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rfc:rfc6127

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.

Arkko & Townsley Informational [Page 1] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

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.

Arkko & Townsley Informational [Page 2] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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.

Arkko & Townsley Informational [Page 3] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

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

Arkko & Townsley Informational [Page 5] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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

Arkko & Townsley Informational [Page 6] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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.

Arkko & Townsley Informational [Page 7] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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

Arkko & Townsley Informational [Page 10] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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

Arkko & Townsley Informational [Page 14] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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.

Arkko & Townsley Informational [Page 15] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

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

Arkko & Townsley Informational [Page 16] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 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.

Arkko & Townsley Informational [Page 17] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 [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.

Arkko & Townsley Informational [Page 18] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

 [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.

Arkko & Townsley Informational [Page 19] RFC 6127 IPv4 and IPv6 Co-Existence May 2011

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

Arkko & Townsley Informational [Page 20]

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