Network Working Group J. Arkko
Internet-Draft Ericsson
Intended status: Standards Track February 13, 2010
Expires: August 17, 2010
Guidelines for Using IPv6 Transition Mechanisms
draft-arkko-ipv6-transition-guidelines-00
Abstract
IPv6 is essential as the Internet continues to grow beyond its
originally envisioned design limits. Yet, IPv6 deployment requires
some effort, resources, and expertise. The availability of many
different deployment models is one reason why expertise is required.
This document discusses the IPv6 deployment models and migration
tools, and recommends ones that have been found to work well in many
common situations.
Status of this Memo
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This Internet-Draft will expire on August 17, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Principles . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Choosing a Deployment Model . . . . . . . . . . . . . . . 5
4. Guidelines for IPv6 Deployment . . . . . . . . . . . . . . . . 6
4.1. Native Dual Stack . . . . . . . . . . . . . . . . . . . . 7
4.2. Crossing IPv4 Islands . . . . . . . . . . . . . . . . . . 8
4.3. IPv6-Only Core Network . . . . . . . . . . . . . . . . . . 9
4.4. Unilateral Deployment . . . . . . . . . . . . . . . . . . 9
5. Further Reading . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
IPv6 is essential as the Internet continues to grow beyond its
originally envisioned design limits. The tremendous success of the
Internet has strained the IPv4 address space, which is no longer
sufficient to fuel future growth. At the time of writing this, the
IANA "free pool" contained only 24 unallocated unicast IPv4 /8
prefixes. This is sufficient only for the next 2-3 years at best.
Accordingly, many organizations have employed or are planning to
employ IPv6 in their networks. Yet, IPv6 deployment requires some
effort, resources, and expertise. This is largely a natural part of
maintaining and evolving a network: new requirements simply have to
be taken into account in the normal planning, procurement and update
cycles. Very large networks have successfully adopted IPv6 alongside
IPv4, with surprisingly little effort.
However, in order to successfully make this transition, some amount
of new expertise is required. Different types of experience will be
required: basic understanding of IPv6 mechanisms, debugging tools,
product capabilities and caveats when used with IPv6, and so on. The
availability of many different IPv6 deployment models and tools is an
additional reason why expertise is required. These models and tools
have been developed over the years at the IETF, some for specific
circumstances and others for more general use. They differ greatly
in their principles of operation. Over time, views about the best
ways to employ the tools have evolved. Given the large number of
possible options and ongoing development for more, network managers
are understandably confused about what the recommended approach for
IPv6 deployment is.
As a result, it is useful to provide guidance about the applicability
of various deployment models and migration tools. This document
discusses these models and tools, and recommends those that have been
found to work well in many common situations.
The rest of this document is organized as follows. Section 2
introduces some terminology, Section 3 discusses some of the general
principles behind choosing particular deployment models and tools,
and Section 4 goes through the recommended deployment models for
common situations
2. Terminology
"NAT44" in this document means an IPv4-to-IPv4 address translator,
both "Basic NAT" and "Network Address/Port Translator (NAPT)" as
defined by [RFC2663].
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3. Principles
The goals, constraints, and opportunities for IPv6 deployment differ
from one case to another. There is no single right model for IPv6
deployment, just like there is no one and only model IPv4 network
design. Some guidelines can be given, however. Common deployment
models that have been found to work well are discussed in Section 4,
and the small set of standardized IETF migration tools support these
models. But first it may be useful to discuss some general
principles that guide our thinking about what is a good deployment
model.
3.1. Goals
The primary goal is to enable IPv6 for communications. For the vast
majority of networks there are two secondary goals, enabling co-
existence between IPv4 and IPv6, and to allow the IPv4 network
continue operate under address scarcity. However, we should not make
it desirable to stay on IPv4 indefinitely. As a result, good
deployment models both reduce pain from IPv4 address shortage and
have incentives for starting to move communications to IPv6.
It is important to start the deployment process in a timely manner.
Most of the effort is practical -- network component choices, network
management, planning, implementation -- and at the time of writing
this, reasonably easily achievable. There is no particular advantage
to avoiding dealing with IPv6 as a part the normal network planning
cycle. The primary migration tools already exist, and while
additional features continue to be developed it is not expected that
they radically change what networks have to do. In other words,
there is no point in waiting for an improved design.
There are only a few exceptional networks where co-existence with
IPv4 is not a consideration at all. These networks are typically new
deployments, strictly controlled by a central authority, and have no
need to deal with legacy devices. For instance, specialized sensor
networks that communicate only to designated servers can easily be
deployed as IPv6-only networks. In most other networks IPv4 has to
be considered. A typical requirement is that older, IPv4-only
devices must be accommodated. Most networks that cross
administrative boundaries or allow end user equipment have such
requirements. Even in situations where the network consists of only
new, IPv6-capable devices it is typically required that the devices
can communicate with the IPv4 Internet.
It is expected that after a period of supporting both IPv4 and IPv6,
IPv4 can eventually be turned off. This happens gradually. For
instance, a service provider network might stop providing IPv4
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service within its own network, while still allowing its IPv6
customers to access the rest of the IPv4 Internet.
3.2. Choosing a Deployment Model
The first requirement is that the model or tool actually allows
communications to flow. While this sounds too obvious to even state,
it is sometimes not easy to ensure that a proposed model does not
have failure modes related to supporting older devices, for instance.
A network that is not serving all of its users is not fulfilling its
task.
The ability to communicate is also far more important than fine-
grained performance differences. In general, it is not productive to
focus on optimization of a design that is designed to be temporary,
such as a migration solution necessarily is. Consequently, existing
tools are often preferred over new ones, even if for some specific
circumstance it would be possible to construct a slightly more
efficient design.
Similarly, migration tools that can be disposed after a period of co-
existence are preferred over tools that require more permanent
changes. Such permanent changes may incur costs even after the
transition to IPv6 has been completed.
Looking back on the deployment of Internet technology, some of the
factors that have been important for success include
[RFC5218, fred.shanghai]
o The ability to offer a valuable service. In the case of the
Internet, connectivity has been this service.
o The ability to deploy the solution in an incremental fashion.
o Simplicity. This has been a key factor in making it possible for
all types of devices to support the Internet protocols.
o Openly available implementations. These make it easier for
researchers, start-ups and others to build on or improve existing
components.
o The ability to scale. The IPv4 Internet grew far larger than its
original designers had anticipated, and scaling limits only became
apparent 20-30 years later.
o The design supports robust interoperability rather than mere
correctness. This is important in order to ensure that the
solution works in different circumstances and in an imperfectly
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controlled world.
These factors are also important when choosing IPv6 migration tools.
It is also essential that any chosen designs allow the network to be
maintained, serviced, diagnosed and measured. The ability of the
network to operate under many different circumstances and surprising
conditions is a key. Any large network that employs brittle
components will incur significant support costs. For instance, it is
generally a bad assumption that large number of devices have specific
software or configuration.
Properly executed IPv6 deployment normally involves a step-wise
approach where individual functions or parts of the network are
updated at different times. For instance, IPv6 connectivity has to
be established and tested before DNS entries with IPv6 addresses can
be entered. Or specific services can be moved to support IPv6
earlier than others. In general, most deployment models employ a
very similar network architecture for both IPv4 and IPv6. The
principle of changing only the minimum amount necessary is applied
here.
4. Guidelines for IPv6 Deployment
This section presents a number of common scenarios along with
recommended deployment tools for them. We start from the default,
most simple deployment situation where native connectivity is
available and both IP versions are used. Since native IPv6
connectivity not available in all networks, our second scenario looks
at ways of arranging such connectivity over the IPv4 Internet. The
third scenario is more advanced and looks at a service provider
network that runs only on IPv6 but which is still capable of
providing both IPv6 and IPv4 services. The fourth and most advanced
scenario focuses unilateral deployment of IPv6, i.e., being able to
employ IPv6 for a particular communication flow even if the other end
is still on IPv4.
Note that there are many other possible deployment models and
existing specifications to support such models. These other models
are by no means frowned upon. However, they are not expected to be
the mainstream deployment models, and consequently, the associated
specifications are typically not IETF standards track RFCs. Network
managers should not adopt these non-mainstream models lightly,
however, as there is little guarantee that they work well.
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4.1. Native Dual Stack
In this deployment model there is direct connectivity to both the
IPv4 and IPv6 Internet, and a desire to support both IP versions
within the network. This model is applicable to most networks - be
it a home, enterprise, service provider, or a content provider
network.
The purpose of this model is to support any type of device and
communication, and to make it an end-to-end choice which IP version
is used between the peers. There are minimal assumptions about the
capabilities and configuration of hosts in these networks. Native
connectivity avoids problems associated with the configuration of
tunnels and Maximum Transfer Unit (MTU) settings. As a result, these
networks are robust and reliable. Accordingly, this is the
recommended deployment model for most networks, and supported by IETF
standards such as dual stack [RFC4213] and address selection
[RFC3484].
The challenges associated with this model are two-fold. First, while
dual-stack allows each individual network to deploy IPv6 on their
own, actual use still requires participation from all parties between
the peers. For instance, the peer must be reachable over IPv6, have
an IPv6 address to itself, and advertise such an address in the
relevant naming service (such as the DNS). This can create a
situation where IPv6 has been turned on in a network but there is
little actual traffic. One direct way to affect this situation is to
ensure that major destinations of traffic are prepared to receive
IPv6 traffic. Current Internet traffic is highly concentrated on
selected content provider networks, and making a change in even a
small number of these networks can have significant effects. This
was recently observed when YouTube started supporting IPv6
[networkworld.youtube].
The second challenge is that not all applications deal gracefully
with situations where one of the alternative destination addresses
works unreliably. For instance, if IPv6 connectivity is unreliable
it may take a long time for some applications to switch over to IPv4.
As a result, many content providers are shying away from advertising
IPv6 addresses in DNS. This in turn exacerbates the first challenge.
Long term, the use of modern application toolkits and APIs solves
this problem. In the short term content providers and user network
managers have made a mutual agreement a requirement to resolve names
to IPv6 addresses. Such agreements are similar to peering agreements
and are necessary for the time being. Nevertheless, there are many
types of traffic in the Internet and only some of it requires such
careful coordination. Popular peer-to-peer systems can automatically
and reliably employ IPv6 connectivity where it is available, for
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instance.
Despite these challenges the native dual stack connectivity model
remains the recommended approach. It is responsible for a large part
of the forward progress on world-wide IPv6 deployment. The largest
IPv6 networks employ this approach.
The original intent of dual stack was to deploy both IP versions
alongside each other before IPv4 addresses were to run out. As we
know, this never happened and deployment now has to take place with
limited IPv4 addresses. Employing dual stack together with a
traditional IPv4 address translator (NAT44) is a very common
configuration. If the address translator is acceptable for the
network from a pure IPv4 perspective, this model can be recommended
from a dual stack perspective as well. The advantage of IPv6 in this
model is that it allows direct addressing of specific nodes in the
network, creating a contrast to the translated IPv4 service and
allowing the construction of IPv6-based applications that offer more
functionality.
There may also be situations where a traditional IPv4 address
translator is no longer sufficient. For instance, in typical
residential networks, each subscriber is given one global IPv4
address, and the subscriber's NAT44 device may use this address with
as many devices as it can handle. As IPv4 address space becomes more
constrained and without substantial movement to IPv6, it is expected
that service providers will be pressured to assign a single global
IPv4 address to multiple subscribers. Indeed, in some deployments
this is already the case. The dual stack model is still applicable
even in these networks, but the NAT44 functionality may need to be
relocated and enhanced. On some networks it is possible to employ
overlapping private address space [I-D.miles-behave-l2nat]
[I-D.arkko-dual-stack-extra-lite]. Other networks may require a
combination of NAT44 enhancements and tunneling. These scenarios are
discussed further in Section 4.3.
4.2. Crossing IPv4 Islands
Native IPv6 connectivity is not always available, but fortunately it
can established using tunnels. Tunneling introduces some additional
complexity and has a risk of MTU or other misconfigurations. It
should only be used when establishing native connectivity is not
possible. Typically, this involves crossing another administrative
domain or a router that cannot be easily re-configured.
There are several types tunneling mechanisms, including manually
configured IPv6-over-IPv4 tunnels [RFC4213], automatic host-based
tunnels [RFC4380] and the use of VPN or mobility tunnels to carry
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both IPv4 and IPv6 [RFC4301] [RFC5454] [RFC5555]. More advanced
solutions provide a mesh-based framework of tunnels [RFC5565].
There are no major challenges with tunneling beyond the possible
configuration and MTU problems. Tunneling is very widely deployed
both for IPv6 connectivity and other reasons, and well understood.
In general, the IETF recommends that tunneling be used if it is
necessary to cross a segment of IP version X when communicating from
IP version Y to Y. An alternative design would be to employ protocol
translation twice. However, this design involves problems similar to
those created by IPv4 address translation and is largely untried
technology in any larger scale.
4.3. IPv6-Only Core Network
An emerging deployment model uses IPv6 as the dominant protocol at a
service provider network, and tunnels IPv4 through this network in a
manner similar to the one described in the previous section. There
are several motivations for choosing this deployment model:
o There may not be enough public or private addresses to support
network management functions in an end-to-end fashion, without
segmenting the network into small parts with overlapping address
space.
o IP address sharing among subscribers may involve new address
translation nodes within the service provider's network. IPv6 can
be used to reach these nodes. Normal IPv4 routing is insufficient
for this purpose, as the same addresses would be used in several
parts of the network.
o It may be simpler for the service provider to employ a single-
version network.
The recommended tool for this model, Dual Stack Lite, is in its final
stages of specification in the SOFTWIRE working group
[I-D.ietf-softwire-dual-stack-lite]. Dual Stack Lite provides both
relief for IPv4 address shortage and makes forward progress on IPv6
deployment, by moving service provider networks and IPv4 traffic over
IPv6. As a result, providing IPv6 service becames easy.
4.4. Unilateral Deployment
Our final deployment model breaks the requirement that all parties
must upgrade to IPv6 before any actual communications use IPv6. This
model makes sense when two following three conditions are met:
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o There is sufficient control of the devices in the network so that
it can be guaranteed they all support IPv6.
o The devices within the network need to access the global IPv4
network.
o There is a reason to switch to a single IP version network. This
reason may be a desire to test such an advanced configuration, or
merely the desire to simplify the network. It is expected that as
IPv6 deployment progresses, the second reason comes more
prevalent.
There are two types of recommended tools for this model. One type of
a tool is appropriate if all traffic is web-based; a simple web proxy
is then capable of mapping all internal IPv6 traffic to the
appropriate global Internet traffic (either IPv4 or IPv6). Another
type of a tool is more general purpose, and is being progressed in
the BEHAVE working group [I-D.ietf-behave-v6v4-framework]
[I-D.ietf-behave-v6v4-xlate] [I-D.ietf-behave-v6v4-xlate-stateful].
This tool allows the translation of IPv6 traffic to IPv4.
5. Further Reading
Various aspects of IPv6 deployment have been covered in several RFCs.
Of particular interest may be the basic dual stack definition
[RFC4213], application aspects [RFC4038], deployment in Internet
Service Provider Networks [RFC4029], deployment in enterprise
networks [RFC4057] [RFC4852], and considerations in specific access
networks such as cellular networks [RFC3314] [RFC3574] [RFC4215] or
802.16 networks [RFC5181].
6. Security Considerations
This document has no impact on the security properties of specific
IPv6 transition tools.
7. IANA Considerations
This document has no IANA implications.
8. References
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8.1. Normative References
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC5454] Tsirtsis, G., Park, V., and H. Soliman, "Dual-Stack Mobile
IPv4", RFC 5454, March 2009.
[RFC5555] Soliman, H., "Mobile IPv6 Support for Dual Stack Hosts and
Routers", RFC 5555, June 2009.
[RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
8.2. Informative References
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC3314] Wasserman, M., "Recommendations for IPv6 in Third
Generation Partnership Project (3GPP) Standards",
RFC 3314, September 2002.
[RFC3574] Soininen, J., "Transition Scenarios for 3GPP Networks",
RFC 3574, August 2003.
[RFC4029] Lind, M., Ksinant, V., Park, S., Baudot, A., and P.
Savola, "Scenarios and Analysis for Introducing IPv6 into
ISP Networks", RFC 4029, March 2005.
[RFC4038] Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.
Castro, "Application Aspects of IPv6 Transition",
RFC 4038, March 2005.
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[RFC4057] Bound, J., "IPv6 Enterprise Network Scenarios", RFC 4057,
June 2005.
[RFC4215] Wiljakka, J., "Analysis on IPv6 Transition in Third
Generation Partnership Project (3GPP) Networks", RFC 4215,
October 2005.
[RFC4852] Bound, J., Pouffary, Y., Klynsma, S., Chown, T., and D.
Green, "IPv6 Enterprise Network Analysis - IP Layer 3
Focus", RFC 4852, April 2007.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
[RFC5181] Shin, M-K., Han, Y-H., Kim, S-E., and D. Premec, "IPv6
Deployment Scenarios in 802.16 Networks", RFC 5181,
May 2008.
[RFC5218] Thaler, D. and B. Aboba, "What Makes For a Successful
Protocol?", RFC 5218, July 2008.
[I-D.ietf-softwire-dual-stack-lite]
Durand, A., Droms, R., Haberman, B., Woodyatt, J., Lee,
Y., and R. Bush, "Dual-stack lite broadband deployments
post IPv4 exhaustion",
draft-ietf-softwire-dual-stack-lite-03 (work in progress),
February 2010.
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-06 (work in progress),
February 2010.
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-09 (work in
progress), February 2010.
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-08 (work in
progress), January 2010.
[I-D.arkko-townsley-coexistence]
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Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6 Co-
Existence Scenarios", draft-arkko-townsley-coexistence-03
(work in progress), July 2009.
[I-D.miles-behave-l2nat]
Miles, D. and M. Townsley, "Layer2-Aware NAT",
draft-miles-behave-l2nat-00 (work in progress),
March 2009.
[]
Arkko, J. and L. Eggert, "Scalable Operation of Address
Translators with Per-Interface Bindings",
draft-arkko-dual-stack-extra-lite-00 (work in progress),
February 2010.
[fred.shanghai]
Baker, F., "The view from IPv6 Operations WG (and we'll
talk about translation)", Presentation in the China Mobile
Workshop on IPv6 Deployment in Cellular Networks,
Shanghai, China, November 2009.
[networkworld.youtube]
Marsan, C., "YouTube support of IPv6 seen in dramatic
traffic spike", Network World article http://
www.networkworld.com/news/2010/020110-youtube-ipv6.html,
February 2010.
Appendix A. Acknowledgments
The author would like to thank the many people who have engaged in
discussions around this topic over the years. This work is
particularly in debt to Fred Baker and his November 6th, 2009
presentation in a workshop in Shanghai [fred.shanghai]. In addition,
the author would like to thank Mark Townsley with whom the author
wrote an earlier document [I-D.arkko-townsley-coexistence], and thank
Dave Thaler, Alain Durand, Randy Bush, and Dan Wing who have always
provided valuable guidance in this field.
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Author's Address
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
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