multi6 J. Abley
Internet-Draft ISC
Expires: October 2, 2003 B. Black
Layer8 Networks
V. Gill
AOL Time Warner
April 3, 2003
Goals for IPv6 Site-Multihoming Architectures
draft-ietf-multi6-multihoming-requirements-04
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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. Existing IPv4 site-multihoming practices
provide a set of capabilities that it would ideally be accommodated
by the adopted site-multihoming architecture in IPv6, and a set of
limitations that must be overcome, relating in particular to
scalability.
This document outlines a set of goals for proposed new IPv6
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site-multihoming architectures.
1. Introduction
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 [7]. Additionally, there has been an
enormous growth in the number of multihomed sites. For purposes of
redundancy and load-sharing, the multihomed address blocks, which are
almost always a longer prefix than the provider aggregate, are
announced along with the larger, covering aggregate originated by the
provider. This contributes to the rapidly-increasing size of the
global routing table. This explosion places significant 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 which 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
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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 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 requirement
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
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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 proposal should not be substantially more
complex to deploy and operate 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.
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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 are 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.
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 [7].
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 solution may require changes to IPv6 router implementations, but
these changes must 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 must 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 RFC 2373 [3], RFC 2460 [5], RFC 2553 [6] and other basic
IPv6 specifications current in November 2001. That is to say, if a
host can work in a single-homed site, it must still be able to work
in a multihomed site, even if it cannot benefit from
site-multihoming.
It would be compatible with this requirement for such a host to lose
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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 session 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, scaleable and
securable.
3.2.5 Operations and Management
It should be posssible 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.
3.2.7 Multiple Solutions
There may be more than one approach to multihoming, provided all
approaches are orthogonal (e.g. 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 as possible.
4. Security Considerations
A multihomed site should not be more vulnerable to security breaches
than a traditionally IPv4-multihomed site.
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Normative 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] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[4] Hinden, R., O'Dell, M. and S. Deering, "An IPv6 Aggregatable
Global Unicast Address Format", RFC 2374, July 1998.
[5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[6] Gilligan, R., Thomson, S., Bound, J. and W. Stevens, "Basic
Socket Interface Extensions for IPv6", RFC 2553, March 1999.
[7] Huston, G., "Analyzing the Internet's BGP Routing Table",
January 2001.
Authors' Addresses
Joe Abley
Internet Software Consortium
10805 Old River Road
Komoka, ON N0L 1R0
Canada
Phone: +1 519 641 4368
EMail: jabley@isc.org
Benjamin Black
Layer8 Networks
EMail: ben@layer8.net
Vijay Gill
AOL Time Warner
EMail: vijaygill9@aol.com
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