Internet DRAFT - draft-ietf-ngtrans-6to4anycast


INTERNET DRAFT		                                    C. Huitema
<draft-ietf-ngtrans-6to4anycast-03.txt>                      Microsoft          
Expires September 21, 2001                              March 21, 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.

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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". 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 
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 

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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 

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, 6to4 Relay 
anycast address, 6to4 IPv6 relay anycast address, and Equivalent 
IPv4 unicast address.

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.

The value of the address is "2002:XXXX:XX01::". [Derived from the 

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6to4 Relay anycast address, TBD IANA]

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 

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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 
whole Internet.

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 

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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.

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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.

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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 

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	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 March 21, 2001. All Rights 

This document and translations of it may be copied and furnished to 
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or assist in its implementation may be prepared, copied, published 
and distributed, in whole or in part, without restriction of any 
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are included on all such copies and derivative works.  However, this 
document itself may not be modified in any way, such as by removing 
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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.

This document and the information contained herein is provided on an 

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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 
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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, Andrew 
Partan and Dave Thaler.

12	References

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

[RADB] Introducing the RADB. Merit Networks,

13	Author's Addresses

Christian Huitema

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Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399


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