Discovering PREF64 in Router Advertisements
draft-ietf-6man-ra-pref64-01

Abstract

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

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 December 30, 2019.

Copyright Notice

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Table of Contents

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

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 (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 ([RFC4861]);

DNS64: a mechanism for synthesizing AAAA records from A records ([RFC6147]);

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:

• 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.
• 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.
• 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 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.

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

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 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 10.0.0.0/8 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 Length > 2)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length |                  Reserved                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


				      

Figure 1: NAT64 Prefix Option Format

Fields:

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):

• 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);
• The pref64 option presents in a single RA more than once;
• 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 lifetime..

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

• PCP-discovered prefixes ([RFC7225]), if supported;
• Pref64 discovered via RA Option;
• 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 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.

The IANA registry for these options is:

• https://www.iana.org/assignments/icmpv6-parameters

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

11.2. Informative References

Authors' Addresses