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Versions: 00 01 02 03 rfc4116                                           
Network Working Group                                           J. Abley
Internet-Draft                             Metromedia Fiber Network Inc.
Expires: December 24, 2001                                      B. Black
                                                         Layer8 Networks
                                                                 V. Gill
                                           Metromedia Fiber Network Inc.
                                                           June 25, 2001


         IPv4 Multihoming Motivation, Practices and Limitations
                  draft-ietf-multi6-v4-multihoming-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 24, 2001.

Copyright Notice

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

Abstract

   Multihoming is an essential component of service for enterprises [3]
   which are part of the Internet.  This draft describes some of the
   motivations, practices and limitations of multihoming as it is
   achieved in the IPv4 world today.

   The context for this discussion is the requirements analysis for site
   multihoming in IPv6, which is described in a companion draft [1].




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1. Introduction

   Multihoming is an essential component of service for enterprises
   which are part of the Internet.  Current IPv4 multihoming practices
   have been added on to the CIDR architecture [2], which assumes that
   routing table entries can be aggregated based upon a hierarchy of
   customers and service providers.

   Multihoming is a mechanism by which enterprises can currently satisfy
   a number of high-level requirements, and is widely used in the IPv4
   network today.  There are some practical limitations, however,
   including concerns of how well (or, if) the current practice will
   scale as the network continues to grow.






































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2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].

   An "enterprise" is an entity autonomously operating a network using
   TCP/IP and, in particular, determining the addressing plan and
   address assignments within that network.  This is the definition of
   "enterprise" used in [3].

   A "transit provider" is an enterprise which provides connectivity to
   the Internet to one or more other enterprises.  The connectivity
   provided extends beyond the transit provider's own network.

   A "multi-homed" enterprise is one with more than one transit
   provider.  "Multihoming" is the practice of being multi-homed.

   A "multi-attached" enterprise is one with more than one point of
   layer-3 interconnection to a single transit provider.

   The term "re-homing" denotes a transition of an enterprise between
   two states of connectedness, due to a change in the connectivity
   between the enterprise and its transit providers.



























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3. Motivations for Multihoming

3.1 Redundancy

   By multihoming, an enterprise can insulate itself from certain
   failure modes within one or more transit providers, as well as
   failures in the network providing interconnection with one or more
   transit providers.

   Examples of failure modes from which an enterprise can obtain some
   degree of protection by multi-homing are:

   o  Physical link 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
      peering.

   Some of these failure modes may be protected against by multi-
   attaching to a single transit provider, rather than multi-homing.

3.2 Load Sharing

   By multihoming, an enterprise can distribute both inbound and
   outbound traffic between multiple transit providers.

   Sometimes it is not possible to increase transit capacity to a single
   transit provider because that provider does not have sufficient spare
   capacity to sell.  In this case a solution is to acquire additional
   transit capacity through a different provider.  This scenario is
   common in bandwidth-starved stubs of the Internet where, for example,
   transit demand outpaces under-sea cable deployment.

3.3 Performance

   By multihoming, an enterprise can protect itself from performance
   difficulties between transit providers.

   For example, suppose enterprise E obtains transit from transit
   providers T1 and T2, and there is long-term congestion between T1 and
   T2.  By multihoming between T1 and T2, E is able to ensure that in
   normal operation none of its traffic is carried over the congested
   interconnection T1-T2.



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3.4 Policy

   An enterprise may choose to load-share for a variety of policy
   reasons outside technical scope (e.g.  cost, acceptable use
   conditions, etc).

   For example, enterprise E homed to transit provider T1 may be able to
   identify a particular range of addresses within its network that
   correspond to non-real-time traffic (e.g.  a network containing mail
   and Usenet/NNTP servers).  It may be advantageous to shift inbound
   traffic destined for that range of addresses to transit-provider T2,
   since T2 charges less for traffic.







































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4. Features of IPv4 Multihoming

4.1 Simplicity

   The current multihoming solution is not without complexity, but in
   practice it quite straightforward to deploy and maintain by virtue of
   the fact that it is well-known, tried and tested.

4.2 Transport-Layer Survivability

   The current multihoming solution provides session survivability for
   transport-layer protocols; i.e.  exchange of data between devices on
   the multi-homed enterprise network and devices elsewhere on the
   Internet may proceed with no greater interruption than that
   associated with the transient packet loss during a re-homing event.

   New transport-layer sessions are able to be created following a re-
   homing event.

4.3 Inter-Provider Traffic Engineering

   A multi-homed enterprise may influence routing decisions beyond its
   immediate transit providers by advertising a strategic mixture of
   carefully-aimed long prefixes and covering shorter-prefix routes.
   This precise effects of such egress policy are often difficult to
   predict, but an approximation of the desired objective is often easy
   to accomplish.  This can provide a similar mechanisms to that
   described in Section 3.3, except that the networks whose traffic is
   being influenced are not transit providers of the enterprise itself.

4.4 Load Control

   The current multihoming solution places control of traffic flow in
   the hands of the organisation responsible for the multi-homed
   interconnections with transit providers.  A single-homed customer of
   a multi-homed enterprise may vary the demand for traffic that they
   impose on the enterprise, and may influence differential traffic load
   between transit providers; however, the basic mechanisms for
   congestion control and route propagation are in the hands of the
   enterprise, not the customer.

4.5 Impact on Routers

   The routers at the boundary of a multi-homed enterprise are usually
   required to participate in BGP sessions with the interconnected
   routers of transit providers.  Other routers within the enterprise
   have no special requirements beyond those of single-homed
   enterprises' routers.



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4.6 Impact on Hosts

   There are no requirements of hosts beyond those of single-homed
   enterprises' hosts.

4.7 Interactions between Hosts and the Routing System

   There are no requirements for interaction between routers and hosts
   beyond those of single-homed enterprises' routers and hosts.










































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5. Limitations of IPv4 Multihoming

5.1 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 [5].









































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6. Security Considerations

   Security considerations are not discussed in this draft.
















































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References

   [1]  Black, B., Gill, V. and J. Abley, "Requirements for IP
        Multihoming Architectures (work-in-progress)", I-D draft-ietf-
        multi6-v4-multihoming-01, June 2001,
        <http://www.automagic.org/~jabley/draft-ietf-multi6-multihoming-
        requirements-01.txt>.

   [2]  Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter-
        Domain Routing (CIDR): an Address Assignment and Aggregation
        Strategy", RFC 1519, September 1993.

   [3]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E.
        Lear, "Address Allocation for Private Internets", RFC 1918,
        February 1996.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.

   [5]  Huston, G., "Analyzing the Internet's BGP Routing Table",
        January 2001.


Authors' Addresses

   Joe Abley
   Metromedia Fiber Network Inc.
   2204 Pembroke Court
   Burlington, ON  L7P 3X8
   Canada

   Phone: +1 905 319 9064
   EMail: jabley@mfnx.net


   Benjamin Black
   Layer8 Networks

   EMail: ben@layer8.net












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   Vijay Gill
   Metromedia Fiber Network Inc.
   8075 Leesburg Pike
   Suite 300
   Vienna, VA  22182
   US

   Phone: +1 410 262 0660
   EMail: vgill@mfnx.net










































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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

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



















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