Network Working Group J. Abley
Request for Comments: 3582 ISC
Category: Informational B. Black
Layer8 Networks
V. Gill
AOL Time Warner
August 2003

Goals for IPv6 Site-Multihoming Architectures

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 (2003). All Rights Reserved.

Abstract

This document outlines a set of goals for proposed new IPv6 site-
multihoming architectures. It is recognised that this set of goals
is ambitious and that some goals may conflict with others. The
solution or solutions adopted may only be able to satisfy some of the
goals presented here.

1. IntrodUCtion

Site-multihoming, i.e., connecting to more than one IP service
provider, is an essential component of service for many sites which
are part of the Internet.

Current IPv4 site-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.

However, it appears that this hierarchy is being supplanted by a
dense mesh of interconnections [6]. Additionally, there has been an
enormous growth in the number of multihomed sites. For purposes of
redundancy and load-sharing, the multihomed address blocks are
introduced into the global table even if they are covered by a
provider aggregate. This contributes to the rapidly-increasing size
of both the global routing table and the turbulence exhibited within
it, and places stress on the inter-provider routing system.

Continued growth of both the Internet and the practice of site-
multihoming will seriously exacerbate this stress. The site-
multihoming architecture for IPv6 should allow the routing system to
scale more pleasantly.

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.

3. Multihoming Goals

3.1. Capabilities of IPv4 Multihoming

The following capabilities of current IPv4 multihoming practices
should be supported by an IPv6 multihoming architecture.

3.1.1. Redundancy

By multihoming, a site should be able to insulate itself from certain
failure modes within one or more transit providers, as well as
failures in the network providing interconnection among one or more
transit providers.

Infrastructural commonalities below the IP layer may result in
connectivity which is apparently diverse, sharing single points of
failure. For example, two separate DS3 circuits ordered from
different suppliers and connecting a site to independent transit
providers may share a single conduit from the street into a building;
in this case, physical disruption (sometimes referred to as
"backhoe-fade") of both circuits may be eXPerienced due to a single
incident in the street. The two circuits are said to "share fate".

The multihoming architecture should accommodate (in the general case,
issues of shared fate notwithstanding) continuity of connectivity
during the following failures:

o Physical 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.

3.1.2. Load Sharing

By multihoming, a site should be able to distribute both inbound and
outbound traffic between multiple transit providers. This goal is
for concurrent use of the multiple transit providers, not just the
usage of one provider over one interval of time and another provider
over a different interval.

3.1.3. Performance

By multihoming, a site should be able to protect itself from
performance difficulties directly between the site's transit
providers.

For example, suppose site E oBTains transit from transit providers T1
and T2, and there is long-term congestion between T1 and T2. The
multihoming architecture should allow E to ensure that in normal
operation, none of its traffic is carried over the congested
interconnection T1-T2. The process by which this is achieved should
be a manual one.

A multihomed site should be able to distribute inbound traffic from
particular multiple transit providers according to the particular
address range within their site which is sourcing or sinking the
traffic.

3.1.4. Policy

A customer may choose to multihome for a variety of policy reasons
beyond technical scope (e.g., cost, acceptable use conditions, etc.)
For example, customer C homed to ISP A may wish to shift traffic of a
certain class or application, NNTP, for example, to ISP B as matter
of policy. A new IPv6 multihoming proposal should provide support
for site-multihoming for external policy reasons.

3.1.5. Simplicity

As any proposed multihoming solution must be deployed in real
networks with real customers, simplicity is paramount. The current
multihoming solution is quite straightforward to deploy and maintain.

A new IPv6 multihoming solution should not be substantially more
complex to deploy and operate (for multihomed sites or for the rest
of the Internet) than current IPv4 multihoming practices.

3.1.6. Transport-Layer Survivability

Multihoming solutions should provide re-homing transparency for
transport-layer sessions; i.e., exchange of data between devices on
the multihomed site and devices elsewhere on the Internet may proceed
with no greater interruption than that associated with the transient
packet loss during the re-homing event.

New transport-layer sessions should be able to be created following a
re-homing event.

Transport-layer sessions include those involving transport-layer
protocols such as TCP, UDP and SCTP over IP. Applications which
communicate over raw IP and other network-layer protocols may also
enjoy re-homing transparency.

3.1.7. Impact on DNS

Multi-homing solutions either should be compatible with the observed
dynamics of the current DNS system, or the solutions should
demonstrate that the modified name resolution system required to
support them is readily deployable.

3.1.8. Packet Filtering

Multihoming solutions should not preclude filtering packets with
forged or otherwise inappropriate source IP addresses at the
administrative boundary of the multihomed site, or at the
administrative boundaries of any site in the Internet.

3.2. Additional Requirements

3.2.1. 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 [6].

A new IPv6 multihoming architecture should scale to accommodate
orders of magnitude more multihomed sites without imposing
unreasonable requirements on the routing system.

3.2.2. Impact on Routers

The solutions may require changes to IPv6 router implementations, but
these changes should be either minor, or in the form of logically
separate functions added to existing functions.

Such changes should not prevent normal single-homed operation, and
routers implementing these changes should be able to interoperate
fully with hosts and routers not implementing them.

3.2.3. Impact on Hosts

The solution should not destroy IPv6 connectivity for a legacy host
implementing RFC3513 [3], RFC2460 [4], RFC3493 [5], and other
basic IPv6 specifications current in April 2003. That is to say, if
a host can work in a single-homed site, it should still be able to
work in a multihomed site, even if it cannot benefit from site-
multihoming.

It would be compatible with this goal for such a host to lose
connectivity if a site lost connectivity to one transit provider,
despite the fact that other transit provider connections were still
operational.

If the solution requires changes to the host stack, these changes
should be either minor, or in the form of logically separate
functions added to existing functions.

If the solution requires changes to the socket API and/or the
transport layer, it should be possible to retain the original socket
API and transport protocols in parallel, even if they cannot benefit
from multihoming.

The multihoming solution may allow host or application changes if
that would enhance transport-layer survivability.

3.2.4. Interaction between Hosts and the Routing System

The solution may involve interaction between a site's hosts and its
routing system; such an interaction should be simple, scalable and
securable.

3.2.5. Operations and Management

It should be possible for staff responsible for the operation of a
site to monitor and configure the site's multihoming system.

3.2.6. Cooperation between Transit Providers

A multihoming strategy may require cooperation between a site and its
transit providers, but should not require cooperation (relating
specifically to the multihomed site) directly between the transit
providers.

The impact of any inter-site cooperation that might be required to
facilitate the multihoming solution should be examined and assessed
from the point of view of operational practicality.

3.2.7. Multiple Solutions

There may be more than one approach to multihoming, provided all
approaches are orthogonal (i.e., each approach addresses a distinct
segment or category within the site multihoming problem). Multiple
solutions will incur a greater management overhead, however, and the
adopted solutions should attempt to cover as many multihoming
scenarios and goals as possible.

4. Security Considerations

A multihomed site should not be more vulnerable to security breaches
than a traditionally IPv4-multihomed site.

Any changes to routing practices made to accommodate multihomed sites
should not cause non-multihomed sites to become more vulnerable to
security breaches.

5. Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

6. Normative References

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

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

[3] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC3513, April 2003.

[4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC2460, December 1998.

[5] Gilligan, R., Thomson, S., Bound, J., McCann, J. and W. Stevens,
"Basic Socket Interface Extensions for IPv6", RFC3493, February
2003.

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

7. Authors' Addresses

Joe Abley
Internet Software Consortium
950 Charter Street
Redwood City, CA 94063
USA

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

Benjamin Black
Layer8 Networks

EMail: ben@layer8.net

Vijay Gill
AOL Time Warner

EMail: vijaygill9@aol.com

8. Full Copyright Statement

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