Network Working Group                                         S. Jiang
Internet Draft                                                  D. Guo
Intended status: Informational            Huawei Technologies Co., Ltd
Expires: March 22, 2010                                   B. Carpenter
                                                University of Auckland
                                                    September 24, 2009

       An Incremental Carrier-Grade NAT (CGN) for IPv6 Transition

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   This Internet-Draft is submitted to IETF in full conformance with the
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   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) approach 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. An Incremental CGN Approach..................................4
      2.1. Incremental CGN Approach Overview.......................4
      2.2. Choice of tunnelling technology.........................5
      2.3. Behaviour of Dual-stack Home Gateway....................5
      2.4. Behaviour of Dual-stack Carrier-Grade NAT...............6
      2.5. Impact for existing end hosts and remaining networks....6
      2.6. Discussion..............................................6
   3. Migration towards IPv6 Core Network..........................7
      3.1. Legacy communication in Phase 2.........................8
   4. Security Considerations......................................8
   5. IANA Considerations..........................................8
   6. Acknowledgements.............................................9
   7. Change Log...................................................9
   8. References...................................................9
      8.1. Normative References....................................9
      8.2. 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 in
   all cases. Therefore, network supporting functions and/or 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. Theoretically, DS-Lite
   can be used with double encapsulation (IPv4-in-IPv6-in-IPv4) but this
   seems even less likely to be accepted by an ISP and is not discussed

   This document proposes an incremental CGN approach for IPv6
   transition. The approach 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 technology does not affect
   legacy IPv4 hosts with global IPv4 addresses at all. It is suitable

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   for the initial stage of IPv4/IPv6 migration. It also supports
   transition towards dual-stack or IPv6-only ISP networks.

2. An Incremental CGN Approach

   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
   approach described in this draft addresses both of these pressures by
   proceeding in two phases.

2.1. Incremental CGN Approach Overview

   The incremental CGN approach 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 approach with IPv4 ISP network

   DS HG = Dual-Stack Home Gateway (CPE).

   The above figure shows only Phase 1, in which the ISP has not
   significantly changed its IPv4 network. This approach enables IPv4
   hosts to access the IPv4 Internet and IPv6 hosts to access the IPv6

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   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 such
   mechanisms is out of scope for this document

   Two new types of devices need to be deployed in this approach: 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.

2.2. 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. Modified GRE [RFC2784] with
   additional auto-configuration mechanism is also suitable to support
   v6-over-v4 tunnelling. Other tunnelling mechanisms such as 6over4
   [RFC2529], 6to4 [RFC3056], the Intra-Site Automatic Tunnel Addressing
   Protocol (ISATAP) [RFC5214] or Virtual Enterprise Traversal (VET)
   [VET] are also considered. 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

2.3. 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
   records the v4-v4 address mapping information for inbound packets,
   just like normal NAT does.

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   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 records 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.

2.4. 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 records 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 records 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.

2.5. Impact for existing end hosts and remaining networks

   This approach 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, if they are upgraded to become dual-
   stack hosts, can access IPv6 Internet through IPv4 ISP network by
   using IPv6-over-IPv4 tunnel technologies.

2.6. Discussion

   For IPv4 traffic, this approach 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 technology than v4-v4 NAT is chosen for IPv4 address
   sharing, for example [APLUSP], the present approach could be suitably
   modified, for example replacing the v4-v4 NAT function by the A+P
   gateway function.

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   However, for IPv6 traffic, a user behind the DS HG will see normal
   IPv6 service. We therefore observe that an IPv6 tunnel MTU of at
   least 1500 bytes would ensure that the mechanism does not cause
   excessive fragmentation of IPv6 traffic nor excessive IPv6 path MTU
   discovery interactions.

   However, for IPv6 traffic, a user behind the DS HG will see normal
   IPv6 service. This, and the absence of NAT problems for IPv6, will
   create an incentive for users and application service providers to
   prefer IPv6.

   ICMP filtering [RFC4890] function may be included as part of CGN

3. Migration towards IPv6 Core Network

   If the core network transits to IPv6, this approach 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 while the dual-stack CGN only keeps 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 approach 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
   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.

   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.

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3.1. 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, IPv4/IPv6 intercommunication
   may become issues. [DSLInter] describes a proposal to enhance DS-lite
   solution with an additional feature to ease interconnection between
   IPv4 and IPv6 realms. Furthermore, 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. Each ISP can 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". Current work in the IETF is
   focussed on one particular proposal [NAT64].

   The NAT64 service can 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 DSLite tunnels in Phase 2. This seems undesirable.

4. 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
   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].

5. IANA Considerations

   This draft does not request any IANA action.

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6. Acknowledgements

   Useful comments were made by Fred Baker, Dan Wing, Fred Templin,
   Seiichi Kawamura, Remi Despres, Janos Mohacsi, Mohamed Boucadair,
   Shin Miyakawa and other members of the IETF V6OPS working group.

7. 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

   draft-jiang-v6ops-incremental-cgn-01, revised after comments at v6ops
   mailing list, 2009-06-30

   draft-jiang-v6ops-incremental-cgn-02, remove normative parts (to be
   documented in other WGs), 2009-07-06

   draft-jiang-v6ops-incremental-cgn-03, revised after comments at v6ops
   mailing list, 2009-09-24

8. References

8.1. Normative References

   [RFC2529] B. Carpenter, and C. Jung, "Transmission of IPv6 over IPv4
             Domains without Explicit Tunnels", RFC2529, March 1999.

   [RFC2784] D. Farinacci, T. Li, S. Hanks, D. Meyer and P. Traina,
             "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

8.2. Informative References

   [RFC2663] P. Srisuresh and M. Holdrege, "IP Network Address
             Translator (NAT) Terminology and Considerations", RFC 2663,
             August 1999.

   [RFC2766] G. Tsirtsis and P. Srisuresh, "Network Address Translation
             - Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [RFC2993] T. Hain, "Architectural Implications of NAT", RFC 2993,
             November 2000.

   [RFC3053] A. Durand, P. Fasano, I. Guardini and D. Lento, "IPv6
             Tunnel Broker", RFC 3053, January 2001.

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   [RFC3056] B. Carpenter and K. Moore, "Connection of IPv6 Domains via
             IPv4 Clouds", RFC 3056, February 2001.

   [RFC4213] E. Nordmark and R. Gilligan, "Basic Transition Mechanisms
             for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4798] J. De Clercq, D. Ooms, S. Prevost and F. Le Faucheur,
             "Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
             Edge Routers (6PE)", RFC 4798, February 2007.

   [RFC4864] G. Van de Velde, T. Hain, R. Droms, B. Carpenter and E.
             Klein, "Local Network Protection for IPv6", RFC 4864, May

   [RFC4890] E. Davies and J. Mohacsi, "Recommendations for Filtering
             ICMPv6 Messages in Firewalls", RFC 4890, May 2007.

   [RFC4891] R. Graveman, "Using IPsec to Secure IPv6-in-IPv4 Tunnels",
             RFC4891, May 2007.

   [RFC4966] C. Aoun 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 and J. Woodyatt, "Dual-
             stack lite broadband deployments post IPv4 exhaustion",
             draft-durand-softwire-dual-stack-lite-01, work in progress.

   [IPUSAGE] G. Huston, IPv4 Address Report, March 2009,

   [6RD]    R. Despres, "IPv6 Rapid Deployment on IPv4 infrastructures
             (6rd)", draft-despres-6rd, work in progress.

   [6CPE]   H. Singh, "IPv6 CPE Router Recommendations", draft-wbeebee-
             ipv6-cpe-router, 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,
             work in progress.

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   [APLUSP] R. Bush, O. Maennel, J. Zorz, S. Bellovin and 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.

   [DSLInter] M. Boucadair, et al, "Stateless IPv4-IPv6 Interconnection
             in the Context of DS-lite Deployment", draft-boucadair-
             dslite-interco-v4v6, work in progress.

   [NAT64]  M. Bagnulo, P. Matthews and I. van Beijnum, "NAT64: Network
             Address and Protocol Translation from IPv6 Clients to IPv4
             Servers", draft-bagnulo-behave-nat64, 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

   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

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland, 1142
   New Zealand

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