure shall be used.

[0085] In the case of multiple source 5G ProSe Layer-3 End UEs communicate with one target 5G ProSe Layer-3 End UE, the PC5 link between the 5G ProSe Layer-3 UE-to-UE Relay and the target 5G ProSe Layer-3 End UE can be shared per RSC while the PC5 links may be established individually between the source 5G ProSe Layer- 3 End UEs and the 5G ProSe Layer-3 UE-to-UE Relay per RSC. For the shared PC5 link, the Layer-2 link modification procedure shall be used.

[0086] Layer-2 link management over PC5 reference point for 5G ProSe Layer-3 UE-to-UE Relay

[0087] For the 5G ProSe Communication via 5G ProSe Layer-3 UE-to-UE Relay as described in clause 6.7.1 of TS 23.304:

[0088] In step 5, the Direct Communication Request message includes User Info ID of source 5G ProSe Layer-3 End UE, User Info ID of 5G ProSe Layer-3 UE-to-UE Relay, User Info ID of target 5G ProSe Layer-3 End UE, RSC and Security Information to the target 5G ProSe Layer-3 End UE.

[0089] In step 7, the Direct Communication Accept message includes User Info ID of source 5G ProSe Layer-3 End UE, User Info ID of 5G ProSe Layer-3 UE-to-UE Relay, User Info ID of target 5G ProSe Layer-3 End UE, RSC, QoS Info, IP Address Configuration.

[0090] If the PC5 link is used for transferring Ethernet traffic, the target 5G ProSe Layer-3 End UE sends its Ethernet MAC address to the 5G ProSe Layer-3 UE-to-UE Relay in the Direct Communication Accept message.

[0091] In step 9, the Direct Communication Accept message includes User Info ID of source 5G ProSe Layer-3 End UE, User Info ID of 5G ProSe Layer-3 UE-to-UE Relay, User Info ID of target 5G ProSe Layer-3 End UE, RSC, QoS Info, IP Address Configuration and IP address or Ethernet MAC address of target 5G ProSe Layer-3 End UE if received before step 9. [0092] FIG. 4 is a flow chart illustrating a process 400, according to an embodiment, that is performed by relay 102. Process 400 may begin in step s402. Step s402 comprises receiving a CP message transmitted by a UE (UE 101 or UE 103) to the relay 102, where the CP message comprises a DLL address of the UE. Step s404 comprises maintaining (s404) (e.g., creating) an association between the DLL address and a link established with the UE. In some embodiments, receiving the CP message comprises receiving a DLL PDU (e.g., PDCP PDU, RLC PDU, MAC PDU) having a header and a payload, wherein the payload contains the CP message (or at least a portion thereof).

[0093] In some embodiments, the process further comprises enabling security protection with the end UE, and the CP message is transmitted by the end UE after the security protection is enabled with the end UE.

[0094] In some embodiments, the end UE is a source UE, the process further comprises transmitting to a target UE a message comprising the DLL address of the source UE, the process further comprises enabling security protection with the target UE, the message comprising the DLL address of the source UE is transmitted to the target UE after the security protection with the target UE is enabled, the process further comprises obtaining a DLL address of the target UE from a message (m207) transmitted by the target UE, and transmitting to the source UE a message (m208, m258) comprising the DLL address of the target UE.

[0095] In some embodiments, the end UE is a target UE, the CP message is an accept message, and the process further comprises transmitting to a source UE a message (m208, m258) comprising the DLL address of the target UE.

[0096] In some embodiments, the DLL address of the end UE is a Media Access Control, MAC, address.

[0097] In some embodiments, the relay UE is a UE-to-UE Relay, and the end UE is 5G ProSe End UE.

[0098] In some embodiments, the relay UE is a UE-to-Network Relay, and the end UE is 5G ProSe

Remote UE.

[0099] In some embodiments, receiving the CP message comprises receiving a DLL protocol data unit, PDU, having a header and a payload, wherein the payload contains at least a portion of the CP message comprising the DLL address of the end UE.

[00100] In some embodiments, the DLL address is an Ethernet MAC address, the process further comprises determining whether the Ethernet MAC address is used by another UE, and the process further comprises transmitting to the end UE a message indicating there is an Ethernet MAC address conflict in response to determining that the Ethernet MAC address is used by another UE. [00101] In some embodiments, the end UE is a source UE, the link established with the end UE is a PC5 link, and the relay UE is configured such that, if the PC5 link is used for transferring Ethernet traffic, the relay UE sends the Ethernet MAC address of the source UE to a target UE after security is enabled with the target UE.

[00102] FIG. 5 is a flow chart illustrating a process 500, according to an embodiment, that is performed by either UE 101 or UE 103. Process 500 may begin in step s502. Step s502 comprises the UE creating a CP message (e.g., a sidelink control plane message), wherein the CP message comprises a data link layer, DLL, address of the UE. Step s504 comprises wirelessly transmitting the CP message to relay 102. In some embodiments, transmitting the CP message to relay 102 comprises creating a DLL PDU (e.g., MAC frame, RLC frame, PDCP PDU) comprising a payload comprising the CP message (or at least a portion thereof), wherein the DLL PDU is addressed to a relay; and transmitting the DLL PDU to relay 102.

[00103] In some embodiments, the end UE is a source UE, and the method comprises the source UE initiating a link establishment procedure with the relay UE prior to wirelessly transmitting the CP message to the relay UE, and the source UE is configured such that, if the link to be established is to be used for transferring Ethernet traffic, the source UE transmits the CP message comprising the Ethernet MAC address after security protection is enabled with the relay UE.

[00104] In some embodiments, the end UE is a source UE and the CP message is a security mode complete message that is responsive to a security command message (m202) transmitted by the relay UE, or the end UE is a target UE and the CP message is an accept message (m207) that is responsive to a communication request message (m204) transmitted by the relay UE.

[00105] In some embodiments, the process further comprises, after transmitting the CP message, receiving from the relay a message (m208, m258) comprising a DLL address of a target UE, the DLL address of the target UE is an Ethernet MAC address of the target UE, and the process further comprises: after receiving the message comprising the Ethernet MAC address of the target UE, creating a DLL protocol data unit, PDU, comprising a payload comprising at least a portion of an Ethernet frame; and transmitting the DLL PDU to the relay UE, wherein the destination address of the DLL PDU is a DLL address of the relay UE, and the destination address of the Ethernet frame is the Ethernet MAC address of the target UE.

[00106] In some embodiments, transmitting the CP message to the relay UE comprises: creating a DLL protocol data unit, PDU, comprising a payload comprising at least a portion of the CP message comprising the DLL address of the end UE, wherein the DLL PDU is addressed to the relay UE; and transmitting the DLL PDU to the relay UE. [00107] In some embodiments, the process further comprises, prior to transmitting to the relay the CP message comprising the DLL address of the end UE: transmitting to the relay a direct communication request, DCR; and performing a security process for establishing security with the relay UE.

[00108] FIG. 6 is a flow chart illustrating a process 600, according to an embodiment, that is performed by relay 102. Process 600 may begin in step s602. Step s602 comprises receiving from a first UE (e.g. UE 101 or UE 103) a first CP message (e.g. message m203 or m207) comprising a DLL address of the first UE. Step s604 comprises transmitting to a second UE a second CP message (e.g., message m206, m208, m254, m258) comprising the DLL address of the first UE.

[00109] In some embodiments, the process further comprises maintaining an association between the DLL address of the first UE and a link established between the first UE and the relay UE, and the link established with the end UE is a PC5 link used for transferring Ethernet traffic.

[00110] In some embodiments, the process further comprise enabling security protection with the second UE, the relay UE is configured to transmit to the second UE the second CP message comprising the DLL address of the first UE after the security protection with the target UE is enabled, and the process further comprise obtaining a DLL address of the second UE from a message (m207) transmitted by the second UE to the relay UE, and the process further comprise transmitting to the first UE a message (m208, m258) comprising the DLL address of the second UE.

[00111] In some embodiments, the DLL address of the first UE is an Ethernet MAC address, the process further comprise determining whether the Ethernet MAC address is used by another UE, and the process further comprise, in response to determining that the Ethernet MAC address is used by another UE, transmitting to the first UE a message indicating there is an Ethernet MAC address conflict.

[00112] In some embodiments, the first UE is a source UE, the second UE is a target UE, the link established with the source UE is a PC5 link, and the relay UE is configured such that, if the PC5 link is used for transferring Ethernet traffic, the relay UE sends the DLL address of the source UE to the target UE after security is enabled with the target UE.

[00113] FIG. 7 is a block diagram of UEs 101, 102, and 103 according to some embodiments. As shown in FIG. 7, each UE may comprise: processing circuitry (PC) 702, which includes one or more processors (P) 755 (e.g., one or more general purpose microprocessors and/or one or more other processors, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), and the like); communication circuitry 748, which is coupled to an antenna arrangement 749 comprising one or more antennas and which comprises a transmitter (Tx) 745 and a receiver (Rx) 747 for enabling the UE to transmit data and receive data (e.g., wirelessly transmit/receive data); and a storage unit (a.k.a., "data storage system”) 708, which may include one or more non-volatile storage devices and/or one or more volatile storage devices. In embodiments where PC 702 includes a programmable processor, a computer readable storage medium (CRSM) 742 may be provided. CRSM 742 may store a computer program (CP) 743 comprising computer readable instructions (CRI) 744. CRSM 742 may be a non-transitory computer readable medium, such as, magnetic media (e.g., a hard disk), optical media, memory devices (e.g., random access memory, flash memory), and the like. In some embodiments, the CRI 744 of computer program 743 is configured such that when executed by PC 702, the CRI causes the UE to perform steps described herein (e.g., steps described herein with reference to the flow charts). In other embodiments, the UE may be configured to perform steps described herein without the need for code. That is, for example, PC 702 may consist merely of one or more ASICs. Hence, the features of the embodiments described herein may be implemented in hardware and/or software.

[00114] Summary of Various Additional Embodiments

[00115] A1 . A user equipment, UE, relay, the UE relay comprising: a receiver for receiving from a UE a control plane, CP, message comprising a DLL address of the UE; and processing circuitry configured to cause the UE relay to maintain (e.g., create) an association between the DLL address and a link established with the UE.

[00116] A2. The UE relay of embodiment A1, wherein the UE is a source UE, and the CP message is a security mode complete message (e.g., an extended version of the current 3GPP PROSE DIRECT LINK SECURITY MODE COMPLETE message).

[00117] A3. The UE relay of embodiment A1 or A2, wherein the UE is a source UE, and the UE relay further comprises a transmitter for transmitting to a target UE a message comprising the DLL address of the source UE.

[00118] A4. The UE relay of embodiment A3, wherein the message transmitted to the target UE is a security message or a link modification request message.

[00119] A5. The UE relay of embodiment A3 or A4, wherein the UE relay is further configured to: obtain a

DLL address of the target UE from a message transmitted by the target UE to the UE relay; transmit to the source UE a message comprising the DLL address of the target UE.

[00120] A6. The UE relay of embodiment A1, wherein the UE is a target UE, and the CP message is a communication accept message (e.g., an extended version of the current 3GPP Direct Communication Accept message).

[00121] A7. The UE relay of embodiment A6, wherein the UE relay is further configured to transmit to a source UE a message comprising the DLL address of the target UE. [00122] A8. The UE relay of any one of embodiments A1-A7, wherein the DLL address of the UE is an

Ethernet MAC address.

[00123] A9. The UE relay of any one of embodiments A1-A8, wherein the UE relay is a UE-to-UE Relay, and the UE is 5G ProSe End UE.

[00124] A10. The UE relay of any one of embodiments A1-A8, wherein the UE relay is a UE-to-Network

Relay, and the UE is 5G ProSe Remote UE.

[00125] A11. The UE relay of any one of embodiments A1-A10, wherein receiving the CP message comprises receiving a DLL PDU (e.g., PDCP PDU or RLC PDU) having a header and a payload, wherein the payload contains at least a portion of the CP message.

[00126] B1 . A user equipment, UE„ the UE comprising: a receiver; and a transmitter, wherein the UE is configured to employ the transmitter to transmit to a relay a control plane, CP, message comprising a data link layer, DLL, address of the UE.

[00127] B2. The UE of embodiment B1 , wherein the UE is a source UE and the CP message is a security mode complete message that is responsive to a security command message transmitted by a UE relay, or the UE is a target UE and the CP message is a communication accept message that is responsive to a communication request message transmitted by the UE relay.

[00128] B3. The UE of embodiment B1 or B2, wherein the DLL address of the source UE is an Ethernet

MAC address.

[00129] B4. The UE of embodiment B1, B2, or B3, wherein the UE transmits the CP message to the relay by transmitting to the relay a layer-2 PDU having a header and a payload, the payload containing at least a portion of the CP message.

[00130] C1. A method performed by a user equipment, UE, relay, the method comprising: receiving from a

UE a control plane, CP, message comprising a DLL address of the UE; and maintaining (e.g., creating) an association between the DLL address and a link established with the UE.

[00131] C2. The method of embodiment C1, wherein the CP message is a security mode complete message.

[00132] C3. The method of embodiment C1 or C2, wherein the UE is a source UE, and the method further comprising the UE relay transmitting to a target UE a message comprising the DLL address of the source UE.

[00133] C4. The method of embodiment C3, wherein the message transmitted to the target UE is a security mode complete message or a link modification request message. [00134] C5. The method of embodiment C3 or C4, further comprising: obtaining a DLL address of the target UE from a message transmitted by the target UE to the UE relay; transmitting to the source UE a message comprising the DLL address of the target UE.

[00135] C6. The method of embodiment C1, wherein the UE is a target UE, the method further comprises transmitting to the target UE a communication request message, and the CP message is a communication accept message responsive to the communication request message transmitted by the UE relay to the target UE.

[00136] C7. The method of embodiment C6, further comprising transmitting to a source UE a message comprising the DLL address of the target UE.

[00137] C8. The method of any one of embodiments C1-C7, wherein the DLL address of the UE is an

Ethernet MAC address.

[00138] C9. The method of any one of embodiments C1-C8, wherein receiving the CP message comprises receiving a DLL PDU (e.g., PDCP PDU, RLC PDU, MAC PDU) having a header and a payload, wherein the payload contains at least a portion of the CP message.

[00139] D1. A method performed by a user equipment, UE„ the method comprising: creating a control plane, CP, message (e.g., a sidelink control plane message), wherein the CP message comprises a data link layer, DLL, address of the UE; and wirelessly transmitting the CP message to a UE relay.

[00140] D2. The method of embodiment D1, wherein the UE is a source UE, and the CP message is a security mode complete message (e.g., an extended version of the current 3GPP PROSE DIRECT LINK SECURITY MODE COMPLETE message).

[00141] D3. The method of embodiment D1 or D2, further comprising after transmitting the CP message, receiving from the relay a message comprising a DLL address of a target UE.

[00142] D4. The method of embodiment D3, further comprising: after receiving the message comprising the

DLL address of the target UE, creating a first DLL protocol data unit, PDU, comprising a payload comprising at least a portion of a second DLL PDU (e.g., an Ethernet frame); and transmitting the first DLL PDU to the UE relay, wherein the destination address of the first DLL PDU is a DLL address of the UE relay, and the destination address of the second DLL PDU is the DLL address of the target UE.

[00143] D5. The method of embodiment D1, wherein the UE is a target UE, and the CP message is a communication accept message (e.g., an extended version of the current 3GPP Direct Communication Accept message). [00144] D6. The method of any one of embodiments D1-D6, wherein the DLL address of the UE is an

Ethernet MAC address.

[00145] D7. The method of embodiments D1-D6, wherein transmitting the CP message to the UE relay comprises: creating a DLL protocol data unit, PDU (e.g., MAC frame, RLC frame, PDCP PDU), comprising a payload comprising at least a portion of the CP message, wherein the DLL PDU is addressed to the UE relay; and transmitting the DLL PDU to the UE relay.

[00146] E1 . A user equipment, UE, relay, the UE relay comprising: a receiver for receiving from a first UE a first control plane, CP, message comprising a DLL address of the first UE; and a transmitter for transmitting to a second UE a second CP message comprising the DLL address of the first UE.

[00147] E2. The UE relay of embodiment E1, further comprising processing circuitry configured to cause the UE relay to maintain (e.g., create) an association between the DLL address of the first UE and a link established between the first UE and the UE relay.

[00148] E3. The UE relay of embodiment E1 or E2, wherein the first UE is a source UE, and the first CP message is a security mode complete message (e.g., an extended version of the current 3GPP PROSE DIRECT LINK SECURITY MODE COMPLETE message).

[00149] E4. The UE relay of embodiment E1 or E2, wherein the first UE is a target UE, and the first CP message is a communication accept message (e.g., an extended version of the current 3GPP Direct Communication Accept message).

[00150] E5. The UE relay of any one of embodiments E1-E4, wherein the second CP message is a security mode complete message, a communication accept message, a link modification request message, or a link modification accept message.

[00151] E6. The UE relay of any one of embodiments E1-E5, wherein the first DLL address of the first UE is a first Ethernet MAC address and the second DLL address of the second UE is a second Ethernet MAC address.

[00152] F1. A method performed by a user equipment, UE, relay, the method comprising: receiving from a first UE a first control plane, CP, message comprising a DLL address of the first UE; and transmitting to a second UE a second CP message comprising the DLL address of the first UE.

[00153] F2. The method of embodiment F1, further comprising maintaining an association between the DLL address of the first UE and a link established between the first UE and the UE relay. [00154] F3. The method of embodiment F1 or F2, wherein the first UE is a source UE, and the first CP message is a security mode complete message (e.g., an extended version of the current 3GPP PROSE DIRECT LINK SECURITY MODE COMPLETE message).

[00155] F4. The method of embodiment F1 or F2, wherein the first UE is a target UE, and the first CP message is a communication accept message (e.g., an extended version of the current 3GPP Direct Communication Accept message).

[00156] F5. The method of any one of embodiments F1-F4, wherein the second CP message is a security mode complete message, a communication accept message, a link modification request message, or a link modification accept message.

[00157] F6. The method of any one of embodiments F1-F5, wherein the first DLL address of the first UE is a first Ethernet MAC address and the second DLL address of the second UE is a second Ethernet MAC address.

[00158] F7. The method of any one of embodiments F1-F6, wherein transmitting the second CP message to the second UE comprises transmitting to the second UE a DLL PDU having a header and a payload, the payload containing at least a portion of the CP message comprising the DLL address of the first UE.

[00159] G1 . A computer program comprising instructions, executable by processing circuitry of a UE, for configuring the UE to perform the method of any one of embodiments C1-C8, D1-D5, or G1-G7.

[00160] G2. A carrier containing the computer program of embodiment H1, wherein the carrier is one of an electronic signal, an optical signal, a radio signal, and a computer readable storage medium.

[00161] Conclusion

[00162] As disclosed above, in one embodiment this disclosure extends the Direct Communication Request message with an indication of Ethernet traffic, so relay 102 and target UE 103 will know the link between the relay and target is established for Ethernet traffic. Target UE 103 may include its Ethernet MAC address in the Direct Communication Accept or Link Modification Accept message sent to relay 102 and the UE relay may forward the target UE's Ethernet MAC address to the source UE, and, likewise, forward the source UE's Ethernet MAC address to the target UE. This enables the source (or target) UE to create Ethernet frames to be sent to the target (or source) UE with destination address set to the target (or source ) UE's MAC address.

[00163] As also disclosed above, in another embodiment, a UE (source UE or target UE) provides its Ethernet MAC address to the relay and the relay then maintains an association between this Ethernet MAC address and the link (e.g., PC5 link) established between the UE and the relay for use in routing Ethernet PDUs (a.k.a., Ethernet frames). This is useful for when the relay is a UE-to-Network because the relay can avoid having to broadcast to all UEs connected to the relay for the same RSC the Ethernet frames received from the network with a destination Ethernet MAC address unknown to the relay. This is also useful when the relay is a UE-to-UE relay because, if the relay serves several UEs, it enable the relay to avoid having to broadcast to all other UEs connected to the relay for the same RSC the Ethernet frames sent by one of the UEs with a destination Ethernet MAC address unknown to the relay.

[00164] While various embodiments are described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.

[00165] As used herein transmitting a message "to” or "toward” an intended recipient encompasses transmitting the message directly to the intended recipient or transmitting the message indirectly to the intended recipient (i.e., one or more other nodes are used to relay the message from the source node to the intended recipient). Likewise, as used herein receiving a message "from” a sender encompasses receiving the message directly from the sender or indirectly from the sender (i.e., one or more nodes are used to relay the message from the sender to the receiving node). Further, as used herein "a” means "at least one” or "one or more.”

[00166] Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel.

Claims

1. A relay user equipment, UE, (102) the relay UE (102) comprising: a receiver (747) for receiving a control plane, CP, message (m201, m203, m205, m206) transmitted by an end UE (101, 103) to the relay UE, the CP message comprising a data link layer, DLL, address of the end UE; and processing circuitry (702) configured to cause the relay UE to maintain an association between the DLL address of the end UE and a link established with the end UE.

2. The relay UE of claim 1, wherein the relay UE is further configured to enable security protection with the end UE, and the CP message is transmitted by the end UE to the relay UE after the security protection is enabled with the end UE.

3. The relay UE of claim 1 or 2, wherein the end UE is a source UE (101), and the relay UE further comprises a transmitter (745) for transmitting to a target UE (103) a message comprising the DLL address of the source UE.

4. The relay UE of claim 3, wherein the relay UE is further configured to enable security protection with the target UE, and the relay UE is further configured to transmit the message comprising the DLL address of the source UE to the target UE after the security protection with the target UE is enabled.

5. The relay UE of claim 3 or 4, wherein the relay UE is further configured to obtain a DLL address of the target UE from a message (m207) transmitted by the target UE to the relay UE, and the relay UE is further configured to transmit to the source UE (101) a message (m208, m258) comprising the DLL address of the target UE.

6. The relay UE of claim 1, wherein the end UE is a target UE (103), and the CP message is an accept message.

7. The relay UE of claim 6, wherein the relay UE is further configured to transmit to a source UE a message (m208, m258) comprising the DLL address of the target UE.

8. The relay UE of any one of claims 1-7, wherein the DLL address of the end UE is an Ethernet Media Access Control, MAC, address.

9. The relay UE of any one of claims 1-8, wherein the relay UE is a UE-to-UE Relay, and the end UE is 5G ProSe End UE.

10. The relay UE of any one of claims 1-8, wherein the relay UE is a UE-to-Network Relay, and the end UE is 5G ProSe Remote UE.

11. The relay UE of any one of claims 1-10, wherein receiving the CP message comprises receiving a DLL protocol data unit, PDU, having a header and a payload, wherein the payload contains at least a portion of the CP message comprising the DLL address of the end UE.

12. The relay UE of any one of claims 1-11, wherein the DLL address is an Ethernet MAC address, the relay UE is further configured to determine whether the Ethernet MAC address is used by another UE, and the relay UE is further configured to transmit to the end UE a message indicating there is an Ethernet MAC address conflict in response to determining that the Ethernet MAC address is used by another UE.

13. The relay UE of any one of claims 1-12, wherein the link established with the end UE is a PC5 link used for transferring Ethernet traffic.

14. The relay UE of claim 1, wherein the end UE is a source UE, the link established with the end UE is a PC5 link, the relay UE is further configured such that, if the PC5 link is used for transferring Ethernet traffic, the relay UE sends the Ethernet MAC address of the source UE to a target UE after security is enabled with the target UE.

15. An end user equipment, UE, (101, 103), the end UE comprising: a receiver (747); and a transmitter (745), wherein the end UE is configured to employ the transmitter to transmit to a relay UE (102) a control plane, CP, message comprising an Ethernet Media Access Control, MAC, address of the UE.

16. The end UE of claim 15, wherein the end UE is a source UE, and the source UE is further configured to initiate a link establishment procedure with the relay UE prior to transmitting the CP message comprising the Ethernet MAC address, and the source UE is further configured such that, if the link to be established is to be used for transferring Ethernet traffic, the source UE transmits the CP message comprising the Ethernet MAC address after security protection is enabled with the relay UE.

17. The end UE of claim 15, wherein the end UE is a source UE (101) and the CP message is a security mode complete message that is responsive to a security command message (m202) transmitted by the relay UE.

18. The end UE of claim 15, wherein the end UE is a target UE (103) and the CP message is an accept message (m207) that is responsive to a communication request message (m204) transmitted by the relay UE (102).

19. The end UE of any one of claims 15-18, wherein the UE transmits the CP message to the relay by transmitting to the relay a layer-2 protocol data unit, PDU, having a header and a payload, the payload containing at least a portion of the CP message comprising the Ethernet MAC address.

20. The end UE of any one of claims 15-19, wherein the end UE is further configured to employ the transmitter to transmit to the relay a direct communication request, DCR, prior to employing the transmitter to transmit to the relay the CP message comprising the Ethernet MAC address of the end UE.

21. A relay user equipment, UE, (102), the relay UE comprising: a receiver (747) for receiving from a first UE (101, 103) a first control plane, CP, message (m203, m207) comprising a data link layer, DLL, address of the first UE; and a transmitter (745) for transmitting to a second UE (101, 103) a second CP message (m206, m208, m254, m258) comprising the DLL address of the first UE, wherein the second CP message is not the first CP message.

22. The relay UE of claim 21, further comprising processing circuitry (702) configured to cause the relay UE to maintain an association between the DLL address of the first UE and a link established between the first UE and the relay UE.

23. The relay UE of claim 22, wherein the link established with the end UE is a P25 link used for transferring Ethernet traffic.

24. The relay UE of any one of claims 21-23, wherein the relay UE is further configured to enable security protection with the second UE, and the relay UE is further configured to transmit to the second UE the second CP message comprising the DLL address of the first UE after the security protection with the target UE is enabled.

25. The relay UE of claim 24, wherein the relay UE is further configured to obtain a DLL address of the second UE from a message (m207) transmitted by the second UE to the relay UE, and the relay UE is further configured to transmit to the first UE a message (m208, m258) comprising the DLL address of the second UE.

26. The relay UE of any one of claims 21-25, wherein the DLL address of the first UE is an Ethernet MAC address, the relay UE is further configured to determine whether the Ethernet MAC address is used by another UE, and the relay UE is further configured to transmit to the first UE a message indicating there is an Ethernet MAC address conflict in response to determining that the Ethernet MAC address is used by another UE.

27. The relay UE of claim 21, wherein the first UE is a source UE, the second UE is a target UE, the link established with the source UE is a P25 link, and the relay UE is further configured such that, if the P25 link is used for transferring Ethernet traffic, the relay UE sends the DLL address of the source UE to the target UE after security is enabled with the target UE.

28. A method (400) performed by a relay user equipment, UE, (102), the method comprising: receiving (s402) a control plane, CP, message (m201, m203, m205, m206) transmitted by an end UE (101, 103), the CP message comprising a data link layer, DLL, address of the end UE; and maintaining (s404) an association between the DLL address of the end UE and a link established with the end UE.

29. The method of claim 28, wherein the method further comprises enabling security protection with the end UE, and the CP message is transmitted by the end UE after the security protection is enabled with the end UE.

30. The method of claim 28 or 29, wherein the end UE is a source UE (101), the method further comprises transmitting to a target UE (103) a message comprising the DLL address of the source UE, the method further comprises enabling security protection with the target UE, the message comprising the DLL address of the source UE is transmitted to the target UE after the security protection with the target UE is enabled, the method further comprises obtaining a DLL address of the target UE from a message (m207) transmitted by the target UE, and transmitting to the source UE a message (m208, m258) comprising the DLL address of the target UE.

31 . The method of claim 28, wherein the end UE is a target UE (103), the CP message is an accept message, and the method further comprises transmitting to a source UE a message (m208, m258) comprising the DLL address of the target UE.

32. The method of any one of claims 28-31, wherein the DLL address of the end UE is a Media Access Control, MAC, address.

33. The method of any one of claims 28-32, wherein the relay UE is a UE-to-UE Relay, and the end UE is 5G ProSe End UE.

34. The method of any one of claims 28-32, wherein the relay UE is a UE-to-Network Relay, and the end UE is 5G ProSe Remote UE.

35. The method of any one of claims 28-34, wherein receiving the CP message comprises receiving a DLL protocol data unit, PDU, having a header and a payload, wherein the payload contains at least a portion of the CP message comprising the DLL address of the end UE.

36. The method of any one of claims 28-35, wherein the DLL address is an Ethernet MAC address, the method further comprises determining whether the Ethernet MAC address is used by another UE, and the method further comprises transmitting to the end UE a message indicating there is an Ethernet MAC address conflict in response to determining that the Ethernet MAC address is used by another UE.

37. The method of claim 28, wherein the end UE is a source UE, the link established with the end UE is a PC5 link, and the relay UE is configured such that, if the PC5 link is used for transferring Ethernet traffic, the relay UE sends the Ethernet MAC address of the source UE to a target UE after security is enabled with the target UE.

38. A method (500) performed by an end user equipment, UE, (101, 103), the method comprising: creating (s502) a control plane, CP, message, wherein the CP message comprises a data link layer, DLL, address of the end UE; and wirelessly transmitting (s504) the CP message to a UE relay (102).

39. The method of claim 38, wherein the end UE is a source UE, and the method comprises the source UE initiating a link establishment procedure with the relay UE prior to wirelessly transmitting the CP message to the relay UE, and the source UE is configured such that, if the link to be established is to be used for transferring Ethernet traffic, the source UE transmits the CP message comprising the Ethernet MAC address after security protection is enabled with the relay UE. T1

40. The method of claim 38, wherein the end UE is a source UE (101) and the CP message is a security mode complete message that is responsive to a security command message (m202) transmitted by the relay UE, or the end UE is a target UE (103) and the CP message is an accept message (m207) that is responsive to a communication request message (m204) transmitted by the relay UE (102).

41 . The method of any one of claims 38-40, wherein the method further comprises, after transmitting the CP message, receiving from the relay a message (m208, m258) comprising a DLL address of a target UE (103), the DLL address of the target UE is an Ethernet MAC address of the target UE, and the method further comprises: after receiving the message comprising the Ethernet MAC address of the target UE, creating a DLL protocol data unit, PDU, comprising a payload comprising at least a portion of an Ethernet frame; and transmitting the DLL PDU to the relay UE, wherein the destination address of the DLL PDU is a DLL address of the relay UE, and the destination address of the Ethernet frame is the Ethernet MAC address of the target UE.

42. The method of any one of claims 38-41, wherein transmitting the CP message to the relay UE comprises: creating a DLL protocol data unit, PDU, comprising a payload comprising at least a portion of the CP message comprising the DLL address of the end UE, wherein the DLL PDU is addressed to the relay UE; and transmitting the DLL PDU to the relay UE.

43. The method of any one of claims 38-42, wherein the method further comprises, prior to transmitting to the relay the CP message comprising the DLL address of the end UE: transmitting to the relay a direct communication request, DCR; and performing a security process for establishing security with the relay UE.

44. A method (600) performed by a user equipment, UE, relay (102), the method comprising: receiving (s602) from a first UE (101, 103) a first control plane, CP, message (m203, m207) comprising a DLL address of the first UE; and transmitting (s604) to a second UE (101, 103) a second CP message (m206, m208, m254, m258) comprising the DLL address of the first UE, wherein the second CP message is not the first CP message.

45. The method of claim 44, wherein the method further comprises maintaining an association between the DLL address of the first UE and a link established between the first UE and the relay UE, and the link established with the end UE is a PC5 link used for transferring Ethernet traffic.

46. The method of any one of claims 44-45, wherein the method further comprise enabling security protection with the second UE, the relay UE is configured to transmit to the second UE the second CP message comprising the DLL address of the first UE after the security protection with the target UE is enabled, and the method further comprise obtaining a DLL address of the second UE from a message (m207) transmitted by the second UE to the relay UE, and the method further comprise transmitting to the first UE a message (m208, m258) comprising the DLL address of the second UE.

47. The method of any one of claims 44-46, wherein the DLL address of the first UE is an Ethernet MAC address, the method further comprise determining whether the Ethernet MAC address is used by another UE, and the method further comprise, in response to determining that the Ethernet MAC address is used by another UE, transmitting to the first UE a message indicating there is an Ethernet MAC address conflict.

48. The method of claim 44, wherein the first UE is a source UE, the second UE is a target UE, the link established with the source UE is a PC5 link, and the relay UE is configured such that, if the PC5 link is used for transferring Ethernet traffic, the relay UE sends the DLL address of the source UE to the target UE after security is enabled with the target UE.

49. A computer program (743) comprising instructions (744), executable by processing circuitry (702) of a UE (101, 102, 103), for configuring the UE to perform the method of any one of above method claims.

50. A carrier containing the computer program of claim 49, wherein the carrier is one of an electronic signal, an optical signal, a radio signal, and a computer readable storage medium (742).