Randomized and Changing MAC Address: Context, Network Impacts, and Use Cases for Wi-Fi Network
draft-ietf-madinas-use-cases-16

Internet Engineering Task Force                                 J. Henry
Internet-Draft                                             Cisco Systems
Intended status: Informational                                    Y. Lee
Expires: 7 June 2025                                             Comcast
                                                         4 December 2024

 Randomized and Changing MAC Address: Context, Network Impacts, and Use
                        Cases for Wi-Fi Network
                    draft-ietf-madinas-use-cases-16

Abstract

   To limit the privacy issues created by the association between a
   device, its traffic, its location, and its user in Wi-Fi
   [IEEE_802.11] network, client and client Operating System vendors
   have started implementing MAC address randomization.  When such
   randomization happens, some in-network states may break, which may
   affect network connectivity and user experience.  At the same time,
   devices may continue using other stable identifiers, defeating the
   MAC address randomization purposes.

   This document lists various network environments and a range of
   network services that may be affected by such randomization.  This
   document then examines settings where the user experience may be
   affected by in-network state disruption.  Last, this document
   examines two existing frameworks to maintain user privacy while
   preserving user quality of experience and network operation
   efficiency.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 7 June 2025.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  MAC Address as Identity: User vs. Device  . . . . . . . . . .   4
     2.1.  Privacy of MAC Address  . . . . . . . . . . . . . . . . .   6
   3.  The Actors: Network Functional Entities and Human Entities  .   6
     3.1.  Network Functional Entities . . . . . . . . . . . . . . .   7
     3.2.  Human-related Entities  . . . . . . . . . . . . . . . . .   9
   4.  Degrees of Trust  . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Environment . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Network Services  . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Device Identification and Associated Problems . . . . . .  13
     6.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Existing Frameworks  . . . . . . . . . . . . . . . .  20
     A.1.  802.1X with WPA2 / WPA3 . . . . . . . . . . . . . . . . .  20
     A.2.  OpenRoaming . . . . . . . . . . . . . . . . . . . . . . .  20
     A.3.  Proprietary RCM schemes . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [IEEE_802.11] (or 'Wi-Fi') technology has revolutionized
   communications and become the preferred, and sometimes the only
   technology used by devices such as laptops, tablets, and Internet of
   Things (IoT) devices.  Wi-Fi is an over-the-air medium that allows
   attackers with surveillance equipment to monitor WLAN packets and
   track the activity of WLAN devices.  It is also sometimes possible
   for attackers to monitor the WLAN packets behind the Wi-Fi Access
   Point (AP) over the wired Ethernet.  Once the association between a
   device and its user is made, identifying the device and its activity
   is sufficient to deduce information about what the user is doing,

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   without the user's consent.

   To reduce the risks of identifying a device only by the MAC address,
   client OS vendors have started implementing Randomized and Changing
   MAC addresses (RCM).  By randomizing the MAC address, it becomes
   harder to use the MAC address to construct a persistent association
   between a flow of data packets and a device, assuming no other
   visible unique identifiers or stable patterns are in use.  When
   individual devices are no longer easily identifiable, it also becomes
   difficult to associate a series of network packet flows in a
   prolonged period with a particular individual using one specific
   device if the device randomizes the MAC address governed by the OS
   privacy policies.

   However, such address change may affect the user experience and the
   efficiency of legitimate network operations.  For a long time,
   network designers and implementers relied on the assumption that a
   given machine, in a network implementing [IEEE_802] technologies,
   would be represented by a unique network MAC address that would not
   change over time.  When this assumption is broken, network
   communication may be disrupted.  For example, sessions established
   between the end-device and network services may break and packets in
   transit may suddenly be lost.  If multiple clients implement
   aggressive (e.g., once an hour or shorter) MAC address randomization
   without coordination with network services, some network services
   such as MAC address cache in the AP and the upstream layer-2 switch
   may not be able to handle the load that may result in unexpected
   network interruption.

   At the same time, some network services rely on the end station (as
   defined by the [IEEE_802] Standard, also called in this document
   device, or machine) providing an identifier, which can be the MAC
   address or another value.  If the client implements MAC address
   randomization but continues sending the same static identifier, then
   the association between a stable identifier and the station continues
   despite the RCM scheme.  There may be environments where such
   continued association is desirable, but others where user privacy has
   more value than any continuity of network service state.

   It is useful for implementations of client and network devices to
   enumerate services that may be affected by RCM and evaluate possible
   framework to maintain both the quality of user experience and network
   efficiency while RCM happens, and user privacy is strengthened.  This
   document presents these assessments and recommendations.

   This document is organized as follows.  Section 2 discusses the
   current status of using MAC address as identity.  Section 3 discusses
   various actors in the network that will be impacted by MAC address

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   randomization.  Section 4 examines the degrees of trust between
   personal devices and the entities at play in a network domain.
   Section 5 discusses various network environments that will be
   impacted.  Section 6 analyzes some existing network services that
   will be impacted.  Finally, Appendix A includes some frameworks that
   are being worked on.

2.  MAC Address as Identity: User vs. Device

   The Media Access Control (MAC) layer of IEEE 802 technologies defines
   rules to control how a device accesses the shared medium.  In a
   network where a machine can communicate with one or more other
   machines, one such rule is that each machine needs to be identified
   either as the target destination of a message or as the source of a
   message (and the target destination of the answer).  Initially
   intended as a 48-bit (6 octets) value in the first versions of the
   [IEEE_802] Standard, other Standards under the [IEEE_802] umbrella
   allow this address to take an extended format of 64 bits (8 octets)
   which enable a larger number of MAC addresses to coexist as the 802
   technologies became widely adopted.

   Regardless of the address length, different networks have different
   needs, and several bits of the first octet are reserved for specific
   purposes.  In particular, the first bit is used to identify the
   destination address as an individual (bit set to 0) or a group
   address (bit set to 1).  The second bit, called the Universally or
   Locally Administered (U/L) Address Bit, indicates whether the address
   has been assigned by a universal or local administrator.  Universally
   administered addresses have this bit set to 0.  If this bit is set to
   1, the entire address is considered locally administered (clause 8.4
   of [IEEE_802]).  Note that universally administered MAC addresses are
   required to register to IEEE while locally administered MAC addresses
   are not.

   The intent of this provision is important for the present document.
   The [IEEE_802] Standard recognized that some devices (e.g., smart
   thermostats) may never change their attachment network and will not
   need a globally unique MAC address to prevent address collision
   against any other device in any other network.  The U/L bit can be
   set to signal to the network that the MAC Address is intended to be
   locally unique (not globally unique).  The 802 Standard [IEEE_802]
   did not initially define the MAC Address allocation schema when the
   U/L bit is set to 1.  It states the address must be unique in a given
   broadcast domain (i.e., the space where the MAC addresses of devices
   are visible to one another).

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   It is also important to note that the purpose of the Universal
   version of the address was to avoid collisions and confusion, as any
   machine could connect to any network, and each machine needs to
   determine if it is the intended destination of a message or its
   response.  Clause 8.4 of [IEEE_802] reminds network designers and
   operators that all potential members of a network need to have a
   unique identifier in that network (if they are going to coexist in
   the network without confusion on which machine is the source or
   destination or any message).  The advantage of an administrated
   address is that a node with such an address can be attached to any
   Local Area Network (LAN) in the world with an assurance that its
   address is unique in that network.

   With the rapid development of wireless technologies and mobile
   devices, this scenario became very common.  With a vast majority of
   networks implementing [IEEE_802] radio technologies at the access,
   the MAC address of a wireless device can appear anywhere on the
   planet and collisions should still be avoided.  However, the same
   evolution brought the distinction between two types of devices that
   the [IEEE_802] Standard generally referred to as ‘nodes in a
   network’. Their definition is found in the [IEEE_802E] Recommended
   Practice stated in Section 6.2 of [IEEE_802].

   1.  Shared Service Device, whose functions are used by enough people
       that the device itself, functions, or its traffic cannot be
       associated with a single or small group of people.  Examples of
       such devices include switches in a dense network, [IEEE_802.11]
       (WLAN) access points in a crowded airport, task-specific device
       (e.g., barcode scanners).

   2.  Personal Device, which is a machine or node primarily used by a
       single person or small group of people, and so that any
       identification of the device or its traffic can also be
       associated with the identification of the primary user or their
       online activity.

   Identifying the device is trivial if it has a unique MAC address.
   Once this unique MAC address is established, detecting any elements
   that directly or indirectly identify the user of the device
   (Personally Identifiable Information, or PII) is enough to link the
   MAC address to that user.  Then, any detection of traffic that can be
   associated with the device will also be linked to the known user of
   that device (Personally Correlated Information, or PCI).

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2.1.  Privacy of MAC Address

   This possible identification or association presents a privacy issue,
   especially with wireless technologies.  For most of them, and in
   particular for [IEEE_802.11], the source and destination MAC
   addresses are not encrypted even in networks that implement
   encryption (so that each machine can easily detect if it is the
   intended target of the message before attempting to decrypt its
   content, and also identify the transmitter, to use the right
   decryption key when multiple unicast keys are in effect).

   This identification of the user associated with a node was clearly
   not the intent of the 802 MAC address.  A logical solution to remove
   this association is to use a locally administered address instead and
   change the address in a fashion that prevents a continuous
   association between one MAC address and some PII.  However, other
   network devices on the same LAN implementing a MAC layer also expect
   each device to be associated with a MAC address that would persist
   over time.  When a device changes its MAC address, other devices on
   the same LAN may fail to recognize that the same machine is
   attempting to communicate with them.  Additionally, upper protocol
   layers (e.g., application layer) have been designed with the
   assumption that each node on the LAN using these services will have a
   MAC address that would remain consistent over time.  This type of MAC
   address is referred to as 'persistent' MAC address in this document.
   This assumption sometimes adds to the PII confusion, for example in
   the case of Authentication, Authorization, and Accounting (AAA)
   services [RFC3539] authenticating the user of a machine and
   associating the authenticated user to the device MAC address.  Other
   services solely focus on the machine (e.g., DHCPv4 [RFC2131]), but
   still expect each device to use a persistent MAC address, for example
   to re-assign the same IP address to a returning device.  Changing the
   MAC address may disrupt these services.

3.  The Actors: Network Functional Entities and Human Entities

   The risk of service disruption is weighed against the privacy
   benefits.  However, the plurality of actors involved in the exchanges
   tends to blur the boundaries of what privacy violations should be
   protected against.  It is therefore useful to list the actors
   associated with the network exchanges, either because they actively
   participate in these exchanges, or because they can observe them.
   Some actors are functional entities, while some others are human (or
   related) entities.

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3.1.  Network Functional Entities

   Network communications based on IEEE 802 technologies commonly rely
   on station identifiers based on a MAC address.  This MAC address is
   utilized by several types of network functional entities such as
   applications or devices that provide a service related to network
   operations.

   1.  Wireless access network infrastructure devices (e.g., WLAN access
       points or controllers): these devices participate in IEEE 802 LAN
       operations.  As such, they need to identify each machine as a
       source or destination to successfully continue exchanging frames.
       As a device changes its network attachment (roams) from one
       access point to another, the access points can exchange
       contextual information, (e.g., device MAC, keying material),
       allowing the device session to continue seamlessly.  These access
       points can also inform devices further in the wired network about
       the roam to ensure that layer-2 frames are redirected to the new
       device access point.

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   2.  Other network devices operating at the MAC layer: many wireless
       network access devices (e.g., [IEEE_802.11] access points) are
       conceived as layer-2 devices, and as such, they bridge a frame
       from one medium (e.g., [IEEE_802.11] or Wi-Fi) to another (e.g.,
       [IEEE_802.3] or Ethernet).  This means that a wireless device MAC
       address often exists on the wire beyond the wireless access
       device.  Devices connected to this wire also implement
       [IEEE_802.3] technologies and, as such, operate on the
       expectation that each device is associated with a MAC address
       that persists for the duration of continuous exchanges.  For
       example, switches and bridges associate MAC addresses to
       individual ports (so as to know to which port to send a frame
       intended for a particular MAC address).  Similarly, AAA services
       can validate the identity of a device and use the device's MAC
       address as a first pointer to the device identity (before
       operating further verification).  Similarly, some networking
       devices offer layer-2 filtering policies that may rely on the
       connected MAC addresses. 802.1X-enabled [IEEE_802.1X] devices may
       also selectively put the interface in a blocking state until a
       connecting device is authenticated.  These services then use the
       MAC address as a first pointer to the device identity to allow or
       block data traffic.  This list is not exhaustive.  Multiple
       services are defined for [IEEE_802.3] networks, and multiple
       services defined by the IEEE 802.1 working group are also
       applicable to [IEEE_802.3] networks.  Wireless access points may
       also connect to other mediums than [IEEE_802.3] (e.g., DOCSIS
       [DOCSIS]), which also implements mechanisms under the umbrella of
       the general 802 Standard, and therefore expect the unique and
       persistent association of a MAC address to a device.

   3.  Network devices operating at upper layers: some network devices
       provide functions and services above the MAC layer.  Some of them
       also operate a MAC layer function: for example, routers provide
       IP forwarding services but rely on the device MAC address to
       create the appropriate frame structure.  Other devices and
       services operate at upper layers but also rely upon the 802
       principles of unique MAC-to-device mapping.  For example, Address
       Resolution Protocol (ARP) [RFC826] and Neighbor Discovery
       Protocol (NDP) [RFC4861] use MAC address to create the mapping of
       an IP address to a MAC address for packet forwarding.  If a
       device changes its MAC address without a mechanism to notify the
       layer-2 switch it is connected to or the provider of a service
       that expects a stable MAC-to-device mapping, the provider of the
       service and traffic forwarding may be disrupted.

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3.2.  Human-related Entities

   Humans may actively participate in the network structure and
   operations, or be observers at any point of the network lifecycle.
   Humans could be wireless device users or people operating wireless
   networks.

   1.  Over-The-Air (OTA) observers: as the transmitting or receiving
       MAC address is usually not encrypted in wireless 802-technologies
       exchanges, and as any protocol-compatible device in range of the
       signal can read the frame header.  As such OTA observers are able
       to read the MAC addresses of individual transmissions.  Some
       wireless technologies also support techniques to establish
       distances or positions, allowing the observer, in some cases, to
       uniquely associate the MAC address with a physical device and its
       associated location.  An OTA observer may have a legitimate
       reason to monitor a particular device, for example, for IT
       support operations.  However, another actor might also monitor
       the same device to obtain PII or PCI.

   2.  Wireless access network operators: some wireless access networks
       host devices that meet specific requirements, such as device type
       (e.g., IoT-only networks, factory operational networks).
       Therefore, operators can attempt to identify the devices (or the
       users) connecting to the networks under their care.  They can use
       the MAC address to represent an identified device.

   3.  Network access providers: wireless access networks are often
       considered beyond the first 2 layers of the OSI model.  For
       example, several regulatory or legislative bodies can group all
       OSI layers into their functional effect of allowing network
       communication between machines.  In this context, entities
       operating access networks can see their liability associated with
       the activity of devices communicating through the networks that
       these entities operate.  In other contexts, operators assign
       network resources based on contractual conditions (e.g., fee,
       bandwidth fair share).  In these scenarios, these operators may
       attempt to identify the devices and the users of their networks.
       They can use the MAC address to represent an identified device.

   4.  Over-The-Wired internal (OTWi) observers: because the device
       wireless MAC address continues to be present over the wire if the
       infrastructure connection device (e.g., access point) functions
       as a layer-2 bridge, observers may be positioned over the wire
       and read transmission MAC addresses.  Such capability supposes
       that the observer has access to the wired segment of the
       broadcast domain where the frames are exchanged.  A broadcast
       domain is a logical segment of a network in which devices can

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       send, receive, and monitor data frames from all other devices
       within the same segment.  In most networks, such capability
       requires physical access to an infrastructure wired device in the
       broadcast domain (e.g., switch closet), and is therefore not
       accessible to all.

   5.  Over-The-Wired external (OTWe) observers: beyond the broadcast
       domain, frame headers are removed by a routing device, and a new
       layer-2 header is added before the frame is transmitted to the
       next segment.  The personal device MAC address is not visible
       anymore unless a mechanism copies the MAC address into a field
       that can be read while the packet travels onto the next segment
       (e.g., pre- [RFC4941] and pre-[RFC7217] IPv6 addresses built from
       the MAC address).  Therefore, unless this last condition exists,
       OTWe observers are not able to see the device's MAC address.

4.  Degrees of Trust

   The surface of PII exposures that can drive MAC address randomization
   depends on (1) the environment where the device operates, (2) the
   presence and nature of other devices in the environment, and (3) the
   type of network the device is communicating through.  Consequently, a
   device can use an identifier (such as a MAC address) that can persist
   over time if trust with the environment is established, or that can
   be temporal if an identifier is required for a service in an
   environment where trust has not been established.  Note that trust is
   not binary.  It is useful to distinguish what trust a personal device
   may establish with the different entities at play in a network domain
   where a MAC address may be visible:

   1.  Full trust: there is environment where a personal device
       establishes a trust relationship and can share a persistent
       device identity with the access network devices (e.g., access
       point and WLAN Controller), the services beyond the access point
       in the layer-2 broadcast domain (e.g., DHCPv4, AAA), without fear
       that observers or network actors may access PII that would not be
       shared willingly.  The personal device (or its user) also has
       confidence that its identity is not shared beyond the layer-2
       broadcast domain boundary.

   2.  Selective trust: in another environment, a device may selectively
       share a persistent identity with some elements of the layer-2
       broadcast domain but not others.  That persistent identity may or
       may not be the same for different services.

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   3.  Zero trust: in another environment, a device may be unwilling to
       share any persistent identity with any local entity reachable
       through the AP.  It may generate a temporal identity to each of
       them.  That temporal identity may or may not be the same for
       different services.

5.  Environment

   The trust relationship depends on the relationship between the user
   of a personal device and the operator of a network service that the
   personal device may use.  It is useful to observe the typical trust
   structure of common environments:

   A.  Residential settings under the control of the user: this is
       typical of a home network with Wi-Fi in the LAN and Internet
       connection.  In this environment, traffic over the Internet does
       not expose the MAC address of the internal device if it is not
       copied to another field before routing happens.  The wire segment
       within the broadcast domain is under the control of the user and
       is therefore usually not at risk of hosting an eavesdropper.
       Full trust is typically established at this level among users and
       with the network elements.  The device trusts the access point
       and all layer-2 domain entities beyond the access point.
       However, unless the user has full access control over the
       physical space where the Wi-Fi transmissions can be detected,
       there is no guarantee that an eavesdropper will not observe the
       communications.  As such, it is common to assume that, even in
       this environment, attackers may still be able to monitor the
       unencrypted information such as MAC addresses.

   B.  Managed residential settings: examples of this type of
       environments include shared living facilities and other
       collective environments where an operator manages the network for
       the residents.  The OTA exposure is similar to that of a home.  A
       number of devices larger than in a standard home may be present,
       and the operator may be requested to provide IT support to the
       residents.  In this setting, the operator may need to identify a
       device activity in real-time.  It may also need to analyze logs
       to understand a past reported issue.  For both activities, a
       device identification associated with the session is needed.
       Full trust is often established in this environment, at the scale
       of a series of a few sessions, not because it is assumed that no
       eavesdropper would observe the network activity, but because it
       is a common condition for the managed operations.

   C.  Public guest networks: public hotspots in shopping malls, hotels,
       stores, train stations, and airports are typical examples of this
       environment.  Some guest network operators may be legally

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       mandated to identify devices or users while some guest network
       operators may be legally mandated to untrack devices or users.
       In this environment, trust is commonly not established with any
       element of the layer-2 broadcast domain (Zero trust model by
       default).

   D.  Enterprises with Bring-Your-Own-Device (BYOD): users may be
       provided with corporate devices or may bring their own devices.
       The devices are not directly under the control of a corporate IT
       team.  Trust may be established as the device joins the network.
       Some enterprise models will mandate full trust.  Others,
       consistent with the BYOD model, will allow selective trust.

   E.  Managed enterprises: in this environment, users are typically
       provided with corporate devices, and all connected devices are
       managed, for example, through a Mobile Device Management (MDM)
       profile installed on the device.  Full trust is created as the
       MDM profile is installed.

6.  Network Services

   Different network environments provide different levels of network
   services, from simple to complex.  At its simplest level, a network
   can provide a wireless connecting device basic IP communication
   service (e.g., DHCPv4 [RFC2131] or SLAAC [RFC4862]) and an ability to
   connect to the Internet (e.g., DNS service or relay, and routing in
   and out through a local gateway).  The network can also offer more
   advanced services, such as managed instant messaging service, file
   storage, printing, and/or local web service.  Larger and more complex
   networks can also incorporate more advanced services, from AAA to AR/
   VR applications.  To the network, its top priority is to provide the
   best Quality of Experience to its users.  Often the network contains
   policies which help to make forwarding decision based on the network
   conditions, the device, and the user identity associated to the
   device.  For example, in a hospital private network, the network may
   contain policy to give highest priority to doctors' Voice-Over-IP
   packets.  In another example, an enterprise network may contain
   policy to allow applications from a group of authenticated devices to
   use ECN [RFC3168] for congestion and/or DSCP [RFC8837] for
   classification to signal the network for specific network policy.  In
   this configuration, the network is required to associate the data
   packets to an identity to validate the legitimacy of the marking.
   Before RCM, many network systems use MAC address as a persistent
   identity to create an association between user and device.  After RCM
   being implemented, the association is broken.

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6.1.  Device Identification and Associated Problems

   Wireless access points and controllers use the MAC address to
   validate the device connection context, including protocol
   capabilities, confirmation that authentication was completed, Quality
   of Service or security profiles, and encryption keying material.
   Some advanced access points and controllers also include upper layer
   functions whose purpose is covered below.  A device changing its MAC
   address, without another recorded device identity, would cause the
   access point and the controller to lose the relation between a
   connection context and the corresponding device.  As such, the
   layer-2 infrastructure does not know that the device (with its new
   MAC address) is authorized to communicate through the network.  The
   encryption keying material is not identified anymore (causing the
   access point to fail to decrypt the device packets and fail to select
   the right key to send encrypted packets to the device).  In short,
   the entire context needs to be rebuilt, and a new session restarted.
   The time consumed by this procedure breaks any flow that needs
   continuity or short delay between packets on the device (e.g., real-
   time audio, video, AR/VR, etc.)  For example: the [IEEE_802.11i]
   Standard recognizes that a device may leave the network and come back
   after a short time window.  As such, the standard suggests that the
   infrastructure should keep the context for a device for a while after
   the device was last seen.  MAC address randomization in this context
   can cause resource exhaustion on the wireless infrastructure and the
   flush of contexts, including for devices that are simply in temporal
   sleep mode.

   Some network equipment such as Multi-Layer routers and Wi-Fi Access
   Points which serve both layer-2 and layer-3 in the same device rely
   on ARP [RFC826], and NDP [RFC4861], to build the MAC-to-IP table for
   packet forwarding.  Aggressive MAC randomization from many devices in
   a short time interval may cause the layer-2 switch to exhaust its
   resources, holding in memory traffic for a device whose port location
   can no longer be found.  As these infrastructure devices also
   implement a cache (to remember the port position of each known
   device), to frequent randomized MAC address changes can cause
   resource exhaustion and the flush of older MAC addresses, including
   for devices that did not change their MAC to a new randomized one.
   For the RCM device, these effects translate into session
   discontinuity and return traffic losses.

   In wireless contexts, 802.1X [IEEE_802.1X] authenticators rely on the
   device and user identity validation provided by an AAA server to
   change the interface from a blocking state to a forwarding state.
   The MAC address is used to verify that the device is in the
   authorized list, and to retrieve the associated key used to decrypt
   the device traffic.  A change in MAC address causes the port to be

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   closed to the device data traffic until the AAA server confirms the
   validity of the new MAC address.  Consequently, MAC address
   randomization can disrupt the device traffic and strain the AAA
   server.

   DHCPv4 servers [RFC2131], without a unique identification of the
   device, lose track of which IP address is validly assigned.  Unless
   the RCM device releases the IP address before changing its MAC
   address, DHCPv4 servers are at risk of scope exhaustion, causing new
   devices (and RCM devices) to fail to obtain a new IP address.  Even
   if the RCM device releases the IP address before changing the MAC
   address, the DHCPv4 server typically holds the released IP address
   for a certain duration, in case the leaving MAC returns.  As the
   DHCPv4 [RFC2131] server cannot know if the release is due to a
   temporal disconnection or a MAC randomization, the risk of scope
   address exhaustion exists even in cases where the IP address is
   released.

   Network devices with self-assigned IPv6 addresses (e.g., with SLAAC
   defined in [RFC6620]) and devices using static IP addresses rely on
   mechanisms like Optimistic Duplicate Address Detection (DAD)
   [RFC4429] and NDP [RFC4861] for peer devices to establish the
   association between the target IP address and a MAC address, and
   these peers may cache this association in memory.  Changing the MAC
   address, even at disconnection-reconnection phase, without changing
   the IP address, may disrupt the stability of these mappings for these
   peers, if the change occurs within the caching period.  Note that
   this behavior is against standard operation and existing privacy
   recommendations.  Implementations must avoid changing MAC address
   while maintaining previously assigned IP address without consulting
   the network.

   Routers keep track of which MAC address is on which interface, so
   they can form the proper Data Link header when forwarding a packet to
   a segment where MAC addresses are used.  MAC address randomization
   can cause MAC address cache exhaustion, but also the need for
   frequent Address Resolution Protocol (ARP), Reverse Address
   Resolution Protocol (RARP) [RFC826], Neighbor Solicitation and,
   Neighbor Advertisement [RFC4861] exchanges.

   In residential settings (environment type A in Section 5), policies
   can be in place to control the traffic of some devices (e.g.,
   parental control or block-list filters).  These policies are often
   based on the device's MAC address.  MAC address randomization removes
   the possibility for such control.

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   In residential settings (environment type A) and in enterprises
   (environment types D and E), device recognition and ranging may be
   used for IoT-related functionalities (door unlock, preferred light
   and temperature configuration, etc.)  These functions often rely on
   the detection of the device's wireless MAC address.  MAC address
   randomization breaks the services based on such model.

   In managed residential settings (environment types B) and in
   enterprises (environment types D and E), the network operator is
   often requested to provide IT support.  With MAC address
   randomization, real-time support is only possible if the user can
   provide the current MAC address.  Service improvement support is not
   possible if the MAC address that the device had at the (past) time of
   the reported issue is not known at the time the issue is reported.

   In industrial environments, policies are associated with each group
   of objects, including IoT devices.  MAC address randomization may
   prevent an IoT device from being identified properly, thus leading to
   network quarantine and disruption of operations.

6.2.  Use Cases

   Section 6.1 discusses different environments, different settings, and
   the expectations of users and network operators.  Table 1 summarizes
   the expected degree of trust, network admin responsibility,
   complexity of supported network services, and network support
   expectation from the user for the different use cases defined in
   Section 5.

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   +=======================+===========+=======+========+=============+
   |       Use Cases       |   Trust   |Network|Network |   Network   |
   |                       |   Degree  | Admin |Services|   Support   |
   |                       |           |       |        | Expectation |
   +=======================+===========+=======+========+=============+
   |    (A) Residential    |    Full   |  User | Medium |     Low     |
   |   settings under the  |   trust   |       |        |             |
   |  control of the user  |           |       |        |             |
   +-----------------------+-----------+-------+--------+-------------+
   |      (B) Managed      | Selective |   IT  | Medium |    Medium   |
   |  residential settings |   trust   |       |        |             |
   +-----------------------+-----------+-------+--------+-------------+
   |    (C) Public guest   |    Zero   |  ISP  | Simple |     Low     |
   |        networks       |   trust   |       |        |             |
   +-----------------------+-----------+-------+--------+-------------+
   |  (D) Enterprises with | Selective |   IT  |Complex |    Medium   |
   | Bring-Your-Own-Device |   trust   |       |        |             |
   |         (BYOD)        |           |       |        |             |
   +-----------------------+-----------+-------+--------+-------------+
   |      (E) Managed      |    Full   |   IT  |Complex |     High    |
   |      enterprises      |   trust   |       |        |             |
   +-----------------------+-----------+-------+--------+-------------+

                            Table 1: Use Cases

   For example, a Home network is sometimes considered to be trusted and
   safe, where users are not worried about other users (or the home
   network admin) seeing their MAC address.  Users expect a simple
   procedure to connect to their home network.  All devices in the home
   network often trust each other.  The Home network can also include
   many IoT devices, which need to be simple to onboard and manage.
   Home users typically expect the network operator to protect the home
   network from external threats (i.e., attacks from the Internet).
   Home users also typically expect certain policy features (e.g.,
   Parental Control).  Most home users do not expect to need networking
   skills to manage their home network.  Such an environment may lead to
   full-trust conditions.  Although trust may typically exist between
   allowed actors, there is no guarantee that an eavesdropper isn't
   monitoring the traffic from outside or a rogue IoT device isn't
   monitoring local traffic from within.  The reality often limits the
   effectiveness of trust in most home scenarios.

   On the other end of the spectrum, Public Wi-Fi is often viewed as
   completely untrusted, with users not expecting to trust other users
   or actors inside or outside the layer-2 domain.  Privacy is one of
   the major concerns for users.  Most users connecting to Public Wi-Fi
   only require simple Internet connectivity services and expect limited
   to no technical support.

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   Existing technical frameworks that address some of the requirements
   of the use cases listed above in Appendix A.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   Privacy considerations are discussed throughout this document.

9.  Informative References

   [DOCSIS]   "DOCSIS 4.0 Physical Layer Specification Version I06, DOI
              CM-SP-CM-OSSIv4.0", CableLabs DOCSIS , March 2022,
              <https://www.cablelabs.com/specifications/CM-SP-CM-
              OSSIv4.0?v=I06>.

   [I-D.ietf-radext-deprecating-radius]
              DeKok, A., "Deprecating Insecure Practices in RADIUS",
              Work in Progress, Internet-Draft, draft-ietf-radext-
              deprecating-radius-05, 26 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-radext-
              deprecating-radius-05>.

   [I-D.tomas-openroaming]
              Tomas, B., Grayson, M., Canpolat, N., Cockrell, B., and S.
              Gundavelli, "WBA OpenRoaming Wireless Federation, Work in
              Progress, Internet-Draft, draft-tomas-openroaming-03", 25
              July 2024, <https://datatracker.ietf.org/doc/html/draft-
              tomas-openroaming-03>.

   [IEEE_802] "IEEE Std 802 - IEEE Standard for Local and Metropolitan
              Area Networks: Overview and Architecture, DOI 10.1109/
              IEEESTD.2014.6847097", IEEE 802 , 30 June 2014,
              <https://ieeexplore.ieee.org/document/6847097>.

   [IEEE_802.11]
              "IEEE 802.11-2020 - Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE
              802.11 , 11 February 2021,
              <https://standards.ieee.org/ieee/802.11/7028/>.

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   [IEEE_802.11bh]
              "IEEE 802.11bh-2023 - Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications Amendment 8
              : Operation with Randomized and Changing MAC Addresses",
              IEEE 802.11bh , 19 July 2023,
              <https://ieeexplore.ieee.org/document/10214483>.

   [IEEE_802.11i]
              "IEEE 802.11i-2004 - Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) specifications: Amendment
              6: Medium Access Control (MAC) Security Enhancements, DOI
              10.1109/IEEESTD.2004.94585", IEEE 802.11i , 24 July 2004,
              <https://ieeexplore.ieee.org/document/1318903>.

   [IEEE_802.1X]
              "IEEE 802.1X-2020 - IEEE Standard for Local and
              Metropolitan Area Networks--Port-Based Network Access
              Control, DOI 10.1109/IEEESTD.2020.9018454", IEEE 802.1X ,
              28 February 2020,
              <https://ieeexplore.ieee.org/document/9018454>.

   [IEEE_802.3]
              "IEEE 802.3-2018 - IEEE Standard for Ethernet", IEEE
              802.3 , 31 August 2018,
              <https://standards.ieee.org/ieee/802.3/7071/>.

   [IEEE_802E]
              "IEEE 802E-2020 - IEEE Recommended Practice for Privacy
              Considerations for IEEE 802(R) Technologies", IEEE 802E ,
              13 November 2020,
              <https://standards.ieee.org/ieee/802E/6242/>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3539]  Aboba, B. and J. Wood, "Authentication, Authorization and
              Accounting (AAA) Transport Profile", RFC 3539,
              DOI 10.17487/RFC3539, June 2003,
              <https://www.rfc-editor.org/info/rfc3539>.

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   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [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>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC6614]  Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
              "Transport Layer Security (TLS) Encryption for RADIUS",
              RFC 6614, DOI 10.17487/RFC6614, May 2012,
              <https://www.rfc-editor.org/info/rfc6614>.

   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses",
              RFC 6620, DOI 10.17487/RFC6620, May 2012,
              <https://www.rfc-editor.org/info/rfc6620>.

   [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>.

   [RFC826]   Plummer, D., "An Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <https://www.rfc-editor.org/info/rfc826>.

   [RFC8837]  Jones, P., Dhesikan, S., Jennings, C., and D. Druta,
              "Differentiated Services Code Point (DSCP) Packet Markings
              for WebRTC QoS", RFC 8837, DOI 10.17487/RFC8837, January
              2021, <https://www.rfc-editor.org/info/rfc8837>.

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Appendix A.  Existing Frameworks

A.1.  802.1X with WPA2 / WPA3

   In typical enterprise Wi-Fi environment, 802.1X authentication
   [IEEE_802.1X] coupled with WPA2 or WPA3 [IEEE_802.11i] encryption
   schemes are commonly used for onboarding a Wi-Fi device.  This allows
   the mutual identification of the client device or the user of the
   device and an authentication authority.  The authentication exchange
   does not occur in clear text, and the user or the device identity can
   be concealed from unauthorized observers.  However, the
   authentication authority is in most cases under the control of the
   same entity as the network access provider.  This may lead to expose
   the user or device identity to the network owner.

   This scheme is well-adapted to enterprise environment, where a level
   of trust is established between the user and the enterprise network
   operator.  In this scheme, MAC address randomization can occur
   through brief disconnections and reconnections (under the rules of
   [IEEE_802.11bh]).  Authentication may then need to reoccur, with an
   associated cost of service disruption and additional load on the
   enterprise infrastructure, and an associated benefit of limiting the
   exposure of a continuous MAC address to external observers.  The
   adoption of this scheme is limited outside of the enterprise
   environment by the requirement to install an authentication profile
   on the end device, which would be recognized and accepted by a local
   authentication authority and its authentication server.  Such a
   server is uncommon in a home environment, and the procedure to
   install a profile is cumbersome for most untrained users.  The
   likelihood that a user or device profile would match a profile
   recognized by a public Wi-Fi authentication authority is also fairly
   limited.  This may restrict the adoption of this scheme for public
   Wi-Fi as well.  Similar limitations are found in hospitality
   environment.  Hospitality environment refers to space provided by
   hospitality industry, which includes but not limited to hotels,
   stadiums, restaurants, concert halls and hospitals.

A.2.  OpenRoaming

   In order to alleviate some of the limitations listed above, the
   Wireless Broadband Alliance (WBA) OpenRoaming Standard introduces an
   intermediate trusted relay between local venues (places where some
   public Wi-Fi is available) and sources of identity
   [I-D.tomas-openroaming].  The federation structure extends the type
   of authorities that can be used as identity sources (compared to
   traditional enterprise-based 802.1X [IEEE_802.1X] scheme for Wi-Fi),
   and facilitates the establishment of trust between local networks and
   an identity provider.  Such a procedure increases the likelihood that

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   one or more identity profiles for the user or the device will be
   recognized by a local network.  At the same time, authentication does
   not occur to the local network.  This may offer the possibility for
   the user or the device to keep their identity obfuscated from the
   local network operator, unless that operator specifically expresses
   the requirement to disclose such identity (in which case the user has
   the option to accept or decline the connection and associated
   identity exposure).

   The OpenRoaming scheme seems well-adapted to public Wi-Fi and
   hospitality environment.  It defines a framework to protect the
   identity from unauthorized entities while to permit mutual
   authentication between the device or the user and a trusted identity
   provider.  Just like with standard 802.1X [IEEE_802.1X] scheme for
   Wi-Fi, authentication allows for the establishment of WPA2 or WPA3
   keys [IEEE_802.11i] that can then be used to encrypt the
   communication between the device and the access point.  The
   encryption adds extra protection to prevent the network traffic from
   being eavesdropped.

   MAC address randomization can occur through brief disconnections and
   reconnections (under the rules of [IEEE_802.11bh]).  Authentication
   may then need to reoccur, with an associated cost of service
   disruption and additional load on the venue and identity provider
   infrastructure, and an associated benefit of limiting the exposure of
   a continuous MAC address to external observers.  Limitations of this
   scheme include the requirement to first install one or more profiles
   on the client device.  This scheme also requires the local network to
   support RADSEC [RFC6614] and the relay function, which may not be
   common in small hotspot networks and home environment.

   It is worth noting that, as part of collaborations between IETF
   MADINAS and WBA around OpenRoaming, some RADIUS privacy enhancements
   have been proposed in the IETF RADEXT group.  For instance,
   [I-D.ietf-radext-deprecating-radius] describes good practices in the
   use of Chargeable-User-Identity (CUI) between different visited
   networks, making it better suited for Public Wi-Fi and Hospitality
   use cases.

A.3.  Proprietary RCM schemes

   Most client device operating system vendors offer RCM schemes,
   enabled by default (or easy to enable) on client devices.  With these
   schemes, the device changes its MAC address, when not associated,
   after having used a given MAC address for a semi-random duration
   window.  These schemes also allow for the device to manifest a
   different MAC address in different SSIDs.

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   Such a randomization scheme enables the device to limit the duration
   of exposure of a single MAC address to observers.  In
   [IEEE_802.11bh], MAC address randomization is not allowed during a
   given association session, and MAC address randomization can only
   occur through disconnection and reconnection.  Authentication may
   then need to reoccur, with an associated cost of service disruption
   and additional load on the venue and identity provider
   infrastructure, directly proportional to the frequency of the
   randomization.  The scheme is also not intended to protect from the
   exposure of other identifiers to the venue network (e.g., DHCP option
   012 [host name] visible to the network between the AP and the DHCPv4
   server).

Authors' Addresses

   Jerome Henry
   Cisco Systems
   United States of America
   Email: jerhenry@cisco.com

   Yiu L. Lee
   Comcast
   1800 Arch Street
   Philadelphia, PA 19103
   United States of America
   Email: yiu_lee@comcast.com

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