IPv6 Maintenance                                              L. Colitti
Internet-Draft                                                J. Linkova
Intended status: Standards Track                                  Google
Expires: April May 5, 2020                                   October 3,                                    November 2, 2019

              Discovering PREF64 in Router Advertisements


   This document specifies a Router Advertisement Neighbor Discovery option to be used in
   Router Advertisements to communicate NAT64 prefixes to hosts.

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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.  Usage Guidelines  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Option format . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   6.  Handling Multiple NAT64 Prefixes  . . . . . . . . . . . . . .   6
   7.  PREF64 Consistency  . . . . . . . . . . . . . . . . . . . . .   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 Neighbor Discovery [RFC4861] option to be
   used in Router Advertisements to communicate the NAT64 prefix prefixes to hosts.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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  Enabling DNS64 functions on end hosts.  In particular:

      *  Local DNSSEC validation (DNS64 in stub-resolver mode).  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.

      *  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 [RFC7858] or DNS-over-HTTPS [RFC8484] server
         with which the host has an existing trust relationship).

      *  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.

   o  Enabling NAT64 address translation functions on end hosts.  For

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

      *  464XLAT [RFC6877]. 464XLAT requires the host be aware of the
         NAT64 prefix.

3.  Why include the NAT64 prefix in Router Advertisements

   Fate sharing: NAT64 requires routing to be configured.  IPv6 routing
   configuration requires receiving an IPv6 Router Advertisement
   [RFC4861].  Therefore using Router Advertisements to provide hosts
   with NAT64 prefix ensures that NAT64 reachability information shares
   fate with the rest of network configuration on the host.

   Atomic configuration: including the NAT64 prefix in the Router
   Advertisement minimizes the number of packets required to configure a
   host.  Only one packet (a Router Advertisement) is required to
   complete the network configuration.  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 prefix).

   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
   Neighbor Discovery [RFC4861] so just a minor extension to the
   existing implementation is required.  Other options such as [RFC7225]
   require implementing other protocols (e.g.  PCP [RFC7225]) which
   could be considered an obstacle for deployment.

4.  Usage Guidelines

   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 is using the RFC1918
   address space, e.g. internally and 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 2001:db8:a:b::/96 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.. NAT64.

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     |     Scaled Lifetime     | PLC |
     |                                                               |
     +                                                               +
     |              Highest 96 bits of the Prefix                    |
     +                                                               +
     |                                                               |

                   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. The
            sender MUST set the length to 2.  The receiver MUST ignore
            the PREF64 option if the length field value is not 2.


   Scaled   13-bit unsigned integer. The maximum time in units of 8
   Lifetime seconds over which this NAT64 prefix MAY be used. The value
            of the Scaled Lifetime field SHOULD by default be set to the
            lesser of 3 x MaxRtrAdvInterval divided by 8, or 8191. The
            receiver MUST multiply the Scaled Lifetime value by 8 (for
            example, by logical left shift) to calculate the maximum
            time in seconds the prefix MAY be used. Lifetime of 0
            indicates that the prefix SHOULD NOT be used anymore. Router
            vendors SHOULD allow administrators to specify non-zero
            lifetime values which are not divisible by 8. In such cases
            the router SHOULD round the provided value up to the lesser
            of nearest integer divisible by 8, or 65528 and divide the
            result by 8 (or just perform a logical right-shift by 3) and
            set the Scaled Lifetime field to the resulting value.

   PLC      3-bit unsigned integer. This field encodes the NAT64 Prefix
   (Prefix  Length defined in [RFC6052]. The PLC field values 0, 1, 2,
   Length   3, 4 and 5 indicate the NAT64 prefix length of 96, 64, 56,
   Code)    48, 40 and 32 bits respectively. The receiver MUST ignore
            the PREF64 option if the prefix length code field is not set
            to one of those values.

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

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 single 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 lifetime.. lifetime.

   When different PREF64 are discovered by using multiple mechanisms,
   hosts SHOULD select one source of information 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 immediately before waiting for PCP to complete, and therefore
   some traffic may not reflect any more detailed configuration provided
   by PCP.

7.  PREF64 Consistency

   Section 6.2.7 of [RFC4861] recommends that routers inspect RAs sent
   by other routers to ensure that all routers onlink advertise the
   consistent information.  Routers SHOULD inspect valid PREF64 options
   received on a given link and verify the consistency.  Detected
   inconsistencies indicate that one or more routers might be
   misconfigured.  Routers SHOULD log such cases to system or network
   management.  Routers SHOULD check and compare the following

   o  set of PREF64 with non-zero lifetime;

   o  set of PREF64 with zero lifetime.


   Provisioning Domain (PvD, [RFC7556])-aware routers MUST only compare
   information scoped to the same implicit or explicit PvD.

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 reduces the attack surface to
   be abused targeted by malicious attackers to provide hosts with invalid configuration.
   configuration as compared to distributing the configuration through
   multiple different mechanisms that need to be secured independently.

   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, David Farmer, Nick Heatley, Robert Hinden, Martin Hunek,
   Tatuya Jinmei, Erik Kline, Suresh Krishnan, David Lamparter, Jordi
   Palet Martinez, Tommy Pauly, Alexandre Petrescu, Michael Richardson,
   David Schinazi, Ole Troan, Bernie Volz.

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,

   [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,

   [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,

   [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,

   [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>.

11.2.  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,

   [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,

   [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,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

11.3.  URIs

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

Authors' Addresses

   Lorenzo Colitti
   Shibuya 3-21-3
   Shibuya, Tokyo  150-0002

   Email: lorenzo@google.com

   Jen Linkova
   1 Darling Island Rd
   Pyrmont, NSW  2009

   Email: furry@google.com