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rfc:rfc3068

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 Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 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."

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 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

Huitema Standards Track [Page 1] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

 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.

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 192.88.99.0/24

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
 prefix, 192.88.99.1.

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::".

Huitema Standards Track [Page 2] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

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
 network.

Huitema Standards Track [Page 3] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

 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
 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

Huitema Standards Track [Page 4] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

 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
 performance.

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.

Huitema Standards Track [Page 5] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

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
 operation.

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.

Huitema Standards Track [Page 6] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

 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.

Huitema Standards Track [Page 7] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

 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
 standards-related documentation can be found in BCP-11.  Copies of
 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 can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

10 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.

11 References

 [RFC3056] Carpenter, B. and K. Moore "Connection of IPv6 Domains via
           IPv4 Clouds", RFC 3056, February 2001.
 [RADB]    Introducing the RADB. Merit Networks,
           http://www.radb.net/docs/intro.html.

12 Author's Address

 Christian Huitema
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052-6399
 EMail: huitema@microsoft.com

Huitema Standards Track [Page 8] RFC 3068 An Anycast Prefix for 6to4 Relay Routers June 2001

13 Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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
 Internet organizations, except as needed for the purpose of
 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
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Huitema Standards Track [Page 9]

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