Signaling DHCPv6 Prefix per Client Availability to Hosts
draft-ietf-6man-pio-pflag-12

IPv6 Maintenance                                              L. Colitti
Internet-Draft                                                J. Linkova
Updates: 4861, 4862 (if approved)                             X. Ma, Ed.
Intended status: Standards Track                                  Google
Expires: 11 April 2025                                      D. Lamparter
                                                            NetDEF, Inc.
                                                          8 October 2024

        Signaling DHCPv6 Prefix per Client Availability to Hosts
                      draft-ietf-6man-pio-pflag-12

Abstract

   This document defines a "P" flag in the Prefix Information Option
   (PIO) of IPv6 Router Advertisements (RAs).  The flag is used to
   indicate that the network prefers that clients use the RFC9663
   deployment model instead of using individual adresses in the on-link
   prefix assigned using Stateless Address Autoconfiguration (SLAAC) or
   DHCPv6 address assignment.

   This document updates RFC4862 to indicate that the Autonomous flag in
   a PIO needs to be ignored if the PIO has the P flag set.  It also
   updates RFC4861 to specify that the P flag indicates DHCPv6 Prefix
   Delegation support for clients.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 April 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Colitti, et al.           Expires 11 April 2025                 [Page 1]
Internet-Draft                 pio-p-flag                   October 2024

   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  P Flag Overview . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Router Behaviour  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Client Behaviour  . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Processing the P Flag . . . . . . . . . . . . . . . . . .   6
     7.2.  Using Delegated Prefix(es)  . . . . . . . . . . . . . . .   7
     7.3.  Absence of PIOs with P bit set  . . . . . . . . . . . . .   8
     7.4.  On-link Communication . . . . . . . . . . . . . . . . . .   8
     7.5.  Source Address Selection  . . . . . . . . . . . . . . . .   9
   8.  Multihoming . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Modifications to RFC-Mandated Behaviour . . . . . . . . . . .   9
     9.1.  Changes to RFC4861  . . . . . . . . . . . . . . . . . . .   9
     9.2.  Changes to RFC4862  . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     13.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   [RFC9663] documents an IPv6 address assignment model where IPv6
   devices obtain dedicated prefixes from the network via DHCPv6 Prefix
   Delegation (DHCPv6-PD, [RFC8415]).  This model provides devices with
   large IPv6 address space they can use to create addresses for
   communication, individually number virtual machines (VM)s or
   containers, or extend the network to downstream devices.  It also
   provides scalability benefits on large networks because network
   infrastructure devices do not need to maintain per-address state,
   such as IPv6 neighbor cache, Source Address Validation Improvement
   (SAVI, [RFC7039]) mappings, Virtual eXtensible Local Area Network

Colitti, et al.           Expires 11 April 2025                 [Page 2]
Internet-Draft                 pio-p-flag                   October 2024

   (VXLAN, [RFC7348]) routes, etc.

   On networks with fewer devices, however, this model may not be
   appropriate, because scaling to support multiple individual IPv6
   addresses per device is less of a concern.  Also, many home networks
   currently offer prefix delegation but assume that a limited number of
   specialized devices and/or applications will require delegated
   prefixes, and thus do not allocate enough address space to offer
   prefixes to every device that connects to the network.  For example,
   if clients enable [RFC9663] on a home network that only receives a
   /60 from the ISP, and each client obtains a /64 prefix, then the
   network will run out of prefixes after 15 devices have been
   connected.

   Therefore, to safely roll out [RFC9663] implementations on the client
   side, it is necessary to have a mechanism for the network to signal
   to the client which address assignment method is preferred.

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

   Node: a device that implements IPv6, [RFC8200].

   Host: any node that is not a router, [RFC8200].

   Client: a node which connects to a network and acquires addresses.
   The node may wish to obtain addresses for its own use, or may be a
   router that wishes to extend the network to its physical or virtual
   subsystems, or both.  It may be either a host or a router as defined
   by [RFC8200].

   DHCPv6-PD: DHCPv6 ([RFC8415]) mechanism to delegate IPv6 prefixes to
   clients.

   DHCPv6 IA_NA: Identity Association for Non-temporary Addresses
   (Section 21.4 of [RFC8415]).

   DHCPv6 IA_PD: Identity Association for Prefix Delegation
   (Section 21.21 of [RFC8415]).

Colitti, et al.           Expires 11 April 2025                 [Page 3]
Internet-Draft                 pio-p-flag                   October 2024

   ND: Neighbor Discovery, [RFC4861].

   On-link address: an address that is assigned to an interface on a
   specified link ([RFC4861]).

   On-link prefix: a prefix that is assigned to a specified link.

   Off-link: the opposite of "on-link" (see [RFC4861]).

   PIO: Prefix Information Option, [RFC4862].

   SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].

4.  Rationale

   The network administrator might want to indicate to clients that
   requesting a prefix via DHCPv6-PD and using that prefix for address
   assignment (see [RFC9663]) should be preferred over using individual
   addresses from the on-link prefix.  The information is passed to the
   client via a P flag in the Prefix Information Option (PIO).  The
   reason for it being a PIO flag is as follows:

   *  The information must be contained in the Router Advertisement
      because it must be available to the client before it decides to
      form IPv6 addresses from the PIO prefix using SLAAC.  Otherwise,
      the client might use SLAAC to form IPv6 addresses from the PIO
      provided and start using them, even if a unique per-client prefix
      is available via DHCPv6-PD.  Forming addresses via SLAAC is
      suboptimal because if the client later acquires a prefix using
      DHCPv6-PD, it can either use both the prefix and SLAAC addresses,
      reducing the scalability benefits of using DHCPv6-PD, or can
      remove the SLAAC addresses, which would be disruptive for
      applications that are using them.

   *  This information is specific to the particular prefix being
      announced.  For example, a network administrator might want
      clients to assign global addresses from delegated prefixes, but
      use the PIO prefix to form Unique Local Unicast (ULA, [RFC4193])
      addresses.  Also, in a multihoming situation, one upstream network
      might choose to assign prefixes via prefix delegation, and another
      via PIOs.

   Note that setting the 'P' flag in a PIO expresses the network
   operator's preference as to whether clients should attempt using
   DHCPv6-PD instead of performing individual address configuration on
   the prefix.  For clients that honor this preference by requesting
   prefix delegation, the actual delegated prefix will necessarily be a
   prefix different from the one from the PIO.

Colitti, et al.           Expires 11 April 2025                 [Page 4]
Internet-Draft                 pio-p-flag                   October 2024

5.  P Flag Overview

   The P flag (also called DHCPv6-PD preferred flag) is a 1-bit PIO
   flag, located after the R flag ([RFC6275]).  The presence of a PIO
   with the P flag set indicates that the network prefers that clients
   use Prefix Delegation instead of acquiring individual addresses via
   SLAAC or DHCPv6 address assignment.  This implies that the network
   has a DHCPv6 server capable of making DHCPv6 Prefix Delegations to
   every device on the network, as described in [RFC9663].

   The resulting format of the Prefix Information Option is as follows
   (see Figure 1):

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |    Length     | Prefix Length |L|A|R|P| Rsvd1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Valid Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Preferred Lifetime                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Reserved2                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                            Prefix                             +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                Figure 1

   The P flag is independent of the value of the M and O flags in the
   Router Advertisement.  If the network desires to delegate prefixes to
   devices that support DHCPv6 Prefix Delegation but do not support the
   P flag, it SHOULD also set the M or O bits in the RA to 1, because
   some devices, such as [RFC7084] Customer Edge (CE) routers, might not
   initiate DHCPv6 Prefix Delegation if both the M and O bits are set to
   zero.

Colitti, et al.           Expires 11 April 2025                 [Page 5]
Internet-Draft                 pio-p-flag                   October 2024

6.  Router Behaviour

   Routers SHOULD set the P flag to zero by default, unless explicitly
   configured by the administrator, and SHOULD allow the operator to set
   the P flag value for any given prefix advertised in a PIO.  Routers
   MUST allow the P flag to be configured separately from the A flag.
   In particular, enabling or disabling the P flag MUST not trigger
   automatic changes in the A flag value set by the router.

7.  Client Behaviour

7.1.  Processing the P Flag

   This specification only applies to clients which support DHCPv6
   Prefix Delegation.  Clients which do not support DHCPv6 prefix
   delegation MUST ignore the P flag.  The P flag is meaningless for
   link-local prefixes and any Prefix Information Option containing the
   link-local prefix MUST be ignored as specified in Section 5.5.3 of
   [RFC4862].  In the following text, all prefixes are assumed not to be
   link-local.

   For each interface, the client MUST keep a list of every prefix that
   was received from a PIO with the P flag set and currently has a non-
   zero Preferred Lifetime.  The list affects the behaviour of the
   DHCPv6 client as follows:

   *  When a prefix's Preferred Lifetime becomes zero, either because
      the Preferred Lifetime expires or because the client receives a
      PIO for the prefix with a zero Preferred Lifetime, the prefix MUST
      be removed from the list.

   *  When the length of the list increases to one, the client SHOULD
      start requesting prefixes via DHCPv6 prefix delegation unless it
      is already doing so.

   *  When the length of the list decreases to zero, the client SHOULD
      stop requesting or renewing prefixes via DHCPv6 prefix delegation
      if it has no other reason to do so.  The lifetimes of any prefixes
      already obtained via DHCPv6 are unaffected.

Colitti, et al.           Expires 11 April 2025                 [Page 6]
Internet-Draft                 pio-p-flag                   October 2024

   *  If the client has already received delegated prefix(es) from one
      or more servers, then any time a prefix is added to or removed
      from the list, the client MUST consider this to be a change in
      configuration information as described in Section 18.2.12 of
      [RFC8415].  In that case the client MUST perform a REBIND, unless
      the list is now empty.  This is in addition to performing a REBIND
      in the other cases required by that section.  Issuing a REBIND
      allows the client to obtain new prefixes if necessary, for example
      when the network is being renumbered.  It also refreshes state
      related to the delegated prefix(es).

   When a client requests a prefix via DHCPv6-PD, it MUST use the prefix
   length hint (Section 18.2.4 of [RFC8415]) to request a prefix that is
   short enough to form addresses via SLAAC.

   In order to achieve the scalability benefits of using DHCPv6-PD, the
   client SHOULD prefer to form addresses from the delegated prefix
   instead of using individual addresses in the on-link prefix(es).
   Therefore, when the client requests a prefix using DHCPv6-PD, the
   client SHOULD NOT use SLAAC to obtain IPv6 addresses from PIOs with
   the P and A bits set.  Similarly, if all PIOs processed by the client
   have the P bit set, the client SHOULD NOT request individual IPv6
   addresses from DHCPv6, i.e., it SHOULD NOT include any IA_NA options
   in SOLICIT ([RFC8415]) messages.  The client MAY continue to use
   addresses that are already configured.

   If the client does not obtain any suitable prefixes via DHCPv6-PD
   that are suitable for SLAAC, it MAY choose to disable further
   processing of the P flag on that interface, allowing the client to
   fall back to other address assignment mechanisms, such as forming
   addresses via SLAAC (if the PIO has the A flag set to 1) and/or
   requesting individual addresses via DHCPv6.

7.2.  Using Delegated Prefix(es)

   If the delegated prefix is too long to be used for SLAAC, the client
   MUST ignore it, as Section 7 of [RFC9663] requires the network to
   provide a SLAAC-suitable prefix to clients.  If the prefix is shorter
   than required for SLAAC, the client SHOULD accept it, allocate one or
   more longer prefix suitable for SLAAC and use the prefixes as
   described below.

   For every accepted prefix:

   *  The client MAY form as many IPv6 addresses from the prefix as it
      chooses.

Colitti, et al.           Expires 11 April 2025                 [Page 7]
Internet-Draft                 pio-p-flag                   October 2024

   *  The client MAY use the prefix to provide IPv6 addresses to
      internal components such as virtual machines or containers.

   *  The client MAY use the prefix to allow devices directly connected
      to it to obtain IPv6 addresses.  For example, the client MAY route
      traffic for that prefix to the interface and send a Router
      Advertisement containing a PIO for the prefix on the interface.
      That interface MUST NOT be the interface the prefix is obtained
      from.  If the client advertises the prefix on an interface, and it
      has formed addresses from the prefix, then it MUST act as though
      the addresses were assigned to that interface for the purposes of
      Neighbour Discovery and Duplicate Address Detection.

   The client MUST NOT send or forward packets with destination
   addresses within a delegated prefix to the interface that it obtained
   the prefix on, as this can cause a routing loop.  This problem will
   not occur if the client has assigned the prefix to another interface.
   Another way the client can prevent this problem is to add to its
   routing table a high-metric discard route for the delegated prefix.

7.3.  Absence of PIOs with P bit set

   The P bit is purely a positive indicator, telling nodes that DHCPv6
   Prefix Delegation is available and the network prefers that nodes use
   it, even if they do not have any other reason to run a Prefix
   Delegation client.  The absence of any PIOs with the P bit does not
   carry any kind of signal to the opposite, and MUST NOT be processed
   to mean that DHCPv6-PD is absent.  In particular, nodes that run
   DHCPv6-PD due to explicit configuration or by default (e.g., to
   extend the network) MUST NOT disable DHCPv6-PD on the absence of PIOs
   with the P bit set.  A very common example of this are CE routers as
   described by [RFC7084].

7.4.  On-link Communication

   When the network delegates unique prefixes to clients, each client
   will consider other client's destination addresses to be off-link,
   because those addresses are from the delegated prefixes and are not
   within any on-link prefix.  When a client sends traffic to another
   client, packets will initially be sent to the default router.  The
   router may respond with an ICMPv6 redirect message (Section 4.5 of
   [RFC4861]).  If the client receives and accepts the redirect, then
   traffic can flow directly from device to device.  Therefore, hosts
   supporting the P flag SHOULD process redirects unless configured
   otherwise.  Hosts which do not process ICMPv6 redirects, and routers,
   which do not act on ICMPv6 redirects, may experience higher latency
   while communicating to prefixes delegated to other clients on the
   same link.

Colitti, et al.           Expires 11 April 2025                 [Page 8]
Internet-Draft                 pio-p-flag                   October 2024

7.5.  Source Address Selection

   For the purpose of source address selection [RFC6724], if the host
   creates any addresses from a delegated prefix, it SHOULD treat those
   addresses as if they were assigned to the interface on which the
   prefix was received.  This includes placing them in the candidate
   set, and associating them with the outgoing interface when
   implementing Rule 5 of the source address selection algorithm.

8.  Multihoming

   In multi-prefix multihoming, the host generally needs to associate
   the prefix with the router that advertised it (see for example,
   [RFC6724] Rule 5.5).  If the host supports Rule 5.5, then it SHOULD
   associate each prefix with the link-local address of the DHCPv6
   server or relay from which it received the REPLY packet.  When
   receiving multiple REPLYs carrying the same prefix from distinct
   link-local addresses, the host SHOULD associate that prefix with all
   of these addresses.  This can commonly happen in networks with
   redundant routers and DHCPv6 servers or relays.

9.  Modifications to RFC-Mandated Behaviour

9.1.  Changes to RFC4861

   This document makes the following changes to Section 4.2 of
   [RFC4861]:

   OLD TEXT:

   ====

   Note: If neither M nor O flags are set, this indicates that no
   information is available via DHCPv6.

   ====

   NEW TEXT:

   ====

   Note: If none of M, O, or P (draft-ietf-6man-pio-pflag) flags are
   set, this indicates that no information is available via DHCPv6.

   ====

Colitti, et al.           Expires 11 April 2025                 [Page 9]
Internet-Draft                 pio-p-flag                   October 2024

9.2.  Changes to RFC4862

   This document makes the following changes to Section 5.5.3 of
   [RFC4862]:

   OLD TEXT:

   ===

   For each Prefix-Information option in the Router Advertisement:

   a) If the Autonomous flag is not set, silently ignore the Prefix
   Information option.

   b) If the prefix is the link-local prefix, silently ignore the Prefix
   Information option.

   c) If the preferred lifetime is greater than the valid lifetime,
   silently ignore the Prefix Information option.  A node MAY wish to
   log a system management error in this case.

   d) If the prefix advertised is not equal to the prefix of an address
   configured by stateless autoconfiguration already in the list of
   addresses associated with the interface (where "equal" means the two
   prefix lengths are the same and the first prefix- length bits of the
   prefixes are identical), and if the Valid Lifetime is not 0, form an
   address (and add it to the list) by combining the advertised prefix
   with an interface identifier of the link as follows:

   ===

   NEW TEXT:

   ===

   For each Prefix-Information option in the Router Advertisement:

   a) If the P flag is set, and the node implements draft-ietf-6man-pio-
   pflag, it SHOULD treat the Autonomous flag as if it was unset, and
   use prefix delegation to obtain addresses as described in draft-ietf-
   6man-pio-pflag.

   b) If the Autonomous flag is not set, silently ignore the Prefix
   Information option.

   c) If the prefix is the link-local prefix, silently ignore the Prefix
   Information option.

Colitti, et al.           Expires 11 April 2025                [Page 10]
Internet-Draft                 pio-p-flag                   October 2024

   d) If the preferred lifetime is greater than the valid lifetime,
   silently ignore the Prefix Information option.  A node MAY wish to
   log a system management error in this case.

   e) If the prefix advertised is not equal to the prefix of an address
   configured by stateless autoconfiguration already in the list of
   addresses associated with the interface (where "equal" means the two
   prefix lengths are the same and the first prefix- length bits of the
   prefixes are identical), and if the Valid Lifetime is not 0, form an
   address (and add it to the list) by combining the advertised prefix
   with an interface identifier of the link as follows:

   ===

10.  Security Considerations

   The mechanism described in this document relies on the information
   provided in the Router Advertisement and therefore shares the same
   security model as SLAAC.  If the network does not implement RA Guard
   [RFC6105], an attacker might send RAs containing the PIO used by the
   network, set the P flag to 1 and force hosts to ignore the A flag.
   In the absence of DHCPv6-PD infrastructure, hosts would either obtain
   no IPv6 addresses or, if they fall back to other IPv6 address
   assignment mechanisms such as SLAAC and IA_NA, would experience
   delays in obtaining IPv6 addresses.  If the network does not support
   DHCPv6-Shield [RFC7610], the attacker could also run a rogue DHCPv6
   server, providing the host with invalid prefixes or other invalid
   configuration information.

   The attacker might force hosts to oscillate between DHCPv6-PD and
   PIO-based SLAAC by sending the same set of PIOs with and then w/o P
   flag set.  That would cause the clients to issue REBIND requests,
   increasing the load on the DHCP infrastructure.  However,
   Section 14.1 of [RFC8415] requires that DHCPv6-PD clients rate limit
   transmitted DHCPv6 messages.

   It should be noted that if the network allows rogue RAs to be sent,
   the attacker would be able to disrupt hosts connectivity anyway, so
   this document doesn't introduce any fundamentally new security
   considerations.

   Security considerations inherent to the PD-per-device model are
   documented in Section 15 of [RFC9663].

Colitti, et al.           Expires 11 April 2025                [Page 11]
Internet-Draft                 pio-p-flag                   October 2024

11.  Privacy Considerations

   The privacy implications of implementing the P flag and using
   DHCPv6-PD to assign prefixes to hosts are similar to privacy
   implications of using DHCPv6 for assigning individual addresses.  If
   the DHCPv6 infrastructure assigns the same prefix to the same client,
   then an observer might be able to identify clients based on the
   highest 64 bits of the client's address.  Those implications and
   recommended countermeasures are discussed in Section 13 of [RFC9663].

   Implementing the P flag support on a host / receiving side enables
   DHCPv6 on that host.  Sending DHCPv6 packets may reveal some minor
   additional information about the host, most prominently the hostname.
   This is not a new concern and would apply for any network which uses
   DHCPv6 and sets 'M' flag in Router Advertisements.

   No privacy considerations result from supporting the P flag on the
   sender side.

12.  IANA Considerations

   This memo requests that IANA allocate bit 3 from the "IPv6 Neighbor
   Discovery Prefix Information Option Flags" registry created by
   [RFC8425] for use as the P flag as described in this document.  The
   following entry should be appended:

    +================+==============================+=================+
    | PIO Option Bit | Description                  | Reference       |
    +================+==============================+=================+
    | 3              | P - DHCPv6-PD preferred flag | [THIS DOCUMENT] |
    +----------------+------------------------------+-----------------+

                                  Table 1

13.  References

13.1.  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>.

Colitti, et al.           Expires 11 April 2025                [Page 12]
Internet-Draft                 pio-p-flag                   October 2024

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [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>.

   [RFC8425]  Troan, O., "IANA Considerations for IPv6 Neighbor
              Discovery Prefix Information Option Flags", RFC 8425,
              DOI 10.17487/RFC8425, July 2018,
              <https://www.rfc-editor.org/info/rfc8425>.

13.2.  Informative References

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [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,
              <https://www.rfc-editor.org/info/rfc7084>.

Colitti, et al.           Expires 11 April 2025                [Page 13]
Internet-Draft                 pio-p-flag                   October 2024

   [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,
              <https://www.rfc-editor.org/info/rfc7039>.

   [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,
              <https://www.rfc-editor.org/info/rfc7348>.

   [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
              Protecting against Rogue DHCPv6 Servers", BCP 199,
              RFC 7610, DOI 10.17487/RFC7610, August 2015,
              <https://www.rfc-editor.org/info/rfc7610>.

   [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>.

   [RFC9663]  Colitti, L., Linkova, J., Ed., and X. Ma, Ed., "Using
              DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
              IPv6 Prefixes per Client in Large Broadcast Networks",
              RFC 9663, DOI 10.17487/RFC9663, October 2024,
              <https://www.rfc-editor.org/info/rfc9663>.

Acknowledgements

   Thanks to Nick Buraglio, Brian Carpenter, Tim Chown, David Farmer,
   Fernando Gont, Susan Hares, Dirk Von Hugo, Mahesh Jethanandani,
   Suresh Krishnan, Ted Lemon, Andrew McGregor, Erik Nordmark, Tomek
   Mrugalski, Michael Richardson, John Scudder, Ole Trøan, Eric Vyncke
   and Timothy Winters for the discussions, reviews, the input and all
   contributions.

Authors' Addresses

   Lorenzo Colitti
   Google
   Shibuya 3-21-3,
   Japan
   Email: lorenzo@google.com

Colitti, et al.           Expires 11 April 2025                [Page 14]
Internet-Draft                 pio-p-flag                   October 2024

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont NSW 2009
   Australia
   Email: furry13@gmail.com, furry@google.com

   Xiao Ma (editor)
   Google
   Shibuya 3-21-3,
   Japan
   Email: xiaom@google.com

   David 'equinox' Lamparter
   NetDEF, Inc.
   San Jose,
   United States of America
   Email: equinox@diac24.net, equinox@opensourcerouting.org

Colitti, et al.           Expires 11 April 2025                [Page 15]