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IETF RFC 7526
Last modified on Wednesday, May 20th, 2015
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Internet Engineering Task Force (IETF) O. Troan
Request for Comments: 7526 Cisco
BCP: 196 B. Carpenter, Ed.
Obsoletes: 3068, 6732 Univ. of Auckland
Category: Best Current Practice May 2015
ISSN: 2070-1721
Deprecating the Anycast Prefix for 6to4 Relay Routers
 Abstract
Experience with the 6to4 transition mechanism defined in RFC 3056
("Connection of IPv6 Domains via IPv4 Clouds") has shown that the
mechanism is unsuitable for widespread deployment and use in the
Internet when used in its anycast mode. Therefore, this document
requests that RFC 3068 ("An Anycast Prefix for 6to4 Relay Routers")
and RFC 6732 ("6to4 Provider Managed Tunnels") be made obsolete and
moved to Historic status. It recommends that future products should
not support 6to4 anycast and that existing deployments should be
reviewed. This complements the guidelines in RFC 6343.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7526.
Troan & Carpenter Best Current Practice PAGE 1 
RFC 7526 Deprecating 6to4 Anycast May 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. 6to4 Operational Problems . . . . . . . . . . . . . . . . . . 4
4. Deprecation . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation Recommendations . . . . . . . . . . . . . . . 5
6. Operational Recommendations . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
Troan & Carpenter Best Current Practice PAGE 2
RFC 7526 Deprecating 6to4 Anycast May 2015
1. Introduction
The original form of the 6to4 transition mechanism [RFC 3056] relies
on unicast addressing. However, its extension specified in "An
Anycast Prefix for 6to4 Relay Routers" [RFC 3068] has been shown to
have severe practical problems when used in the Internet. This
document requests that RFCs 3068 and 6732 be moved to Historic
status, as defined in Section 4.2.4 of [RFC 2026]. It complements the
deployment guidelines in [RFC 6343].
6to4 was designed to help transition the Internet from IPv4 to IPv6.
It has been a good mechanism for experimenting with IPv6, but because
of the high failure rates seen with anycast 6to4 [HUSTON], end users
may end up disabling IPv6 on hosts; this has resulted in some content
providers being reluctant to make content available over IPv6 in the
past.
[RFC 6343] analyzes the known operational issues in detail and
describes a set of suggestions to improve 6to4 reliability, given the
widespread presence of hosts and customer premises equipment that
support it. The advice to disable 6to4 by default has been widely
adopted in recent operating systems, and the failure modes have been
widely hidden from users by many browsers adopting the "Happy
Eyeballs" approach [RFC 6555].
Nevertheless, a measurable amount of 6to4 traffic is still observed
by IPv6 content providers. The remaining successful users of anycast
6to4 are likely to be on hosts using the obsolete policy table
[RFC 3484] (which prefers 6to4 above IPv4) and running without Happy
Eyeballs. Furthermore, they must have a route to an operational
anycast relay and they must be accessing an IPv6 host that has a
route to an operational return relay.
However, experience shows that operational failures caused by anycast
6to4 have continued despite the advice in RFC 6343 being available.
1.1. Related Work
"IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) -- Protocol
Specification" [RFC 5969] explicitly builds on the 6to4 mechanism,
using a service provider prefix instead of 2002::/16. However, the
deployment model is based on service provider support such that 6rd
avoids the problems observed with anycast 6to4.
The framework for "6to4 Provider Managed Tunnels" [RFC 6732] is
intended to help a service provider manage 6to4 anycast tunnels.
This framework only exists because of the problems observed with
anycast 6to4.
Troan & Carpenter Best Current Practice PAGE 3
RFC 7526 Deprecating 6to4 Anycast May 2015
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC 2119].
In this document, the word "deprecate" and its derivatives are used
only in their generic sense of "criticize or express disapproval" and
do not have any specific normative meaning. A deprecated function
might exist in the Internet for many years to allow backwards
compatibility.
3. 6to4 Operational Problems
6to4 is a mechanism designed to allow isolated IPv6 islands to reach
each other using IPv6-over-IPv4 automatic tunneling. To reach the
native IPv6 Internet, the mechanism uses relay routers in both the
forward and reverse direction. The mechanism is supported in many
IPv6 implementations. With the increased deployment of IPv6, the
mechanism has been shown to have a number of shortcomings.
In the forward direction, a 6to4 node will send IPv4-encapsulated
IPv6 traffic to a 6to4 relay that is connected to both the 6to4 cloud
and native IPv6. In the reverse direction, a 2002::/16 route is
injected into the native IPv6 routing domain to attract traffic from
native IPv6 nodes to a 6to4 relay router. It is expected that
traffic will use different relays in the forward and reverse
direction.
One model of 6to4 deployment, described in Section 5.2 of RFC 3056,
suggests that a 6to4 router should have a set of managed connections
(via BGP connections) to a set of 6to4 relay routers. While this
makes the forward path more controlled, it does not guarantee a
functional reverse path. In any case, this model has the same
operational burden as manually configured tunnels and has seen no
deployment in the public Internet.
RFC 3068 adds an extension that allows the use of a well-known IPv4
anycast address to reach the nearest 6to4 relay in the forward
direction. However, this anycast mechanism has a number of
operational issues and problems, which are described in detail in
Section 3 of [RFC 6343]. This document is intended to deprecate the
anycast mechanism.
Troan & Carpenter Best Current Practice PAGE 4
RFC 7526 Deprecating 6to4 Anycast May 2015
Peer-to-peer usage of the 6to4 mechanism exists in the Internet,
likely unknown to many operators. This usage is harmless to third
parties and is not dependent on the anycast 6to4 mechanism that this
document deprecates.
4. Deprecation
This document formally deprecates the anycast 6to4 transition
mechanism defined in [RFC 3068] and the associated anycast IPv4
address 192.88.99.1. It is no longer considered to be a useful
service of last resort.
The prefix 192.88.99.0/24 MUST NOT be reassigned for other use except
by a future IETF Standards Action.
The basic unicast 6to4 mechanism defined in [RFC 3056] and the
associated 6to4 IPv6 prefix 2002::/16 are not deprecated. The
default address selection rules specified in [RFC 6724] are not
modified.
In the absence of 6to4 anycast, "6to4 Provider Managed Tunnels"
[RFC 6732] will no longer be necessary, so they are also deprecated by
this document.
Incidental references to 6to4 should be reviewed and possibly removed
from other IETF documents if and when they are updated. These
documents include RFC 3162, RFC 3178, RFC 3790, RFC 4191, RFC 4213,
RFC 4389, RFC 4779, RFC 4852, RFC 4891, RFC 4903, RFC 5157, RFC 5245,
RFC 5375, RFC 5971, RFC 6071, and RFC 6890.
5. Implementation Recommendations
It is NOT RECOMMENDED to include the anycast 6to4 transition
mechanism in new implementations. If included in any
implementations, the anycast 6to4 mechanism MUST be disabled by
default.
In host implementations, unicast 6to4 MUST also be disabled by
default. All hosts using 6to4 MUST support the IPv6-address-
selection policy described in [RFC 6724].
In router implementations, 6to4 MUST be disabled by default. In
particular, enabling IPv6 forwarding on a device MUST NOT
automatically enable 6to4.
Troan & Carpenter Best Current Practice PAGE 5
RFC 7526 Deprecating 6to4 Anycast May 2015
6. Operational Recommendations
This document does not imply a recommendation for the generalized
filtering of traffic or routes for 6to4 or even anycast 6to4. It
simply recommends against further deployment of the anycast 6to4
mechanism, calls for current 6to4 deployments to evaluate the
efficacy of continued use of the anycast 6to4 mechanism, and makes
recommendations intended to prevent any use of 6to4 from hampering
broader deployment and use of native IPv6 on the Internet as a whole.
Networks SHOULD NOT filter out packets whose source address is
192.88.99.1, because this is normal 6to4 traffic from a 6to4 return
relay somewhere in the Internet. This includes ensuring that traffic
from a local 6to4 return relay with a source address of 192.88.99.1
is allowed through anti-spoofing filters (such as those described in
[RFC 2827] and [RFC 3704]) or through Unicast Reverse Path Forwarding
(uRPF) checks [RFC 5635].
The guidelines in Section 4 of [RFC 6343] remain valid for those who
choose to continue operating anycast 6to4 despite its deprecation.
Current operators of an anycast 6to4 relay with the IPv4 address
192.88.99.1 SHOULD review the information in [RFC 6343] and the
present document, and then consider carefully whether the anycast
relay can be discontinued as traffic diminishes. Internet service
providers that do not operate an anycast relay but do provide their
customers with a route to 192.88.99.1 SHOULD verify that it does in
fact lead to an operational anycast relay, as discussed in
Section 4.2.1 of [RFC 6343]. Furthermore, Internet service providers
and other network providers MUST NOT originate a route to
192.88.99.1, unless they actively operate and monitor an anycast 6to4
relay service as detailed in Section 4.2.1 of [RFC 6343].
Operators of a 6to4 return relay responding to the IPv6 prefix
2002::/16 SHOULD review the information in [RFC 6343] and the present
document, and then consider carefully whether the return relay can be
discontinued as traffic diminishes. To avoid confusion, note that
nothing in the design of 6to4 assumes or requires that return packets
are handled by the same relay as outbound packets. As discussed in
Section 4.5 of RFC 6343, content providers might choose to continue
operating a return relay for the benefit of their own residual 6to4
clients. Internet service providers SHOULD announce the IPv6 prefix
2002::/16 to their own customers if and only if it leads to a
correctly operating return relay as described in RFC 6343. IPv6-only
service providers, including those operating a NAT64 service
[RFC 6146], are advised that their own customers need a route to such
a relay in case a residual 6to4 user served by a different service
provider attempts to communicate with them.
Troan & Carpenter Best Current Practice PAGE 6
RFC 7526 Deprecating 6to4 Anycast May 2015
Operators of "6to4 Provider Managed Tunnels" [RFC 6732] SHOULD
carefully consider when this service can be discontinued as traffic
diminishes.
7. IANA Considerations
The document creating the "IANA IPv4 Special-Purpose Address
Registry" [RFC 6890] included the 6to4 Relay Anycast prefix
(192.88.99.0/24) as Table 10. Per this document, IANA has marked the
192.88.99.0/24 prefix (originally defined by [RFC 3068]) as
"Deprecated (6to4 Relay Anycast)" and added a reference to this RFC.
The Boolean values for the address block 192.88.99.0/24 have been
removed. Redelegation of this prefix for any use requires
justification via an IETF Standards Action [RFC 5226].
8. Security Considerations
There are no new security considerations pertaining to this document.
General security issues with tunnels are listed in [RFC 6169] and more
specifically to 6to4 in [RFC 3964] and [RFC 6324].
9. References
9.1. Normative References
[RFC 2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, DOI 10.17487/RFC 2026, October 1996,
<http://www.rfc-editor.org/info/RFC 2026>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC 2827,
May 2000, <http://www.rfc-editor.org/info/RFC 2827>.
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC 3056, February
2001, <http://www.rfc-editor.org/info/RFC 3056>.
[RFC 3068] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, DOI 10.17487/RFC 3068, June 2001,
<http://www.rfc-editor.org/info/RFC 3068>.
Troan & Carpenter Best Current Practice PAGE 7
RFC 7526 Deprecating 6to4 Anycast May 2015
[RFC 3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC 3704, March
2004, <http://www.rfc-editor.org/info/RFC 3704>.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC 5226, May 2008,
<http://www.rfc-editor.org/info/RFC 5226>.
[RFC 6146] 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/RFC 6146,
April 2011, <http://www.rfc-editor.org/info/RFC 6146>.
[RFC 6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC 6724, September 2012,
<http://www.rfc-editor.org/info/RFC 6724>.
[RFC 6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153,
RFC 6890, DOI 10.17487/RFC 6890, April 2013,
<http://www.rfc-editor.org/info/RFC 6890>.
9.2. Informative References
[HUSTON] Huston, G., "Flailing IPv6", The ISP Column, December
2010,
<http://www.potaroo.net/ispcol/2010-12/6to4fail.html>.
[RFC 3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484,
DOI 10.17487/RFC 3484, February 2003,
<http://www.rfc-editor.org/info/RFC 3484>.
[RFC 3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, DOI 10.17487/RFC 3964, December 2004,
<http://www.rfc-editor.org/info/RFC 3964>.
[RFC 5635] Kumari, W. and D. McPherson, "Remote Triggered Black Hole
Filtering with Unicast Reverse Path Forwarding (uRPF)",
RFC 5635, DOI 10.17487/RFC 5635, August 2009,
<http://www.rfc-editor.org/info/RFC 5635>.
[RFC 5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, DOI 10.17487/RFC 5969, August 2010,
<http://www.rfc-editor.org/info/RFC 5969>.
Troan & Carpenter Best Current Practice PAGE 8
RFC 7526 Deprecating 6to4 Anycast May 2015
[RFC 6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169,
DOI 10.17487/RFC 6169, April 2011,
<http://www.rfc-editor.org/info/RFC 6169>.
[RFC 6324] Nakibly, G. and F. Templin, "Routing Loop Attack Using
IPv6 Automatic Tunnels: Problem Statement and Proposed
Mitigations", RFC 6324, DOI 10.17487/RFC 6324, August 2011,
<http://www.rfc-editor.org/info/RFC 6324>.
[RFC 6343] Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
RFC 6343, DOI 10.17487/RFC 6343, August 2011,
<http://www.rfc-editor.org/info/RFC 6343>.
[RFC 6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC 6555, April
2012, <http://www.rfc-editor.org/info/RFC 6555>.
[RFC 6732] Kuarsingh, V., Ed., Lee, Y., and O. Vautrin, "6to4
Provider Managed Tunnels", RFC 6732, DOI 10.17487/RFC 6732,
September 2012, <http://www.rfc-editor.org/info/RFC 6732>.
Acknowledgements
The authors would like to acknowledge Tore Anderson, Mark Andrews,
Dmitry Anipko, Jack Bates, Cameron Byrne, Ben Campbell, Lorenzo
Colitti, Gert Doering, Nick Hilliard, Philip Homburg, Ray Hunter,
Joel Jaeggli, Victor Kuarsingh, Kurt Erik Lindqvist, Jason Livingood,
Jeroen Massar, Keith Moore, Tom Petch, Daniel Roesen, Mark Townsley,
and James Woodyatt for their contributions and discussions on this
topic.
Special thanks go to Fred Baker, David Farmer, Wes George, and Geoff
Huston for their significant contributions.
Many thanks to Gunter Van de Velde for documenting the harm caused by
non-managed tunnels and stimulating the creation of this document.
Troan & Carpenter Best Current Practice PAGE 9
RFC 7526 Deprecating 6to4 Anycast May 2015
Authors' Addresses
Ole Troan
Cisco
Oslo
Norway
EMail: ot@cisco.com
Brian Carpenter (editor)
Department of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
EMail: brian.e.carpenter@gmail.com
Troan & Carpenter Best Current Practice PAGE 10
RFC TOTAL SIZE: 20894 bytes
PUBLICATION DATE: Wednesday, May 20th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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