1. RFC 7526
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


   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

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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

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RFC 7526                Deprecating 6to4 Anycast                May 2015

1.  Introduction

   The original form of the 6to4 transition mechanism [RFC3056] relies
   on unicast addressing.  However, its extension specified in "An
   Anycast Prefix for 6to4 Relay Routers" [RFC3068] 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 [RFC2026].  It complements the
   deployment guidelines in [RFC6343].

   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

   [RFC6343] 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 [RFC6555].

   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
   [RFC3484] (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" [RFC5969] 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" [RFC6732] is
   intended to help a service provider manage 6to4 anycast tunnels.
   This framework only exists because of the problems observed with
   anycast 6to4.

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RFC 7526                Deprecating 6to4 Anycast                May 2015

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

   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

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

   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 [RFC6343].  This document is intended to deprecate the
   anycast mechanism.

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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 [RFC3068] and the associated anycast IPv4
   address  It is no longer considered to be a useful
   service of last resort.

   The prefix MUST NOT be reassigned for other use except
   by a future IETF Standards Action.

   The basic unicast 6to4 mechanism defined in [RFC3056] and the
   associated 6to4 IPv6 prefix 2002::/16 are not deprecated.  The
   default address selection rules specified in [RFC6724] are not

   In the absence of 6to4 anycast, "6to4 Provider Managed Tunnels"
   [RFC6732] 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

   In host implementations, unicast 6to4 MUST also be disabled by
   default.  All hosts using 6to4 MUST support the IPv6-address-
   selection policy described in [RFC6724].

   In router implementations, 6to4 MUST be disabled by default.  In
   particular, enabling IPv6 forwarding on a device MUST NOT
   automatically enable 6to4.

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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, 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
   is allowed through anti-spoofing filters (such as those described in
   [RFC2827] and [RFC3704]) or through Unicast Reverse Path Forwarding
   (uRPF) checks [RFC5635].

   The guidelines in Section 4 of [RFC6343] 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 SHOULD review the information in [RFC6343] 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 SHOULD verify that it does in
   fact lead to an operational anycast relay, as discussed in
   Section 4.2.1 of [RFC6343].  Furthermore, Internet service providers
   and other network providers MUST NOT originate a route to, unless they actively operate and monitor an anycast 6to4
   relay service as detailed in Section 4.2.1 of [RFC6343].

   Operators of a 6to4 return relay responding to the IPv6 prefix
   2002::/16 SHOULD review the information in [RFC6343] 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
   [RFC6146], 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.

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RFC 7526                Deprecating 6to4 Anycast                May 2015

   Operators of "6to4 Provider Managed Tunnels" [RFC6732] SHOULD
   carefully consider when this service can be discontinued as traffic

7.  IANA Considerations

   The document creating the "IANA IPv4 Special-Purpose Address
   Registry" [RFC6890] included the 6to4 Relay Anycast prefix
   ( as Table 10.  Per this document, IANA has marked the prefix (originally defined by [RFC3068]) as
   "Deprecated (6to4 Relay Anycast)" and added a reference to this RFC.
   The Boolean values for the address block have been
   removed.  Redelegation of this prefix for any use requires
   justification via an IETF Standards Action [RFC5226].

8.  Security Considerations

   There are no new security considerations pertaining to this document.
   General security issues with tunnels are listed in [RFC6169] and more
   specifically to 6to4 in [RFC3964] and [RFC6324].

9.  References

9.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2827]  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/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February
              2001, <http://www.rfc-editor.org/info/rfc3056>.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, DOI 10.17487/RFC3068, June 2001,

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RFC 7526                Deprecating 6to4 Anycast                May 2015

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <http://www.rfc-editor.org/info/rfc3704>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,

9.2.  Informative References

   [HUSTON]   Huston, G., "Flailing IPv6", The ISP Column, December

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484,
              DOI 10.17487/RFC3484, February 2003,

   [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
              6to4", RFC 3964, DOI 10.17487/RFC3964, December 2004,

   [RFC5635]  Kumari, W. and D. McPherson, "Remote Triggered Black Hole
              Filtering with Unicast Reverse Path Forwarding (uRPF)",
              RFC 5635, DOI 10.17487/RFC5635, August 2009,

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification",
              RFC 5969, DOI 10.17487/RFC5969, August 2010,

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   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169,
              DOI 10.17487/RFC6169, April 2011,

   [RFC6324]  Nakibly, G. and F. Templin, "Routing Loop Attack Using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations", RFC 6324, DOI 10.17487/RFC6324, August 2011,

   [RFC6343]  Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
              RFC 6343, DOI 10.17487/RFC6343, August 2011,

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

   [RFC6732]  Kuarsingh, V., Ed., Lee, Y., and O. Vautrin, "6to4
              Provider Managed Tunnels", RFC 6732, DOI 10.17487/RFC6732,
              September 2012, <http://www.rfc-editor.org/info/rfc6732>.


   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

   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.

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RFC 7526                Deprecating 6to4 Anycast                May 2015

Authors' Addresses

   Ole Troan

   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

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  1. RFC 7526