Signalling DHCPv6 Prefix per Client Availability to Hosts
draft-ietf-6man-pio-pflag-09

IPv6 Maintenance                                              L. Colitti
Internet-Draft                                                J. Linkova
Updates: 4862 (if approved)                                   X. Ma, Ed.
Intended status: Standards Track                                  Google
Expires: 16 February 2025                                   D. Lamparter
                                                            NetDEF, Inc.
                                                          15 August 2024

       Signalling DHCPv6 Prefix per Client Availability to Hosts
                      draft-ietf-6man-pio-pflag-09

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 draft-ietf-
   v6ops-dhcp-pd-per-device deployment model instead of using individual
   adresses in the on-link prefix assigned using SLAAC or DHCPv6 IA_NA.

Status of This Memo

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

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   This Internet-Draft will expire on 16 February 2025.

Copyright Notice

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

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   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  P Flag Overview . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Router Behaviour  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Host Behaviour  . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Processing the P Flag . . . . . . . . . . . . . . . . . .   5
     6.2.  Using Delegated Prefix(es)  . . . . . . . . . . . . . . .   6
     6.3.  Absence of PIOs with P bit set  . . . . . . . . . . . . .   7
     6.4.  On-link Communication . . . . . . . . . . . . . . . . . .   7
     6.5.  Source Address Selection  . . . . . . . . . . . . . . . .   8
   7.  Multihoming . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Modifications to RFC-Mandated Behavior  . . . . . . . . . . .   8
     8.1.  Changes to RFC4862  . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [I-D.ietf-v6ops-dhcp-pd-per-device] documents an IPv6 address
   assignment model where IPv6 devices obtain dedicated prefixes from
   the network via DHCPv6 Prefix Delegation [RFC8415].

   This model provides devices with large amounts of address space that
   they can use to individually number VMs 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 state per address: IPv6 neighbor cache, SAVI mappings
   ([RFC7039]), VXLAN routes, etc.  On smaller networks, scaling to
   support multiple individual IPv6 addresses is less of a concern,
   because many home routers support hundreds of neighbor cache entries.

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   Also, many smaller 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 hosts enable
   [I-D.ietf-v6ops-dhcp-pd-per-device] on a home network that only
   provides a /60, and each host obtains a /64 prefix, then the network
   will run out of prefixes after 15 devices connect.

   Therefore, to safely roll out [I-D.ietf-v6ops-dhcp-pd-per-device]
   implementations on the client side, it is necessary to have a
   mechanism for the network to signal to the host 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.  Rationale

   The network administrator might want to indicate to hosts that
   requesting a prefix via DHCPv6-PD and using that prefix for address
   assignment (see [I-D.ietf-v6ops-dhcp-pd-per-device]) should be
   preferred over using individual addresses from the on-link prefix.
   The information is passed to the host 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 host before it decides to form
      IPv6 addresses from the PIO prefix using SLAAC.  Otherwise, the
      host might use SLAAC to form IPv6 addresses from the PIO provided
      and start using them, even if a unique per-host prefix is
      available via DHCPv6-PD.  Forming addresses via SLAAC is
      suboptimal because if the host 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 hosts
      to assign global addresses from delegated prefixes, but use the

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      PIO prefix to form ULA 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 option expresses the
   operator's preference as to whether hosts should attempt using
   DHCPv6-PD instead of performing individual address configuration on
   the prefix.  For hosts that honor this preference by requesting
   prefix delegation, the actual delegated prefix will necessarily be a
   prefix different from the one from the PIO.

4.  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 that the network prefers that
   hosts 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
   [I-D.ietf-v6ops-dhcp-pd-per-device].

   Adding the P flag reduces the PIO Reserved1 field ([RFC4861],
   [RFC8425]) from 5 bits to 4 bits.  The resulting format of the Prefix
   Information Option is as follows:

        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

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   The P flag is independent from 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] CE routers, might not initiate DHCPv6
   Prefix Delegation if both the M and O bits are set to zero.

5.  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.

6.  Host Behaviour

   This section uses the term host to refer to any node that processes
   Router Advertisements.  This includes both hosts and nodes such as CE
   Routers [RFC7084] which forward packets but also listen to Router
   Advertisements.

6.1.  Processing the P Flag

   This specification only applies to hosts which support acting as
   DHCPv6 Prefix Delegation clients.  Hosts 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 host 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 due to
      expiration or due to the receipt of a PIO with a Preferred
      Lifetime of zero, the prefix MUST be removed from the list.

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

   *  When the length of the list decreases to zero, the host 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.

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   *  If the host 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 host MUST consider this to be a change in
      configuration information as described in Section 18.2.12 of
      [RFC8415], and it MUST perform a REBIND, unless it is going to
      stop the DHCPv6 client because the list became empty.  This is in
      addition to performing a REBIND in the other cases required by
      that section.  Issuing a REBIND allows the host to obtain new
      prefixes if necessary, e.g., when the network is being renumbered.
      It also refreshes state related to the delegated prefix(es).

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

   If the host 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 host 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.

6.2.  Using Delegated Prefix(es)

   If the delegated prefix is too long to be used for SLAAC, the host
   MUST ignore it.  If the prefix is shorter than required for SLAAC,
   the host SHOULD accept it, allocate one or more longer prefix
   suitable for SLAAC and use the prefixes as described below.

   For every accepted prefix:

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

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

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   *  The host MAY use the prefix to allow devices directly connected to
      it to obtain IPv6 addresses, e.g., by routing traffic for that
      prefix to the interface and sending a Router Advertisement
      containing the prefix on the interface.  If the host does so, and
      it has 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 host MUST NOT forward packets with destination addresses within a
   delegated prefix to the interface that it obtained the prefix on, as
   this will cause a routing loop.  This problem will not occur if the
   host has assigned the prefix to a downstream interface.  If the host
   has not assigned the prefix to a downstream interface, then one way
   to prevent this problem this is to add to its routing table a high-
   metric discard route for the delegated prefix.  Similarly, the host
   MUST NOT send packets with destination addresses in the delegated
   prefix to the interface that it obtained the prefix on.

6.3.  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].

6.4.  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.

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6.5.  Source Address Selection

   For the purpose of source address selection [RFC6724], if the host
   forms 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.

7.  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 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 relays.

8.  Modifications to RFC-Mandated Behavior

8.1.  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.

   ===

   NEW TEXT: Insert the following text after "For each Prefix-
   Information option in the Router Advertisement:" but before "If the
   Autonomous flag is not set, silently ignore the Prefix Information
   option.":

   ===

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   a) If the P flag is set, the node 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.

   ===

9.  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 doesn't 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 [I-D.ietf-v6ops-dhcp-pd-per-device].

10.  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
   [I-D.ietf-v6ops-dhcp-pd-per-device].

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   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.

11.  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

12.  References

12.1.  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>.

   [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>.

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   [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>.

12.2.  Informative References

   [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>.

   [I-D.ietf-v6ops-dhcp-pd-per-device]
              Colitti, L., Linkova, J., and X. Ma, "Using DHCPv6-PD to
              Allocate Unique IPv6 Prefix per Client in Large Broadcast
              Networks", Work in Progress, Internet-Draft, draft-ietf-
              v6ops-dhcp-pd-per-device-08, 3 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              dhcp-pd-per-device-08>.

   [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>.

   [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>.

   [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>.

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Acknowledgements

   Thanks to Nick Buraglio, Brian Carpenter, Tim Chown, David Farmer,
   Fernando Gont, Suresh Krishnan, Ted Lemon, Andrew McGregor, Tomek
   Mrugalski, Michael Richardson, Ole Trøan, Timothy Winters for the
   discussions, the input and all contributions.

Authors' Addresses

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

   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

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