Internet Engineering Task Force (IETF) J. Palet Martinez
Request for Comments: 8683 The IPv6 Company
Category: Informational November 2019
ISSN: 2070-1721
Additional Deployment Guidelines for NAT64/464XLAT in Operator and
Enterprise Networks
Abstract
This document describes how Network Address and Protocol Translation
from IPv6 Clients to IPv4 Servers (NAT64) (including 464XLAT) can be
deployed in an IPv6 network -- whether it's cellular ISP, broadband
ISP, or enterprise -- and the possible optimizations. This document
also discusses issues to be considered when having IPv6-only
connectivity, such as: a) DNS64, b) applications or devices that use
literal IPv4 addresses or non-IPv6-compliant APIs, and c) IPv4-only
hosts or applications.
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 candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8683.
Copyright Notice
Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Requirements Language
3. NAT64 Deployment Scenarios
3.1. Known to Work
3.1.1. Service Provider NAT64 with DNS64
3.1.2. Service Provider Offering 464XLAT Using DNS64
3.1.3. Service Provider Offering 464XLAT, without Using DNS64
3.2. Known to Work under Special Conditions
3.2.1. Service Provider NAT64 without DNS64
3.2.2. Service-Provider NAT64; DNS64 in IPv6 Hosts
3.2.3. Service-Provider NAT64; DNS64 in the IPv4-Only Remote
Network
3.3. Comparing the Scenarios
4. Issues to be Considered
4.1. DNSSEC Considerations and Possible Approaches
4.1.1. Not Using DNS64
4.1.2. DNSSEC Validator Aware of DNS64
4.1.3. Stub Validator
4.1.4. CLAT with DNS Proxy and Validator
4.1.5. ACL of Clients
4.1.6. Mapping Out IPv4 Addresses
4.2. DNS64 and Reverse Mapping
4.3. Using 464XLAT with/without DNS64
4.4. Foreign DNS
4.4.1. Manual Configuration of DNS
4.4.2. DNS Privacy/Encryption Mechanisms
4.4.3. Split DNS and VPNs
4.5. Well-Known Prefix (WKP) vs. Network-Specific Prefix (NSP)
4.6. IPv4 Literals and Non-IPv6-Compliant APIs
4.7. IPv4-Only Hosts or Applications
4.8. CLAT Translation Considerations
4.9. EAM Considerations
4.10. Incoming Connections
5. Summary of Deployment Recommendations for NAT64/464XLAT
6. Deployment of 464XLAT/NAT64 in Enterprise Networks
7. Security Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Example of Broadband Deployment with 464XLAT
Appendix B. CLAT Implementation
Appendix C. Benchmarking
Acknowledgements
Author's Address
1. Introduction
Stateful NAT64 [RFC6146] describes a stateful IPv6-to-IPv4
translation mechanism that allows IPv6-only hosts to communicate with
IPv4-only servers using unicast UDP, TCP, or ICMP by means of IPv4
public address sharing among multiple IPv6-only hosts. Unless
otherwise stated, references to NAT64 (function) in this document
should be interpreted as Stateful NAT64.
The translation of the packet headers is done using the IP/ICMP
translation algorithm defined in [RFC7915]; algorithmically
translating the IPv4 addresses to IPv6 addresses, and vice versa, is
done following [RFC6052].
DNS64 [RFC6147] is in charge of the synthesis of AAAA records from
the A records, so it only works for applications making use of DNS.
It was designed to avoid changes in both the IPv6-only hosts and the
IPv4-only server, so they can use a NAT64 function. As discussed in
Section 5.5 of [RFC6147], a security-aware and validating host has to
perform the DNS64 function locally.
However, the use of NAT64 and/or DNS64 presents three drawbacks:
1. Because DNS64 [RFC6147] modifies DNS answers, and DNSSEC is
designed to detect such modifications, DNS64 [RFC6147] may
potentially break DNSSEC, depending on a number of factors such
as the location of the DNS64 function (at a DNS server or
validator, at the end host, ...), how it has been configured, if
the end hosts are validating, etc.
2. Because of the need to use DNS64 [RFC6147] or an alternative
"host/application built-in" mechanism for address synthesis,
there may be an issue for NAT64 [RFC6146] because it doesn't work
when IPv4 literal addresses or non-IPv6-compliant APIs are being
used.
3. NAT64 alone was not designed to provide a solution for IPv4-only
hosts or applications that are located within a network and
connected to a service provider IPv6-only access link, as it was
designed for a very specific scenario (see Section 2.1 of
[RFC6144]).
The drawbacks discussed above may come into play if part of an
enterprise network is connected to other parts of the same network or
to third-party networks by means of IPv6-only connectivity. This is
just an example that may apply to many other similar cases. All of
them are deployment specific.
Accordingly, the use of "operator", "operator network", "service
provider", and similar terms in this document are interchangeable
with equivalent cases of enterprise networks; other cases may be
similar as well. This may be also the case for "managed end-user
networks".
Note that if all the hosts in a network were performing address
synthesis, as described in Section 7.2 of [RFC6147], some of the
drawbacks may not apply. However, it is unrealistic to expect that
in today's world, considering the high number of devices and
applications that aren't yet IPv6 enabled. In this document, the
case in which all hosts provide synthesis will be considered only for
specific scenarios that can guarantee it.
An analysis of stateful IPv4/IPv6 mechanisms is provided in
[RFC6889].
This document looks into different possible NAT64 [RFC6146]
deployment scenarios, including IPv4-IPv6-IPv4 (464 for short) and
similar ones that were not documented in [RFC6144], such as 464XLAT
[RFC6877] in operator (broadband and cellular) and enterprise
networks; it provides guidelines to avoid operational issues.
This document also explores the possible NAT64 deployment scenarios
(split in "known to work" and "known to work under special
conditions"), providing a quick and generic comparison table among
them. Then, the document describes the issues that an operator needs
to understand, which will allow the best approach/scenario to be
defined for each specific network case. A summary provides some
recommendations and decision points. A section with clarifications
on the usage of this document for enterprise networks is also
provided. Finally, Appendix A provides an example of a broadband
deployment using 464XLAT and hints for a customer-side translator
(CLAT) implementation.
[RFC7269] already provides information about NAT64 deployment options
and experiences. This document and [RFC7269] are complementary; they
both look into different deployment considerations. Furthermore,
this document considers the updated deployment experience and newer
standards.
The target deployment scenarios in this document may also be covered
by other IPv4-as-a-Service (IPv4aaS) transition mechanisms. Note
that this is true only for broadband networks; in the case of
cellular networks, the only supported solution is the use of
NAT64/464XLAT. So, it is out of scope of this document to provide a
comparison among the different IPv4aaS transition mechanisms, which
are analyzed in [IPv6-TRANSITION].
Consequently, this document should not be used as a guide for an
operator or enterprise to decide which IPv4aaS is the best one for
its own network. Instead, it should be used as a tool for
understanding all the implications, including relevant documents (or
even specific parts of them) for the deployment of NAT64/464XLAT and
for facilitating the decision process regarding specific deployment
details.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. NAT64 Deployment Scenarios
DNS64 (see Section 7 of [RFC6147]) provides three deployment
scenarios, depending on the location of the DNS64 function. However,
since the publication of that document, other deployment scenarios
and NAT64 use cases need to be considered in actual networks, despite
the fact that some of them were specifically ruled out by the
original NAT64/DNS64 work.
Consequently, the perspective in this document is to broaden those
scenarios and include a few new ones. However, in order to reduce
the number of possible cases, we work under the assumption that the
service provider wants to make sure that all the customers have a
service without failures. This means considering the following
assumptions for the worst possible case:
a. There are hosts that will be validating DNSSEC.
b. IPv4 literal addresses and non-IPv6-compliant APIs are being
used.
c. There are IPv4-only hosts or applications beyond the IPv6-only
link (e.g., tethering in cellular networks).
This document uses a common set of possible "participant entities":
1. An IPv6-only access network (IPv6).
2. An IPv4-only remote network/server/service (IPv4).
3. A NAT64 function (NAT64) in the service provider.
4. A DNS64 function (DNS64) in the service provider.
5. An external service provider offering the NAT64 function and/or
the DNS64 function (extNAT64/extDNS64).
6. A 464XLAT customer-side translator (CLAT).
Note that the nomenclature used in parentheses is the one that, for
short, will be used in the figures. Note: for simplicity, the boxes
in the figures don't mean they are actually a single device; they
represent one or more functions as located in that part of the
network (i.e., a single box with NAT64 and DNS64 functions can
actually be several devices, not just one).
The possible scenarios are split in two general categories:
1. Known to work.
2. Known to work under special conditions.
3.1. Known to Work
The scenarios in this category are known to work, as there are well-
known existing deployments from different operators using them. Each
one may have different pros and cons, and in some cases, the trade-
offs may be acceptable for some operators.
3.1.1. Service Provider NAT64 with DNS64
In this scenario (Figure 1), the service provider offers both the
NAT64 and DNS64 functions.
This is the most common scenario as originally considered by the
designers of NAT64 [RFC6146] and DNS64 [RFC6147]; however, it may
also have the implications related to the DNSSEC.
This scenario may also fail to solve the issues of IPv4 literal
addresses, non-IPv6-compliant APIs, or IPv4-only hosts or
applications behind the IPv6-only access network.
+----------+ +----------+ +----------+
| | | NAT64 | | |
| IPv6 +--------+ + +--------+ IPv4 |
| | | DNS64 | | |
+----------+ +----------+ +----------+
Figure 1: NAT64 with DNS64
A similar scenario (Figure 2) exists if the service provider offers
only the DNS64 function; the NAT64 function is provided by an
outsourcing agreement with an external provider. All the
considerations in the previous paragraphs of this section are the
same for this sub-case.
+----------+ +----------+
| | | |
| extNAT64 +--------+ IPv4 |
| | | |
+----+-----+ +----------+
|
|
+----------+ +----+-----+
| | | |
| IPv6 +--------+ DNS64 +
| | | |
+----------+ +----------+
Figure 2: NAT64 in an External Service Provider
This is equivalent to the scenario (Figure 3) where the outsourcing
agreement with the external provider is to provide both the NAT64 and
DNS64 functions. Once more, all the considerations in the previous
paragraphs of this section are the same for this sub-case.
+----------+ +----------+
| extNAT64 | | |
| + +-------+ IPv4 |
| extDNS64 | | |
+----+-----+ +----------+
|
+----------+ |
| | |
| IPv6 +-------------+
| |
+----------+
Figure 3: NAT64 and DNS64 in an External Provider
One additional equivalent scenario (Figure 4) exists if the service
provider only offers the NAT64 function; the DNS64 function is from
an external provider with or without a specific agreement among them.
This is a common scenario today, as several "global" service
providers provide free DNS/DNS64 services, and users often configure
their DNS manually. This will only work if both the NAT64 and DNS64
functions are using the Well-Known Prefix (WKP) or the same Network-
Specific Prefix (NSP). All the considerations in the previous
paragraphs of this section are the same for this sub-case.
Of course, if the external DNS64 function is agreed with the service
provider, then this case is similar to the ones already depicted in
this scenario.
+----------+
| |
| extDNS64 |
| |
+----+-----+
|
|
+----------+ +----+-----+ +----------+
| | | | | |
| IPv6 +--------+ NAT64 +--------+ IPv4 |
| | | | | |
+----------+ +----------+ +----------+
Figure 4: NAT64; DNS64 by an External Provider
3.1.2. Service Provider Offering 464XLAT Using DNS64
464XLAT [RFC6877] describes an architecture that provides IPv4
connectivity across a network, or part of it, when it is only
natively transporting IPv6. The need to support the CLAT function in
order to ensure the IPv4 service continuity in IPv6-only cellular
deployments has been suggested in [RFC7849].
In order to do that, 464XLAT [RFC6877] relies on the combination of
existing protocols:
1. The CLAT is a stateless IPv4-to-IPv6 translator (NAT46) [RFC7915]
implemented in the end-user device or Customer Edge Router (CE),
located at the "customer edge" of the network.
2. The provider-side translator (PLAT) is a stateful NAT64
[RFC6146], implemented typically in the operator network.
3. Optionally, DNS64 [RFC6147] may allow an optimization: a single
translation at the NAT64, instead of two translations
(NAT46+NAT64), when the application at the end-user device
supports IPv6 DNS (uses AAAA Resource Records).
Note that even if the provider-side translator is referred to as PLAT
in the 464XLAT terminology [RFC6877], for simplicity and uniformity
across this document, it is always referred to as NAT64 (function).
In this scenario (Figure 5), the service provider deploys 464XLAT
with a DNS64 function.
As a consequence, the DNSSEC issues remain, unless the host is doing
the address synthesis.
464XLAT [RFC6877] is a very simple approach to cope with the major
NAT64+DNS64 drawback: not working with applications or devices that
use literal IPv4 addresses or non-IPv6-compliant APIs.
464XLAT [RFC6877] has been used mainly in IPv6-only cellular
networks. By supporting a CLAT function, end-user device
applications can access IPv4-only end networks / applications,
despite the fact that those applications or devices use literal IPv4
addresses or non-IPv6-compliant APIs.
In addition, in the cellular network example above, if the User
Equipment (UE) provides tethering, other devices behind it will be
presented with a traditional Network Address Translation from IPv4 to
IPv4 (NAT44), in addition to the native IPv6 support, so clearly it
allows IPv4-only hosts behind the IPv6-only access network.
Furthermore, as discussed in [RFC6877], 464XLAT can be used in
broadband IPv6 network architectures, by implementing the CLAT
function at the CE.
The support of this scenario in a network offers two additional
advantages:
* DNS load optimization: A CLAT should implement a DNS proxy (per
[RFC5625]) so that only IPv6-native queries and AAAA records are
sent to the DNS64 server. Otherwise, doubling the number of
queries may impact the DNS infrastructure.
* Connection establishment delay optimization: If the UE/CE
implementation is detecting the presence of a DNS64 function, it
may issue only the AAAA query, instead of both the AAAA and A
queries.
In order to understand all the communication possibilities, let's
assume the following representation of two dual-stack (DS) peers:
+-------+ .-----. .-----.
| | / \ / \
.-----. | Res./ | / IPv6- \ .-----. / IPv4- \
/ Local \ | SOHO +--( only )---( NAT64 )---( only )
/ \ | | \ flow /\ `-----' \ flow /
( Dual- )--+ IPv6 | \ / \ / \ /
\ Stack / | CE | `--+--' \ .-----. / `--+--'
\ Peer / | with | | \ / Remote\/ |
`-----' | CLAT | +---+----+ / \ +---+----+
| | |DNS/IPv6| ( Dual- ) |DNS/IPv4|
+-------+ | with | \ Stack / +--------+
| DNS64 | \ Peer /
+--------+ `-----'
Figure A: Representation of 464XLAT among Two Peers with DNS64
In this case, the possible communication paths, among the IPv4/IPv6
stacks of both peers, are as follows:
a. Local-IPv6 to Remote-IPv6: Regular DNS and native IPv6 among
peers.
b. Local-IPv6 to Remote-IPv4: DNS64 and NAT64 translation.
c. Local-IPv4 to Remote-IPv6: Not possible unless the CLAT
implements Explicit Address Mappings (EAMs) as indicated by
Section 4.9. In principle, it is not expected that services are
deployed in the Internet when using IPv6 only, unless there is
certainty that peers will also be IPv6 capable.
d. Local-IPv4 to Remote-IPv4: DNS64, CLAT, and NAT64 translations.
e. Local-IPv4 to Remote-dual-stack using EAM optimization: If the
CLAT implements EAM as indicated by Section 4.9, instead of using
the path d. above, NAT64 translation is avoided, and the flow
will use IPv6 from the CLAT to the destination.
The rest of the figures in this section show different choices for
placing the different elements.
+----------+ +----------+ +----------+
| IPv6 | | NAT64 | | |
| + +--------+ + +--------+ IPv4 |
| CLAT | | DNS64 | | |
+----------+ +----------+ +----------+
Figure 5: 464XLAT with DNS64
A similar scenario (Figure 6) exists if the service provider only
offers the DNS64 function; the NAT64 function is provided by an
outsourcing agreement with an external provider. All the
considerations in the previous paragraphs of this section are the
same for this sub-case.
+----------+ +----------+
| | | |
| extNAT64 +--------+ IPv4 |
| | | |
+----+-----+ +----------+
|
|
+----------+ +----+-----+
| IPv6 | | |
| + +--------+ DNS64 +
| CLAT | | |
+----------+ +----------+
Figure 6: 464XLAT with DNS64; NAT64 in an External Provider
In addition, it is equivalent to the scenario (Figure 7) where the
outsourcing agreement with the external provider is to provide both
the NAT64 and DNS64 functions. Once more, all the considerations in
the previous paragraphs of this section are the same for this sub-
case.
+----------+ +----------+
| extNAT64 | | |
| + +--------+ IPv4 |
| extDNS64 | | |
+----+-----+ +----------+
|
+----------+ |
| IPv6 | |
| + +-------------+
| CLAT |
+----------+
Figure 7: 464XLAT with DNS64; NAT64 and DNS64 in an External Provider
3.1.3. Service Provider Offering 464XLAT, without Using DNS64
The major advantage of this scenario (Figure 8), using 464XLAT
without DNS64, is that the service provider ensures that DNSSEC is
never broken, even if the user modifies the DNS configuration.
Nevertheless, some CLAT implementations or applications may impose an
extra delay, which is induced by the dual A/AAAA queries (and the
wait for both responses), unless Happy Eyeballs v2 [RFC8305] is also
present.
A possible variation of this scenario is when DNS64 is used only for
the discovery of the NAT64 prefix. In the rest of the document, it
is not considered a different scenario because once the prefix has
been discovered, the DNS64 function is not used, so it behaves as if
the DNS64 synthesis function is not present.
In this scenario, as in the previous one, there are no issues related
to IPv4-only hosts (or IPv4-only applications) behind the IPv6-only
access network, as neither are related to the usage of IPv4 literals
or non-IPv6-compliant APIs.
The support of this scenario in a network offers one advantage:
* DNS load optimization: A CLAT should implement a DNS proxy (per
[RFC5625]) so that only IPv6 native queries are sent to the DNS64
server. Otherwise, doubling the number of queries may impact the
DNS infrastructure.
As indicated earlier, the connection establishment delay optimization
is achieved only in the case of devices, Operating Systems, or
applications that use Happy Eyeballs v2 [RFC8305], which is very
common.
As in the previous case, let's assume the representation of two dual-
stack peers:
+-------+ .-----. .-----.
| | / \ / \
.-----. | Res./ | / IPv6- \ .-----. / IPv4- \
/ Local \ | SOHO +--( only )---( NAT64 )---( only )
/ \ | | \ flow /\ `-----' \ flow /
( Dual- )--+ IPv6 | \ / \ / \ /
\ Stack / | CE | `--+--' \ .-----. / `--+--'
\ Peer / | with | | \ / Remote\/ |
`-----' | CLAT | +---+----+ / \ +---+----+
| | |DNS/IPv6| ( Dual- ) |DNS/IPv4|
+-------+ +--------+ \ Stack / +--------+
\ Peer /
`-----'
Figure B: Representation of 464XLAT among Two Peers without DNS64
In this case, the possible communication paths, among the IPv4/IPv6
stacks of both peers, are as follows:
a. Local-IPv6 to Remote-IPv6: Regular DNS and native IPv6 among
peers.
b. Local-IPv6 to Remote-IPv4: Regular DNS, CLAT, and NAT64
translations.
c. Local-IPv4 to Remote-IPv6: Not possible unless the CLAT
implements EAM as indicated by Section 4.9. In principle, it is
not expected that services are deployed in the Internet using
IPv6 only, unless there is certainty that peers will also be
IPv6-capable.
d. Local-IPv4 to Remote-IPv4: Regular DNS, CLAT, and NAT64
translations.
e. Local-IPv4 to Remote-dual-stack using EAM optimization: If the
CLAT implements EAM as indicated by Section 4.9, instead of using
the path d. above, NAT64 translation is avoided, and the flow
will use IPv6 from the CLAT to the destination.
Notice that this scenario works while the local hosts/applications
are dual stack (which is the current situation) because the
connectivity from a local IPv6 to a remote IPv4 is not possible
without a AAAA synthesis. This aspect is important only when there
are IPv6-only hosts in the LANs behind the CLAT and they need to
communicate with remote IPv4-only hosts. However, it is not a
sensible approach from an Operating System or application vendor
perspective to provide IPv6-only support unless, similar to case c
above, there is certainty of peers supporting IPv6 as well. An
approach to a solution for this is also presented in [OPT-464XLAT].
The following figures show different choices for placing the
different elements.
+----------+ +----------+ +----------+
| IPv6 | | | | |
| + +--------+ NAT64 +--------+ IPv4 |
| CLAT | | | | |
+----------+ +----------+ +----------+
Figure 8: 464XLAT without DNS64
This is equivalent to the scenario (Figure 9) where there is an
outsourcing agreement with an external provider for the NAT64
function. All the considerations in the previous paragraphs of this
section are the same for this sub-case.
+----------+ +----------+
| | | |
| extNAT64 +--------+ IPv4 |
| | | |
+----+-----+ +----------+
|
+----------+ |
| IPv6 | |
| + +-------------+
| CLAT |
+----------+
Figure 9: 464XLAT without DNS64; NAT64 in an External Provider
3.2. Known to Work under Special Conditions
The scenarios in this category are known not to work unless
significant effort is devoted to solving the issues or they are
intended to solve problems across "closed" networks instead of as a
general Internet access usage. Even though some of the different
pros, cons, and trade-offs may be acceptable, operators have
implementation difficulties, as their expectations of NAT64/DNS64 are
beyond the original intent.
3.2.1. Service Provider NAT64 without DNS64
In this scenario (Figure 10), the service provider offers a NAT64
function; however, there is no DNS64 function support at all.
As a consequence, an IPv6 host in the IPv6-only access network will
not be able to detect the presence of DNS64 by means of [RFC7050] or
learn the IPv6 prefix to be used for the NAT64 function.
This can be sorted out as indicated in Section 4.1.1.
Regardless, because of the lack of the DNS64 function, the IPv6 host
will not be able to obtain AAAA synthesized records, so the NAT64
function becomes useless.
An exception to this "useless" scenario is to manually configure
mappings between the A records of each of the IPv4-only remote hosts
and the corresponding AAAA records with the WKP or NSP used by the
service-provider NAT64 function, as if they were synthesized by a
DNS64 function.
This mapping could be done by several means, typically at the
authoritative DNS server or at the service-provider resolvers by
means of DNS Response Policy Zones (RPZs) [DNS-RPZ] or equivalent
functionality. DNS RPZ may have implications in DNSSEC if the zone
is signed. Also, if the service provider is using an NSP, having the
mapping at the authoritative server may create troubles for other
parties trying to use a different NSP or WKP, unless multiple DNS
"views" (split-DNS) are also being used at the authoritative servers.
Generally, the mappings alternative will only make sense if a few
sets of IPv4-only remote hosts need to be accessed by a single
network (or a small number of them), which supports IPv6 only in the
access. This will require some kind of mutual agreement for using
this procedure; this should not be a problem because it won't
interfere with Internet use (which is a "closed service").
In any case, this scenario doesn't solve the issue of IPv4 literal
addresses, non-IPv6-compliant APIs, or IPv4-only hosts within that
IPv6-only access network.
+----------+ +----------+ +----------+
| | | | | |
| IPv6 +--------+ NAT64 +--------+ IPv4 |
| | | | | |
+----------+ +----------+ +----------+
Figure 10: NAT64 without DNS64
3.2.2. Service-Provider NAT64; DNS64 in IPv6 Hosts
In this scenario (Figure 11), the service provider offers the NAT64
function but not the DNS64 function. However, the IPv6 hosts have a
built-in DNS64 function.
This may become common if the DNS64 function is implemented in all
the IPv6 hosts/stacks. This is not common at the time of writing but
may become more common in the near future. This way, the DNSSEC
validation is performed on the A record, and then the host can use
the DNS64 function in order to use the NAT64 function without any
DNSSEC issues.
This scenario fails to solve the issue of IPv4 literal addresses or
non-IPv6-compliant APIs, unless the IPv6 hosts also support Happy
Eyeballs v2 (Section 7.1 of [RFC8305]).
Moreover, this scenario also fails to solve the problem of IPv4-only
hosts or applications behind the IPv6-only access network.
+----------+ +----------+ +----------+
| IPv6 | | | | |
| + +--------+ NAT64 +--------+ IPv4 |
| DNS64 | | | | |
+----------+ +----------+ +----------+
Figure 11: NAT64; DNS64 in IPv6 Hosts
3.2.3. Service-Provider NAT64; DNS64 in the IPv4-Only Remote Network
In this scenario (Figure 12), the service provider offers the NAT64
function only. The IPv4-only remote network offers the DNS64
function.
This is not common, and it doesn't make sense that a remote network,
not deploying IPv6, is providing a DNS64 function. Like the scenario
depicted in Section 3.2.1, it will only work if both sides are using
the WKP or the same NSP, so the same considerations apply. It can
also be tuned to behave as in Section 3.1.1.
This scenario fails to solve the issue of IPv4 literal addresses or
non-IPv6-compliant APIs.
Moreover, this scenario also fails to solve the problem of IPv4-only
hosts or applications behind the IPv6-only access network.
+----------+ +----------+ +----------+
| | | | | IPv4 |
| IPv6 +--------+ NAT64 +--------+ + |
| | | | | DNS64 |
+----------+ +----------+ +----------+
Figure 12: NAT64; DNS64 in IPv4-Only Hosts
3.3. Comparing the Scenarios
This section compares the different scenarios, including possible
variations (each one represented in the previous sections by a
different figure), while considering the following criteria:
a. DNSSEC: Are there hosts validating DNSSEC?
b. Literal/APIs: Are there applications using IPv4 literals or non-
IPv6-compliant APIs?
c. IPv4 only: Are there hosts or applications using IPv4 only?
d. Foreign DNS: Does the scenario survive if the user, Operating
System, applications, or devices change the DNS?
e. DNS load opt. (DNS load optimization): Are there extra queries
that may impact the DNS infrastructure?
f. Connect. opt. (connection establishment delay optimization): Is
the UE/CE only issuing the AAAA query or also the A query and
waiting for both responses?
In the table below, the columns represent each of the scenarios from
the previous sections by the figure number. The possible values are
as follows:
"-" means the scenario is "bad" for that criterion.
"+" means the scenario is "good" for that criterion.
"*" means the scenario is "bad" for that criterion; however, it
is typically resolved with the support of Happy Eyeballs v2
[RFC8305].
In some cases, "countermeasures", alternative or special
configurations, may be available for the criterion designated as
"bad". So, this comparison is considering a generic case as a quick
comparison guide. In some cases, a "bad" criterion is not
necessarily a negative aspect; it all depends on the specific needs/
characteristics of the network where the deployment will take place.
For instance, in a network that only has IPv6-only hosts and apps
using DNS and IPv6-compliant APIs, there is no impact using only
NAT64 and DNS64, but if the hosts validate DNSSEC, that criterion is
still relevant.
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| Item / Figure | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
+===============+===+===+===+===+===+===+===+===+===+====+====+====+
| DNSSEC | - | - | - | - | - | - | - | + | + | + | + | + |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| Literal/APIs | - | - | - | - | + | + | + | + | + | - | - | - |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| IPv4-only | - | - | - | - | + | + | + | + | + | - | - | - |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| Foreign DNS | - | - | - | - | + | + | + | + | + | - | + | - |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| DNS load opt. | + | + | + | + | + | + | + | + | + | + | + | + |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
| Connect. opt. | + | + | + | + | + | + | + | * | * | + | + | + |
+---------------+---+---+---+---+---+---+---+---+---+----+----+----+
Table 1: Scenario Comparison
As a general conclusion, we should note if the network must support
applications using any of the following:
* IPv4 literals
* non-IPv6-compliant APIs
* IPv4-only hosts or applications
Then, only the scenarios with 464XLAT, a CLAT function, or equivalent
built-in local address synthesis features will provide a valid
solution. Furthermore, those scenarios will also keep working if the
DNS configuration is modified. Clearly, depending on if DNS64 is
used or not, DNSSEC may be broken for those hosts doing DNSSEC
validation.
All the scenarios are good in terms of DNS load optimization, and in
the case of 464XLAT, it may provide an extra degree of optimization.
Finally, all of the scenarios are also good in terms of connection
establishment delay optimization. However, in the case of 464XLAT
without DNS64, the usage of Happy Eyeballs v2 is required. This is
not an issue as it is commonly available in actual Operating Systems.
4. Issues to be Considered
This section reviews the different issues that an operator needs to
consider for a NAT64/464XLAT deployment, as they may develop specific
decision points about how to approach that deployment.
4.1. DNSSEC Considerations and Possible Approaches
As indicated in the security considerations for DNS64 (see Section 8
of [RFC6147]) because DNS64 modifies DNS answers and DNSSEC is
designed to detect such modifications, DNS64 may break DNSSEC.
When a device connected to an IPv6-only access network queries for a
domain name in a signed zone, by means of a recursive name server
that supports DNS64, the result may be a synthesized AAAA record. In
that case, if the recursive name server is configured to perform
DNSSEC validation and has a valid chain of trust to the zone in
question, it will cryptographically validate the negative response
from the authoritative name server. This is the expected DNS64
behavior: the recursive name server actually "lies" to the client
device. However, in most of the cases, the client will not notice
it, because generally, they don't perform validation themselves;
instead, they rely on the recursive name servers.
In fact, a validating DNS64 resolver increases the confidence on the
synthetic AAAA, as it has validated that a non-synthetic AAAA doesn't
exist. However, if the client device is oblivious to NAT64 (the most
common case) and performs DNSSEC validation on the AAAA record, it
will fail as it is a synthesized record.
The best possible scenario from a DNSSEC point of view is when the
client requests that the DNS64 server perform the DNSSEC validation
(by setting the DNSSEC OK (DO) bit to 1 and the CD bit to 0). In
this case, the DNS64 server validates the data; thus, tampering may
only happen inside the DNS64 server (which is considered as a trusted
part, thus, its likelihood is low) or between the DNS64 server and
the client. All other parts of the system (including transmission
and caching) are protected by DNSSEC [Threat-DNS64].
Similarly, if the client querying the recursive name server is
another name server configured to use it as a forwarder, and it is
performing DNSSEC validation, it will also fail on any synthesized
AAAA record.
All those considerations are extensively covered in Sections 3, 5.5,
and 6.2 of [RFC6147].
DNSSEC issues could be avoided if all the signed zones provide IPv6
connectivity together with the corresponding AAAA records. However,
this is out of the control of the operator needing to deploy a NAT64
function. This has been proposed already in [DNS-DNSSEC].
An alternative solution, which was considered while developing
[RFC6147], is that the validators will be DNS64 aware. Then, they
can perform the necessary discovery and do their own synthesis.
Since that was standardized sufficiently early in the validator
deployment curve, the expectation was that it would be okay to break
certain DNSSEC assumptions for networks that were stuck and really
needing NAT64/DNS64.
As already indicated, the scenarios in the previous section are
simplified to look at the worst possible case and for the most
perfect approach. A DNSSEC breach will not happen if the end host is
not doing validation.
The figures in previous studies indicate that DNSSEC broken by using
DNS64 makes up about 1.7% [About-DNS64] of the cases. However, we
can't negate that this may increase as DNSSEC deployment grows.
Consequently, a decision point for the operator must depend on the
following question: Do I really care about that percentage of cases
and the impact on my help desk, or can I provide alternative
solutions for them? Some possible solutions may be exist, as
depicted in the next sections.
4.1.1. Not Using DNS64
One solution is to avoid using DNS64, but as already indicated, this
is not possible in all the scenarios.
The use of DNS64 is a key component for some networks, in order to
comply with traffic performance metrics, monitored by some
governmental bodies and other institutions [FCC] [ARCEP].
One drawback of not having a DNS64 on the network side is that it's
not possible to heuristically discover NAT64 [RFC7050].
Consequently, an IPv6 host behind the IPv6-only access network will
not be able to detect the presence of the NAT64 function, nor learn
the IPv6 prefix to be used for it, unless it is configured by
alternative means.
The discovery of the IPv6 prefix could be solved, as described in
[RFC7050], by means of adding the relevant AAAA records to the
ipv4only.arpa. zone of the service-provider recursive servers, i.e.,
if using the WKP (64:ff9b::/96):
ipv4only.arpa. SOA . . 0 0 0 0 0
ipv4only.arpa. NS .
ipv4only.arpa. AAAA 64:ff9b::192.0.0.170
ipv4only.arpa. AAAA 64:ff9b::192.0.0.171
ipv4only.arpa. A 192.0.0.170
ipv4only.arpa. A 192.0.0.171
An alternative option is the use of DNS RPZ [DNS-RPZ] or equivalent
functionalities. Note that this may impact DNSSEC if the zone is
signed.
Another alternative, only valid in environments with support from the
Port Control Protocol (PCP) (for both the hosts or CEs and for the
service-provider network), is to follow "Discovering NAT64 IPv6
Prefixes Using the Port Control Protocol (PCP)" [RFC7225].
Other alternatives may be available in the future. All them are
extensively discussed in [RFC7051]; however, due to the deployment
evolution, many considerations from that document have changed. New
options are being documented, such as using Router Advertising
[PREF64] or DHCPv6 options [DHCPv6-OPTIONS].
Simultaneous support of several of the possible approaches is
convenient and will ensure that clients with different ways to
configure the NAT64 prefix successfully obtain it. This is also
convenient even if DNS64 is being used.
Also of special relevance to this section is [IPV4ONLY-ARPA].
4.1.2. DNSSEC Validator Aware of DNS64
In general, by default, DNS servers with DNS64 function will not
synthesize AAAA responses if the DO flag was set in the query.
In this case, since only an A record is available, if a CLAT function
is present, the CLAT will, as in the case of literal IPv4 addresses,
keep that traffic flow end to end as IPv4 so DNSSEC is not broken.
However, this will not work if a CLAT function is not present because
the hosts will not be able to use IPv4 (which is the case for all the
scenarios without 464XLAT).
4.1.3. Stub Validator
If the DO flag is set and the client device performs DNSSEC
validation, and the Checking Disabled (CD) flag is set for a query,
the DNS64 recursive server will not synthesize AAAA responses. In
this case, the client could perform the DNSSEC validation with the A
record and then synthesize the AAAA responses [RFC6052]. For that to
be possible, the client must have learned the NAT64 prefix beforehand
using any of the available methods (see [RFC7050], [RFC7225],
[PREF64], and [DHCPv6-OPTIONS]). This allows the client device to
avoid using the DNS64 function and still use NAT64 even with DNSSEC.
If the end host is IPv4 only, this will not work if a CLAT function
is not present (which is the case for all scenarios without 464XLAT).
Instead of a CLAT, some devices or Operating Systems may implement an
equivalent function by using Bump-in-the-Host [RFC6535] as part of
Happy Eyeballs v2 (see Section 7.1 of [RFC8305]). In this case, the
considerations in the above paragraphs are also applicable.
4.1.4. CLAT with DNS Proxy and Validator
If a CE includes CLAT support and also a DNS proxy, as indicated in
Section 6.4 of [RFC6877], the CE could behave as a stub validator on
behalf of the client devices. Then, following the same approach
described in Section 4.1.3, the DNS proxy will actually "lie" to the
client devices, which, in most cases, will not be noticed unless they
perform validation by themselves. Again, this allows the client
devices to avoid the use of the DNS64 function but to still use NAT64
with DNSSEC.
Once more, this will not work without a CLAT function (which is the
case for all scenarios without 464XLAT).
4.1.5. ACL of Clients
In cases of dual-stack clients, AAAA queries typically take
preference over A queries. If DNS64 is enabled for those clients, it
will never get A records, even for IPv4-only servers.
As a consequence, in cases where there are IPv4-only servers, and
those are located in the path before the NAT64 function, the clients
will not be able to reach them. If DNSSEC is being used for all
those flows, specific addresses or prefixes can be left out of the
DNS64 synthesis by means of Access Control Lists (ACLs).
Once more, this will not work without a CLAT function (which is the
case for all scenarios without 464XLAT).
4.1.6. Mapping Out IPv4 Addresses
If there are well-known specific IPv4 addresses or prefixes using
DNSSEC, they can be mapped out of the DNS64 synthesis.
Even if this is not related to DNSSEC, this "mapping-out" feature is
quite commonly used to ensure that addresses [RFC1918] (for example,
used by LAN servers) are not synthesized to AAAA.
Once more, this will not work without a CLAT function (which is the
case for all scenarios without 464XLAT).
4.2. DNS64 and Reverse Mapping
When a client device using DNS64 tries to reverse-map a synthesized
IPv6 address, the name server responds with a CNAME record that
points the domain name used to reverse-map the synthesized IPv6
address (the one under ip6.arpa) to the domain name corresponding to
the embedded IPv4 address (under in-addr.arpa).
This is the expected behavior, so no issues need to be considered
regarding DNS reverse mapping.
4.3. Using 464XLAT with/without DNS64
In case the client device is IPv6 only (either because the stack or
application is IPv6 only or because it is connected via an IPv6-only
LAN) and the remote server is IPv4 only (either because the stack is
IPv4 only or because it is connected via an IPv4-only LAN), only
NAT64 combined with DNS64 will be able to provide access between
both. Because DNS64 is then required, DNSSEC validation will only be
possible if the recursive name server is validating the negative
response from the authoritative name server, and the client is not
performing validation.
Note that at this stage of the transition, it is not expected that
applications, devices, or Operating Systems are IPv6 only. It will
not be a sensible decision for a developer to work on that direction,
unless it is clear that the deployment scenario fully supports it.
On the other hand, an end user or enterprise network may decide to
run IPv6 only in the LANs. In case there is any chance for
applications to be IPv6 only, the Operating System may be responsible
for either doing a local address synthesis or setting up some kind of
on-demand VPN (IPv4-in-IPv6), which needs to be supported by that
network. This may become very common in enterprise networks, where
"Unique IPv6 Prefix per Host" [RFC8273] is supported.
However, when the client device is dual stack and/or connected in a
dual-stack LAN by means of a CLAT function (or has a built-in CLAT
function), DNS64 is an option.
1. With DNS64: If DNS64 is used, most of the IPv4 traffic (except if
using literal IPv4 addresses or non-IPv6-compliant APIs) will not
use the CLAT and will instead use the IPv6 path, so only one
translation will be done at the NAT64. This may break DNSSEC,
unless measures as described in the previous sections are taken.
2. Without DNS64: If DNS64 is not used, all the IPv4 traffic will
make use of the CLAT, so two translations are required (NAT46 at
the CLAT and NAT64 at the PLAT), which adds some overhead in
terms of the extra NAT46 translation. However, this avoids the
AAAA synthesis and consequently will never break DNSSEC.
Note that the extra translation, when DNS64 is not used, takes place
at the CLAT, which means no extra overhead for the operator.
However, it adds potential extra delays to establish the connections
and has no perceptible impact for a CE in a broadband network, but it
may have some impact on a battery-powered device. The cost for a
battery-powered device is possibly comparable to the cost when the
device is doing a local address synthesis (see Section 7.1 of
[RFC8305]).
4.4. Foreign DNS
Clients, devices, or applications in a service-provider network may
use DNS servers from other networks. This may be the case if
individual applications use their own DNS server, the Operating
System itself or even the CE, or combinations of the above.
Those "foreign" DNS servers may not support DNS64; as a consequence,
those scenarios that require a DNS64 may not work. However, if a
CLAT function is available, the considerations in Section 4.3 will
apply.
If the foreign DNS supports the DNS64 function, incorrect
configuration parameters may be provided that, for example, casue cause WKP
or NSP to become unmatched or result in a case such as the one
described in Section 3.2.3.
Having a CLAT function, even if using foreign DNS without a DNS64
function, ensures that everything will work, so the CLAT must be
considered to be an advantage despite user configuration errors. As
a result, all the traffic will use a double translation (NAT46 at the
CLAT and NAT64 at the operator network), unless there is support for
EAM (Section 4.9).
An exception is the case where there is a CLAT function at the CE
that is not able to obtain the correct configuration parameters
(again, causing WKP or NSP to become unmatched).
However, it needs to be emphasized that if there is no CLAT function
(which is the case for all scenarios without 464XLAT), an external
DNS without DNS64 support will disallow any access to IPv4-only
destination networks and will not guarantee the correct DNSSEC
validation, so it will behave as in Section 3.2.1.
In summary, the consequences of using foreign DNS depends on each
specific case. However, in general, if a CLAT function is present,
most of the time there will not be any issues. In the other cases,
the access to IPv6-enabled services is still guaranteed for
IPv6-enabled hosts, but it is not guaranteed for IPv4-only hosts nor
is the access to IPv4-only services for any hosts in the network.
The causes of "foreign DNS" could be classified in three main
categories, as depicted in the following subsections.
4.4.1. Manual Configuration of DNS
It is becoming increasingly common that end users, or even devices or
applications, configure alternative DNS in their Operating Systems
and sometimes in CEs.
4.4.2. DNS Privacy/Encryption Mechanisms
Clients or applications may use mechanisms for DNS privacy/
encryption, such as DNS over TLS (DoT) [RFC7858], DNS over DTLS
[RFC8094], DNS queries over HTTPS (DoH) [RFC8484], or DNS over QUIC
(DoQ) [QUIC-CONNECTIONS].
Currently, those DNS privacy/encryption options are typically
provided by the applications, not the Operating System vendors. At
the time this document was written, the DoT and DoH standards have
declared DNS64 (and consequently NAT64) out of their scope, so an
application using them may break NAT64, unless a correctly configured
CLAT function is used.
4.4.3. Split DNS and VPNs
When networks or hosts use "split-DNS" (also called Split Horizon,
DNS views, or private DNS), the successful use of DNS64 is not
guaranteed. This case is analyzed in Section 4 of [RFC6950].
A similar situation may happen with VPNs that force all the DNS
queries through the VPN and ignore the operator DNS64 function.
4.5. Well-Known Prefix (WKP) vs. Network-Specific Prefix (NSP)
Section 3 of "IPv6 Addressing of IPv4/IPv6 Translator" [RFC6052]
discusses some considerations that are useful to an operator when
deciding if a WKP or an NSP should be used.
Considering that discussion and other issues, we can summarize the
possible decision points to as follows:
a. The WKP MUST NOT be used to represent non-global IPv4 addresses.
If this is required because the network to be translated uses
non-global addresses, then an NSP is required.
b. The WKP MAY appear in interdomain routing tables, if the operator
provides a NAT64 function to peers. However, in this case,
special considerations related to BGP filtering are required, and
IPv4-embedded IPv6 prefixes longer than the WKP MUST NOT be
advertised (or accepted) in BGP. An NSP may be a more
appropriate option in those cases.
c. If several NAT64s use the same prefix, packets from the same flow
may be routed to a different NAT64 in case of routing changes.
This can be avoided by either using different prefixes for each
NAT64 function or ensuring that all the NAT64s coordinate their
state. Using an NSP could simplify that.
d. If DNS64 is required and users, devices, Operating Systems, or
applications may change their DNS configuration and deliberately
choose an alternative DNS64 function, the alternative DNS64 will
most likely use the WKP by default. In that case, if an NSP is
used by the NAT64 function, clients will not be able to use the
operator NAT64 function, which will break connectivity to
IPv4-only destinations.
4.6. IPv4 Literals and Non-IPv6-Compliant APIs
A host or application using literal IPv4 addresses or older APIs,
which aren't IPv6 compliant, behind a network with IPv6-only access
will not work unless any of the following alternatives are provided:
* CLAT (or an equivalent function).
* Happy Eyeballs v2 (Section 7.1 of [RFC8305]).
* Bump-in-the-Host [RFC6535] with a DNS64 function.
Those alternatives will solve the problem for an end host. However,
if the end host is providing "tethering" or an equivalent service to
other hosts, that needs to be considered as well. In other words, in
a cellular network, these alternatives resolve the issue for the UE
itself, but this may not be the case for hosts connected via the
tethering.
Otherwise, the support of 464XLAT is the only valid and complete
approach to resolve this issue.
4.7. IPv4-Only Hosts or Applications
IPv4-only hosts or an application behind a network with IPv6-only
access will not work unless a CLAT function is present.
464XLAT is the only valid approach to resolve this issue.
4.8. CLAT Translation Considerations
As described in "IPv6 Prefix Handling" (see Section 6.3 of
[RFC6877]), if the CLAT function can be configured with a dedicated
/64 prefix for the NAT46 translation, then it will be possible to do
a more efficient stateless translation.
Otherwise, if this dedicated prefix is not available, the CLAT
function will need to do a stateful translation, for example, perform
stateful NAT44 for all the IPv4 LAN packets so they appear as coming
from a single IPv4 address; in turn, the CLAT function will perform a
stateless translation to a single IPv6 address.
A possible setup, in order to maximize the CLAT performance, is to
configure the dedicated translation prefix. This can be easily
achieved automatically, if the broadband CE or end-user device is
able to obtain a shorter prefix by means of DHCPv6-PD [RFC8415] or
other alternatives. The CE can then use a specific /64 for the
translation. This is also possible when broadband is provided by a
cellular access.
The above recommendation is often not possible for cellular networks,
when connecting smartphones (as UEs): generally they don't use
DHCPv6-PD [RFC8415]. Instead, a single /64 is provided for each
Packet Data Protocol (PDP) context, and prefix sharing [RFC6877] is
used. In this case, the UEs typically have a build-in CLAT function
that is performing a stateful NAT44 translation before the stateless
NAT46.
4.9. EAM Considerations
"Explicit Address Mappings for Stateless IP/ICMP Translation"
[RFC7757] provides a way to configure explicit mappings between IPv4
and IPv6 prefixes of any length. When this is used, for example, in
a CLAT function, it may provide a simple mechanism in order to avoid
traffic flows between IPv4-only nodes or applications and dual-stack
destinations to be translated twice (NAT46 and NAT64), by creating
mapping entries with the Global Unicast Address (GUA) of the
IPv6-reachable destination. This optimization of NAT64 usage is very
useful in many scenarios, including Content Delivery Networks (CDNs)
and caches, as described in [OPT-464XLAT].
In addition, it may also provide a way for IPv4-only nodes or
applications to communicate with IPv6-only destinations.
4.10. Incoming Connections
The use of NAT64, in principle, disallows IPv4 incoming connections,
which may still be needed for IPv4-only peer-to-peer applications.
However, there are several alternatives that resolve this issue:
a. Session Traversal Utilities for NAT (STUN) [RFC5389], Traversal
Using Relays around NAT (TURN) [RFC5766], and Interactive
Connectivity Establishment (ICE) [RFC8445] are commonly used by
peer-to-peer applications in order to allow incoming connections
with IPv4 NAT. In the case of NAT64, they work as well.
b. The Port Control Protocol (PCP) [RFC6887] allows a host to
control how incoming IPv4 and IPv6 packets are translated and
forwarded. A NAT64 may implement PCP to allow this service.
c. EAM [RFC7757] may also be used in order to configure explicit
mappings for customers that require them. This is used, for
example, by Stateless IP/ICMP Translation for IPv6 Data Center
Environments (SIIT-DC) [RFC7755] and SIIT-DC Dual Translation
Mode (SIIT-DC-DTM) [RFC7756].
5. Summary of Deployment Recommendations for NAT64/464XLAT
It has been demonstrated that NAT64/464XLAT is a valid choice in
several scenarios (IPv6-IPv4 and IPv4-IPv6-IPv4), being the
predominant mechanism in the majority of the cellular networks, which
account for hundreds of millions of users [ISOC]. NAT64/464XLAT
offer different choices of deployment, depending on each network
case, needs, and requirements. Despite that, this document is not an
explicit recommendation for using this choice versus other IPv4aaS
transition mechanisms. Instead, this document is a guide that
facilitates evaluating a possible implementation of NAT64/464XLAT and
key decision points about specific design considerations for its
deployment.
Depending on the specific requirements of each deployment case, DNS64
may be a required function, while in other cases, the adverse effects
may be counterproductive. Similarly, in some cases, a NAT64
function, together with a DNS64 function, may be a valid solution
when there is a certainty that IPv4-only hosts or applications do not
need to be supported (see Sections 4.6 and 4.7). However, in other
cases (i.e., IPv4-only devices or applications that need to be
supported), the limitations of NAT64/DNS64 may indicate that the
operator needs to look into 464XLAT as a more complete solution.
For broadband-managed networks (where the CE is provided or
suggested/supported by the operator), in order to fully support the
actual user's needs (i.e., IPv4-only devices and applications and the
usage of IPv4 literals and non-IPv6-compliant APIs), the 464XLAT
scenario should be considered. In that case, it must support a CLAT
function.
If the operator provides DNS services, they may support a DNS64
function to avoid, as much as possible, breaking DNSSEC. This will
also increase performance, by reducing the double translation for all
the IPv4 traffic. In this case, if the DNS service is offering
DNSSEC validation, then it must be in such a way that it is aware of
the DNS64. This is considered the simpler and safer approach, and it
may be combined with other recommendations described in this
document:
* DNS infrastructure MUST be aware of DNS64 (Section 4.1.2).
* Devices running CLAT SHOULD follow the indications in "Stub
Validator" (see Section 4.1.3). However, this may be out of the
control of the operator.
* CEs SHOULD include a DNS proxy and validator (Section 4.1.4).
* "ACL of Clients" (see Section 4.1.5) and "Mapping Out IPv4
Addresses" (see Section 4.1.6) MAY be considered by operators,
depending on their own infrastructure.
This "increased performance" approach has the disadvantage of
potentially breaking DNSSEC for a small percentage of validating end
hosts versus the small impact of a double translation taking place in
the CE. If CE performance is not an issue, which is the most
frequent case, then a much safer approach is to not use DNS64 at all,
and consequently, ensure that all the IPv4 traffic is translated at
the CLAT (Section 4.3).
If DNS64 is not used, at least one of the alternatives described in
Section 4.1.1 must be followed in order to learn the NAT64 prefix.
The operator needs to consider that if the DNS configuration is
modified (see Sections 4.4, 4.4.2, and 4.4.3), which most likely
cannot be avoided, a foreign non-DNS64 could be used instead of
configuring a DNS64. In a scenario with only a NAT64 function, an
IPv4-only remote host will no longer be accessible. Instead, it will
continue to work in the case of 464XLAT.
Similar considerations need to be made regarding the usage of a NAT64
WKP vs. NSP (Section 4.5), as they must match the configuration of
DNS64. When using foreign DNS, they may not match. If there is a
CLAT and the configured foreign DNS is not a DNS64, the network will
keep working only if other means of learning the NAT64 prefix are
available.
For broadband networks, as described in Section 4.8, the CEs
supporting a CLAT function SHOULD support DHCPv6-PD [RFC8415] or
alternative means for configuring a shorter prefix. The CE SHOULD
internally reserve one /64 for the stateless NAT46 translation. The
operator must ensure that the customers are allocated prefixes
shorter than /64 in order to support this optimization. One way or
another, this is not impacting the performance of the operator
network.
Operators may follow "Deployment Considerations" (Section 7 of
[RFC6877]) for suggestions on how to take advantage of traffic-
engineering requirements.
For cellular networks, the considerations regarding DNSSEC may appear
to be out of scope because UEs' Operating Systems commonly don't
support DNSSEC. However, applications running on them may, or it may
be an Operating System "built-in" support in the future. Moreover,
if those devices offer tethering, other client devices behind the UE
may be doing the validation; hence, proper DNSSEC support by the
operator network is relevant.
Furthermore, cellular networks supporting 464XLAT [RFC6877] and
"Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"
[RFC7050] allow a progressive IPv6 deployment, with a single Access
Point Name (APN) supporting all types of PDP context (IPv4, IPv6, and
IPv4v6). This approach allows the network to automatically serve
every possible combination of UEs.
If the operator chooses to provide validation for the DNS64 prefix
discovery, it must follow the advice from "Validation of Discovered
Pref64::/n" (see Section 3.1 of [RFC7050]).
One last consideration is that many networks may have a mix of
different complex scenarios at the same time; for example, customers
that require 464XLAT and those that don't, customers that require
DNS64 and those that don't, etc. In general, the different issues
and the approaches described in this document can be implemented at
the same time for different customers or parts of the network. That
mix of approaches doesn't present any problem or incompatibility;
they work well together as a matter of appropriate and differentiated
provisioning. In fact, the NAT64/464XLAT approach facilitates an
operator offering both cellular and broadband services to have a
single IPv4aaS for both networks while differentiating the deployment
key decisions to optimize each case. It's even possible to use
hybrid CEs that have a main broadband access link and a backup via
the cellular network.
In an ideal world, we could safely use DNS64 if the approach proposed
in [DNS-DNSSEC] were followed, avoiding the cases where DNSSEC may be
broken. However, this will not solve the issues related to DNS
privacy and split DNS.
The only 100% safe solution that also resolves all the issues is, in
addition to having a CLAT function, not using a DNS64 but instead
making sure that the hosts have a built-in address synthesis feature.
Operators could manage to provide CEs with the CLAT function;
however, the built-in address synthesis feature is out of their
control. If the synthesis is provided by either the Operating System
(via its DNS resolver API) or the application (via its own DNS
resolver) in such way that the prefix used for the NAT64 function is
reachable for the host, the problem goes away.
Whenever feasible, using EAM [RFC7757] as indicated in Section 4.9
provides a very relevant optimization, avoiding double translations.
Applications that require incoming connections typically provide a
means for that already. However, PCP and EAM, as indicated in
Section 4.10, are valid alternatives, even for creating explicit
mappings for customers that require them.
6. Deployment of 464XLAT/NAT64 in Enterprise Networks
The recommendations in this document can also be used in enterprise
networks, campuses, and other similar scenarios (including managed
end-user networks).
This includes scenarios where the NAT64 function (and DNS64 function,
if available) are under the control of that network (or can be
configured manually according to that network's specific
requirements), and there is a need to provide IPv6-only access to any
part of that network, or it is IPv6 only connected to third-party
networks.
An example is the IETF meeting network itself, where both NAT64 and
DNS64 functions are provided, presenting in this case the same issues
as per Section 3.1.1. If there is a CLAT function in the IETF
network, then there is no need to use DNS64, and it falls under the
considerations of Section 3.1.3. Both scenarios have been tested and
verified already in the IETF network.
The following figures represent a few of the possible scenarios.
Figure 13 provides an example of an IPv6-only enterprise network
connected with a dual stack to the Internet using local NAT64 and
DNS64 functions.
+----------------------------------+
| Enterprise Network |
| +----------+ +----------+ | +----------+
| | IPv6- | | NAT64 | | | IPv4 |
| | only +--------+ + | +-------+ + |
| | LANs | | DNS64 | | | IPv6 |
| +----------+ +----------+ | +----------+
+----------------------------------+
Figure 13: IPv6-Only Enterprise with NAT64 and DNS64
Figure 14 provides an example of a DS enterprise network connected
with DS to the Internet using a CLAT function, without a DNS64
function.
+----------------------------------+
| Enterprise Network |
| +----------+ +----------+ | +----------+
| | IPv6 | | | | | IPv4 |
| | + +--------+ NAT64 | +-------+ + |
| | CLAT | | | | | IPv6 |
| +----------+ +----------+ | +----------+
+----------------------------------+
Figure 14: DS Enterprise with CLAT, DS Internet, without DNS64
Finally, Figure 15 provides an example of an IPv6-only provider with
a NAT64 function, and a DS enterprise network by means of their own
CLAT function, without a DNS64 function.
+----------------------------------+
| Enterprise Network |
| +----------+ +----------+ | +----------+
| | IPv6 | | | | IPv6 | |
| | + +--------+ CLAT | +--------+ NAT64 |
| | IPv4 | | | | only | |
| +----------+ +----------+ | +----------+
+----------------------------------+
Figure 15: DS Enterprise with CLAT and IPv6-Only Access, without
DNS64
7. Security Considerations
This document does not have new specific security considerations
beyond those already reported by each of the documents cited. For
example, DNS64 [RFC6147] already describes the DNSSEC issues.
As already described in Section 4.4, note that there may be
undesirable interactions, especially if using VPNs or DNS privacy,
which may impact the correct performance of DNS64/NAT64.
Note that the use of a DNS64 function has privacy considerations that
are equivalent to regular DNS, and they are located in either the
service provider or an external service provider.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/info/rfc5389>.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<https://www.rfc-editor.org/info/rfc5625>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.
[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, DOI 10.17487/RFC6144,
April 2011, <https://www.rfc-editor.org/info/rfc6144>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.
[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,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6535] Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
Using "Bump-in-the-Host" (BIH)", RFC 6535,
DOI 10.17487/RFC6535, February 2012,
<https://www.rfc-editor.org/info/rfc6535>.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.
[RFC7225] Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
Port Control Protocol (PCP)", RFC 7225,
DOI 10.17487/RFC7225, May 2014,
<https://www.rfc-editor.org/info/rfc7225>.
[RFC7757] Anderson, T. and A. Leiva Popper, "Explicit Address
Mappings for Stateless IP/ICMP Translation", RFC 7757,
DOI 10.17487/RFC7757, February 2016,
<https://www.rfc-editor.org/info/rfc7757>.
[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
"IP/ICMP Translation Algorithm", RFC 7915,
DOI 10.17487/RFC7915, June 2016,
<https://www.rfc-editor.org/info/rfc7915>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8273] Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,
<https://www.rfc-editor.org/info/rfc8273>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<https://www.rfc-editor.org/info/rfc8375>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
9.2. Informative References
[About-DNS64]
Linkova, J., "Let's talk about IPv6 DNS64 & DNSSEC", June
2016, <https://blog.apnic.net/2016/06/09/lets-talk-
ipv6-dns64-dnssec/>.
[ARCEP] ARCEP, "Service client des operateurs : les mesures de
qualite de service", April 2018, <https://www.arcep.fr/
cartes-et-donnees/nos-publications-chiffrees/service-
client-des-operateurs-mesures-de-la-qualite-de-service/
service-client-des-operateurs-les-mesures-de-qualite-de-
service.html>.
[DHCPv6-OPTIONS]
Li, L., Cui, Y., Liu, C., Wu, J., Baker, F., and J. Palet,
"DHCPv6 Options for Discovery NAT64 Prefixes", Work in
Progress, Internet-Draft, draft-li-intarea-nat64-prefix-
dhcp-option-02, 20 April 2019,
<https://tools.ietf.org/html/draft-li-intarea-nat64-
prefix-dhcp-option-02>.
[DNS-DNSSEC]
Byrne, C. and J. Palet, "IPv6-Ready DNS/DNSSSEC
Infrastructure", Work in Progress, Internet-Draft, draft-
bp-v6ops-ipv6-ready-dns-dnssec-00, 10 October 2018,
<https://tools.ietf.org/html/draft-bp-v6ops-ipv6-ready-
dns-dnssec-00>.
[DNS-RPZ] Vixie, P. and V. Schryver, "DNS Response Policy Zones
(RPZ)", Work in Progress, Internet-Draft, draft-vixie-
dnsop-dns-rpz-00, 23 June 2018,
<https://tools.ietf.org/html/draft-vixie-dnsop-dns-rpz-
00>.
[DNS64-Benchm]
Lencse, G. and Y. Kadobayashi, "Benchmarking DNS64
Implementations: Theory and Practice",
DOI 10.1016/j.comcom.2018.05.005, pp. 61-74, no. 1,
vol. 127, Computer Communications , September 2018,
<https://www.sciencedirect.com/science/article/pii/
S0140366418302184?via%3Dihub>.
[DNS64-BM-Meth]
Lencse, G., Georgescu, M., and Y. Kadobayashi,
"Benchmarking Methodology for DNS64 Servers",
DOI 10.1016/j.comcom.2017.06.004, pp. 162-175, no. 1,
vol. 109, Computer Communications , September 2017,
<https://www.sciencedirect.com/science/article/pii/
S0140366416305904?via%3Dihub>.
[FCC] FCC, "Measuring Broadband America Mobile 2013-2018
Coarsened Data", December 2018, <https://www.fcc.gov/
reports-research/reports/measuring-broadband-america/
measuring-broadband-america-mobile-2013-2018>.
[IPV4ONLY-ARPA]
Cheshire, S. and D. Schinazi, "Special Use Domain Name
'ipv4only.arpa'", Work in Progress, Internet-Draft, draft-
cheshire-sudn-ipv4only-dot-arpa-14, 3 November 2018,
<https://tools.ietf.org/html/draft-cheshire-sudn-ipv4only-
dot-arpa-14>.
[IPv6-TRANSITION]
Lencse, G., Palet, J., Howard, L., Patterson, R., and I.
Farrer, "Pros and Cons of IPv6 Transition Technologies for
IPv4aaS", Work in Progress, Internet-Draft, draft-lmhp-
v6ops-transition-comparison-03, 6 July 2019,
<https://tools.ietf.org/html/draft-lmhp-v6ops-transition-
comparison-03>.
[ISOC] ISOC, "State of IPv6 Deployment 2018", June 2018,
<https://www.internetsociety.org/resources/2018/state-of-
ipv6-deployment-2018/>.
[OPT-464XLAT]
Palet, J. and A. D'Egidio, "464XLAT Optimization", Work in
Progress, Internet-Draft, draft-palet-v6ops-464xlat-opt-
cdn-caches-03, 8 July 2019, <https://tools.ietf.org/html/
draft-palet-v6ops-464xlat-opt-cdn-caches-03>.
[PREF64] Colitti, L. and J. Linkova, "Discovering PREF64 in Router
Advertisements", Work in Progress, Internet-Draft, draft-
ietf-6man-ra-pref64-06, 3 October 2019,
<https://tools.ietf.org/html/draft-ietf-6man-ra-
pref64-06>.
[QUIC-CONNECTIONS]
Huitema, C., Shore, M., Mankin, A., Dickinson, S., and J.
Iyengar, "Specification of DNS over Dedicated QUIC
Connections", Work in Progress, Internet-Draft, draft-
huitema-quic-dnsoquic-07, 7 September 2019,
<https://tools.ietf.org/html/draft-huitema-quic-dnsoquic-
07>.
[RFC6889] Penno, R., Saxena, T., Boucadair, M., and S. Sivakumar,
"Analysis of Stateful 64 Translation", RFC 6889,
DOI 10.17487/RFC6889, April 2013,
<https://www.rfc-editor.org/info/rfc6889>.
[RFC6950] Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
"Architectural Considerations on Application Features in
the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,
<https://www.rfc-editor.org/info/rfc6950>.
[RFC7051] Korhonen, J., Ed. and T. Savolainen, Ed., "Analysis of
Solution Proposals for Hosts to Learn NAT64 Prefix",
RFC 7051, DOI 10.17487/RFC7051, November 2013,
<https://www.rfc-editor.org/info/rfc7051>.
[RFC7269] Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64
Deployment Options and Experience", RFC 7269,
DOI 10.17487/RFC7269, June 2014,
<https://www.rfc-editor.org/info/rfc7269>.
[RFC7755] Anderson, T., "SIIT-DC: Stateless IP/ICMP Translation for
IPv6 Data Center Environments", RFC 7755,
DOI 10.17487/RFC7755, February 2016,
<https://www.rfc-editor.org/info/rfc7755>.
[RFC7756] Anderson, T. and S. Steffann, "Stateless IP/ICMP
Translation for IPv6 Internet Data Center Environments
(SIIT-DC): Dual Translation Mode", RFC 7756,
DOI 10.17487/RFC7756, February 2016,
<https://www.rfc-editor.org/info/rfc7756>.
[RFC7849] Binet, D., Boucadair, M., Vizdal, A., Chen, G., Heatley,
N., Chandler, R., Michaud, D., Lopez, D., and W. Haeffner,
"An IPv6 Profile for 3GPP Mobile Devices", RFC 7849,
DOI 10.17487/RFC7849, May 2016,
<https://www.rfc-editor.org/info/rfc7849>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8219] Georgescu, M., Pislaru, L., and G. Lencse, "Benchmarking
Methodology for IPv6 Transition Technologies", RFC 8219,
DOI 10.17487/RFC8219, August 2017,
<https://www.rfc-editor.org/info/rfc8219>.
[RFC8585] Palet Martinez, J., Liu, H. M.-H., and M. Kawashima,
"Requirements for IPv6 Customer Edge Routers to Support
IPv4-as-a-Service", RFC 8585, DOI 10.17487/RFC8585, May
2019, <https://www.rfc-editor.org/info/rfc8585>.
[RIPE-690] RIPE, "Best Current Operational Practice for Operators:
IPv6 prefix assignment for end-users - persistent vs non-
persistent, and what size to choose", October 2017,
<https://www.ripe.net/publications/docs/ripe-690>.
[Threat-DNS64]
Lencse, G. and Y. Kadobayashi, "Methodology for the
identification of potential security issues of different
IPv6 transition technologies: Threat analysis of DNS64 and
stateful NAT64", DOI 10.1016/j.cose.2018.04.012,
pp. 397-411, no. 1, vol. 77, Computers & Security , August
2018, <https://www.sciencedirect.com/science/article/pii/
S0167404818303663?via%3Dihub>.
Appendix A. Example of Broadband Deployment with 464XLAT
This section summarizes how an operator may deploy an IPv6-only
network for residential/SOHO customers, supporting IPv6 inbound
connections, and IPv4-as-a-Service (IPv4aaS) by using 464XLAT.
Note that an equivalent setup could also be provided for enterprise
customers. If they need to support IPv4 inbound connections, several
mechanisms, depending on specific customer needs, allow it; see
[RFC7757].
Conceptually, most of the operator network could be IPv6 only
(represented in the next figures as "IPv6-only flow"), or even if
part of the network is actually dual stack, only IPv6 access is
available for some customers (i.e., residential customers). This
part of the network connects the IPv6-only subscribers (by means of
IPv6-only access links) to the IPv6 upstream providers and to the
IPv4-Internet by means of NAT64 (PLAT in the 464XLAT terminology).
The traffic flow from and back to the CE to services available in the
IPv6 Internet (or even dual-stack remote services, when IPv6 is being
used) is purely native IPv6 traffic, so there are no special
considerations about it.
From the DNS perspective, there are remote networks with IPv4 only
that will typically have only IPv4 DNS (DNS/IPv4) or will at least be
seen as IPv4 DNS from the CE perspective. On the operator side, the
DNS, as seen from the CE, is only IPv6 (DNS/IPv6), and it also has a
DNS64 function.
On the customer LANs side, there is actually one network, which of
course could be split into different segments. The most common setup
will be dual-stack segments, using global IPv6 addresses and
[RFC1918] for IPv4, in any regular residential / Small Office, Home
Office (SOHO) IPv4 network. In the figure below, it is represented
as tree segments to show that the three possible setups are valid
(IPv6 only, IPv4 only, and dual stack).
.-----. +-------+ .-----. .-----.
/ IPv6- \ | | / \ / \
( only )--+ Res./ | / IPv6- \ .-----. / IPv4- \
\ LANs / | SOHO +--( only )--( NAT64 )--( only )
`-----' | | \ flow / `-----' \ flow /
.-----. | IPv6 | \ / \ /
/ IPv4- \ | CE | `--+--' `--+--'
( only )--+ with | | |
\ LANs / | CLAT | +---+----+ +---+----+
`-----' | | |DNS/IPv6| |DNS/IPv4|
.-----. +---+---+ | with | +--------+
/ Dual- \ | | DNS64 |
( Stack )------| +--------+
\ LANs /
`-----'
Figure 16: CE Setup with Built-In CLAT, with DNS64
In addition to the regular CE setup, which typically will be access-
technology dependent, the steps for the CLAT function configuration
can be summarized as follows:
1. Discovery of the PLAT (NAT64) prefix: It may be done using
[RFC7050], [RFC7225] in those networks where PCP is supported, or
other alternatives that may be available in the future, such as
Router Advertising [PREF64] or DHCPv6 options [DHCPv6-OPTIONS].
2. If the CLAT function allows stateless NAT46 translation, a /64
from the pool typically provided to the CE by means of DHCPv6-PD
[RFC8415] needs to be set aside for that translation. Otherwise,
the CLAT is forced to perform an intermediate stateful NAT44
before the stateless NAT46, as described in Section 4.8.
A more detailed configuration approach is described in [RFC8585].
The operator network needs to ensure that the correct responses are
provided for the discovery of the PLAT prefix. It is highly
recommended that [RIPE-690] be followed in order to ensure that
multiple /64s are available, including the one needed for the NAT46
stateless translation.
The operator needs to understand other issues, as described
throughout this document, in order to make relevant decisions. For
example, if several NAT64 functions are needed in the context of
scalability / high availability, an NSP should be considered (see
Section 4.5).
More complex scenarios are possible, for example, if a network offers
multiple NAT64 prefixes, destination-based NAT64 prefixes, etc.
If the operator decides not to provide a DNS64 function, then this
setup will be the same as the following figure. This will also be
the setup that will be seen from the perspective of the CE, if a
foreign DNS is used and consequently is not the operator-provided
DNS64 function.
.-----. +-------+ .-----. .-----.
/ IPv6- \ | | / \ / \
( only )--+ Res./ | / IPv6- \ .-----. / IPv4- \
\ LANs / | SOHO +--( only )--( NAT64 )--( only )
`-----' | | \ flow / `-----' \ flow /
.-----. | IPv6 | \ / \ /
/ IPv4- \ | CE | `--+--' `--+--'
( only )--+ with | | |
\ LANs / | CLAT | +---+----+ +---+----+
`-----' | | |DNS/IPv6| |DNS/IPv4|
.-----. +---+---+ +--------+ +--------+
/ Dual- \ |
( Stack )------|
\ LANs /
`-----'
Figure 17: CE Setup with Built-In CLAT, without DNS64
In this case, the discovery of the PLAT prefix needs to be arranged
as indicated in Section 4.1.1.
In addition, if the CE doesn't have a built-in CLAT function, the
customer can choose to set up the IPv6 operator-managed CE in bridge
mode (and optionally use an external router). Or, for example, if
there is an access technology that requires some kind of media
converter (Optical Network Termination (ONT) for fiber to the home
(FTTH), Cable Modem for Data-Over-Cable Service Interface
Specification (DOCSIS), etc.), the complete setup will look like
Figure 18. Obviously, there will be some intermediate configuration
steps for the bridge, depending on the specific access technology/
protocols, which should not modify the steps already described in the
previous cases for the CLAT function configuration.
+-------+ .-----. .-----.
| | / \ / \
| Res./ | / IPv6- \ .-----. / IPv4- \
| SOHO +--( only )--( NAT64 )--( only )
| | \ flow / `-----' \ flow /
| IPv6 | \ / \ /
| CE | `--+--' `--+--'
| Bridge| | |
| | +---+----+ +---+----+
| | |DNS/IPv6| |DNS/IPv4|
+---+---+ +--------+ +--------+
|
.-----. +---+---+
/ IPv6- \ | |
( only )--+ IPv6 |
\ LANs / | Router|
`-----' | |
.-----. | with |
/ IPv4- \ | CLAT |
( only )--+ |
\ LANs / | |
`-----' | |
.-----. +---+---+
/ Dual- \ |
( Stack )------|
\ LANs /
`-----'
Figure 18: CE Setup with Bridged CLAT, without DNS64
Several routers (i.e., the operator-provided CE and the downstream
user-provided router) that enable simultaneous routing and/or CLAT
should be avoided to ensure that multiple NAT44 and NAT46 levels are
not used and that the operation of multiple IPv6 subnets is correct.
In those cases, the use of the Home Networking Control Protocol
(HNCP) [RFC8375] is suggested.
Note that the procedure described here for the CE setup can be
simplified if the CE follows [RFC8585].
Appendix B. CLAT Implementation
In addition to the regular set of features for a CE, a CLAT CE
implementation requires support for:
* [RFC7915] for the NAT46 function.
* [RFC7050] for the PLAT prefix discovery.
* [RFC7225] for the PLAT prefix discovery if PCP is supported.
* [PREF64] for the PLAT prefix discovery by means of Router
Advertising.
* [DHCPv6-OPTIONS] for the PLAT prefix discovery by means of DHCP.
* If stateless NAT46 is supported, a mechanism to ensure that
multiple /64 are available, such as DHCPv6-PD [RFC8415], must be
used.
There are several Open Source implementations of CLAT, such as:
* Android: https://github.com/ddrown/android_external_android-clat
* Jool: https://www.jool.mx
* Linux: https://github.com/toreanderson/clatd
* OpenWRT: https://git.openwrt.org/?p=openwrt%2Fopenwrt.git&a=search
&h=refs%2Ftags%2Fv19.07.0-rc1&st=commit&s=464xlat
* VPP: https://git.fd.io/vpp/tree/src/plugins/nat
Appendix C. Benchmarking
A benchmarking methodology for IPv6 transition technologies has been
defined in [RFC8219]. NAT64 and 464XLAT are addressed among the
single- and double-translation technologies, respectively. DNS64 is
addressed in Section 9, and the methodology is elaborated in
[DNS64-BM-Meth] of that document.
Several documents provide references to benchmarking results, for
example, for DNS64 [DNS64-Benchm].
Acknowledgements
The author would like to acknowledge the inputs of Gabor Lencse,
Andrew Sullivan, Lee Howard, Barbara Stark, Fred Baker, Mohamed
Boucadair, Alejandro D'Egidio, Dan Wing, Mikael Abrahamsson, and Eric
Vyncke.
Conversations with Marcelo Bagnulo, one of the coauthors of NAT64 and
DNS64, and email correspondence via the IETF mailing lists with Mark
Andrews have been very useful for this work.
Work on this document was inspired by Christian Huitema, who
suggested that DNS64 should never be used when deploying CLAT in the
IETF network.
Author's Address
Jordi Palet Martinez
The IPv6 Company
Molino de la Navata, 75
28420 La Navata - Galapagar Madrid
Spain
Email: jordi.palet@theipv6company.com
URI: http://www.theipv6company.com/