Method for changing a value of an extended unique identifier of a non-ap station associated with an ap station

Abstract

The present invention concerns a method of communication between a first station and a second station, the first station being capable of changing an identifier from a current identifier to a new identifier, the method comprising at the first station, during a margin period setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station for received frames, and setting one of the current or the new identifier as valid identifier of the first station for emitted frames.

Description

[0001] METHOD FOR CHANGING A VALUE OF AN EXTENDED UNIQUE IDENTIFIER OF A NON-AP STATION ASSOCIATED WITH AN AP STATION

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to wireless communications and more specifically to user privacy during wireless communications.

[0004] BACKGROUND OF INVENTION

[0005] The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.

[0006] Today, the evolution of wireless systems has brought privacy concerns at the forefront, driven by user demand and requirements of the General Data Protection Regulation (GDPR). The global wireless industry is faced with the growing need to protect users’ personally identifiable information from increasingly sophisticated user tracking and user profiling activities, while continuing to improve wireless services and the user experience.

[0007] In particular, the Media Access Control (MAC) address of a user device constitutes a piece of data that can be used to track this user. Indeed, the access points (APs) of wireless networks can monitor the locations of mobile devices (tablets, laptops, mobile phones, etc.) of a user without his / her consent, by means of their MAC addresses. This is because mobile phones are configured to discover surrounding access points to wireless networks. As a user moves, his / her mobile phone sends requests to determine if there are any access points nearby, these requests identifying the mobile phone which sends these requests and including in particular the MAC address of the mobile phone. Access points that hear these requests can respond. In the context of Wi-Fi networks as defined by IEEE 802.11 standards (Wi-Fi is a trademark), this procedure is called Probe Request / Response exchange.

[0008] So even when a mobile phone is not connected to a Wi-Fi network, surrounding access points may receive its MAC address. It is then possible to track a user by reconstructing his / her trajectory from access points to which his / her mobile phone has sent its MAC address. In addition, if the mobile phone has been associated with one of the access points (i.e., the user has connected to an associated Wi-Fi network through that access point) and the user has provided personal identification information (name, place of residence, etc.) in the past, the access point may have recorded in a database the MAC address of the phone in association with the identification information. Therefore, even if the user is not connected to the Wi-Fi network, this identity information could be recovered by comparing the MAC address contained in a Probe Request to the MAC address used for the past association.

[0009] In the context of Wi-Fi networks, a solution has been proposed by the IEEE 802.11 working group to limit the risk of a user being tracked, and consists in dynamically modifying the MAC address of the user device. This mechanism is called Randomized and Changing MAC (RCM) procedure. It has been originally introduced as a privacy enhancing feature in the 802.11aq Pre-Association Service Discovery Task Group and finally included in the standard IEEE Std 802.11-2020. It comprises periodical change of the MAC address of a non-AP station or STA (i.e., a station which is not an access point) to a random value, while the non-AP station is not associated with a network (or, equivalently, with an access point). The non-AP station may construct the randomized MAC address from the locally administered address space as defined in IEEE Std 802®- 2014 and IEEE Std 802c™-2017.

[0010] More specifically, a new Management Information Base (MIB) variable controllable by an external management entity has been specified. This variable is called ‘dotH MACPrivacyActivated’. When dotH MACPrivacyActivated is set to “true”, the non- AP station can apply specific mechanisms for enhancing the privacy at MAC level, including RCM.

[0011] The MAC address, or Elll-48 address, of a device is an Extended Unique Identifier (EUI) composed of 48 bits. It can be administered universally or locally. A universally administered address is uniquely assigned to the device by the manufacturer. On the contrary, a locally administered address is assigned to the device by a software or a network administrator, and replaces the physical burned-in address. The second- least-significant bit of the first octet of the MAC address, i.e., the seventh bit of the first octet of the address, also referred to as “U / L bit” (for “Universal / Local bit”), indicates whether it is universally (when set to 0) or locally (when set to 1) administered. The leastsignificant bit of the first octet of the MAC address, i.e., the eighth bit of the first octet of the address, also referred to as “l / G bit” (for “Individual / Group bit”), indicates whether the frame is sent to only one receiving device (when set to 0, indicating unicast transmission) or to a plurality of devices (when set to 1 , indicating multicast transmission). When RCM mechanism is operated in the non-AP station, the MAC address of the non-AP station is randomly changed (for instance periodically). More specifically, the ll / L bit is set to 1 , the l / G bit is set to 0, and the remaining 46 bits are randomly generated by using a pseudorandom function (PRF), or can be obtained by any other means (for instance received from AP, or taken in a predefined list of addresses).

[0012] In the scope of the IEEE802.11bi, standard currently under drafting, solutions have been proposed to change the MAC address of a station while associated, that are all relying on the fact that both AP and non-AP station operate the change of MAC address at the same instant on their respective side. Obviously, a change of MAC address of a station while not associated is not an issue. This instant can be dynamically determined (duration between 2 changes can be variable along time), or can be based on a fixed frequency. The problem of all those proposed solutions is that the instant of change of the MAC address of a station needs to be precisely synchronized on both side (AP and non-AP). Without an accurate synchronization, a receiving station may discard frames addressed with a MAC address considered as valid only by the emitter side.

[0013] To overcome this problem, it has been recently proposed to rely on an instant defined by an absolute time (Timing synchronization function or TSF time), or on an instant that is relative to the reception of a frame (typically a beacon frame). Unfortunately, the proposed solutions still suffer from a risk of frame discarding upon reception since the proposed solutions relies on beacon frames reception and cannot guaranty a perfect synchronization. The solution of the prior art cannot guaranty a perfect synchronization between the non-AP station’s clock and the AP’s clock. One of the reasons is that the internal quartz that support the 1MHz clock of the 802.11 stations (AP or non-AP station) can have a drift up to 100ppm as defined in the 802.11 standards. This 100ppm clock drift will for instance end up with a relative clock drift difference of up to 20us per 100ms, which is a typical interval between two beacon receptions, between an AP and a non-AP station. This difference by itself doesn’t seems that important, but may allow 1 frame discard and generate retransmission, and associated congestion window evolution slowing down the future emission of frames.

[0014] In addition to this relatively small drift, a much bigger problem occurs when a non- AP station sleeps, namely goes in doze state, for a long period of time, because the clock drift may then be much higher upon station awakening. For instance, if a station sleeps for 10 minutes, the clock drifts can be up to 120ms because the station cannot receive beacons able to resynchronize its internal clock while asleep. In this last situation, the awakening station may use the wrong MAC address until the reception of a new beacon frame, typically up to 100ms later if the first received beacon is well received. This will create a large number of discarded frames and associated retransmissions.

[0015] As previously explained, a perfect synchronization (corresponding to less than one internal clock tic = 1us) is very difficult to achieve and the industry generally admits that an acceptable clock drift is around 25us. There is thus a need for a method for a station (AP or non-AP station) to avoid discarding frames upon MAC address change, even if their respective clocks are not perfectly synchronized.

[0016] SUMMARY OF THE INVENTION

[0017] The present invention has been devised to address one or more of the foregoing concerns.

[0018] According to a first aspect of the invention there is provided a method of communication between a first station and a second station, the first station being capable of changing an identifier from a current identifier to a new identifier, the method comprising at the first station, during a margin period:

[0019] - setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station for receiving frames, and

[0020] - setting one of the current or the new identifier as valid identifier of the first station for emitting frames.

[0021] In an embodiment, the set of valid identifiers apply to the receiver address (RA) field in received frames.

[0022] In an embodiment, the one identifier applies to the transmitter address (TA) field in emitted frames.

[0023] According to another aspect of the invention there is provided a method of communication between a first station and a second station, the first station being capable of changing an identifier from a current identifier to a new identifier, the method comprising at the second station, during a margin period:

[0024] - setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station for receiving frames, and

[0025] - setting one of the current or the new identifier as valid identifier of the first station for emitting frames.

[0026] In an embodiment, the set of valid identifiers apply to the transmitter address (TA) field in received frames.

[0027] In an embodiment, the one identifier applies to the receiver address (RA) field in emitted frames. In an embodiment, the method further comprises determining a usage period start time at which the identifier of the first station is changed from the current identifier to the new identifier.

[0028] In an embodiment, the margin period is a forward margin period starting prior the start of the usage period.

[0029] In an embodiment, the method further comprises, upon reception of a frame addressed to the first station with the new identifier during the forward margin period, shifting forward the start time of the usage period, or a following usage period, based on the time difference between the reception of the frame and the determined usage period start time.

[0030] In an embodiment, the margin period is a backward margin period ending after the start of the usage period.

[0031] In an embodiment, the method further comprises, upon reception of a frame addressed to the first station with the current identifier during the backward margin period, shifting backward the start time of a following usage period based on the time of reception of the frame.

[0032] In an embodiment, the first station is a non-access point (AP) station and the second station is an AP station.

[0033] In an embodiment, station identifiers are MAC addresses.

[0034] According to another aspect of the invention there is provided a station comprising:

[0035] - means for changing an identifier of the first station from a current identifier to a new identifier;

[0036] - means for setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station, during a margin period, for receiving frames, and

[0037] - means for setting one of the current or the new identifier as valid identifier of the first station, during the margin period, for emitting frames.

[0038] According to another aspect of the invention there is provided a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless device, causes the wireless device to perform the method of the invention.

[0039] At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit", "module" or "system". Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.

[0040] Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible, non-transitory carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, a hard disk drive, a magnetic tape device or a solid-state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g., a microwave or RF signal.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which:

[0043] Figure 1 illustrates an example of a network system in which some embodiments of the invention may be implemented;

[0044] Figure 2a illustrates an example of steps performed at a non-AP station or an AP station before changing a non-AP MAC address, according to some embodiments of the invention;

[0045] Figure 2b illustrates an example of steps performed at a non-AP station or an AP Station after changing a non-AP MAC address, according to some embodiments of the invention;

[0046] Figure 3 illustrates an example of steps carried out by a non-AP station or by an AP station to initiate the period of change of MAC address;

[0047] Figure 4 illustrates an example of steps carried out by a non-AP station or by an AP station for closing the period of change of the MAC address of a non-AP-station, according to some embodiments of the invention;

[0048] Figure 5 illustrates a first example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station;

[0049] Figure 6 illustrates an example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station, according to some embodiments of the invention applied to a case of a non-AP station clock being late compared to the AP;

[0050] Figure 7 illustrates an example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station, according to some embodiments of the invention applied to a case of a non-AP station clock being in advance compared to the AP;

[0051] Figures 8 and 9 illustrate the handling of TA and RA addresses both at the non- AP station and AP sides for different examples of the forward and backward margin periods; and

[0052] Figure 10 schematically illustrates an example of a communication device configured to implement at least some embodiments of the present invention.

[0053] DETAILED DESCRIPTION OF THE INVENTION

[0054] The RCM procedure is a method for obfuscating simultaneous multiple parameters (including the MAC Address) of a CPE (Client Privacy Enhancement) Client while it is associated with a CPE AP. It is based on the standardized PRF (section 12.7.1.2 -IEEE Std 802.11-2020) executed in parallel by the CPE Client and the CPE AP with the same input parameters.

[0055] This procedure is made up of 3 mains steps:

[0056] 1) During, or after association, encrypted information (SERCM key) is shared between AP and non-AP STA;

[0057] 2) Upon AP or non-AP STA request, both AP and non-AP STA compute and generate new uncorrelated values or new masks with a single execution of the standardized PRF performed in parallel;

[0058] 3) At the starting of a transition period, both AP and non-AP STA initiate the obfuscation of the CPE parameters.

[0059] Purpose of this document is to focus on the period surrounding the change of CPE Parameters also known as Frame Anonymization (FA) parameters.

[0060] Instants of change of the CPE Parameters define successive usage periods (1 , ... , n, n+1 , ...), also referred to as ‘Epoch’, or Enhanced Data Privacy (EDP) Epoch, during each of which given values of the CPE Parameters (e.g., MAC address) are to be used. In some implementations, a usage period starts with a transition period during which both old values (pertaining to previous usage period ‘n’) and new values (pertaining to current usage period ‘n+1’) of the CPE parameters may be considered. In some implementations, constraints may be applied to the use of old values during the transition period. For example, it may be allowed to use an old MAC address, @MAC(n), as transmitter address for the transmission of already generated and buffered frames, but the old MAC address @MAC(n) can no longer be used for the generation of a new frame since the current usage period (n+1) has started. In other implementations, the change of CPE parameters between two usage periods is strictly applied, i.e., the transition period is not implemented and only CPE parameters (n) of a given usage period ‘n’ can be used.

[0061] According to embodiments of the invention, two new phases (periods) are introduced in the “Seamless Enhanced RCM” (SERCM) procedure to overcome the lack of perfect synchronization of the clocks at the AP and at the non-AP stations. The periods are aimed to absorb the potential clocks drift between the stations and the AP.

[0062] A first period, referred to as forward margin, is defined before a next usage period starts, between an RCM ready time and the start of the next usage period (and thus the start of the transition period, if present). The forward margin is useful if the station is late relatively to the AP. The forward margin allows to decode received frames addressed with the new MAC address, @MAC(n+1), although the usage period has not started yet (at the station / receiving side). In other words, during this first period, the station is ready to receive frames addressed with its new MAC address ‘n+1’ but the station is not allowed to use this new MAC address in transmitted frames until the new usage period ‘n+1’ effectively starts.

[0063] A second period, referred to as backward margin, is defined after a usage period starts, between the start of the usage period and an RCM done time. The backward margin is useful if the station is in advance of the AP. The backward margin allows to decode received frames addressed with the old MAC address, @MAC(n), although the usage period has already started (at the receiving side). In other words, during this second period, the station is still configured to receive frames addressed with its old MAC address ‘n’ but the station is not allowed anymore to use this old MAC address in transmitted frames.

[0064] If a transition period is present, the backward margin may be defined between the end of the transition period end and RCM done time. This way, old MAC address ‘n’ may still be used during the transition period in transmitted frames to flush buffered frames generated with @MAC(n). In a variant, the backward margin is not present (implemented) if the transition period is present. In this case, the transition period is acting as a backward margin with a relaxed constraint regarding the use of the old MAC address in the transmitted frame. If the transition period is used in replacement of the backward margin, the duration of the transition period duration may be adjusted accordingly, for example to last until RCM done time.

[0065] The RCM (n) ready time may be determined relatively to the usage period (n) start time by removing a margin time, Mt, that may be fixed and determined by the AP as a parameter of the BSS (e.g., transmitted as an information element during the association procedure).

[0066] Alternatively, the margin time Mt may be determined as a proportion of the usage period (Epoch) duration. This is especially beneficial in case of usage periods of variable durations; e.g., random durations. In this case, the margin may reflect the maximum clock shift between an AP and a station and between two MAC address changes. If the time until the next MAC address change is noted Tc, then the margin time, Mt, can be determined for example by the formula Mt = 100ppm x 2 x Tc; 100ppm being the minimum precision for the internal quartz in the 802.11 specification. For example, for a Tc = 10 minutes, Mt = 100ppm x 2 x 600s = 120ms.

[0067] For the RCM (n) done time determination, in an embodiment, a same margin value as for the RCM ready time is applied but after the usage period (n) start time. In a variant, if the transition period is present, the duration of the transition period is subtracted from the calculated margin so that the RCM (n) done time occurs substantially a margin time after the start of the period, independently on whether the transition period is present or not.

[0068] If the time between two CPE parameters, e.g., MAC addresses, change is determined in multiple of the Target Beacon Transmission Time (TBTT), the Tc can be determined by multiplying the number of beacons between the two CPE parameters change by the TBTT value. This value could then be used for the determination of the margin time Mt.

[0069] In an embodiment, the RCM (n+1) ready time of usage period n+1 is equal to the RCM (n) done time of the previous usage period n. In this embodiment, as soon as the previous change of MAC address is done, the station is ready for the next one.

[0070] The MAC address change time, between AP and non-AP, is then secured by introducing margins in the MAC address change time.

[0071] Global user privacy is enhanced by allowing a station to change a set of its CPE parameters even in case of clock drift or loss of beacon counter synchronization.

[0072] According to some embodiments of the invention, it is provided a method for changing a value of an identifier of a non-access point (non-AP) station, for example a MAC address or an Extended Unique Identifier (EUI) of the non-AP station, associated with an access point (AP) station. The non-AP station and the AP station both initiate at the same time an identifier change process, and have the same duration to perform the actual identifier change. During a transition period (period of time between the initiation and the termination of the identifier change procedure), both the new and current identifiers are valid, and can be used by the AP station and / or the non-AP station. To do so, a new identifier called transient identifier is associated with the non-AP station. At the end of the transition period, the current identifier is replaced by the transient identifier, and the current identifier is not used any more.

[0073] During the transition period, the AP station and the non-AP station monitor the identifiers used for the frame transmission to determine if the identifier change is effective or not. In order to reduce the transition duration, stations may use a different identifier than the emitter identifier to acknowledge the reception of the frame depending on the transmission of buffered frames with the current identifier or the new one.

[0074] Note that AP or non-AP Station may request MAC Address change of a non-AP STA.

[0075] The usage period duration can be exchanged during the association procedure, for instance in a dedicated information element broadcasted in AP’s beacon frames, or in probe request or in probe response frames that can be exchanged during the association procedure. In this case, AP may initiate regular change of all non-AP STAs (at once). Alternatively, the duration may be indicated in a EUI change request frame. In such embodiment, non-AP Station may initiate its MAC Address change by sending a change request to the AP.

[0076] In order to secure the duration of the transition period, both the AP station and the non-AP station may initiate a timer with the value of the transition period.

[0077] For the sake of illustration, the examples provided hereafter are directed to changing the value of a MAC address of a non-AP station. Changing the value of another identifier of a non-AP station, for example its station AID or EUI, applies similarly.

[0078] Figure 1 illustrates an example of a network system in which some embodiments of the invention may be implemented.

[0079] For the sake of illustration, Figure 1 represents an 802.11 network (i.e., a Wi-Fi network) system 100 comprising four wireless devices: an access point station (AP) 105 and three non-AP stations (non-AP STAs) 110a, 110b, and 110c. Of course, the number of non-AP stations 110a, 110b, and 110c may be different from three. AP station 105 provides wireless connections between non-AP stations 110a, 110b, 110c and a wider network, such as the Internet (not represented). The connection of one of non-AP station 110a, 110b, and 110c to AP 105 may be performed by a standardized process called association. Once a non-AP station is associated with the AP station, the non-AP station can send data to the network and receive data from the network through the AP station.

[0080] AP station 105 may comprise, be implemented as, or known as a Node B, Radio Network Controller (RNC), evolved Node B (eNB), 5G Next generation base station (gNB), Base Station Controller (BSC), Base Transceiver Station (BTS), Base Station (BS), Transceiver Function (TF), Radio Router, Radio Transceiver, Basic Service Set (BSS), Extended Service Set (ESS), Radio Base Station (RBS), or some other terminology. It can be a standalone product or it may be integrated in a device, for instance in a broadband remote access server (BRAS).

[0081] Non-AP stations 110a, 110b, and / or 110c may comprise, be implemented as, or known as a subscriber station, a subscriber unit, a mobile station (MS), a remote station, a remote terminal, a user terminal (UT), a user agent, a user device, a user equipment (UE), a user station (STA), or some other terminology. In some implementations, a non- AP station may be or may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol (SIP) phone, a wireless local loop (WLL) station, a personal digital assistant (PDA), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or a smartphone), a computer (e.g., a laptop), a tablet, a portable communication device, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a global positioning system (GPS) device, or any other suitable device that is configured to communicate via a wireless or wired medium. In some aspects, some of non-AP stations 110a, 110b, and 110c may be wireless nodes. Such a wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as the Internet or a cellular network) via a wired or wireless communication link.

[0082] AP station 105 manages a set of stations that together organize their accesses to the wireless medium for communication purposes. All the stations (AP station 105 and non-AP stations 110a, 110b, and 110c) form a service set, which may be referred to as basic service set, BSS (although other terminology can be used). It is noted that AP station 105 may manage more than one BSS: each BSS is thus uniquely identified by a specific basic service set identifier (BSSID) and managed by a separate virtual AP station implemented in physical AP station 105.

[0083] Figure 2a illustrates an example of steps performed at a non-AP station or an AP station before changing a non-AP MAC address, according to some embodiments of the invention.

[0084] The algorithm illustrated by Figure 2a is carried out by a non-AP station or an AP station at a RCM ready time, for example the time referenced 600 or 605 in Figure 6 or in Figure 7. As a reminder, the RCM ready time is the beginning of the forward margin period introduced in this disclosure.

[0085] At step 200, a new MAC address, noted @MAC (n+1), that will be used to replace a current one noted @MAC(n), is determined. Associated obfuscated parameters are determined along with the new MAC address. These obfuscated parameters are parameters that need to be obfuscated in association with the change of the MAC address. The method for the MAC address and the parameters determination is out of scope of the present invention. Step 200 is preferably executed at the beginning of the forward margin, but it is clear that step 200 may be executed at any time prior this time. The important point being that the @MAC(n+1) and associated parameters are determined at RCM ready time with the same result at both the AP and the non-AP side.

[0086] Unfortunately, as already mentioned, RCM ready time may be significantly different at the AP and non-AP sides, so to avoid unwanted discard of frames in case of a significant difference, step 210 is executed.

[0087] Step 210 adds the @MAC(n+1) that will be used in the future, in the list of the MAC addresses that are considered as valid in received frames.

[0088] In the case of a non-AP, this is the list of the MAC addresses as they appear in the RA (Receiver Address) field of the received frames that will not be ignored by the non-AP station and will be decoded. For a non-AP station, this list typically corresponds to the unicast MAC addresses of the non-AP station, and the group MAC addresses to which the non-AP station belongs. Usually, a non-AP station has only one unicast MAC address fully valid, meaning valid as emitting MAC address or receiving MAC address, at a time, but in some embodiments, a non-AP station may have a set of MAC addresses fully valid. Step 210 may then correspond to the change of at least one of the fully valid MAC addresses.

[0089] In the case of an AP station, this list corresponds to the list of MAC addresses of the non-AP stations associated with the AP that will be received in the TA (Transmitter Address) filed on the received frame. This list is used to discard frames received by the AP that are not sent by registered stations. This list of course is not used during the association procedure.

[0090] By adding the new address @MAC (n+1) in the list of valid addresses in received frames, the non-AP station accepts the reception and decoding of frames having their RA field corresponding to the new address even if the receiving station has not yet started its usage period, i.e., the station still uses the current address @MAC(n) in the TA field of frames it transmits.

[0091] It is important to note that at that time, the new MAC address @MAC(n+1) is not fully valid, it is only valid in received frames, but cannot be used in transmitted frames.

[0092] Figure 2b illustrates an example of steps performed at a non-AP station or an AP Station after changing a non-AP MAC address, according to some embodiments of the invention.

[0093] The algorithm illustrated in Figure 2b is carried out by a non-AP station or an AP station at a RCM done time, meaning the end of the backward margin period, for example the time referenced 610 or 615 in Figure 6 or in Figure 7.

[0094] After the transition period is considered as finished by a station, the station stops using the current address, @MAC(n) in transmitted frames. This means that during the backward margin period, the station can still receive frames with the current address or the new one, while transmitting frames can only use the new address.

[0095] Then at the end of the backward margin period, the station executes step 220.

[0096] Step 220 removes the current MAC address @MAC(n) from the list of MAC addresses that are considered valid in received frames. By doing so, the station or non- AP station will then discard any received frame addressed using this MAC address.

[0097] In step 230, since the @MAC(n) is no more valid neither in received frames, nor in frames for transmission, the @MAC(n) can be discarded (not used anymore by the station), and all associated obfuscation parameters can be removed from the internal memory of the station. Step 230 then allows to free the memory dedicated for the storage of the MAC address and associated parameters.

[0098] Figure 3 illustrates an example of steps carried out by a non-AP station or by an AP station to initiate the period of change of MAC address (transition period), according to some embodiments of the invention. Figure 3 illustrates steps of the algorithm executed by a non-AP station at time 611 and respectively by the AP at time 621 of the Figure 6.

[0099] At step 300, the station replaces the current MAC address @MAC(n) by the new MAC address @MAC (n+1) as the effective address of the non-AP station changing its MAC address. By doing so, each new frame generated by the non-AP station for transmission is addressed with the @MAC(n+1) by setting the TA field value to the @MAC (n+1) value. It is to be noted, that in the current disclosure, the new address was already set for reception during the forward margin period. This means that, contrary to the prior art, step 300 sets the new address for the generation of frames to be transmitted. On the AP side, step 300 set the effective MAC address value of the non- AP station to the @MAC(n+1) in its internal list of registered non-AP stations. As a consequence, each new frame generated by the AP to be transmitted to the non-AP station will be addressed using the @MAC(n+1) by setting the RA field of the frame with the @MAC(n+1) of the non-AP station.

[0100] After step 300 is executed, step 310 is executed and frames that was buffered for transmission prior the execution of the step 300 are sent in priority. This step allows to avoid discarding the frames already buffered and addressed with the @MAC(n) and then avoid breaking on-going transmission. During a transition period, both @MAC(n) and @MAC(n+1) are considered fully valid (addresses are valid in received frames and transmitted frames).

[0101] In some embodiments, the transition period is null (RCM start time = RCM End time). This means that buffered traffic with @MAC(n) are not transmitted in priority and will be discarded at step 410 of Figure 4. This advantageously allows simpler implementations.

[0102] Figure 4 illustrates an example of steps carried out by a non-AP station or by an AP station for closing the period of change of the MAC address of a non-AP-station, meaning at the end of the transition period, according to some embodiments of the invention.

[0103] The steps of the algorithm of Figure 4 are executed at the end of the transition period, for example at max SERCM change end time that corresponds to the transition end times 625 or 616 of Figure 6.

[0104] At step 400, the station removes the current MAC address @MAC(n) from the list of MAC address that are valid for transmitted frames. This means that after execution of step 400, the station is not allowed anymore to send a frame that is addressed using the current address @MAC(n). For example, the AP cannot send a frame to the non-AP station by setting the RA field of the transmitted frame to the @MAC(n) value, and non- AP station shall not send a frame to the AP with a TA field set to the current MAC address @MAC(n). It is to be noted that the current MAC address @MAC(n) stay valid for received frames during the backward margin period, which was not the case in the prior art.

[0105] At step 410, since the emission of frames addressed with the current MAC address is not allowed anymore, potential frames buffered for transmission or retransmission addressed with the current MAC address are discarded.

[0106] After execution of step 410, the backward margin period begins according to embodiments. During this period, all transmitted frames are transmitted with the new MAC address @MAC(n+1), while received frames can still use the current or the new MAC address.

[0107] In summary, the algorithms of Figures 2a, 2b, 3 and 4 aim at achieving the following behaviour at a station, non-AP or AP according to embodiments of the invention:

[0108] Before the RCM ready time, meaning before the forward margin period, only the current MAC address is valid for both transmitted and received frames.

[0109] At the RCM ready time, the station begins to accept received frames with the new MAC address, the current one being still valid for transmitted frames.

[0110] At the beginning of the transition period (if present), the new MAC address is used for generating frames to be transmitted, while both addresses are valid for received and transmitted frames.

[0111] At the end of the transition period corresponding to the beginning of the backward margin period, only the new MAC address can be used for transmitting frames while both addresses are valid for receiving frames.

[0112] At the end of the backward margin period, only the new MAC address is valid for both transmitted and received frames.

[0113] Figure 5 illustrates a first example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station, according to some embodiments.

[0114] The MAC address changing procedure basically comprises two phases: a first phase during which new MAC addresses (and potentially all associated parameters that need to be obfuscated like the sequence numbers for instance) for one or more non-AP stations are computed and during which the AP station and the one or more non-AP stations identify the effective change start time that corresponds to the beginning of a usage period. Then a second phase corresponds to an effective change of the MAC addresses of these non-AP stations (when the usage period starts). The effective change of a MAC address starts at a time called SERCM change start time and ends at the latest at a time called maximum SERCM change end time, from which the new calculated MAC address is used for data exchanges between the considered non-AP stations and the AP station with which they are associated. Therefore, during the changing procedure, each considered non-AP station changes its MAC address from a current value @MAC(n) to a new value @MAC(n+1). During a transition period, both current and new values for the MAC address are valid.

[0115] The SERCM change start time 510, representing the beginning of the transition period, can be determined either by determining an absolute instant of change (typically by determining a specific value of the TSF), or by determining a specific beacon transmission (typically by determining a beacon index value that correspond to the instant of change).

[0116] The new MAC address(es) must be calculated by the considered non-AP station(s) and by the AP station prior to its effective usage during the transition period.

[0117] At the SERCM change start time 510, the non-AP station(s) and the AP station start the effective change address procedure and, at the maximum SERCM change end time 515 (or earlier), they modify their respective registry by updating the MAC address of each of the considered non-AP stations from @MAC(n) to @MAC(n+1).

[0118] In other words, at the starting of the transition period, both AP and non-AP STA initiate the MAC address change for the changing non-AP STA.

[0119] The way the AP station and the non-AP stations determine the new MAC addresses to be used is out of scope of the disclosure. The AP station and the non-AP stations may, for instance, store a list of MAC addresses, and each time a change of MAC address must be performed, the next value in the list is chosen as the new MAC address. Such embodiments could however present security problems, if a third party had access to the list. Alternatively, the same function may be used by a non-AP station and the AP station with which it is associated to determine, for example, an index of the next MAC address to use in a predetermined list of MAC addresses. This index may be advantageously determined randomly. Another MAC address selection method can be used. For example, it may be based on the usage of a pseudorandom function (PRF) with the same input parameters. Therefore, both a non-AP station and the AP station with which it is associated obtain the same address value @MAC(n+1).

[0120] In order to guaranty the effectiveness of the MAC address change with regard to the privacy enhancement of the stations, the station changing its MAC address shall also obfuscate a set of parameters (for example the sequence numbers, scrambler seed, AID) that can help a potential eavesdropper to track the station.

[0121] Back to Figure 5 and by reference to Figure 1 , it is illustrated a changing procedure initiated by AP station 105 and intended to non-AP stations 110a and 110b of the BSS for which the initiation procedure has been performed. In other words, according to these embodiments, AP station 105 indicates to non-AP stations 110a and 110b for which the initiation procedure has been performed that they have to change their respective MAC addresses, and non-AP stations 110a and 110b initiate the change of their respective MAC addresses at the same time (the SERCM change start time 510).

[0122] For the sake of illustration, the SERCM change start time 510 may be expressed in terms of a number of Target Beacon Transmission Times (TBTTs) (as illustrated in Figure 5). Of course, the SERCM change start time 510 may be expressed differently, for example as an actual time based on the TSF transmitted in the beacons.

[0123] According to IEEE 802.11 standards, an AP station periodically (every TBTT) transmits beacon frames to the non-AP stations of the BSS, which are management frames containing information relative to the network. Therefore, beacon frames may include a field storing an item of information to indicate a SERCM change date. For example, such an item of information may represent a value of a counter that is decremented within each successive beacon frame transmitted by the AP station so as to indicate that a change of MAC address is in progress and to indicate the SERCM change date. For example, each beacon frame transmitted by AP station 105 to non-AP stations 110a and 110b of the BSS in addition to the TSF timer, indicated in every beacon for time synchronization of the associated non-AP STA, may include a beacon counter updated at each beacon transition (increased or decreased) as illustrated by beacons 500, 505 and 510.

[0124] The new addresses of non-AP stations 110a and 110b are determined at any time between steps 500 and 510, that it to say after requesting the MAC address change and before starting the effective MAC address change. All the transmissions between AP station 105 and non-AP stations 110a and 110b, occurring after max. SERCM change end time 515, are then performed with the new MAC addresses of non-AP stations 110a and 110b.

[0125] The transition duration (referenced 520) between SERCM change start time 510 and max. SERCM change end time 515 advantageously allows the AP station and the non-AP stations to transmit the frames addressed with the current MAC address @MAC(n) that were buffered in their transmission buffer. This allows to change the MAC address without breaking the on-going transmissions, making it possible that frequent MAC address changes do not have negative effects on the performances.

[0126] Even if the embodiments described by reference to Figure 5 use beacon frames, it has to be understood that other types of frames may be used similarly, for instance using an absolute time for the SERCM change start time.

[0127] Figure 6 illustrates an example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station, according to some embodiments of the invention applied to a case of a non-AP station clock being late compared to the AP.

[0128] Figure 6 then illustrates from the point of view of a non-AP station the addresses that are valid for emission (TA filed value of the transmitted frames) or reception (RA field value of the received frames) along the different steps of the Randomized and changing MAC address procedure according to some embodiment of the invention.

[0129] For sake of simplicity, only the point of view on the non-AP station is described in this Figure, but the steps executed on the AP side at the different instant of the SRCM procedure are described in more detail in Figures 2a, 2b, 3 and 4.

[0130] In Figure 6, the time difference between the non-AP station and the AP station is indicated by the non-AP clock drift 630.

[0131] In this example, the non-AP station is late so its RCM ready time 605 occurs after the AP RCM ready time 600.

[0132] At instant 605, the non-AP station executed the steps 200 and 210 of the Figure 2a to allow the non-AP station to be ready to receive frames that are already addressed to it using the new MAC address, @MAC(n+1), even if the non-AP station hasn’t started the usage period. This mechanism allows for instance frames sent by the AP at the beginning of its transition period, and addressed by the new MAC address @MAC(n+1), to be correctly decoded by the non-AP station despite its late internal clock. It is here interesting to notice that in an alternative embodiment, the non-AP station receiving a frame addressed with the new @MAC(n+1) after the RCM ready time and before its future transition start time, may determine that it is late compared to the AP and may shift its transition period and RCM done time by the difference between its theorical future transition start time 611 and the instant of reception of the first frame sent by the AP and addressed with the new MAC address @MAC(n+1). Clock of the non-AP station may be adjusted accordingly. The clock drift between the AP and the non-AP station may then be reduced.

[0133] This alternative embodiment applicable only on the non-AP side allows the non- AP station to shorten its SERCM procedure and then to free its internal memory earlier. In addition, if all non-AP stations implement such alternative embodiment, the AP may drastically reduce the time between the end of its transition period 615 and its RCM done time 610 since the probability for the AP to receive a frame addressed with the current MAC address @MAC(n) after the end of its transition period is very low (all non-AP station shifting their transition period if they are late). This may reduce the time the AP have to maintain in memory the current and new MAC addresses (and associated parameters) for all the associated non-AP station.

[0134] In another embodiment, the transition period may be null. In this embodiment, if there are some frames already addressed with an old MAC address that are buffered for transmission at non-AP or AP side, those frames will be discarded at the starting time of the usage period (thus end time of the transition period is assumed to occur immediately after the start time).

[0135] At instant 611 , the non-AP station initiates the effective change of MAC address entering the usage period (and transition period) by executing steps 300 and 310 of Figure 3. After steps 300 and 310 are executed, both the current and new MAC addresses (@MAC(n) and @MAC(n+1) are fully valid at the same time. This means that during the transition period, the non-AP station can send or receive frames addressed with any of the fully valid MAC addresses. Typically, the non-AP station can receive frames with an RA field set to the @MAC(n) or @MAC(n+1), and can send frames with a TA field set to the @MAC(n) or @MAC(n+1).

[0136] At the instant 616, the non-AP station ends the transition period by executing steps 400 and 410 of Figure 5. After the end of the transition period, (instant 616), the non-AP station may receive frames from the AP with a RA field addressed with the current MAC address @MAC(n), but should not send a frame to the AP with a TA field set to the current MAC address @MAC (n) anymore and should only use the new MAC address @MAC(n+1) instead.

[0137] At instant 615, the non-AP station execute the steps 220 and 230 of Figure 2b to terminate the SRCM procedure and free its internal memory.

[0138] Figure 7 illustrates an example of a sequence of steps for operating a procedure for changing the MAC address of a non-AP station associated with an AP station, according to some embodiments of the invention applied to a case of a non-AP station clock being in advance compared to the AP.

[0139] This Figure is very similar to Figure 6 and highlights the case where the non-AP station is in advance compared to the AP station.

[0140] The steps executed at the different instant of the SERCM procedure are the same as the ones described in relation to Figure 6 except that the alternative embodiment to correct the clock drift, based on the analysis of what MAC addresses are used by the AP, may be implemented differently. For example, receiving one or more frames from the AP addressed with an old MAC address after the transition period ends, means that the non-AP station is in advance relatively to the AP. The non-AP may consider then the time of receiving those frames (or preferably the last received frame) as a reference to estimate a clock drift and make an appropriate adjustment to reduce it.

[0141] Note that if the RCM done time is appropriate determined, no frame addressed with an old MAC address should be received by the non-AP station after the RCM done time.

[0142] Figure 8 illustrates the handling of TA and RA addresses both at the non-AP station and AP sides. AP’s clock is considered as the reference. Clock drift may be positive or negative. The considered margins are set to handle both situations. Transition period is not represented (may or may not be present). Focus is made on the periods of validity and / or use of the MAC addresses. The usage periods (n) are for example the Epoch periods.

[0143] At the non-AP station side:

[0144] • TA address of frames sent by STA;

[0145] • Valid RA addresses of frames received by STA from AP.

[0146] At the AP side:

[0147] Valid TA addresses of frames received by AP from STA;

[0148] RA address of frames sent by AP to STA. In some embodiment, we can have a regular variable epoch period (~ 10 min);

[0149] • Randomized around a 10 minutes average value.

[0150] • Computed, without information exchange between STA and AP, the STA MAC address change start time.

[0151] The goal of such embodiment is to make it difficult for eavesdroppers to determine the instant of change with no overhead.

[0152] The RCM Procedure synchronization (“margin” management) can then be summarized as follow:

[0153] • Before epoch transition: RCM Ready: p x TBTT before the Epoch transition.

[0154] • Station sends MPDlls addressed only with Old MAC address.

[0155] • Station should be ready to receive MPDlls addressed with Old MAC address or (future) new MAC address.

[0156] • Goal: avoid synchro issue (non-AP STA late)

[0157] • At epoch transition: duration ~ n x TBTT

[0158] • Retransmission of old MPDll uses param from old epoch.

[0159] • A-MPDll Aggregation, TXOP contains either only old MPDlls or only new MPDUs.

[0160] • Station may send buffered MPDUs already addressed with Old MAC address.

[0161] • Station sends new MPDUs addressed with new MAC address.

[0162] • Station may receive MPDUs addresses with Old or new MAC address

[0163] • Goal: allow soft transition (no communication break)

[0164] • After epoch transition: RCM Done: p x TBTT after transition

[0165] • Station sends MPDUs addressed only with New MAC address.

[0166] • Station should be ready to receive MPDUs addressed with New or Old MAC address.

[0167] • Goal: avoid synchro issue (non-AP STA in advance)

[0168] In an implementation, particularly where the transition duration is long enough to ignore the backward margin, a non-AP STA or an AP STA can handle a state machine for performing embodiments of the present invention. In this implementation, for sake of clarity, we assume a station can register to an EDP Epoch sequence to indicate to its AP that the station will follow the successive changes of addresses according to the scheme of Figure 8 for instance. According to the implementation, to handle the RCM mechanism once associated, each CPE non-AP station may handle an RCM state variable that can take the following values:

[0169] • RCM Idle: FA parameters will not change until next EDP Epoch transition.

[0170] • RCM Ready: A change of FA parameters will occur in a certain period of time (referred to as dot11 EpochStartTimeMargin).

[0171] • RCM Transition: A change of FA parameters just occurred and both current and old FA parameters can be used following specific rules.

[0172] According to the implementation, the backward margin is ignored; thus, RCM Idle state corresponds to the state after epoch transition (including backward margin and RCM Done) that lasts until next RCM Ready state starts.

[0173] For the purpose of the description of this state machine, we call dot11 EpochStartTimeMargin the duration of the RCM Ready state (identified as forward margin above), and dotH EpochTransitionTime the maximum duration of the RCM Transition state.

[0174] The values of the above two durations may be stored as Management Information Base (MIB) variables in the internal memory of each device. Those values may be, for instance, transmitted by the AP during association process or in beacons, or setup with predefined default values.

[0175] Default value for the RCM State is RCM Idle state.

[0176] To account for clock drifts, all the CPE non-AP MLDs registered to an EDP Epoch Sequence may enter in RCM Ready state a dot11 EpochStartTimeMargin before the start of next EDP Epoch start time of the sequence.

[0177] The CPE AP MLD may consider all the CPE non-AP MLDs registered an EDP Epoch Sequence in RCM Ready state a dot11 EpochStartTimeMargin before the start of the next EDP Epoch of the sequence.

[0178] In RCM Ready state, each CPE non-AP MLD registered to an EDP Epoch Sequence may accept to receive from its AP-MLD:

[0179] Any frame that uses the FA parameter set established for the current EDP Epoch.

[0180] Individually addressed frames that use the FA parameter set established for the next EDP Epoch. In RCM Ready state, a CPE non-AP MLD may only transmit or re-transmit frames using FA parameters associated to the current EDP Epoch.

[0181] An AP-MLD may accept to receive from its associated non-AP MLD in RCM Ready state:

[0182] • Any frame that uses the FA parameter set established for the current EDP Epoch.

[0183] • Individually addressed frames that use the FA parameter set established for the next EDP Epoch.

[0184] A CPE AP MLD may only transmit or re-transmit frames, to a non-AP MLD in RCM Ready state, using FA parameters associated to the current EDP Epoch.

[0185] In RCM Ready state, a CPE non-AP MLD may switch to RCM Transition state at the next EDP Epoch start time of its registered EDP Epoch Sequence, for a dotH EpochTransitionTime maximum duration.

[0186] A CPE AP MLD may consider all the non-AP MLDs, registered to an EDP Epoch Sequence, in RCM Transition state at the next EDP Epoch start time of the sequence.

[0187] In RCM Transition state, a non-AP MLD may accept:

[0188] • Any frame received using the current FA parameter set.

[0189] • Individually addressed frames using the FA parameter set associated to the immediately preceding EDP Epoch in its registered EDP Epoch Sequence.

[0190] In RCM Transition state, a non-AP MLD may not transmit frames using different FA parameters in a single TXOP.

[0191] A non-AP MLD may re-transmit frames originally using FA parameter set associated to the immediately preceding EDP Epoch.

[0192] A non-AP MLD may transmit new frames using FA parameter set associated to the current EDP Epoch.

[0193] A dotH EpochTransitionTime after entering the RCM Transition state, a non-AP MLD may flush all remaining buffered traffic for transmission or re-transmission using any FA parameter set different from the current FA parameter set, and enter in RCM Idle state.

[0194] In RCM Idle state, any non-AP MLD may only accept, transmit, or re-transmit frames using the current FA Parameter set.

[0195] An AP-MLD may only accept frames using current FA Parameter set from non-

[0196] AP MLD in RCM Idle state. An AP-MLD may only transmit or re-transmit frames, to a non-AP MLD in RCM Idle state, using the current FA Parameter set.

[0197] Figure 9 illustrates an embodiment where the RCM(n-1) done and RCM(n) ready events are both set at the same time (substantially at the middle of a period in this example). In this embodiment, there are always (at least) two RA MAC addresses at the station side that are valid for reception from the AP. At the AP side, there are always (at least) two TA MAC addresses that are valid for reception from the station.

[0198] Note that more than two simultaneous MAC addresses can also be envisaged. For example, the station may have three RA MAC addresses valid at reception by setting, e.g., RCM(n) ready at the start of period n-1 and RCM(n) done at the end of period n.

[0199] This embodiment is simple to implement as the forward and backward margins (RCM ready and RCM done) do not need to be determined precisely based on the clock drift. Large enough margins (greater than worse case) just need to be considered. Also, although two MAC addresses are considered at the same time (either RA at the station side or TA at the AP side), the MAC addresses are deterministically obtained and known to belong to the right station the AP is associated with. Thus, there is no risk to accept frames not received from, or not intended to, the right targeted stations.

[0200] Figure 10 schematically illustrates an example of a communication device that may correspond any of the stations described by reference to Figure 1 , of a wireless network, configured to implement at least some embodiments of the present invention. The communication device, referenced 1000, may preferably be a device such as a micro-computer, a workstation, or a light portable device. Communication device 1000 may comprise a communication bus 1013 to which may be connected:

[0201] - a central processing unit 1001 , such as a processor, denoted CPU;

[0202] - a memory 1003, denoted MEM, for storing an executable code of methods or steps of the methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the methods; and

[0203] - at least two communication interfaces 1002 and 1002’ connected to the wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards, via transmitting and receiving antennas 1004 and 1004’, respectively. Preferably, communication bus 1013 may provide communication and interoperability between the various elements included in the communication device 1000 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 1000 directly or by means of another element of the communication device 1000.

[0204] The executable code may be stored in a memory that may either be read only, a hard disk, or on a removable digital medium such as for example a disk. According to an optional variant, the executable code of the programs can be received by means of the communication network, via the interface 1002 or 1002’, in order to be stored in the memory 1003 of communication device 1000 before being executed.

[0205] In some embodiments, communication device 1000 may be a programmable apparatus which uses software to implement embodiments of the invention. However, alternatively, some embodiments of the present invention may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC).

[0206] Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a “non-transitory computer-readable storage medium”) to perform the functions of one or more of the above-described embodiment(s) and / or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and / or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard-disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), etc.), a flash memory device, a memory card, and the like.

[0207] Expressions such as “comprise”, “include”, “incorporate”, “contain”, “is” and “have” are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa.

[0208] A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed may be combined without departing from the scope of the invention.

Claims

CLAIMS1 . A method of communication between a first station and a second station, the first station being capable of changing an identifier from a current identifier to a new identifier, the method comprising at the first station, during a margin period:- setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station for receiving frames, and- setting one of the current or the new identifier as valid identifier of the first station for emitting frames.

2. The method of claim 1 , wherein the set of valid identifiers apply to the receiver address (RA) field in received frames.

3. The method of claim 1 , wherein the one identifier applies to the transmitter address (TA) field in emitted frames.

4. A method of communication between a first station and a second station, the first station being capable of changing an identifier from a current identifier to a new identifier, the method comprising at the second station, during a margin period:- setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station for receiving frames, and- setting one of the current or the new identifier as valid identifier of the first station for emitting frames.

5. The method of claim 1 , wherein the set of valid identifiers apply to the transmitter address (TA) field in received frames.

6. The method of claim 1 , wherein the one identifier applies to the receiver address (RA) field in emitted frames.

7. The method of any preceding claims, further comprising determining a usage period start time at which the identifier of the first station is changed from the current identifier to the new identifier.

8. The method of claim 7, wherein the margin period is a forward margin period starting prior the start of the usage period.

9. The method of claim 8, further comprising, upon reception of a frame addressed to the first station with the new identifier during the forward margin period, shifting forward the start time of the usage period, or a following usage period, based on the time difference between the reception of the frame and the determined usage period start time.

10. The method of claim 7, wherein the margin period is a backward margin period ending after the start of the usage period.

11. The method of claim 10, further comprising, upon reception of a frame addressed to the first station with the current identifier during the backward margin period, shifting backward the start time of a following usage period based on the time of reception of the frame.

12. The method of any preceding claims wherein the first station is a non-access point (AP) station and the second station is an AP station.

13. The method of any preceding claims wherein station identifiers are MAC addresses.

14. A station comprising: means for changing an identifier of the first station from a current identifier to a new identifier; means for setting a set of identifiers, including the current and the new identifiers, as valid identifiers of the first station, during a margin period, for receiving frames, and means for setting one of the current or the new identifier as valid identifier of the first station, during the margin period, for emitting frames.

15. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless device, causes the wireless device to perform the method of Claim 1 or 4.

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