Method, apparatus and system directed to change of WTRU to WTRU relay

Abstract

To provide methods, apparatuses, systems, etc., directed to performing a change of relay.SOLUTION: A WTRU may transmit a link modification request message to a target WTRU via a first relay WTRU for requesting a change of relay. The WTRU may receive a link modification accept message from the target WTRU via the first relay WTRU, indicating a (e.g., proposed) second relay WTRU (e.g., identifier). The WTRU may transmit a direct communication request message including the second relay WTRU identifier for indicating to the target WTRU acceptance of the proposed second relay WTRU. The WTRU may receive a direct communication accept message from the target WTRU via the second relay WTRU, indicating the traffic may be relayed via the second relay WTRU.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Patent Application No. 63 / 007,008, filed August 4, 2020, which is incorporated herein by reference in its entirety. [Background technology]

[0002] The present disclosure relates to network communications, including but not exclusively to methods, apparatus, systems, etc., directed to relays between wireless transmit / receive units (WTRUs). Two WTRUs that may not be able to communicate directly may communicate using a relay WTRU. The relay WTRU may relay data between both WTRUs, allowing them to communicate. Summary of the Invention

[0003] Disclosed herein are methods, apparatuses, systems, etc. directed to implementing a relay change. In one embodiment, a WTRU may send a link modification request message to a target WTRU via a first relay WTRU to request a relay change. The WTRU may receive a link modification accept message from the target WTRU via the first relay WTRU, the link modification accept message indicating a second relay WTRU (e.g., an identifier). The WTRU may send a direct communication request message including a second relay WTRU identifier to indicate acceptance (e.g., confirmation) of the second relay WTRU to the target WTRU. The WTRU may receive a direct communication accept message from the target WTRU via the second relay WTRU, indicating that traffic may be relayed via the second relay WTRU.

[0004] Although various embodiments are described and / or claimed herein in which apparatus, systems, devices, etc., and / or any elements thereof are configured to perform an operation, process, algorithm, function, etc., and / or any portion thereof, it should be understood that any embodiment described and / or claimed herein assumes that any apparatus, system, device, etc., and / or any element thereof performs any operation, process, algorithm, function, etc., and / or any portion thereof (and vice versa). [Brief explanation of the drawings]

[0005] A more detailed understanding can be had from the following detailed description, given by way of example in conjunction with the accompanying drawings. The figures in such drawings, like the detailed description, are examples. Therefore, the figures and detailed description should not be considered limiting, as other equally effective examples are possible and likely. Also, like reference numerals in the figures indicate like elements. [Figure 1A] FIG. 1 is a system diagram illustrating an example communication system in which one or more disclosed embodiments may be implemented. [Figure 1B] 1B is a system diagram illustrating an exemplary wireless transmit / receive unit (WTRU) that may be used within the communications system shown in FIG. 1A, according to one embodiment. [Figure 1C] 1B is a system diagram illustrating an example radio access network (RAN) and an example core network (CN) that may be used within the communication system shown in FIG. 1A, according to one embodiment. [Figure 1D] 1B is a system diagram illustrating a further exemplary RAN and a further example of a CN that may be used within the communication system shown in FIG. 1A, according to one embodiment. [Figure 2] 1 illustrates an example of a Layer 2 WTRU to WTRU relaying method. [Figure 3] FIG. 1 illustrates an example of a Layer 3 WTRU to WTRU relaying method. [Figure 4]FIG. 1 illustrates an example of an end-to-end PC5 unicast link method. [Figure 5] 10 shows an example of a method for a change of relay from a Layer 2 WTRU to a WTRU triggered by a source WTRU. [Figure 6] 10 illustrates an example of a method for a change of relay from a Layer 2 WTRU to a WTRU triggered by a relay WTRU. [Figure 7] 10 illustrates an example method for a change of relay from a Layer 2 WTRU to a WTRU based on a link identifier exchange. [Figure 8] 1 illustrates an example of a method for relay change from Layer 3 WTRU to WTRU based on PC5 signaling. [Figure 9] FIG. 10 illustrates an example method for a change of relay from a Layer 3 WTRU to a WTRU based on an end-to-end PC5 unicast link. [Figure 10] FIG. 10 shows an example of a method for relay change at a source WTRU. [Figure 11] FIG. 10 shows an example of a method for relay change at a target WTRU. [Figure 12] FIG. 10 shows another example of a method for relay change at a source WTRU. [Figure 13] FIG. 10 illustrates another example of a method for changing relays at a target WTRU. [Figure 14] FIG. 10 shows another example of a method for relay change at a source WTRU. [Figure 15] FIG. 10 illustrates another example of a method for changing relays at a target WTRU. DETAILED DESCRIPTION OF THE INVENTION

[0006] A detailed description of illustrative embodiments will now be described with reference to various figures. While the description provides detailed examples of possible implementations, it should be noted that the details are intended to be illustrative and in no way limit the scope of the present application. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments and / or examples disclosed herein. However, it will be understood that such embodiments and examples can be practiced without some or all of the specific details set forth herein. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the following description. Furthermore, embodiments and examples not specifically described herein can be practiced in place of, or in combination with, embodiments and other examples explicitly, implicitly, and / or inherently described, disclosed, or otherwise provided herein (collectively "provided").

[0007] Exemplary Communication Network 1A illustrates an exemplary communication system 100 in which one or more disclosed embodiments may be implemented. Communication system 100 may be a multiple-access system that provides content, such as voice, data, video, messaging, broadcasts, etc., to multiple wireless users. Communication system 100 may enable multiple wireless users to access such content through sharing of system resources, including wireless bandwidth. For example, the communication system 100 may employ one or more channel access methods such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), zero-tail unique-word DFT-Spread OFDM (ZT UW DTS-s OFDM), unique word OFDM (UW-OFDM), resource block filtered OFDM, filter bank multicarrier (FBMC), etc.

[0008] 1A, communications system 100 may include wireless transmit / receive units (WTRUs) 102a, 102b, 102c, 102d, RANs 104 / 113, CNs 106 / 115, public switched telephone network (PSTN) 108, the Internet 110, and other networks 112, although it will be understood that the disclosed embodiments contemplate any number of WTRUs, base stations, networks, and / or network elements. Each of WTRUs 102a, 102b, 102c, 102d may be any type of device configured to operate and / or communicate in a wireless environment. By way of example, the WTRUs 102a, 102b, 102c, 102d, any of which may be referred to as a “station” and / or “STA,” may be configured to transmit and / or receive wireless signals and may include user equipment (UE), mobile stations, fixed or mobile subscriber units, subscription-based units, pagers, cellular phones, personal digital assistants (PDAs), smartphones, laptops, netbooks, personal computers, wireless sensors, hotspots or Mi-Fi devices, Internet of Things (IoT) devices, watches or other wearables, head-mounted displays (HMDs), vehicles, drones, medical devices and applications (e.g., remote surgery), industrial devices and applications (e.g., robots and / or other wireless devices operating in industrial and / or automated processing chain situations), consumer electronics devices, devices operating on commercial and / or industrial wireless networks, etc. Any of the WTRUs 102a, 102b, 102c, and 102d may be referred to interchangeably as a UE.

[0009] The communications system 100 may also include a base station 114a and / or a base station 114b. Each of the base stations 114a, 114b may be any type of device configured to wirelessly interface with at least one of the WTRUs 102a, 102b, 102c, 102d to facilitate access to one or more communications networks, such as the CN 106 / 115, the Internet 110, and / or other networks 112. By way of example, the base stations 114a, 114b may be a base transceiver station (BTS), a Node B, an eNodeB, a Home Node B, a Home eNodeB, a gNB, an NR Node B, a site controller, an access point (AP), a wireless router, etc. Although the base stations 114a, 114b are each shown as a single element, it will be understood that the base stations 114a, 114b may include any number of interconnected base stations and / or network elements.

[0010] The base station 114a may be part of the RAN 104 / 113, which may also include other base stations and / or network elements (not shown), such as a base station controller (BSC), a radio network controller (RNC), a relay node, etc. The base station 114a and / or base station 114b may be configured to transmit and / or receive radio signals on one or more carrier frequencies, which may be referred to as a cell (not shown). These frequencies may be licensed spectrum, unlicensed spectrum, or a combination of licensed and unlicensed spectrum. A cell may provide wireless service coverage for a particular geographic area, which may be relatively fixed or may change over time. A cell may be further divided into cell sectors. For example, the cell associated with the base station 114a may be divided into three sectors. Thus, in one embodiment, the base station 114a may include three transceivers, i.e., one transceiver for each sector of the cell. In one embodiment, the base station 114a may employ multiple-input multiple output (MIMO) technology and may utilize multiple transceivers for each sector of the cell, for example, using beamforming to transmit and / or receive signals in desired spatial directions.

[0011] The base stations 114a, 114b may communicate with one or more of the WTRUs 102a, 102b, 102c, 102d over an air interface 116, which may be any suitable wireless communication link (e.g., radio frequency (RF), microwave, centimeter wave, micrometer wave, infrared (IR), ultraviolet (UV), visible light, etc.). The air interface 116 may be established using any suitable radio access technology (RAT).

[0012] More specifically, as noted above, the communications system 100 may be a multiple-access system and may use one or more channel access schemes, such as, for example, CDMA, TDMA, FDMA, OFDMA, SC-FDMA, etc. For example, the base station 114 a and the WTRUs 102 a, 102 b, 102 c in the RAN 104 / 113 may implement a radio technology such as Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA), which may establish the air interface 115 / 116 / 117 using wideband CDMA (WCDMA). WCDMA may include communications protocols such as High-Speed ​​Packet Access (HSPA) and / or Evolved HSPA (HSPA+). HSPA may include High-Speed ​​Downlink (DL) Packet Access (HSDPA) and / or High-Speed ​​Uplink Packet Access (HSUPA).

[0013] In one embodiment, the base station 114a and the WTRUs 102a, 102b, 102c may implement a radio technology such as Evolved UMTS Terrestrial Radio Access (E-UTRA), which may establish the air interface 116 using Long Term Evolution (LTE) and / or LTE-Advanced (LTE-A) and / or LTE-Advanced Pro (LTE-A Pro).

[0014] In one embodiment, the base station 114a and the WTRUs 102a, 102b, 102c may implement a radio technology such as NR radio access, which may establish the air interface 116 using New Radio (NR).

[0015] In one embodiment, the base station 114a and the WTRUs 102a, 102b, 102c may implement multiple radio access technologies. For example, the base station 114a and the WTRUs 102a, 102b, 102c may jointly implement LTE radio access and NR radio access, e.g., using dual connectivity (DC) principles. Thus, the air interface utilized by the WTRUs 102a, 102b, 102c may be characterized by multiple types of radio access technologies and / or transmissions transmitted to / from multiple types of base stations (e.g., eNBs and gNBs).

[0016] In other embodiments, the base station 114a and the WTRUs 102a, 102b, 102c may implement a wireless technology such as IEEE 802.11 (i.e., Wireless Fidelity, WiFi), IEEE 802.16 (i.e., Worldwide Interoperability for Microwave Access, WiMAX), CDMA2000, CDMA2000 1X, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), Interim Standard 95 (IS-95), Interim Standard 856 (IS-856), Global System for Mobile communications (GSM), Enhanced Data rates for GSM Evolution (EDGE), GSM EDGE (GERAN), or the like.

[0017] 1A may be, for example, a wireless router, a Home Node B, a Home eNode B, or an access point and may utilize any suitable RAT to facilitate wireless connectivity in a local area such as a location such as a business, a home, a vehicle, a campus, an industrial facility, an air corridor (e.g., for use by drones), a road, etc. In one embodiment, the base station 114b and the WTRUs 102c, 102d may implement a radio technology such as IEEE 802.11 to establish a wireless local area network (WLAN). In one embodiment, the base station 114b and the WTRUs 102c, 102d may implement a radio technology such as IEEE 802.15 to establish a wireless personal area network (WPAN). In yet another embodiment, the base station 114b and the WTRUs 102c, 102d may establish a picocell or a femtocell using a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE, LTE-A, LTE-A Pro, NR, etc.). As shown in FIG. 1A, the base station 114b may have a direct connection to the Internet 110. Thus, the base station 114b may not need to access the Internet 110 through the CN 106 / 115.

[0018] The RAN 104 / 113 may communicate with the CN 106 / 115, which may be any type of network configured to provide voice, data, application, and / or voice over internet protocol (VoIP) services to one or more of the WTRUs 102a, 102b, 102c, 102d. The data may have various quality of service (QoS) requirements, such as different throughput, latency, error tolerance, reliability, data throughput, and mobility requirements. The CN 106 / 115 may provide call control, billing services, mobile location-based services, prepaid calls, Internet connectivity, video distribution, and / or perform high-level security functions such as user authentication. Although not shown in FIG. 1A , it will be understood that the RAN 104 / 113 and / or the CN 106 / 115 may communicate directly or indirectly with other RANs employing the same RAT as the RAN 104 / 113 or a different RAT. For example, in addition to being connected to the RAN 104 / 113, which may utilize NR radio technology, the CN 106 / 115 may also communicate with another RAN (not shown) employing GSM, UMTS, CDMA2000, WiMAX, E-UTRA, or WiFi radio technology.

[0019] The CN 106 / 115 may also serve as a gateway for the WTRUs 102a, 102b, 102c, 102d to access the PSTN 108, the Internet 110, and / or other networks 112. The PSTN 108 may include a public switched telephone network providing plain old telephone service (POTS). The Internet 110 may include a global system of interconnected computer networks and devices that use common communication protocols, such as the transmission control protocol (TCP), user datagram protocol (UDP), and / or the internet protocol (IP) of the TCP / IP Internet protocol suite. The network 112 may include wired and / or wireless communication networks owned and / or operated by other service providers. For example, the network 112 may include another CN connected to one or more RANs, which may employ the same RAT as the RAN 104 / 113 or a different RAT.

[0020] Some or all of the WTRUs 102a, 102b, 102c, 102d in the communications system 100 may include multi-mode capabilities (e.g., the WTRUs 102a, 102b, 102c, 102d may include multiple transceivers for communicating with different wireless networks over different wireless links.) For example, the WTRU 102c shown in FIG. 1A may be configured to communicate with a base station 114a that may use a cellular-based wireless technology and a base station 114b that may use an IEEE 802 wireless technology.

[0021] 1B is a system diagram illustrating an example WTRU 102. As shown in FIG. 1B, the WTRU 102 may include, among other things, a processor 118, a transceiver 120, a transmit / receive element 122, a speaker / microphone 124, a keypad 126, a display / touchpad 128, non-removable memory 130, removable memory 132, a power source 134, a global positioning system (GPS) chipset 136, and / or other peripherals 138. It will be understood that the WTRU 102 may include any sub-combination of the foregoing elements while remaining consistent with an embodiment.

[0022] The processor 118 may be a general-purpose processor, a special-purpose processor, a conventional processor, a digital signal processor (DSP), multiple microprocessors, one or more microprocessors associated with a DSP core, a controller, a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) circuit, any other type of integrated circuit (IC), a state machine, etc. The processor 118 may perform signal coding, data processing, power control, input / output processing, and / or any other functionality that enables the WTRU 102 to operate in a wireless environment. The processor 118 may be coupled to the transceiver 120, which may be coupled to the transmit / receive element 122. While FIG. 1B depicts the processor 118 and the transceiver 120 as separate components, it will be understood that the processor 118 and the transceiver 120 may be integrated together in an electronic package or chip.

[0023] The transmit / receive element 122 may be configured to transmit signals to or receive signals from a base station (e.g., base station 114a) over the air interface 116. For example, in one embodiment, the transmit / receive element 122 may be an antenna configured to transmit and / or receive RF signals. In one embodiment, the transmit / receive element 122 may be an emitter / detector configured to transmit and / or receive IR, UV, or visible light signals, for example. In yet another embodiment, the transmit / receive element 122 may be configured to transmit and / or receive both RF and light signals. It will be understood that the transmit / receive element 122 may be configured to transmit and / or receive any combination of wireless signals.

[0024] 1B as a single element, the WTRU 102 may include any number of transmit / receive elements 122. More specifically, the WTRU 102 may use MIMO technology. Thus, in one embodiment, the WTRU 102 may include two or more transmit / receive elements 122 (e.g., multiple antennas) for transmitting and receiving wireless signals over the air interface 116.

[0025] The transceiver 120 may be configured to modulate signals transmitted by the transmit / receive element 122 and demodulate signals received by the transmit / receive element 122. As mentioned above, the WTRU 102 may have multi-mode capabilities. Thus, the transceiver 120 may include multiple transceivers to enable the WTRU 102 to communicate via multiple RATs, such as NR and IEEE 802.11.

[0026] The processor 118 of the WTRU 102 may be coupled to and may receive user-entered data from a speaker / microphone 124, a keypad 126, and / or a display / touchpad 128 (e.g., a liquid crystal display (LCD) display unit or an organic light-emitting diode (OLED) display unit). The processor 118 may also output user data to the speaker / microphone 124, the keypad 126, and / or the display / touchpad 128. Furthermore, the processor 118 may access information from and store data in any type of suitable memory, such as non-removable memory 130 and / or removable memory 132. The non-removable memory 130 may include random-access memory (RAM), read-only memory (ROM), a hard disk, or any other type of memory storage device. The removable memory 132 may include a subscriber identity module (SIM) card, a memory stick, a secure digital (SD) memory card, etc. In other embodiments, the processor 118 may access information and store data in memory that is not physically located on the WTRU 102, such as on a server or home computer (not shown).

[0027] The processor 118 may receive power from the power source 134, but may also be configured to distribute and / or control the power to other components in the WTRU 102. The power source 134 may be any suitable device for providing power to the WTRU 102. For example, the power source 134 may include one or more dry batteries (e.g., nickel-cadmium (NiCd), nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion), etc.), solar cells, fuel cells, etc.

[0028] The processor 118 may also be coupled to a GPS chipset 136, which may be configured to provide location information (e.g., longitude and latitude) regarding the current location of the WTRU 102. In addition to or instead of information from the GPS chipset 136, the WTRU 102 may receive location information from a base station (e.g., base stations 114a, 114b) over the air interface 116 and / or determine its location based on the timing of signals being received from two or more nearby base stations. It will be appreciated that the WTRU 102 may obtain location information by way of any suitable location-determination method while remaining consistent with an embodiment.

[0029] The processor 118 may further be coupled to other peripherals 138, which may include one or more software and / or hardware modules that provide additional features, functionality, and / or wired or wireless connectivity. For example, the peripherals 138 may include an accelerometer, an electronic compass, a satellite transceiver, a digital camera (for photos and / or videos), a universal serial bus (USB) port, a vibration device, a television transceiver, a hands-free headset, a Bluetooth module, a frequency modulated (FM) radio unit, a digital music player, a media player, a video game player module, an internet browser, a virtual reality and / or augmented reality (VR / AR) device, an activity tracker, etc. The peripheral device 138 may include one or more sensors, which may be one or more of a gyroscope, an accelerometer, a Hall effect sensor, a magnetometer, a direction sensor, a proximity sensor, a temperature sensor, a time sensor, a geolocation sensor, an altimeter, a light sensor, a touch sensor, a magnetometer, a barometer, a gesture sensor, a biometric sensor, and / or a humidity sensor.

[0030] The WTRU 102 may include a full-duplex radio where transmission and reception of some or all of the signals (e.g., associated with a particular subframe for both the UL (e.g., for transmission) and downlink (e.g., for reception)) may be parallel and / or simultaneous. The full-duplex radio may include an interference management unit 139 for reducing and or substantially eliminating self-interference through either hardware (e.g., chokes) or processor-mediated signal processing (e.g., via a separate processor (not shown) or processor 118). In one embodiment, the WTRU 102 may include a half-duplex radio for transmission and reception of either some or all of the signals (e.g., associated with a particular subframe for either the UL (e.g., for transmission) or downlink (e.g., for reception)).

[0031] 1C is a system diagram illustrating the RAN 104 and the CN 106 according to one embodiment. As mentioned above, the RAN 104 may communicate with the WTRUs 102a, 102b, 102c over the air interface 116 using E-UTRA radio technology. The RAN 104 may also communicate with the CN 106.

[0032] The RAN 104 may include eNode-Bs 160a, 160b, and 160c, although it will be understood that the RAN 104 may include any number of eNode-Bs while remaining consistent with an embodiment. The eNode-Bs 160a, 160b, and 160c may each include one or more transceivers for communicating with the WTRUs 102a, 102b, and 102c over the air interface 116. In an embodiment, the eNode-Bs 160a, 160b, and 160c may implement MIMO technology. Thus, the eNode-B 160a may, for example, use multiple antennas to transmit wireless signals to and / or receive wireless signals from the WTRU 102a.

[0033] Each of the eNode-Bs 160a, 160b, 160c may be associated with a particular cell (not shown) and may be configured to handle radio resource management decisions, handover decisions, user scheduling, etc. in the UL and / or DL. As shown in FIG. 1C, the eNode-Bs 160a, 160b, 160c may communicate with each other via an X2 interface.

[0034] 1C may include a mobility management entity (MME) 162, a serving gateway (SGW) 164, and a packet data network (PDN) gateway (or PGW) 166. Although each of the foregoing elements is illustrated as part of the CN 106, it will be understood that any of these elements may be owned and / or operated by an entity other than the CN operator.

[0035] The MME 162 may be connected to each of the eNode-Bs 160a, 160b, 160c in the RAN 104 via an S1 interface and may function as a control node. For example, the MME 162 may be responsible for authenticating users of the WTRUs 102a, 102b, 102c, activating / deactivating bearers, selecting a particular serving gateway during initial attach of the WTRUs 102a, 102b, 102c, etc. The MME 162 may provide a control plane function for switching between the RAN 104 and other RANs (not shown) that employ other radio technologies such as GSM and / or WCDMA.

[0036] The SGW 164 may be connected to each of the eNode-Bs 160a, 160b, 160c in the RAN 104 via an S1 interface. The SGW 164 may generally route and forward user data packets to and from the WTRUs 102a, 102b, 102c. The SGW 164 may perform other functions, such as anchoring the user plane during inter-eNode-B handovers, triggering paging when DL data is available to the WTRUs 102a, 102b, 102c, and managing and storing the context of the WTRUs 102a, 102b, 102c.

[0037] The SGW 164 may be connected to a PGW 166, which may provide the WTRUs 102a, 102b, 102c with access to packet-switched networks, such as the Internet 110, to facilitate communications between the WTRUs 102a, 102b, 102c and IP-enabled devices.

[0038] The CN 106 may facilitate communications with other networks. For example, the CN 106 may provide the WTRUs 102a, 102b, 102c with access to circuit-switched networks, such as the PSTN 108, to facilitate communications between the WTRUs 102a, 102b, 102c and traditional landline communications devices. For example, the CN 106 may include or communicate with an IP gateway (e.g., an IP multimedia subsystem (IMS) server) that serves as an interface between the CN 106 and the PSTN 108. Furthermore, the CN 106 may provide the WTRUs 102a, 102b, 102c with access to other networks 112, which may include other wired and / or wireless networks owned and / or operated by other service providers.

[0039] Although the WTRU is depicted in FIGS. 1A-1D as a wireless terminal, it is contemplated that in certain representative embodiments, such a terminal may use a wired communication interface (e.g., temporarily or permanently) with the communication network.

[0040] In a representative embodiment, the other network 112 may be a WLAN.

[0041] A WLAN in infrastructure Basic Service Set (BSS) mode may have an access point (AP) of the BSS and one or more stations (STAs) associated with the AP. The AP may have access or interface to a Distribution System (DS) or another type of wired / wireless network that carries traffic into and / or out of the BSS. Traffic originating from outside the BSS to a STA may arrive through the AP and be delivered to the STA. Traffic originating from a STA to a destination outside the BSS may be sent to the AP and transmitted to the respective destination. Traffic between STAs within the BSS may be transmitted, for example, through the AP; the source STA may send traffic to the AP, which may deliver the traffic to the destination STA. Traffic between STAs within the BSS may be considered and / or referred to as peer-to-peer traffic. Peer-to-peer traffic may be transmitted between a source STA and a destination STA (e.g., directly between them) in a direct link setup (DLS). In certain representative embodiments, the DLS may use 802.11e DLS or 802.11z tunneled DLS (TDLS). A WLAN using an Independent BSS (IBSS) mode may not have an AP, and STAs within or using the IBSS (e.g., all of the STAs) may communicate directly with each other. The IBSS mode of communication may be referred to herein as an "ad hoc" communication mode.

[0042] When using the 802.11ac infrastructure mode of operation or a similar mode of operation, an AP may transmit beacons on a fixed channel, such as a primary channel. The primary channel may be a fixed width (e.g., a 20 MHz wide bandwidth) or a width that is dynamically set via signaling. The primary channel may be the operating channel of the BSS and may be used by STAs to establish a connection with the AP. In certain representative embodiments, Carrier Sense Multiple Access with Collision Avoidance (CSMA / CA) may be implemented, for example, in an 802.11 system. With CSMA / CA, STAs (e.g., all STAs), including the AP, may sense the primary channel. If the primary channel is sensed / detected and / or determined to be busy by a particular STA, the particular STA may back off. One STA (e.g., only one station) may transmit at any given time in a given BSS.

[0043] High Throughput (HT) STAs may use 40 MHz wide channels for communication, which may be formed, for example, through a combination of a primary 20 MHz channel and adjacent or non-adjacent 20 MHz channels.

[0044] A Very High Throughput (VHT) STA may support 20 MHz, 40 MHz, 80 MHz, and / or 160 MHz-wide channels. The 40 MHz and / or 80 MHz-wide channels may be formed by combining contiguous 20 MHz channels. A 160 MHz channel may be formed by combining eight contiguous 20 MHz channels or by combining two non-contiguous 80 MHz channels, which may be referred to as an 80+80 configuration. For the 80+80 configuration, after channel encoding, the data may pass through a segment parser that may split the data into two streams. Inverse Fast Fourier Transform (IFFT) processing and time-domain processing may be performed separately on each stream. The streams may be mapped to two 80 MHz channels, and the data may be transmitted by the transmitting STA. At the receiver of the receiving STA, the operations described above for the 80+80 configuration may be reversed and the combined data may be transmitted to the Medium Access Control (MAC).

[0045] Sub-1 GHz operating modes are supported by 802.11af and 802.11ah. Channel operating bandwidths and carriers are reduced in 802.11af and 802.11ah compared to those used in 802.11n and 802.11ac. 802.11af supports 5 MHz, 10 MHz, and 20 MHz bandwidths in the TV White Space (TVWS) spectrum, while 802.11ah supports 1 MHz, 2 MHz, 4 MHz, 8 MHz, and 16 MHz bandwidths using non-TVWS spectrum. According to representative embodiments, 802.11ah may support meter-type control / machine-type communications, such as MTC devices within macro coverage areas. MTC devices may have limited capabilities, including, for example, support for (e.g., only for) specific and / or limited bandwidths. MTC devices may include batteries with above-threshold battery life (e.g., to maintain very long battery life).

[0046] WLAN systems that can support multiple channels and channel bandwidths, such as 802.11n, 802.11ac, 802.11af, and 802.11ah, include a channel that can be designated as a primary channel. The primary channel can have a bandwidth equal to the maximum common operating bandwidth supported by all STAs in the BSS. The bandwidth of the primary channel can be configured and / or limited by the STAs among all STAs operating in the BSS that support the minimum bandwidth operating mode. In an 802.11ah example, the primary channel can be 1 MHz wide for STAs (e.g., MTC-type devices) that support (e.g., only) the 1 MHz mode, even if the AP and other STAs in the BSS support 2 MHz, 4 MHz, 8 MHz, 16 MHz, and / or other channel bandwidth operating modes. Carrier sensing and / or Network Allocation Vector (NAV) configuration can depend on the condition of the primary channel. For example, if the primary channel is busy due to a STA (that only supports 1 MHz mode of operation) transmitting to the AP, the entire available frequency band may be considered busy, even though most of the frequency band may remain idle and be available for use.

[0047] In the United States, the available frequency band that can be used by 802.11ah is 902MHz to 928MHz. In South Korea, the available frequency band is 917.5MHz to 923.5MHz. In Japan, the available frequency band is 916.5MHz to 927.5MHz. The total bandwidth available for 802.11ah is 6MHz to 26MHz depending on the country code.

[0048] 1D is a system diagram illustrating the RAN 113 and the CN 115 according to one embodiment. As mentioned above, the RAN 113 may communicate with the WTRUs 102a, 102b, 102c over the air interface 116 using NR radio technology. The RAN 113 may also communicate with the CN 115.

[0049] The RAN 113 may include gNBs 180a, 180b, and 180c, although it will be understood that the RAN 113 may include any number of gNBs while remaining consistent with an embodiment. The gNBs 180a, 180b, and 180c may each include one or more transceivers for communicating with the WTRUs 102a, 102b, and 102c over the air interface 116. In an embodiment, the gNBs 180a, 180b, and 180c may implement MIMO technology. For example, the gNBs 180a and 180b may utilize beamforming to transmit and / or receive signals to and / or from the WTRUs 102a, 102b, and 102c. Thus, the gNB 180a may, for example, transmit wireless signals to and / or receive wireless signals from the WTRU 102a using multiple antennas. In one embodiment, the gNBs 180a, 180b, and 180c may implement carrier aggregation technology. For example, the gNB 180a may transmit multiple component carriers to the WTRU 102a (not shown). A subset of these component carriers may be on an unlicensed spectrum, and the remaining component carriers may be on a licensed spectrum. In one embodiment, the gNBs 180a, 180b, and 180c may implement coordinated multi-point (CoMP) technology. For example, the WTRU 102a may receive coordinated transmissions from the gNBs 180a and 180b (and / or 180c).

[0050] The WTRUs 102a, 102b, 102c may communicate with the gNBs 180a, 180b, 180c using transmissions associated with scalable numerology. For example, the OFDM symbol spacing and / or OFDM subcarrier spacing may vary for different transmissions, different cells, and / or different portions of the wireless transmission spectrum. The WTRUs 102a, 102b, 102c may communicate with the gNBs 180a, 180b, 180c using subframes or transmission time intervals (TTIs) of varying or scalable lengths (e.g., including varying numbers of OFDM symbols and / or varying lengths of absolute time).

[0051] The gNBs 180a, 180b, 180c may be configured to communicate with the WTRUs 102a, 102b, 102c in a standalone configuration and / or a non-standalone configuration. In a standalone configuration, the WTRUs 102a, 102b, 102c may communicate with the gNBs 180a, 180b, 180c without accessing another RAN (e.g., eNode-Bs 160a, 160b, 160c, etc.). In a standalone configuration, the WTRUs 102a, 102b, 102c may utilize one or more of the gNBs 180a, 180b, 180c as mobility anchor points. In a standalone configuration, the WTRUs 102a, 102b, 102c may communicate with the gNBs 180a, 180b, 180c using signals in unlicensed bands. In a non-standalone configuration, the WTRUs 102a, 102b, 102c may communicate with and connect to a gNB 180a, 180b, 180c while also communicating with and connecting to another RAN, such as an eNode-B 160a, 160b, 160c. For example, the WTRUs 102a, 102b, 102c may implement DC principles to communicate with one or more gNBs 180a, 180b, 180c and one or more eNode-Bs 160a, 160b, 160c substantially simultaneously. In a non-standalone configuration, the eNode-Bs 160a, 160b, 160c may act as mobility anchors for the WTRUs 102a, 102b, 102c, while the gNBs 180a, 180b, 180c may provide additional coverage and / or throughput for serving the WTRUs 102a, 102b, 102c.

[0052] Each of the gNBs 180a, 180b, 180c may be associated with a particular cell (not shown) and may be configured to handle radio resource management decisions, handover decisions, scheduling of users in the UL and / or DL, support for network slicing, dual connectivity, interworking between NR and E-UTRA, routing of user plane data to User Plane Functions (UPFs) 184a, 184b, routing of control plane information to Access and Mobility Management Functions (AMFs) 182a, 182b, etc. As shown in FIG. 1D , the gNBs 180a, 180b, 180c may communicate with each other via an Xn interface.

[0053] 1D may include at least one AMF 182a, 182b, at least one UPF 184a, 184b, at least one Session Management Function (SMF) 183a, 183b, and possibly a Data Network (DN) 185a, 185b. While each of the foregoing elements is illustrated as part of the CN 115, it will be understood that any of these elements may be owned and / or operated by an entity other than the CN operator.

[0054] The AMF 182a, 182b may be connected to one or more of the gNBs 180a, 180b, 180c in the RAN 113 via an N2 interface and may function as a control node. For example, the AMF 182a, 182b may be responsible for authenticating users of the WTRUs 102a, 102b, 102c, supporting network slicing (e.g., handling different PDU sessions with different requirements), selecting a particular SMF 183a, 183b, managing registration areas, terminating NAS signaling, mobility management, etc. Network slicing may be used by the AMF 182a, 182b to customize the CN support of the WTRUs 102a, 102b, 102c based on the type of service utilizing the WTRUs 102a, 102b, 102c. For example, different network slices may be established for different use cases, such as services relying on ultra-reliable low latency (URLLC) access, services relying on enhanced massive mobile broadband (eMBB) access, services for machine type communication (MTC) access, and / or the like. The AMF 182 may provide a control plane function for switching between the RAN 113 and other RANs (not shown) that employ other radio technologies, such as LTE, LTE-A, LTE-A Pro, and / or non-3GPP access technologies, such as WiFi.

[0055] The SMFs 183a, 183b may be connected to the AMFs 182a, 182b in the CN 115 via an N11 interface. The SMFs 183a, 183b may also be connected to the UPFs 184a, 184b in the CN 115 via an N4 interface. The SMFs 183a, 183b may select and control the UPFs 184a, 184b and configure the routing of traffic through the UPFs 184a, 184b. The SMFs 183a, 183b may perform other functions, such as managing and allocating UE IP addresses, managing PDU sessions, controlling policy enforcement and QoS, providing downlink data notification, etc. The PDU session type may be IP-based, non-IP-based, Ethernet-based, etc.

[0056] The UPFs 184a, 184b may be connected to one or more of the gNBs 180a, 180b, 180c in the RAN 113 via an N3 interface, which may provide the WTRUs 102a, 102b, 102c with access to packet-switched networks such as the Internet 110 to facilitate communications between the WTRUs 102a, 102b, 102c and IP-enabled devices. The UPFs 184a, 184b may perform other functions such as routing and forwarding packets, enforcing user plane policies, supporting multi-homed PDU sessions, handling user plane QoS, buffering downlink packets, providing mobility anchoring, etc.

[0057] The CN 115 may facilitate communication with other networks. For example, the CN 115 may include or communicate with an IP gateway (e.g., an IP Multimedia Subsystem (IMS) server) that acts as an interface between the CN 115 and the PSTN 108. Additionally, the CN 115 may provide the WTRUs 102a, 102b, 102c with access to other networks 112, which may include other wired and / or wireless networks owned and / or operated by other service providers. In one embodiment, the WTRUs 102a, 102b, 102c may be connected to local data networks (DNs) 185a, 185b through the UPFs 184a, 184b via an N3 interface to the UPFs 184a, 184b and an N6 interface between the UPFs 184a, 184b and the DNs 185a, 185b.

[0058] 1A-1D and the corresponding description thereof, one or more or all of the functions described herein with respect to one or more of the WTRUs 102a-d, base stations 114a-b, eNode-Bs 160a-c, MME 162, SGW 164, PGW 166, gNBs 180a-c, AMFs 182a-ab, UPFs 184a-b, SMFs 183a-b, DNs 185a-b, and / or any other devices described herein may be performed by one or more emulation devices (not shown). The emulation devices may be one or more devices configured to emulate one or more or all of the functions described herein. For example, the emulation devices may be used to test other devices and / or simulate network and / or WTRU functions.

[0059] The emulation devices may be designed to implement one or more tests of other devices in a lab environment and / or an operator network environment. For example, one or more emulation devices may perform one or more or all functions while fully or partially implemented and / or deployed as part of a wired and / or wireless communication network to test other devices in the communication network. One or more emulation devices may perform one or more or all functions while temporarily implemented or deployed as part of a wired and / or wireless communication network. The emulation devices may be directly coupled to another device for testing purposes and / or may perform testing using terrestrial wireless communication.

[0060] One or more emulation devices may perform one or more functions, inclusive, while not being implemented or deployed as part of a wired and / or wireless communication network. For example, the emulation devices may be utilized in test scenarios in a test lab and / or in an undeployed (e.g., test) wired and / or wireless communication network to implement testing of one or more components. One or more emulation devices may be test equipment. Direct RF coupling and / or wireless communication via RF circuitry (which may include, e.g., one or more antennas) may be used by the emulation devices to transmit and / or receive data.

[0061] In the embodiments described herein, proximity-based services may be referred to as "ProSe." ProSe communication 5 (PC5) may represent a reference point (e.g., interface) between two WTRUs or any number of WTRUs. Both control and user plane messages may be exchanged between WTRUs via the PC5 reference point (e.g., interface).

[0062] According to an embodiment, a WTRU-to-WTRU relay may be a WTRU acting as a relay between two peer WTRUs. A WTRU-to-WTRU relay may be configured to relay (e.g., forward) data from a source WTRU to a target WTRU.

[0063] In the embodiments described herein, the terms "WTRU-to-WTRU relay," "R-WTRU," and "relay WTRU" may be used interchangeably.

[0064] In the embodiments described herein, the terms "source WTRU," "S-WTRU," "initiating WTRU," "peer WTRU," and "WTRU1" may be used interchangeably.

[0065] In the embodiments described herein, the terms "target WTRU," "T-WTRU," "responding WTRU," "peer WTRU," "WTRU2," "WTRU3," and "WTRU4" may be used interchangeably.

[0066] In the embodiments described herein, the term "user information" may refer to an application layer identifier. For example, a WTRU may be identified at the application layer based on this identifier. The user information may be used in direct communication (e.g., either a request or an acceptance) messages to identify either the source or target WTRU.

[0067] For communication over the PC5 reference point, either an IP address or an IP prefix (collectively IP address / prefix) may be used. An IP address may be either an IPv4 address or an IPv6 address. A prefix may be an IPv6 prefix.

[0068] In the embodiments described herein, the terms "layer-2" (L2) and "layer-3" (L3) may refer to the data link layer and network layer, respectively, of the Open Standard Interconnection (OSI) reference model.

[0069] Example of Layer 2 WTRU to WTRU Relay Method FIG. 2 illustrates an example Layer 2 WTRU-to-WTRU relay method 200. According to an embodiment, in step 210, the relay WTRU 20 may be provisioned with relay policy parameters. In step 211, the target WTRUs 22, 23, 24 may determine a destination layer-2 identifier (L2ID) for signaling reception. The source WTRU 21 may send a direct communication request (DCR) message 212 to the relay WTRU 20, for example, in broadcast mode, which may trigger a WTRU discovery process by sending a broadcast DCR message 213 to the peer WTRUs 22, 23, 24. The relay WTRU 20 may receive the DCR message 212. The relay WTRU 20 may verify whether it is configured to relay this application. For example, the relay WTRU 20 may compare the ProSe application identifier with its provisioned relay policy / parameters. For example, the relay WTRU 20 may assign (e.g., to itself) a relay layer-2 identifier (R-L2-ID) for the source WTRU 21 (e.g., related to the L2 ID of the source (S-L2-ID) of the source WTRU 21). The R-L2-ID and S-L2-ID may be stored, for example, in a local mapping table at the relay WTRU 20. The relay WTRU 20 may override (e.g., set) the source field of the message with its R-L2-ID and include (e.g., add) its (e.g., unique) relay identifier (RID) as a relay indication.

[0070] According to an embodiment, the target WTRU 23 may initiate (e.g., trigger) authentication and security establishment 214 with the source WTRU 21 via the relay WTRU 20. After security may be established, the target WTRU 23 may send a direct communication accept (DCA) message 215 to the relay WTRU 20, which may be forwarded 216 to the source WTRU 21.

[0071] According to an embodiment, a (e.g., secured, end-to-end PC5 unicast) link 260 may be set up between the source WTRU 21 and the target WTRU 23 via the relay WTRU 20. For example, the source 21 and target 23 WTRUs may not know their respective peer WTRU L2IDs. The source 21 and target 23 WTRUs may send messages to, and receive messages from, the relay WTRU 20. According to an embodiment, a security association and (e.g., PC5 unicast) link may be established (e.g., directly) between the source WTRU 21 and the target WTRU 23. For example, the source 21 and target 23 WTRUs may detect that communications may pass through a relay WTRU based on detecting either a relay indication and a relay identifier (RID) included in a received message.

[0072] According to an embodiment, the relay WTRU 20 may maintain a mapping table containing, for example, a mapping of peer WTRU L2IDs and corresponding R-L2IDs that may be self-assigned. After receiving a message, the relay WTRU 20 may look up, based on the mapping table, a source and destination identifier (L2ID) to be used to forward the message to the target WTRU. Before forwarding the message, the relay WTRU 20 may update the source and destination fields of the received message with the L2ID and R-L2ID of the corresponding WTRU. For example, upon receiving a message from the target WTRU 23, the relay WTRU 20 may use the R-L2-ID of the received message to look up, based on the mapping table, the L2ID of the source WTRU 21. The relay WTRU 20 may set the source field of the message to the R-L2ID and the destination field to the L2ID of the source WTRU 21. The relay WTRU 20 may transmit the message to the source WTRU 21.

[0073] According to an embodiment, a (e.g., management) unicast link may be established between a WTRU 21, 23 (e.g., either the source or target WTRU) and a relay WTRU 20. For example, the (e.g., management) unicast link may be used to manage other links, such as links that pass through the relay WTRU 20 and that are associated with the same RID as the (e.g., management) unicast link. For example, the (e.g., management) unicast link may be secured (e.g., either integrity or confidentiality protected) between any of the source 21, target 23, and relay 20 WTRUs.

[0074] Example of Layer 3 WTRU to WTRU relay method 3 illustrates an example Layer 3 WTRU-to-WTRU relay method 300. According to an embodiment, in step 3100, the source 31 and target 32 ​​WTRUs may be configured to use WTRU-to-WTRU relay. In step 3101, the WTRU 30 may be configured to act as a relay WTRU. In step 311, the relay 30 WTRU may announce its relay capabilities, for example, based on WTRU-to-WTRU relay discovery. The relay advertisement (e.g., of relay capabilities) may be received by either the source 31 or target 32 ​​WTRU.

[0075] According to an embodiment, either the source WTRU 31 or the target WTRU 32 (e.g., intended for use in a ProSe WTRU-to-WTRU relay) may establish a (e.g., respective PC5 unicast) link with the relay WTRU 30 as described above. For example, any of steps and messages 312, 313, 314, 315, and 316 of Figure 3 correspond to steps and messages 212, 213, 214, 215, and 216, respectively, of Figure 2.

[0076] According to an embodiment, the relay WTRU 30 may allocate an IP address / prefix to either the source WTRU 31 or the target WTRU 32, for example, as part of a (e.g., PC5 unicast) link establishment procedure. An association between the user information (e.g., user information) of the WTRUs 31, 32 and the allocated IP address / prefix may be stored, for example, in a domain name server (DNS) entry. The relay WTRU 30 may, for example, handle the DNS server.

[0077] According to an embodiment, to communicate with the target WTRU 32, the source WTRU 31 may send a DNS query 321 for the target WTRU 32 to the relay WTRU 30 via a unicast link. For example, the DNS query 321 may include target user information (e.g., user information of the target WTRU). According to an embodiment, to discover a ProSe service via the relay WTRU 30, the source WTRU 31 may send the DNS query 321 to the relay WTRU 30. For example, the DNS query 321 may include the ProSe service (e.g., the ProSe service type). The relay WTRU may respond to the DNS query with a DNS response 322 that includes the IP address / prefix of the target WTRU 32 (e.g., or of the ProSe service).

[0078] According to an embodiment, the source WTRU 31 may send IP-encapsulated (e.g., either IP or non-IP) data 323 to the target WTRU 32 via a (e.g., PC5 unicast) link to the relay WTRU 30 (which may have returned, e.g., the IP address / prefix of the target WTRU 32). The relay WTRU 30 may act as an IP router (e.g., between the source WTRU 31 and the target WTRU 32). For example, the relay WTRU 30 may forward the packet 324 to the corresponding (e.g., PC5 unicast) link towards the target WTRU 32. For example, each (e.g., PC5 unicast) link may be treated as an IP interface.

[0079] According to an embodiment, a WTRU may be in proximity to multiple (e.g., any number) relay WTRUs. For example, the WTRU may select a relay WTRU from among any relay WTRUs in its proximity according to any criteria. For example, a (e.g., PC5 unicast) link may be established with the selected relay WTRU. For example, the WTRU may send a DNS query for the target WTRU to any number of relay candidates and select the first relay candidate to return a positive response to the DNS query for the target WTRU as the relay WTRU for the target WTRU. In another example, a relay WTRU may be selected from among the relay candidates for the target WTRU based on a received signal level (e.g., either the highest signal level or a signal level higher than a certain value).

[0080] End-to-end PC5 unicast link method example 4 illustrates an example end-to-end PC5 unicast link method 400. According to an embodiment, a first PC5 unicast link 410 may be established between the source WTRU 41 and the relay WTRU 40. According to an embodiment, a second PC5 unicast link 411 may be established between the target WTRU 41 and the relay WTRU 40. According to an embodiment, at step 415, user information of the source WTRU 41 may be associated with a first IP address / prefix. User information of the target WTRU 42 may be associated with a second IP address / prefix. According to an embodiment, the source WTRU 41 may send a DNS query 420 to the relay WTRU 40, the DNS query including user information of the target WTRU 42. The relay WTRU 40 may send a DNS response 430 to the source WTRU 41, the DNS response including the second IP address / prefix (e.g., associated with the target WTRU user information). According to an embodiment, the source WTRU 41 may send a DCR message 440 to the target WTRU 42. According to an embodiment, the target WTRU 42 may send a DCA message 450 to the source WTRU 41 .

[0081] According to an embodiment, an end-to-end (E-to-E) PC5 unicast link 460 may be established over the IP layer between the source WTRU 41 and the target WTRU 42. For example, (e.g., all) E-to-E unicast link packets sent between the source WTRU 41 and the target WTRU 42 may be encapsulated in IP packets (e.g., using either the first or second IP address / prefix as the source / destination IP address) and forwarded (e.g., routed) by the relay WTRU 40.

[0082] The embodiments described herein may enable a source WTRU to reselect a relay WTRU proximate to the source WTRU (e.g., select another, different relay WTRU). For example, the source WTRU and target WTRU may communicate through a first relay WTRU (e.g., Relay #1). For example, the source WTRU may move away from Relay #1 and discover that other relay WTRUs (e.g., Relay #2 and Relay #3) may be better candidates than Relay #1. For example, either Relay #2 or Relay #3 may provide a stronger signal and may be available (e.g., to perform WTRU-to-WTRU relaying). The embodiments described herein may enable the source WTRU to select another relay (e.g., either Relay #2 or Relay #3) to continue communicating with the target WTRU.

[0083] The embodiments described herein may enable, for example, providing a mechanism for rerouting in the event of a WTRU change. For example, the embodiments described herein may enable a source WTRU to set up communications via a new relay WTRU (e.g., Relay #2), switch data traffic, and maintain service continuity via the new relay WTRU to continue (e.g., continue uninterrupted) communications with the target WTRU.

[0084] The embodiments described herein may enable WTRU-to-WTRU relay reselection (e.g., by the WTRU) and path switching to be handled in an efficient and guaranteed manner. The embodiments described herein may enable signaling, in general, and for security in particular, to be reduced (e.g., minimized) for WTRU-to-WTRU relay reselection (e.g., by the WTRU) and path switching.

[0085] According to an embodiment, for a Layer 2 WTRU-to-WTRU relay method, the source and target WTRUs may establish a security context based on a root key (which may be referred to herein as KD) and a session key (which may be referred to herein as KD session). For example, KD may be generated from mutual authentication of the source and target WTRUs, and the KD session may be derived from KD. For example, an identifier of the KD session (KD session ID) may serve as an identifier of the security context. Both the integrity and confidentiality keys may be derived from the KD session, for example.

[0086] According to an embodiment, a relay change from an L2 WTRU to a WTRU may be triggered by the source WTRU. For example, the source WTRU may send a link modification request message to the target WTRU (e.g., via the current relay WTRU). The current relay WTRU may be referred to herein as RID1. The link modification request message may include any of a relay reselection indication, a list of potential relay identifiers (RIDs), a security establishment method, and a (e.g., first) token (which may be referred to herein as token A). The link modification request message may include security parameters, such as nonce 1 (e.g., any number used once for encryption), an identifier of a root key (e.g., either a KD ID or the most significant bits of a new KD session identifier), etc. According to an embodiment, the session establishment method may be set to any of (1) “advance,” (2) “reuse the same security context,” and (3) “at link establishment.” Security parameters may be included in the link request modification message for the “advance” and “reuse the same security context” session establishment methods. The link request modification message for the "link establishment" session establishment method may not include security parameters. According to an embodiment, token A may be protected (e.g., either integrity and confidentiality) using, for example, the current security context (e.g., used in communication via the current relay WTRU).

[0087] According to an embodiment, the target WTRU may select a new relay (e.g., RID2) from a list of potential RIDs. The target WTRU may, for example, be self-assigned (e.g., a priori) a new L2 ID. If the security establishment method may be set to "a priori," the target WTRU may establish a security context (e.g., derive security keys) a priori for later use via the new relay WTRU (e.g., RID2). For example, a new KD session may be derived from a root key KD, nonce 1, and nonce 2, for example, generated by the target WTRU.

[0088] According to an embodiment, the target WTRU may send a link modify accept message, for example, including the selected RID, the derived (e.g., new) L2 ID, the accepted security establishment method, and / or token B. Security parameters may be included in the link modify accept message, for example (e.g., nonce 2, or / and the LSB of the new KD session identifier). According to an embodiment, token B may be protected (e.g., integrity and / or confidentiality) using, for example, the current security context.

[0089] According to an embodiment, the target WTRU may monitor its new L2 ID for receipt of messages.

[0090] According to an embodiment, the source WTRU may previously establish a security context (e.g., derive security keys) (e.g., derive a KD session similar to the target WTRU) for use via the new relay WTRU (RID2).

[0091] According to an embodiment, the source WTRU may send (e.g., broadcast) a DCR message indicating either a new relay WTRU identifier (RID2), a new L2ID of the target WTRU, and token B. According to an embodiment, token B may be protected (e.g., integrity and / or confidentiality) using either the new established security context or the current security context.

[0092] According to an embodiment, a new relay WTRU (RID2) may receive a (e.g., broadcast) DCR. For example, the new relay WTRU may check whether the DCR destination can be set to RID2. The new relay WTRU may extract the target WTRU's L2ID from the DCR and set the destination field of the DCR message to the target WTRU's L2ID. The new relay WTRU may send a (e.g., unicast) DCR message to the target WTRU.

[0093] According to an embodiment, the target WTRU may use token B to associate the new unicast link being established (e.g., via RID2) with (e.g., other current) unicast links (e.g., via RID1). Establishing a security context in advance may allow the target WTRU to skip the security establishment procedure and, for example, send a DCA message including token A to the source WTRU (e.g., directly). The message may be secured, for example, using a security context that may have been previously derived in advance.

[0094] According to an embodiment, the source WTRU may associate the new unicast link being established (e.g., via RID2) with the (e.g., other current) unicast link (e.g., via RID1) using token A. According to an embodiment, the source WTRU may switch traffic (e.g., from the current unicast link) to the new unicast link (e.g., via RID2).

[0095] According to an embodiment, a relay change from an L2 WTRU to a WTRU may be triggered by the relay WTRU. According to an embodiment, the relay WTRU may, for example, send a link modification request message to the source WTRU indicating that a new relay WTRU may be selected and traffic may be switched to the new connection. The relay WTRU may, for example, send this information over a management link established with the source WTRU. The relay WTRU may indicate which other (e.g., PC5 unicast) link the relay change may apply to (e.g., toward which target WTRU). For example, the link modification request message may include an "other link" indication, and the L2 ID may identify the (e.g., PC5 unicast) link being switched to. For example, the link modification request message may (e.g., also) include a replacement RID.

[0096] According to an embodiment, the source WTRU may trigger a change of relay by using the methods described above (e.g., for a change triggered by the target WTRU). According to an embodiment, the source WTRU may indicate that the RID should be replaced by the target WTRU.

[0097] According to an embodiment, the L2WTRU to WTRU relay change method may include exchanging a link identifier (ID). According to an embodiment, the link ID may be associated with a unicast link, for example, during link establishment.

[0098] For example, a WTRU-to-WTRU relay change may be triggered. The source WTRU may send (e.g., broadcast) a DCR message to establish a new link via the new relay WTRU. The source WTRU may include a link ID associated with the link via the relay WTRU to be changed and the ID of the target WTRU.

[0099] According to an embodiment, the relay WTRU may forward the DCR, including the link ID, to the target WTRU.

[0100] According to an embodiment, the target WTRU may receive the DCR message and may locate (eg, identify) the previous unicast link to be replaced based on the link ID.

[0101] According to an embodiment, the target WTRU may respond (eg, send) the DCA message.

[0102] According to an embodiment, PC5 signaling may be relayed between the source and target WTRU. For example, the source WTRU may send a link modification request to the current relay WTRU (e.g., RID1). The source WTRU may indicate that the message is for "relay reselection" and may be directed to the target WTRU.

[0103] According to an embodiment, the current relay WTRU (eg, RID1) may forward a link modification request message to the target WTRU based on the "relay reselection" indication.

[0104] According to an embodiment, the target WTRU may send a link modification accept message including, for example, the selected relay WTRU (eg, RID2) and the IP address / prefix of the target WTRU to be used with RID2.

[0105] According to an embodiment, the current relay WTRU (eg, RID1) may forward a link modification accept message to the source WTRU based on the "relay reselection" indication.

[0106] According to an embodiment, the source WTRU may send a new link modification ack message to (e.g., towards) the target WTRU, including the source WTRU's IP address / prefix to be used with RID2, for example, with a "relay reselection" indication.

[0107] According to an embodiment, the current relay WTRU (eg, RID1) may forward a new link modification ack message to the target WTRU based on the "relay reselection" indication.

[0108] According to an embodiment, traffic may be moved to RID2. The source and target WTRUs may use IP addresses / prefixes exchanged via link modify messages. According to an embodiment, a (e.g., PC5 unicast) link may be established between the source / target WTRUs and the new relay WTRU (RID2) without a DNS query to discover peer WTRUs (e.g., possibly through many relays).

[0109] According to an embodiment, the Layer 3 WTRU to WTRU relay change method may be based on a link on the user plane (eg, E-to-E PC5 unicast).

[0110] For example, the source WTRU may send a link modification request message (eg, via RID1) to the target WTRU that includes either a "relay reselection" indication and a list (eg, identifiers) of candidate relays (RIDs).

[0111] According to an embodiment, the target WTRU may, for example, select a new relay WTRU (RID2) from a list of RIDs, and may send a link modification accept message including the selected RID and the target WTRU's IP address / prefix to be used with the selected RID.

[0112] According to an embodiment, the source WTRU may send a new link modify ack message containing the source WTRU's IP address / prefix to be used with the selected RID.

[0113] According to an embodiment, the source / target WTRU may establish a user plane (e.g., PC5 unicast) link over RID2 and may switch traffic from the previous link over RID1 to the new link over RID2 using the IP addresses / prefixes exchanged via the link modify message. According to an embodiment, the source / target WTRU may change relay WTRUs without any DNS queries to discover peer WTRUs (e.g., possibly through many relays).

[0114] According to an embodiment, the Layer 3 WTRU to WTRU relay change method may be based on message exchange over the user plane, for example, either the source and target WTRUs may exchange user plane messages (e.g., application specific) to trigger the relay change.

[0115] Example of a Method for a Change of Relay from a Layer 2 WTRU to a WTRU Triggered by a Source WTRU 5 illustrates an example Layer 2 method 500 for a Layer 2 WTRU-to-WTRU relay change triggered by a source WTRU. A (e.g., PC5 end-to-end unicast) link 505 may have been established between the source WTRU 51 and the target WTRU 52 via a first relay WTRU 501 in accordance with any embodiment described herein. For example, the link 505 between the source WTRU 51 and the target WTRU 52 via the first relay WTRU 501 may be referred to herein as the first link. Both the source and target WTRUs may send and / or receive messages (e.g., packets) to / from the first relay WTRU 501, which is configured to handle the Layer 2 WTRU-to-WTRU relay method. For example, the first relay WTRU may forward messages based on Layer 2 addressing.

[0116] In step 510, the source WTRU 51 may be instructed to change relays according to criteria. For example, the WTRU may detect conditions for a relay change. For example, the condition may be based on a (e.g., low) signal level received from the first relay WTRU 501. For example, a relay change may be triggered based on a decrease in the signal level. In another example, a relay change may be triggered in a condition where the signal level falls below a certain (e.g., threshold) value. More generally, the condition may be based on any metric that represents the link quality with the first relay WTRU (e.g., QoS, latency, packet loss, ...) and may indicate a decrease in link quality. In another example, a trigger for a relay change may be received (e.g., from the first relay WTRU 501). According to an embodiment, the source WTRU 51 may perform a discovery procedure to obtain (e.g., a list of) relay WTRU candidates. A relay WTRU candidate may be any WTRU in the vicinity of the source WTRU 51 that is capable of relaying from the WTRU to the WTRU. For example, a relay WTRU candidate may be identified by a relay identifier (RID).

[0117] According to an embodiment, the source WTRU 51 may send a link modification request message 520 to the target WTRU 52 via the first relay WTRU 501, e.g., over the first link. According to an embodiment, the link modification request message 520 may include (e.g., first information indicative of) any of: a relay reselection indication (e.g., indicating a request for a change of relay), a reselection reason (e.g., unsatisfied QoS, low signal, relay maintenance), a list of identifiers of candidate relays (RIDs), and a source identifier, such as, for example, a token (which may be referred to herein as token A). Token A may be any type of identifier that may be used to associate the first link with a replacement (e.g., second) link at either the source or target WTRU. For example, token A may be a source (e.g., link) identifier. According to an embodiment, the link modification request message 520 may include any number of (e.g., proposed) security establishment methods and their parameters. For example, the parameters may include security parameters for security establishment (e.g., a list of supported security algorithms). In another example, the parameters may include security parameters (eg, a context count value) for reuse of an existing security context.

[0118] According to an embodiment, the link modification request message 520 may include information indicating a security establishment method, which may be set to a value indicating, for example, any of “advance,” “reselect same,” and “at link establishment.” According to an embodiment, “advance” may indicate that security may be established during the link modification process (e.g., before the link establishment process with the new relay). The link modification request message 520 may include security parameters (e.g., all) for establishing security. According to an embodiment, “reuse same” may indicate that a security context used on a (e.g., current) link via the first relay WTRU 51 may be reused on a (e.g., new) link established with the second (e.g., selected) relay WTRU 52. For example, parameters (e.g., context count values) from a security context that may not be reused may be included in the security parameters with the (e.g., corresponding new) values ​​that are used. According to an embodiment, “at link establishment” may indicate that a security context may be established during the link establishment procedure with the second (e.g., selected) relay WTRU 52. For example, the link modify request message 520 may include security parameters.

[0119] According to an embodiment, the link modification request message 520 may not include any indication of any security establishment method. A link modification request message 520 without a security establishment method indication may, for example, indicate that a security context may be established during the link establishment procedure with the second relay WTRU 502 (e.g., similar to “at link establishment”). In another example, a link modification request message 520 without a security establishment method indication may indicate that the (e.g., same) security context used (e.g., already) on the (e.g., current) link via the first relay WTRU 51 may be reused on the (e.g., new) link established via the second (e.g., selected) relay WTRU 52. This may allow skipping (e.g., avoiding, bypassing) a direct security mode procedure that may be performed (e.g., normally) during the link establishment procedure.

[0120] According to an embodiment, tokens may be exchanged during a link modification procedure between the source WTRU 51 (e.g., via the first relay WTRU 501) and the target WTRU 52. The token may be used, for example, during a link establishment procedure (e.g., via the second relay WTRU 502) to associate a current link (e.g., via the first relay WTRU 501) with a new link (e.g., via the second relay WTRU 502). For example, the token may also represent a relay reselection indication.

[0121] According to an embodiment, the token may be a (e.g., self-generated random) number. For example, using a token may make it possible to avoid replay attacks. For example, a new token may be generated when (e.g., each time) a link modification message may be sent.

[0122] According to an embodiment, the token may include information that points to the current link (e.g., via the first relay WTRU 501). For example, the information may be the source WTRU link identifier (e.g., the L2 ID of the source WTRU 51), the L2 ID of the target WTRU 52, or the KD session ID used for communication on the first relay WTRU 501.

[0123] According to an embodiment, in step 530, the target WTRU 52 may select the second relay WTRU 502, for example, from the list of relay WTRU candidates received in the link modification request message 520. For example, the second relay WTRU 502 may be selected from the list of candidates based on a signal quality metric, a candidate load, or provisioned parameters. According to an embodiment, the target WTRU 52 may select a security establishment method based on a security establishment method received (e.g., proposed) from the source WTRU 51.

[0124] According to an embodiment, the target WTRU 52 may select an “ahead” security establishment method. The target WTRU 52 may (e.g., ahead) establish a security context for the link to be established (e.g., via the selected relay) based on security parameters from the source WTRU 51 (e.g., retrieved from the link modification request message 520) and based on its own security parameters. The target WTRU 52 may derive security keys, for example, based on the source WTRU security parameters and its own security parameters. The exchange of security information (e.g., lists of preferred and selected algorithms, nonces, etc.) during the link modification procedure may enable the source and target WTRUs to derive security keys ahead and skip (e.g., avoid, bypass) the direct security mode procedure that may (e.g., normally) be performed during the link establishment procedure.

[0125] According to an embodiment, the target WTRU 52 may select the “reuse” security establishment method. The target WTRU 52 may obtain a (e.g., new) security context for the new link (e.g., via the first relay WTRU 51) based at least in part on a (e.g., existing) security context that may be used on the current link (e.g., via the second relay WTRU 51). For example, some parameters may not be reused from the existing security context (e.g., count, KD session ID). These parameters may be retrieved from the security parameters included in the link modification request message 520 and associated with the (e.g., new) security context to be used via the second relay WTRU 52. According to an embodiment, the link modification request message 520 may not include any security parameters from the source WTRU 51. For example, the target WTRU 52 may use (e.g., preserve) either default and legacy security parameters in the (e.g., new) security context. According to an embodiment, the (e.g., current) security context may be updated with the new security parameters, and the WTRU may be permitted to reuse the already established security context. The "reuse" security establishment method may allow the WTRU to skip (eg, avoid, bypass) direct security mode procedures that may be performed (eg, normally) during a link establishment procedure.

[0126] According to an embodiment, the target WTRU 52 may process direct security mode procedures 580, 590 to derive a new security context during the link establishment procedure via the selected second relay WTRU 502, e.g., to generate fresh session keys, fresh integrity and confidentiality keys, and (e.g., in some cases) a fresh root key. For example, the direct security mode procedures 580, 590 may be processed on the condition that the link modification request message 520 indicated a "link establishment" security establishment method. For example, the direct security mode procedures 580, 590 may be processed on the condition that the link modification request message 520 did not indicate any security establishment methods. For example, the direct security mode procedures 580, 590 may be processed on the condition that the target WTRU 52 does not support any of the security establishment methods indicated in the link modification request message 520.

[0127] According to an embodiment, the direct security mode procedures 580, 590 may be skipped (eg, may not be processed) if either the "prior" and "reuse" security establishment methods can be processed.

[0128] According to an embodiment, the target WTRU 52 may send a link modification accept message 540 to the source WTRU 51 via the first relay WTRU 501. The link modification accept message 540 may include an identifier of the (e.g., selected) second relay WTRU 502, e.g., a target identifier such as a token (e.g., token B) for associating the second (e.g., new) link with the first (e.g., current) link, a security establishment method (e.g., selected by the target WTRU 52), and any of the security parameters (e.g., second information indicating any of them). For example, the target identifier (e.g., token B) may be any kind of identifier that can be used to associate the second (e.g., new) link to be established (e.g., to replace) with the first link. For example, the target identifier (e.g., token B) may be a target (e.g., link) identifier (e.g., L2 ID of the target WTRU 52). For example, the link modification accept message 540 may indicate a “prior” security establishment method and may include a (e.g., selected) security algorithm. In another example, the link modification accept message 540 may indicate a "reuse" security establishment method and may include, for example, a count (e.g., as a security parameter). According to an embodiment, the link modification accept message 540 may include an identifier of the target WTRU (T-WTRU-ID). For example, the T-WTRU-ID may be, for example, a new Layer 2 ID previously generated by the target WTRU 502. The T-WTRU-ID may be used, for example, when establishing a new unicast link via the second relay WTRU 502. In another example, the T-WTRU-ID may be user information of the target WTRU 502 (e.g., an application layer ID of the target WTRU 502).

[0129] According to an embodiment, the security establishment method included in the link modification request message 520 (e.g., proposed by the source WTRU 51) may not be supported (e.g., not acceptable) by the target WTRU 52. For example, the target WTRU 52 may proceed with (e.g., revert to) the “at link establishment” method. The target WTRU 52 may notify the source WTRU 51 via the link modification accept message 540 (e.g., by including an indication of the security establishment method selected by the target WTRU 52). For example, the source WTRU 51 may have provided security parameters for the “ahead” and “at link establishment” methods, and the target WTRU 52 may select either the “ahead” or “at link establishment” method. For example, the selection may be based on, for example, provisioning information or the capabilities of the target WTRU. For example, a preferred order for selection may be set, e.g., “ahead” first, “reuse” second, and “at link establishment” third.

[0130] According to an embodiment, the target WTRU 52 may protect the token (e.g., Token B) before including it in the message, for example, based on a security establishment method. For example, the target WTRU 52 may use a new security context or a previously used security context, as in the case of a link via the first relay WTRU 51.

[0131] According to an embodiment, the security establishment method may be “ahead,” e.g., the source WTRU may receive a link modification accept message 540 indicating that the target WTRU 52 has selected the “ahead” security establishment method. The source WTRU 51 may use security information from the target WTRU 52 (e.g., security parameters received from the target WTRU 52) and its own security information (e.g., a selected algorithm, nonce, etc.) to derive (e.g., ahead) a security context (e.g., security keys) for the link to be established via the selected second relay WTRU 52 (e.g., RID2). According to an embodiment, the security establishment method may be “reuse,” e.g., the source WTRU may receive a link modification accept message 540 indicating that the target WTRU 52 has selected the “reuse” security establishment method. The source WTRU 51 may complete (e.g., update) the security context based on the provided security parameters. The updated security context may be used via the selected second relay WTRU 52. According to an embodiment, the source WTRU 51 may transmit (e.g., broadcast) a DCR message 550 including (e.g., a selected) relay identifier (RID) (e.g., received from the target WTRU 52), a target WTRU identifier (T-WTRU ID) (e.g., one of user information and Layer 2 ID), and any (e.g., third information indicating any of them) a target identifier (e.g., token) (e.g., token B) received from the target WTRU 52. For example, the DCR message 550 may be transmitted to the target WTRU via the selected relay.

[0132] According to an embodiment, the broadcast DCR message 550 may be sent as clear text (e.g., unencrypted), allowing the selected relay to have access to the contents (e.g., parameters) of the DCR message 550. For example, the source WTRU 51 may secure (e.g., encrypt) token B using a security context (e.g., security keys) that was pre-derived or reused from its link with the first relay WTRU 51 (e.g., based on a security establishment method).

[0133] According to an embodiment, in step 560, the second relay WTRU 502 may receive the DCR message 550 from the source WTRU 51. For example, the second relay WTRU 502 may check whether the (e.g., selected) relay identifier (RID) of the DCR message 560 may match its own RID. According to an embodiment, the second relay WTRU 502 may forward the DCR message (e.g., only) if the (e.g., selected) relay identifier (RID) of the DCR message 560 matches its own RID. For example, if the relay identifier of the received DCR may not match its own RID, the second relay WTRU 502 may delete the message without forwarding it.

[0134] According to an embodiment, if the (e.g., selected) relay identifier (RID) of the DCR message 560 may match its own RID, and if the DCR message 560 may include an identifier of a target WTRU (T-WTRU ID), the second relay WTRU 502 may forward the message 570 to the target WTRU identified by the T-WTRU ID, e.g., in a unicast mode. For example, the forwarded DCR message 570 may include the received (e.g., unchanged) token (token B). Token B may be used, for example, to identify a first (e.g., current) link to be replaced with a second (e.g., new) link. If the DCR message 550 may not include any target WTRU identifier, the DCR message 550 may be forwarded 570 in a broadcast mode.

[0135] According to an embodiment, if the DCR message 550 may include a Layer 2 ID as a T-WTRU ID, the DCR message may be forwarded to this (e.g., specific) Layer 2 ID (e.g., in unicast mode instead of broadcast mode) 570. If the DCR message 550 may include user information, such as a T-WTRU ID, the T-WTRU ID may remain in the forwarded DCR message 570 (e.g., payload), and the forwarded DCR message 570 may be broadcast.

[0136] According to an embodiment, the second relay WTRU 502 may remove the (e.g., selected) relay identifier (RID) received from the source WTRU in the DCR message 550. For example, the forwarded DCR message 570 (e.g., sent to the target WTRU 52) may not include the (e.g., selected) relay identifier (RID).

[0137] According to an embodiment, the target WTRU 52 may receive a forwarded DCR message 570 (e.g., from the source WTRU via a selected relay WTRU). For example, the forwarded DCR message 570 may include an identifier (e.g., a token) for identifying a first (e.g., current) link to be replaced with a second (e.g., new) link. For example, the target WTRU 52 may check that any of the T-WTRU IDs and identifiers (e.g., tokens) included in the DCR message 570 can match any of its own T-WTRU IDs and target identifiers (e.g., tokens). For example, the target WTRU 52 may use its T-WTRU ID to look up information associated with the currently established new link (e.g., the selected RID, the established security context (e.g., key), the target WTRU's token, the source WTRU's token, etc.). If the identifier (e.g., token) included in the DCR message 570 can be confidentiality protected, the target WTRU 52 may decrypt the identifier (e.g., token) using the security context (e.g., key) associated with the new link. For example, the target WTRU may verify (e.g., check) that the identifier (e.g., token) may match its target identifier (e.g., token) previously sent to the source WTRU. For example, the target WTRU may verify (e.g., check) that the DCR message 570 may be received from its (e.g., previously) selected RID (e.g., RID2). For example, the target WTRU 52 may use the identifier (e.g., token) to look up the previous link to be replaced by the new link. The association between the application layer identifier and the unicast link via the first relay WTRU (RID1) may be updated with the new unicast link via the second relay WTRU (RID2), for example, once (e.g., after) traffic may be switched to the new link (via the second relay WTRU 52), e.g., in step 5120.According to an embodiment, the security keys may be (e.g., already) derived by the target WTRU 52, and the target WTRU 52 may skip (e.g., avoid, bypass) triggering the direct security mode (DSM) procedures 580, 590. According to an embodiment, the security keys may be reused from a previous link (e.g., via the first relay WTRU 501), and the target WTRU 52 may skip (e.g., avoid, bypass) triggering the DSM procedures 580, 590.

[0138] According to an embodiment, if a security context (e.g., a key) has not been pre-established or if a security context (e.g., a key) from a previous link is not reused, the target WTRU 52 may initiate (e.g., trigger) a direct security mode procedure via the second relay WTRU 52, for example, by sending a DSM command message 580 to the source WTRU 51 via the second relay WTRU 502. According to an embodiment, the DSM command 580 may include the source WTRU token (e.g., token A).

[0139] According to an embodiment, the source WTRU 51 may use the token of the received DSM command 580 (e.g., token A) to verify that the message can be associated with a previous link with the first relay WTRU 501. For example, the source WTRU 51 may send a direct security mode complete message 590 to the target WTRU 52 via the second relay WTRU 502. According to an embodiment, security may be established on the link (e.g., the source and target WTRUs may obtain a security context).

[0140] According to an embodiment, for example, if the identifier (e.g., token) included in the received DCR message 570 matches a target identifier (e.g., token B) that may have been included in the link modification accept message 540, the target WTRU 52 may transmit a DCA message 5100 that may include an identifier (e.g., token) from the source WTRU 51 (e.g., token A received from the link modification request message 520). For example, the DCA message 5100 may be secured based on a security key associated with this link via the second relay WTRU 502. According to an embodiment, the token may not be included in the DCA message 5100 if a DSM process may be performed.

[0141] According to an embodiment, the source WTRU 51 may receive the DCA message 5100 via the second relay WTRU 502. The source WTRU 51 may retrieve any of the security keys and security parameters. If the DCA message 5100 includes (e.g., a matching identifier for) the source WTRU's token (e.g., source identifier, Token A), the source WTRU 51 may associate the DCA message 5100 with the previous link via the first relay WTRU 501 based on the token. In step 5110, a (e.g., new PC5 unicast) link may be established between the source WTRU 51 and the target WTRU 52 via the second relay WTRU 502.

[0142] According to an embodiment, in step 5120, either the Source 51 or Target 52 WTRU may switch data traffic to a (e.g., new PC5 unicast) link via the second relay WTRU 502. In other words, either the Source 51 or Target 52 WTRU may stop sending packets (intended for the Target 52 and Source 51 WTRUs, respectively) to the first relay WTRU 501 and may send them to the second relay WTRU 502.

[0143] According to an embodiment, either the source or target WTRU may send a link release message via the first relay WTRU 501 to terminate the previous link (not shown).

[0144] Example of a Method for a Relay WTRU-to-WTRU Relay Change Triggered by a Relay WTRU 6 illustrates an example method 600 for a change of relay from a Layer 2 WTRU to a WTRU triggered by a relay WTRU 601. According to an embodiment, the change of relay WTRU 601 may be triggered by the (e.g., current) relay WTRU 601 (which may be referred to herein as the first relay WTRU). For example, the first relay WTRU 601 may be scheduled for maintenance. In another example, the first relay WTRU 601 may be configured to offload some traffic (e.g., connections).

[0145] According to an embodiment, a (e.g., PC5 end-to-end unicast) link 603 may have been established between the source WTRU 61 and the target WTRU 62 via the first relay WTRU 601 according to an embodiment described herein. Both the source and target WTRUs may send and / or receive messages (e.g., packets) to / from the first relay WTRU 601, which is configured to handle Layer 2 WTRU-to-WTRU relaying methods. According to an embodiment, a management link 605 may be established between the source WTRU 61 and the first relay WTRU 601.

[0146] According to an embodiment, the first relay WTRU 601 may notify the source WTRU 61 that a relay change may be processed. For example, the first relay WTRU 601 may send, for example, a link modification request message, a link release message, or any other PC5-S message over the management link 605. The message may include (e.g., new) information indicating either a (e.g., pending) relay change and a timer indicating that the first relay WTRU 601 may not be available upon expiration of the timer. The source WTRU 61 may initiate the relay change with the target WTRU 62 (e.g., according to embodiments described herein). A new link may be established via the second relay WTRU 602. Traffic may be moved (e.g., switched) from the initial link via the first relay WTRU 601 to the new link via the second relay WTRU 602.

[0147] According to an embodiment, the first relay WTRU 601 may send a link modification request message 610 to the source WTRU 61, for example, via the management link 605. For example, the link modification request message 610 may include either information indicating "other links" or (e.g., a list, a set) of link identifiers representing (e.g., identifying) the links to be moved to another relay WTRU. For example, the links may be identified by a pair of a source WTRU layer-2 ID (S-WTRU-L2ID) and a target WTRU layer-2 ID (T-WTRU-L2ID), which may be used to send PC5-S (e.g., signaling) messages and user traffic over the links. For example, the first relay WTRU 601 may include a replacement relay ID (RID) and any number of candidate RIDs. For example, several relay WTRUs with similar (e.g., the same) capabilities may be in the same region. The embodiments described herein may allow the network to select a replacement relay WTRU without the source WTRU 61 having to go through a relay discovery procedure.

[0148] According to an embodiment, the source WTRU 61 may receive the link modification request message 610. For example, the source WTRU 61 may determine that the link modification request message 610 may be applicable to another link (e.g., not the management link) based on the “other link” indication. For example, the source WTRU 61 may obtain the link to which the link modification request message 610 may be applied based on the Layer 2 ID (e.g., included in the link modification request message 610). For example, the source WTRU 61 may determine that a relay WTRU (e.g., identified by a replacement RID) may be reachable by the source WTRU 61. The source WTRU 61 may send a link modification request message 620 to the target WTRU 62 via the first relay WTRU 601 (e.g., via the other link indicated in the link modification request message 610). The link modification request message 620 may include similar (e.g., the same) parameters as the link modification request message 520 described in FIG. 5. The link modification request message 620 may (e.g., additionally) include a replacement RID. For example, a link modification request message 620 that includes a (eg, single) replacement ID may not include a set (eg, list) of candidate RIDs.

[0149] According to an embodiment, the target WTRU 62 may determine whether the link modification request message 620 includes a replacement RID. For example, the target WTRU 62 may determine that the relay WTRU identified by the replacement RID may be reachable (e.g., accessible) by the target WTRU 62. For example, the replacement RID may be selected if the replacement RID is reachable. If the replacement RID may not be reachable, the target WTRU 62 may select another RID, for example, from a list of candidate RIDs. In another example, the target WTRU 62 may initiate (e.g., trigger) a relay discovery procedure to select an RID (e.g., if a list of candidate RIDs cannot be provided). According to an embodiment, the target WTRU may send a link modification accept (e.g., link modification response) message 630 to the source WTRU 61, which may include the selected RID. According to an embodiment, the link modification accept message 630 may include similar (e.g., the same) parameters as the link modification accept message 540 described in FIG. 5.

[0150] According to an embodiment, the source WTRU 61 may transmit a broadcast DCR message 640, for example, including similar (eg, the same) parameters as the DCR message 550 described in FIG.

[0151] According to an embodiment, the second relay WTRU 602 (e.g., identified by the selected RID) may receive the DCR message 640. For example, the second relay WTRU 602 may forward the DCR message 650 to the target WTRU 62.

[0152] According to an embodiment, the target WTRU 62 may send a DCA message 660 that includes similar (e.g., the same) parameters as, for example, the DCA message 5100 described in Figure 5. For example, the target WTRU 62 may skip (e.g., bypass, avoid) security establishment (DSM command / completion) if a security context (e.g., key) can be established (e.g., already) or if a security context (e.g., key) can be reused from a previous link via the first relay WTRU 601.

[0153] According to an embodiment, the source WTRU 61 may receive the DCA message 660 via the second relay WTRU 602. According to an embodiment, a link 670 may be established via the second relay WTRU 602.

[0154] According to an embodiment, either the Source 61 or Target 62 WTRU may switch data traffic 680 to the (e.g., new) link 670 via the second relay WTRU 602. In other words, either the Source 61 or Target 62 WTRU may stop sending packets 680 (intended for the Target 62 and Source 61 WTRUs, respectively) to the first relay WTRU 601 and send them to the second relay WTRU 602.

[0155] According to an embodiment, the source WTRU 61 may send a link modification accept message 690 to the first relay WTRU 601 via the management link 605. This message may indicate that the link may have been modified (e.g., moved to another relay as requested).

[0156] According to an embodiment, the link 603 between the source WTRU 61 and the target WTRU 62 via the first relay WTRU 601 may be released 6100. For example, the link release may be initiated (e.g., triggered) by either the source 61 or target 62 WTRU.

[0157] According to an embodiment, the management link 605 between the source WTRU 61 and the first relay WTRU 601 may be released 6110. For example, the management link 605 may be used to manage other links, e.g., links with other target WTRUs. For example, the management link 605 may be released if (e.g., only if) other links from the source WTRU 61 via the first relay WTRU 601 may exist (e.g., may be established).

[0158] Example Method for Layer 2 WTRU to WTRU Relay Change Based on Link ID Exchange 7 illustrates an example method 700 for changing a relay from a Layer 2 WTRU to a WTRU based on a link identifier (ID) exchange. According to an embodiment, a link ID may be assigned to a (e.g., PC5 unicast) link, for example, during a PC5 link establishment procedure 710. For example, a new relay WTRU may be selected. For example, the source WTRU 71 may include the link ID of the link to be moved in the DCR message 740. For example, the target WTRU 72 may associate the PC5 link via the new relay with the PC5 link via the initial relay WTRU.

[0159] According to an embodiment, a first (e.g., PC5 unicast) link may be established between the source WTRU 71 and the target WTRU 72 via the first relay WTRU 701. The source WTRU 71 may send a DCR (e.g., broadcast) message 711 to the first relay WTRU 701 that may be rebroadcast by the first relay WTRU 701 and received by the target WTRU 72, for example, as described in FIG.

[0160] According to an embodiment, the target WTRU 72 may send a DCA message 712 to the source WTRU 71, for example, similar to that described in Figure 2. According to an embodiment, the DCA message 712 may include, for example, a link ID (LID) assigned (e.g., allocated) to the (e.g., currently established PC5 unicast) link by the target WTRU 72. The link ID may enable locating (e.g., retrieving, identifying) the context of the link (e.g., any of the source / destination L2 ID, security context, and application layer ID).

[0161] According to an embodiment, the source WTRU 71 may detect, for example, a condition for a relay change in step 730. In another example, the source WTRU 71 may receive a trigger to change the relay (e.g., a link modification request from the first relay WTRU 701).

[0162] According to an embodiment, the source WTRU 71 may initiate a (e.g., PC5 unicast) link establishment procedure by, for example, sending a DCR message 740 (e.g., LID1) including the link ID of the PC5 link to be switched and an identifier (T-WTRU ID) of the target WTRU 72, such as user information of the target WTRU 72.

[0163] According to an embodiment, the source WTRU 71 may select a replacement relay and send a DCR message 740 to the selected relay (eg, RID2), for example, by including the LID (LID1) in the broadcast DCR message 740.

[0164] According to an embodiment, the second relay WTRU 702 may forward the DCR message 750 similar to that described in FIG.

[0165] According to an embodiment, in step 760, the target WTRU 760 may associate the link established via the first relay WTRU 701 (e.g., the first link) with the link established via the second relay WTRU 702 (e.g., the second link), for example, based on the received link ID (LID1).

[0166] According to an embodiment, a DCA message 770 may be sent from the target WTRU 72 to the source WTRU 71 via the second relay WTRU 702. For example, the DCA message 770 may include link IDs. According to an embodiment, the link IDs in the DCA message 770 may be a newly assigned one (e.g., LID2) and one already assigned (LID1).

[0167] According to an embodiment, either the source 71 or target 72 WTRU may initiate (e.g., trigger) a PC5 link release procedure 780 for the (e.g., first) PC5 link via the first relay WTRU 701, for example, after the (e.g., second) PC5 link may be established via the second relay WTRU 702.

[0168] Example of a method for changing relay from Layer 3 WTRU to WTRU based on PC5 signaling 8 illustrates an example method 800 for a Layer 3 WTRU-to-WTRU relay change based on PC5 signaling. According to an embodiment, a Layer 3 relay WTRU 801 may forward data between a source WTRU 81 and a target WTRU 82 based on two (e.g., separate PC5 unicast) links 803, 804 established between the relay WTRU 801 and the source WTRU 81 and target WTRU 82, respectively, as described in FIG. 3. The source WTRU 81 and target WTRU 82 may communicate, for example, by exchanging IP packets relayed by the first relay WTRU 802 via the (e.g., PC5 unicast) links 803, 804.

[0169] According to an embodiment, a first (e.g., PC5 unicast) link 803 may be established between the source WTRU 81 and the first relay WTRU 801. A second (e.g., PC5 unicast) link 804 may be established between the target WTRU 82 and the first relay WTRU 801. The source 81 and target 82 WTRUs may exchange IP packets via the first relay WTRU 801 based on the first link 803 and the second link 804. According to an embodiment, a third (e.g., PC5 unicast) link 805 may be established between the source WTRU 81 and the second relay WTRU 802.

[0170] According to an embodiment, the source WTRU 81 may detect, for example, a condition for a relay change. In another example, the source WTRU 81 may receive a trigger to change relays (e.g., a link modification request from the first relay WTRU 801). According to an embodiment, the source WTRU 81 may send, for example, a link modification request message 810 to the first relay WTRU 801 (e.g., on the first link 803) to be forwarded to the target WTRU 82 to request a relay change. For example, the link modification request message 810 may include (e.g., first information indicative of) any of: a relay reselection indication (e.g., indicating a request for a relay change), a reselection reason (e.g., QoS, low signal, or relay maintenance), and a list of candidate relay WTRUs (e.g., RIDs). The link modification request message 810 may indicate either the IP address / prefix of the source WTRU 81 and the IP address / prefix of the target WTRU 82 (e.g., for communication between the source WTRU 81 and the target WTRU 82 via the first relay WTRU 801).

[0171] According to an embodiment, a list of candidate relay WTRUs may be obtained (eg, selected) based on potentially already established (eg, PC5 unicast) links with such relay WTRUs (RIDs).

[0172] According to an embodiment, the IP addresses / prefixes of any number of target WTRUs may be included in the link modification request message 810, for example, if the source WTRU 81 is communicating with any number of target WTRUs via the first relay WTRU 801.

[0173] According to an embodiment, in step 820, the first relay WTRU 801 may determine (e.g., detect) whether the link modification request message 810 may be directed to the target WTRU 82 based on the relay reselection indication and based on the IP address / prefix of the target WTRU 82 (e.g., which may be included in the link modification request message 810).

[0174] According to an embodiment, the first relay WTRU 801 may determine (e.g., verify) that the source WTRU IP address / prefix included in the link modification request message 810 may be valid. This may allow, for example, to include the IP address of another WTRU to avoid a WTRU carrying out an attack by tearing down this other WTRU connection.

[0175] According to an embodiment, the first relay WTRU 801 may send a link modification request message 821 to the IP address / prefix of the target WTRU 82 (e.g., included in message 810). If the link modification request message 810 may include multiple IP addresses / prefixes (e.g., of multiple target WTRUs), the first relay WTRU 801 may send a link modification request to each of these target WTRUs (e.g.,).

[0176] According to an embodiment, at step 830, the target WTRU 82 may determine that communications with the source WTRU 81 may be processed via another relay WTRU, e.g., based on a relay reselection indication. For example, the target WTRU 82 may obtain (e.g., select) a relay WTRU (e.g., identifier) ​​RID from a list of candidate relay identifiers (RIDs). For example, the target WTRU 82 may obtain (e.g., select) an RID (e.g., of a selected relay WTRU) with which a (e.g., PC5 unicast) link may already be established. In another example, the target WTRU 82 may establish a fourth (e.g., new) link 831 with the selected RID (e.g., the selected relay WTRU, identified by

[0177] According to an embodiment, the target WTRU 82 may send a link modification accept message 840 to the first relay WTRU 801, for example, over the second link 804, to be forwarded to the source WTRU 81. The link modification accept message 840 may include (e.g., second information indicative of) any of the following: a relay reselection indication, an IP address / prefix of the source WTRU 81, an IP address / prefix of the target WTRU 82 via the first relay WTRU 801, e.g., an IP address / prefix of the target WTRU 82 for communication between the source WTRU and the target WTRU via the second (e.g., selected) relay WTRU 802, and an identifier of the second (e.g., selected) relay WTRU 802 (RID2).

[0178] According to an embodiment, the first relay WTRU 801 may send a link modification accept message 850 to the source WTRU 81 (e.g., based on the relay reselection indication). For example, the link modification accept message 850 may be sent to the source WTRU 81, e.g., over the first link 803, using the IP address / prefix of the source WTRU 81 (e.g., included in message 840).

[0179] According to an embodiment, the source WTRU 81 may extract (e.g., look up) the selected RID (e.g., RID2) from the received link modify accept message 850. The source WTRU 81 may send a link modify acknowledge (ack) message 860 including, for example, the IP address / prefix of the source WTRU associated with RID2 to the first relay WTRU 801, for example, over the first link 803 (e.g., for communication between the source WTRU 81 and the target WTRU 82 via the selected relay WTRU 802). The link modify ack message 860 may be sent by the source WTRU 81 to acknowledge the change of relay towards the target WTRU 82.

[0180] According to an embodiment, the first relay WTRU 801 may send a link modification ack message 870 to the target WTRU 82 .

[0181] According to an embodiment, either the source or target WTRU may switch IP traffic (e.g., from the first link 803 and the second link 804, respectively) to the third link 805 and the fourth link 831, respectively, via the second relay WTRU 802. Exchanging the respective IP addresses / prefixes in the link modification messages (request / accept) allows either the source or target WTRU to skip (e.g., avoid, not perform) DNS queries with the second relay WTRU 802.

[0182] Example of a method for Layer 3 WTRU-to-WTRU relay change based on an end-to-end PC5 unicast link 9 illustrates an example method 900 for Layer 3 WTRU-to-WTRU relay change based on an end-to-end (e.g., PC5 unicast) link 905. According to an embodiment, the end-to-end (e.g., PC5 unicast) link 905 may be established between a source 91 and a target 92 WTRU.

[0183] According to an embodiment, a first (e.g., PC5 unicast) link 903 may be established between a source WTRU 91 and the first relay WTRU 901. A second (e.g., PC5 unicast) link 904 may be established between a target WTRU 92 and the first relay WTRU 901. The source 91 and target 92 WTRUs may exchange IP packets via the first relay WTRU 901 based on the first link 903 and the second link 904.

[0184] According to an embodiment, the source 91 and target 92 WTRUs may establish, for example, via the user plane, a (e.g., end-to-end PC5 unicast) link 905. For example, the source 91 and target 92 WTRUs may exchange PC5 signaling messages encapsulated in IP packets and relayed via the first relay WTRU 901.

[0185] According to an embodiment, the source WTRU 91 may detect, for example, a condition for a relay change. In another example, the source WTRU 91 may receive a trigger to change relays (e.g., a link modification request from the first relay WTRU 901). According to an embodiment, the source WTRU 91 may send a link modification request message 910 to the target WTRU 92, for example, via the end-to-end link 905 via the first relay WTRU 901. For example, the link modification request message 910 may include any of a relay reselection indication (e.g., indicating a request for a relay change), a reselection reason (e.g., QoS, low signal, or relay maintenance), and a list of candidate relay WTRUs (e.g., RIDs).

[0186] According to an embodiment, a list of candidate relay WTRUs may be obtained (eg, selected) based on potentially already established (eg, PC5 unicast) links with such relay WTRUs (RIDs).

[0187] According to an embodiment, at step 920, the target WTRU 92 may determine (e.g., detect) whether the link 905 can be switched through another relay WTRU based on the relay reselection indication. For example, the target WTRU 92 may obtain (e.g., select) an RID from a list of candidate RIDs. For example, the target WTRU 92 may obtain (e.g., select) an RID with which a (e.g., PC5 unicast) link may already be established. In another example, the target WTRU 92 may establish a (e.g., new) link with the selected RID.

[0188] According to an embodiment, the target WTRU 92 may send a link modification accept message 930 including any of a relay reselection indication, the IP address / prefix of the target WTRU 92 via the second relay WTRU 902, and an identifier of the second relay WTRU 902 (RID2).

[0189] According to an embodiment, the source WTRU 91 may receive a link modify accept message 930 from the target WTRU. For example, the source WTRU 91 may extract the selected RID (RID2) from the link modify accept message 930. For example, the source WTRU 91 may send to the target WTRU 92 a link modify ack message 940 including, for example, an IP address / prefix associated with RID2.

[0190] According to an embodiment, the source 91 and target 92 WTRUs may establish a link 950 (eg, end-to-end PC5 unicast) via a second relay WTRU 902 (RID2).

[0191] According to an embodiment, either the source or target WTRU may switch IP traffic to link 950 via second relay WTRU 902 .

[0192] Example of using DNS to get target WTRU and select new relay According to an embodiment, the source WTRU may use DNS to determine to which other relay WTRU the target WTRU can connect. For example, the source WTRU may select a second relay WTRU (RID2), e.g., itself. According to an embodiment, for example, a link modification request message including the selected RID and any of the source WTRU's IP address / prefix via the selected RID may be sent by the source WTRU to the target WTRU via the first relay WTRU.

[0193] According to an embodiment, the target WTRU may return a link modification accept message to accept the selected relay WTRU (RID2).

[0194] According to an embodiment, the source WTRU may initiate (e.g., trigger) the establishment of a (e.g., PC5 unicast) link via the second relay WTRU (RID2), and traffic may be switched via the second relay WTRU (RID2).

[0195] According to an embodiment, messages such as, for example, either change relay request and accept messages may be exchanged via the user plane. For example, the first relay WTRU may forward IP packets without inspecting them (e.g., to process the contents of the message). Either the source or target WTRU may select a new relay WTRU. For example, the selection of a new relay may be based on a DNS query. In another example, the selection of a new relay may be based on messages exchanged between the source WTRU and the target WTRU.

[0196] 10 illustrates an example method 1000 for a relay change at a source WTRU. According to an embodiment, in step 1010, the source WTRU may send a link modification request to the target WTRU via the first relay WTRU to request a relay change. According to an embodiment, the source WTRU may send the link modification request based on detecting a condition for a relay change. According to an embodiment, the source WTRU may send the link modification request based on receiving a trigger for a relay change from the first relay WTRU. According to an embodiment, the trigger for a relay change may include an initial link modification request received from the first relay WTRU. According to an embodiment, the initial link modification request may include either a replacement relay identifier or a list of identifiers identifying candidates for replacing the relay.

[0197] According to an embodiment, the source WTRU may receive a link modification acceptance from the target WTRU via the first relay WTRU in step 1020. The link modification acceptance may include an identifier of the second relay WTRU.

[0198] According to an embodiment, in step 1030, the source WTRU may transmit a direct communication request including an identifier of the second relay WTRU, for example, indicating acceptance (eg, confirmation) of the second relay WTRU to the target WTRU.

[0199] According to an embodiment, in step 1040, the source WTRU may receive, via the second relay WTRU, a direct communication acceptance from the target WTRU, for example, indicating that traffic may be relayed via the second relay WTRU.

[0200] According to an embodiment, in step 1050, the source WTRU may transmit packets destined for the target WTRU via the second relay WTRU.

[0201] FIG. 11 illustrates an example method 1100 for a change of relay at a target WTRU. According to an embodiment, in step 1110, the target WTRU may receive a link modification request from the source WTRU via the first relay WTRU to request a change of relay. According to an embodiment, in step 1120, the target WTRU may send a link modification accept to the source WTRU via the first relay WTRU. For example, the link modification accept may include an identifier of the second relay WTRU. According to an embodiment, in step 1130, the target WTRU may receive a direct communication request from the source WTRU via the second relay WTRU. According to an embodiment, in step 1140, the target WTRU may send a direct communication accept to the source WTRU via the second relay WTRU. According to an embodiment, in step 1150, the target WTRU may send a packet intended for the source WTRU via the second relay WTRU.

[0202] According to an embodiment, either the initial link modification request or the link modification request may include a list of identifiers identifying candidates to replace the relay. According to an embodiment, either the source WTRU or the target WTRU may select an identifier of a second relay WTRU in the list of identifiers.

[0203] According to an embodiment, the link modification request may include any of a relay reselection indication, a reason indication, a first token, a security establishment method indication, and security parameters.

[0204] According to an embodiment, the security establishment method indication may indicate establishing a security context for the second link according to security parameters included in the link modification request. According to an embodiment, the security context for the second link may be established prior to establishing the second link via the second relay WTRU.

[0205] According to an embodiment, the security establishment method indication may indicate reusing the same security context for the second link via the second relay WTRU as already used for the first link via the first relay WTRU.

[0206] According to an embodiment, the link modification request may indicate establishing a security context for the second link when establishing the second link via the second relay WTRU.

[0207] According to an embodiment, the link modification acceptance may further include any of a selected security establishment method indication, a target WTRU identifier, and a second token.

[0208] According to an embodiment, the direct communication request may further include any of the target WTRU identifier and the second token.

[0209] According to an embodiment, the direct communication acceptance message may include a first token.

[0210] 12 illustrates an example method 1200 for a (e.g., Layer 2-based) change of relay at a source WTRU. According to an embodiment, at step 1210, a link modification request message may be sent by the source WTRU to the target WTRU over a first link via a first relay WTRU to request a change of relay. For example, the link modification request message may include first information indicating any of a relay reselection indication, a list of candidate relay identifiers, and a source identifier.

[0211] According to embodiments, the source WTRU may send a link modification request based on detecting a condition for a relay change. According to embodiments, the source WTRU may send a link modification request based on receiving a trigger for a relay change from a first relay WTRU. According to embodiments, the trigger for a relay change may include an initial link modification request received from the first relay WTRU. According to embodiments, the initial link modification request message may include any number of link identifiers (e.g., a link identifier and / or multiple link identifiers) representing the link to be moved and any number of candidate relay identifiers (e.g., identifying any number of candidate relays).

[0212] According to an embodiment, a link modification accept message may be received by the source WTRU from the target WTRU via the first relay WTRU in step 1230. For example, the link modification accept message may include second information indicating either a relay identifier of the selected relay WTRU and a target identifier for associating the second one with the first link.

[0213] According to an embodiment, a direct communication request (DCR) message may be sent by the source WTRU to the target WTRU via the selected relay WTRU in step 1240. For example, the DCR message may include third information indicating a target identifier for identifying the first link to be replaced with the second link.

[0214] According to an embodiment, in step 1250, a direct communication accept (DCA) message may be received by the source WTRU from the target WTRU via the selected relay WTRU.

[0215] According to an embodiment, in step 1260, data traffic may be switched by the source WTRU from the first link to the second link via the selected relay WTRU, provided that the identifier included in the received DCA message matches the source identifier.

[0216] 13 illustrates an example method 1300 for a (e.g., Layer 2-based) change of relay at a target WTRU. According to an embodiment, at step 1310, a link modification request message may be received by the target WTRU from the source WTRU over a first link via a first relay WTRU to request a change of relay. For example, the link modification request message may include first information indicating any of a relay reselection indication, a list of candidate relay identifiers, and a source identifier.

[0217] According to an embodiment, in step 1320, a relay WTRU may be selected by the target WTRU based on the list of candidate relay identifiers.

[0218] According to an embodiment, a link modification accept message may be sent by the target WTRU to the source WTRU via the first relay WTRU in step 1330. For example, the link modification accept message may include second information indicating either a relay identifier of the selected relay WTRU and a target identifier for associating the second link with the first link.

[0219] According to an embodiment, in step 1340, a direct communication request (DCR) message may be received by the target WTRU from the source WTRU via the selected relay WTRU.

[0220] According to an embodiment, in step 1350, a Direct Communication Accept (DCA) message may be sent by the target WTRU to the source WTRU via the selected relay WTRU, provided that the identifier included in the received DCR message matches the target identifier.

[0221] According to an embodiment, in step 1360, data traffic may be switched by the target WTRU from the first link to the second link via the selected relay WTRU.

[0222] According to an embodiment, for either the source WTRU or the target WTRU, the same security context already used for the first link via the first relay WTRU may be reused for the second link via the selected relay WTRU.

[0223] According to an embodiment, for either the source WTRU or the target WTRU, the link modification request message may include any of a cause indication, a security establishment method indication, and security parameters.

[0224] According to an embodiment, for either the source WTRU or the target WTRU, the security establishment method indication may indicate establishing a security context for the second link according to security parameters included in the link modification request message, and the security context for the second link may be established prior to establishing the second link via the selected relay WTRU.

[0225] According to an embodiment, for either the source WTRU or the target WTRU, the link modification request message may indicate that a security context for the second link is to be established when establishing the second link via the selected relay WTRU.

[0226] 14 illustrates an example method 1400 for a (e.g., Layer 3-based) change of relay at a source WTRU. According to an embodiment, at step 1410, a link modification request message for forwarding to a target WTRU may be sent by the source WTRU to a first relay WTRU over a first link to request a change of relay. For example, the link modification request message may include first information indicating either a relay reselection indication or a list of candidate relay identifiers.

[0227] According to an embodiment, transmitting the link modification request message may be based on detecting a condition for a relay change. According to an embodiment, transmitting the link modification request message may be based on receiving a trigger for a relay change from the first relay WTRU. According to an embodiment, the trigger for a relay change may include an initial link modification request message received from the first relay WTRU.

[0228] According to an embodiment, in step 1430, a link modification accept message may be received by the source WTRU over the first link from the first relay WTRU, the link modification accept message originating from the target WTRU. For example, the link modification accept message may include second information indicating any of a relay reselection indication, an identifier of the selected relay WTRU, and a target WTRU IP address for communication between the source WTRU and the target WTRU via the selected relay WTRU.

[0229] According to an embodiment, in step 1440, a link modification acknowledgement message to be forwarded to the target WTRU may be sent by the source WTRU to the first relay WTRU over the first link to acknowledge the change of relay. For example, the link modification acknowledgement message may include third information indicating any of the source WTRU IP addresses for communication between the source WTRU and the target WTRU via the selected relay WTRU.

[0230] According to an embodiment, in step 1450, IP traffic (eg, directed to the target WTRU) may be switched by the source WTRU from the first link to the second link via the selected relay WTRU.

[0231] 15 illustrates an example method 1500 for a (e.g., Layer 3-based) change of relay at a target WTRU. According to an embodiment, at step 1510, a link modification request message may be received by the target WTRU over a first link from a first relay WTRU, the link modification request message originating from the source WTRU to request a change of relay. For example, the link modification request message may include first information indicating either a relay reselection indication or a list of candidate relay identifiers.

[0232] According to an embodiment, in step 1520, a relay WTRU may be selected by the target WTRU based on the list of candidate relay identifiers.

[0233] According to an embodiment, a link modification accept message may be sent by the target WTRU to the first relay WTRU over the first link to be forwarded to the source WTRU in step 1530. For example, the link modification accept message may include second information indicating either a relay reselection indication, an identifier of the selected relay WTRU, and a target WTRU IP address for communication between the source WTRU and the target WTRU via the selected relay WTRU.

[0234] According to an embodiment, in step 1540, a link modification acknowledgment message may be received by the target WTRU over the first link from the first relay WTRU, the link modification acknowledgment message originating from the source WTRU to acknowledge the change of relay. For example, the link modification acknowledgment message may include third information indicating a source WTRU IP address for communications between the source WTRU and the target WTRU via the selected relay WTRU. According to an embodiment, in step 1550, IP traffic (e.g., directed to the source WTRU) may be switched by the target WTRU from the first link to the second link via the selected relay WTRU.

[0235] According to an embodiment, for either the source WTRU or the target WTRU, the first information may further indicate either a first source WTRU IP address or a first target WTRU IP address, and the first source WTRU IP address and the first target WTRU IP address are used for communication between the source WTRU and the target WTRU via the first relay WTRU.

[0236] According to an embodiment, for either the source WTRU or the target WTRU, the second information may further indicate either a first source WTRU IP address or a first target WTRU IP address to be used for communication between the source WTRU and the target WTRU via the first relay WTRU.

[0237] According to an embodiment, for either the source WTRU or the target WTRU, the list of candidate relay identifiers may include identifiers of candidate relays that already have an established link with the source WTRU.

[0238] According to an embodiment, for either the source WTRU or the target WTRU, the selected relay WTRU (802) may be selected from a list of candidate relays that already have an established link with the target WTRU.

[0239] According to an embodiment, for either the source WTRU or the target WTRU, the first information may further indicate a plurality of target IP addresses of a plurality of target WTRUs, respectively, with which the source WTRU may communicate via the first relay WTRU.

[0240] conclusion Although features and elements are described above in particular combinations, those skilled in the art will understand that each feature or element may be used alone or in any combination with the other features and elements. Furthermore, the methods described herein may be implemented in a computer program, software, or firmware embodied in a computer-readable medium for execution by a computer or processor. Examples of computer-readable media include electronic signals (transmitted via wired or wireless connections) and computer-readable storage media. Examples of computer-readable storage media include, but are not limited to, read-only memory (ROM), random-access memory (RAM), registers, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks and digital versatile disks (DVDs). A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, UE, terminal, base station, RNC, or any host computer.

[0241] Although not explicitly stated, the embodiments described herein may be used in any combination or subcombination, for example, the principles described herein are not limited to the variations described, but rather any arrangement of variations and embodiments may be used.

[0242] Additionally, any features, variations, or embodiments described in the methods are compatible with an apparatus device including means for processing the disclosed methods, compatible with a device with a processor configured to process the disclosed methods, compatible with a computer program product including program code instructions, and compatible with a non-transitory computer-readable storage medium storing program instructions.

[0243] While features and elements are described above in particular combinations, those skilled in the art will understand that each feature or element may be used alone or in any combination with the other features and elements. Furthermore, the methods described herein may be implemented in a computer program, software, or firmware embodied in a computer-readable medium for execution by a computer or processor. Examples of non-transitory computer-readable storage media include, but are not limited to, read-only memory (ROM), random-access memory (RAM), registers, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, and optical media such as magneto-optical media and CD-ROM disks and digital versatile disks (DVDs). A processor in association with software may be used to implement a radio frequency transceiver for use in the WTRU 102, UE, terminal, base station, RNC, or any host computer.

[0244] Furthermore, in the above embodiments, processing platforms, computing systems, controllers, and other devices including processors are described. These devices may include at least one central processing unit ("Central Processing Unit (CPU") and memory. In accordance with the practices of those skilled in the art of computer programming, references to acts and symbolic representations of operations or instructions may be performed by various CPUs and memories. Such acts and operations or instructions may be referred to as being "executed," "executed by a computer," or "executed by a CPU."

[0245] Those of ordinary skill in the art will understand that the operations and symbolically represented operations or instructions include the manipulation of electrical signals by a CPU. The electrical system represents data bits that can cause a resulting transformation or reduction of the electrical signals, and maintains the data bits in memory locations in a memory system, thereby reconfiguring or otherwise altering the operation of the CPU and the processing of other signals. The memory locations in which the data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties that correspond to or represent the data bits. It should be understood that exemplary embodiments are not limited to the above-mentioned platforms or CPUs, and that other platforms and CPUs may support the provided methods.

[0246] The data bits may also be maintained on computer-readable media, including magnetic disks, optical disks, and any other volatile (e.g., random access memory ("RAM")) or non-volatile (e.g., read-only memory ("ROM")) mass storage system readable by a CPU. The computer-readable media may include cooperative or interconnected computer-readable media that reside exclusively on a processing system or that are distributed among multiple interconnected processing systems, which may be local or remote to a processing system. Representative embodiments are not limited to the memories described above, and it will be understood that other platforms and memories may support the described methods.

[0247] In an exemplary embodiment, any of the operations, processes, etc. described herein may be implemented as computer-readable instructions stored on a computer-readable medium, which may be executed by a processor of a mobile, a network element, and / or any other computing device.

[0248] There is little distinction between hardware and software implementations of aspects of the system. The use of hardware or software is generally a design choice that implies a cost vs. efficiency trade-off (e.g., in certain situations, but not always, the choice between hardware and software may be significant). There may be a variety of vehicles (e.g., hardware, software, and / or firmware) in which the processes and / or systems and / or other techniques described herein may be effective, and the preferred vehicle may vary depending on the context in which the processes and / or systems and / or other techniques are deployed. For example, if an implementer determines that speed and accuracy are paramount, the implementer may select a primarily hardware and / or firmware vehicle. If flexibility is paramount, the implementer may select a primarily software implementation. Alternatively, the implementer may select some combination of hardware, software, and / or firmware.

[0249] The foregoing detailed description has illustrated various embodiments of devices and / or processes through the use of block diagrams, flowcharts, and / or examples. To the extent that such block diagrams, flowcharts, and / or examples include one or more functions and / or operations, those skilled in the art will appreciate that each function and / or operation in such block diagrams, flowcharts, or examples may be individually and / or collectively implemented by a wide range of hardware, software, firmware, or substantially any combination thereof. Suitable processors include, by way of example, a general-purpose processor, a special-purpose processor, a conventional processor, a digital signal processor (DSP), multiple microprocessors, one or more microprocessors associated with a DSP core, a controller, a microcontroller, an application specific integrated circuit (ASIC), an application specific standard product (ASSP), a field programmable gate array (FPGA) circuit, any other type of integrated circuit (IC), and / or a state machine.

[0250] While features and elements have been provided above in particular combinations, those of ordinary skill in the art will understand that each feature or element can be used alone or in any combination with other features and elements. The present disclosure is not limited in terms of the specific embodiments described herein, which are intended as examples of various aspects. It will be apparent to those skilled in the art that many modifications and variations can be made without departing from the spirit and scope of the invention. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly stated as such. Functionally equivalent methods and apparatuses within the scope of the present disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing description. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is understood that the present disclosure is not limited to any particular method or system.

[0251] It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, "station" and its abbreviation "STA," "user equipment" and its abbreviation "UE," when referred to herein, may mean or include (i) a wireless transmit and / or receive unit (WTRU), such as a described infrastructure; (ii) any of several embodiments of a WTRU, such as a described infrastructure; (iii) a wireless-enabled and / or wired-enabled (e.g., tethered) device configured with some or all of the structure and functionality of an illustrated WTRU (e.g., a described infrastructure); (iii) a wireless-enabled and / or wired-enabled device configured with less than all of the structure and functionality of a WTRU (e.g., a described infrastructure); or (iv) others. Details of an exemplary WTRU that may represent any of the UEs enumerated herein are provided below with respect to FIGS. 1A-1D.

[0252] In certain exemplary embodiments, portions of the subject matter described herein may be implemented via application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), digital signal processors (DSPs), and / or other integrated formats. However, those skilled in the art will recognize that certain aspects of the embodiments disclosed herein may be equivalently implemented in an integrated circuit, in whole or in part, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more processors (e.g., as one or more programs running on one or more microprocessors), as firmware, or as substantially any combination thereof, and that designing circuitry and / or writing software and / or firmware code is within the skill of those skilled in the art in light of this disclosure. Furthermore, those skilled in the art will recognize that the mechanisms of the subject matter described herein may be distributed as program products in various forms, and that exemplary embodiments of the subject matter described herein apply regardless of the particular type of signal-bearing medium used to actually effect the distribution. Examples of signal bearing media include, but are not limited to, recordable-type media such as floppy disks, hard disk drives, CDs, DVDs, digital tape, computer memory, and transmission-type media such as digital and / or analog communications media (e.g., fiber optic cables, wave guides, wired communications links, wireless communications links, etc.).

[0253] The subject matter described herein may, in some cases, depict different components that are contained within or connected to different other components. It should be understood that such illustrated architectures are merely examples, and that in fact many other architectures that achieve the same functionality may be implemented. Conceptually, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality may be achieved. Thus, any two components combined herein to achieve a particular function can be viewed as “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated may also be considered to be “operably connected” or “operably coupled” to each other to achieve the desired functionality, and any two components so associated may also be considered to be “operably coupleable” to each other to achieve the desired functionality. Examples of operably coupleable include, but are not limited to, physically matable and / or physically interacting components, wirelessly interacting and / or wirelessly interacting components, and / or logically interacting and / or logically interacting components.

[0254] With respect to the use of virtually any plural and / or singular term herein, those skilled in the art can convert from plural to singular and / or from singular to plural as appropriate to the context and / or application. Various singular / plural permutations may be expressly set forth herein for purposes of clarity.

[0255] In general, those skilled in the art will understand that terms used in this specification, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including, but not limited to," the term "having" should be interpreted as "having at least," and the term "comprises" should be interpreted as "including, but not limited to"). Furthermore, where a specific number of recitations of an introduced claim are intended, such intention will be explicitly set forth in the claim; in the absence of such recitation, those skilled in the art will understand that no such intention exists. For example, where only one item is intended, the term "single" or similar language may be used. To assist in understanding, the following appended claims and / or description of this specification may include the use of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed as meaning that the introduction of a claim recitation by the indefinite article "a" or "an" limits any particular claim containing such an introduced claim recitation to embodiments containing only one such recitation, even if the same claim contains the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should be interpreted to mean "at least one" or "one or more"). The same applies to the use of definite articles used to introduce claim recitations. Furthermore, those skilled in the art will recognize that even when a specific number of recitations of an introduced claim are explicitly recited, such recitation should be interpreted to mean at least the recited number (e.g., the simple recitation "two recitations" without other modifiers means at least two recitations, or more than two recitations).

[0256] Furthermore, when notation similar to "at least one of A, B, and C" is used, such structure is generally intended as the meaning that one of ordinary skill in the art would understand the notation (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, a system having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together). When notation similar to "at least one of A, B, or C" is used, such structure is generally intended as the meaning that one of ordinary skill in the art would understand the notation (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, a system having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together). Those skilled in the art will further appreciate that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" should be understood to include the possibilities of "A" or "B" or "A and B." Furthermore, as used herein, the term "any of," followed by a list of items and / or a list of categories of items, is intended to include "any of," "any combination of," "any plurality of," and / or "any combination of" the items and / or categories of items, individually or in combination with other items and / or other categories of items. Furthermore, as used herein, the terms "set" or "group" are intended to include any number of items, including zero. Furthermore, as used herein, the term "number" is intended to include any number, including zero.

[0257] Furthermore, where features or aspects of the disclosure are described in terms of a Markush group, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0258] As will be understood by those skilled in the art, for all purposes, including in terms of providing a written description, all ranges disclosed herein encompass any possible subranges and combinations of subranges. Any recited range can be readily recognized as fully descriptive and allowing the same range to be broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range described herein can be easily broken down into a lower third, middle third, upper third, etc. Additionally, as will be understood by those skilled in the art, all terms such as "up to," "at least," "greater than," "less than," etc., refer to ranges that are inclusive of the recited number and that can be further broken down into subranges as described above. Finally, as will be understood by those skilled in the art, ranges include individual elements. Thus, for example, a group having 1 to 3 cells refers to a group having 1, 2, or 3 cells. Similarly, a group having 1 to 5 cells refers to a group having 1, 2, 3, 4, or 5 cells, and so on.

[0259] Furthermore, the claims should not be read as limited to the provided order or to the provided elements unless specifically so stated. Furthermore, the use of the term "means for" in any claim is intended to invoke 35 U.S.C. 112, paragraph 6, or means-plus-function claim format, and any claim without the term "means for" is not so intended.

[0260] A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit / receive unit (WTRU), user equipment (UE), terminal, base station, mobility management entity (MME), or evolved packet core (EPC), or any host computer. The WTRU may be used in conjunction with modules implemented in hardware and / or software, such as, for example, a software defined radio (SDR), and may also be implemented in other components, such as a camera, a video camera module, a video phone, a speaker phone, a vibration device, a speaker, a microphone, a television transceiver, a hands-free headset, a keyboard, a Bluetooth module, a frequency modulation (FM) radio unit, a near field communication (NFC) module, an LCD display unit, an organic light emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an internet browser, and / or a wireless local area network (WLAN) or ultra wide band (UWB) module.

[0261] Although the present invention has been described with respect to a communications system, it is contemplated that the system may be implemented in software on a microprocessor / general purpose computer (not shown). In particular embodiments, one or more of the functions of the various components may be implemented in software controlling a general purpose computer.

[0262] Moreover, although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications of the details can be made within the scope of the claims and their equivalents without departing from the invention.

[0263] Throughout this disclosure, those skilled in the art will understand that certain exemplary embodiments may be used alternatively or in combination with other exemplary embodiments.

[0264] While features and elements are described above in particular combinations, those skilled in the art will understand that each feature or element can be used alone or in any combination with the other features and elements. Furthermore, the methods described herein may be implemented in a computer program, software, or firmware embodied in a computer-readable medium for execution by a computer or processor. Examples of non-transitory computer-readable storage media include, but are not limited to, read-only memory (ROM), random-access memory (RAM), registers, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, and optical media such as magneto-optical media and CD-ROM disks and digital versatile disks (DVDs). A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, UE, terminal, base station, RNC, or any host computer.

[0265] Furthermore, in the above embodiments, processing platforms, computing systems, controllers, and other devices including processors are described. These devices may include at least one central processing unit ("CPU") and memory. In accordance with the practices of those skilled in the art of computer programming, references to acts and symbolic representations of operations or instructions may be performed by various CPUs and memories. Such acts and operations or instructions may be referred to as being "executed," "executed by a computer," or "executed by a CPU."

[0266] Those of ordinary skill in the art will understand that the operations and symbolically represented operations or instructions involve the manipulation of electrical signals by a CPU. An electrical system represents data bits that can cause a resulting transformation or reduction of the electrical signals, and maintains the data bits in memory locations in a memory system, thereby reconfiguring or otherwise altering the operation of the CPU and the processing of other signals. The memory locations in which the data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties that correspond to or represent the data bits.

[0267] The data bits may also be maintained on computer-readable media, including magnetic disks, optical disks, and any other volatile (e.g., random access memory ("RAM")) or non-volatile (e.g., read-only memory ("ROM")) mass storage system readable by a CPU. The computer-readable media may include cooperative or interconnected computer-readable media that reside exclusively on a processing system or that are distributed among multiple interconnected processing systems, which may be local or remote to a processing system. Representative embodiments are not limited to the memories described above, and it will be understood that other platforms and memories may support the described methods.

[0268] Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), multiple microprocessors, one or more microprocessors associated with a DSP core, a controller, a microcontroller, an application specific integrated circuit (ASIC), an application specific standard product (ASSP), a field programmable gate array (FPGA) circuit, any other type of integrated circuit (IC), and / or a state machine.

[0269] Although the present invention has been described with respect to a communications system, it is contemplated that the system may be implemented in software on a microprocessor / general purpose computer (not shown). In particular embodiments, one or more of the functions of the various components may be implemented in software controlling a general purpose computer.

[0270] Moreover, although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications of the details can be made within the scope of the claims and their equivalents without departing from the invention.

Claims

1. 1. A method implemented in a source wireless transmit / receive unit (WTRU), wherein a first link is established between the source WTRU and a first relay WTRU, and a second link is established between the source WTRU and a second relay WTRU, the method comprising: sending a link modification request message to the first relay WTRU over the first link, the link modification request message being a request for a change of relay for the target WTRU, the link modification request message including a relay reselection indication, a list of candidate relay identifiers, and a first IP address of the target WTRU; receiving a link modification accept message on the first link from the first relay WTRU, the link modification accept message indicating that the target WTRU accepts the request for the change of relay, the link modification accept message including the relay reselection indication, an identifier of the second relay WTRU, and a second IP address of the target WTRU to be used for communication via the second relay WTRU; sending a link modification confirmation message over the first link to the first relay WTRU, the link modification confirmation message confirming the target WTRU's acceptance of the request for the change of relay, the link modification confirmation message including a second IP address of the source WTRU to be used for communication via the second relay WTRU; switching IP traffic destined for the target WTRU from the first link to the second link via the second relay WTRU; A method comprising:

2. The method of claim 1 , wherein sending the link modification request message is based on detecting a condition for the change of relay.

3. The method of claim 1 , wherein sending the link modification request message is based on receiving a trigger for the change of relay from the first relay WTRU.

4. The method of claim 3 , wherein receiving the trigger for the change of relay comprises receiving an initial link modification request message from the first relay WTRU.

5. The method of claim 1 , wherein if the source WTRU is communicating with multiple target WTRUs via the first relay WTRU, the link modification request message indicates IP addresses of the multiple target WTRUs.

6. The method of claim 1 , wherein the link modification request message further indicates a first IP address of the source WTRU.

7. 10. The method of claim 1, wherein the first IP address of the target WTRU and the second IP address of the target WTRU are the same IP address.

8. The method of claim 1 , wherein the first IP address of the target WTRU is different from the second IP address of the target WTRU.

9. The method of claim 6 , wherein the first IP address of the source WTRU and the second IP address of the source WTRU are the same IP address.

10. The method of claim 6 , wherein the first IP address of the source WTRU is different from the second IP address of the source WTRU.

11. A source wireless transmit / receive unit (WTRU), wherein a first link is established between the source WTRU and a first relay WTRU, and a second link is established between the source WTRU and a second relay WTRU, the source WTRU comprising: a processor; and a transceiver operatively coupled to the processor, wherein the source WTRU: sending a link modification request message to the first relay WTRU over the first link, the link modification request message being a request for a change of relay for the target WTRU, the link modification request message including a relay reselection indication, a list of candidate relay identifiers, and a first IP address of the target WTRU; receiving a link modification accept message on the first link from the first relay WTRU, the link modification accept message indicating that the target WTRU accepts the request for the change of relay, the link modification accept message including the relay reselection indication, an identifier of the second relay WTRU, and a second IP address of the target WTRU to be used for communication via the second relay WTRU; sending a link modification confirmation message over the first link to the first relay WTRU, the link modification confirmation message confirming the target WTRU's acceptance of the request for the change of relay, the link modification confirmation message including a second IP address of the source WTRU to be used for communication via the second relay WTRU; Switching IP traffic destined for the target WTRU from the first link to the second link via the second relay WTRU. The source WTRU is configured to:

12. 12. The source WTRU of claim 11, wherein being configured to transmit the link modification request message comprises being configured to transmit the link modification request message based on detecting a condition for the change of relay.

13. 12. The source WTRU of claim 11, wherein being configured to transmit the link modification request message comprises being configured to transmit the link modification request message based on receiving a trigger for the change of relay from the first relay WTRU.

14. The source WTRU of claim 13 , wherein being configured to receive the trigger for the change of relay comprises being configured to receive an initial link modification request message from the first relay WTRU.

15. The source WTRU of claim 11 , wherein if the source WTRU is communicating with multiple target WTRUs via the first relay WTRU, the link modification request message indicates IP addresses of the multiple target WTRUs.

16. The source WTRU of claim 11 , wherein the link modification request message further indicates a first IP address of the source WTRU.

17. The source WTRU of claim 11 , wherein the first IP address of the target WTRU and the second IP address of the target WTRU are the same IP address.

18. The source WTRU of claim 11 , wherein the first IP address of the target WTRU is different from the second IP address of the target WTRU.

19. The source WTRU of claim 16 , wherein the first IP address of the source WTRU and the second IP address of the source WTRU are the same IP address.

20. The source WTRU of claim 16 , wherein the first IP address of the source WTRU is different from the second IP address of the source WTRU.

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