





Internet Engineering Task Force (IETF)                          J. Arkko
Request for Comments: 6127                                      Ericsson
Category: Informational                                      M. Townsley
ISSN: 2070-1721                                                    Cisco
                                                                May 2011


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

Abstract

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

   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 co-existence work.  This document is published as a
   historical record of the thinking at the time, but hopefully will
   also help readers understand the rationale behind current IETF tools
   for co-existence and transition.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6127.







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Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Scenarios .......................................................4
      2.1. Reaching the IPv4 Internet .................................4
           2.1.1. NAT444 ..............................................5
           2.1.2. Distributed NAT .....................................6
           2.1.3. Recommendation ......................................8
      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 ................................14
   3. Security Considerations ........................................16
   4. Conclusions ....................................................16
   5. References .....................................................17
      5.1. Normative References ......................................17
      5.2. Informative References ....................................17
   Appendix A. Acknowledgments .......................................20

1.  Introduction

   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 co-existence work.  This document is published as a
   historical record of the thinking at the time, but hopefully will
   also help readers understand the rationale behind current IETF tools
   for co-existence and transition.








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   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 that 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, and not
   replace, existing ones.  For instance, dual-stack continues to be the
   most 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.






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2.  Scenarios

   This section identifies five deployment scenarios that 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 consider for each scenario what
   type of new equipment must be introduced in the network, and where;
   which nodes must be changed in some way; and which nodes must work
   together in an interoperable manner via a new or existing protocol.

   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
   to allow more hosts behind the gateway (GW) than 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 the
   Internet, but nonetheless is the dominant method of access today,
   particularly to residential subscribers.



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   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, however, this
   becomes operational overhead for the Internet Service Provider (ISP),
   for which the ISP will no longer be able to rely on the customer and
   the seller of the GW device.

   This new address translation node has been termed a "Carrier Grade
   NAT", or CGN [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 approach is known as "NAT444" or "Double-NAT" and is discussed
   further in [NAT-PT].

   It is important to note that while multiplexing of IPv4 addresses is
   occurring here at multiple levels, there is no aggregation of NAT
   state between the GW and the CGN.  Every flow that is originated in
   the subscriber home is represented as duplicate state in the GW and
   the CGN.  For example, if there are 4 PCs in a subscriber home, each
   with 25 open TCP sessions, both the GW and the 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 that can accept a
   lease for a global IPv4 address would very likely accept a translated



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   IPv4 address as well, the CGN is neither transparent to the GW nor to
   the subscriber.  In short, it is a very different service model to
   offer a translated IPv4 address versus 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 [ISP-SHARED-ADDR], ambiguous routes in the GW could
   lead to misdirected or black-holed traffic.  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 [IPv4-SPACE-ISSUES].

   Network operations that 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, operations,
   accounting, logging and legal intercept, Quality of Service (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
   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



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   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 the GW.

   If the connection between the GW and the 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 the 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--->
                             (distributed,
                           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 [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 (Application Level Gateways (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 fairly stateless manner, forwarding
   traffic 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
   how to handle fragmented packets, but the benefit is that we are no
   longer burdened by two layers of NAT as in NAT444.





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   Not all access architectures provide a natural point-to-point link
   between the GW and the 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 [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--->
                            (distributed,
                            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 be 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 the CGN in order to keep
   most of the NAT functionality in the GW.

   Given the assumption of a point-to-point link between the GW and the
   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 10.0.0.0/8 address space (or all RFC 1918
   [RFC1918] 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 deals 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 the 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




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   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 methods 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).

   o  Addressing the subscriber's NAT devices in a Double-NAT
      arrangement.

   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.




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

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

             Figure 5: Running IPv4 over an 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:

                   +----+                               +-------------+
     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 [DUAL-STACK-LITE], this is referred to as "dual-stack lite",
   bowing to the fact that it is dual-stack at the gateway, but not at
   the network.  As introduced in Section 2.1, if the CGN here is a full
   functioning NAT, hosts behind a dual-stack lite gateway can support
   IPv4-only and IPv6-enabled applications across an IPv6-only network
   without provisioning a unique IPv4 address to each gateway.  In fact,
   every gateway may have the same address.

   While the high-level problem space in this scenario is how 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



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   of transitioning 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 the 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 that 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
   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 enterprise IPv6-
   only scenario bears a striking resemblance to the Section 2.2
   scenario as well, if one considers a particularly large enterprise
   network that begins to resemble a service provider network.

   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



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   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 operating systems.  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 on 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.

   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 an 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.  Different types of translation schemes are discussed in
   [NAT-PT], [RFC6144], [RFC6145], and [RFC6052].  NAT64 is one example
   of a translation scheme falling under this category [RFC6147]
   [RFC6146].

   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 in which it is 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
   unchanged.








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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
   purposes.

   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 (that is, without upgrading the server farm to IPv6).

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

   If the clients are IPv4-only as well, the problem is a hard one.
   This issue is 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 the 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 one or more known prefixes.  The GW at the edge
   of the server farm is aware of the mapping, as is the Domain Name







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   System (DNS).  AAAA records for each server name are 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 Application Level Gateway for DNS (DNS-ALG) is
   needed in this case, contrary to what NAT-PT would require, for
   instance.

   This is very similar to the Section 2.3 scenario 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/firewall 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 cases where the hosts are all IPv6-capable, 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 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
   unchanged.

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 set up 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



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   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 regarding this scenario are about timing and
   priority.  While the expectation that this scenario may be of
   importance one day is readily acceptable, at the time of this
   writing, there are few or no IPv6-only servers of importance (beyond
   some 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 such that we see IPv6-only
   servers, the vast majority of hosts will either have IPv6
   connectivity or the ability to tunnel IPv6 over IPv4 in one way or
   another.

   This discussion makes assumptions about what a "server" is as well.
   For the majority of applications seen on the IPv4 Internet to date,
   this distinction has been more or less clear.  This clarity 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.  However, current notions of a "server" are beginning
   to shift, as we see more and more pressure to 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 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 from based on past
   experience with applications.

   It is recommended that the IETF address 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.








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3.  Security Considerations

   Security aspects of the individual solutions are discussed in more
   depth elsewhere, for instance in [DUAL-STACK-LITE], [RFC6144],
   [RFC6147], [RFC6145], [RFC6146], [NAT-PT], and [RFC4966].  This
   document highlights just three issues:

   o  Any type of translation may have an impact on how certain
      protocols can pass through.  For example, IPsec needs support for
      NAT traversal, and the proliferation of NATs implies an even
      higher reliance on these mechanisms.  It may also require
      additional support for new types of translation.

   o  Some solutions have a need to modify results obtained from DNS.
      This may have an impact on DNS security, as discussed in
      [RFC4966].  Minimization or even elimination of such problems is
      essential, as discussed in [RFC6147].

   o  Tunneling solutions have their own security issues, for instance
      the need to secure tunnel endpoint discovery or to avoid opening
      up denial-of-service or reflection vulnerabilities [RFC6169].

4.  Conclusions

   The authors believe that the scenarios outlined in this document are
   among the top of the list of those that should 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 the Section 2.2 scenario
   with a combination of existing tools (tunneling, IPv4 NATs) and some
   minor new ones (DHCP options) [DUAL-STACK-LITE].  Similarly, the
   Behave working group was rechartered to address scenarios from
   Sections 2.3, 2.4, and 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



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   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 too far for perfection could result in losing these
   opportunities if we are not attentive.  Our goal with this document
   is to support the development of tools to help minimize the tangible
   problems that we are experiencing now, as well as those problems that
   we can best anticipate down the road, in hopes of steering the
   Internet on its best course from here.

5.  References

5.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., de 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.

5.2.  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.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

   [NAT-PT]   Wing, D., Ward, D., and A. Durand, "A Comparison of
              Proposals to Replace NAT-PT", Work in Progress,
              September 2008.






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   [DUAL-STACK-LITE]
              Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", Work in Progress, May 2011.

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

   [INTAREA-TUNNELS]
              Touch, J. and M. Townsley, "Tunnels in the Internet
              Architecture", Work in Progress, March 2010.

   [v6OPS-APBP]
              Despres, R., "A Scalable IPv4-IPv6 Transition Architecture
              Need for an Address-Port-Borrowing-Protocol (APBP)", Work
              in Progress, July 2008.

   [HUSTON-IPv4]
              Huston, G., "IPv4 Address Report", available
              at http://www.potaroo.net, December 2010.

   [NISHITANI-CGN]
              Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
              A., and H. Ashida, "Common Requirements for IP Address
              Sharing Schemes", Work in Progress, March 2011.

   [ISP-SHARED-ADDR]
              Yamagata, I., Miyakawa, S., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "ISP Shared Address", Work in Progress,
              September 2010.









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   [IPv4-SPACE-ISSUES]
              Azinger, M. and L. Vegoda, "Issues Associated with
              Designating Additional Private IPv4 Address Space", Work
              in Progress, January 2011.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.












































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Appendix A.  Acknowledgments

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

Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   EMail: jari.arkko@piuha.net


   Mark Townsley
   Cisco
   Paris  75006
   France

   EMail: townsley@cisco.com



























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