Structured Local Address Plan (SLAP) Quadrant Selection Option for DHCPv6
RFC 8948

Document Type RFC - Proposed Standard (December 2020; No errata)
Authors Carlos Bernardos  , Alain Mourad 
Last updated 2020-12-01
Replaces draft-bernardos-dhc-slap-quadrant
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Internet Engineering Task Force (IETF)                     CJ. Bernardos
Request for Comments: 8948                                          UC3M
Category: Standards Track                                      A. Mourad
ISSN: 2070-1721                                             InterDigital
                                                           December 2020

   Structured Local Address Plan (SLAP) Quadrant Selection Option for
                                 DHCPv6

Abstract

   The IEEE originally structured the 48-bit Media Access Control (MAC)
   address space in such a way that half of it was reserved for local
   use.  In 2017, the IEEE published a new standard (IEEE Std 802c) with
   a new optional Structured Local Address Plan (SLAP).  It specifies
   different assignment approaches in four specified regions of the
   local MAC address space.

   The IEEE is developing protocols to assign addresses (IEEE P802.1CQ).
   There is also work in the IETF on specifying a new mechanism that
   extends DHCPv6 operation to handle the local MAC address assignments.

   This document 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.  A new DHCPv6 option
   (QUAD) is defined for this purpose.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8948.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction
     1.1.  Problem Statement
       1.1.1.  Wi-Fi (IEEE 802.11) Devices
       1.1.2.  Hypervisor: Functions That Are and Are Not Migratable
   2.  Terminology
   3.  DHCPv6 Extensions
     3.1.  Address Assignment from the Preferred SLAP Quadrant
           Indicated by the Client
     3.2.  Address Assignment from the Preferred SLAP Quadrant
           Indicated by the Relay
   4.  DHCPv6 Option Definition
     4.1.  QUAD Option
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Example Uses of Quadrant Selection Mechanisms
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The IEEE structures the 48-bit MAC address space in such a way that
   half of it is reserved for local use (where the Universal/Local (U/L)
   bit is set to 1).  In 2017, the IEEE published a new standard
   [IEEEStd802c] that defines a new optional Structured Local Address
   Plan (SLAP) that specifies different assignment approaches in four
   specified regions of the local MAC address space.  These four
   regions, called SLAP quadrants, are briefly described below (see
   Figure 1 and Table 1 for details):

   *  In SLAP Quadrant 01, Extended Local Identifier (ELI) MAC addresses
      are assigned based on a 24-bit Company ID (CID), which is assigned
      by the IEEE Registration Authority (RA).  The remaining bits are
      specified as an extension by the CID assignee or by a protocol
      designated by the CID assignee.

   *  In SLAP Quadrant 11, Standard Assigned Identifier (SAI) MAC
      addresses are assigned based on a protocol specified in an IEEE
      802 standard.  For 48-bit MAC addresses, 44 bits are available.
      Multiple protocols for assigning SAIs may be specified in IEEE
      standards.  Coexistence of multiple protocols may be supported by
      limiting the subspace available for assignment by each protocol.

   *  In SLAP Quadrant 00, Administratively Assigned Identifier (AAI)
      MAC addresses are assigned locally by an administrator.  Multicast
      IPv6 packets use a destination address starting in 33-33, so AAI
      addresses in that range should not be assigned.  For 48-bit MAC
      addresses, 44 bits are available.

   *  SLAP Quadrant 10 is "Reserved for future use" where MAC addresses
      may be assigned using new methods yet to be defined or until then
      by an administrator as in the AAI quadrant.  For 48-bit MAC
      addresses, 44 bits would be available.

          LSB                MSB
          M  X  Y  Z  -  -  -  -
          |  |  |  |
          |  |  |  +------------ SLAP Z-bit
          |  |  +--------------- SLAP Y-bit
          |  +------------------ X-bit (U/L) = 1 for locally assigned
          +--------------------- M-bit (I/G) (unicast/group)

          Legend:
          LSB: Least Significant Bit
          MSB: Most Significant Bit

    Figure 1: IEEE 48-Bit MAC Address Structure (in IEEE Representation)

     +==========+=======+=======+=======================+============+
     | Quadrant | Y-bit | Z-bit | Local Identifier Type | Local      |
     |          |       |       |                       | Identifier |
     +==========+=======+=======+=======================+============+
     | 01       | 0     | 1     | Extended Local        | ELI        |
     +----------+-------+-------+-----------------------+------------+
     | 11       | 1     | 1     | Standard Assigned     | SAI        |
     +----------+-------+-------+-----------------------+------------+
     | 00       | 0     | 0     | Administratively      | AAI        |
     |          |       |       | Assigned              |            |
     +----------+-------+-------+-----------------------+------------+
     | 10       | 1     | 0     | Reserved              | Reserved   |
     +----------+-------+-------+-----------------------+------------+

                          Table 1: SLAP Quadrants

1.1.  Problem Statement

   The IEEE is developing mechanisms to assign addresses
   [IEEE-P802.1CQ-Project].  And [RFC8947] specifies a new mechanism
   that extends DHCPv6 operation to handle the local MAC address
   assignments.  This document 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 the MAC
   addresses in the quadrant requested by the relay or client.

   In the following, we describe two application scenarios in which a
   need arises to assign local MAC addresses according to preferred SLAP
   quadrants.

1.1.1.  Wi-Fi (IEEE 802.11) Devices

   Today, most Wi-Fi devices come with interfaces that have a "burned-
   in" MAC address, allocated from the universal address space using a
   24-bit Organizationally Unique Identifier (OUI) (assigned to IEEE 802
   interface vendors).  However, recently, the need to assign local
   (instead of universal) MAC addresses has emerged particularly in the
   following two scenarios:

   *  IoT (Internet of Things): In general, composed of constrained
      devices [RFC7228] with constraints such as available power and
      energy, memory, and processing resources.  Examples of this
      include sensors and actuators for health or home automation
      applications.  Given the increasingly high number of these
      devices, manufacturers might prefer to equip devices with
      temporary MAC addresses used only at first boot.  These temporary
      MAC addresses would just be used to send initial DHCP packets to
      available DHCP servers.  IoT devices typically need a single MAC
      address for each available network interface.  Once the server
      assigns a MAC address, the device would abandon its temporary MAC
      address.  Home automation IoT devices typically do not move
      (however, note that there is an increase of mobile smart health
      monitoring devices).  For this type of device, in general, any
      type of SLAP quadrant would be good for assigning addresses, but
      ELI/SAI quadrants might be more suitable in some scenarios.  For
      example, the device might need to use an address from the CID
      assigned to the IoT communication device vendor, thus preferring
      the ELI quadrant.

   *  Privacy: Today, MAC addresses allow the exposure of user locations
      making it relatively easy to track user movements.  One of the
      mechanisms considered to mitigate this problem is the use of local
      random MAC addresses: changing them every time the user connects
      to a different network.  In this scenario, devices are typically
      mobile.  Here, AAI is probably the best SLAP quadrant from which
      to assign addresses; it is the best fit for randomization of
      addresses, and it is not required for the addresses to survive
      when changing networks.

1.1.2.  Hypervisor: Functions That Are and Are Not Migratable

   In large-scale virtualization environments, thousands of virtual
   machines (VMs) are active.  These VMs are typically managed by a
   hypervisor, which is in charge of spawning and stopping VMs as
   needed.  The hypervisor is also typically in charge of assigning new
   MAC addresses to the VMs.  If a DHCP solution is in place for that,
   the hypervisor acts as a DHCP client and requests that available DHCP
   servers assign one or more MAC addresses (an address block).  The
   hypervisor does not use those addresses for itself, but rather it
   uses them to create new VMs with appropriate MAC addresses.  If we
   assume very large data-center environments, such as the ones that are
   typically used nowadays, it is expected that the data center is
   divided in different network regions, each one managing its own local
   address space.  In this scenario, there are two possible situations
   that need to be tackled:

   *  Migratable functions: If a VM (providing a given function) needs
      to be migrated to another region of the data center (e.g., for
      maintenance, resilience, end-user mobility, etc.), the VM's
      networking context needs to migrate with the VM.  This includes
      the VM's MAC address(es).  Since the assignments from the ELI/SAI
      SLAP quadrants are governed by rules per IEEE Std 802c, they are
      more appropriate for use to ensure MAC address uniqueness globally
      in the data center.

   *  Functions that are not migratable: If a VM will not be migrated to
      another region of the data center, there are no requirements
      associated with its MAC address.  In this case, it is simpler to
      allocate it from the AAI SLAP quadrant, which does not need to be
      unique across multiple data centers (i.e., each region can manage
      its own MAC address assignment without checking for duplicates
      globally).

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Where relevant, the DHCPv6 terminology from [RFC8415] also applies
   here.  Additionally, the following definitions are updated for this
   document.

   address        Unless specified otherwise, a link-layer (or MAC)
                  address, as specified in [IEEEStd802].  The address is
                  6 or 8 bytes long.

   address block  A number of consecutive link-layer addresses.  An
                  address block is expressed as a first address plus a
                  number that designates the number of additional
                  (extra) addresses.  A single address can be
                  represented by the address itself and zero extra
                  addresses.

   client         A node that is interested in obtaining link-layer
                  addresses.  It implements the basic DHCP mechanisms
                  needed by a DHCP client, as described in [RFC8415],
                  and supports the options (IA_LL and LLADDR) specified
                  in [RFC8947] as well as the new option (QUAD)
                  specified in this document.  The client may or may not
                  support IPv6 address assignment and prefix delegation,
                  as specified in [RFC8415].

   IA_LL          Identity Association for Link-Layer Address, an
                  identity association (IA) used to request or assign
                  link-layer addresses.  Section 11.1 of [RFC8947]
                  provides details on the IA_LL option.

   LLADDR         Link-layer address option that is used to request or
                  assign a block of link-layer addresses.  Section 11.2
                  of [RFC8947] provides details on the LLADDR option.

   relay          A node that acts as an intermediary to deliver DHCP
                  messages between clients and servers.  A relay, based
                  on local knowledge and policies, may include the
                  preferred SLAP quadrant in a QUAD option sent to the
                  server.  The relay implements basic DHCPv6 relay agent
                  functionality, as described in [RFC8415].

   server         A node that manages link-layer address allocation and
                  is able to respond to client queries.  It implements
                  basic DHCP server functionality, as described in
                  [RFC8415], and supports the options (IA_LL and LLADDR)
                  specified in [RFC8947] as well as the new option
                  (QUAD) specified in this document.  The server may or
                  may not support IPv6 address assignment and prefix
                  delegation, as specified in [RFC8415].

3.  DHCPv6 Extensions

3.1.  Address Assignment from the Preferred SLAP Quadrant Indicated by
      the Client

   Next, we describe the protocol operations for a client to select a
   preferred SLAP quadrant using the DHCPv6 signaling procedures
   described in [RFC8947].  The signaling flow is shown in Figure 2.

    +--------+                            +--------+
    | DHCPv6 |                            | DHCPv6 |
    | client |                            | server |
    +--------+                            +--------+
        |                                      |
        +----1. Solicit(IA_LL(LLADDR,QUAD))--->|
        |                                      |
        |<--2. Advertise(IA_LL(LLADDR,QUAD))---+
        |                                      |
        +---3. Request(IA_LL(LLADDR,QUAD))---->|
        |                                      |
        |<------4. Reply(IA_LL(LLADDR))--------+
        |                                      |
        .                                      .
        .          (timer expiring)            .
        .                                      .
        |                                      |
        +---5. Renew(IA_LL(LLADDR,QUAD))------>|
        |                                      |
        |<-----6. Reply(IA_LL(LLADDR))---------+
        |                                      |

              Figure 2: DHCPv6 Signaling Flow (Client-Server)

   1.  Link-layer addresses (i.e., MAC addresses) are assigned in
       blocks.  The smallest block is a single address.  To request an
       assignment, the client sends a Solicit message with an IA_LL
       option in the message.  The IA_LL option MUST contain an LLADDR
       option.  In order to indicate the preferred SLAP quadrant(s), the
       IA_LL option includes the new OPTION_SLAP_QUAD option in the
       IA_LL-option field (alongside the LLADDR option).

   2.  The server, upon receiving an IA_LL option in a Solicit message,
       inspects its contents.  For each of the entries in the
       OPTION_SLAP_QUAD, in order of the preference field (highest to
       lowest), the server checks if it has a configured MAC address
       pool matching the requested quadrant identifier and an available
       range of addresses of the requested size.  If suitable addresses
       are found, the server sends back an Advertise message with an
       IA_LL option containing an LLADDR option that specifies the
       addresses being offered.  If the server manages a block of
       addresses belonging to a requested quadrant, the addresses being
       offered MUST belong to a requested quadrant.  If the server does
       not have a configured address pool matching the client's request,
       it SHOULD return the IA_LL option with the addresses being
       offered belonging to a quadrant managed by the server (following
       a local policy to select from which of the available quadrants).
       If the server has a configured address pool of the correct
       quadrant but no available addresses, it MUST return the IA_LL
       option containing a Status Code option with status set to
       NoAddrsAvail.

   3.  The client waits for available servers to send Advertise
       responses and picks one server, as defined in Section 18.2.9 of
       [RFC8415].  The client SHOULD NOT pick a server that does not
       advertise an address in the requested quadrant(s).  The client
       then sends a Request message that includes the IA_LL container
       option with the LLADDR option copied from the Advertise message
       sent by the chosen server.  It includes the preferred SLAP
       quadrant(s) in a new QUAD IA_LL option.

   4.  Upon reception of a Request message with an IA_LL container
       option, the server assigns requested addresses.  The server MAY
       alter the allocation at this time (e.g., by reducing the address
       block).  It then generates and sends a Reply message back to the
       client.  Upon receiving a Reply message, the client parses the
       IA_LL container option and may start using all provided
       addresses.  Note that a client that has included a Rapid Commit
       option in the Solicit message may receive a Reply message in
       response to the Solicit message and skip the Advertise and
       Request message steps above (following standard DHCPv6
       procedures).

   5.  The client is expected to periodically renew the link-layer
       addresses, as governed by T1 and T2 timers.  This mechanism can
       be administratively disabled by the server sending "infinity" as
       the T1 and T2 values (see Section 7.7 of [RFC8415]).  The client
       sends a Renew option after T1.  It includes the preferred SLAP
       quadrant(s) in the new QUAD IA_LL option, so in case the server
       is unable to extend the lifetime on the existing address(es), the
       preferred quadrants are known for the allocation of any "new"
       (i.e., different) addresses.

   6.  The server responds with a Reply message with an IA_LL option
       that includes an LLADDR option with extended lifetime.

   The client SHOULD check if the received MAC address comes from one of
   the requested quadrants.  It MAY repeat the process selecting a
   different DHCP server.

3.2.  Address Assignment from the Preferred SLAP Quadrant Indicated by
      the Relay

   Next, we describe the protocol operations for a relay to select a
   preferred SLAP quadrant using the DHCPv6 signaling procedures
   described in [RFC8947].  This is useful when a DHCPv6 server is
   operating over a large infrastructure split in different network
   regions, where each region might have different requirements.  The
   signaling flow is shown in Figure 3.

   +--------+                  +--------+                     +--------+
   | DHCPv6 |                  | DHCPv6 |                     | DHCPv6 |
   | client |                  | relay  |                     | server |
   +--------+                  +--------+                     +--------+
      |                            |                                |
      +-----1. Solicit(IA_LL)----->|                                |
      |                            +----2. Relay-forward            |
      |                            |    (Solicit(IA_LL),QUAD)------>|
      |                            |                                |
      |                            |<---3. Relay-reply              |
      |                            |    (Advertise(IA_LL(LLADDR)))--+
      |<4. Advertise(IA_LL(LLADDR))+                                |
      |-5. Request(IA_LL(LLADDR))->|                                |
      |                            +-6. Relay-forward               |
      |                            | (Request(IA_LL(LLADDR)),QUAD)->|
      |                            |                                |
      |                            |<--7. Relay-reply               |
      |                            |   (Reply(IA_LL(LLADDR)))-------+
      |<--8. Reply(IA_LL(LLADDR))--+                                |
      |                            |                                |
      .                            .                                .
      .                    (timer expiring)                         .
      .                            .                                .
      |                            |                                |
      +--9. Renew(IA_LL(LLADDR))-->|                                |
      |                            |--10. Relay-forward             |
      |                            |  (Renew(IA_LL(LLADDR)),QUAD)-->|
      |                            |                                |
      |                            |<---11. Relay-reply             |
      |                            |     (Reply(IA_LL(LLADDR)))-----+
      |<--12. Reply(IA_LL(LLADDR))-+                                |
      |                            |                                |

           Figure 3: DHCPv6 Signaling Flow (Client-Relay-Server)

   1.   Link-layer addresses (i.e., MAC addresses) are assigned in
        blocks.  The smallest block is a single address.  To request an
        assignment, the client sends a Solicit message with an IA_LL
        option in the message.  The IA_LL option MUST contain an LLADDR
        option.

   2.   The DHCP relay receives the Solicit message and encapsulates it
        in a Relay-forward message.  The relay, based on local knowledge
        and policies, includes in the Relay-forward message the QUAD
        option with the preferred quadrant.  The relay might know which
        quadrant to request based on local configuration (e.g., the
        served network contains IoT devices only, thus requiring ELI/
        SAI) or other means.  Note that if a client sends multiple
        instances of the IA_LL option in the same message, the DHCP
        relay MAY only add a single instance of the QUAD option.

   3.   Upon receiving a relayed message containing an IA_LL option, the
        server inspects the contents for instances of OPTION_SLAP_QUAD
        in both the Relay-forward message and the client's message
        payload.  Depending on the server's policy, the instance of the
        option to process is selected (see the end of this section).
        For each of the entries in OPTION_SLAP_QUAD, in order of the
        preference field (highest to lowest), the server checks if it
        has a configured MAC address pool matching the requested
        quadrant identifier and an available range of addresses of the
        requested size.  If suitable addresses are found, the server
        sends back an Advertise message with an IA_LL option containing
        an LLADDR option that specifies the addresses being offered.
        This message is sent to the Relay in a Relay-reply message.  If
        the server supports the semantics of the preferred quadrant
        included in the QUAD option and manages a block of addresses
        belonging to a requested quadrant, then the addresses being
        offered MUST belong to a requested quadrant.  The server SHOULD
        apply the contents of the relay's supplied QUAD option for all
        of the client's IA_LLs, unless configured to do otherwise.

   4.   The relay sends the received Advertise message to the client.

   5.   The client waits for available servers to send Advertise
        responses and picks one server, as defined in Section 18.2.9 of
        [RFC8415].  The client then sends a Request message that
        includes the IA_LL container option with the LLADDR option
        copied from the Advertise message sent by the chosen server.

   6.   The relay forwards the received Request in a Relay-forward
        message.  It adds, in the Relay-forward, a QUAD IA_LL option
        with the preferred quadrant.

   7.   Upon reception of the forwarded Request message with IA_LL
        container option, the server assigns requested addresses.  The
        server MAY alter the allocation at this time.  It then generates
        and sends a Reply message in a Relay-reply message back to the
        relay.

   8.   Upon receiving a Reply message, the client parses the IA_LL
        container option and may start using all provided addresses.

   9.   The client is expected to periodically renew the link-layer
        addresses, as governed by T1 and T2 timers.  This mechanism can
        be administratively disabled by the server sending "infinity" as
        the T1 and T2 values (see Section 7.7 of [RFC8415]).  The client
        sends a Renew option after T1.

   10.  This message is forwarded by the relay in a Relay-forward
        message, including a QUAD IA_LL option with the preferred
        quadrant.

   11.  The server responds with a Reply message, including an LLADDR
        option with extended lifetime.  This message is sent in a Relay-
        reply message.

   12.  The relay sends the Reply message back to the client.

   The server SHOULD implement a configuration parameter to deal
   with the case where the client's DHCP message contains an instance of
   OPTION_SLAP_QUAD and the relay adds a second instance in its Relay-
   forward message.  This parameter configures the server to process
   either the client's or the relay's instance of option QUAD.  It is
   RECOMMENDED that the default for such a parameter is to process the
   client's instance of the option.

   The client MAY check if the received MAC address belongs to a
   quadrant it is willing to use/configure and MAY decide based on that
   whether to use/configure the received address.

4.  DHCPv6 Option Definition

4.1.  QUAD Option

   The QUAD option is used to specify the preferences for the selected
   quadrants within an IA_LL.  The option MUST be encapsulated either in
   the IA_LL-options field of an IA_LL option or in a Relay-forward
   message.

   The format of the QUAD option is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       OPTION_SLAP_QUAD        |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   quadrant-1  |    pref-1     |   quadrant-2  |    pref-2     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  quadrant-n-1 |   pref-n-1    |   quadrant-n  |    pref-n     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4: QUAD Option Format

   option-code     OPTION_SLAP_QUAD (140).

   option-len      2 * number of included (quadrant, preference).  This
                   is a 2-byte field containing the total length of all
                   (quadrant, preference) pairs included in the option.

   quadrant-n      Identifier of the quadrant (0: AAI, 1: ELI, 2:
                   Reserved by IEEE [IEEEStd802c], and 3: SAI).  Each
                   quadrant MUST only appear once at most in the option.
                   This is a 1-byte field.

   pref-n          Preference associated to quadrant-n.  A higher value
                   means a more preferred quadrant.  This is a 1-byte
                   field.

   A quadrant identifier value MUST only appear, at most, once in the
   option. If an option includes more than one occurrence of the same
   quadrant identifier, only the first occurrence is processed, and the
   rest MUST be ignored by the server.

   If the same preference value is used for more than one quadrant, the
   server MAY select which quadrant should be preferred (if the server
   can assign addresses from all or some of the quadrants with the same
   assigned preference).  Note that this is not a simple list of
   quadrants ordered by preference with no preference value, but a list
   of quadrants with explicit preference values.  This way it can
   support the case whereby a client really has no preference between
   two or three quadrants, leaving the decision to the server.

   If the client or relay agent provides the OPTION_SLAP_QUAD, the
   server MUST use the quadrant-n/pref-n values to order the selection
   of the quadrants.  If the server can provide an assignment from one
   of the specified quadrants, it SHOULD proceed with the assignment.
   If the server does not have a configured address pool matching any of
   the specified quadrant-n fields or if the server has a configured
   address pool of the correct quadrant but no available addresses, it
   MUST return the IA_LL option containing a status of NoAddrsAvail.

   There is no requirement that the client or relay agent order the
   quadrant/pref values in any specific order; hence, servers MUST NOT
   assume that quadrant-1/pref-1 have the highest preference (except if
   there is only one set of values).

   For cases where a server may not be configured to have pools for the
   client or relay quadrant preferences, clients and relays SHOULD
   specify all quadrants in the QUAD option to assure the client gets an
   address (or addresses) -- if any are available.  Specifying all
   quadrants also results in a QUAD option supporting server responding
   like a non-QUAD option supporting server, i.e., an address (or
   addresses) from any available quadrants can be returned.

5.  IANA Considerations

   IANA has assigned the QUAD (140) option code from the "Option Codes"
   subregistry of the "Dynamic Host Configuration Protocol for IPv6
   (DHCPv6)" registry maintained at <http://www.iana.org/assignments/
   dhcpv6-parameters>:

   Value:  140
   Description:  OPTION_SLAP_QUAD
   Client ORO:  No
   Singleton Option:  Yes
   Reference:  RFC 8948

6.  Security Considerations

   See [RFC8415] and [RFC7227] for the DHCPv6 security and privacy
   considerations.  See [RFC8200] for the IPv6 security considerations.

   Also, see [RFC8947] for security considerations regarding link-layer
   address assignments using DHCP.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

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

7.2.  Informative References

   [IEEE-P802.1CQ-Project]
              IEEE, "P802.1CQ - Standard for Local and Metropolitan Area
              Networks: Multicast and Local Address Assignment",
              <https://standards.ieee.org/project/802_1CQ.html>.

   [IEEEStd802]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Overview and Architecture", IEEE Std 802-2014,
              DOI 10.1109/IEEESTD.2014.6847097, June 2014,
              <https://doi.org/10.1109/IEEESTD.2014.6847097>.

   [IEEEStd802c]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Overview and Architecture -- Amendment 2: Local
              Medium Access Control (MAC) Address Usage", IEEE Std 802c-
              2017, DOI 10.1109/IEEESTD.2017.8016709, August 2017,
              <https://doi.org/10.1109/IEEESTD.2017.8016709>.

   [RFC7227]  Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
              S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
              BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
              <https://www.rfc-editor.org/info/rfc7227>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7548]  Ersue, M., Ed., Romascanu, D., Schoenwaelder, J., and A.
              Sehgal, "Management of Networks with Constrained Devices:
              Use Cases", RFC 7548, DOI 10.17487/RFC7548, May 2015,
              <https://www.rfc-editor.org/info/rfc7548>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

Appendix A.  Example Uses of Quadrant Selection Mechanisms

   This appendix describes some examples of how the quadrant preference
   mechanisms could be used.

   First, let's take an IoT scenario as an example.  An IoT device might
   decide on its own the SLAP quadrant it wants to use to obtain a local
   MAC address, using the following information to make the decision:

   *  Type of IoT deployment: For example, industrial, domestic, rural,
      etc.  For small deployments, such as domestic ones, the IoT device
      itself can decide to use the AAI quadrant (this might not even
      involve the use of DHCP, by the device just configuring a random
      address computed by the device itself).  For large deployments,
      such as industrial or rural ones, where thousands of devices might
      coexist, the IoT can decide to use the ELI or SAI quadrants.

   *  Mobility: If the IoT device can move, then it might prefer to
      select the SAI or AAI quadrants to minimize address collisions
      when moving to another network.  If the device is known to remain
      fixed, then the ELI is probably the most suitable one to use.

   *  Managed/Unmanaged: Depending on whether the IoT device is managed
      during its lifetime or cannot be reconfigured [RFC7548], the
      decision of what quadrant is more appropriate might be different.
      For example, if the IoT device can be managed (e.g., configured)
      and network topology changes might occur during its lifetime
      (e.g., due to changes on the deployment, such as extensions
      involving additional devices), this has an impact on the preferred
      quadrant (e.g., to avoid potential collisions in the future).

   *  Operation / Battery Lifetime: Depending on the expected lifetime
      of the device, a different quadrant might be preferred (as before,
      to minimize potential address collisions in the future).

   The previous parameters are considerations that the device vendor/
   administrator may wish to use when defining the IoT device's
   MAC address request policy (i.e., how to select a given SLAP
   quadrant).  IoT devices are typically very resource constrained, so
   there may only be a simple decision-making process based on
   preconfigured preferences.

   We now take the Wi-Fi device scenario, considering, for example, that
   a laptop or smartphone connects to a network using its built-in MAC
   address.  Due to privacy/security concerns, the device might want to
   configure a local MAC address.  The device might use different
   parameters and context information to decide, not only which SLAP
   quadrant to use for the local MAC address configuration, but also
   when to perform a change of address (e.g., it might be needed to
   change address several times).  This information includes, but it is
   not limited to:

   *  Type of network the device is connected: public, work, home.

   *  Trusted network: Yes/No.

   *  First time visited network: Yes/No.

   *  Network geographical location.

   *  Mobility: Yes (the device might change its network attachment
      point) / No (the device is not going to change its network
      attachment point).

   *  Operating System (OS) network profile, including security/trust-
      related parameters: Most modern OSs keep metadata associated with
      the networks they can attach to as, for example, the level of
      trust the user or administrator assigns to the network.  This
      information is used to configure how the device behaves in terms
      of advertising itself on the network, firewall settings, etc.  But
      this information can also be used to decide whether or not to
      configure a local MAC address, from which SLAP quadrant it should
      be assigned, and how often it may be assigned.

   *  Triggers coming from applications regarding location privacy: An
      app might request that the OS maximize location privacy (due to
      the nature of the application), and this might require the OS to
      force the use of a local MAC address or the local MAC address to
      be changed.

   This information can be used by the device to select the SLAP
   quadrant.  For example, if the device is moving around (e.g., while
   connected to a public network in an airport), it is likely that it
   might change access points several times; therefore, it is best to
   minimize the chances of address collision, using the SAI or AAI
   quadrants.  If the device is not expected to move and is attached to
   a trusted network (e.g., in some scenarios at work), then it is
   probably best to select the ELI quadrant.  These are just some
   examples of how to use this information to select the quadrant.

   Additionally, the information can also be used to trigger subsequent
   changes of MAC address to enhance location privacy.  Besides,
   changing the SLAP quadrant might also be used as an additional
   enhancement to make it harder to track the user location.

   Last, if we consider the data-center scenario, a hypervisor might
   request local MAC addresses be assigned to virtual machines.  As in
   the previous scenarios, the hypervisor might select the preferred
   SLAP quadrant using information provided by the cloud management
   system or virtualization infrastructure manager running on top of the
   hypervisor.  This information might include, but is not limited to:

   *  Migratable VM: If the function implemented by the VM is subject to
      be moved to another physical server or not, this has an impact on
      the preference for the SLAP quadrant, as the ELI and SAI quadrants
      are better suited for supporting migration in a large data center.

   *  VM connectivity characteristics: For example, standalone, part of
      a pool, and part of a service graph/chain.  If the connectivity
      characteristics of the VM are known, this can be used by the
      hypervisor to select the best SLAP quadrant.

Acknowledgments

   The authors would like to thank Bernie Volz for his very valuable
   comments on this document.  We also want to thank Ian Farrer, Tomek
   Mrugalski, Éric Vyncke, Tatuya Jinmei, Carl Wallace, Ines Robles, Ted
   Lemon, Jaime Jimenez, Robert Wilton, Benjamin Kaduk, Barry Leiba,
   Alvaro Retana, and Murray Kucherawy for their very detailed and
   helpful reviews.  And thanks to Roger Marks and Antonio de la Oliva
   for comments related to IEEE work and references.

   The work in this document has been supported by the H2020 5GROWTH
   (Grant 856709) and 5G-DIVE projects (Grant 859881).

Authors' Addresses

   Carlos J. Bernardos
   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/

   Alain Mourad
   InterDigital Europe

   Email: Alain.Mourad@InterDigital.com
   URI:   http://www.InterDigital.com/