Network Working Group M. Bagnulo
Internet-Draft Huawei Labs at UC3M
Intended status: Standards Track November 11, 2007
Expires: May 14, 2008
IPv6 - IPv4 Translators (NAT64) - Problem Statement and Analysis
draft-bagnulo-v6ops-6man-nat64-pb-statement-00
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Abstract
RFC 4966 published on July 2007 deprecates the NAT-PT tool, the
mechanism defined by the IETF to enable communications between IPv4
only nodes with IPv6 only nodes, letting the dual-stack approach as
the preferred mechanism to enable nodes to be able to communicate
with v6 and v4 nodes. However, there are several reasons why the
dual stack approach may not be adequate for a number of scenarios.
For once, the dual-stack approach imposes the management of two
networks in a site, the v6 one and the v4 one, increasing the costs
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of using IPv6. In addition, as IPv4 public address space is
depleted, it will no longer possible to access to IPv4 public
addresses, making dual stack nodes even less attractive. It is then
considered necessary to explore alternative mechanisms that enable
communication of v6 nodes and v4 nodes, other than the deprecated
NAT-PT. In order to do that, the first that is needed is to
understand what capabilities are required and what constrains affect
the design space. The goal of this document is to state the
different capabilities that can be required to IPv4 - IPv6
translators (hereafter called NAT64) and the constraints that affect
the solution space.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Supported application behavior . . . . . . . . . . . . . . . . 4
4. Placement of the NAT64 mechanisms . . . . . . . . . . . . . . . 5
5. v4 addressing consideration . . . . . . . . . . . . . . . . . . 6
6. Name-space considerations . . . . . . . . . . . . . . . . . . . 7
7. Other considerations . . . . . . . . . . . . . . . . . . . . . 8
7.1. Transparency considerations . . . . . . . . . . . . . . . . 8
8. Security considerations . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
RFC 4966 published on July 2007 deprecates the NAT-PT tool, the
mechanism defined by the IETF to enable communications between IPv4
only nodes with IPv6 only nodes, letting the dual-stack approach as
the preferred mechanism to enable nodes to be able to communicate
with v6 and v4 nodes. However, there are several reasons why the
dual stack approach may not be adequate for a number of scenarios.
For once, the dual-stack approach imposes the management of two
networks in a site, the v6 one and the v4 one, increasing the costs
of using IPv6. In addition, as IPv4 public address space is
depleted, it will no longer possible to access to IPv4 public
addresses, making dual stack nodes even less attractive. It is then
considered necessary to explore alternative mechanisms that enable
communication of v6 nodes and v4 nodes, other than the deprecated
NAT-PT. In order to do that, the first that is needed is to
understand what capabilities are required and what constrains affect
the design space. The goal of this document is to state the
different capabilities that can be required to IPv4 - IPv6
translators (hereafter called NAT64) and the constraints that affect
the solution space.
2. Scenario
The scenario of operation of NAT64-type of mechanisms is the
following. We have a IPv4-only node in the v4 Internet and an IPv6-
only node in the v6 Internet. The goal is to enable communications
between these two nodes. In order to do that, we install a NAT64 box
with two interfaces, one in the v4 Internet and the other one in the
v6 Internet. We assume that there is v6 communication between the
IPv6-only node and the v6 interface of the NAT64 and that there is v4
connectivity between the IPv4-only node and the v4 interface of the
NAT64 box. The type of v4 addressing that is used in the v4-only
node and in the v4 interface of the NAT64 is part of the design
choices that are analyzed below.
+----------------------------------+-------------------------------+
| v4 Internet | v6 Internet |
| +------+ +------------+ +------+ |
| | IPv4 | | NAT64 | | IPv6 | |
| | only |-------------------| v4 | v6 |---------------| only | |
| | node | | Translator | | node | |
| +------+ +------------+ +------+ |
| | |
+----------------------------------+-------------------------------+
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3. Supported application behavior
The general purpose of NAT64 type of mechanisms is to enable
communication between a v4-only node and a v6-only node. However,
there is wide range of type of communication, when considering how
they handle the IP addresses. So, in order to properly characterize
the problem in hand, we need to do a more depth analysis of the
different application behavior in terms of the usage of their IP
addresses. We will next present a taxonomy of the behavior of the
application with respect of how they use the IP address and the
support of the different type of behavior will impose a different set
of constraints to the design of a NAT64 mechanisms. It is then
important to decide which type of application behavior will be
supported before starting to design a NAT64 mechanism. The proposed
taxonomy is heavily based on the one presented in section 1.1 of
draft-ietf-shim6-app-refer-00.txt.
The proposed application behavior taxonomy is the following:
Short-lived local handle. The IP addresses is never retained by the
application. The only usage is for the application to pass it from
the DNS APIs (e.g., getaddrinfo()) and the API to the protocol stack
(e.g., connect() or sendto()). This type of communication can be
either initiated by the v4-only node or by the v6-only node,
resulting in two type of behaviors, v4-initiated short lived local
handle and v6-initiated short lived local handle.
Long-lived application associations. The IP address is retained by
the application for several instances of communication. However, it
is always the same node that initiates the communication. This type
of communication can be either initiated by the v4-only node or by
the v6-only node, resulting in two type of behaviors, v4-initiated
long-lived associations and v6-initiated long-lived associations.
Callbacks. The application at one end retrieves the IP address of
the peer and uses that to later communicate "back" to the peer. This
type of communication can be either initiated by the v4-only node or
by the v6-only node, resulting in two type of behaviors, v4-initiated
callback, meaning that the initial communication is initiated by the
v6-only node, and later the v4-only node initiates the callback, and
v6-initiated callback, meaning that the initial communication is
initiated by the v4-only node, and later the v6-only node initiates
the callback .
Referrals. In an application with more than two parties, party B
takes the IP address of party A and passes that to party C. After
this party C uses the IP address to communicate with A. In this type
of communication, the following 6 sub-cases are possible.
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o A and B are v6-only nodes and C is a v4-only node;
o A and C are v6-only nodes and B is a v4-only node,
o B and C are v6-only nodes and A is a v4-only node,
o A and B are v4-only nodes and C is a v6-only node;
o A and C are v4-only nodes and B is a v6-only node,
o B and C are v4-only nodes and A is a v6-only node,
"Identity" comparison. Some applications might retain the IP
address, not as a means to initiate communication as in the above
cases, but as a means to compare whether a peer is the same as
another peer. While this is insecure in general, it might be
something which is used e.g., when TLS is used. This type of
communication results in two sub-cases, when the v4-only node
performs comparison of the v6-only node identity, and when the v6-
only node performs comparison of the v4-only node identity
Discussion: is there another type of application that embed IP
addresses in the application data that doesn't fit in the previous
cases?
4. Placement of the NAT64 mechanisms
Another aspect that is critical to design a NAT64 mechanism is the
placement of the mechanisms involved. In other words, what elements
can be modified/updated to support the NAT64 mechanisms. We assume
that the NAT64 box supports a set of mechanisms that are the core
part of the solution, but some approaches may require the
modification of additional elements. In particular, we can identify
the following additional elements that may require modification to
support a NAT64 approach.
Modification to v4-only nodes: one option is to require modification
to existent v4-only nodes in order to support the NAT64 mechanism.
This option would impose high deployment costs, because the existent
base of v4-only nodes is really big and there is no incentives for
the v4-only nodes to install such mechanism, since it seems unlikely
that v4-only nodes will have a strong need to communicate with v6-
only nodes (at least at the initial stages of v6 deployment).
However, it may be possible that this is the only viable solution for
supporting some type of application behavior.
Modification to v6-only nodes: Another option is to require
modifications to v6-only nodes. This option seems much more
acceptable, since the existent base of v6-nodes is relatively small
and there would be a strong incentive for v6-only nodes to
communicate with v4-only nodes, since most of the contents are
available only in v4 today. However, imposing modifications to v6-
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only nodes does make deployment of the solution more difficult, since
update of current v6-implementations is needed. In addition, there
is an architectural consideration, that we would be imposing v6-only
nodes to support "NAT hacks" in order to enable communication with
the v4 world, and that those modifications may stay forever, even
when the need for communication with the v4-Internet is not so
pressing.
Modification to both v4-only nodes and v6-only nodes. Another option
is to require updates to both v4-only nodes and also to v6-only
nodes. Needless to say that this would be the option with higher
deployment costs.
No modification. Another option is that the NAT64 mechanisms does
not require modification to any host and that the mechanism is fully
contained in the NAT64 box. This was the case of the previously
defined NAT-PT approach. However, it may be challenging to design a
solution with this constraint that does not suffer the limitations
suffered by the NAT-PT mechanism that lead the IETF community to
deprecate it.
Another consideration related to the modification imposed by a NAT64
approach is about what elements in the nodes need to be updated. In
particular, it is important to determine if only the IP layer on the
affected nodes needs to be modified or f other elements in the nodes
needs to be updated. In particular, it is critical to determine if
applications need to e modified in order to support the NAT64
mechanism.
5. v4 addressing consideration
We assume that both the v6-only nodes and the v6 interface of the
NAT64 boxes will have routable IPv6 addresses. However, on the v4
side, there are more options. Either the v4 interface of the NAT64
boxes and/or the v4-only nodes can have either v4 private addresses
or v4 public addresses. Actually, it is possible that the different
combinations make sense. It seems clear that the case where public
v4 addresses are used in both the v4 interface of the NAT64 box and
the v4-only nodes is relevant. The case where the v4-only node has a
private v4 address and the NAT64 box has a public address seems also
possible, but here it seems reasonable to assume that a NAT box will
exist between the v4 only node and the NAT64 box. The case where
both the v4 node and the NAT64 box have v4 private addresses could
also make sense, since this could apply to a scenario where a site
that has v4 private addresses and v6 addresses could try to use a
NAT64 box internally. The last case, where the v4 node has public
address and the NAT64 box has a private address seems harder to
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justify though.
Another consideration related to v4 addressing of the NAT64 approach
is the number of addresses required by the NAT64 box. It is possible
that some NAT64 approaches require a pool of v4 addresses instead of
a single v4 address. Considering the status of the v4 address space
consumption, it may not be feasible to use a NAT64 approach that
require a big number of v4 public addresses.
6. Name-space considerations
One of the major choices that are faced when designing a NAT4
mechanism that enable communication initiated by the v4-only node
towards a v6-only node. In this case, the v4 only node needs to
identify the v6 only node and the problem is that there is no mean to
permanently map the v6 address space in the v4 address space. So in
order to enable a v4-only node to identify a v6-only node a name
space other than the IPv4 address space is needed. We will next
discuss some options that could be considered to identify v6 nodes in
the v4 world.
A first option is to use IPv4 addresses to identify IPv6 nodes. The
problem is that the v6 address space is much bigger than the v4
address space, so it is not possible to do permanent mapping between
these two. This basically implies that dynamic mapping between a
given v4 address and different v6 addresses are established. While
this works for some type of application behavior, it does not support
others, such as communications initiated by a v4 node towards a v6
node in a general case (it is possible for a given subset of v6
nodes, but not as a general solution)
A second option is to use IPv6 addresses themselves. In this case,
the IPv4 node is aware of the IPv6 address of the destination and it
uses it to identify the target at the NAT64 box. This option would
likely imply modifications in the v4 nodes.
A third option is to use FQDN to identify nodes. In this case v4
nodes identify v6 nodes using FQDNs, which is already supported in
the v4 world. The difficulties with such a approach is that DNS ALG
are likely to be required.
A fourth option is to use a combination of IPv4 address, transport
protocol and port for identification of a v6 node or a v6 flow.
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7. Other considerations
7.1. Transparency considerations
Another design option that need to be addressed is whether the NAT64
box will be transparent or not to the end nodes. One option is that
the nodes are aware of the NAT64 box and they establish a
communication with it to configure their translation. Th other
option is that the NAT64 box is transparent and that the end nodes
just establish the communication (even if they know they are
communicating with a node that supports a different version of IP,
there is no explicit communication between the end nodes and the
NAT64 box).
8. Security considerations
TBD
Author's Address
Marcelo Bagnulo
Huawei Labs at Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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