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
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
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
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
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
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
This document formally deprecates the anycast 6to4 transition
mechanism defined in [RFC3068] and the associated anycast IPv4
address 126.96.36.199. It is no longer considered to be a useful
service of last resort.
The prefix 188.8.131.52/24 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
In the absence of 6to4 anycast, "6to4 Provider Managed Tunnels"
[RFC6732] will no longer be necessary, so they are also deprecated by
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.
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
184.108.40.206, 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 220.127.116.11
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
18.104.22.168 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 22.214.171.124 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
126.96.36.199, 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.
[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
[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.
Brian Carpenter (editor)
Department of Computer Science
University of Auckland