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rfc:rfc4116

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).

Abley, et al. Informational [Page 1] RFC 4116 IPv4 Multihoming July 2005

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

Abley, et al. Informational [Page 2] RFC 4116 IPv4 Multihoming July 2005

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).

Abley, et al. Informational [Page 3] RFC 4116 IPv4 Multihoming July 2005

 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.

Abley, et al. Informational [Page 4] RFC 4116 IPv4 Multihoming July 2005

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.

Abley, et al. Informational [Page 5] RFC 4116 IPv4 Multihoming July 2005

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".

Abley, et al. Informational [Page 6] RFC 4116 IPv4 Multihoming July 2005

 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.

Abley, et al. Informational [Page 7] RFC 4116 IPv4 Multihoming July 2005

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).

Abley, et al. Informational [Page 8] RFC 4116 IPv4 Multihoming July 2005

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.

Abley, et al. Informational [Page 9] RFC 4116 IPv4 Multihoming July 2005

 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.

Abley, et al. Informational [Page 10] RFC 4116 IPv4 Multihoming July 2005

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.

Abley, et al. Informational [Page 11] RFC 4116 IPv4 Multihoming July 2005

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

Abley, et al. Informational [Page 12] RFC 4116 IPv4 Multihoming July 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Abley, et al. Informational [Page 13]

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