IPv6 Maintenance                                              L. Colitti
Internet-Draft                                                J. Linkova
Intended status: Standards Track                                  Google
Expires: December 30, 2019                                 June 28, January 23, 2020                                  July 22, 2019

              Discovering PREF64 in Router Advertisements


   This document specifies a Router Advertisement option to communicate
   NAT64 prefixes to clients.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Use cases for communicating the NAT64 prefix to hosts . . . .   3
   3.  Why include the NAT64 prefix in Router Advertisements . . . .   3
   4.  Semantics . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Option format . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Handling Multiple NAT64 Prefixes  . . . . . . . . . . . . . .   6
   7.  Multihoming . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   NAT64 [RFC6146] with DNS64 [RFC6147] is a widely-deployed mechanism
   to provide IPv4 access on IPv6-only networks.  In various scenarios,
   the host must be aware of the NAT64 prefix in use by the network.
   This document specifies a Router Advertisement [RFC4861] option to
   communicate the NAT64 prefix to hosts.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   Pref64 (or NAT64 prefix): an IPv6 prefix used for IPv6 address
   synthesis [RFC6146];

   NAT64: Network Address and Protocol Translation from IPv6 Clients to
   IPv4 Servers ([RFC6146]);

   RA: Router Advertisement, a message used by IPv6 routers to advertise
   their presence together with various link and Internet parameters

   DNS64: a mechanism for synthesizing AAAA records from A records

2.  Use cases for communicating the NAT64 prefix to hosts

   On networks employing NAT64, it is useful for hosts to know the NAT64
   prefix for several reasons, including the following:

   o  Local DNSSEC validation.  As discussed in [RFC6147] section 2, the
      stub resolver in the host "will try to obtain (real) AAAA RRs, and
      in case they are not available, the DNS64 function will synthesize
      AAAA RRs for internal usage."  This is required in order to use
      DNSSEC on a NAT64 network.

   o  IPv4 address literals on an IPv6-only host.  As described in
      [RFC8305] section 7.1, IPv6-only hosts connecting to IPv4 address
      literals can resolve the IPv4 literal to an IPv6 address.

   o  464XLAT [RFC6877]. 464XLAT is widely deployed and requires that
      the host be aware of the NAT64 prefix.

   o  Trusted DNS server.  AAAA synthesis is required for the host to be
      able to use a DNS server not provided by the network (e.g., a DNS-
      over-TLS server with which the host has an existing trust

   o  Networks with no DNS64 server.  Hosts that support AAAA synthesis
      and that are aware of the NAT64 prefix in use do not need the
      network to perform the DNS64 function at all.

3.  Why include the NAT64 prefix in Router Advertisements

   Fate sharing: NAT64 requires a routing to be configured.  IPv6
   routing configuration requires receiving an IPv6 Router Advertisement
   [RFC4861].  Compared to currently-deployed NAT64 prefix discovery
   methods such as [RFC7050], including the NAT64 prefix in the Router
   Advertisement minimizes the number of packets required to configure a
   host.  This speeds up the process of connecting to a network that
   supports NAT64/DNS64, and simplifies host implementation by removing
   the possibility that the host can have an incomplete layer 3
   configuration (e.g., IPv6 addresses and prefixes, but no NAT64

   Updatability: it is possible to change the NAT64 prefix at any time,
   because when it changes, it is possible to notify hosts by sending a
   new Router Advertisement.

   Deployability: all IPv6 hosts and networks are required to support
   [RFC4861].  Other options such as [RFC7225] require implementing
   other protocols.

4.  Semantics

   To support prefix lengths defined in ([RFC6052]) this option contains
   the prefix length field.  However as /96 prefix is considered to be
   the most common usecase, the prefix length field is optional and only
   presents for non-/96 prefixes.  It allows to keep the option length
   to a minimum (16 bytes) for the most common case and increase it to
   20 bytes for non-/96 prefixes only (see Section 5 below for more

   This option specifies exactly one NAT64 prefix for all IPv4
   destinations.  If the network operator desires to route different
   parts of the IPv4 address space to different NAT64 devices, this can
   be accomplished by routing more specifics of the NAT64 prefix to
   those devices.  For example, if the operator would like to route through NAT64 device A and the rest of the IPv4 space
   through NAT64 device B, and the operator's NAT64 prefix is
   2001:db8:a:b::/96, then the operator can route
   2001:db8:a:b::a00:0/104 to NAT64 A and 2001:db8:a:b::/64 to NAT64 B.

   This option may appear more than once in a Router Advertisement (e.g.
   in case of graceful renumbering the network from one NAT64 prefix to
   another).  Host behaviour with regards to synthesizing IPv6 addresses
   from IPv4 addresses SHOULD follow the recommendations given in
   Section 3 of [RFC7050], limited to the NAT64 prefixes that have non-
   zero lifetime.

   In a network (or a provisioning domain) that provides both IPv4 and
   NAT64, it may be desirable for certain IPv4 addresses not to be
   translated.  An example might be private address ranges that are
   local to the network/provisioning domain and should not be reached
   through the NAT64.  This type of configuration cannot be conveyed to
   hosts using this option, or through other NAT64 prefix provisioning
   mechanisms such as [RFC7050] or [RFC7225].  This problem does not
   apply in IPv6-only networks, because in such networks, the host does
   not have an IPv4 address and cannot reach any IPv4 destinations
   without the NAT64.  The multihoming and multiple provisioning domains
   scenarios are discussed in Section 7.

5.  Option format
      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     |           Lifetime            |
     |                                                               |
     +                                                               +
     |              Highest 96 bits of the Prefix                    |
     +                                                               +
     |                                                               |
     | Lowest bits (96-127) of the prefix (optional, if Prefix Length > 2) |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                  Reserved                     | Prefix Length
     |                           Reserved                            |

                   Figure 1: NAT64 Prefix Option Format


   Type     8-bit identifier of the Pref64 option type as assigned by
            IANA: TBD
   Length   8-bit unsigned integer.  The length of the option (including
            the Type and Length fields) is in units of 8 octets. If the
            prefix length is 96 bits the sender MUST set the Length to 2
            and include the 96 bits of the prefix in the option. If the
            prefix length is not 96 bits then the sender MUST set the
            length to 3 and include all 128 bits of the prefix in the
            Prefix field and set the Prefix Length field to the prefix
            length.  The receiver MUST ignore the Pref64 option if the
            length field value is 1. If the Length field value exceeds
            3, the receiver MUST utilize the first 21 octets and ignore
            the rest of the option.

   Lifetime 16-bit unsigned integer.  The maximum time in seconds over
            which this NAT64 prefix MAY be used. The value of Lifetime
            SHOULD by default be set to lesser of  3 x MaxRtrAdvInterval
            or 65535 seconds.  A value of zero means that the prefix
            MUST no longer be used.

   Highest  96-bit unsigned integer. Contains bits 0 - 95 of the NAT64
   96 bits  prefix.
   of the

   Lowest   32-bit unsigned integer. Contains bits 96 - 127 of the NAT64
   bits of  prefix.

   Prefix   8-bit unsigned integer. Optional field which present only if
   Length   the prefix length is not 96 bits. The sender MUST set it
            only to one
   Length of the following values: 32, 40, 48, 56, 64
            ([RFC6052]. The receiver MUST ignore the Pref64 option if
            the prefix length value is not set to one of those numbers.

   Reserved A 3-byte 7-byte unused field.  It MUST be initialized to zero by
            the sender and MUST be ignored by the receiver. This field
            is optional and presents only if the prefix length is not 96

6.  Handling Multiple NAT64 Prefixes

   In some cases a host may receive multiple NAT64 prefixes from
   different sources.  Possible scenarios include (but are not limited

   o  the host is using multiple mechanisms to discover Pref64 prefixes
      (e.g. by using PCP ([RFC7225]) and/or by resolving IPv4-only fully
      qualified domain name ([RFC7050]) in addition to receiving the
      Pref64 RA option);

   o  The pref64 option presents in a single RA more than once;

   o  the host receives multiple RAs with different Pref64 prefixes on
      one or multiple interfaces.

   When multiple Pref64 were discovered via RA Pref64 Option (the Option
   presents more than once in a singe RA or multiple RAs were received),
   host behaviour with regards to synthesizing IPv6 addresses from IPv4
   addresses SHOULD follow the recommendations given in Section 3 of
   [RFC7050], limited to the NAT64 prefixes that have non-zero

   When different Pref64 are discovered by using multiple mechanisms,
   hosts SHOULD select one source of infromation only.  The RECOMMENDED
   order is:

   o  PCP-discovered prefixes ([RFC7225]), if supported;

   o  Pref64 discovered via RA Option;

   o  Pref64 resolving IPv4-only fully qualified domain name ([RFC7050])

   Note that if the network provides Pref64 both via this RA option and
   [RFC7225], hosts that receive the Pref64 via RA option may choose to
   use it imediately before waiting for PCP to complete, and therefore
   some traffic may not reflect any more detailed configuration provided
   by PCP.

7.  Multihoming

   Like most IPv6 configuration information, the Pref64 option is
   specific to the network on which it is received.  For example, a
   Pref64 option received on a particular wireless network may not be
   usable unless the traffic is also sourced on that network.
   Similarly, a host connected to a cellular network that povides NAT64
   generally cannot use that NAT64 for destinations reached through a
   VPN tunnel that terminates outside that network.

   Thus, correct use of this option on a multihomed host generally
   requires the host to support the concept of multiple Provisioning
   Domains (PvD, a set of configuration information associated with a
   network, [RFC7556]) and to be able to use these PvDs.

   This issue is not specific to the Pref64 RA option and, for example,
   is quite typical for DNS resolving on multihomed hosts (e.g. a host
   might resolve a destination name by using the corporate DNS server
   via the VPN tunnel but then send the traffic via its Internet-facing

8.  IANA Considerations

   The IANA is requested to assign a new IPv6 Neighbor Discovery Option
   type for the PREF64 option defined in this document.

                         | Option Name   | Type  |
                         | PREF64 option | (TBD) |

                                  Table 1

   The IANA registry for these options is:

      https://www.iana.org/assignments/icmpv6-parameters [1]

9.  Security Considerations

   Because Router Advertisements are required in all IPv6 configuration
   scenarios, on IPv6-only networks, Router Advertisements must already
   be secured, e.g., by deploying RA guard [RFC6105].  Providing all
   configuration in Router Advertisements increases security by ensuring
   that no other protocols can be abused by malicious attackers to
   provide hosts with invalid configuration.

   The security measures that must already be in place to ensure that
   Router Advertisements are only received from legitimate sources
   eliminate the problem of NAT64 prefix validation described in section
   3.1 of [RFC7050].

10.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   review and feedback: Mikael Abrahamsson, Mark Andrews, Brian E
   Carpenter, Nick Heatley, Martin Hunek, Tatuya Jinmei, Erik Kline,
   Michael Richardson, David Schinazi.

11.  References

11.1.  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,

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,

11.2.  Informative References

              Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
              Shao, "Discovering Provisioning Domain Names and Data",
              draft-ietf-intarea-provisioning-domains-05 (work in
              progress), June 2019.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [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,

   [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,

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,

   [RFC7225]  Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
              Port Control Protocol (PCP)", RFC 7225,
              DOI 10.17487/RFC7225, May 2014,

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

11.3.  URIs

   [1] https://www.iana.org/assignments/icmpv6-parameters

Authors' Addresses

   Lorenzo Colitti
   Roppongi 6-10-1
   Minato, Tokyo  106-6126

   Email: lorenzo@google.com

   Jen Linkova
   1 Darling Island Rd
   Pyrmont, NSW  2009

   Email: furry@google.com