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Using DHCP-PD to Allocate Unique IPv6 Prefix per Device in Broadcast Networks

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Lorenzo Colitti , Jen Linkova , Xiao Ma
Last updated 2023-05-18
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v6ops Working Group                                           L. Colitti
Internet-Draft                                               Google, LLC
Intended status: Informational                           J. Linkova, Ed.
Expires: 19 November 2023                                     X. Ma, Ed.
                                                             18 May 2023

  Using DHCP-PD to Allocate Unique IPv6 Prefix per Device in Broadcast


   This document discusses the IPv6 deployment scenario when individual
   devices connected to broadcast networks (like WiFi hotspots or
   enterprise networks) are allocated unique prefixes via DHCP-PD.

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
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on 19 November 2023.

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   Copyright (c) 2023 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Design Principles . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Prefix Length Considerations  . . . . . . . . . . . . . . . .   4
   5.  Routing Considerations  . . . . . . . . . . . . . . . . . . .   5
     5.1.  First-Hop Routers Requirements  . . . . . . . . . . . . .   5
     5.2.  Topologies with Multiple First-Hop Routers  . . . . . . .   5
     5.3.  Preventing Routing Loops  . . . . . . . . . . . . . . . .   6
   6.  DHCPv6-PD Server Considerations . . . . . . . . . . . . . . .   6
   7.  Antispoofing and SAVI Interaction . . . . . . . . . . . . . .   7
   8.  Migration Strategies and Co-existence with SLAAC Using Prefixes
           From PIO  . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Applicability and Limitations . . . . . . . . . . . . . . . .   9
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     14.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Unlike IPv4, IPv6 allows (and often requires) hosts to have multiple
   addresses.  At the very least, a host can be expected to have one
   link-local address, one temporary address and, in most cases, one
   stable global address.  On an IPv6-only network the device would need
   to have a dedicated 464XLAT address, which brings the total number of
   addresses to 4.  If the network is multihomed and uses two different
   prefixes, or during graceful renumbering (when the old prefix is
   deprecated), or if an enterprise uses ULAs, the number of global
   addresses can easily double, bringing the total number of addresses
   to 7.  Devices running containers/namespaces might need even more
   addresses per physical host.  On one hand multiple addresses could be
   considered as a significant advantage of IPv6.  On the other hand,
   however, they are sometimes seen as a drawback for the following

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   *  Increased number of addresses introduces scalability issues on the
      network infrastructure.  Network devices need to maintain various
      types of tables/hashes (Neighbor Cache on first-hop routers,
      Neighbor Discovery Proxy caches on L2 devices etc).  On VXLAN
      [RFC7348] networks each address might be represented as a route so
      8 addresses instead of 1 requires the devices to support 8 times
      more routes, etc.

   *  An operator might need to know all addresses used by a given
      device in the past for forensics or troubleshooting purposes.

   *  If an infrastructure device resources are exhausted, the device
      might drop some IPv6 addresses from the corresponding tables,
      while the address owner might be still using the address to send
      traffic.  This leads to traffic blackholing and degraded customer

   [RFC7934] discusses this aspect and explicitly states that IPv6
   deployments SHOULD NOT limit the number of IPv6 addresses a host can
   have.  However it's been observed that networks do impose such
   limits, likely in an attempt to protect the network resources and
   prevent DoS attacks.  The most common scenario of network-imposed
   limitations is Neighbor Discovery (ND) proxy.  Many enterprise-scale
   wireless solutions implement ND proxy to reduce amount of broadcast
   and multicast downstream (AP to clients) traffic.  To perform ND
   proxy a device usually maintains a table, containing IPv6 and MAC
   addresses of connected clients.  At least some implementations have
   hardcoded limits on how many IPv6 addresses per a single MAC such a
   table can contain.  When the limit is exceeded the behaviour is
   implementation-dependent.  Some vendors just fail to install N+1
   address to the table.  Other delete the oldest entry for this MAC and
   replace it with the new address.  In any case the affected addresses
   lose network connectivity without receiving any implict signal, with
   traffic being silently dropped.

   It would be beneficial for IPv6 deployments to address the above
   mentioned scalability issues while still allowing devices to have
   multiple IPv6 addresses.  One of the very promising approaches is
   allocating an unique IPv6 prefix per host ([RFC8273]).  The same
   principle has been actively used in cellular IPv6 deployments
   ([RFC6459]).  However it's very uncommon in enterprise-style
   networks, where nodes are usually connected to broadcast segments/
   VLANs and each segment has a single shared subnet assigned.  This
   document expands the approach defined in [RFC8273] to allocate an
   unique IPv6 prefix per device using DHCP-PD.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  Design Principles

   Instead of all devices on a given segment forming addresses from the
   same shared prefix assigned to that segment:

   *  An device acts as DHCP-PD client and requests a prefix via
      DHCPv6-PD by sending an IA_PD request.

   *  The first-hop router acts as a DHCPv6-PD relay and sends the
      request to the DHCPv6-PD servers.  In smaller networks it's
      entirely possible for the first-hop router to act as a DHCPv6-PD
      server and assign the prefix from a larger pool allocated for the
      given segment or the whole site.

   *  The allocated prefix is installed into the first-hop router
      routing table as a route pointing to the client's link-local
      address.  For the router and all other infrastructure devices that
      prefix is considered off-link, so traffic to that prefix does not
      trigger any ND packets.

   *  The device uses the delegated prefix to form addresses and/or
      extend the network, e.g., as described in [RFC7278].  For example,
      the device can include the prefix into Router Advertisements sent
      to virtual systems or to any other devices connected to its
      downstream interface.

4.  Prefix Length Considerations

   DHCPv6 prefix delegation supports delegating prefixes of any size.
   However at the time of writing, the only prefix size that will allow
   the device to use SLAAC is 64 bits (see also [RFC7421]).  As a result
   delegating a prefix suitable for forming addresses using SLAAC allows
   the client to provide limitless addresses to IPv6 nodes connected to
   it (e.g., virtual machines, tethered devices), because all IPv6 nodes
   are required to support SLAAC.  In other words, it allows devices to
   extend the network arbitrarily, similarily to using NAT in IPv4 but
   with full support for end-to-end communication.  Chosing longer
   prefixes would require the device and any connected system to use
   some other form of address assignment and therefore would drastically
   limit the applicability of the proposed solution.  The extensive

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   analysis provided in [RFC7421] is fully applicable to selecting the
   delegated prefix in the proposed deployment model.

   Section 9.2 of [RFC7934] demonstrates that if a network uses to address hosts, /40 would be sufficient to provide each
   device with /64.  In multi-site networks the calculations might get
   more complex as each site IPv6 prefix needs to be larger enough to be
   globally routable and accepted by eBGP peers, for example /48.  Let's
   consider an enterprise network which has 8000 sites (~2^13).  Imagine
   that site has up to 64 (2^6) different network types and each network
   requires its own /48.  So each network can provide /64 to 65K clients
   (an equivalent of using /16 IPv4 subnet to address devices).  In that
   case such an enterprise would need /29 (48 - 6 - 13) to provide /64
   to its devices.  Networks of such size usually have multiple
   allocations from RIRs so /29 sounds reasonable.  In real life there
   are very few networks of that scale and a single /32 would be
   sufficient for most deployments.

5.  Routing Considerations

5.1.  First-Hop Routers Requirements

   The design described in this document is targeted to large networks
   were the number of clients combined with multiple IPv6 addresses per
   client creates scalability issues.  In such networks DHCPv6 servers
   are usually deployed as dedicated systems, so the first-hop routers
   act as DHCP relays.  To delegate IPv6 prefixes to devices the first
   hop router needs to implement DHCPv6-PD relay functions and meet the
   requirements defined in [RFC8987].

   In particular, if the same DHCPv6-PD pool is used for clients
   connected to multiple routers, dynamic routing protocols are required
   to propagate the routes to the allocated prefixes.  Each relay needs
   to advertise the learned delegated leases as per requirement R-4
   specified in Section 4.2 of [RFC8987].

5.2.  Topologies with Multiple First-Hop Routers

   Traditionally DHCPv6-PD is used in environments where a DHCPv6-PD
   client (a home CPE, for example) is connected to a single router
   which performs DHCPv6-PD relay functions.  In the topology with
   redundant first-hop routers, all those routers need to snoop DHCPv6
   traffic, install the delegated prefixes into its routing table and,
   if needed, advertise those prefixes to the network.  That means that
   all relays the device is connected to must be able to snoop DHCPv6-PD
   traffic, in particular Reply messages sent by the server (as those
   messages contain the delegated prefix).  Normally the client uses
   multicast to reach all servers or an individual server (see

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   Section 14 of [RFC8415]).  As per Section 18.4 of [RFC8415] the
   server is not supposed to accept unicast traffic when it is not
   explicitly configured to do, and unicast transmission is only allowed
   for some messages and only if the Server Unicast option ([RFC8415],
   Section 21.12) is used.  Therefore, in the topologies with multiple
   first-hop routers the DHCPv6 servers MUST be configured not to use
   the Server Unicast option (it should be noted that
   [I-D.dhcwg-dhc-rfc8415bis] deprecates the Server Unicast option
   exactly because it is not compatible with multiple relays topology).
   Therefore as long as the Server Unicast option is not used, all
   first-hop routers shall be able to install the route for the
   delegates prefix.

   To ensure that routes to the delegated prefixes are preserved even if
   a relay is rebooted or replaced, the operator MUST ensure that all
   relays in the network infrastructure support DHCPv6 Bulk Leasequery
   as defined in [RFC5460].  While Section 4.3 of [RFC8987] lists
   keeping active prefix delegations in persistent storage as an
   alternative to DHCPv6 Bulk Leasequery, relying on persistent storage
   has the following drawbacks:

   *  In a network with multiple relays, network state can change
      significantly while the relay was rebooting (new prefixes
      delegated, some prefixes expiring etc).

   *  Persistent storage might not be preserved if the router is
      physically replaced.

5.3.  Preventing Routing Loops

   To prevent routing loops caused by traffic to unused addresses from
   the delegated prefix the client MUST drop all packets to such
   addresses (see the requirement WPD-5 in Section 4.2 of [RFC7084]).

6.  DHCPv6-PD Server Considerations

   The following requirements are applicable to the DHCPv6-PD server
   delegating prefixes to devices:

   *  The server MUST follow [RFC8168] recommendations on processing
      prefix-length hints.

   *  The server MUST provide a prefix short enough for the client to
      assign addresses to its interfaces and connected systems via

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   *  If the server receives the same SOLICIT message from the same
      client multiple times through multiple relays, it MUST reply to
      all of them with the same prefix(es).  This ensures that all the
      relays will correctly configure routes to the delegated prefixes.

   *  The server MUST NOT send the Server Unicast option to the client
      unless the network topology guarantees that no client is connected
      to a segment with multiple relays (see Section 5.2).

7.  Antispoofing and SAVI Interaction

   Enabling the unicast Reverse Path Forwarding (uRPF) on the first-hop
   router interfaces towards clients provides the first layer of defence
   agains spoofing.  If the malicious client sends a spoofed packet it
   would be dropped by the router unless the spoofed address belongs to
   a prefix delegated to another client on the same interface.
   Therefore the malicious client can only spoof addresses already
   delegated to another client on the same segment or another device
   link-local address.

   Source Address Validation Improvement (SAVI, [RFC7039]) provides more
   reliable protection against address spoofing.  Administrators
   deploying the proposed solution on the SAVI-enabled infastructure
   should ensure that SAVI perimeter devices support DHCPv6-PD snooping
   to create the correct binding for the delegated prefixes (see
   [RFC7513]).  Using FCFS SAVI ([RFC6620]) for protecting link-local
   addresses and creating SAVI bindings for DHCPv6-PD assigned prefixes
   would prevent spoofing.

   It should be noted that using DHCPv6-PD makes it harder for an
   attacker to select the spoofed source address.  When all devices are
   using the same shared subnet to form addresses, the attacker might
   learn addresses used by other devices by listening to multicast
   Neighbor Solicitations and Neighbour Advertisements.  In DHCPv6-PD
   environments, however, the attacker can only learn about other
   devices global addresses by listening to multicast DHCPv6 messages,
   which are not transmitted so often.

8.  Migration Strategies and Co-existence with SLAAC Using Prefixes From

   It would be beneficial for the network to explicitly indicate its
   support of DHCPv6-PD for connected devices.

   *  In small networks (e.g. home ones), where the number of devices is
      not too high, the number of available prefixes becomes a limiting
      factor.  If every phone or laptop in a home network would request
      an unique prefix suitable for SLAAC, the home network might run

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      out of prefixes, if the prefix allocated to the CPE by its ISP is
      too small (e.g. if an ISP allocates /60, it would only allow 16
      devices to request /64).  So while the enterprise network
      administrator might want all phones in the network to request a
      prefix, it would be highly undesirable for the same phone to
      request a prefix when connecting to a home network.

   *  When the network supports both a unique prefix per device and a
      PIO with A=1 as address assignment methods, it's highly desirable
      for the device NOT to use the PIO prefix to form global addresses
      and only use the prefix delegated via DHCPv6-PD.  Starting both
      SLAAC using the PIO prefix and DHCPv6-PD and deprecating that
      SLAAC addresses after receiving a delegated prefix would be very
      disruptive for applications.  If the device continues to use
      addresses formed from the PIO prefix it would not only undermine
      the benefits of the proposed solution (see Section 9), but would
      also introduce complexity and unpredictability in the source
      address selection.  Therefore the device needs to know what
      address assignment method to use and whether to use the prefix in
      PIO or not, if the network provides the PIO with A flag set.

   To allow the network to signal DHCPv6-PD support,
   [I-D.collink-6man-pio-pflag] defines a new PIO flag, indicating that
   DHCPv6-PD is preferred method of obtaining prefixes.

9.  Benefits

   The proposed solution provides the following benefits:

   *  The network devices resources (e.g. memory) need to scale to the
      number of devices, not the number of IPv6 addresses.  The first-
      hop routers have a single route per device pointing to the
      device's link-local address.

   *  If all devices connected to the given network segment support this
      mode of operation and can generate addresses from the delegated
      prefixes, there is no reason to advertise a common prefix assigned
      to that segment in PIO with 'A' flag set.  Therefore it is
      possible to remove the global shared prefix from that network
      segment and the router interface completely, so no global
      addresses are on-link for the segment.  This would lead to
      reducing the attack surface for Neighbor Discovery attacks
      described in [RFC6583].

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   *  DHCP-PD logs and first-hop routers routing tables provide complete
      information on IPv6 to MAC mapping, which can be used for
      forensics and troubleshooting.  Such information is much less
      dynamic than ND cache and therefore it's much easier for an
      operator to collect and process it.

   *  A dedicated prefix per device allows the network administrator to
      create per-host security policies (ACLs) even if the device is
      using temporary addresses.  This mitigates one of the issues
      described in [I-D.gont-opsec-ipv6-addressing].

   *  The cost of having multiple addresses is offloaded to the end
      devices.  Devices are free to create and use as many addresses as
      they need.

   *  Fate sharing: all global addresses used by a given device are
      routed as a single prefix.  Either all of them work or not, which
      makes the failures easier to diagnoze and mitigate.

   *  Ability to extend the network transparently.  If the device use
      SLAAC, delegating a prefix allows the device to provide
      connectivity to other hosts, like it is possible in IPv4 with NAT.

10.  Applicability and Limitations

   Delegating a unique prefix per device provides all the benefits of
   both SLAAC and DHCPv6 address allocation, but at the cost of greater
   address space usage.  This design would substantially benefit some
   networks (see Section 9), in which the addional cost of an additional
   service (DHCPv6 prefix delegation) and allocating a larger amount of
   address space can easily be justified.  Examples of such networks
   include but are not limited to:

   *  Large-scale networks where even 3-5 addresses per client might
      introduce scalability issues.

   *  Networks with high number of virtual hosts, so physical devices
      require multiple addresses.

   *  Managed networks where extensive troubleshooting, device traffic
      logging or forensics might be required.

   In smaller networks, such as home networks, with smaller address
   space and lower number of clients, SLAAC is a better and simpler

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11.  Privacy Considerations

   Eventually, if/when the vast majority of devices support the proposed
   mechanism, an eavesdropper/information collector might be able to
   correlate the prefix to the device.  To mitigate the threat the
   device might implement a mechanism similar to SLAAC temporary
   extensions ([RFC8981]) but for delegated prefixes:

   *  The device requests another prefix.

   *  Upon receiving the new prefix the device deprecates all addresses
      from the old one.

   *  After some time (shall it be T2 from IA_PD for the original
      prefix?) the device sends RELEASE for the old prefix.

12.  IANA Considerations

   This memo includes no request to IANA.

13.  Security Considerations

   A malicious or just misbehaving device might exhaust the DHCP-PD pool
   by sending a large number of requests with various DUIDs.  This is
   not a new issue as the same attack might be implemented in DHCPv4 or
   DHCPv6 for IA_NA requests.  To prevent a misbehaving client from
   denying service to other clients, the DHCPv6 server or relay MUST
   support limiting the number of prefixes delegated to a given client
   at any given time.

   A malicious client might request a prefix and then release it very
   quickly, causing routing convergence events on the relays.  The
   probability of such attack can be reduced if the network rate limits
   the amount of broadcast and multicast messages from the client.

   Delegating the same prefix for the same device introduces privacy
   concerns.  The proposed mitigation is discussed in Section 11.

   Spoofing scenarios and prevention mechanisms are discussed in
   Section 7.

14.  References

14.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
              DOI 10.17487/RFC5460, February 2009,

   [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,

   [RFC8168]  Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint
              Issues", RFC 8168, DOI 10.17487/RFC8168, May 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8273]  Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
              per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,

   [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,

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,

   [RFC8987]  Farrer, I., Kottapalli, N., Hunek, M., and R. Patterson,
              "DHCPv6 Prefix Delegating Relay Requirements", RFC 8987,
              DOI 10.17487/RFC8987, February 2021,

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14.2.  Informative References

   [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
              T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, DOI 10.17487/RFC6459, January 2012,

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583,
              DOI 10.17487/RFC6583, March 2012,

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,

   [RFC7278]  Byrne, C., Drown, D., and A. Vizdal, "Extending an IPv6
              /64 Prefix from a Third Generation Partnership Project
              (3GPP) Mobile Interface to a LAN Link", RFC 7278,
              DOI 10.17487/RFC7278, June 2014,

   [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
              "Host Address Availability Recommendations", BCP 204,
              RFC 7934, DOI 10.17487/RFC7934, July 2016,

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              Colitti, L. and J. Linkova, "Signalling DHCPv6 Prefix
              Delegation Availability to Hosts", Work in Progress,
              Internet-Draft, draft-collink-6man-pio-pflag-00, 27 March
              2023, <

              Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T.
              Winters, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", Work in Progress, Internet-Draft, draft-dhcwg-
              dhc-rfc8415bis-00, 7 October 2022,

              Gont, F. and G. Gont, "Implications of IPv6 Addressing on
              Security Operations", Work in Progress, Internet-Draft,
              draft-gont-opsec-ipv6-addressing-00, 2 February 2023,


   Thanks to Nick Buraglio, Brian Carpenter, Gert Doering, David Farmer,
   Fernando Gont, Nick Hilliard, Bob Hinden, Martin Hunek, Erik Kline,
   David Lamparter, Andrew McGregor, Tomek Mrugalski, Pascal Thubert,
   Ole Troan, Eduard Vasilenko, Timothy Winters, Chongfeng Xie for the
   discussions, their input and all contribution.


Authors' Addresses

   Lorenzo Colitti
   Google, LLC
   Shibuya 3-21-3,

   Jen Linkova (editor)
   1 Darling Island Rd
   Pyrmont NSW 2009

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   Xiao Ma (editor)
   Shibuya 3-21-3,

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