Network Working Group                                           J. Arkko
Internet-Draft                                                  Ericsson
Intended status: Informational                               M. Townsley
Expires: January 14, 2010                                          Cisco
                                                           July 13, 2009

           IPv4 Run-Out and IPv4-IPv6 Co-Existence Scenarios

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on January 14, 2010.

Copyright Notice

   Copyright (c) 2009 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
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   When IPv6 was designed, it was expected that the transition from IPv4
   to IPv6 would occur more smoothly and expeditiously than experience

Arkko & Townsley        Expires January 14, 2010                [Page 1]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   has revealed.  The growth of the IPv4 Internet and predicted
   depletion of the free pool of IPv4 address blocks on a foreseeable
   horizon has highlighted an urgent need to revisit IPv6 deployment
   models.  This document provides an overview of deployment scenarios
   with the goal of helping to understand what types of additional tools
   the industry needs to assist in IPv4 and IPv6 co-existence and

   This document was originally created as input to the Montreal co-
   existence interim meeting in October 2008, which led to the
   rechartering of the Behave and Softwire working groups to take on new
   IPv4 and IPv6 coexistence work.  This document is published as a
   historical record of the thinking at the time.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Reaching the IPv4 Internet . . . . . . . . . . . . . . . .  4
       2.1.1.  NAT444 . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.2.  Distributed NAT  . . . . . . . . . . . . . . . . . . .  7
       2.1.3.  Recommendation . . . . . . . . . . . . . . . . . . . .  9
     2.2.  Running out of IPv4 Private Address Space  . . . . . . . .  9
     2.3.  Enterprise IPv6 Only Networks  . . . . . . . . . . . . . . 11
     2.4.  Reaching Private IPv4 Only Servers . . . . . . . . . . . . 13
     2.5.  Reaching IPv6 Only Servers . . . . . . . . . . . . . . . . 15
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

Arkko & Townsley        Expires January 14, 2010                [Page 2]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

1.  Introduction

   When IPv6 was designed, it was expected that IPv6 would be enabled,
   in part or in whole, while continuing to run IPv4 side-by-side on the
   same network nodes and hosts.  This method of transition is referred
   to as "Dual-Stack" [RFC4213] and has been the prevailing method
   driving the specifications and available tools for IPv6 to date.

   Experience has shown that large-scale deployment of IPv6 takes time,
   effort, and significant investment.  With IPv4 address pool depletion
   on the foreseeable horizon [Huston.IPv4], network operators and
   Internet Service Providers are being forced to consider network
   designs that no longer assume the same level of access to unique
   global IPv4 addresses.  IPv6 alone does not alleviate this concern
   given the basic assumption that all hosts and nodes will be Dual-
   Stack until the eventual sunsetting of IPv4-only equipment.  In
   short, the time-frames for the growth of the IPv4 Internet, the
   universal deployment of Dual-Stack IPv4 and IPv6, and the final
   transition to an IPv6-dominant Internet are not in alignment with
   what was originally expected.

   While Dual-Stack remains the most well-understood approach to
   deploying IPv6 today, current realities dictate a re-assessment of
   the tools available for other deployment models that are likely to
   emerge.  In particular, the implications of deploying multiple layers
   of IPv4 address translation need to be considered, as well as those
   associated with translation between IPv4 and IPv6 which led to the
   deprecation of [RFC2766] as detailed in [RFC4966].  This document
   outlines some of the scenarios where these address and protocol
   translation mechanisms could be useful, in addition to methods where
   carrying IPv4 over IPv6 may be used to assist in transition to IPv6
   and co-existence with IPv4.  We purposefully avoid a description of
   classic Dual-Stack methods, as well as IPv6 over IPv4 tunneling.
   Instead, this document focuses on scenarios which are driving tools
   we have historically not been developing standard solutions around.

   It should be understood that the scenarios in this document represent
   new deployment models and are intended to complement, not replace
   existing ones.  For instance, Dual-Stack continues to be the most
   well understood and recommended deployment model.  Note that Dual-
   Stack is not limited to situations where all hosts can acquire public
   IPv4 addresses.  A common deployment scenario is running Dual-Stack
   on the IPv6 side with public addresses and on the IPv4 side with just
   one public address and a traditional IPv4 NAT.  Generally speaking,
   offering native connectivity with both IP versions is preferred over
   the use of translation or tunneling mechanisms when sufficient
   address space is available.

Arkko & Townsley        Expires January 14, 2010                [Page 3]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

2.  Scenarios

   This section identifies five deployment scenarios which we believe
   have a significant level of near to medium term demand somewhere on
   the globe.  We will discuss these in the following sections, while
   walking through a bit of the design space to get an understanding of
   the types of tools that could be developed to solve each.  In
   particular, we want the reader to be consider what type of new
   equipment must be introduced in the network and where for each
   scenario, which nodes must be changed in some way, and which nodes
   must work together in an interoperable manner via a new or existing

   The five scenarios are:

   o  Reaching the IPv4 Internet with less than one global IPv4 address
      per subscriber or subscriber household available (Section 2.1).

   o  Running a large network needing more addresses than those
      available in private RFC 1918 address space (Section 2.2).

   o  Running an IPv6-only network for operational simplicity as
      compared to Dual-Stack, while still needing access to the global
      IPv4 Internet for some, but not all, connectivity (Section 2.3).

   o  Reaching one or more privately addressed IPv4 only servers via
      IPv6 (Section 2.4).

   o  Accessing IPv6-only servers from IPv4 only clients (Section 2.5).

2.1.  Reaching the IPv4 Internet

                    +----+                       +---------------+
   IPv4 host(s)-----+ GW +------IPv4-------------| IPv4 Internet |
                    +----+                       +---------------+

   <---private v4--->NAT<--------------public v4----------------->

            Figure 1: Accessing the IPv4 Internet today

   Figure 1 shows a typical model for accessing the IPv4 Internet today,
   with the gateway device implementing a Network Address and Port
   Translation (NAPT, or more simply referred to in this document as
   NAT).  The NAT function serves a number of purposes, one of which is

Arkko & Townsley        Expires January 14, 2010                [Page 4]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   to allow more hosts behind the GW as there are IPv4 addresses
   presented to the Internet.  This multiplexing of IP addresses comes
   at great cost to the original end-to-end model of Internet, but
   nonetheless is the dominant method of access today, particularly to
   residential subscribers.

   Taking the typical residential subscriber as an example, each
   subscriber line is allocated one global IPv4 address for it to use
   with as many devices as the NAT GW and local network 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.

2.1.1.  NAT444

   When there is less than one address per subscriber at a given time,
   address multiplexing must be performed at a location where visibility
   to more than one subscriber can be realized.  The most obvious place
   for this is within the service provider network itself, requiring the
   service provider to acquire and operate NAT equipment to allow
   sharing of addresses across multiple subscribers.  For deployments
   where the GW is owned and operated by the customer, this becomes
   operational overhead for the Internet Service Provider (ISP) that it
   will no longer be able to rely on the customer and the seller of the
   GW device for.

   This new address translation node has been termed a "Carrier Grade
   NAT", or CGN [I-D.nishitani-cgn].  The CGN's insertion into the ISP
   network is shown in Figure 2.

                    +----+                   +---+  +-------------+
   IPv4 host(s)-----+ GW +------IPv4---------+CGN+--+IPv4 Internet|
                    +----+                   +---+  +-------------+

   <---private v4--->NAT<----private v4------>NAT<----public v4--->

                Figure 2: Employing two NAT devices, NAT444

   This solution approach is known as "NAT444" or "Double-NAT" and is
   discussed further in [I-D.wing-nat-pt-replacement-comparison].

   It is important to note that while multiple levels of multiplexing of
   IPv4 addresses is occurring here, there is no aggregation of NAT

Arkko & Townsley        Expires January 14, 2010                [Page 5]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   state between the GW and CGN.  Every flow that is originated in the
   subscriber home is represented as duplicate state in the GW and CGN.
   For example, if there are 4 PCs in a subscriber home, each with 25
   open TCP sessions, both the GW and CGN must track 100 sessions each
   for that subscriber line.

   NAT444 has the enticing property that it seems, at first glance, that
   the CGN can be deployed without any change to the GW device or other
   node in the network.  While it is true that a GW which can accept a
   lease for a global IPv4 address would very likely accept a translated
   IPv4 address as well, the CGN is neither transparent to the GW or the
   subscriber.  In short, it is a very different service model to offer
   a translated IPv4 address vs. a global IPv4 address to a customer.
   While many things may continue to work in both environments, some
   end-host applications may break, and GW port-mapping functionality
   will likely cease to work reliably.  Further, if addresses between
   the subscriber network and service provider network overlap,
   ambiguous routes in the GW could lead to misdirected or black-holed
   traffic [I-D.shirasaki-isp-shared-addr].  Resolving this overlap
   through allocation of new private address space is difficult, as many
   existing devices rely on knowing what address ranges represent
   private addresses.

   Network operations which had previously been tied to a single IPv4
   address for a subscriber would need to be considered when deploying
   NAT444 as well.  These may include troubleshooting and OAM,
   accounting, logs (including legal intercept), QoS functions, anti-
   spoofing and security, backoffice systems, etc.  Ironically, some of
   these considerations overlap with the kinds of considerations one
   needs to perform when deploying IPv6.

   Consequences aside, NAT444 service is already being deployed in some
   networks for residential broadband service.  It is safe to assume
   that this trend will likely continue in the face of tightening IPv4
   address availability.  The operational considerations of NAT444 need
   to be well documented.

   NAT444 assumes that the global IPv4 address offered to a residential
   subscriber today will simply be replaced with a single translated
   address.  In order to try and circumvent performing NAT twice, and
   since the address being offered is no longer a global address, a
   service provider could begin offering a subnet of translated IPv4
   addresses in hopes that the subscriber would route IPv4 in the GW
   rather than NAT.  The same would be true if the GW was known to be an
   IP-unaware bridge.  This makes assumptions on whether the ISP can
   enforce policies, or even identify specific capabilities, of the GW.
   Once we start opening the door to making changes at the GW, we have
   increased the potential design space considerably.  The next section

Arkko & Townsley        Expires January 14, 2010                [Page 6]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   covers the same problem scenario of reaching the IPv4 Internet in the
   face of IPv4 address depletion, but with the added wrinkle that the
   GW can be updated or replaced along with the deployment of a CGN (or
   CGN-like) node.

2.1.2.  Distributed NAT

   Increasingly, service providers offering "triple-play" services own
   and manage a highly-functional GW in the subscriber home.  These
   managed GWs generally have rather tight integration with the service
   provider network and applications.  In these types of deployments, we
   can begin to consider what other possibilities exist besides NAT444
   by assuming cooperative functionality between the CGN and GW.

   If the connection between the GW and CGN is a point-to-point link (a
   common configuration between the GW and the "IP-Edge" in a number of
   access architectures), NAT-like functionality may be "split" between
   the GW and CGN rather than performing NAT444 as described in the
   previous section.

                 one frac addr            one public addr
                    +----+                   +---+  +-------------+
   IPv4 host(s)-----+ GW +-----p2p link------+CGN+--+IPv4 Internet|
                    +----+                   +---+  +-------------+

   <---private v4--->            NAT             <----public v4--->
                           over a p2p link)

               Figure 3: Distributed-NAT service

   In this approach, multiple GWs share a common public IPv4 address,
   but with separate, non-overlapping, port ranges.  Each such address/
   port range pair is defined as a "fractional address".  Each home
   gateway can use the address as if it were its own public address,
   except that only a limited port range is available to the gateway.
   The CGN is aware of the port ranges, which may be assigned in
   different ways, for instance during DHCP lease acquisition or
   dynamically when ports are needed [I-D.despres-v6ops-apbp].  The CGN
   directs traffic to the fractional address towards that subscriber's
   GW device.  This method has the advantage that the more complicated
   aspects of the NAT function (ALGs, port-mapping, etc.) remain in the
   GW, augmented only by the restricted port-range allocated to the
   fractional address for that GW.  The CGN is then free to operate in a

Arkko & Townsley        Expires January 14, 2010                [Page 7]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   fairly stateless manner, forwarding based on IP address and port
   ranges and not tracking any individual flows from within the
   subscriber network.  There are obvious scaling benefits to this
   approach within the CGN node, with the tradeoff of complexity in
   terms of the number of nodes and protocols that must work together in
   an interoperable manner.  Further, the GW is still receiving a global
   IPv4 address, albeit only a "portion" of one in terms of available
   port usage.  There are still outstanding questions in terms of how to
   handle protocols that run directly over IP and cannot use the divided
   port number ranges, and handling of fragmented packets, but the
   benefit is that we are no longer burdened by two layers of NAT as in

   Not all access architectures provide a natural point to point link
   between the GW and CGN to tie into.  Further, the CGN may not be
   incorporated into the IP Edge device in networks that do have point-
   to-point links.  For these cases, we can build our own point-to-point
   link using a tunnel.  A tunnel is essentially a point to point link
   that we create when needed [I-D.touch-intarea-tunnels].  This is
   illustrated in Figure 4.

                 one frac addr            one public addr
                    +----+                   +---+  +-------------+
   IPv4 host(s)-----+ GW +======tunnel=======+CGN+--+IPv4 Internet|
                    +----+                   +---+  +-------------+

   <---private v4--->            NAT             <----public v4--->
                            over a tunnel)

          Figure 4: Point-to-point link created through a tunnel

   Figure 4 is essentially the same as Figure 3, except the data link is
   created with a tunnel.  The tunnel could created in any number of
   ways depending on the underlying network.

   At this point, we have used a tunnel or point-to-point link with
   coordinated operation between the GW and CGN in order to keep most of
   the NAT functionality in the GW.

   Given the assumption of a point-to-point link between GW and CGN, the
   CGN could perform the NAT function, allowing private, overlapping,
   space to all subscribers.  For example, each subscriber GW may be
   assigned the same address space (or all RFC 1918 [RFC1918]

Arkko & Townsley        Expires January 14, 2010                [Page 8]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   space for that matter).  The GW then becomes a simple "tunneling
   router" and the CGN takes on the full NAT role.  One can think of
   this design as effectively a layer-3 VPN, but with Virtual-NAT tables
   rather than Virtual-Routing tables.

2.1.3.  Recommendation

   This section dealt strictly with the problem of reaching the IPv4
   Internet with limited public address space for each device in a
   network.  We explored combining NAT functions and tunnels between the
   GW and CGN to obtain similar results with different design tradeoffs.
   The methods presented can be summarized as:

   a. Double-NAT (NAT444)

   b. Single-NAT at CGN with a subnet and routing at the GW

   c. Tunnel/link + Fractional IP (NAT at GW, port-routing at CGN)

   d. Tunnel/link + Single NAT with overlapping RFC 1918 ("Virtual NAT"
      tables and routing at the GW)

   In all of the above, the GW could be logically moved into a single
   host, potentially eliminating one level of NAT by that action alone.
   As long as the hosts themselves need only a single IPv4 address,
   methods b and d obviously are of little interest.  This leaves
   methods a and c as the more interesting methods in cases where there
   is no analogous GW device (such as a campus network).

   This document recommends the development of new guidelines and
   specifications to address the above methods.  Cases where the home
   gateway both can and cannot be modified should be addressed.

2.2.  Running out of IPv4 Private Address Space

   In addition to public address space depletion, very large privately
   addressed networks are reaching exhaustion of RFC 1918 space on local
   networks as well.  Very large service provider networks are prime
   candidates for this.  Private address space is used locally in ISPs
   for a variety of things, including:

   o  control and management of service provider devices in subscriber
      premises (cable modems, set-top boxes, and so on) and

   o  addressing the subscriber's NAT devices in a double NAT
      arrangement, and

Arkko & Townsley        Expires January 14, 2010                [Page 9]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   o  "walled garden" data, voice, or video services.

   Some providers deal with this problem by dividing their network into
   parts, each on its own copy of the private space.  However, this
   limits the way services can be deployed and what management systems
   can reach what devices.  It is also operationally complicated in the
   sense that the network operators have to understand which private
   scope they are in.

   Tunnels were used in the previous section to facilitate distribution
   of a single global IPv4 address across multiple endpoints without
   using NAT, or to allow overlapping address space to GWs or hosts
   connected to a CGN.  The kind of tunnel or link was not specified.
   If the tunnel used carries IPv4 over IPv6, the portion of the IPv6
   network traversed naturally need not be IPv4 capable, and need not
   utilize IPv4 addresses, private or public, for the tunnel traffic to
   traverse the network.  This is shown in Figure 5.

                            IPv6-only network
                    +----+                     +---+  +-------------+
   IPv4 host--------+ GW +=======tunnel========+CGN+--+IPv4 Internet|
                    +----+                     +---+  +-------------+

   <---private v4---->  <-----  v4 over v6 ----->  <---public v4---->

             Figure 5: Running IPv4 over IPv6-only network

   Each of the four approaches (a, b, c and d) from the Section 2.1
   scenario could be applied here, and for brevity each iteration is not
   specified in full here.  The models are essentially the same, just
   that the tunnel is over an IPv6 network and carries IPv4 traffic.
   Note that while there are numerous solutions for carrying IPv6 over
   IPv4, this reverse mode is somewhat of an exception (one notable
   exception being the Softwire working group, as seen in [RFC4925]).

   Once we have IPv6 to the GW (or host, if we consider the GW embedded
   in the host), enabling IPv6 and IPv4 over the IPv6 tunnel allows for
   Dual-Stack operation at the host or network behind the GW device.
   This is depicted in Figure 6:

Arkko & Townsley        Expires January 14, 2010               [Page 10]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

                   +----+                               +-------------+
     IPv6 host-----+    |            +------------------+IPv6 Internet|
                   |    +---IPv6-----+                  +-------------+
   Dual-Stack host-+ GW |
                   |    |                        +---+  +-------------+
     IPv4 host-----+    +===v4 over v6 tunnel====+CGN+--+IPv4 Internet|
                   +----+                        +---+  +-------------+

   <-----------private v4 (partially in tunnel)-->NAT<---public v4---->
   <-----------------------------public v6---------------------------->

        Figure 6: "Dual-Stack Lite" operation over an IPv6-only network

   In [I-D.durand-dual-stack-lite] this is referred to as "Dual-Stack
   Lite" bowing to the fact that it is Dual-Stack at the host, but not
   at the network.  As introduced in Section 2.1, if the CGN here is a
   full functioning NAT, a Dual-Stack Lite host can run IPv4-only and
   IPv6-enabled applications across an IPv6-only network without
   provisioning a unique IPv4 addresses to each host.  In fact, every
   host may have the same address.

   While the high-level problem space in this scenario is to alleviate
   local usage of IPv4 addresses within a service provider network, the
   solution direction identified with IPv6 has interesting operational
   properties that should be pointed out.  By tunneling IPv4 over IPv6
   across the service provider network, the separate problems of
   transition the service provider network to IPv6, deploying IPv6 to
   subscribers, and continuing to provide IPv4 service can all be
   decoupled.  The service provider could deploy IPv6 internally, turn
   off IPv4 internally, and still carry IPv4 traffic across the IPv6
   core for end users.  In the extreme case, all of that IPv4 traffic
   need not be provisioned with different IPv4 addresses for each
   endpoint as there is not IPv4 routing or forwarding within the
   network.  Thus, there are no issues with IPv4 renumbering, address
   space allocation, etc. within the network itself.

   It is recommended that the IETF develop tools to address this
   scenario for both a host and GW.  It is assumed that both endpoints
   of the tunnel can be modified to support these new tools.

2.3.  Enterprise IPv6 Only Networks

   This scenario is about allowing an IPv6-only host or a host which has
   no interfaces connected to an IPv4 network, to reach servers on the
   IPv4 internet.  This is an important scenario for what we sometimes
   call "greenfield" deployments.  One example is an enterprise network
   that wishes to operate only IPv6 for operational simplicity, but

Arkko & Townsley        Expires January 14, 2010               [Page 11]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   still wishes to reach the content in the IPv4 Internet.  For
   instance, a new office building may be provisioned with IPv6-only.
   This is shown in Figure 7.

                             +----+                  +-------------+
                             |    +------------------+IPv6 Internet+
                             |    |                  +-------------+
   IPv6 host-----------------+ GW |
                             |    |                  +-------------+
                             |    +------------------+IPv4 Internet+
                             +----+                  +-------------+

   <-------------------------public v6----------------------------->
   <-------public v6--------->NAT<----------public v4-------------->

            Figure 7: Enterprise IPv6-only network

   Other cases that have been mentioned include "greenfield" wireless
   service provider networks and sensor networks.  This bears a striking
   resemblance to Section 2.2 as well, if one considers the service
   provider network to simply be a very special kind of enterprise

   In the Section 2.2 scenario, we dipped into design space enough to
   illustrate that the service provider was able to implement an IPv6-
   only network to ease their addressing problems via tunneling.  This
   came at the cost of touching two devices on the edges of this
   network; both the GW and the CGN have to support IPv6 and the
   tunneling mechanism over IPv6.  The greenfield enterprise scenario is
   different from that one in the sense that there is only one place
   that the enterprise can easily modify: the border between its network
   and the IPv4 Internet.  Obviously, the IPv4 Internet operates the way
   it already does.  But in addition, the hosts in the enterprise
   network are commercially available devices, personal computers with
   existing operating systems.  While we consider in this scenario that
   all of the devices on the network are "modern" Dual-Stack capable
   devices, we do not want to have to rely upon kernel-level
   modifications to these OSes.  This restriction drives us to a "one
   box" type of solution, where IPv6 can be translated into IPv4 to
   reach the public Internet.  This is one situation where new or
   improved IETF specifications could have an effect to the user
   experience in these networks.  In fairness, it should be noted that
   even a network-based solution will take time and effort to deploy.
   This is essentially, again, a tradeoff between one new piece of
   equipment in the network, or a cooperation between two.

Arkko & Townsley        Expires January 14, 2010               [Page 12]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   One approach to deal with this environment is to provide an
   application level proxy at the edge of the network (GW).  For
   instance, if the only application that needs to reach the IPv4
   Internet is the web, then a HTTP/HTTPS proxy can easily convert
   traffic from IPv6 into IPv4 on the outside.

   Another more generic approach is to employ an IPv6 to IPv4 translator
   device.  This is discussed in
   [I-D.wing-nat-pt-replacement-comparison].  NAT64 is an one example of
   a translation scheme falling under this category

   Translation will in most cases have some negative consequences for
   the end-to-end operation of Internet protocols.  For instance, the
   issues with Network Address Translation - Protocol Translation
   (NAT-PT) [RFC2766] have been described in [RFC4966].  It is important
   to note that the choice of translation solution and the assumptions
   about the network where they are used impact the consequences.  A
   translator for the general case has a number of issues that a
   translator for a more specific situation may not have at all.

   It is recommended that the IETF develop tools to address this
   scenario.  These tools need to allow existing IPv6 hosts to operate

2.4.  Reaching Private IPv4 Only Servers

   This section discusses the specific problem of IPv4-only capable
   server farms that no longer can be allocated a sufficient number of
   public addresses.  It is expected that for individual servers,
   addresses are going to be available for a long time in a reasonably
   easy manner.  However, a large server farm may require a large enough
   block of addresses that it is either not feasible to allocate one or
   it becomes economically desirable to use the addresses for other

   Another use case for this scenario involves a service provider that
   is capable of acquiring a sufficient number of IPv4 addresses, and
   has already done so.  However, the service provider also simply
   wishes to start to offer an IPv6 service but without yet touching the
   server farm by upgrading it to IPv6.

   One option available in such a situation is to move those servers and
   their clients to IPv6.  However, moving to IPv6 is not just the cost
   of the IPv6 connectivity, but the cost of moving the application
   itself away to IPv6.  So, in this case the server farm is IPv4 only,
   there is an increasing cost for IPv4 connectivity, and an expensive
   bill for moving server infrastructure to IPv6.  What can be done?

Arkko & Townsley        Expires January 14, 2010               [Page 13]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   If the clients are IPv4-only as well, the problem is a hard one, and
   dealt with in more depth in Section 2.5.  However, there are
   important cases where large sets of clients are IPv6-capable.  In
   these cases it is possible to place the server farm in private IPv4
   space and arrange some of gateway service from IPv6 to IPv4 to reach
   the servers.  This is shown in Figure 8.

   IPv6 Host(s)-------(Internet)-----+ GW +------Private IPv4 Servers

   <---------public v6--------------->NAT<------private v4---------->

          Figure 8: Reaching servers in private IPv4 space

   One approach to implement this is to use NAT64 to translate IPv6 into
   private IPv4 addresses.  The private IPv4 addresses are mapped into
   IPv6 addresses within a known prefix(es).  The GW at the edge of the
   server farm is aware of the mapping, as is DNS.  AAAA records for
   each server name is given an IPv6 address that corresponds to the
   mapped private IPv4 address.  Thus, each privately addressed IPv4
   server is given a public IPv6 presentation.  No DNS application level
   gateway (DNS-ALG) is needed in this case, contrary to what NAT-PT
   required, for instance.

   This is very similar to Section 2.3 where we typically think of a
   small site with IPv6 needing to reach the public IPv4 Internet.  The
   difference here is that we assume not a small IPv6 site, but the
   whole of the IPv6 Internet needing to reach a small IPv4 site.  This
   example was driven by the enterprise network with IPv4 servers, but
   could be scaled down to the individual subscriber home level as well.
   Here, the same technique could be used to, say, access an IPv4 webcam
   in the home from the IPv6 internet.  All that is needed is the
   ability to update AAAA records appropriately, an IPv6 client (which
   could use Teredo [RFC4380] or some other method to obtain IPv6
   reachability), and the NAT64 mechanism.  In this sense, this method
   looks much like a "NAT/FW bypass" function.

   An argument could be made that since the host is likely Dual-Stack,
   existing port mapping services or NAT traversal techniques could be
   used to reach the private space instead of IPv6.  This would have to
   be done anyway if the hosts are not all IPv6-capable or connected.
   However, in the case that they are, the alternative techniques force
   additional limitations on the use of port numbers.  In the case of
   IPv6 to IPv4 translation, the full port space would be available for

Arkko & Townsley        Expires January 14, 2010               [Page 14]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   each server even in the private space.

   It is recommended that the IETF develop tools to address this
   scenario.  These tools need to allow existing IPv4 servers to operate

2.5.  Reaching IPv6 Only Servers

   This scenario is predicted to become increasingly important as IPv4
   global connectivity sufficient for supporting server-oriented content
   becomes significantly more difficult to obtain than global IPv6
   connectivity.  Historically, the expectation has been that for
   connectivity to IPv6-only devices, devices would either need to be
   IPv6 connected, or Dual-Stack with the ability to setup an IPv6 over
   IPv4 tunnel in order to access the IPv6 Internet.  Many "modern"
   device stacks have this capability, and for them this scenario does
   not present a problem as long as a suitable gateway to terminate the
   tunnel and route the IPv6 packets is available.  But, for the server
   operator, it may be a difficult proposition to leave all IPv4-only
   devices without reachability.  Thus, if a solution for IPv4-only
   devices to reach IPv6-only servers were realizable, the benefits
   would be clear.  Not only could servers move directly to IPv6 without
   trudging through a difficult Dual-Stack period, but they could do so
   without risk of losing connectivity with the IPv4-only Internet.

   Unfortunately, realizing this goal is complicated by the fact that
   IPv4 to IPv6 is considered "hard" since of course IPv6 has a much
   larger address space than IPv4.  Thus, representing 128 bits in 32
   bits is not possible, barring the use of techniques similar to NAT64,
   which uses IPv6 addresses to represent IPv4 addresses as well.

   The main questions about this scenario are about the timing and
   priority.  While the expectation that this scenario may be of
   importance one day is readily acceptable, at time of this writing
   there are little or no IPv6-only servers of importance beyond
   contrived cases that the authors are aware of.  The difficulty of
   making a decision about this case is that, quite possibly, when there
   is sufficient pressure on IPv4 in order to see IPv6-only servers, the
   vast majority of hosts either have IPv6 connectivity, or the ability
   to tunnel IPv6 over IPv4 one way or another.

   This discussion makes assumptions about what is a "server" as well.
   For the majority of applications seen on the IPv4 Internet to date,
   this distinction has been more or less clear.  This is perhaps in no
   small part due to the overhead today in creating a truly end to end
   application in the face of the fragmented addressing and reachability
   brought on by the various NATs and firewalls employed today.  This is
   beginning to shift, however, as we see more and more pressure to

Arkko & Townsley        Expires January 14, 2010               [Page 15]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   connect people to one another in an end-to-end fashion -- with peer-
   to-peer techniques, for instance -- rather than simply content server
   to client.  Thus, if we consider an "IPv6-only server" as what we
   classically consider as an "IPv4 server" today, there may not be a
   lot of demand for this in the near future.  However, with a more
   distributed model of the Internet in mind there may be more
   opportunities to employ IPv6-only "servers" that we would normally
   extrapolate based on past experience with applications.

   It is recommended that IETF addresses this scenario, though perhaps
   with a slightly lower priority than the others.  In any case, when
   new tools are developed to support this, it should be obvious that we
   cannot assume any support for updating legacy IPv4 hosts in order to
   reach the IPv6-only servers.

3.  Security Considerations

   Security aspects of the individual solutions are discussed in more
   depth elsewhere, for instance in
   [I-D.wing-nat-pt-replacement-comparison].  It is important to note
   that some of the solutions may have impacts on how IPsec or DNS
   Security can work through translation devices.  Minimization or even
   elimination of such problems is essential.

4.  IANA Considerations

   This document has no actions for IANA.

5.  Conclusions

   The authors believe that the scenarios outlined in this document are
   among the top of the list of those that should to be addressed by the
   IETF community in short order.  For each scenario, there are clearly
   different solution approaches with implementation, operations and
   deployment tradeoffs.  Further, some approaches rely on existing or
   well-understood technology, while some require new protocols and
   changes to established network architecture.  It is essential that
   these tradeoffs be considered, understood by the community at large,
   and in the end well-documented as part of the solution design.

   After writing the initial version of this document, the Softwire
   working group was rechartered to address Section 2.2 scenario with a
   combination of existing tools (tunneling, IPv4 NATs) and some minor
   new ones (DHCP options) [I-D.durand-dual-stack-lite].  Similarly, the
   Behave working group was rechartered to address scenarios from

Arkko & Townsley        Expires January 14, 2010               [Page 16]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

   Section 2.3, Section 2.4, and Section 2.5.  At the time this document
   is being published, proposals to address scenarios from Section 2.1
   are still under consideration for new IETF work items.

   This document set out to list scenarios that are important for the
   Internet community.  While it introduces some design elements in
   order to understand and discuss tradeoffs, it does not list detailed
   requirements.  In large part, the authors believe that exhaustive and
   detailed requirements would not be helpful at the expense of
   embarking on solutions given our current state of affairs.  We do not
   expect any of the solutions to be perfect when measured from all
   vantage points.  When looking for opportunities to deploy IPv6,
   reaching for perfection too far could become its own demise if we are
   not attentive to this.  Our goal with this document is to support
   development of tools to help minimize the tangible problems that we
   are experiencing now, as well as those that we can best anticipate
   down the road, in hopes of steering the Internet on its best course
   from here.

6.  References

6.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

6.2.  Informative References

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

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC4925]  Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
              Problem Statement", RFC 4925, July 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.

Arkko & Townsley        Expires January 14, 2010               [Page 17]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009

              Wing, D., Ward, D., and A. Durand, "A Comparison of
              Proposals to Replace NAT-PT", Internet-Draft wing-nat-pt-
              replacement-comparison-00, September 2008.

              Durand, A., "Dual-stack lite broadband deployments post
              IPv4 exhaustion", draft-durand-dual-stack-lite-00 (work in
              progress), July 2008.

              Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network
              Address and Protocol Translation from IPv6 Clients to IPv4
              Servers", draft-bagnulo-behave-nat64-03 (work in
              progress), March 2009.

              Touch, J. and M. Townsley, "Tunnels in the Internet
              Architecture", draft-touch-intarea-tunnels-00 (work in
              progress), July 2008.

              Despres, R., "A Scalable IPv4-IPv6 Transition Architecture
              Need for an address-port-borrowing-protocol (APBP)",
              draft-despres-v6ops-apbp-01 (work in progress), July 2008.

              Huston, G., "The IPv4 Internet Report", available
              at, August 2008.

              Nishitani, T., Miyakawa, S., Nakagawa, A., and H. Ashida,
              "Common Functions of Large Scale NAT (LSN)",
              draft-nishitani-cgn-02 (work in progress), June 2009.

              Shirasaki, Y., Miyakawa, S., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "ISP Shared Address",
              draft-shirasaki-isp-shared-addr-02 (work in progress),
              March 2009.

Appendix A.  Acknowledgments

   Discussions with a number of people including Dave Thaler, Thomas
   Narten, Marcelo Bagnulo, Fred Baker, Remi Depres, Lorenzo Colitti,
   Dan Wing, and feedback during the Internet Area open meeting at
   IETF-72 were essential to the creation of the content in this

Arkko & Townsley        Expires January 14, 2010               [Page 18]

Internet-Draft         IPv4 and IPv6 Co-Existence              July 2009


Authors' Addresses

   Jari Arkko
   Jorvas  02420


   Mark Townsley
   Paris  75006


Arkko & Townsley        Expires January 14, 2010               [Page 19]