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RFC 3068 - An Anycast Prefix for 6to4 Relay Routers


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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

   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::".

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.

   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

   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.

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.

   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
   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

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
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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Acknowledgement

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

 

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