Link-Layer Address Assignment Mechanism for DHCPv6
RFC 8947



Internet Engineering Task Force (IETF)                           B. Volz
Request for Comments: 8947                                         Cisco
Category: Standards Track                                   T. Mrugalski
ISSN: 2070-1721                                                      ISC
                                                           CJ. Bernardos
                                                                    UC3M
                                                           December 2020

           Link-Layer Address Assignment Mechanism for DHCPv6

Abstract

   In certain environments, e.g., large-scale virtualization
   deployments, new devices are created in an automated manner.  Such
   devices may have their link-layer addresses assigned in an automated
   fashion.  With sufficient scale, the likelihood of a collision using
   random assignment without duplication detection is not acceptable.
   Therefore, an allocation mechanism is required.  This document
   proposes an extension to DHCPv6 that allows a scalable approach to
   link-layer address assignments where preassigned link-layer address
   assignments (such as by a manufacturer) are not possible or are
   unnecessary.

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/rfc8947.

Copyright Notice

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

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Terminology
   4.  Deployment Scenarios
     4.1.  Scenario: Proxy Client Mode
     4.2.  Scenario: Direct Client Mode
   5.  Mechanism Overview
   6.  Design Assumptions
   7.  Information Encoding
   8.  Requesting Addresses
   9.  Renewing Addresses
   10. Releasing Addresses
   11. Option Definitions
     11.1.  Identity Association for Link-Layer Addresses Option
     11.2.  Link-Layer Addresses Option
   12. Selecting Link-Layer Addresses for Assignment to an IA_LL
   13. IANA Considerations
   14. Security Considerations
   15. Privacy Considerations
   16. References
     16.1.  Normative References
     16.2.  Informative References
   Appendix A.  IEEE 802c Summary
   Acknowledgments
   Authors' Addresses

1.  Introduction

   There are several deployment types that deal with a large number of
   devices that need to be initialized.  One of them is a scenario where
   virtual machines (VMs) are created on a massive scale.  Typically,
   the new VM instances are assigned a link-layer address, but random
   assignment does not scale well due to the risk of a collision (see
   Appendix A.1 of [RFC4429]).  Another use case is Internet of Things
   (IoT) devices (see [RFC7228]).  The huge number of such devices could
   strain the IEEE's available Organizationally Unique Identifier (OUI)
   global address space.  While there is typically no need to provide
   global link-layer address uniqueness for such devices, a link-layer
   assignment mechanism allows for conflicts to be avoided inside an
   administrative domain.  For those reasons, it is desired to have some
   form of mechanism that would be able to assign locally unique Media
   Access Control (MAC) addresses.

   This document proposes a new mechanism that extends DHCPv6 operation
   to handle link-layer address assignments.

   Since DHCPv6 [RFC8415] is a protocol that can allocate various types
   of resources (non-temporary addresses, temporary addresses, prefixes,
   as well as many options) and has the necessary infrastructure to
   maintain such allocations (numerous server and client
   implementations, large deployed relay infrastructure, and supportive
   solutions such as leasequery and failover), it is a good candidate to
   address the desired functionality.

   While this document presents a design that should be usable for any
   link-layer address type, some of the details are specific to IEEE 802
   48-bit MAC addresses [IEEEStd802].  Future documents may provide
   specifics for other link-layer address types.

   IEEE 802 originally set aside half of the 48-bit MAC address space
   for local use (where the Universal/Local (U/L) bit is set to 1).  In
   2017, IEEE published an amendment [IEEEStd802c] that divides this
   space into quadrants with differentiated address rules.  More details
   are in Appendix A.

   IEEE is also developing protocols and procedures for assignment of
   locally unique addresses (IEEE 802.1CQ).  This work may serve as an
   alternative protocol for assignment.  For additional background, see
   [IEEE-P802.1CQ-Project].

2.  Requirements Language

   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.

3.  Terminology

   The DHCP terminology relevant to this specification from [RFC8415]
   applies here.  The following definitions either modify those
   definitions as to how they are used in this document or define new
   terminology used herein.

   address         Unless specified otherwise, a link-layer (or MAC)
                   address, as specified in [IEEEStd802].  The address
                   is typically six octets long, but some network
                   architectures may use different lengths.

   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 new options specified in this
                   document (IA_LL and LLADDR).  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.  See Section 11.1 for details
                   on the IA_LL option.

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

   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 new options specified in
                   this document (IA_LL and LLADDR).  The server may or
                   may not support IPv6 address assignment and prefix
                   delegation as specified in [RFC8415].

4.  Deployment Scenarios

   This mechanism is designed to be generic and usable in many
   deployments, but there are two scenarios it attempts to address in
   particular: (i) proxy client mode and (ii) direct client mode.

4.1.  Scenario: Proxy Client Mode

   This mode is used when an entity acts as a DHCP client that requests
   that available DHCP servers assign one or more addresses (an address
   block) for the DHCP client to then assign to the final end devices to
   use.  Large-scale virtualization is one application scenario for
   proxy client mode.  In such environments, this entity is often called
   a "hypervisor" and is frequently required to spawn new VMs.  The
   hypervisor needs to assign new addresses to those machines.  The
   hypervisor does not use those addresses for itself, but rather it
   uses them to create new VMs with appropriate addresses.  It is worth
   pointing out the cumulative nature of this scenario.  Over time, the
   hypervisor is likely to increase its address use.  Some obsolete VMs
   will be deleted; their addresses are potentially eligible for reuse
   by new VMs.

4.2.  Scenario: Direct Client Mode

   This mode can be used when an entity acts as a DHCP client that
   requests that available DHCP servers assign one or more addresses (an
   address block) for its own use.  This usage scenario is related to
   IoT (see Section 1).  Upon first boot, for each interface, the device
   uses a temporary address, as described in [IEEEStd802.11] and IEEE
   802.1CQ [IEEE-P802.1CQ-Project], to send initial DHCP packets to
   available DHCP servers wherein the device requests a single address
   for that network interface.  Once the server assigns an address, the
   device abandons its temporary address and uses the assigned (leased)
   address.

   Note that a client that operates as above that does not have a
   globally unique link-layer address on any of its interfaces MUST NOT
   use a link-layer-based DHCP Unique Identifier (DUID).  For more
   details, refer to Section 11 of [RFC8415].

   Also, a client that operates as above may run into issues if the
   switch it is connected to prohibits or restricts link-layer address
   changes.  This may limit where this capability can be used or may
   require the administrator to adjust the configuration of the
   switch(es) to allow a change in address.

5.  Mechanism Overview

   In the scenarios described in Section 4, the protocol operates in
   fundamentally the same way.  The device requesting an address, acting
   as a DHCP client, will send a Solicit message with an IA_LL option to
   all available DHCP servers.  That IA_LL option MUST include an LLADDR
   option specifying the link-layer-type and link-layer-len, and it may
   include a specific address or address block as a hint for the server.
   Each available server responds with either a Reply message with
   committed address(es) (if Rapid Commit was requested and honored) or
   an Advertise message with offered address(es).  The client selects a
   server's response, as governed by [RFC8415].  If necessary, the
   client sends a Request message; the target server will then assign
   the address(es) and send a Reply message.  Once a Reply is received,
   the client can start using those address(es).

   Normal DHCP mechanisms are in use.  The client is expected to
   periodically renew the addresses as governed by T1 and T2 timers and
   to stop using the address once the valid lifetime expires.  Renewals
   can be administratively disabled by the server sending "infinity" as
   the T1 and T2 values (see Section 7.7 of [RFC8415]).  An
   administrator may make the address assignment permanent by specifying
   use of the "infinity" valid lifetime, as defined in Section 7.7 of
   [RFC8415].

   The client can release addresses when they are no longer needed by
   sending a Release message (see Section 18.2.7 of [RFC8415]).

   Figure 9 in [RFC8415] shows a timeline diagram of the messages
   exchanged between a client and two servers for the typical life cycle
   of one or more leases.

   Confirm and Information-request messages are not used in link-layer
   address assignment.  Decline should technically never be needed, but
   see Section 12 for one situation where this message is needed.

   Clients implementing this mechanism SHOULD use the Rapid Commit
   option, as specified in Sections 5.1 and 18.2.1 of [RFC8415], to
   obtain addresses with a two-message exchange when possible.

   Devices supporting this proposal MAY support the reconfigure
   mechanism, as defined in Section 18.2.11 of [RFC8415].  If supported
   by both server and client, the reconfigure mechanism allows the
   administrator to immediately notify clients that the configuration
   has changed and triggers retrieval of relevant changes immediately,
   rather than after the T1 timer elapses.  Since this mechanism
   requires implementation of Reconfiguration Key Authentication
   Protocol (see Section 20.4 of [RFC8415]), small-footprint devices may
   choose not to support it.

6.  Design Assumptions

   One of the essential aspects of this mechanism is its cumulative
   nature, especially in the hypervisor scenario.  The server-client
   relationship does not look like other DHCP transactions in the
   hypervisor scenario.  In a typical environment, there would be one
   server and a rather small number of hypervisors, possibly even only
   one.  However, over time, the number of addresses requested by the
   hypervisor(s) will increase as more VMs are spawned.

   Another aspect crucial for efficient design is the observation that a
   single client acting as hypervisor will likely use thousands of
   addresses.  An approach similar to what is used for IPv6 address or
   prefix assignment (IA container with all assigned addresses listed,
   one option for each address) would not work well.  Therefore, the
   mechanism should operate on address blocks rather than single values.
   A single address can be treated as an address block with just one
   address.

   The DHCP mechanisms are reused to a large degree, including message
   and option formats, transmission mechanisms, relay infrastructure,
   and others.  However, a device wishing to support only link-layer
   address assignment is not required to support full DHCP.  In other
   words, the device may support only assignment of link-layer addresses
   but not IPv6 addresses or prefixes.

7.  Information Encoding

   A client MUST send an LLADDR option encapsulated in an IA_LL option
   to specify the link-layer-type and link-layer-len values.  For link-
   layer-type 1 (Ethernet) and 6 (IEEE 802 Networks), a client sets the
   link-layer-address field to:

   1.  All zeroes if the client has no hint as to the starting address
       of the unicast address block.  This address has the IEEE 802
       individual/group bit set to 0 (individual).

   2.  Any other value to request a specific block of address starting
       with the specified address.

   Encoding information for other link-layer-types may be added in the
   future by publishing an RFC that specifies those values.

   A client sets the extra-addresses field to either 0 for a single
   address or the size of the requested address block minus 1.

   A client MUST set the valid-lifetime field to 0 (this field MUST be
   ignored by the server).

8.  Requesting Addresses

   The 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 inside.  The IA_LL option MUST contain
   an LLADDR option, as specified in Section 7.

   The server, upon receiving an IA_LL option, inspects its content and
   may offer an address or addresses for each LLADDR option according to
   its policy.  The server MAY take into consideration the address block
   requested by the client in the LLADDR option.  However, the server
   MAY choose to ignore some or all parameters of the requested address
   block.  In particular, the server may send either a different
   starting address or a smaller number of addresses than requested.
   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 is unable to provide any addresses, it MUST
   return the IA_LL option containing a Status Code option (see
   Section 21.13 of [RFC8415]) with status set to NoAddrsAvail.

   Note that servers that do not support the IA_LL option will ignore
   the option and not return it in Advertise (and Reply) messages.
   Clients that send IA_LL options MUST treat this as if the server
   returned the NoAddrsAvail status for these IA_LL option(s).

   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.

   The client MUST process the address block(s) returned in the
   Advertise, rather than what it included in the Solicit message, and
   may consider the offered address block(s) in selecting the Advertise
   message to accept.  The server may offer a smaller number of
   addresses or different addresses from those requested.  A client MUST
   NOT use resources returned in an Advertise message except to select a
   server and in sending the Request message to that server; resources
   are only useable by a client when returned in a Reply message.

   Upon reception of a Request message with the IA_LL container option,
   the server assigns the requested addresses.  The server allocates a
   block of addresses according to its configured policy.  The server
   MAY assign a different block or smaller block size than requested in
   the Request message.  The server 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.  It MUST restart
   its T1 and T2 timers using the values specified in the IA_LL option.

   The client MUST use the address block(s) returned in the Reply
   message, which may be a smaller block(s) or may have a different
   address(es) than requested.

   A client that has included a Rapid Commit option in the Solicit
   message may receive a Reply in response to the Solicit message and
   skip the Advertise and Request message steps above (see
   Section 18.2.1 of [RFC8415]).

   A client that changes its link-layer address on an interface SHOULD
   follow the recommendations in Section 7.2.6 of [RFC4861] to inform
   its neighbors of the new link-layer address quickly.

9.  Renewing Addresses

   Address renewals follow the normal DHCP renewals processing described
   in Section 18.2.4 of [RFC8415].  Once the T1 timer elapses, the
   client starts sending Renew messages with the IA_LL option containing
   an LLADDR option for the address block being renewed.  The server
   responds with a Reply message that contains the renewed address
   block.  The server MUST NOT shrink or expand the address block.  Once
   a block is assigned and has a non-zero valid lifetime, its size,
   starting address, and ending address MUST NOT change.

   If the requesting client needs additional addresses (e.g., in the
   hypervisor scenario because addresses need to be assigned to new
   VMs), it MUST send an IA_LL option with a different Identity
   Association IDentifier (IAID) to create another "container" for more
   addresses.

   If the client is unable to renew before the T2 timer elapses, it
   starts sending Rebind messages, as described in Section 18.2.5 of
   [RFC8415].

10.  Releasing Addresses

   The client may decide to release a leased address block.  A client
   MUST release the block in its entirety.  A client releases an address
   block by sending a Release message that includes an IA_LL option
   containing the LLADDR option for the address block to release.  The
   Release transmission mechanism is described in Section 18.2.7 of
   [RFC8415].

   Note that if the client is releasing the link-layer address it is
   using, it MUST stop using this address before sending the Release
   message (as per [RFC8415]).  In order to send the Release message,
   the client MUST use another address (such as the one originally used
   to initiate DHCPv6 to provide an allocated link-layer address).

11.  Option Definitions

   This mechanism uses an approach similar to the existing mechanisms in
   DHCP.  There is one container option (the IA_LL option) that contains
   the actual address or addresses, represented by an LLADDR option.
   Each LLADDR option represents an address block, which is expressed as
   a first address with a number that specifies how many additional
   addresses are included.

11.1.  Identity Association for Link-Layer Addresses Option

   The Identity Association for Link-Layer Addresses option (the IA_LL
   option) is used to carry an IA_LL, the parameters associated with the
   IA_LL, and the address blocks associated with the IA_LL.

   The format of the IA_LL 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_IA_LL         |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        IAID (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          T1 (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          T2 (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                         IA_LL-options                         .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: IA_LL Option Format

   option-code     OPTION_IA_LL (138).

   option-len      12 + length of IA_LL-options field.

   IAID            The unique identifier for this IA_LL; the IAID must
                   be unique among the identifiers for all of this
                   client's IA_LLs.  The number space for IA_LL IAIDs is
                   separate from the number space for other IA option
                   types (i.e., IA_NA, IA_TA, and IA_PD).  A 4-octet
                   field containing an unsigned integer.

   T1              The time interval after which the client should
                   contact the server from which the addresses in the
                   IA_LL were obtained to extend the valid lifetime of
                   the addresses assigned to the IA_LL; T1 is a time
                   duration relative to the current time expressed in
                   units of seconds.  A 4-octet field containing an
                   unsigned integer.

   T2              The time interval after which the client should
                   contact any available server to extend the valid
                   lifetime of the addresses assigned to the IA_LL; T2
                   is a time duration relative to the current time
                   expressed in units of seconds.  A 4-octet field
                   containing an unsigned integer.

   IA_LL-options   Options associated with this IA_LL.  A variable-
                   length field (12 octets less than the value in the
                   option-len field).

   An IA_LL option may only appear in the options area of a DHCP
   message.  A DHCP message may contain multiple IA_LL options (though
   each must have a unique IAID).

   The status of any operations involving this IA_LL is indicated in a
   Status Code option (see Section 21.13 of [RFC8415]) in the IA_LL-
   options field.

   Note that an IA_LL has no explicit "lifetime" or "lease length" of
   its own.  When the valid lifetimes of all of the addresses in an
   IA_LL have expired, the IA_LL can be considered to be expired.  T1
   and T2 are included to give servers explicit control over when a
   client recontacts the server about a specific IA_LL.

   In a message sent by a client to a server, the T1 and T2 fields MUST
   be set to 0.  The server MUST ignore any values in these fields in
   messages received from a client.

   In a message sent by a server to a client, the client MUST use the
   values in the T1 and T2 fields for the T1 and T2 times, unless those
   values in those fields are 0.  The values in the T1 and T2 fields are
   the number of seconds until T1 and T2.

   As per Section 7.7 of [RFC8415], the value 0xffffffff is taken to
   mean "infinity" and should be used carefully.

   The server selects the T1 and T2 times to allow the client to extend
   the lifetimes of any address block in the IA_LL before the lifetimes
   expire, even if the server is unavailable for some short period of
   time.  Recommended values for T1 and T2 are .5 and .8 times the
   shortest valid lifetime of the address blocks in the IA that the
   server is willing to extend, respectively.  If the "shortest" valid
   lifetime is 0xffffffff ("infinity"), the recommended T1 and T2 values
   are also 0xffffffff.  If the time at which the addresses in an IA_LL
   are to be renewed is to be left to the discretion of the client, the
   server sets T1 and T2 to 0.  The client MUST follow the rules defined
   in Section 14.2 of [RFC8415].

   If a client receives an IA_LL with T1 greater than T2, and both T1
   and T2 are greater than 0, the client discards the IA_LL option and
   processes the remainder of the message as though the server had not
   included the invalid IA_LL option.

   The IA_LL-options field typically contains one or more LLADDR options
   (see Section 11.2).  If a client does not include an LLADDR option in
   a Solicit or Request message, the server MUST treat this as a request
   for a single address and that the client has no hint as to the
   address it would like.

11.2.  Link-Layer Addresses Option

   The Link-Layer Addresses option is used to specify an address block
   associated with an IA_LL.  The option must be encapsulated in the
   IA_LL-options field of an IA_LL option.  The LLaddr-options field
   encapsulates those options that are specific to this address block.

   The format of the Link-Layer Addresses 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_LLADDR        |          option-len           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       link-layer-type         |        link-layer-len         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                     link-layer-address                        .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      extra-addresses                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      valid-lifetime                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                      LLaddr-options                           .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: LLADDR Option Format

   option-code          OPTION_LLADDR (139).

   option-len           12 + link-layer-len field value + length of
                        LLaddr-options field.  Assuming a link-layer-
                        address length of 6 and no extra options, the
                        option-len would be 18.

   link-layer-type      The link-layer type MUST be a valid hardware
                        type assigned by IANA, as described in
                        [RFC5494], and registered in the "Hardware
                        Types" registry at
                        <https://www.iana.org/assignments/arp-
                        parameters>.  A 2-octet field containing an
                        unsigned integer.

   link-layer-len       Specifies the length, in octets, of the link-
                        layer-address field (typically 6 for a link-
                        layer-type of 1 (Ethernet) and 6 (IEEE 802
                        Networks)).  This is to accommodate link layers
                        that may have variable-length addresses.  A
                        2-octet field containing an unsigned integer.

   link-layer-address   Specifies the address of the first link-layer
                        address that is being requested or assigned
                        depending on the message.  A client MAY send a
                        special value to request any address.  For link-
                        layer types 1 and 6, see Section 7 for details
                        on this field.  A link-layer-len length octet
                        field containing an address.

   extra-addresses      Specifies the number of additional addresses
                        that follow the address specified in link-layer-
                        address.  For a single address, 0 is used.  For
                        example, link-layer-address 02:04:06:08:0a and
                        extra-addresses 3 designate a block of four
                        addresses, starting from 02:04:06:08:0a and
                        ending with 02:04:06:08:0d (inclusive).  A
                        4-octet field containing an unsigned integer.

   valid-lifetime       The valid lifetime for the address(es) in the
                        option, expressed in units of seconds.  A
                        4-octet field containing an unsigned integer.

   LLaddr-options       Any encapsulated options that are specific to
                        this particular address block.  Currently, there
                        are no such options defined, but there may be in
                        the future.

   In a message sent by a client to a server, the valid lifetime field
   MUST be set to 0.  The server MUST ignore any received value.

   In a message sent by a server to a client, the client MUST use the
   value in the valid lifetime field for the valid lifetime for the
   address block.  The value in the valid lifetime field is the number
   of seconds remaining in the lifetime.

   As per Section 7.7 of [RFC8415], the valid lifetime of 0xffffffff is
   taken to mean "infinity" and should be used carefully.

   More than one LLADDR option can appear in an IA_LL option.

12.  Selecting Link-Layer Addresses for Assignment to an IA_LL

   A server selects link-layer addresses to be assigned to an IA_LL
   according to the assignment policies determined by the server
   administrator and the requirements of that address space.

   Link-layer addresses are typically specific to a link and the server
   SHOULD follow the steps in Section 13.1 of [RFC8415] to determine the
   client's link.

   For IEEE 802 MAC addresses (see [IEEEStd802] as amended by
   [IEEEStd802c]):

   1.  Server administrators SHOULD follow the IEEE 802 Specifications
       with regard to the unicast address pools made available for
       assignment (see Appendix A and [IEEEStd802c]) -- only address
       space reserved for local use or with the authorization of the
       assignee may be used.

   2.  Servers MUST NOT allow administrators to configure address pools
       that would cross the boundary of 2^(42) bits (for 48-bit MAC
       addresses) to avoid issues with changes in the first octet of the
       address and the special bits therein (see Appendix A).  Clients
       MUST reject assignments where the assigned block would cross this
       boundary (they MUST decline the allocation -- see Section 18.2.8
       of [RFC8415]).

   3.  A server MAY use options supplied by a relay agent or client to
       select the quadrant (see Appendix A) from which addresses are to
       be assigned.  This MAY include options like those specified in
       [RFC8948].

13.  IANA Considerations

   IANA has assigned the OPTION_IA_LL (138) 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:        138
   Description:  OPTION_IA_LL
   Client ORO:   No
   Singleton Option:  No
   Reference:    RFC 8947

   IANA has assigned the OPTION_LLADDR (139) 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:        139
   Description:  OPTION_LLADDR
   Client ORO:   No
   Singleton Option:  No
   Reference:    RFC 8947

14.  Security Considerations

   See Section 22 of [RFC8415] and Section 23 of [RFC7227] for the DHCP
   security considerations.  See [RFC8200] for the IPv6 security
   considerations.

   As discussed in Section 22 of [RFC8415]:

   |  DHCP lacks end-to-end encryption between clients and servers;
   |  thus, hijacking, tampering, and eavesdropping attacks are all
   |  possible as a result.

   In some network environments, it is possible to secure them, as
   discussed later in Section 22 of [RFC8415].

   If not all parties on a link use this mechanism to obtain an address
   from the space assigned to the DHCP server, there is the possibility
   of the same link-layer address being used by more than one device.
   Note that this issue would exist on these networks even if DHCP were
   not used to obtain the address.

   Server implementations SHOULD consider configuration options to limit
   the maximum number of addresses to allocate (both in a single request
   and in total) to a client.  However, note that this does not prevent
   a bad client actor from pretending to be many different clients and
   consuming all available addresses.

15.  Privacy Considerations

   See Section 23 of [RFC8415] for the DHCP privacy considerations.

   For a client requesting a link-layer address directly from a server,
   as the address assigned to a client will likely be used by the client
   to communicate on the link, the address will be exposed to those able
   to listen in on this communication.  For those peers on the link that
   are able to listen in on the DHCP exchange, they would also be able
   to correlate the client's identity (based on the DUID used) with the
   assigned address.  Additional mechanisms, such as the ones described
   in [RFC7844], can also be used to improve anonymity by minimizing
   what is exposed.

   As discussed in Section 23 of [RFC8415], DHCP servers and hypervisors
   may need to consider the implications of assigning addresses
   sequentially.  Though in general, this is only of link-local concern
   unlike for IPv6 address assignment and prefix delegation, as these
   may be used for communication over the Internet.

16.  References

16.1.  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>.

   [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>.

   [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>.

16.2.  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", IEEE
              STD 802-2014, DOI 10.1109/IEEESTD.2014.6847097,
              <https://doi.org/10.1109/IEEESTD.2014.6847097>.

   [IEEEStd802.11]
              IEEE, "IEEE Standard for Information technology--
              Telecommunications and information exchange between
              systems Local and metropolitan area networks--Specific
              requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE Std
              802.11, DOI 10.1109/IEEESTD.2016.7786995,
              <https://doi.org/10.1109/IEEESTD.2016.7786995>.

   [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,
              <https://doi.org/10.1109/IEEESTD.2017.8016709>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <https://www.rfc-editor.org/info/rfc2464>.

   [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>.

   [RFC5494]  Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
              for the Address Resolution Protocol (ARP)", RFC 5494,
              DOI 10.17487/RFC5494, April 2009,
              <https://www.rfc-editor.org/info/rfc5494>.

   [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>.

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

   [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>.

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

Appendix A.  IEEE 802c Summary

   This appendix provides a brief summary of IEEE 802c [IEEEStd802c].

   The original IEEE 802 specifications assigned half of the 48-bit MAC
   address space to local use -- these addresses have the U/L bit set to
   1 and are locally administered with no imposed structure.

   In 2017, the IEEE issued the IEEE Std 802c specification, which
   defines a new optional "Structured Local Address Plan (SLAP) that
   specifies different assignment approaches in four specified regions
   of the local MAC address space".  Under this plan, there are four
   SLAP quadrants that use different assignment policies.

   The first octet of the MAC address Z and Y bits define the quadrant
   for locally assigned addresses (X-bit is 1).  In IEEE representation,
   these bits are as follows:

       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)

                            Figure 3: SLAP Bits

   The SLAP quadrants are:

     +==========+=======+=======+=======================+============+
     | 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

   MAC addresses derived from an Extended Local Identifier (ELI) are
   based on an assigned Company ID (CID), which is 24 bits (including
   the M, X, Y, and Z bits) for 48-bit MAC addresses.  This leaves 24
   bits for the locally assigned address for each CID for unicast (M-bit
   = 0) and also for multicast (M-bit = 1).  The CID is assigned by the
   IEEE Registration Authority (RA).

   MAC addresses derived from a Standard Assigned Identifier (SAI) are
   assigned by 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.

   MAC addresses derived from an Administratively Assigned Identifier
   (AAI) are assigned locally.  Administrators manage the space as
   needed.  Note that multicast IPv6 packets [RFC2464] 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.

   The last quadrant is reserved for future use.  While this quadrant
   may also be used similar to AAI space, administrators should be aware
   that future specifications may define alternate uses that could be
   incompatible.

Acknowledgments

   Thanks to the DHC Working Group participants that reviewed this
   document and provided comments and support.  With special thanks to
   Ian Farrer for his thorough reviews and shepherding of this document
   through the IETF process.  Thanks also to directorate reviewers
   Samita Chakrabarti, Roni Even, and Tianran Zhou and IESG members
   Martin Duke, Benjamin Kaduk, Murray Kucherawy, Warren Kumari, Barry
   Leiba, Alvaro Retana, Éric Vyncke, and Robert Wilton for their
   suggestions.  And thanks to Roger Marks, Robert Grow, and Antonio de
   la Oliva for comments related to IEEE work and references.

Authors' Addresses

   Bernie Volz
   Cisco Systems, Inc.
   300 Beaver Brook Rd
   Boxborough, MA 01719
   United States of America

   Email: volz@cisco.com

   Tomek Mrugalski
   Internet Systems Consortium, Inc.
   PO Box 360
   Newmarket, NH 03857
   United States of America

   Email: tomasz.mrugalski@gmail.com

   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/