    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/rfc7526.

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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
       past.

       [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",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "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
       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 [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 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 [RFC3056] and the
       associated 6to4 IPv6 prefix 2002::/16 are not deprecated.  The
       default address selection rules specified in [RFC6724] are not
       modified.

       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
       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 [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
       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
       [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
       192.88.99.1 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 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 [RFC6343].  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 [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
       diminishes.

    7.  IANA Considerations

       The document creating the "IANA IPv4 Special-Purpose Address
       Registry" [RFC6890] 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 [RFC3068]) 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 [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,
                  <http://www.rfc-editor.org/info/rfc2026>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [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,
                  <http://www.rfc-editor.org/info/rfc3068>.

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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,
                  <http://www.rfc-editor.org/info/rfc5226>.

       [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,
                  <http://www.rfc-editor.org/info/rfc6724>.

       [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,
                  <http://www.rfc-editor.org/info/rfc6890>.

    9.2.  Informative References

       [HUSTON]   Huston, G., "Flailing IPv6", The ISP Column, December
                  2010,
                  <http://www.potaroo.net/ispcol/2010-12/6to4fail.html>.

       [RFC3484]  Draves, R., "Default Address Selection for Internet
                  Protocol version 6 (IPv6)", RFC 3484,
                  DOI 10.17487/RFC3484, February 2003,
                  <http://www.rfc-editor.org/info/rfc3484>.

       [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
                  6to4", RFC 3964, DOI 10.17487/RFC3964, December 2004,
                  <http://www.rfc-editor.org/info/rfc3964>.

       [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,
                  <http://www.rfc-editor.org/info/rfc5635>.

       [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
                  Infrastructures (6rd) -- Protocol Specification",
                  RFC 5969, DOI 10.17487/RFC5969, August 2010,
                  <http://www.rfc-editor.org/info/rfc5969>.

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    RFC 7526                Deprecating 6to4 Anycast                May 2015

       [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
                  Concerns with IP Tunneling", RFC 6169,
                  DOI 10.17487/RFC6169, April 2011,
                  <http://www.rfc-editor.org/info/rfc6169>.

       [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,
                  <http://www.rfc-editor.org/info/rfc6324>.

       [RFC6343]  Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
                  RFC 6343, DOI 10.17487/RFC6343, August 2011,
                  <http://www.rfc-editor.org/info/rfc6343>.

       [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>.

    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.

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    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

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