IPv6 Maintenance L. Colitti
Internet-Draft E. Kline
Intended status: Standards Track J. Linkova
Expires: September 25, 2019 Google
March 24, 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

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

1.2. Terminology

Pref64: an IPv6 prefix used for IPv6 address synthesis [RFC6146];

PvD: Provisioning Domain, a set of network configuration information; for more information, see [RFC7556].

PvD-aware host A host that supports the association of network configuration information into PvDs and the use of these PvDs. Also named PvD-aware node in [RFC7556].

RA: Router Advertisement, a message used by IPv6 routers to advertise their presence together with various link and Internet parameters ([RFC4861]);

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:

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 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 [RFC4861]. Other options such as [RFC7225] require implementing other protocols.

4. Semantics

This option only supports a NAT64 prefix length of 96 bits, as this is by the most common configuration used by hosts and supporting variable prefix length would significantly increase the option size. Networks using one of the other prefix lengths supported in ([RFC6052]) can use other mechanisms such as [RFC7050] or [RFC7225]. If different prefix lengths become common, another RA option can be created to configure them.

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 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 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.

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            |
|                                                               |
+                            Prefix                             +
|                                                               |
+                                                               +
|                                                               |


Figure 1: NAT64 Prefix Option Format


Type 8-bit identifier of the RDNSS 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. A host MUST ignore the NAT64 prefix option if the length field value is 1. If the Length field value exceeds 2, the host MUST utilize the first 16 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.
Prefix The 96-bit NAT64 prefix.

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 to):

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 lifetime..

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

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 be PVD-aware.

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 interface).

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)

The IANA registry for these options is:

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, Brian E Carpenter, Nick Heatley, Tatuya Jinmei, 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

[I-D.ietf-intarea-provisioning-domains] Pfister, P., Vyncke, E., Pauly, T., Schinazi, D. and W. Shao, "Discovering Provisioning Domain Names and Data", Internet-Draft draft-ietf-intarea-provisioning-domains-04, March 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.
[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., "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.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017.

Authors' Addresses

Lorenzo Colitti Google Roppongi 6-10-1 Minato, Tokyo 106-6126 JP EMail: lorenzo@google.com
Erik Kline Google Roppongi 6-10-1 Minato, Tokyo 106-6126 JP EMail: ek@google.com
Jen Linkova Google 1 Darling Island Rd Pyrmont, NSW 2009 AU EMail: furry@google.com