Network Working Group S. Jiang
Internet Draft D. Guo
Intended status: Informational Huawei Technologies Co., Ltd
Expires: November 6, 2009 B. Carpenter
University of Auckland
May 8, 2009
An Incremental Carrier-Grade NAT (CGN) for IPv6 Transition
draft-jiang-v6ops-incremental-cgn-00.txt
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Abstract
Global IPv6 deployment was slower than originally expected in the
last ten years. As IPv4 address exhaustion gets closer, the IPv4/IPv6
transition issues become more critical and complicated. Host-based
transition mechanisms are not able to meet the requirements while
most end users are not sufficiently expert to configure or maintain
these transition mechanisms. Carrier Grade NAT with integrated
transition mechanisms can simplify the operation of end users during
the IPv4/IPv6 migration or coexistence period. This document proposes
an incremental Carrier-Grade NAT (CGN) solution for IPv6 transition.
It can provide IPv6 access services for IPv6-enabled end hosts and
IPv4 access services for IPv4 end hosts while remaining most of
legacy IPv4 ISP networks unchanged. It is suitable for the initial
stage of IPv4/IPv6 migration. Unlike CGN alone, it also supports and
encourages transition towards dual-stack or IPv6-only ISP networks.
Table of Contents
1. Introduction................................................3
2. Terminology.................................................4
3. An Incremental CGN Solution..................................4
3.1. Incremental CGN Solution Overview.......................4
3.2. Choice of tunnelling technology.........................5
3.3. Behaviour of Dual-stack Home Gateway....................6
3.4. Behaviour of Dual-stack Carrier-Grade NAT...............6
3.5. Impact for end hosts and remaining networks.............7
3.6. Discussion.............................................7
4. Migration towards IPv6 Core Network..........................7
4.1. Legacy communication in Phase 2.........................8
5. Security Considerations......................................8
6. IANA Considerations.........................................9
7. Acknowledgements............................................9
8. Change Log [RFC Editor please remove]........................9
9. References..................................................9
9.1. Normative References....................................9
9.2. Informative References..................................9
Author's Addresses............................................11
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1. Introduction
Up to now, global IPv6 deployment does not happen as was expected 10
years ago. The progress was much slower than originally expected.
Network providers were hesitant to take the first move while IPv4 was
and is still working well. However, IPv4 address exhaustion is now
confirmed to happen soon. The dynamically-updated IPv4 Address Report
[IPUSAGE] has analyzed this issue. It predicts early 2011 for IANA
unallocated address pool exhaustion and middle 2012 for RIR
unallocated address pool exhaustion. Based on this fact, the Internet
industry appears to have reached consensus that global IPv6
deployment is inevitable and has to be done quite quickly.
IPv4/IPv6 transition issues therefore become more critical and
complicated for the soon-coming global IPv6 deployment. Host-based
transition mechanisms alone are not able to meet the requirements.
They are too complicated for most end users who do not have enough
technical knowledge to configure or maintain these transition
mechanisms. New transition mechanisms with simple user-side operation
are needed.
Carried Grade NAT (CGN) alone creates operational problems, but does
nothing to help IPv4/IPv6 transition. In fact it allows ISPs to delay
the transition, and therefore causes double transition costs (once to
add CGN, and again to support IPv6).
Carrier-Grade NAT that integrates multiple transition mechanisms can
simplify the operation of end user services during the IPv4/IPv6
migration or coexistence period. CGNs are deployed on the network
side and managed/maintained by professionals. On the user side, new
CPE devices may be needed too. They may be provided by network
providers, depending on the specific business model. Dual-stack lite
[DSLite] is a CGN-based solution that supports transition, but it
requires the ISP to upgrade its network to IPv6 immediately. Many
ISPs hesitate to do this as the first step.
This document proposes an incremental CGN solution for IPv6
transition. The solution is similar to DSLite, but the other way
around. Technically, it mainly combines v4-v4 NAT with v6-over-v4
tunnelling functions along with some minor adjustment. It can provide
IPv6 access services for IPv6-enabled end hosts and IPv4 access
services for IPv4 end hosts, while leaving most of legacy IPv4 ISP
networks unchanged. The deployment of this solution does not affect
legacy IPv4 hosts with global IPv4 addresses at all. It is suitable
for the initial stage of IPv4/IPv6 migration. It also supports
transition towards dual-stack or IPv6-only ISP networks.
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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 [RFC2119].
3. An Incremental CGN Solution
Most ISP networks are still IPv4. Network providers are starting to
provide IPv6 access services for end users. However, at the initial
stage of IPv4/IPv6 migration, IPv4 connectivity and traffic would be
the majority for most ISP networks. ISPs would like to minimize the
changes on their IPv4 networks. Switching the whole ISP network into
IPv6-only would be considered as a radical strategy. Switching the
whole ISP network to dual stack is less radical, but introduces
operational costs and complications. Although some ISPs have
successfully deployed dual stack routers, others prefer not to do
this as their first step in IPv6. However, they currently face two
urgent pressures - to compensate for an immediate shortage of IPv4
addresses by deploying some method of address sharing, and to prepare
actively for the deployment of IPv6 address space and services. The
solution described in this draft addresses both of these pressures by
proceeding in two phases.
3.1. Incremental CGN Solution Overview
The incremental CGN solution we propose is illustrated as the
following figure.
+-------------+
|IPv6 Internet|
+-------------+
|
+-------------+----------+
+-----+ +--+ | IPv4 ISP +--+--+ | +--------+
|v4/v6|----|DS|=====+==========| CGN |-------+---| IPv4 |
|Host | |HG| | Network +-----+ | | |Internet|
+-----+ +--+ +--------------------+---+ +--------+
_ _ _ _ _ _ _ _ _ _ _ |
()_6_o_4_ _t_u_n_n_e_l_() +---------------------+
| Existing IPv4 hosts |
+---------------------+
Figure 1: Phase 1 of incremental CGN solution with IPv4 ISP network
DS HG = Dual-Stack Home Gateway (CPE).
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The above figure shows only Phase 1, in which the ISP has not
significantly changed its IPv4 network. This solution enables IPv4
hosts to access the IPv4 Internet and IPv6 hosts to access the IPv6
Internet. A dual stack host can be treated as an IPv4 host when it
uses IPv4 access service and as an IPv6 host when it uses IPv6 access
service. In order to enable IPv4 hosts to access IPv6 Internet and
IPv6 hosts to access IPv4 Internet, NAT-PT [RFC2766, RFC4966] (or its
replacement) can be integrated with CGN. The integration of NAT-PT is
out of scope for this document
Two new types of devices need to be deployed in this solution: a
dual-stack home gateway, which may follow the requirements of [6CPE],
and dual-stack Carrier-Grade NAT. The dual-stack home gateway
integrates IPv4 forwarding and v6-over-v4 tunnelling functions. It
may integrate v4-v4 NAT function, too. The dual-stack CGN integrates
v6-over-v4 tunnelling and carrier-grade v4-v4 NAT functions.
3.2. Choice of tunnelling technology
In principle, this model will work with any form of tunnel between
the DS HG and the dual-stack CGN. However, tunnels that require
individual configuration are clearly undesirable because of their
operational cost. Configured tunnels based directly on [RFC4213] are
therefore not suitable. A tunnel broker according to [RFC3053] would
also have high operational costs.
Modified 6RD [6RD] technology appears suitable to support v6-over-v4
tunnelling with low operational cost. 6RD was designed for an IPv4
ISP scenario and allows re-use of slightly modified existing support
for 6to4 [RFC3056]. 6RD using a 32-bit IPv6 prefix from the ISP's
address space will allow each CPE to receive a 64-bit prefix
corresponding to its IPv4 address. If it is desired to delegate 56-
bit prefixes to each customer, the 6RD prefix must be of 24 bits, as
illustrated below. In that case, the ISP must have a general IPv6
prefix shorter than /24.
+----------------------.------------------------------+
| 6RD-relay IPv6 prefix| IPv4 address |
| of the ISP | of the customer site |
+----------------------'------------------------------+
<----- 24 bits -----><--------- 32 bits ---------->
Figure 2: format of a 56-bit 6RD prefix
Modified GRE [RFC2784] with additional auto-configuration mechanism
is also suitable to support v6-over-v4 tunnelling.
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Other tunnelling mechanisms such as ISATAP [RFC5214] or VET [VET]
could also be considered, but 6RD seems well adapted.
If the ISP has an entirely MPLS infrastructure between the CPE and
the dual-stack CGN, it would also be possible to consider a 6PE
[RFC4798] tunnel directly over MPLS. This would, however, only be
suitable for an advanced CPE that is unlikely to be found as a home
gateway, and is not further discussed here.
3.3. Behaviour of Dual-stack Home Gateway
When a dual-stack home gateway receives a data packet from an end
host, it firstly checks whether the packet is IPv4 or IPv6. For IPv4
data, the HG can directly forward it if there is no v4-v4 NAT running
on the HG. Or the HG translates packet source address from a HG-scope
private IPv4 address into a CGN-scope private IPv4 address. The HG
should record the v4-v4 address mapping information for inbound
packets, just like normal NAT does.
For IPv6 data, the HG needs to encapsulate the data into an IPv4
tunnel, which has the dual-stack CGN as the other end. Then the HG
sends the new IPv4 packet towards CGN.
The HG should record the mapping information between the tunnel and
the source IPv6 address for inbound packets if HG uplinks to more
than one CGN. Detailed considerations for the use of multiple CGNs by
one HG are for further study.
3.4. Behaviour of Dual-stack Carrier-Grade NAT
When a dual-stack CGN receives a data packet from a dual-stack home
gateway, it firstly checks whether the packet is a normal IPv4 packet
or a v6-over-v4 tunnel packet. For a normal IPv4 packet, the CGN
translates packet source address from a CGN-scope private IPv4
address into a public IPv4 address, and then send it to IPv4 Internet.
The CGN should record the v4-v4 address mapping information for
inbound packets, just like normal NAT does. For a v6-over-v4 tunnel
packet, the CGN needs to decapsulate it into the original IPv6 packet
and then send it to IPv6 Internet. The CGN should record the mapping
information between the tunnel and the source IPv6 address for
inbound packets.
Depending on the deployed location of the CGN, it may use v6-over-v4
tunnels to connect to the IPv6 Internet.
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3.5. Impact for end hosts and remaining networks
This solution does not affect the remaining networks at all. Legacy
IPv4 ISP networks and their IPv4 devices remain in use. The existing
IPv4 hosts, shown as the right box in Figure 1, either having global
IPv4 addresses or behind v4-v4 NAT can connect to IPv4 Internet as it
is now. Of course, these hosts can access IPv6 Internet through IPv4
ISP network by using IPv4-over-IPv6 tunnel technologies.
3.6. Discussion
It should be noted that for IPv4 traffic, this solution inherits all
the problems of CGN (e.g., scaling, and the difficulty of supporting
well-known ports for inbound traffic). Application layer problems
created by double NAT are for further study.
If a different solution than v4-v4 NAT is chosen for IPv4 address
sharing, for example [APLUSP], the present solution could be suitably
modified, for example replacing the v4-v4 NAT function by the A+P
gateway function.
However, for IPv6 traffic, a user behind the DS HG will see normal
IPv6 service. It is strongly recommended that all IPv6 tunnels
support an MTU of at least 1500 bytes, to ensure that the mechanism
usually does not cause fragmentation of IPv6 traffic. This, and the
absence of NAT problems for IPv6, will create an incentive for users
and application service providers to prefer IPv6.
4. Migration towards IPv6 Core Network
When the core network starts transition to IPv6, this solution can
easily be transited into Phase 2, in which the ISP network is either
dual-stack or IPv6-only.
For dual-stack ISP networks, dual-stack home gateways can simply
switch off the v6-over-v4 function and forward both IPv6 and IPv4
traffic directly; the dual-stack CGN should only keep its v4-v4 NAT
function. However, this is considered an unlikely choice, since we
expect ISPs to choose the approach described here because they want
to avoid dual-stack deployment completely.
For IPv6-only ISP networks, the dual-stack lite solution [DSLite],
which also needs dual-stack home gateway and CGN devices, can be
adopted for Phase 2. The best business model for this solution is
that CPE has integrated the functions for both Phase 1 and 2, and can
automatically detect the change. For example, the DS HG can use the
appearance of IPv6 Route Advertisement messages or DHCPv6 messages as
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a signal that Phase 2 has started. Then when ISPs decide to switch
from Phase 1 to Phase 2, it may be that only a configuration change
or a minor software update is needed on the CGNs. The DS HG will then
switch automatically to DSLite mode. The only impact on the home user
will be to receive a different IPv6 prefix.
Note that if the 6RD mechanism is used in Phase 1, the user may have
a /64 prefix during Phase 1, but could get a shorter prefix such as
/56 in Phase 2. This would be an improved service offering available
as a result of the Phase 1 to Phase 2 transition.
It will not be necessary for all customers of a given ISP to switch
from Phase 1 to Phase 2 simultaneously; in fact it will be
operationally better to switch small groups of customers (e.g. all
those connected to a single point of presence). This is a matter of
planning and scheduling.
4.1. Legacy communication in Phase 2
We do not expect to see IPv6-only public services as long as there is
an IPv4-only customer base in the world, for obvious commercial
reasons. However, especially in Phase 2, home users may encounter the
problem of reaching legacy IPv4-only public services from IPv6-only
clients. This problem could already exist in Phase 1, but will become
more serious as time goes on. It is proposed that each ISP should
provide its IPv6-only customers with a network-layer translation
service to satisfy this need. Such a service is not fully defined at
this time, so we refer to it non-specifically as NAT64.
We propose that the NAT64 service should be provided as a common
service located at the border between the ISP and the IPv4 Internet,
beyond the dual stack CGN from the customer's viewpoint. It may be
integrated into CGN devices too. The question has been asked why it
is better to do this than to distribute the NAT64 function by
locating it in (or near) the home gateway, so that relevant
translation state resides only in the HG. While this might be
suitable in Phase 1, when the ISP still provides full IPv4
connectivity, it would force all translated traffic into DS-Lite
tunnels in Phase 2. This seems undesirable.
5. Security Considerations
Security issues associated with NAT have been documented in [RFC2663]
and [RFC2993].
Further security analysis will be needed to understand double NAT
security issues and tunnel security issues. However, since the tunnel
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proposed here exists entirely within a single ISP network, between
the CPE and the CGN, the threat model is relatively simple. [RFC4891]
describes how to protect tunnels using IPSec, but it is not clear
whether this would be an important requirement. An ISP could deem its
infrastructure to have sufficient security without additional
protection of the tunnels.
The dual-stack home gateway will need to provide basic security for
IPv6 [6CPESec]. Other aspects are described in [RFC4864].
6. IANA Considerations
This draft does not request any IANA action.
7. Acknowledgements
Shin Miyakawa discussed a related proposal at IETF72.
Useful comments were made by Fred Templin, and other members of the
IETF V6OPS working group.
8. Change Log [RFC Editor please remove]
draft-jiang-incremental-cgn-00, original version, 2009-02-27
draft-jiang-v6ops-incremental-cgn-00, revised after comments at
IETF74, 2009-04-23
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2784] D. Farinacci, T. Li, S. Hanks, D. Meyer, P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
9.2. Informative References
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC 2663,
August 1999.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address Translation
- Protocol Translation (NAT-PT)", RFC 2766, February 2000.
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[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[RFC3053] A. Durand, P. Fasano, I. Guardini, D. Lento, "IPv6 Tunnel
Broker", RFC3053, January 2001.
[RFC3056] B. Carpenter, K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC3056, February 2001.
[RFC4213] E. Nordmark, R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", RFC4213, October 2005.
[RFC4798] J. De Clercq, D. Ooms, S. Prevost, F. Le Faucheur,
"Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
Edge Routers (6PE)", RFC4798, February 2007.
[RFC4864] G. Van de Velde, T. Hain, R. Droms, B. Carpenter, E. Klein,
"Local Network Protection for IPv6", RFC4864, May 2007.
[RFC4891] R. Graveman, "Using IPsec to Secure IPv6-in-IPv4 Tunnels",
RFC4891, May 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.
[RFC5214] F. Templin, T. Gleeson, and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[DSLite] A. Durand, R. Droms, B. Haberman, J. Woodyatt, "Dual-stack
lite broadband deployments post IPv4 exhaustion", draft-
durand-softwire-dual-stack-lite-01, work in progress.
[IPUSAGE] Huston, G., IPv4 Address Report, March 2009,
http://www.potaroo.net/tools/ipv4/index.html.
[6RD] R. Despres, "IPv6 Rapid Deployment on IPv4 infrastructures
(6rd)", draft-despres-6rd-03, work in progress.
[6CPE] H. Singh, "IPv6 CPE Router Recommendations", draft-wbeebee-
ipv6-cpe-router-04, work in progress.
[6CPESec] J. Woodyatt, "Recommended Simple Security Capabilities in
Customer Premises Equipment for Providing Residential IPv6
Internet Service", draft-ietf-v6ops-cpe-simple-security-05,
work in progress.
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[APLUSP] R. Bush, O. Maennel, J. Zorz, S. Bellovin, L. Cittadini,
"The A+P Approach to the IPv4 Address Shortage", draft-
ymbk-aplusp, work in progress.
[VET] F. Templin, "Virtual Enterprise Traversal (VET)", draft-
templin-autoconf-dhcp, work in progress.
Author's Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
KuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base, Hai-Dian District, Beijing 100085
P.R. China
Phone: 86-10-82836774
Email: shengjiang@huawei.com
Dayong Guo
Huawei Technologies Co., Ltd
KuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base, Hai-Dian District, Beijing 100085
P.R. China
Phone: 86-10-82836284
Email: guoseu@huawei.com
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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