State of Affairs for Randomized and Changing Media Access Control (MAC) Addresses
RFC 9724



Internet Engineering Task Force (IETF) JC. Zúñiga
Request for Comments: 9724 Cisco
Category: Informational CJ. Bernardos, Ed.
ISSN: 2070-1721 UC3M
 A. Andersdotter
 Safespring AB
 March 2025

State of Affairs for Randomized and Changing Media Access Control (MAC)
 Addresses

Abstract

 Internet users are becoming more aware that their activity over the
 Internet leaves a vast digital footprint, that communications might
 not always be properly secured, and that their location and actions
 can be tracked. One of the main factors that eases tracking of
 Internet users is the wide use of long-lasting, and sometimes
 persistent, identifiers at various protocol layers. This document
 focuses on Media Access Control (MAC) addresses.

 There have been several initiatives within the IETF and the IEEE 802
 standards committees to address some of the privacy issues involved.
 This document provides an overview of these activities to help
 coordinate standardization activities in these bodies.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.

 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Not all documents
 approved by the IESG are candidates for any level of Internet
 Standard; see Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9724.

Copyright Notice

 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document. Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document. Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1. Introduction
 2. Background
 2.1. MAC Address Usage
 2.2. MAC Address Randomization
 2.3. Privacy Workshop, Tutorial, and Experiments at IETF and
 IEEE 802 Meetings
 3. Activities Relating to Randomized and Changing MAC Addresses in
 the IEEE 802
 4. Recent Activities Related to MAC Address Randomization in the
 WBA
 5. IPv6 Address Randomization in the IETF
 6. Taxonomy of MAC Address Selection Policies
 6.1. Per-Vendor OUI MAC Address (PVOM)
 6.2. Per-Device Generated MAC Address (PDGM)
 6.3. Per-Boot Generated MAC Address (PBGM)
 6.4. Per-Network Generated MAC Address (PNGM)
 6.5. Per-Period Generated MAC Address (PPGM)
 6.6. Per-Session Generated MAC Address (PSGM)
 7. OS Current Practices
 8. IANA Considerations
 9. Security Considerations
 10. Informative References
 Acknowledgments
 Authors' Addresses

1. Introduction

 Privacy is becoming a huge concern, as more and more devices are
 connecting to the Internet either directly (e.g., via Wi-Fi) or
 indirectly (e.g., via a smartphone using Bluetooth). This ubiquitous
 connectivity, together with the lack of proper education about
 privacy, makes it very easy to track/monitor the location of users
 and/or eavesdrop on their physical and online activities. This is
 due to many factors, such as the vast digital footprint that users
 leave on the Internet with or without their consent and the weak (or
 even null) authentication and encryption mechanisms used to secure
 communications. A digital footprint may include information shared
 on social networks, cookies used by browsers and servers for various
 reasons, connectivity logs that allow tracking of a user's Layer 2
 (L2) address (i.e., MAC address) or Layer 3 (L3) address, web
 trackers, etc.

 Privacy concerns affect all layers of the protocol stack, from the
 lower layers involved in the access to the network (e.g., MAC/L2 and
 L3 addresses can be used to obtain the location of a user) to higher-
 layer protocol identifiers and user applications [CSCN2015]. In
 particular, IEEE 802 MAC addresses have historically been an easy
 target for tracking users [wifi_tracking].

 There have been several initiatives within the IETF and the IEEE 802
 standards committees to address some of these privacy issues. This
 document provides an overview of these activities to help coordinate
 standardization activities within these bodies.

2. Background

2.1. MAC Address Usage

 Most mobile devices used today are Wi-Fi enabled (i.e., they are
 equipped with an IEEE 802.11 wireless local area network interface).
 Like any other kind of network interface based on IEEE 802 such as
 Ethernet (i.e., IEEE 802.3), Wi-Fi interfaces have an L2 address
 (also referred to as a MAC address) that can be seen by anybody who
 can receive the radio signal transmitted by the network interface.
 The format of these addresses (for 48-bit MAC addresses) is shown in
 Figure 1.

 +--------+--------+---------+--------+--------+---------+
 | Organizationally Unique | Network Interface |
 | Identifier (OUI) | Controller (NIC) Specific |
 +--------+--------+---------+--------+--------+---------+
 / \
 / \
 / \ b0 (I/G bit):
 / \ 0: unicast
 / \ 1: multicast
 / \
 / \ b1 (U/L bit):
 +--+--+--+--+--+--+--+--+ 0: globally unique (OUI enforced)
 |b7|b6|b5|b4|b3|b2|b1|b0| 1: locally administered
 +--+--+--+--+--+--+--+--+

 Figure 1: IEEE 802 MAC Address Format (for 48-Bit Addresses)

 MAC addresses can be either universally or locally administered.
 Universally and locally administered addresses are distinguished by
 setting the second least significant bit of the most significant byte
 of the address (the U/L bit).

 A universally administered address is uniquely assigned to a device
 by its manufacturer. Most physical devices are provided with a
 universally administered address, which is composed of two parts:

 Organizationally Unique Identifier (OUI): The first three octets in
 transmission order, which identify the organization that issued
 the identifier.

 Network Interface Controller (NIC) Specific: The following three
 octets, which are assigned by the organization that manufactured
 the NIC, in such a way that the resulting MAC address is globally
 unique.

 Locally administered addresses override the burned-in address, and
 they can be set up by either the network administrator or the
 Operating System (OS) of the device to which the address pertains.
 However, as explained in later sections of this document, there are
 new initiatives in the IEEE 802 and other organizations to specify
 ways in which these locally administered addresses should be
 assigned, depending on the use case.

2.2. MAC Address Randomization

 Since universally administered MAC addresses are by definition
 globally unique, when a device uses this MAC address over a shared
 medium to transmit data -- especially over the air -- it is
 relatively easy to track this device by simple medium observation.
 Since a device is usually directly associated to an individual, this
 poses a privacy concern [link_layer_privacy].

 MAC addresses can be easily observed by a third party, such as a
 passive device listening to communications in the same L2 network.
 In an 802.11 network, a device (also known as an IEEE 802.11 station
 or STA) exposes its MAC address in two different situations:

 * While actively scanning for available networks, the MAC address is
 used in the Probe Request frames sent by the device.

 * Once associated to a given Access Point (AP), the MAC address is
 used in frame transmission and reception, as one of the addresses
 used in the unicast address fields of an IEEE 802.11 frame.

 One way to address this privacy concern is by using randomly
 generated MAC addresses. IEEE 802 addressing includes one bit to
 specify if the hardware address is locally or globally administered.
 This allows local addresses to be generated without the need for any
 global coordination mechanism to ensure that the generated address is
 still unique within the local network. This feature can be used to
 generate random addresses, which decouple the globally unique
 identifier from the device and therefore make it more difficult to
 track a user device from its MAC/L2 address
 [enhancing_location_privacy].

 Note that there are reports [contact_tracing_paper] of some mobile
 OSes reporting persistently (every 20 minutes or so) on MAC addresses
 (as well as other information), which would defeat MAC address
 randomization. While these practices might have changed by now, it
 is important to highlight that privacy-preserving techniques should
 be conducted while considering all layers of the protocol stack.

2.3. Privacy Workshop, Tutorial, and Experiments at IETF and IEEE 802
 Meetings

 As an outcome to the STRINT W3C/IAB Workshop [strint], a tutorial
 titled "Pervasive Surveillance of the Internet - Designing Privacy
 into Internet Protocols" [privacy_tutorial] was given at the IEEE 802
 Plenary meeting in San Diego in July of 2014. The tutorial provided
 an update on the recent developments regarding Internet privacy, the
 actions undertaken by other Standards Development Organizations
 (SDOs) like the IETF, and guidelines that were being followed when
 developing new Internet protocol specifications (e.g., the
 considerations described in [RFC6973]). The tutorial highlighted
 some privacy concerns that apply specifically to link-layer
 technologies and provided suggestions on how IEEE 802 could help
 address them.

 Following the discussions and interest within the IEEE 802 community,
 on 18 July 2014, the IEEE 802 Executive Committee (EC) created the
 IEEE 802 EC Privacy Recommendation Study Group (SG)
 [ieee_privacy_ecsg]. The work and discussions from the group have
 generated multiple outcomes, such Project Authorization Requests
 (PARs) that resulted in the following documents:

 * "IEEE Recommended Practice for Privacy Considerations for IEEE
 802(R) Technologies" [IEEE_802E]

 * "IEEE Standard for Local and Metropolitan Area Networks: Overview
 and Architecture - Amendment 2: Local Medium Access Control (MAC)
 Address Usage" [IEEE_802c]

 In order to test the effects of MAC address randomization,
 experiments were conducted at the IETF and IEEE 802 meetings between
 November 2014 and March 2015 -- IETF 91, IETF 92, and the IEEE 802
 Plenary in Berlin. The purpose of the experiments was to evaluate
 the use of MAC address randomization from two different perspectives:
 (1) the effect on the connectivity experience of the end user, as
 well as any effect on applications and OSes, and (2) the potential
 impact on the network infrastructure itself. Some of the findings
 were published in [CSCN2015].

 During the experiments, it was observed that the probability of
 address duplication in a network is negligible. The experiments also
 revealed that other protocol identifiers (e.g., the DHCP client
 identifier) can be correlated and can therefore still be used to
 track an individual. Hence, effective privacy tools should not work
 in isolation at a single layer; instead; they should be coordinated
 with other privacy features at higher layers.

 Since then, MAC address randomization has been further implemented by
 mobile OSes to provide better privacy for mobile phone users when
 connecting to public wireless networks [privacy_ios]
 [privacy_windows] [privacy_android].

3. Activities Relating to Randomized and Changing MAC Addresses in the
 IEEE 802

 Practical experiences with Randomized and Changing MAC addresses
 (RCM) in devices (some of which are explained in Section 6) helped
 researchers fine-tune their understanding of attacks against
 randomization mechanisms [when_mac_randomization_fails]. Within the
 IEEE 802.11 group, these research experiences eventually formed the
 basis for a specified mechanism that randomizes MAC addresses, which
 was introduced in IEEE Std 802.11aq [IEEE_802.11aq] in 2018.

 More recent developments include turning on MAC address randomization
 in mobile OSes by default, which has an impact on the ability of
 network operators to customize services [rcm_user_experience_csd].
 Therefore, follow-on work in the IEEE 802.11 mapped effects of a
 potentially large uptake of randomized MAC identifiers on a number of
 commonly offered operator services in 2019 [rcm_tig_final_report].
 In the summer of 2020, this work emanated in two new standards
 projects. The purpose of these projects was to develop mechanisms
 that do not decrease user privacy but enable an optimal user
 experience when (1) the MAC address of a device in an Extended
 Service Set (a group of interconnected IEEE 802.11 wireless access
 points and stations that form a single logical network) is randomized
 or changes [rcm_user_experience_par] and (2) user privacy solutions
 described in IEEE Std 802.11 [rcm_privacy_par] apply.

 IEEE Std 802 [IEEE_802], as of the amendment IEEE 802c-2017
 [IEEE_802c], specifies a local MAC address space structure known as
 the Structured Local Address Plan (SLAP) [RFC8948]. The SLAP
 designates a range of Extended Local Identifiers for subassignment
 within a block of addresses assigned by the IEEE Registration
 Authority via a Company ID. A range of local MAC addresses is
 designated for Standard Assigned Identifiers to be specified by IEEE
 802 standards. Another range of local MAC addresses is designated
 for Administratively Assigned Identifiers, which are subject to
 assignment by a network administrator.

 IEEE Std 802E-2020 ("IEEE Recommended Practice for Privacy
 Considerations for IEEE 802(R) Technologies") [IEEE_802E] recommends
 the use of temporary and transient identifiers if there are no
 compelling reasons for a newly introduced identifier to be permanent.
 This recommendation is part of the basis for the review of user
 privacy solutions for IEEE Std 802.11 devices (also known as Wi-Fi
 devices) as part of the RCM efforts [rcm_privacy_csd]. Annex I of
 IEEE Std 802.1AEdk-2023 ("MAC Privacy Protection") [IEEE_802.1AEdk]
 discusses privacy considerations in bridged networks.

 As of 2024, two task groups in IEEE 802.11 are dealing with issues
 related to RCM:

 * The IEEE 802.11bh task group, which is looking at mitigating the
 repercussions that RCM creates on 802.11 networks and related
 services.

 * The IEEE 802.11bi task group, which is chartered to define
 modifications to the IEEE Std 802.11 MAC specification
 [IEEE_802.11] to specify new mechanisms that address and improve
 user privacy.

4. Recent Activities Related to MAC Address Randomization in the WBA

 In the Wireless Broadband Alliance (WBA), the Testing and
 Interoperability Work Group has been looking at issues related to MAC
 address randomization and has identified a list of potential impacts
 of these changes to existing systems and solutions, mainly related to
 Wi-Fi identification.

 As part of this work, the WBA has documented a set of use cases that
 a Wi-Fi Identification Standard should address in order to scale and
 achieve longer-term sustainability of deployed services (see
 [wba_paper]).

5. IPv6 Address Randomization in the IETF

 [RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
 IPv6, which typically results in hosts configuring one or more
 "stable" addresses composed of a network prefix advertised by a local
 router and an Interface Identifier (IID). [RFC8064] formally updated
 the original IPv6 IID selection mechanism to avoid generating the IID
 from the MAC address of the interface (via EUI64), as this
 potentially allowed for tracking of a device at L3. Additionally,
 the prefix part of an IP address provides meaningful insights of the
 physical location of the device in general, which together with the
 IID based on the MAC address, made it easier to perform global device
 tracking.

 [RFC8981] identifies and describes the privacy issues associated with
 embedding MAC stable addressing information into IPv6 addresses (as
 part of the IID). It describes an extension to IPv6 SLAAC that
 causes hosts to generate temporary addresses with randomized IIDs for
 each prefix advertised with autoconfiguration enabled. Changing
 addresses over time limits the window of time during which
 eavesdroppers and other information collectors may trivially perform
 address-based network-activity correlation when the same address is
 employed for multiple transactions by the same host. Additionally,
 it reduces the window of exposure of a host as being accessible via
 an address that becomes revealed as a result of active communication.
 These temporary addresses are meant to be used for a short period of
 time (hours to days) and then deprecated. Deprecated addresses can
 continue to be used for already-established connections but are not
 used to initiate new connections. New temporary addresses are
 generated periodically to replace temporary addresses that expire.
 To generate temporary addresses, a node produces a sequence of
 temporary global scope addresses from a sequence of IIDs that appear
 to be random in the sense that (1) it is difficult for an outside
 observer to predict a future address (or identifier) based on a
 current one and (2) it is difficult to determine previous addresses
 (or identifiers) knowing only the present one. Temporary addresses
 should not be used by applications that listen for incoming
 connections (as these are supposed to be waiting on permanent/well-
 known identifiers). If a node changes network and comes back to a
 previously visited one, the temporary addresses that the node would
 use will be different, which might be an issue in certain networks
 where addresses are used for operational purposes (e.g., filtering or
 authentication). [RFC7217], summarized next, partially addresses the
 problems aforementioned.

 [RFC7217] describes a method to generate IIDs that are stable for
 each network interface within each subnet but change as a host moves
 from one network to another. This method enables the "stability"
 properties of the IIDs specified in [RFC4291] to be kept, while still
 mitigating address-scanning attacks and preventing correlation of the
 activities of a host as it moves from one network to another. The
 method defined to generate the IPv6 IID is based on computing a hash
 function that takes the following as input: information that is
 stable and associated to the interface (e.g., a local IID), stable
 information associated to the visited network (e.g., the IEEE 802.11
 Service Set Identifier (SSID)), the IPv6 prefix, a secret key, and
 some other additional information. This basically ensures that a
 different IID is generated when one of the input fields changes (such
 as the network or the prefix) but that the IID is the same within
 each subnet.

 To mitigate the privacy threats posed by the use of MAC-derived IIDs,
 [RFC8064] recommends that nodes implement [RFC7217] as the default
 scheme for generating stable IPv6 addresses with SLAAC.

 In addition to the documents above, [RFC8947] proposes a DHCPv6
 extension that:

 | allows a scalable approach to link-layer address assignments where
 | preassigned link-layer address assignments (such as by a
 | manufacturer) are not possible or are unnecessary.

 And [RFC8948] proposes DHCPv6 extensions that:

 | enable a DHCPv6 client or a DHCPv6 relay to indicate a preferred
 | SLAP quadrant to the server so that the server may allocate MAC
 | addresses in the quadrant requested by the relay or client.

 In addition to MAC and IP addresses, some DHCP options that carry
 unique identifiers can also be used for tracking purposes. These
 identifiers can enable device tracking even if the device
 administrator takes care of randomizing other potential
 identifications like link-layer addresses or IPv6 addresses.
 [RFC7844] introduces anonymity profiles that are:

 | designed for clients that wish to remain anonymous to the visited
 | network

 and that:

 | provide guidelines on the composition of DHCP or DHCPv6 messages,
 | designed to minimize disclosure of identifying information.

 [RFC7844] also indicates that the link-layer address, IP address, and
 DHCP identifier shall evolve in synchrony.

6. Taxonomy of MAC Address Selection Policies

 This section documents different policies for MAC address selection.
 Some OSes might use a combination of multiple policies.

 | Note: The naming convention for the terms defined in this
 | section aligns with 802.11/Wi-Fi terminology in that the "A"
 | for "address" is not included in the acronym. For example,
 | "PVOM" stands for "Per-Vendor OUI MAC address", and "PNGM"
 | stands for "Per-Network Generated MAC address".

6.1. Per-Vendor OUI MAC Address (PVOM)

 This form of MAC address selection is the historical default.

 The vendor obtains an OUI from the IEEE. This is a 24-bit prefix
 (including two upper bits that are set specifically) that is assigned
 to the vendor. The vendor generates a unique 24-bit value for the
 lower 24 bits, forming the 48-bit MAC address. It is not unusual for
 the 24-bit value to be used as an incrementing counter that was
 assigned at the factory and burnt into non-volatile storage.

 Note that IEEE Std 802.15.4 [IEEE_802.15.4] uses 64-bit MAC
 addresses, and the IEEE assigns 32-bit prefixes. The IEEE has
 indicated that there may be a future Ethernet specification that uses
 64-bit MAC addresses.

6.2. Per-Device Generated MAC Address (PDGM)

 This form of MAC address is randomly generated by the device, usually
 upon first boot. The resulting MAC address is stored in non-volatile
 storage and is used for the rest of the device lifetime.

6.3. Per-Boot Generated MAC Address (PBGM)

 This form of MAC address is randomly generated by the device each
 time the device is booted. The resulting MAC address is *not* stored
 in non-volatile storage. It does not persist across power cycles.
 This case may sometimes be a PDGM where the non-volatile storage is
 no longer functional (or has failed).

6.4. Per-Network Generated MAC Address (PNGM)

 This form of MAC address is generated each time a new network
 attachment is created.

 This is typically used with Wi-Fi networks (i.e., 802.11 networks)
 where the network is identified by an SSID Name. The generated
 address is stored in non-volatile storage, indexed by the SSID. Each
 time the device returns to a network with the same SSID, the device
 uses the saved MAC address.

 It is possible to use PNGM for wired Ethernet connections through
 some passive observation of network traffic (such as spanning tree
 protocols [IEEE_802.1Q], the Link Layer Discovery Protocol (LLDP)
 [IEEE_802.1AB], DHCP, or Router Advertisements) to determine which
 network has been attached.

6.5. Per-Period Generated MAC Address (PPGM)

 This form of MAC address is generated periodically, typically around
 every twelve hours. Like PNGM, it is used primarily with Wi-Fi.

 When the MAC address changes, the station disconnects from the
 current session and reconnects using the new MAC address. This will
 involve a new 802.1x session, as well as obtaining or refreshing a
 new IP address (e.g., using DHCP or SLAAC).

 If DHCP is used, then a new DHCP Unique Identifier (DUID) is
 generated so as to not link to the previous connection; this usually
 results in the allocation of new IP addresses.

6.6. Per-Session Generated MAC Address (PSGM)

 This form of MAC address is generated on a per-session basis. How a
 session is defined is implementation-dependent, for example, a
 session might be defined by logging in to a portal, VPN, etc. Like
 PNGM and PPGM, it is used primarily with Wi-Fi.

 Since the address only changes when a new session is established,
 there is no disconnection/reconnection involved.

7. OS Current Practices

 By default, most modern OSes (especially mobile ones) do implement
 some MAC address randomization policies. Since the mechanism and
 policies that OSes implement can evolve with time, the content is
 hosted at <https://wiki.ietf.org/en/group/madinas/RFC9724>. For
 completeness, a snapshot of the content at the time of publication of
 this document is included below. Note that the extensive testing
 reported in this document was conducted in 2021, but no significant
 changes have been detected at the time of publication of this
 document.

 Table 1 summarizes current practices for Android and iOS at the time
 of writing this document (the original source is available at
 [private_mac]) and also includes updates based on findings from the
 authors.

 +=============================================+===================+
 | Android 10+ | iOS 14+ |
 +=============================================+===================+
 | The randomized MAC address is bound to the | The randomized |
 | SSID. | MAC address is |
 | | bound to the |
 | | Basic SSID. |
 +---------------------------------------------+-------------------+
 | The randomized MAC address is stable across | The randomized |
 | reconnections for the same network. | MAC address is |
 | | stable across |
 | | reconnections for |
 | | the same network. |
 +---------------------------------------------+-------------------+
 | The randomized MAC address does not get re- | The randomized |
 | randomized when the device forgets a Wi-Fi | MAC address is |
 | network. | reset when the |
 | | device forgets a |
 | | Wi-Fi network. |
 +---------------------------------------------+-------------------+
 | MAC address randomization is enabled by | MAC address |
 | default for all the new Wi-Fi networks. | randomization is |
 | But if the device previously connected to a | enabled by |
 | Wi-Fi network identifying itself with the | default for all |
 | real MAC address, no randomized MAC address | the new Wi-Fi |
 | will be used (unless manually enabled). | networks. |
 +---------------------------------------------+-------------------+

 Table 1: Android and iOS MAC Address Randomization Practices

 In September 2021, we performed some additional tests to evaluate how
 OSes that are widely used behave regarding MAC address randomization.
 Table 2 summarizes our findings; the rows in the table show whether
 the OS performs address randomization per network (PNGM according to
 the taxonomy introduced in Section 6), performs address randomization
 per new connection (PSGM), performs address randomization daily (PPGM
 with a period of 24 hours), supports configuration per SSID, supports
 address randomization for scanning, and supports address
 randomization for scanning by default.

 +=================+===============+=========+=========+=====+
 | OS | Linux (Debian | Android | Windows | iOS |
 | | "bookworm") | 10 | 10 | 14+ |
 +=================+===============+=========+=========+=====+
 | Random. per | Y | Y | Y | Y |
 | net. (PNGM) | | | | |
 +-----------------+---------------+---------+---------+-----+
 | Random. per | Y | N | N | N |
 | connec. (PSGM) | | | | |
 +-----------------+---------------+---------+---------+-----+
 | Random. daily | N | N | Y | N |
 | (PPGM) | | | | |
 +-----------------+---------------+---------+---------+-----+
 | SSID config. | Y | N | N | N |
 +-----------------+---------------+---------+---------+-----+
 | Random. for | Y | Y | Y | Y |
 | scan | | | | |
 +-----------------+---------------+---------+---------+-----+
 | Random. for | N | Y | N | Y |
 | scan by default | | | | |
 +-----------------+---------------+---------+---------+-----+

 Table 2: Observed Behavior in Different OSes (as of
 September 2021)

 According to [privacy_android], starting with Android 12, Android
 uses non-persistent randomization in the following situations:

 * A network suggestion application specifies that non-persistent
 randomization be used for the network (through an API).

 * The network is an open network that hasn't encountered a captive
 portal, and an internal config option is set to do so (by default,
 it is not).

8. IANA Considerations

 This document has no IANA actions.

9. Security Considerations

 Privacy considerations regarding tracking the location of a user
 through the MAC address of a device are discussed throughout this
 document. Given the informational nature of this document, no
 protocols/solutions are specified, but the current state of affairs
 is documented.

 Any future specification in this area would need to look into
 security and privacy aspects, such as (but not limited to) the
 following:

 * Mitigating the problem of location privacy while minimizing the
 impact on upper layers of the protocol stack

 * Providing the means for network operators to authenticate devices
 and authorize network access, despite the MAC addresses changing
 according some pattern

 * Providing the means for the device not to use MAC addresses that
 it is not authorized to use or that are currently in use

 A major conclusion of the work in IEEE Std 802E [IEEE_802E] concerned
 the difficulty of defending privacy against adversaries of any
 sophistication. Individuals can be successfully tracked by
 fingerprinting, using aspects of their communication other than MAC
 addresses or other permanent identifiers.

10. Informative References

 [contact_tracing_paper]
 Leith, D. J. and S. Farrell, "Contact Tracing App Privacy:
 What Data Is Shared By Europe's GAEN Contact Tracing
 Apps", IEEE INFOCOM 2021 - IEEE Conference on Computer
 Communications, DOI 10.1109/INFOCOM42981.2021.9488728, May
 2021, <https://ieeexplore.ieee.org/document/9488728>.

 [CSCN2015] Bernardos, CJ., Zúñiga, JC., and P. O'Hanlon, "Wi-Fi
 Internet Connectivity and Privacy: Hiding your tracks on
 the wireless Internet", 2015 IEEE Conference on Standards
 for Communications and Networking (CSCN),
 DOI 10.1109/CSCN.2015.7390443, October 2015,
 <https://doi.org/10.1109/CSCN.2015.7390443>.

 [enhancing_location_privacy]
 Gruteser, M. and D. Grunwald, "Enhancing Location Privacy
 in Wireless LAN Through Disposable Interface Identifiers:
 A Quantitative Analysis", Mobile Networks and
 Applications, vol. 10, no. 3, pp. 315-325,
 DOI 10.1007/s11036-005-6425-1, June 2005,
 <https://doi.org/10.1007/s11036-005-6425-1>.

 [IEEE_802] IEEE, "IEEE Standard for Local and Metropolitan Area
 Networks: Overview and Architecture", IEEE Std 802-2014,
 DOI 10.1109/IEEESTD.2014.6847097, June 2014,
 <https://doi.org/10.1109/IEEESTD.2014.6847097>.

 [IEEE_802.11]
 IEEE, "IEEE Standard for Information Technology--
 Telecommunications and Information Exchange between
 Systems - Local and Metropolitan Area Networks--Specific
 Requirements - Part 11: Wireless LAN Medium Access Control
 (MAC) and Physical Layer (PHY) Specifications", IEEE
 Std 802.11-2020, DOI 10.1109/IEEESTD.2021.9363693,
 February 2021,
 <https://doi.org/10.1109/IEEESTD.2021.9363693>.

 [IEEE_802.11aq]
 IEEE, "IEEE Standard for Information technology--
 Telecommunications and information exchange between
 systems Local and metropolitan area network--Specific
 requirements Part 11: Wireless LAN Medium Access Control
 (MAC) and Physical Layer (PHY) Specifications Amendment 5:
 Preassociation Discovery", IEEE Std 802.11aq-2018,
 DOI 10.1109/IEEESTD.2018.8457463, August 2018,
 <https://doi.org/10.1109/IEEESTD.2018.8457463>.

 [IEEE_802.15.4]
 IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
 Std 802.15.4-2024, DOI 10.1109/IEEESTD.2024.10794632,
 December 2024,
 <https://doi.org/10.1109/IEEESTD.2024.10794632>.

 [IEEE_802.1AB]
 IEEE, "IEEE Standard for Local and metropolitan area
 networks - Station and Media Access Control Connectivity
 Discovery", IEEE Std 802.1AB-2016,
 DOI 10.1109/IEEESTD.2016.7433915, March 2016,
 <https://doi.org/10.1109/IEEESTD.2016.7433915>.

 [IEEE_802.1AEdk]
 IEEE, "IEEE Standard for Local and metropolitan area
 networks-Media Access Control (MAC) Security - Amendment
 4: MAC Privacy protection", IEEE Std 802.1AEdk-2023,
 DOI 10.1109/IEEESTD.2023.10225636, August 2023,
 <https://doi.org/10.1109/IEEESTD.2023.10225636>.

 [IEEE_802.1Q]
 IEEE, "IEEE Standard for Local and Metropolitan Area
 Networks--Bridges and Bridged Networks", IEEE Std 802.1Q-
 2022, DOI 10.1109/IEEESTD.2022.10004498, December 2022,
 <https://doi.org/10.1109/IEEESTD.2022.10004498>.

 [IEEE_802c]
 IEEE, "IEEE Standard for Local and Metropolitan Area
 Networks:Overview and Architecture--Amendment 2: Local
 Medium Access Control (MAC) Address Usage", IEEE Std 802c-
 2017, DOI 10.1109/IEEESTD.2017.8016709, August 2017,
 <https://doi.org/10.1109/IEEESTD.2017.8016709>.

 [IEEE_802E]
 IEEE, "IEEE Recommended Practice for Privacy
 Considerations for IEEE 802(R) Technologies", IEEE Std 
 802E-2020, DOI 10.1109/IEEESTD.2020.9257130, November
 2020, <https://doi.org/10.1109/IEEESTD.2020.9257130>.

 [ieee_privacy_ecsg]
 IEEE 802 LAN/MAN Standards Committee, "IEEE 802 EC Privacy
 Recommendation Study Group",
 <http://www.ieee802.org/PrivRecsg/>.

 [link_layer_privacy]
 O'Hanlon, P., Wright, J., and I. Brown, "Privacy at the
 link-layer", W3C/IAB workshop on Strengthening the
 Internet Against Pervasive Monitoring (STRINT), February
 2014.

 [privacy_android]
 Android Open Source Project, "MAC randomization behavior",
 Android OS Documentation,
 <https://source.android.com/devices/tech/connect/wifi-mac-
 randomization-behavior>.

 [privacy_ios]
 Apple Inc., "Use private Wi-Fi addresses on Apple
 Devices", Apple Support,
 <https://support.apple.com/en-us/102509>.

 [privacy_tutorial]
 Cooper, A., Hardie, T., Zuniga, JC., Chen, L., and P.
 O'Hanlon, "Pervasive Surveillance of the Internet -
 Designing Privacy into Internet Protocols", IEEE 802
 Tutorial, 14 July 2014, <https://mentor.ieee.org/802-
 ec/dcn/14/ec-14-0043-01-00EC-internet-privacy-
 tutorial.pdf>.

 [privacy_windows]
 Microsoft Corporation, "How to use random hardware
 addresses in Windows", Microsoft Support,
 <https://support.microsoft.com/en-us/windows/how-to-use-
 random-hardware-addresses-ac58de34-35fc-31ff-
 c650-823fc48eb1bc>.

 [private_mac]
 Pantaleone, D., "Private MAC address on iOS 14", Wayback
 Machine archive, September 2020,
 <https://web.archive.org/web/20230905111429/
 https://www.fing.com/news/private-mac-address-on-ios-14>.

 [rcm_privacy_csd]
 IEEE 802.11 WG RCM SG, "IEEE 802.11 Randomized And
 Changing MAC Addresses Study Group CSD on user experience
 mechanisms", doc.:IEEE 802.11-20/1346r4, 2020. Download
 available at <https://mentor.ieee.org/802.11/
 dcn/20/11-20-1346-04-0rcm-csd-draft-for-privacy-amendment-
 of-rcm- project.docx>.

 [rcm_privacy_par]
 IEEE 802.11 WG RCM SG, "IEEE 802.11 Randomized And
 Changing MAC Addresses Study Group PAR on privacy
 mechanisms", doc.:IEEE 802.11-19/854r7, 2020. Download
 available at <https://mentor.ieee.org/802.11/
 dcn/20/11-20-0854-07-0rcm-par-proposal-for-privacy.docx>.

 [rcm_tig_final_report]
 IEEE 802.11 WG RCM TIG, "IEEE 802.11 Randomized And
 Changing MAC Addresses Topic Interest Group Report",
 doc.:IEEE 802.11-19/1442r9, 2019. Download available at
 <https://mentor.ieee.org/802.11/ dcn/19/11-19-1442-09-
 0rcm-rcm-tig-draft-report-outline.odt>.

 [rcm_user_experience_csd]
 IEEE 802.11 WG RCM SG, "IEEE 802.11 Randomized And
 Changing MAC Addresses Study Group CSD on user experience
 mechanisms", doc.:IEEE 802.11-20/1117r5, 2020. Download
 available at <https://mentor.ieee.org/802.11/
 dcn/20/11-20-1117-05-0rcm-rcm-sg-proposed-rcm-csd-
 draft.docx>.

 [rcm_user_experience_par]
 IEEE 802.11 WG RCM SG, "IEEE 802.11 Randomized And
 Changing MAC Addresses Study Group PAR on user experience
 mechanisms", doc.:IEEE 802.11-20/742r6, 2020. Download
 available at <https://mentor.ieee.org/802.11/
 dcn/20/11-20-0742-06-0rcm-proposed-par-draft.docx>.

 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
 Architecture", RFC 4291, DOI 10.17487/RFC4291, February
 2006, <https://www.rfc-editor.org/info/rfc4291>.

 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
 Address Autoconfiguration", RFC 4862,
 DOI 10.17487/RFC4862, September 2007,
 <https://www.rfc-editor.org/info/rfc4862>.

 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
 Morris, J., Hansen, M., and R. Smith, "Privacy
 Considerations for Internet Protocols", RFC 6973,
 DOI 10.17487/RFC6973, July 2013,
 <https://www.rfc-editor.org/info/rfc6973>.

 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque
 Interface Identifiers with IPv6 Stateless Address
 Autoconfiguration (SLAAC)", RFC 7217,
 DOI 10.17487/RFC7217, April 2014,
 <https://www.rfc-editor.org/info/rfc7217>.

 [RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
 Profiles for DHCP Clients", RFC 7844,
 DOI 10.17487/RFC7844, May 2016,
 <https://www.rfc-editor.org/info/rfc7844>.

 [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
 "Recommendation on Stable IPv6 Interface Identifiers",
 RFC 8064, DOI 10.17487/RFC8064, February 2017,
 <https://www.rfc-editor.org/info/rfc8064>.

 [RFC8947] Volz, B., Mrugalski, T., and CJ. Bernardos, "Link-Layer
 Address Assignment Mechanism for DHCPv6", RFC 8947,
 DOI 10.17487/RFC8947, December 2020,
 <https://www.rfc-editor.org/info/rfc8947>.

 [RFC8948] Bernardos, CJ. and A. Mourad, "Structured Local Address
 Plan (SLAP) Quadrant Selection Option for DHCPv6",
 RFC 8948, DOI 10.17487/RFC8948, December 2020,
 <https://www.rfc-editor.org/info/rfc8948>.

 [RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
 "Temporary Address Extensions for Stateless Address
 Autoconfiguration in IPv6", RFC 8981,
 DOI 10.17487/RFC8981, February 2021,
 <https://www.rfc-editor.org/info/rfc8981>.

 [strint] W3C/IAB, "STRINT Workshop: A W3C/IAB workshop on
 Strengthening the Internet Against Pervasive Monitoring
 (STRINT)", <https://www.w3.org/2014/strint/>.

 [wba_paper]
 Wireless Broadband Alliance, "Wi-Fi Device Identification
 - A Way Through MAC Randomization", WBA White Paper, July
 2022, <https://wballiance.com/resource/wi-fi-device-
 identification-a-way-through-mac-randomization/>.

 [when_mac_randomization_fails]
 Martin, J., Mayberry, T., Donahue, C., Foppe, L., Brown,
 L., Riggins, C., Rye, E., and D. Brown, "A Study of MAC
 Address Randomization in Mobile Devices and When it
 Fails", arXiv:1703.02874v2, DOI 10.48550/arXiv.1703.02874,
 March 2017, <https://doi.org/10.48550/arXiv.1703.02874>.

 [wifi_tracking]
 Vincent, J., "London's bins are tracking your smartphone",
 The Independent, 9 August 2013,
 <https://www.independent.co.uk/life-style/gadgets-and-
 tech/news/updated-london-s-bins-are-tracking-your-
 smartphone-8754924.html>.

Acknowledgments

 The authors would like to thank Guillermo Sanchez Illan for the
 extensive tests performed on different OSes to analyze their behavior
 regarding address randomization.

 The authors would also like to thank Jerome Henry, Hai Shalom,
 Stephen Farrell, Alan DeKok, Mathieu Cunche, Johanna Ansohn
 McDougall, Peter Yee, Bob Hinden, Behcet Sarikaya, David Farmer,
 Mohamed Boucadair, Éric Vyncke, Christian Amsüss, Roman Danyliw,
 Murray Kucherawy, and Paul Wouters for their reviews and comments on
 previous draft versions of this document. In addition, the authors
 would like to thank Michael Richardson for his contributions on the
 taxonomy section. Finally, the authors would like to thank the IEEE
 802.1 Working Group for its review and comments (see
 <https://datatracker.ietf.org/liaison/1884/>).

Authors' Addresses

 Juan Carlos Zúñiga
 Cisco
 Montreal QC
 Canada
 Email: juzuniga@cisco.com

 Carlos J. Bernardos (editor)
 Universidad Carlos III de Madrid
 Av. Universidad, 30
 28911 Leganes, Madrid
 Spain
 Phone: +34 91624 6236
 Email: cjbc@it.uc3m.es
 URI: http://www.it.uc3m.es/cjbc/

 Amelia Andersdotter
 Safespring AB
 Email: amelia.ietf@andersdotter.cc