Randomized and Changing MAC Address State of Affairs
draft-ietf-madinas-mac-address-randomization-15

MADINAS                                                       JC. Zúñiga
Internet-Draft                                                     CISCO
Intended status: Informational                        CJ. Bernardos, Ed.
Expires: 16 January 2025                                            UC3M
                                                         A. Andersdotter
                                                           Safespring AB
                                                            15 July 2024

          Randomized and Changing MAC Address State of Affairs
            draft-ietf-madinas-mac-address-randomization-15

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 Internet
   users is the wide use of long-lasting, and sometimes persistent,
   identifiers at various protocol layers.  This document focuses on MAC
   addresses.

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

Status of This Memo

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   This Internet-Draft will expire on 16 January 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  MAC address usage . . . . . . . . . . . . . . . . . . . .   3
     2.2.  MAC address randomization . . . . . . . . . . . . . . . .   4
     2.3.  Privacy Workshop, Tutorial and Experiments at IETF and IEEE
           802 meetings  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Randomized and Changing MAC addresses activities at the IEEE
           802 . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Recent MAC randomization-related activities at the WBA  . . .   7
   5.  IPv6 address randomization at the IETF  . . . . . . . . . . .   8
   6.  A taxonomy of MAC address selection policies  . . . . . . . .   9
     6.1.  Per-Vendor OUI MAC address (PVOM) . . . . . . . . . . . .  10
     6.2.  Per-Device Generated MAC address (PDGM) . . . . . . . . .  10
     6.3.  Per-Boot Generated MAC address (PBGM) . . . . . . . . . .  10
     6.4.  Per-Network Generated MAC address (PNGM)  . . . . . . . .  10
     6.5.  Per-Period Generated MAC address (PPGM) . . . . . . . . .  11
     6.6.  Per-Session Generated MAC address (PSGM)  . . . . . . . .  11
   7.  OS current practices  . . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   11. Informative References  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Privacy is becoming a huge concern, as more and more devices are
   getting directly (e.g., via Wi-Fi) or indirectly (e.g., via a
   smartphone using Bluetooth) connected to the Internet.  This
   ubiquitous connectivity, together with the lack of proper education
   about privacy make it very easy to track/monitor the location of
   users and/or eavesdrop 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, for instance
   sharing information on social networks, cookies used by browsers and
   servers for various reasons, connectivity logs that allow tracking of
   a user's Layer-2 (L2/MAC) or Layer-3 (L3) address, web trackers,

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   etc.; and/or the weak (or even null in some cases) authentication and
   encryption mechanisms used to secure communications.

   This privacy concern affects all layers of the protocol stack, from
   the lower layers involved in the access to the network (e.g., the
   MAC/Layer-2 and Layer-3 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 at the IETF and the IEEE 802
   standards committees to overcome some of these privacy issues.  This
   document provides an overview of these activities to help
   coordinating standardization activities within these bodies.

2.  Background

2.1.  MAC address usage

   Most mobile devices used today are WLAN enabled (i.e., they are
   equipped with an IEEE 802.11 wireless local area network interface).
   Wi-Fi interfaces, as any other kind of IEEE 802-based network
   interface, like Ethernet (i.e., IEEE 802.3) have a Layer-2 address
   also referred to as MAC address, which 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)

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   MAC addresses can either be universally administered or locally
   administered.  Universally administered 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: (i)
   the Organizationally Unique Identifier (OUI), which are the first
   three octets in transmission order and identify the organization that
   issued the identifier, and (ii) Network Interface Controller (NIC)
   Specific, which are the following three octets, 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 either be set-up by the network administrator, or by the
   Operating System (OS) of the device to which the address pertains.
   However, as explained in further sections of this document, there are
   new initiatives at 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 layer-2
   network.  In an 802.11 network, a station 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 (a.k.a.  IEEE
      802.11 STA).

   *  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 overcome this privacy concern is by using randomly
   generated MAC addresses.  The IEEE 802 addressing includes one bit to
   specify if the hardware address is locally or globally administered.

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   This allows generating local addresses without the need of 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
   Operating Systems (OSes) reporting persistently (every 20 minutes or
   so) on MAC addresses (among 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 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 on
   "Pervasive Surveillance of the Internet - Designing Privacy into
   Internet Protocols" was given at the IEEE 802 Plenary meeting in San
   Diego [privacy_tutorial] in July of 2014.  The tutorial provided an
   update on the recent developments regarding Internet privacy, the
   actions undertaken by other SDOs such as IETF, and guidelines that
   were being followed when developing new Internet protocol
   specifications (e.g., [RFC6973]).  The tutorial highlighted some
   privacy concerns applicable specifically to link-layer technologies
   and provided suggestions on how IEEE 802 could help addressing them.

   Following the discussions and interest within the IEEE 802 community,
   on 18 July 2014 the IEEE 802 Executive Committee (EC) created an IEEE
   802 EC Privacy Recommendation Study Group (SG) [ieee_privacy_ecsg].
   The work and discussions from the group have generated multiple
   outcomes, such as: 802E PAR (Project Authorization Request, this is
   the means by which standards projects are started within the IEEE.
   PARs define the scope, purpose, and contact points for a new
   project): Recommended Practice for Privacy Considerations for IEEE
   802 Technologies [IEEE_802E], and the 802c PAR: Standard for Local
   and Metropolitan Area Networks - Overview and Architecture Amendment
   - Local Medium Access Control (MAC) Address Usage [IEEE_802c].

   In order to test the effects of MAC address randomization, trials
   were conducted at the IETF and IEEE 802 meetings between November
   2014 and March 2015 - IETF91, IETF92 and IEEE 802 Plenary in Berlin.
   The purpose of the trials was to evaluate the use of MAC address
   randomization from two different perspectives: (i) the effect on the
   connectivity experience of the end-user, also checking if

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   applications and OSes were affected; and (ii) the potential impact on
   the network infrastructure itself.  Some of the findings were
   published in [CSCN2015].

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

   Since then, MAC randomization has further been 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.  Randomized and Changing MAC addresses activities at the IEEE 802

   Practical experiences of Randomized and Changing MAC addresses (RCM)
   in devices (some of them are explained in Section Section 6) helped
   researchers fine-tune their understanding of attacks against
   randomization mechanisms [when_mac_randomization_fails].  At the IEEE
   802.11 group these research experiences eventually formed the basis
   for a specified mechanism introduced in the IEEE 802.11aq in 2018
   which randomize MAC addresses [IEEE_802_11_aq].

   More recent developments include turning on MAC 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
   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
   with the purpose of developing mechanisms that do not decrease user
   privacy but enable an optimal user experience when 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 user
   privacy solutions applicable to IEEE Std 802.11 [rcm_privacy_par].

   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

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   802 standards.  Another range of local MAC addresses is designated
   for Administratively Assigned Identifiers subject to assignment by a
   network administrator.

   "IEEE Std 802E-2020: Recommended Practice for Privacy Considerations
   for IEEE 802 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 (a.k.a.  Wi-Fi) devices as part of the
   RCM [rcm_privacy_csd] efforts.  Annex T of IEEE Std 802.1AEdk-2023:
   MAC Privacy Protection [IEEE802.1AEdk-2023] discusses privacy
   considerations in bridged networks.

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

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

   *  The IEEE 802.11bi task group, which is chartered to define
      modifications to the IEEE Std 802.11 medium access control (MAC)
      specification to specify new mechanisms that address and improve
      user privacy.

4.  Recent MAC randomization-related activities at the WBA

   At the Wireless Broadband Alliance (WBA), the Testing and
   Interoperability Work Group has been looking at the 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, 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.  A first
   version of this document has been liaised with the IETF as part of
   the MAC Address Device Identification for Network and Application
   Services (MADINAS) activities through the "Wi-Fi Identification In a
   post MAC Randomization Era v1.0" paper [wba_paper].

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5.  IPv6 address randomization at 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
   MAC address-based IID, made it easier to perform global device
   tracking.

   [RFC8981] identifies and describes the privacy issues associated with
   embedding MAC stable addressing information into the IPv6 addresses
   (as part of the IID).  It describes an extension to IPv6 SLAAC that
   causes hosts to generate temporary addresses with randomized
   interface identifiers 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 would then be 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.
   In order to do so, a node produces a sequence of temporary global
   scope addresses from a sequence of interface identifiers that appear
   to be random in the sense that it is difficult for an outside
   observer to predict a future address (or identifier) based on a
   current one, and 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, and this 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.

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   [RFC7217] describes a method to generate Interface Identifiers that
   are stable for each network interface within each subnet, but that
   change as a host moves from one network to another.  This method
   enables keeping the "stability" properties of the Interface
   Identifiers specified in [RFC4291], 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 which
   takes as input information that is stable and associated to the
   interface (e.g., a local interface identifier), stable information
   associated to the visited network (e.g., IEEE 802.11 SSID), the IPv6
   prefix, and a secret key, plus some other additional information.
   This basically ensures that a different IID is generated when any of
   the input fields changes (such as the network or the prefix), but
   that the IID is the same within each subnet.

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

   In addition to the former documents, [RFC8947] proposes "an extension
   to DHCPv6 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 unnecessary".  [RFC8948]
   proposes "extensions to DHCPv6 protocols to 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".

   Not only MAC and IP addresses can be used for tracking purposes.
   Some DHCP options carry unique identifiers.  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,
   "designed for clients that wish to remain anonymous to the visited
   network.  The profiles 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.  A taxonomy of MAC address selection policies

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

   Note about the used naming convention: the "M" in MAC is included in
   the acronym, but not the "A" from address.  This allows one to talk
   about a PVOM Address, or PNGM Address.

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6.1.  Per-Vendor OUI MAC address (PVOM)

   This form of MAC address selection is the historical default.

   The vendor obtains an Organizationally Unique Identifier (OUI) from
   the IEEE.  This has been a 24-bit prefix (including two upper bits
   which 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 has not been unusual for the 24-bit value
   to be taken as an incrementing counter, assigned at the factory, and
   burnt into non-volatile storage.

   Note that 802.15.4 use 64-bit MAC addresses, and the IEEE assigns
   32-bit prefixes.  The IEEE has indicated that there may be a future
   Ethernet specification using 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 (802.11) networks where the network
   is identified by an SSID Name.  The generated address is stored on
   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 STP
   [IEEE802.1D-2004], LLDP [IEEE802.1AB-2016], DHCP or Router
   Advertisements to determine which network has been attached.

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6.5.  Per-Period Generated MAC address (PPGM)

   This form of MAC address is generated periodically.  Typical numbers
   are 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 WPA/802.1x session: new EAP, TLS, etc. negotiations.  A
   new DHCP, SLAAC will be done.

   If DHCP is used, then a new DUID is generated so as to not link to
   the previous connection, and the result is usually new IP addresses
   allocated.

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-dependant, for example, a
   session might be defined by logging in a portal, VPN, etc.  Like
   PNGM, it is used primarily with Wi-Fi.

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

7.  OS current practices

   Most modern OSes (especially mobile ones) do implement by default
   some MAC address randomization policy.  Since the mechanism and
   policies OSes implement can evolve with time, the content is now
   hosted at https://github.com/ietf-wg-madinas/draft-ietf-madinas-mac-
   address-randomization/blob/main/OS-current-practices.md.  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, as the time
   of writing this document (original source posted at:
   https://www.fing.com/news/private-mac-address-on-ios-14, latest
   wayback machine's snapshot available here:
   https://web.archive.org/web/20230905111429/https://www.fing.com/news/
   private-mac-address-on-ios-14, updated based on findings from the
   authors).

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    +=============================================+===================+
    | 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 WiFI   | MAC address is    |
    | network                                     | reset when the    |
    |                                             | device forgets a  |
    |                                             | WiFI 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 have performed some additional tests to
   evaluate how most widely used OSes behave regarding MAC address
   randomization.  Table 2 summarizes our findings, where show on
   different rows whether the OS performs address randomization per
   network (PNGM according to the taxonomy introduced in Section 6), per
   new connection (PSGM), daily (PPGM with a period of 24h), supports
   configuration per SSID, supports address randomization for scanning,
   and whether it does that by default.

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       +=================+===============+=========+=========+=====+
       | OS              | Linux (Debian | Android | Windows | iOS |
       |                 |  "bookworm")  |    10   |    10   | 14+ |
       +=================+===============+=========+=========+=====+
       | Random per net. |       Y       |    Y    |    Y    |  Y  |
       | (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 from different OS (as of
                              September 2021)

   According to [privacy_android], starting in Android 12, Android uses
   non-persistent randomization in the following situations: (i) a
   network suggestion app specifies that non-persistant randomization be
   used for the network (through an API); or (ii) 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 this device are discussed throughout this
   document.  Given the informational nature of this document, no
   protocols/solutions are specified, but current state of affairs is
   documented.

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   Any future specification in this area would have to look into
   security and privacy aspects, such as, but not limited to: i)
   mitigating the problem of location privacy while minimizing the
   impact on upper layers of the protocol stack; ii) providing means to
   network operators to authenticate devices and authorize network
   access despite the MAC addresses changing following some pattern;
   and, iii) provide means for the device not to use MAC addresses it is
   not authorized to use or that are currently in use.

   A major conclusion of the work in IEEE Std 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.  Acknowledgments

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

   Authors would like to thank Jerome Henry, Hai Shalom, Stephen Farrel,
   Alan DeKok, Mathieu Cunche, Johanna Ansohn McDougall, Peter Yee, Bob
   Hinden, Behcet Sarikaya, David Farmer, Mohamed Boucadair, Éric
   Vyncke, Christian Amsüss, Roma Danyliw, Murray Kucherawy and Paul
   Wouters for their reviews and comments on previous versions of this
   document.  Authors would also like to thank Michael Richardson for
   his contributions on the taxonomy section.  Finally, authors would
   also like to thank the IEEE 802.1 Working Group for its review and
   comments, performed as part of the Liaison statement on Randomized
   and Changing MAC Address (https://datatracker.ietf.org/
   liaison/1884/).

11.  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 , July 2020.

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

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   [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 , 2005.

   [IEEE802.1AB-2016]
              IEEE 802.1, "IEEE Std 802.1AB-2016: IEEE Standard for
              Local and metropolitan area networks - Station and Media
              Access Control Connectivity Discovery", 2016.

   [IEEE802.1AEdk-2023]
              IEEE 802.1, "IEEE Std 802.1AEdk-2023: IEEE Standard for
              Local and metropolitan area networks-Media Access Control
              (MAC) Security - Amendment 4: MAC Privacy protection",
              2023.

   [IEEE802.1D-2004]
              IEEE 802.1, "IEEE Std 802.1D-2004: IEEE Standard for Local
              and metropolitan area networks: Media Access Control (MAC)
              Bridges", 2004.

   [IEEE_802] IEEE 802, "IEEE Std 802 - IEEE Standard for Local and
              Metropolitan Area Networks: Overview and Architecture",
              IEEE 802 , 2014.

   [IEEE_802c]
              IEEE 802.1 WG - 802 LAN/MAN architecture, "IEEE 802c-2017
              - IEEE Standard for Local and Metropolitan Area
              Networks:Overview and Architecture--Amendment 2: Local
              Medium Access Control (MAC) Address Usage", IEEE 802c ,
              2017.

   [IEEE_802E]
              IEEE 802.1 WG - 802 LAN/MAN architecture, "IEEE 802E-2020
              - IEEE Recommended Practice for Privacy Considerations for
              IEEE 802 Technologies", IEEE 802E , 2020.

   [IEEE_802_11_aq]
              IEEE 802.11 WG - Wireless LAN Working Group, "IEEE
              802.11aq-2018 - 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 Amendment 5:
              Preassociation Discovery", IEEE 802.11 , 2018.

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   [ieee_privacy_ecsg]
              IEEE 802 Privacy EC SG, "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", Contribution at W3C/IAB workshop on
              Strengthening the Internet Against Pervasive Monitoring
              (STRINT) , February 2014.

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

   [privacy_ios]
              Apple, "Use private Wi-Fi addresses in iOS 14, iPadOS 14,
              and watchOS 7",
              <https://support.apple.com/en-us/HT211227>.

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

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

   [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 802.11-20/1346r1 , 2020.

   [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.

   [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.

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   [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 802.11-20/1117r3 , 2020.

   [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 802.11-20/742r5 , 2020.

   [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 C. Bernardos, "Link-Layer
              Address Assignment Mechanism for DHCPv6", RFC 8947,
              DOI 10.17487/RFC8947, December 2020,
              <https://www.rfc-editor.org/info/rfc8947>.

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

   [wba_paper]
              Alliance, W. B., "Wi-Fi Identification Scope for Liasing -
              In a post MAC Randomization Era", doc.:WBA Wi-Fi ID Intro:
              Post MAC Randomization Era v1.0 - IETF liaison , March
              2020.

   [when_mac_randomization_fails]
              Martin, J., Mayberry, T., Donahue, C., Foppe, L., Brown,
              L., Riggins, C., Rye, E.C., and D. Brown, "A Study of MAC
              Address Randomization in Mobile Devices and When it
              Fails", arXiv:1703.02874v2 [cs.CR] , 2017.

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

Authors' Addresses

   Juan Carlos Zúñiga
   CISCO
   Montreal  QC
   Canada
   Email: [email protected]

   Carlos J. Bernardos (editor)
   Universidad Carlos III de Madrid
   Av. Universidad, 30
   28911 Leganes, Madrid
   Spain
   Phone: +34 91624 6236

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   Email: [email protected]
   URI:   http://www.it.uc3m.es/cjbc/

   Amelia Andersdotter
   Safespring AB
   Email: [email protected]

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