|Title||An Anycast Prefix for 6to4 Relay Routers
Network Working Group C. Huitema
Request for Comments: 3068 Microsoft
Category: Standards Track June 2001
An Anycast Prefix for 6to4 Relay Routers
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2001). All Rights Reserved.
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 (Internet Assigned Numbers
Authority) of the "6to4 relay anycast prefix."
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 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 performance.
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 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.
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, 6to4 Relay anycast
address, 6to4 IPv6 relay anycast address, and Equivalent IPv4 unicast
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 126.96.36.199/24
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
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.
The value of the address is "2002:c058:6301::".
2.6 Equivalent IPv4 unicast address
A regular IPv4 address associated with a specific 6to4 Relay Router.
Packets sent to that address are treated by the 6to4 Relay Router as
if they had been sent to the 6to4 Relay anycast address.
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.
Anycast routing is known to cause operational issues: since the
sending 6to4 router does not directly identify the specific 6to4
relay router to which it forwards the packets, it is hard to identify
the responsible router in case of failure, in particular when the
failure is transient or intermittent. Anycast solutions must thus
include adequate monitoring of the routers performing the service, in
order to promptly detect and correct failures, and also adequate
fault isolation procedures, in order to find out the responsible
element when needed, e.g., following a user's complaint.
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].
Each 6to4 relay router that advertise the 6to4 anycast prefix MUST
also provide an equivalent IPv4 unicast address. Packets sent to
that unicast address will follow the same processing path as packets
sent to the anycast address, i.e., be relayed to the IPv6 Internet.
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 initial AS path must contain the
AS number of the announcing AS. The AS path should also include an
indication of the actual router providing the service; there is a
suggestion to perform this function by documenting the router's
equivalent IPv4 address in the BGP aggregator attribute of the path;
further work is needed on this point.
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
multi-homed network, with multiple access points scattered over the
4.4 Monitoring of the 6to4 relay routers
Any 6to4 relay router corresponding to this specification must
include a monitoring function, to check that the 6to4 relay function
is operational. The router must stop injecting the route leading to
the 6to4 anycast prefix immediately if it detects that the relay
function is not operational.
The equivalent IPv4 address may be used to check remotely that a
specific router is operational, e.g., by tunneling a test IPv6 packet
through the router's equivalent unicast IPv4 address. When a domain
deploys several 6to4 relay routers, it is possible to build a
centralized monitoring function by using the list of equivalent IPv4
addresses of these routers.
4.5 Fault isolation
When an error is reported, e.g., by a user, the domain manager should
be able to find the specific 6to4 relay router that is causing the
problem. The first step of fault isolation is to retrieve the
equivalent unicast IPv4 address of the router used by the user. If
the router is located within the domain, this information will have
to be retrieved from the IGP tables. If the service is obtained
through a peering agreement with another domain, the information will
be retrieved from the EGP data, e.g., the BGP path attributes.
The second step is obviously to perform connectivity tests using the
equivalent unicast IPv4 address.
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. We expect
that adequate monitoring tools will be used to guarantee timely
discovery of connectivity losses.
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
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 /24." The actual limit may vary from
network to network, and also over time.
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 Do we need a specific AS number?
A first version of this memo suggested the use of a specific AS
number to designate a virtual AS containing all the 6to4 relay
routers. The rationale was to facilitate the registration of the
access point in databases such as the RADB routing registry [RADB].
Further analysis has shown that this was not required for practical
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 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.
The practical operation of the 6to4 relay routers requires the
development of monitoring and testing tools, and the elaboration of
gradual management practices. While this document provides general
guidelines for the design of tools and practice, we expect that the
actual deployment will be guided by operational experience.
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 routing.
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;
there is no need for any recurring assignment.
9. 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
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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
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, Andrew Partan and
[RFC3056] Carpenter, B. and K. Moore "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[RADB] Introducing the RADB. Merit Networks,
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