





Network Working Group                                           J. Abley
Request for Comments: 4116                                           ISC
Category: Informational                                     K. Lindqvist
                                                Netnod Internet Exchange
                                                               E. Davies
                                                  Independent Researcher
                                                                B. Black
                                                         Layer8 Networks
                                                                 V. Gill
                                                                     AOL
                                                               July 2005


               IPv4 Multihoming Practices and Limitations

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 (2005).

Abstract

   Multihoming is an essential component of service for many Internet
   sites.  This document describes some implementation strategies for
   multihoming with IPv4 and enumerates features for comparison with
   other multihoming proposals (particularly those related to IPv6).




















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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. IPv4 Multihoming Practices ......................................4
      3.1. Multihoming with BGP .......................................4
           3.1.1. Addressing Considerations ...........................4
           3.1.2. AS Number Considerations ............................6
      3.2. Multiple Attachments to a Single Transit Provider ..........6
      3.3. NAT- or RFC2260-based Multihoming ..........................7
   4. Features of IPv4 Multihoming ....................................7
      4.1. Redundancy .................................................7
      4.2. Load Sharing ...............................................8
      4.3. Performance ................................................8
      4.4. Policy .....................................................8
      4.5. Simplicity .................................................9
      4.6. Transport-Layer Survivability ..............................9
      4.7. Impact on DNS ..............................................9
      4.8. Packet Filtering ...........................................9
      4.9. Scalability ................................................9
      4.10. Impact on Routers ........................................10
      4.11. Impact on Hosts ..........................................10
      4.12. Interactions between Hosts and the Routing System ........10
      4.13. Operations and Management ................................10
      4.14. Cooperation between Transit Providers ....................10
   5. Security Considerations ........................................10
   6. Acknowledgements ...............................................10
   7. Informative References .........................................11























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1.  Introduction

   Multihoming is an important component of service for many Internet
   sites.  Current IPv4 multihoming practices have been added on to the
   Classless Inter Domain Routing (CIDR) architecture [RFC1519], which
   assumes that routing table entries can be aggregated based upon a
   hierarchy of customers and service providers.

   Multihoming is a mechanism by which sites can satisfy a number of
   high-level requirements.  It is widely used in the IPv4 Internet.
   There are some practical limitations, however, including concerns as
   to how it would scale with future Internet growth.  This document
   aims to document common IPv4 multihoming practices and enumerate
   their features for comparison with other multihoming approaches.

   There are a number of different ways to route and manage traffic in
   and out of a multihomed site: the majority rely on the routing policy
   capabilities of the inter-domain routing protocol, the Border Gateway
   Protocol, version 4 (BGP) [RFC1771].  This document also discusses a
   multi-homing strategy which does not rely on the capabilities of BGP.

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 [RFC1918].

   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
   and its own direct customer networks.  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
   multihomed.

   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.

   A "multi-attached" site has more than one point of layer-3
   interconnection to a single transit provider.

   Provider-Independent (PI) addresses are globally-unique addresses
   which are not assigned by a transit provider, but are provided by
   some other organisation, usually a Regional Internet Registry (RIR).



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   Provider-Aggregatable (PA) addresses are globally-unique addresses
   assigned by a transit provider to a customer.  The addresses are
   considered "aggregatable" because the set of routes corresponding to
   the PA addresses are usually covered by an aggregate route set
   corresponding to the address space operated by the transit provider,
   from which the assignment was made.

   Note that the words "assign" and "allocate" have specific meanings in
   Regional Internet Registry (RIR) address management policies, but are
   used more loosely in this document.

3.  IPv4 Multihoming Practices

3.1.  Multihoming with BGP

   The general approach for multihoming with BGP is to announce a set of
   routes to two or more transit providers.  This provides the rest of
   the Internet with multiple paths back to the multihomed sites, and
   each transit provider provides an additional possible path for the
   site's outbound traffic.

3.1.1.  Addressing Considerations

3.1.1.1.  PI Addresses

   The site uses PI addresses, and a set of routes covering those PI
   addresses is announced or propagated by two or more transit
   providers.

   Using PI addresses has long been the preferred approach for IPv4
   multihoming.  Until the mid-1990s this was relatively easy to
   accomplish, as the maximum generally accepted prefix length in the
   global routing table was a /24, and little justification was needed
   to obtain a /24 PI assignment.  Since then, RIR address management
   policies have become less liberal in this respect.  Not all RIRs
   support the assignment of address blocks to small, multihomed end-
   users, and those that do support it require justification for blocks
   as large as a /24, which cannot be met by small sites.  As a
   consequence, PI addresses are not available to many sites who wish to
   multihome.

   Each site that uses PI addresses introduces an additional prefix into
   the global routing system.  If this scheme for multihoming became
   widespread, it would present scaling concerns.







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3.1.1.2.  PA Addresses

   The site uses PA addresses assigned by a single transit provider.
   The set of routes covering those PA addresses (the "site route set")
   is announced or propagated by one or more additional transit
   providers.  The transit provider which assigned the PA addresses (the
   "primary transit provider") originates a set of routes which cover
   the site route set.  The primary transit provider often originates or
   propagates the site route set as well as the covering aggregates.

   The use of PA addresses is applicable to sites whose addressing
   requirements are not sufficient to meet the requirements for PI
   assignments by RIRs.  However, in the case where the site route set
   is to be announced or propagated by two or more different transit
   providers, common operational practice still dictates minimum /24
   prefixes, which may be larger than the allocation available to small
   sites.

   There have been well-documented examples of sites filtering long-
   prefix routes which are covered by a transit-providers aggregate.  If
   this practice were to become very widespread, it might limit the
   effectiveness of multihoming using PA addresses.  However, limited
   filtering of this kind can be tolerated because the aggregate
   announcements of the primary transit provider should be sufficient to
   attract traffic from autonomous systems which do not accept the
   covered site route set.  The more traffic that follows the primary
   transit provider's aggregate in the absence of the covered, more-
   specific route, the greater the reliance on that primary transit
   provider.  In some cases, this reliance might result in an effective
   single point of failure.

   Traffic following the primary transit provider's aggregate routes may
   still be able to reach the multihomed site, even in the case where
   the connection between the primary transit provider and the site has
   failed.  The site route set will still be propagating through the
   site's other transit providers.  If that route set reaches (and is
   accepted by) the primary transit provider, connectivity for traffic
   following the aggregate route will be preserved.

   Sites that use PA addresses are usually obliged to renumber if they
   decide not to retain connectivity to the primary transit provider.
   While this is a common requirement for all sites using PA addresses
   (and not just those that are multihomed), it is one that may have
   more frequent impact on sites whose motivation to multihome is to
   facilitate changes of ISP.  A multihomed site using PA addresses can
   still add or drop other service providers without having to renumber.





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3.1.2.  AS Number Considerations

3.1.2.1.  Consistent Origin AS

   A multihomed site may choose to announce routes to two or more
   transit providers from a globally-unique Autonomous System (AS)
   number assigned to the site.  This causes the origin of the route to
   appear consistent when viewed from all parts of the Internet.

3.1.2.2.  Inconsistent Origin AS

   A multihomed site may choose to use a private-use AS number [RFC1930]
   to originate routes to transit providers.  It is normal practice for
   private-use AS numbers to be stripped from AS_PATH attributes before
   they are allowed to propagate from transit providers towards peers.
   Therefore, routes observed from other parts of the Internet may
   appear to have inconsistent origins.

   When using private-use AS numbers, collisions between the use of
   individual numbers by different transit providers are possible.
   These collisions are arguably best avoided by not using private-use
   AS numbers for applications which involve routing across
   administrative domain boundaries.

   A multihomed site may request that their transit providers each
   originate the site's routes from the transit providers' ASes.
   Dynamic routing (for the purposes of withdrawing the site's route in
   the event that connectivity to the site is lost) is still possible,
   in this case, using the transit providers' internal routing systems
   to trigger the externally-visible announcements.

   Operational troubleshooting is facilitated by the use of a consistent
   origin AS.  This allows import policies to be based on a route's true
   origin rather than on intermediate routing details, which may change
   (e.g., as transit providers are added and dropped by the multihomed
   site).

3.2.  Multiple Attachments to a Single Transit Provider

   Multihoming can be achieved through multiple connections to a single
   transit provider.  This imposes no additional load on the global
   routing table beyond that involved in the site being single-attached.
   A site that has solved its multihoming needs in this way is commonly
   referred to as "multi-attached".







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   It is not a requirement that the multi-attached site exchange routing
   information with its transit provider using BGP.  However, in the
   event of failure, some mechanism for re-routing inbound and outbound
   traffic over remaining circuits is required.  BGP is often used for
   this purpose.

   Multi-attached sites gain no advantages from using PI addresses or
   (where BGP is used) globally-unique AS numbers, and have no need to
   be able to justify address assignments of a particular minimum size.
   However, multi-attachment does not protect a site from the failure of
   the single transit provider.

3.3.  NAT- or RFC2260-based Multihoming

   This method uses PA addresses assigned by each transit provider to
   which the site is connected.  The addresses are either allocated to
   individual hosts within the network according to [RFC2260], or the
   site uses Network Address Translation (NAT) to translate the various
   provider addresses into a single set of private-use addresses
   [RFC1918] within the site.  The site is effectively singlehomed to
   more than one transit provider.  None of the transit providers need
   to make any accommodations beyond those typically made for a non-
   multihomed customer.

   This approach accommodates a wide range of sites, from residential
   Internet users to very large enterprises, requires no PI addresses or
   AS numbers, and imposes no additional load on the Internet's global
   routing system.  However, it does not address several common
   motivations for multihoming, most notably transport-layer
   survivability.

4.  Features of IPv4 Multihoming

   The following sections describe some of the features of the
   approaches described in Section 3, in the context of the general
   goals for multihoming architectures presented in [RFC3582].  Detailed
   descriptions and rationale for these goals can be found in that
   document.

4.1.  Redundancy

   All the methods described provide redundancy, which can protect a
   site from some single-point failures.  The degree of protection
   depends on the choice of transit providers and the methods used to
   interconnect the site to those transit providers.






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4.2.  Load Sharing

   All of the methods described provide some measure of load-sharing
   capability.  Outbound traffic can be shared across ISPs using
   appropriate exit selection policies; inbound traffic can be
   distributed using appropriate export policies designed to influence
   the exit selection of remote sites sending traffic back towards the
   multihomed site.

   In the case of RFC2260/NAT multihoming, distribution of inbound
   traffic is controlled by address selection on the host or NAT.

4.3.  Performance

   BGP-speaking sites can employ import policies that cause exit
   selection to avoid paths known to be problematic.  For inbound
   traffic, sites can often employ route export policy, which affords
   different treatment of traffic towards particular address ranges
   within their network.

   It should be noted that this is not a comprehensive capability.  In
   general, there are many traffic engineering goals which can only be
   loosely approximated using this approach.

   In the case of RFC2260/NAT multihoming in the absence of BGP routing
   information, management of outbound traffic is not possible.  The
   path taken by inbound traffic for a particular session can be
   controlled by source address selection on the host or NAT.

4.4.  Policy

   In some circumstances, it is possible to route traffic of a
   particular type (e.g., protocol) via particular transit providers.
   This can be done if the devices in the site which source or sink that
   traffic can be isolated to a set of addresses to which a special
   export policy can be applied.

   An example of this capability is the grouping of budget, best-effort
   Internet customers into a particular range of addresses that is
   covered by a route which is announced preferentially over a single,
   low-quality transit path.

   In the case of RFC2260/NAT multihoming, policies such as those
   described here can be accommodated by appropriate address selection
   on the host or NAT.  More flexible implementations may be possible
   for sessions originated from the multihomed site by selecting an
   appropriate source address on a host or NAT, according to criteria
   such as transport-layer protocols and addresses (ports).



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4.5.  Simplicity

   The current methods used as multihoming solutions are not without
   their complexities, but have proven to be sufficiently simple to be
   used.  They have the advantage of familiarity due to having been
   deployed extensively.

4.6.  Transport-Layer Survivability

   All BGP-based multihoming practices provide some degree of session
   survivability for transport-layer protocols.  However, in cases where
   path convergence takes a long time following a re-homing event,
   sessions may time out.

   Transport-layer sessions will not, in general, survive over a re-
   homing event when using RFC2260/NAT multihoming.  Transport protocols
   which support multiple volatile endpoint addresses may be able to
   provide session stability; however, these transport protocols are not
   in wide use.

   In all the methods described in this document, new transport-layer
   sessions are able to be created following a re-homing event.

4.7.  Impact on DNS

   These multihoming strategies impose no new requirements on the DNS.

4.8.  Packet Filtering

   These multihoming practices do not preclude filtering of packets with
   inappropriate source or destination addresses at the administrative
   boundary of the multihomed site.

4.9.  Scalability

   Current IPv4 multihoming practices are thought to contribute to
   significant observed growth in the amount of state held in the global
   inter-provider routing system.  This is a concern because of both the
   hardware requirements it imposes and the impact on the stability of
   the routing system.  This issue is discussed in greater detail in
   [RFC3221].

   Of the methods presented in this document, RFC2260/NAT multihoming
   and multi-attaching to a single transit provider provide no
   additional state to be held in the global routing system.  All other
   strategies contribute to routing system state bloat.





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   Globally-unique AS numbers are a finite resource.  Thus, widespread
   multihoming that uses strategies requiring assignment of AS numbers
   might lead to increased resource contention.

4.10.  Impact on Routers

   For some of the multihoming approaches described in this document,
   the routers at the boundary of the multihomed site are required to
   participate in BGP sessions with transit provider routers.  Other
   routers within the site generally have no special requirements beyond
   those in singlehomed sites.

4.11.  Impact on Hosts

   There are no requirements of hosts beyond those in singlehomed sites.

4.12.  Interactions between Hosts and the Routing System

   There are no requirements for interaction between routers and hosts
   beyond those in singlehomed sites.

4.13.  Operations and Management

   There is extensive operational experience in managing IPv4-multihomed
   sites.

4.14.  Cooperation between Transit Providers

   Transit providers who are asked to announce or propagate a PA prefix
   covered by some other (primary) transit provider usually obtain
   authorisation first.  However, there is no technical requirement or
   common contractual policy which requires this coordination to take
   place.

5.  Security Considerations

   This document discusses current IPv4 multihoming practices, but
   provides no analysis of the security implications of multihoming.

6.  Acknowledgements

   Special acknowledgement goes to John Loughney for proof-reading and
   corrections.  Thanks also goes to Pekka Savola and Iljitsch van
   Beijnum for providing feedback and contributing text.

   This work was supported by the US National Science Foundation
   (research grant SCI-0427144) and DNS-OARC.




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7.  Informative References

   [RFC1519]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
              Inter-Domain Routing (CIDR): an Address Assignment and
              Aggregation Strategy", RFC 1519, September 1993.

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC2260]  Bates, T. and Y. Rekhter, "Scalable Support for Multi-
              homed Multi-provider Connectivity", RFC 2260,
              January 1998.

   [RFC3221]  Huston, G., "Commentary on Inter-Domain Routing in the
              Internet", RFC 3221, December 2001.

   [RFC3582]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
              Multihoming Architectures", RFC 3582, August 2003.

























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Authors' Addresses

   Joe Abley
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

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


   Kurt Erik Lindqvist
   Netnod Internet Exchange
   Bellmansgatan 30
   Stockholm  S-118 47
   Sweden

   Phone: +46 8 615 85 70
   EMail: kurtis@kurtis.pp.se


   Elwyn B. Davies
   Independent Researcher
   Soham, Cambridgeshire  CB7 5AW
   UK

   Phone: +44 7889 488 335
   EMail: elwynd@dial.pipex.com


   Benjamin Black
   Layer8 Networks

   EMail: ben@layer8.net


   Vijay Gill
   AOL
   12100 Sunrise Valley Dr
   Reston, VA  20191
   US

   Phone: +1 410 336 4796
   EMail: vgill@vijaygill.com






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