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RFC3068 - An Anycast Prefix for 6to4 Relay Routers

王朝other·作者佚名  2008-05-31
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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 RFC2026 [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", RFC3056, 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

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 RFCEditor function is currently provided by the

Internet Society.

 
 
 
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