Randomized and Changing MAC Address state of affairs
draft-ietf-madinas-mac-address-randomization-10

MADINAS                                                       JC. Zuniga
Internet-Draft                                                     CISCO
Intended status: Informational                        CJ. Bernardos, Ed.
Expires: 14 July 2024                                               UC3M
                                                         A. Andersdotter
                                                           Safespring AB
                                                         11 January 2024

          Randomized and Changing MAC Address state of affairs
            draft-ietf-madinas-mac-address-randomization-10

Abstract

   Internet privacy has become a major concern over the past few years.
   Users are becoming more aware that their online activity leaves a
   vast digital footprint, that communications are not always properly
   secured, and that their location and actions can be easily tracked.
   One of the main factors for the location tracking issue is the wide
   use of long-lasting identifiers, such as MAC addresses.

   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, with the intention
   to inform the technical community about them, and help coordinate
   between present and future standardization activities.

Status of This Memo

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   This Internet-Draft will expire on 14 July 2024.

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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  MAC address usage . . . . . . . . . . . . . . . . . . . .   3
     3.2.  MAC address randomization . . . . . . . . . . . . . . . .   4
     3.3.  Privacy Workshop, Tutorial and Experiments at IETF and IEEE
           802 meetings  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Recent RCM activities at the IEEE 802 . . . . . . . . . . . .   6
   5.  Recent MAC randomization-related activities at the WBA  . . .   7
   6.  MAC randomization-related activities at the IETF  . . . . . .   8
   7.  A taxonomy of MAC address selection policies  . . . . . . . .   9
     7.1.  Per-Vendor OUI MAC address (PVOM) . . . . . . . . . . . .  10
     7.2.  Per-Device Generated MAC address (PDGM) . . . . . . . . .  10
     7.3.  Per-Boot Generated MAC address (PBGM) . . . . . . . . . .  10
     7.4.  Per-Network Generated MAC adress (PNGM) . . . . . . . . .  10
     7.5.  Per-Period Generated MAC address (PPGM) . . . . . . . . .  11
     7.6.  Per-Session Generated MAC address (PSGM)  . . . . . . . .  11
   8.  OS current practices  . . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   12. Informative References  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Internet privacy is becoming a huge concern, as more and more mobile
   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 not very secure protocol
   stacks and 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, for
   instance sharing information on social networks, cookies used by
   browsers and servers to provide a better navigation experience,
   connectivity logs that allow tracking of a user's Layer-2 (L2/MAC) or

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   Layer-3 (L3) address, web trackers, 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 actual 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 [wifi_internet_privacy].  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, with the intention
   to inform the community and help coordinate between present and
   futures standardization activities.

2.  Terminology

   The following terms are used in this document:

      MAC: Medium Access Control

3.  Background

3.1.  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).
   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 signal transmitted by the network interface.  The format
   of these addresses is shown in Figure 1.

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           +--------+--------+---------+--------+--------+---------+
           |  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

   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.

3.2.  MAC address randomization

   Since universally administered MAC addresses are by definition
   globally-unique, when a device uses this MAC address 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].

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   MAC addresses can be easily observed by a third party, such as a
   passive device listening to communications in the same 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 (aka 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 address fields of an IEEE 802.11 frame.

   One way to overcome this privacy concern is by using randomly
   generated MAC addresses.  As described in the previous section, the
   IEEE 802 addressing includes one bit to specify if the hardware
   address is locally or globally administered.  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
   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.

3.3.  Privacy Workshop, Tutorial and Experiments at IETF and IEEE 802
      meetings

   As an outcome to the STRINT W3C/IAB Workshop [strint], on July 2014 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].  The Tutorial provided an
   update on the recent developments regarding Internet privacy, the
   actions that other SDOs such as IETF were taking, 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].

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   The work and discussions from the group have generated multiple
   outcomes, such as: 802E PAR: 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, major
   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 experiments 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 applications and operating systems (OSs) were affected; and (ii)
   the potential impact on the network infrastructure itself.  Some of
   the findings were published in [wifi_internet_privacy].

   During the experiments it was observed that the probability of
   address duplication in a network with this characteristics is
   negligible.  The experiments also showed that other protocol
   identifiers 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
   operating systems to provide better privacy for mobile phone users
   when connecting to public wireless networks [privacy_ios],
   [privacy_windows], [privacy_android].

4.  Recent RCM activities at the IEEE 802

   Practical experiences of Randomized And Changing MAC Addresses (RCM)
   in live devices helped researchers fine-tune their understanding of
   attacks against randomization mechanisms
   [when_mac_randomization_fails].  At IEEE 802.11 these research
   experiences eventually formed the basis for a specified mechanism
   introduced in the IEEE 802.11aq in 2018 which randomize MAC addresses
   that recommends mechanisms to avoid pitfalls [IEEE_802_11_aq].

   More recent developments include turning on MAC randomization in
   mobile operating systems by default, which has an impact on the
   ability of network operators to personalize or 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

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   mechanisms that do not decrease user privacy and enable an optimal
   user experience when the MAC address of a device in an Extended
   Service Set 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).The SLAP designates a range
   of Extended Local Identifiers (ELIs) for subassignment within a block
   of addresses assigned by the IEEE Registration Authority via a
   Company ID (CID).  A range of local MAC addresses is designated for
   Standard Assigned Identifiers (SAI) to be specified by IEEE 802
   standards.  Another range of local MAC addresses is designated for
   Administratively Assigned Identifiers (AAI) 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 Recommended
   Practice is part of the basis for the review of user privacy
   solutions for IEEE Std 802.11 (aka 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.

   Currently, 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 will define modifications to
      the IEEE Std 802.11 medium access control (MAC) specification to
      specify new mechanisms that address and improve user privacy.

5.  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.

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   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].

6.  MAC randomization-related activities at the IETF

   Several IP address assignment mechanisms such as the IPv6 stateless
   autoconfiguration techniques (SLAAC) [RFC4862] generate the Interface
   Identifier (IID) of the address from its MAC address (via EUI64),
   which then becomes visible to all IPv6 communication peers.  This
   potentially allows for global tracking of a device at L3 from any
   point on the Internet.  Besides, the prefix part of the address
   provides meaningful insights of the physical location of the device
   in general, which together with the MAC address-based IID, makes it
   easier to perform global device tracking.

   There are some solutions that might mitigate this privacy threat,
   such as the use of temporary addresses [RFC4191], the use of opaque
   IIDs [RFC7217], [I-D.gont-6man-deprecate-eui64-based-addresses].
   Next, we briefly describe how these solutions work.

   [RFC4191] identifies and describes the privacy issues associated with
   embedding MAC stable addressing information into the IPv6 addresses
   (as part of the IID) and describes some mechanisms to mitigate the
   associated problems.  The specification is meant for IPv6 nodes that
   auto-configure IPv6 addresses based on the MAC address (EUI-64
   mechanism).  It defines how to create additional addresses (generally
   known as "temporary addresses") based on a random interface
   identifier for the purpose of initiating outgoing sessions.  These
   "random" or 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.
   The main problem with the temporary addresses is that they should not
   be used by applications that listen for incoming connections (as
   these are supposed to be waiting on permanent/well-known
   identifiers).  Besides, if a node changes network and comes back to a

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   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.

   [RFC7217] defines a method for generating IPv6 IIDs to be used with
   IPv6 Stateless Address Autoconfiguration (SLAAC), such that an IPv6
   address configured using this method is stable within each subnet,
   but the corresponding IID changes when the host moves from one
   network to another.  This method is meant to be an alternative to
   generating Interface Identifiers based on MAC addresses, such that
   the benefits of stable addresses can be achieved without sacrificing
   the security and privacy of users.  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., MAC address or 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.

   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.

7.  A taxonomy of MAC address selection policies

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

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   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.

   The names are all in the form for per-period-of-time-selection.

7.1.  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.

7.2.  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.

7.3.  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).

7.4.  Per-Network Generated MAC adress (PNGM)

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

   This is typically used with WiFi (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.

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   It is possible to use PNGM for wired Ethernet connections through
   some passive observation of network traffic, such as STP, LLDP, DHCP
   or Router Advertisements to determine which network has been
   attached.

7.5.  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
   WiFi (802.11).

   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, Router-Advertisement will be done.  TBD: it is unclear if
   any TLS session-resumption ticket (used by EAP-TLS) can or should be
   retained across a change of the MAC address.

   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.

7.6.  Per-Session Generated MAC address (PSGM)

   This form of MAC address is generated on a per session basis.  Like
   PNGM, it is used primarily with WiFi (802.11).

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

8.  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 snapshop of the content at the time of publication of
   this document is included below.

   Table 1 summarizes current practices for Androiod and iOS, as the
   time of writing this document (original source:
   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      |
    |                                             | BSSID             |
    +---------------------------------------------+-------------------+
    +---------------------------------------------+-------------------+
    | 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 WiFi networks.  But | randomization is  |
    | if the device previously connected to a     | enabled by        |
    | WiFi network identifying itself with the    | default for all   |
    | real MAC address, no randomized MAC address | the new WiFi      |
    | 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, per new connection, daily, supports configuration per SSID,
   supports address randomization for scanning, and whether it does that
   by default.

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    +====================+=======+============+============+=========+
    | OS                 | Linux | Android 10 | Windows 10 | iOS 14+ |
    +====================+=======+============+============+=========+
    | Random per net.    |   Y   |     Y      |     Y      |    Y    |
    +--------------------+-------+------------+------------+---------+
    +--------------------+-------+------------+------------+---------+
    | Random per connec. |   Y   |     N      |     N      |    N    |
    +--------------------+-------+------------+------------+---------+
    +--------------------+-------+------------+------------+---------+
    | Random daily       |   N   |     N      |     Y      |    N    |
    +--------------------+-------+------------+------------+---------+
    +--------------------+-------+------------+------------+---------+
    | SSID config.       |   Y   |     N      |     N      |    N    |
    +--------------------+-------+------------+------------+---------+
    +--------------------+-------+------------+------------+---------+
    | Random. for scan   |   Y   |     Y      |     Y      |    Y    |
    +--------------------+-------+------------+------------+---------+
    +--------------------+-------+------------+------------+---------+
    | Random. for scan   |   N   |     Y      |     N      |    Y    |
    | by default         |       |            |            |         |
    +--------------------+-------+------------+------------+---------+

      Table 2: Observed behavior from different OS (as of September
                                  2022)

   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).

9.  IANA Considerations

   N/A.

10.  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.

   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

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   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.  In particular it has been shown that individuals can
   be successfully tracked by fingerprinting using aspects of their
   communication other than MAC Addresses or other permanent
   identifiers.  Machine learning techniques facilitate fingerprinting
   without the adversary needing to understand the technical reasons for
   the correlation.

11.  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 and Peter Yee
   for their review and comments on previous versions of this document.
   Authors would also like to thank Michael Richardson for his
   contributions on the taxonomy section.

12.  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.

   [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.

   [I-D.gont-6man-deprecate-eui64-based-addresses]
              Gont, F., Cooper, A., Thaler, D., and W. S. LIU,
              "Deprecating EUI-64 Based IPv6 Addresses", Work in
              Progress, Internet-Draft, draft-gont-6man-deprecate-eui64-
              based-addresses-00, 21 October 2013,
              <https://datatracker.ietf.org/doc/html/draft-gont-6man-
              deprecate-eui64-based-addresses-00>.

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   [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.

   [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.

   [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>.

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   [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.

   [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.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

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   [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>.

   [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>.

   [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>.

   [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.

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   [wifi_internet_privacy]
              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.

   [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 Zuniga
   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

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