Site Multihoming in IPv6 (multi6) J. Abley
Internet-Draft Internet Systems Consortium, Inc.
Expires: April 17, 2005 B. Black
Layer8 Networks
V. Gill
AOL
K. Lindqvist
Netnod Internet Exchange
October 17, 2004
IPv4 Multihoming Motivation, Practices and Limitations
draft-ietf-multi6-v4-multihoming-02
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Copyright (C) The Internet Society (2004).
Abstract
Multihoming is an essential component of service for sites which are
part of the Internet. This draft describes some of the motivations,
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practices and limitations of multihoming as it is achieved in the
IPv4 world today. The analysis is done in order to serve as
underlaying documentation to the discussions in the "Site multihoming
for IPv6" working group of the IETF, who are working to a longer term
solution to some of the issues that arise from doing multihoming in
the ways as are described here.
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1. Introduction
Multihoming is an important component of service for many sites which
are part of the Internet. Current IPv4 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 providers.
Multihoming is a mechanism by which sites 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.
The preferred way to multihome in IPv4 is to announce an independent
block of address space over two or more ISPs using BGP. 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 receive a /24. However,
RIR policies today do not generally support the assignment of
netblocks to small multi-homed end-users (e.g. those who might only
be able to justify a /24 assignment), and as a consequence
provider-independent (PI) space is not available to many sites who
wish to multi-home.
An alternative way to multihome in IPv4 is to get address space
allocated to the site from a upstream service provider. The site can
then subscribe to a second Internet connection from a second service
provider, and ask that the second service provider either announce or
accept the announcement over BGP of the address block allocated to
the site from the first service provider.
This second practice has two advantages and two disadvantages for the
multihomed site, as well for the Internet at large. The first
advantage is that this practice is applicable to sites whose
addressing requirements are not sufficient to meet the requirements
for PI assignments by RIRs. The second advantage is that even if the
more specific announcement is filtered, they are still reachable
over the primary ISP by virtue of the aggregate announced by this
ISP. Even when the circuit to the primary ISP is down, this often
works because the primary ISP will generally accept the announcement
over the secondary ISP, so traffic flows from the filtering network
to the primary ISP and then to the secondary ISP in order to arrive
at the multihomed network. While this is common, it is also not
universally true.
The first disadvantage with this approach is that the multihomed
network must depend on the primary ISP for the aggregate. If the
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primary ISP goes down, this will impact reachability to networks that
filter. Most ISPs will cooperate with this "punching holes in an
aggregate" solution to multihoming, but some are reluctant. And when
the multihomed network leaves the primary ISP, they are generally
expected to return the address space because otherwise this ISP would
have to route traffic for a non-customer. The second disadvantage
with the approach of announcing more specific routes out of a PA
block, is that if the site were to change service provider away from
the provider that has allocated the PA block, they will have to
renumber. It is worth noting that the site can change all the other
service providers without having to renumber.
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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
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 is one with more than one point of layer-3
interconnection to a single transit provider.
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3. Motivations for Multihoming
There exists a great wealth of reasons why any single person or
entity would like to multihome their network, in one way or the
other. The reasons for doing this, are to achieve one or more of the
goals as outlined in [5]. A more detailed analysis of the reasoning
behind multihoming configurations are considered out-of-scope for
this document.
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4. Current methods used for IPv4 multihoming
There are a number of ways that a site which wishes to become
multihomed can achieve its objectives today using IPv4. These
methods can broadly be split into five categories as described below.
4.1 Multihoming with PI addresses and AS
The site uses provider-independent (PI) addresses assigned by an RIR.
The routes corresponding to the PI addresses are announced with an
origin AS associated with the multi-homed site to two or more transit
providers.
4.2 Multihoming with your own AS, but PA addresses
The site uses provider-aggregatable (PA) addresses assigned by one
transit provider. The routes corresponding to those PA addresses are
announced with an origin AS associated with the multi-homed site to
two or more transit providers. One of those transit providers
originates a covering supernet for the site's routes.
4.3 Multihoming with your own addresses, and private AS
Another possible way of multihoming is with addresses owned by the
site wishing to multihome, but advertising them without having a
public AS, or with no AS at all. This is done with the site either
sourcing the prefixes in a private AS [3], and having their upstreams
remove those on announcement to the rest of the world, or the
upstreams simply sourcing the prefixes in their AS and then routing
to the organization.
4.4 Multiple attachments to the same ISP
Fourth option is to have multiple connection to the same ISP. This
is fairly popular, but will not have an impact on the global routing
table as both paths are covered by the ISPs aggregate route. A site
that have solved their multihoming needs in this way is commonly
referred to as "multi-attached".
4.5 NAT or RFC2260 based multihoming
This last method might very well be the most commonly used method in
terms of volume. Simply because this is what most residential users
are normally referred to. However, this method is also in use by
very large enterprises, as this is an easy way for large enterprise
networks to avoid advertising multiple prefixes when they connect at
multiple locations.
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This method uses addresses from each of the upstream that an
organization is connected to. Either the addresses are allocated to
nodes inside the network according to the proposal in [4], or the
site uses NAT to translate into private addresses inside the site.
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5. Features of IPv4 Multihoming
The following section analyzes some of the features driving the
choices for various multihoming approaches in todays IPv4 Internet.
As the "Site multihoming for IPv6" working group progresses, they
will have to take similar considerations into account, learning from
IPv4. These considerations are listed in [5], and some of the
operational considerations that needs to be thought of for new
multihoming mechanisms can be found in [6]. Not all approaches
described in this document will support all the features listed
below. Particularly solutions based on RFC2260/NAT [4] based designs
will support a significantly smaller subset of these features.
5.1 Simplicity
The current methods used as multihoming solutions are not all without
complexity, but in practice it is quite straightforward to deploy and
maintain by virtue of the fact that they are well-known, tried and
tested.
5.2 Transport-Layer Survivability
The current multihoming solution provides session survivability for
transport-layer protocols in most cases; i.e. exchange of data
between devices on the multi-homed site 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.
However, there are cases where the current multihoming solutions does
not provide transport-layer survivability. One example is that due
to the large number of ASes in the current Internet routing table,
BGP convergence take longer and longer and the transport-layer might
time-out while waiting for BGP to converge. There are also BGP
implementations in wide use that will take a long time for failure
detection and that might cause TCP timeouts.
In the case a re-homing event occurs, new transport-layer sessions
are able to be created.
5.3 Inter-Provider Traffic Engineering
A multi-homed site 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.
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5.4 Load Control
The current multihoming solution places control of traffic flow in
the hands of the site responsible for the multi-homed
interconnections with transit providers. A single-homed client a
multi-homed site may vary the demand for traffic that they impose on
the site, 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 site, not the client.
5.5 Impact on Routers
The routers at the boundary of a multi-homed site are usually
required to participate in BGP sessions with the interconnected
routers of transit providers. Other routers within the site have no
special requirements beyond those of single-homed site routers.
5.6 Impact on Hosts
There are no requirements of hosts beyond those of single-homed sites
hosts.
5.7 Interactions between Hosts and the Routing System
There are no requirements for interaction between routers and hosts
beyond those of single-homed site routers and hosts.
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6. Limitations of IPv4 Multihoming
6.1 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.
The only method described in this document that doesn't add to the
growth of state in the global inter-provider routing system is the
RFC2260/NAT based method. The use of this method might explains the
relatively higher growth in multihomed sites, compared to the growth
of the state in the global inter-provider routing system.
These mechanisms also add to the consumption of public AS number
resources, when small site wishing to multihome obtain an AS number
specifically for only that purpose. Using a different mechanism
would help to conserve the 16-bit AS number space, and avoid the move
to 32-bit AS numbers.
This issue is discussed in great detail in [7].
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7. Security Considerations
This memo analyzes the IPv4 multihoming practices. This analysis
only includes the description of the mechanisms and partially how
they affect the availability of the site deploying the IPv4
multihoming mechanism. Other security properties of the IPv4
multihoming mechanisms are not analyzed.
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8. IANA Considerations
This document requests no action by IANA.
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9. Acknowledgements
Thanks goes to Pekka Savola and Iljitsch van Beijnum for providing
feedback and suggestions on the text as well as text.
10 Informative 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", RFC 1918,
February 1996.
[3] Hawkinson, J. and T. Bates, "Guidelines for creation, selection,
and registration of an Autonomous System (AS)", RFC 1930, March
1996.
[4] Bates, T. and Y. Rekhter, "Scalable Support for Multi-homed
Multi-provider Connectivity", RFC 2260, January 1998.
[5] Black, B., Gill, V. and J. Abley, "Goals for IP Multihoming
Architectures", RFC 3582, August 2003.
[6] Lear, E., "Things MULTI6 Developers should think about",
Internet-Drafts draft-ietf-multi6-things-to-think-about-00, June
2004.
[7] Huston, G., "Analyzing the Internet's BGP Routing Table",
January 2001.
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
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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
Kurt Erik Lindqvist
Netnod Internet Exchange
Bellmansgatan 30
Stockholm S-118 47
Sweden
Phone: +46 8 615 85 70
EMail: kurtis@kurtis.pp.se
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