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.
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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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