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Network Working Group J. Abley Request for Comments: 3582 ISC Category: Informational B. Black

                                                       Layer8 Networks
                                                               V. Gill
                                                       AOL Time Warner
                                                           August 2003
           Goals for IPv6 Site-Multihoming Architectures

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 This document outlines a set of goals for proposed new IPv6 site-
 multihoming architectures.  It is recognised that this set of goals
 is ambitious and that some goals may conflict with others.  The
 solution or solutions adopted may only be able to satisfy some of the
 goals presented here.

1. Introduction

 Site-multihoming, i.e., connecting to more than one IP service
 provider, is an essential component of service for many sites which
 are part of the Internet.
 Current IPv4 site-multihoming practices have been added on to the
 CIDR architecture [1], which assumes that routing table entries can
 be aggregated based upon a hierarchy of customers and service
 However, it appears that this hierarchy is being supplanted by a
 dense mesh of interconnections [6].  Additionally, there has been an
 enormous growth in the number of multihomed sites.  For purposes of
 redundancy and load-sharing, the multihomed address blocks are
 introduced into the global table even if they are covered by a
 provider aggregate.  This contributes to the rapidly-increasing size
 of both the global routing table and the turbulence exhibited within
 it, and places stress on the inter-provider routing system.

Abley, et al. Informational [Page 1] RFC 3582 IPv6 Site-Multihoming Goals August 2003

 Continued growth of both the Internet and the practice of site-
 multihoming will seriously exacerbate this stress.  The site-
 multihoming architecture for IPv6 should allow the routing system to
 scale more pleasantly.

2. Terminology

 A "site" is an entity autonomously operating a network using IP, and
 in particular, determining the addressing plan and routing policy for
 that network.  This definition is intended to be equivalent to
 "enterprise" as defined in [2].
 A "transit provider" operates a site that directly provides
 connectivity to the Internet to one or more external sites.  The
 connectivity provided extends beyond the transit provider's own site.
 A transit provider's site is directly connected to the sites for
 which it provides transit.
 A "multihomed" site is one with more than one transit provider.
 "Site-multihoming" is the practice of arranging a site to be
 The term "re-homing" denotes a transition of a site between two
 states of connectedness due to a change in the connectivity between
 the site and its transit providers' sites.

3. Multihoming Goals

3.1. Capabilities of IPv4 Multihoming

 The following capabilities of current IPv4 multihoming practices
 should be supported by an IPv6 multihoming architecture.

3.1.1. Redundancy

 By multihoming, a site should be able to insulate itself from certain
 failure modes within one or more transit providers, as well as
 failures in the network providing interconnection among one or more
 transit providers.
 Infrastructural commonalities below the IP layer may result in
 connectivity which is apparently diverse, sharing single points of
 failure.  For example, two separate DS3 circuits ordered from
 different suppliers and connecting a site to independent transit
 providers may share a single conduit from the street into a building;
 in this case, physical disruption (sometimes referred to as
 "backhoe-fade") of both circuits may be experienced due to a single
 incident in the street.  The two circuits are said to "share fate".

Abley, et al. Informational [Page 2] RFC 3582 IPv6 Site-Multihoming Goals August 2003

 The multihoming architecture should accommodate (in the general case,
 issues of shared fate notwithstanding) continuity of connectivity
 during the following failures:
 o  Physical failure, such as a fiber cut, or router failure,
 o  Logical link failure, such as a misbehaving router interface,
 o  Routing protocol failure, such as a BGP peer reset,
 o  Transit provider failure, such as a backbone-wide IGP failure, and
 o  Exchange failure, such as a BGP reset on an inter-provider

3.1.2. Load Sharing

 By multihoming, a site should be able to distribute both inbound and
 outbound traffic between multiple transit providers.  This goal is
 for concurrent use of the multiple transit providers, not just the
 usage of one provider over one interval of time and another provider
 over a different interval.

3.1.3. Performance

 By multihoming, a site should be able to protect itself from
 performance difficulties directly between the site's transit
 For example, suppose site E obtains transit from transit providers T1
 and T2, and there is long-term congestion between T1 and T2.  The
 multihoming architecture should allow E to ensure that in normal
 operation, none of its traffic is carried over the congested
 interconnection T1-T2.  The process by which this is achieved should
 be a manual one.
 A multihomed site should be able to distribute inbound traffic from
 particular multiple transit providers according to the particular
 address range within their site which is sourcing or sinking the

Abley, et al. Informational [Page 3] RFC 3582 IPv6 Site-Multihoming Goals August 2003

3.1.4. Policy

 A customer may choose to multihome for a variety of policy reasons
 beyond technical scope (e.g., cost, acceptable use conditions, etc.)
 For example, customer C homed to ISP A may wish to shift traffic of a
 certain class or application, NNTP, for example, to ISP B as matter
 of policy.  A new IPv6 multihoming proposal should provide support
 for site-multihoming for external policy reasons.

3.1.5. Simplicity

 As any proposed multihoming solution must be deployed in real
 networks with real customers, simplicity is paramount.  The current
 multihoming solution is quite straightforward to deploy and maintain.
 A new IPv6 multihoming solution should not be substantially more
 complex to deploy and operate (for multihomed sites or for the rest
 of the Internet) than current IPv4 multihoming practices.

3.1.6. Transport-Layer Survivability

 Multihoming solutions should provide re-homing transparency for
 transport-layer sessions; i.e., exchange of data between devices on
 the multihomed site and devices elsewhere on the Internet may proceed
 with no greater interruption than that associated with the transient
 packet loss during the re-homing event.
 New transport-layer sessions should be able to be created following a
 re-homing event.
 Transport-layer sessions include those involving transport-layer
 protocols such as TCP, UDP and SCTP over IP.  Applications which
 communicate over raw IP and other network-layer protocols may also
 enjoy re-homing transparency.

3.1.7. Impact on DNS

 Multi-homing solutions either should be compatible with the observed
 dynamics of the current DNS system, or the solutions should
 demonstrate that the modified name resolution system required to
 support them is readily deployable.

3.1.8. Packet Filtering

 Multihoming solutions should not preclude filtering packets with
 forged or otherwise inappropriate source IP addresses at the
 administrative boundary of the multihomed site, or at the
 administrative boundaries of any site in the Internet.

Abley, et al. Informational [Page 4] RFC 3582 IPv6 Site-Multihoming Goals August 2003

3.2. Additional Requirements

3.2.1. Scalability

 Current IPV4 multihoming practices contribute to the significant
 growth currently observed in the state held in the global inter-
 provider routing system; this is a concern, both because of the
 hardware requirements it imposes, and also because of the impact on
 the stability of the routing system.  This issue is discussed in
 great detail in [6].
 A new IPv6 multihoming architecture should scale to accommodate
 orders of magnitude more multihomed sites without imposing
 unreasonable requirements on the routing system.

3.2.2. Impact on Routers

 The solutions may require changes to IPv6 router implementations, but
 these changes should be either minor, or in the form of logically
 separate functions added to existing functions.
 Such changes should not prevent normal single-homed operation, and
 routers implementing these changes should be able to interoperate
 fully with hosts and routers not implementing them.

3.2.3. Impact on Hosts

 The solution should not destroy IPv6 connectivity for a legacy host
 implementing RFC 3513 [3], RFC 2460 [4], RFC 3493 [5], and other
 basic IPv6 specifications current in April 2003.  That is to say, if
 a host can work in a single-homed site, it should still be able to
 work in a multihomed site, even if it cannot benefit from site-
 It would be compatible with this goal for such a host to lose
 connectivity if a site lost connectivity to one transit provider,
 despite the fact that other transit provider connections were still
 If the solution requires changes to the host stack, these changes
 should be either minor, or in the form of logically separate
 functions added to existing functions.
 If the solution requires changes to the socket API and/or the
 transport layer, it should be possible to retain the original socket
 API and transport protocols in parallel, even if they cannot benefit
 from multihoming.

Abley, et al. Informational [Page 5] RFC 3582 IPv6 Site-Multihoming Goals August 2003

 The multihoming solution may allow host or application changes if
 that would enhance transport-layer survivability.

3.2.4. Interaction between Hosts and the Routing System

 The solution may involve interaction between a site's hosts and its
 routing system; such an interaction should be simple, scalable and

3.2.5. Operations and Management

 It should be possible for staff responsible for the operation of a
 site to monitor and configure the site's multihoming system.

3.2.6. Cooperation between Transit Providers

 A multihoming strategy may require cooperation between a site and its
 transit providers, but should not require cooperation (relating
 specifically to the multihomed site) directly between the transit
 The impact of any inter-site cooperation that might be required to
 facilitate the multihoming solution should be examined and assessed
 from the point of view of operational practicality.

3.2.7. Multiple Solutions

 There may be more than one approach to multihoming, provided all
 approaches are orthogonal (i.e., each approach addresses a distinct
 segment or category within the site multihoming problem).  Multiple
 solutions will incur a greater management overhead, however, and the
 adopted solutions should attempt to cover as many multihoming
 scenarios and goals as possible.

4. Security Considerations

 A multihomed site should not be more vulnerable to security breaches
 than a traditionally IPv4-multihomed site.
 Any changes to routing practices made to accommodate multihomed sites
 should not cause non-multihomed sites to become more vulnerable to
 security breaches.

Abley, et al. Informational [Page 6] RFC 3582 IPv6 Site-Multihoming Goals August 2003

5. Intellectual Property Statement

 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 of the 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 implementors 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

6. Normative References

 [1] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter-
     Domain Routing (CIDR): an Address Assignment and Aggregation
     Strategy", RFC 1519, September 1993.
 [2] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
     Lear, "Address Allocation for Private Internets", BCP 5, RFC
     1918, February 1996.
 [3] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
     Addressing Architecture", RFC 3513, April 2003.
 [4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
     Specification", RFC 2460, December 1998.
 [5] Gilligan, R., Thomson, S., Bound, J., McCann, J. and W. Stevens,
     "Basic Socket Interface Extensions for IPv6", RFC 3493, February
 [6] Huston, G., "Commentary on Inter-Domain Routing in the Internet",
     RFC 3221, December 2001.

Abley, et al. Informational [Page 7] RFC 3582 IPv6 Site-Multihoming Goals August 2003

7. Authors' Addresses

 Joe Abley
 Internet Software Consortium
 950 Charter Street
 Redwood City, CA  94063
 Phone: +1 650 423 1317
 Benjamin Black
 Layer8 Networks
 Vijay Gill
 AOL Time Warner

Abley, et al. Informational [Page 8] RFC 3582 IPv6 Site-Multihoming Goals August 2003

8. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  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
 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


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

Abley, et al. Informational [Page 9]

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