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Versions: 00 01 02 RFC 3068

INTERNET DRAFT                                              C. Huitema
<draft-ietf-ngtrans-6to4anycast-02.txt>                      Microsoft
Expires August 19, 2001                              February 19, 2001

An anycast prefix for 6to4 relay routers

Status of this memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups.  Note that other groups may also distribute
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The operation of 6to4 routers requires either that the routers
participate in IPv6 inter-domain routing, or that the routers be
provisioned with a default route. This memo proposes a standard
method to define the default route. It introduces the IANA assigned
"6to4 Relay anycast prefix" from which 6to4 routers can derive the
static "6to4 anycast address".  In order to enable efficient
management of the "6to4 Relay anycast prefix" in IPv4 inter-domain
routing, this memo also documents the reservation by IANA of a "6to4
Autonomous System ID." With this definition, the proposed scheme
guarantees that 6to4 packets will be automatically routed to the
nearest available router. It allows the managers of the 6to4 relay
routers to control the sources authorized to use their resource. It
makes it easy to set up a large number of 6to4 relay routers, thus
enabling scalability.

1       Introduction

According to [RFC3056], there are two deployment options for a 6to4
routing domain, depending on whether or not the domain is using an
IPv6 exterior routing protocol. If a routing protocol is used, then
the 6to4 routers acquire routes to all existing IPv6 networks
through the combination of EGP and IGP. If no IPv6 exterior routing
protocol is used, the 6to4 routers using a given relay router each
have a default IPv6 route pointing to the relay router. This second

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INTERNET DRAFT                                 Anycast Prefix for 6to4

case is typically used by small networks; for these networks,
finding and configuring the default route is in practice a
significant hurdle. In addition, even when the managers of these
networks find an available route, this route often points to a
router on the other side of the Internet, leading to very poor

This memo introduces a "6to4 anycast address" in order to simplify
the configuration of 6to4 routers. It also defines how this address
will be used by 6to4 relay routers, how the corresponding "6to4
anycast prefix" will be advertised in the IGP and in the EGP. The
memo documents the reservation by IANA of the "6to4 relay anycast
prefix" and of a "6to4 Autonomous System ID."

2       Definitions

This memo uses the definitions introduced in [RFC3056], in
particular the definition of a 6to4 router and a 6to4 Relay Router.
It adds the definition of the 6to4 Relay anycast prefix,

2.1     6to4 router (or 6to4 border router)

An IPv6 router supporting a 6to4 pseudo-interface. It is normally
the border router between an IPv6 site and a wide-area IPv4 network.

2.2     6to4 Relay Router

A 6to4 router configured to support transit routing between 6to4
addresses and native IPv6 addresses.

2.3     6to4 Relay anycast prefix

An IPv4 address prefix used to advertise an IPv4  route to an
available 6to4 Relay Router, as defined in this memo.

The value of this prefix is x.x.x.0/nn [Length and value TBD IANA]

2.4     6to4 Relay anycast address

An IPv4 address used to reach the nearest 6to4 Relay Router, as
defined in this memo.

The address corresponds to host number 1 in the 6to4 Relay anycast
prefix, x.x.x.1. [Derived from the 6to4 Relay anycast prefix, TBD

2.5     6to4 IPv6 relay anycast address

The IPv6 address derived from the 6to4 Relay anycast address
according to the rules defined in 6to4, using a null prefix and a
null host identifier.

Huitema                                                       [Page 2]

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The value of the address is "2002:XXXX:XX01::". [Derived from the
6to4 Relay anycast address, TBD IANA]

2.6     6to4 Autonomous System ID

A 16-bit Autonomous system ID, for use in BGP in accordance to this

The value of the 6to4 Autonomous System ID is YYYY. [TBD IANA]

3       Model, requirements

Operation of 6to4 routers in domains that don't run an IPv6 EGP
requires that these routers be configured with a default route to
the IPv6 Internet. This route will be expressed as a 6to4 address.
The packets bound to this route will be encapsulated in IPv4 whose
source will be an IPv4 address associated to the 6to4 router, and
whose destination will be the IPv4 address that is extracted from
the default route. We want to arrive at a model of operation in
which the configuration is automatic.

It should also be easy to set up a large number of 6to4 relay
routers, in order to cope with the demand. The discovery of the
nearest relay router should be automatic; if a router fails, the
traffic should be automatically redirected to the nearest available
router. The managers of the 6to4 relay routers should be able to
control the sources authorized to use their resource.

4       Description of the solution

4.1     Default route in the 6to4 routers

The 6to4 routers are configured with the default IPv6 route (::/0)
pointing to the 6to4 IPv6 anycast address.

4.2     Behavior of 6to4 relay routers

The 6to4 relay routers that follow the specification of this memo
shall advertise the 6to4 anycast prefix, using the IGP of their IPv4
autonomous system, as if it where a connection to an external

The 6to4 relay routers that advertise the 6to4 anycast prefix will
receive packets bound to the 6to4 anycast address. They will relay
these packets to the IPv6 Internet, as specified in [RFC3056].

4.3     Interaction with the EGP

If the managers of an IPv4 autonomous domain that includes 6to4
relay routers want to make these routers available to neighbor ASes,
they will advertise reachability of the 6to4 anycast prefix. When
this advertisement is done using BGP, the AS path leading to the

Huitema                                                       [Page 3]

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6to4 anycast prefix shall include the identifier of the local AS and
the 6to4 Autonomous System ID.

The path to the 6to4 anycast prefix may be propagated using standard
EGP procedures. The whole v6 network will appear to v4 as a single
AS, with multiple peering points scattered over the whole Internet.

5       Discussion of the solution

The initial surfacing of the proposal in the NGTRANS working group
helped us discover a number of issues, such as scaling concerns, the
size of the address prefix, the need for an AS number, and concerns
about risking to stay too long in a transition state.

5.1     Does it scale?

With the proposed scheme, it is easy to first deploy a small number
of relay routers, which will carry the limited 6to4 traffic during
the initial phases of IPv6 deployment. The routes to these routers
will be propagated according to standard peering agreements.

As the demand for IPv6 increases, we expect that more ISPs will
deploy 6to4 relay routers. Standard IPv4 routing procedures will
direct the traffic to the nearest relay router, assuring good

5.2     Discovery and failover

The 6to4 routers send packets bound to the v6 Internet by tunneling
them to the 6to4 anycast address. These packets will reach the
closest 6to4 relay router provided by their ISP, or by the closest
ISP according to inter-domain routing.

The routes to the relay routers will be propagated according to
standard IPv4 routing rules. This ensures automatic discovery.

If a 6to4 relay router somehow breaks, or loses connectivity to the
v6 Internet, it will cease to advertise reachability of the 6to4
anycast prefix. At that point, the local IGP will automatically
compute a route towards the "next best" 6to4 relay router.

5.3     Access control

Only those ASes that run 6to4 relay routers and are willing to
provide access to the v6 network announce a path to the 6to4 anycast
prefix. They can use the existing structure of peering and transit
agreements to control to whom they are willing to provide service,
and possibly to charge for the service.

5.4     Why do we need a large prefix"

In theory, a single IP address, a.k.a. a /32 prefix, would be

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sufficient: all IGPs, and even BGP, can carry routes that are
arbitrarily specific. In practice, however, such routes are almost
guaranteed not to work.

The size of the routing table is of great concern for the managers
of Internet "default free" networks: they don't want to waste a
routing entry, which is an important resource, for the sole benefit
of a small number of Internet nodes. Many have put in place filters
that automatically drop the routes that are too specific; most of
these filters are expressed as a function of the length of the
address prefix, such as "my network will not accept advertisements
for a network that is smaller than a /19." The actual limit may vary
from network to network, and also over time. We consider that a /16,
from the old class B, would be very safe.

It could indeed be argued that using a large network is a waste of
the precious addressing resource. However, this is a waste for the
good cause of actually moving to IPv6, i.e. providing a real relief
to the address exhaustion problem.

5.5     Why do we need a specific AS number?

Erroneous advertisements are a frequent source of errors in inter-
domain routing. A misconfigured AS will advertise that it can reach
some random network, divert the traffic, and effectively cut that
network from some parts of the Internet. As a protection, many
managers of border routers use databases to check the relation
between the advertised network and the last hop in the AS path. If
we use a specific AS to denote that "this is a path to IPv6", then
we can enter the relation between that AS and the 6to4 access prefix
in the databases used to check inter-domain routing.

5.6     Will this slow down the move to IPv6?

Some have expressed a concern that, while the assignment of an
anycast address to 6to4 access routers would make life a bit easier,
it would also tend to leave things in a transition state in
perpetuity. In fact, we believe that the opposite is true.

A condition for easy migration out of the "tunnelling" state is that
it be easy to have connectivity to the "real" IPv6 network; this
means that people trust that opting for a real IPv6 address will not
somehow result in lower performances. So the anycast proposal
actually ensures that we don't stay in a perpetual transition.

6       Future Work

Using a default route to reach the IPv6 Internet has a potential
drawback: the chosen relay may not be on the most direct path to the
target v6 address. In fact, one might argue that, in the early phase
of deployment, a relay close to the 6to4 site would probably not be
the site's ISP or the native destination's ISP... it would probably

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be some third party ISP's relay which would be used for transit and
may have lousy connectivity.  Using the relay closest to the native
destination would more closely match the v4 route, and quite
possibly provide a higher degree of reliability. A potential way to
deal with this issue is to use a "redirection" procedure, by which
the 6to4 router learns the most appropriate route for a specific
destination. This is left for further study.

7       Security Considerations

The generic security risks of 6to4 tunneling and the appropriate
protections are discussed in [RFC3056]. The anycast technique
introduces an additional risk, that a rogue router or a rogue AS
would introduce a bogus route to the 6to4 anycast prefix, and thus
divert the traffic. IPv4 network managers have to guarantee the
integrity of their routing to the 6to4 anycast prefix in much the
same way that they guarantee the integrity of the generic v4

8       IANA Considerations

The purpose of this memo is to document the allocation by IANA of an
IPv4 prefix dedicated to the 6to4 gateways to the native v6
Internet, and an autonomous system number dedicated to a pseudo-AS.
This is a one time effort; there is no need for any recurring

9       Copyright

The following copyright notice is copied from RFC 2026 [Bradner,
1996], Section 10.4, and describes the applicable copyright for this

Copyright (C) The Internet Society XXX 0, 0000. All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

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This document and the information contained herein is provided on an

10      Intellectual Property

The following notice is copied from RFC 2026 [Bradner, 1996],
Section 10.4, and describes the position of the IETF concerning
intellectual property claims made against this document.

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use other technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights.  Information on the
IETF's procedures with respect to rights in standards-track and
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claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification
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The IETF invites any interested party to bring to its attention any
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this standard.  Please address the information to the IETF Executive

11      Acknowledgements

The discussion presented here was triggered by a note that Brad
Huntting sent to the NGTRANS and IPNG working groups. The note
revived previous informal discussions, for which we have to
acknowledge the members of the NGTRANS and IPNG working groups, in
particular Scott Bradner, Randy Bush, Brian Carpenter, Steve
Deering, Bob Fink, Tony Hain, Bill Manning, Keith Moore and Dave

12      References

[RFC3056] B. Carpenter, K. Moore. Connection of IPv6 Domains via
IPv4 Clouds. RFC 3056, February 2001.

13      Author's Addresses

Christian Huitema
Microsoft Corporation

Huitema                                                       [Page 7]

INTERNET DRAFT                                 Anycast Prefix for 6to4

One Microsoft Way
Redmond, WA 98052-6399

Email: huitema@exchange.microsoft.com

Huitema                                                       [Page 8]

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