IPv6 Maintenance L. Colitti
Internet-Draft J. Linkova
Intended status: Standards Track Google
Expires: April 5, 2020 October 3, 2019
Discovering PREF64 in Router Advertisements
draft-ietf-6man-ra-pref64-06
Abstract
This document specifies a Router Advertisement option to communicate
NAT64 prefixes to hosts.
Status of This Memo
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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
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 [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 [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
[RFC4861];
DNS64: a mechanism for synthesizing AAAA records from A records
[RFC6147];
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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] 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
example:
* 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
literal.
* 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
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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 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..
5. Option format
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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 | PLC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Highest 96 bits of the Prefix |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NAT64 Prefix Option Format
Fields:
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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.
Lifetime 13-bit unsigned integer. The maximum time in units of 8
seconds 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 8191. The receiver MUST
multiply the 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 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
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):
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;
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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..
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
information:
o set of PREF64 with non-zero lifetime;
o set of PREF64 with zero lifetime.
PvD-aware routers MUST only compare information scoped to the same
implicit or explicit PvD.
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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, 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,
<https://www.rfc-editor.org/info/rfc2119>.
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[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,
<https://www.rfc-editor.org/info/rfc4861>.
[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,
<https://www.rfc-editor.org/info/rfc6052>.
[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,
<https://www.rfc-editor.org/info/rfc7050>.
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,
<https://www.rfc-editor.org/info/rfc6105>.
[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,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.
[RFC7225] Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
Port Control Protocol (PCP)", RFC 7225,
DOI 10.17487/RFC7225, May 2014,
<https://www.rfc-editor.org/info/rfc7225>.
[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>.
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[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
11.3. URIs
[1] https://www.iana.org/assignments/icmpv6-parameters
Authors' Addresses
Lorenzo Colitti
Google
Shibuya 3-21-3
Shibuya, Tokyo 150-0002
JP
Email: lorenzo@google.com
Jen Linkova
Google
1 Darling Island Rd
Pyrmont, NSW 2009
AU
Email: furry@google.com
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