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INFORMATIONAL
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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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
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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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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
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RFC 4116 IPv4 Multihoming July 2005
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Abley, et al. Informational [Page 13]
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