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RFC 3582 - Goals for IPv6 Site-Multihoming Architectures

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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.

   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".

   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

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.

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.

   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.

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.

7.  Authors' Addresses

   Joe Abley
   Internet Software Consortium
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1317
   EMail: jabley@isc.org

   Benjamin Black
   Layer8 Networks

   EMail: ben@layer8.net

   Vijay Gill
   AOL Time Warner

   EMail: vijaygill9@aol.com

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
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   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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   Internet Society.


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