IPv6 Maintenance                                              L. Colitti
Internet-Draft                                                J. Linkova
Intended status: Standards Track                                  Google
Expires: February 12, April 2, 2020                               August 11,                                September 30, 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 . . . .  Usage Guidelines  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Option format . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   6.  Handling Multiple NAT64 Prefixes  . . . . . . . . . . . . . .   6
   7.  Multihoming . . . .  PREF64 Consistency  . . . . . . . . . . . . . . . . . . . . .   7
   8.  Pref64 Consistency  . . . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10.   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   12.   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  11  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  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]); [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. 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.

   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.


      *  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 DNS-over-TLS server ([RFC7858]) [RFC7858] 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 a routing to be configured.  IPv6 routing
   configuration requires receiving an IPv6 Router Advertisement
   [RFC4861].  Compared  Therefore using Router Advertisements to currently-deployed provide hosts
   with NAT64 prefix discovery
   methods such as [RFC7050], 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. protocols (e.g.  PCP [RFC7225]) which
   could be considered an obstacle for deplyoment.

4.  Semantics  Usage Guidelines

   To support prefix lengths defined in ([RFC6052]) [RFC6052] this option contains
   the prefix length field.  However as /96 prefix is considered to be
   the most common use case, 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) octets) for the most common case and increase
   it to 20 bytes 24 octets for non-/96 prefixes only (see Section 5 below for
   more details).

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

                   Figure 1: NAT64 Prefix Option Format


   Type     8-bit identifier of the Pref64 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 The
            sender MUST set the Length length to 2
            and include 2.  The receiver MUST ignore
            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 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. not 2.

   Lifetime 16-bit 13-bit unsigned integer. The maximum time in seconds units of 8
            secons over which this NAT64 prefix MAY be used. The value
            of Lifetime SHOULD by default be set to the lesser of 3 x
            MaxRtrAdvInterval divided by 8 or 65535 seconds.  A 8192. The reciever MUST
            multiply the Lifetime value of zero means that by 8 to calculate the maximum
            time in seconds the prefix
            MUST no longer MAY be used.

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

   Lowest   32-bit unsigned integer. Contains bits 96 - 127 of SHOULD NOT be used anymore. Router
            vendors SHOULD allow adminstrators to specify non-zero
            lifetime values which are not divisible by 8. In such cases
            the NAT64
   bits router SHOULD round the provided value up to the lesser
            of  prefix. This field is optional nearest integer divisible by 8 or 65536, divide the
            result by 8 and presents only if set the Lifetime field to the      prefix length is not 96 bits.

   Prefix   8-bit resulting

   PL       3-bit unsigned integer. Optional integer.This field which present only if
   Length encodes the NAT64 Prefix
   (Prefix  Length. The PL field values 0,1,2,3,4 and 5 indicate the
   Length)  NAT64 prefix length is not 96 bits. The sender MUST set it
            only to one of the following values: 32, 40, 48, 56, 64
            ([RFC6052]. 96,64,56,48,40 and 32 bits
            respectively. The receiver reciever MUST ignore the Pref64 PREF64 option if
            the prefix length value field is not set to one of those numbers.

   Reserved A 3-byte unused field.  If present it MUST be initialized to
            zero by the sender and MUST be ignored by values.

   Highest  96-bit unsigned integer. Contains bits 0 - 95 of the receiver. This
            field is optional and presents only if NAT64
   96 bits  prefix.
   of the
   prefix length is
            not 96 bits.

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 PREF64 prefixes
      (e.g. by using PCP ([RFC7225]) [RFC7225]) and/or by resolving IPv4-only fully
      qualified domain name ([RFC7050]) [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 PREF64 prefixes on
      one or multiple interfaces.

   When multiple Pref64 PREF64 were discovered via RA Pref64 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..

   When different Pref64 PREF64 are discovered by using multiple mechanisms,
   hosts SHOULD select one source of information only.  The RECOMMENDED
   order is:

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

   o  Pref64  PREF64 discovered via RA Option;

   o  Pref64  PREF64 resolving IPv4-only fully qualified domain name ([RFC7050]) [RFC7050]

   Note that if the network provides Pref64 PREF64 both via this RA option and
   [RFC7225], hosts that receive the Pref64 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.  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 provides 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.  Pref64  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 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 PREF64 with non-zero lifetime;

   o  set of Pref64 PREF64 with zero lifetime.

   PvD-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 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, David Farmer, Nick Heatley, Robert Hinden, Martin Hunek,
   Tatuya Jinmei, Erik Kline, David Lamparter, Jordi Palet Martinez,
   Tommy Pauly, Michael Richardson, David Schinazi.

12. Schinazi, Ole Troan.

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,

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-07 (work in
              progress), June September 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,

   [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