IPv4 Run-Out and IPv4-IPv6 Co-Existence ScenariosEricssonJorvas02420Finlandjari.arkko@piuha.netCiscoParis75006Francetownsley@cisco.comIPv4address depletionIPv6translationNAT-PTdual-stackSoftwireBehaveNATNAT444
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 coexistence work. This document is published as
a historical record of the thinking at the time.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" 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 ,
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 as detailed in
. 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.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 protocol.
The five scenarios are:Reaching the IPv4 Internet with less than one global IPv4
address per subscriber or subscriber household available
().Running a large network needing more addresses than those available in
private RFC 1918 address space
().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
().Reaching one or more privately addressed IPv4 only servers via IPv6
().Accessing IPv6-only servers from IPv4 only clients
().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 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.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 . The CGN's insertion
into the ISP network is shown in Figure 2.This solution approach is known as "NAT444" or "Double-NAT" and
is discussed further in
.It is important to note that while multiple levels of multiplexing
of IPv4 addresses is occurring here, there is no aggregation of NAT
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
. 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 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.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.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
. 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 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 NAT444. 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
. This is illustrated in
Figure 4.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 10.0.0.0/8 address space (or all RFC 1918 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.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:
Double-NAT (NAT444)Single-NAT at CGN with a subnet and routing at the GWTunnel/link + Fractional IP (NAT at GW, port-routing at CGN)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.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:
control and management of service provider devices in subscriber
premises (cable modems, set-top boxes, and so on) andaddressing the subscriber's NAT devices in a double NAT arrangement, and"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. Each of the four approaches (a, b, c and d) from the
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 ). 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:In 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
, 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.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 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.Other cases that have been mentioned include "greenfield" wireless service
provider networks and sensor networks. This bears a striking
resemblance to as well, if
one considers the service provider network to simply be a very special kind
of enterprise network.In the 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.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
. 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)
have been described in .
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 unchanged.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 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?If the clients are IPv4-only as well, the problem is a hard one,
and dealt with in more depth in
. 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.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 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 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 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.
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
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.Security aspects of the individual solutions are discussed in more
depth elsewhere, for instance in
. 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.This document has no actions for IANA.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 scenario with a
combination of existing tools (tunneling, IPv4 NATs) and some minor
new ones (DHCP options)
. Similarly, the Behave working group was rechartered
to address scenarios from
,
, and
.
At the time this document is being published, proposals to address
scenarios from
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.
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 document.