v6ops J. Palet Martinez
Internet-Draft The IPv6 Company
Intended status: Informational A. D'Egidio
Expires: January 12, 2021 Telecentro
July 11, 2020
464XLAT/NAT64 Optimization
draft-ietf-v6ops-464xlat-optimization-02
Abstract
IP/ICMP Translation Algorithm (SIIT) can be used to provide access
for IPv4-only devices or applications to IPv4-only or dual-stack
destinations over IPv6-only infrastructure. In that case, the
traffic flows are translated twice: first from IPv4 to IPv6
(stateless NAT46 at the ingress point to the IPv6-only
infrastructure) and then from IPv6 back to IPv4 (stateful NAT64, at
the egress point). When the destination is IPv6-enabled, the second
translation might be avoided. This document describes a possible
optimization to 464XLAT and MAP-T to avoid translating IPv6 flows
back to IPv4 if the destination is reachable over IPv6. The proposed
solution would significantly reduce the NAT64 utilization in the
operator's network.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on January 12, 2021.
Copyright Notice
Copyright (c) 2020 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
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Possible Optimization . . . . . . . . . . . . . . . . . . . . 3
4. Problem Statement Summary . . . . . . . . . . . . . . . . . . 5
5. Solution Approaches . . . . . . . . . . . . . . . . . . . . . 7
5.1. Approach 1: DNS/Routing-based Solution . . . . . . . . . 7
5.2. Approach 2: NAT46/CLAT/DNS-proxy-EAM-based Solution . . . 8
5.2.1. Detection of IPv4-only hosts or applications . . . . 9
5.2.2. Detection of IPv6-enabled service . . . . . . . . . . 9
5.2.3. Creation of EAMT entries . . . . . . . . . . . . . . 9
5.2.4. Forwarding path via stateful NAT for existing EAMT
entries . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.5. Maintenance of the EAMT entries . . . . . . . . . . . 11
5.2.6. Usage example . . . . . . . . . . . . . . . . . . . . 11
5.2.7. Behavior in case of multiple A/AAAA RRs . . . . . . . 12
5.2.8. Behavior in presence/absence of DNS64 . . . . . . . . 12
5.2.9. Behavior when using literal addresses or non
IPv6-compliant APIs . . . . . . . . . . . . . . . . . 12
5.2.10. Behavior in case of Foreign DNS . . . . . . . . . . . 12
5.2.11. False detection of a dual-stack host as IPv4-only . . 13
5.2.12. Behavior in presence of Happy Eyeballs . . . . . . . 14
5.2.13. Troubleshooting Implications . . . . . . . . . . . . 16
5.3. Approach 3: NAT46/CLAT-provider-EAM-based Solution . . . 16
6. IPv6-only Services become accessible to IPv4-only
devices/apps . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Different transition mechanisms, typically in the group of the so-
called IPv6-only with IPv4aaS (IPv4-as-a-Service), such as 464XLAT
([RFC6877]) or MAP-T ([RFC7599]), allow IPv4-only devices or
applications to connect with IPv4 services in Internet over IPv6-only
infrastructure, by means of a stateless NAT46 SIIT (IP/ICMP
Translation Algorithm) as described by [RFC7915].
This is done by the implementation of SIIT at the CE (Customer Edge)
Router or sometimes at the end-device, for example, the UE (User
Equipment) in cellular networks. This functionality is the CLAT
(Customer Translator) in the case of 464XLAT, while in the case of
MAP-T is called NAT46.
The NAT46/CLAT (WAN side) is connected by IPv6-only to the operator
network, which in turn, will have a reverse translation, the NAT64
([RFC6146]), known as PLAT (Provider Translator) in the case of
464XLAT. This allows to translate the IPv6 flow back to IPv4, in
order to forward it to Internet.
In both cases (NAT46 and NAT64), the translation of the packet
headers is done using the IP/ICMP translation algorithm defined in
[RFC7915]. Translation between IPv4 and IPv6 addresses is done as
per [RFC6052]. The NAT64 prefix should be discovered by the CE by
one or more of the existing mechanisms ([RFC7225], [RFC8781] or
[RFC7050]), and sometimes it is pre-configured at the CE to the WKP.
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. Possible Optimization
In the case of 464XLAT, a DNS64 ([RFC6147]) is (optionally) in charge
of the synthesis of AAAA records from the A records, so the NAT64 can
be used without the need of doing a double-translation by means of
the NAT46/CLAT.
However, the DNS64 is not useful for the IPv4-only devices or
applications in the LANs, as they will not be able to use the AAAA
records, so they are always forced to use the double-translation.
This is the expected behavior, as the original design of NAT64 was
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targeted to connect IPv6-only devices (using DNS) to IPv4-only
services. 464XLAT expanded the solution to also allow IPv4-only
devices (even if not using DNS) to connect to IPv4-only services by
means of IPv6-only access networks.
The optimization solutions presented by this document try to avoid
this double-translation, in the cases when the Internet services, are
already IPv6-enabled. So, in those cases, if the NAT46 already
translated the IPv4 flow to IPv6, it doesn't look necessary to
translate this back to IPv6.
A typical 464XLAT deployment is depicted in Figure 1.
+-------+ .-----. .-----.
| IPv6 | / \ / \
.-----. | CE | / IPv6- \ .-----. / IPv4 \
/ \ | or +--( only )---( NAT64 )---( Internet )
/ Dual- \ | UE | \ Access /\ `-----' \ /
( Stack )--+ | \ / \ \ /
\ LAN's / | with | `--+--' \ .-----. `-----'
\ / | NAT46 | | \ / \
`-----' | CLAT | +---+----+ / IPv6 \
| | | DNS/ | ( Internet )
+-------+ | DNS64 | \ /
+--------+ \ /
`-----'
Figure 1: Typical 464XLAT Deployment
Examples of a topology shown on the above picture includes:
o An IPv6-only residential access network where the CE Router (with
NAT46/CLAT) supports Dual-Stack in the customer LANs.
o An IPv6-only cellular network where a UE uses the NAT46/CLAT for
dual-stack internal applications and other devices connected via
tethering.
If the operator is providing direct access, for example, to Content
Delivery Networks (CDNs), caches, or other resources, and they are
dual-stacked, the situation can be described as shown in Figure 2.
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+-------+ .-----. .-----.
| IPv6 | / \ / \
.-----. | CE | / IPv6- \ .-----. / IPv4 \
/ \ | or +--( only )---( NAT64 )---( Internet )
/ Dual- \ | UE | \ Access /\ `-----' \ /
( Stack )--+ | \ / \ \ /
\ LAN's / | with | `--+--' \ .-----. `--+--'
\ / | NAT46 | | \ / \ \
`-----' | CLAT | +---+----+ / IPv6 \ .--+--.
| | | DNS/ | ( Internet ) / Dual- \
+-------+ | DNS64 | \ /----/ Stack \
+--------+ \ / ( )
`-----' \ CDNs/ /
\ Caches/
`-----'
Figure 2: Typical 464XLAT Deployment with CDNs/Caches
In this case if the flows initiated in the LANs come from IPv4-only
devices or applications, even if the destination resources are
IPv6-enabled, the double-translation is enforced, which has the
following consequences:
o More traffic needs to pass thru the NAT64 devices.
o More NAT64 devices may be needed to handle the additional traffic.
o Additional usage of IPv4 addresses.
o Additional state at the NAT64 devices.
o Additional logging, its relevant storage and processing resources.
Clearly, all those aspects have impact in both, CapEx and OpEx. This
is extremely important when considering that most of the time, the
contents stored in CDNs, caches, and so on, is there for a good
reason: It is frequently accessed resources and/or big. Examples
such as video, audio, software and updates, are very common. So,
this optimization can be highly impacting in many networks.
4. Problem Statement Summary
If the devices or applications in the customer LAN are IPv6-capable,
then the access to the CDNs, caches or other resources, will be made
in an optimized way, by means of IPv6-only, not using the NAT64, as
depicted in Figure 3.
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+-------+ .-----. .-----.
| IPv6 | / \ / \
.-----. | CE | / IPv6- \ .-----. / IPv4 \
/ \ | or +--( only )---( NAT64 )---( Internet )
/ IPv6 \ | UE | \ Access /\ `-----' \ /
( capable )--+ | \ / \ \ /
\ apps / | with | `--+--' \ .-----. `-----'
\ / | NAT46 | | \ / \
`-----' | CLAT | +---+----+ / IPv6 \ .-----.
| | | DNS/ | ( Internet )IPv6/ Dual- \
+-------+ | DNS64 | \ /----/ Stack \
+--------+ \ / ( )
`-----' \ CDNs/ /
\ Caches/
`-----'
<---------------------- end-to-end IPv6 flow ---------------------->
Figure 3: 464XLAT access to CDNs/Caches by IPv6-capable apps
However, if the devices or applications are IPv4-only, for example,
many SmartTVs and Set-Top-Boxes available today, a non-optimal double
translation will occur (NAT46 at the CLAT and NAT64 at the PLAT), as
illustrated in Figure 4.
+-------+ .-----. .-----.
| IPv6 | / \ / \
.-----. | CE | / IPv6- \ .-----. / IPv4 \
/ IPv4- \ | or +--( only )---( NAT64 )---( Internet )
/ only \ | UE | \ Access /\ `-----' \ /
( SmartTV )--+ | \ / \ \ /
\ STB / | with | `--+--' \ .-----. `--+--'
\ VoIP / | NAT46 | | \ / \ \ IPv4
`-----' | CLAT | +---+----+ / IPv6 \ .--+--.
| | | DNS/ | ( Internet ) / Dual- \
+-------+ | DNS64 | \ / / Stack \
+--------+ \ / ( )
`-----' \ CDNs/ /
\ Caches/
`-----'
<-------------------- IPv4 to IPv6 to IPv4 flow -------------------->
Figure 4: 464XLAT access to CDNs/Caches by IPv4-only apps
Clearly, this is a non-optimal situation, as it means that even if
there is a dual-stack service, the NAT46/CLAT translated IPv4 to IPv6
traffic flow, is unnecessarily translated back to IPv4, traversing
the stateful NAT64. This has a direct impact in the need to scale
the NAT64 beyond what will be actually needed, if possible solutions,
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in order to keep using the IPv6 path towards those services, are
considered.
As shown in the Figure 4, this is also the case for many other
services, not just CDNs or caches, such as VoIP access to the
relevant operator infrastructure, which may be also dual-stack. This
is true as well for many other dual-stack or IPv6-enabled services,
which may be directly reachable from the operator infrastructure,
even if they are not part of it, for example peering agreements,
services in IXs, etc. In general, this will become a more frequent
situation for many other services, which are not yet dual-stack.
For simplicity, across the rest of this document, references to CDNs/
caches, should be understood, unless otherwise stated, as any dual-
stacked resources.
This document looks into different possible solution approaches in
order to optimize the IPv4-only SIIT translation providing a direct
path to IPv6-capable services, as depicted in Figure 5.
+-------+ .-----. .-----.
| IPv6 | / \ / \
.-----. | CE | / IPv6- \ .-----. / IPv4 \
/ IPv4- \ | or +--( only )---( NAT64 )---( Internet )
/ only \ | UE | \ Access /\ `-----' \ /
( SmartTV )--+ | \ / \ \ /
\ STB / | with | `--+--' \ .-----. `-----'
\ VoIP / | NAT46 | | \ / \
`-----' | CLAT | +---+----+ / IPv6 \ .-----.
| | | DNS/ | ( Internet )IPv6/ Dual- \
+-------+ | DNS64 | \ /----/ Stack \
+--------+ \ / ( )
`-----' \ CDNs/ /
\ Caches/
`-----'
<------------------------ IPv4 to IPv6 flow ------------------------>
Figure 5: Optimized 464XLAT access to CDNs/Caches by IPv4-only apps
5. Solution Approaches
5.1. Approach 1: DNS/Routing-based Solution
Because the IPv4-only devices will not be able to query for AAAA
records, the NAT46/CLAT/CE will translate the IPv4 addresses from the
A record for the CDN/cache destination, using the WKP or NSP, as
configured by the operator.
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If the CDN/cache provider is able to configure, in the relevant
interfaces of the CDN/caches, the same IPv6 addresses that will
naturally result as the translated destination addresses for the
queried A records, preceded by the WKP or NSP, then having more
specific routing prefixes, will result in traffic to those
destinations being directly forwarded towards those interfaces,
instead of needing to traverse the NAT64.
For example, let's suppose a provider using the WKP (64:ff9b::/96)
and a SmartTV querying for www.example.com:
www.example.com A 192.0.2.1
NAT46/CLAT translated to 64:ff9b::192.0.2.1
CDN IPv6 interface must be 64:ff9b::192.0.2.1
Operator must have a specific route to 64:ff9b::192.0.2.1
Note: Examples using text representation as per Section 2.3 of
[RFC6052] and IPv4 documentation addresses following [RFC5737].
It should be remarked that this approach requires that the path to
the destination is configured in such way (i.e., more specific
routing prefixes), that doesn't traverse the NAT64 devices.
Because the WKP is non-routable, this solution will only be possible
if the CDN/cache is in the same ASN as the provider network, or
somehow interconnected without routing thru Internet.
This solution has the additional drawback of the operational
complexity/issues added to the operation of the CDN/cache, and the
need to synchronize any IPv4 interface address changes with the
relevant IPv6 ones, and possibly with routing.
5.2. Approach 2: NAT46/CLAT/DNS-proxy-EAM-based Solution
If the NAT46/CLAT/CE, as commonly is the case, is also a DNS proxy/
stub resolver, it is possible to modify the behavior and create an
"internal" interaction among both of them.
This approach uses the existing IPv4 and IPv6 addresses in the A and
AAAA records, respectively, so no additional complexity/issues added
to the CDN/caches operations.
The following sub-sections detail this approach and provide a step-
by-step example case. Note that this optimization MUST NOT be
enabled when the WAN link is IPv4-only or dual-stack. In other
words, only can be enabled if the WAN link is IPv6-only and
consequently, the NAT46/CLAT is enabled in the CE.
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5.2.1. Detection of IPv4-only hosts or applications
The assumption is that, typically a dual-stack host will prefer using
IPv6 as the DNS transport. So, when there is a DNS query,
transported with IPv4, for an A record, and there is not a query for
the AAAA record from the same IPv4 source (to the same destination),
the DNS proxy/stub resolver can infer that, most probably, it is an
IPv4-only device or application.
It needs to be remarked that, if the detection of the IPv4-only
device or application is done incorrectly (either not detecting it or
by a false detection), no harm is caused. In the worst case,
optimization will not be performed, at least, at the time being.
However, optimization maybe performed later on, if a new detection
succeeds (for example, another device using the same A record).
5.2.2. Detection of IPv6-enabled service
In the case of an IPv4-only detected device or application, the DNS
proxy/stub resolver MUST actually perform an additional AAAA query,
unless the information is already present in the Additional Section,
as per Section 3 of [RFC3596]. Note that the NAT46/CLAT MUST already
know the WKP or NSP being used in that network. If the response
contains at least one IPv6 address not using the WKP/NSP, it means
that the destination is IPv6-enabled (because at least one of the
IPv6 addresses is not synthesized). This means that it is possible
for the NAT46/CLAT, to create an Explicit Address Mapping
([RFC7757]).
5.2.3. Creation of EAMT entries
This way, an EAM Table (EAMT used for short, across the rest of this
document) is created/maintained automatically by the DNS proxy/stub
resolver in the NAT46/CLAT, and the NAT46/CLAT is responsible to
prioritize any available entries in the EAMT, versus the use of any
synthetic AAAA.
In order to create the EAMT entry, to determine if there is an AAAA
record after an A record query, it is suggested to use the same delay
value (50 milliseconds) as the "Resolution Delay" indicated by Happy
Eyeballs [RFC8305]. This avoids a slight NAT64 overload and flapping
between destination addresses (IPv4/IPv6), which may impact some
applications, at the cost of a small extra delay for the initial
communication setup, when the EAMT entry doesn't yet exist.
Each EAMT entry MUST contain, the fields already described in
[RFC7757] and a few new ones:
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1. ID: EAMT Entry Index (optional).
2. IPv4 address/prefix: By default, the prefix length is 32 bits.
3. IPv6 address/prefix: By default, the prefix length is 128 bits.
4. TTL: Because the optimization will make use of the AAAA (IPv6
address), the TTL for the EAMT entry MUST be set to the same
value as in the AAAA RR. In normal conditions the TTL for both A
and AAAA records, of a given FQDN, should be the same, so this
ensures a proper behavior if there is any DNS mismatch.
5. FQDN: The one that originated the A query for this EAMT entry.
Required in order to ensure a correct detection of cases such as
the use of reverse-proxy with a single IPv4 address to multiple
IPv6 addresses.
6. Valid/Invalid: When set to 1, means that this EAMT entry MUST NOT
be used and consequently no optimization performed. It may be
used also for an explicit configuration (GUI, CLI, provisioning
system, etc.) to disallow optimization for explicitly configured
IPv4 addresses.
7. Auto/Static: When set to 1, means that this EAMT entry has been
manually/statically configured, for example by means of an
explicit configuration (GUI, CLI, provisioning system, etc.), so
it doesn't expire with TTL.
When a new EAMT entry is first automatically created, it is marked as
"Valid" and "Auto" (both bits cleared). If a subsequent A query,
with a different FQDN, results in an IPv4 address that has already an
EAMT entry and a different IPv6 address, it means that some reverse-
proxy or similar functionality is being used by the IPv6-enabled
service. In this case, the existing EAMT entry will be marked as
"Invalid" (bit set). No new EAMT entry is created for that IPv4
address. Otherwise, the optimization will only allow to access the
first set of IPv4/IPv6/FQDN, which may break the access to other FQDN
that share the same IPv4 address and different IPv6 addresses.
In this case the EAMT entry will still expire according the TTL,
which allows to re-enable optimization if a new query for the A
record has changed the situation. For example, maybe the reverse-
proxy has been removed, or there is now only a single device using
it, so at the time being, the optimization is again possible without
creating troubles to other hosts.
Note that when an EAMT entry is marked as "invalid", it will not
affect the devices or applications, as they will still be able to use
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the regular CLAT+NAT64 flow, of course, without the optimization.
Note the newly defined EAMT fields, follow the "extensions" approach
as per section 3.1 of [RFC7757].
5.2.4. Forwarding path via stateful NAT for existing EAMT entries
Following this approach, if there is a valid EAMT entry, for a given
IPv4-destination, the IPv6-native path pointed by the IPv6 address of
that EAMT entry, will take precedence versus the NAT64 path, so the
traffic will not be forwarded to the NAT64.
However, this is not sufficient to ensure that individual
applications are able to keep existing connections. In many cases,
audio and video streaming may use a single TCP connection lasting
from minutes to hours. Instead, the CDN TTLs may be configured in
the range from 10 to 300 seconds in order to allow new resolutions to
switch quickly and to handle large recursive resolvers (with hundreds
of thousands of clients behind them).
Consequently, the EAMT entries should not be used directly to
establish a forwarding path, but instead, to create a stateful NAT
entry for the 4-tuple for the duration of the session/connection.
5.2.5. Maintenance of the EAMT entries
The information in the EAMT MUST be kept timely-synchronized with the
AAAA records TTL's, so the EAMT entries MUST expire on the AAAA TTL
expiry and consequently be deleted.
However, EAMT entries with the Auto/Static bit set, will not be
deleted. This allows users/operators to set explicit rules for
diagnostics or resolution of issues in special situations.
5.2.6. Usage example
Using the same example as in the previous approach:
www.example.com A 192.0.2.1
AAAA 2001:db8::a:b:c:d
EAMT entry 192.0.2.1 2001:db8::a:b:c:d
NAT46/CLAT translated to 2001:db8::a:b:c:d
CDN IPv6 interface already is 2001:db8::a:b:c:d
Operator already has a specific route to 2001:db8::a:b:c:d
The following is an example of the CE behavior after the previous
case has already created an EAMT entry and a reverse-proxy is
detected:
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1. A query for www.another-example.com A RR is received
2. www.another-example.com A 192.0.2.1
3. www.another-example.com AAAA 2001:db8::e:e:f:f
4. A conflict has been detected
5. The existing EAMT entry for 192.0.2.1 is set as invalid
5.2.7. Behavior in case of multiple A/AAAA RRs
Existing DNS proxy/stub resolvers already implement mechanisms for
DNS Load Balancing ([RFC1794]). This should not be modified to
implement the optimization so, if multiple A and/or AAAA records are
available, any of them could be chosen. In other words, the chosen
pair of A/AAAA records doesn't present any different result compared
with a situation when this mechanism is not used.
5.2.8. Behavior in presence/absence of DNS64
464XLAT can be deployed/used with and without a DNS64. However, as
indicated in Section 5.2.2, the EAMT entry is only created when the
service is IPv6-enabled, because the optimization is only relevant
for destinations which already have AAAA records. In those cases
DNS64 is not relevant.
5.2.9. Behavior when using literal addresses or non IPv6-compliant APIs
Because the EAMT entries are only created when the NAT46/CLAT/CE
proxy/stub DNS is being used, any devices or applications that don't
use DNS, will not create the relevant entries.
They may be optimized if devices or applications using DNS, at some
point, query for the same A RRs, or if EAMT entries are statically
configured.
5.2.10. Behavior in case of Foreign DNS
Devices or applications may use DNS servers from other networks. For
a complete description of reasons for that, refer to Section 4.4 of
[RFC8683]. In the case the DNS is modified, or some devices or
applications use other DNS servers, the possible scenarios and the
implications are:
a. Devices configured to use a DNS proxy/resolver which is not the
CE/NAT46/CLAT. In this case this optimization will not work,
because the EAMT entry will not be created based on their own
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flows. Nevertheless, the EAMT entry may be created by other
devices using the same destinations. However, the lack of EAMT
entry, will not impact negatively in the user's devices/
applications (the optimization is not performed). It should be
noticed that users commonly, don't change the configuration of
devices such as SmartTVs or STBs (if they do, some other
functionalities, such as CDN/caches optimizations may not work as
well), so this only happens typically if the vendor is doing it
on-purpose and for good well-known reasons.
b. DNS privacy/encryption. Hosts or applications that use
mechanisms for DNS privacy/encryption, such as DoT ([RFC7858],
[RFC8094]), DoH ([RFC8484]) or DoQ
([I-D.huitema-dprive-dnsoquic]), will not make use of the stub/
proxy resolver, so the same considerations as for the previous
case applies.
c. Users that modify the DNS in their Operating Systems. This is
quite frequent, however commonly Operating Systems are dual-
stack, so aren't part of the problem statement described by this
document and will not be adversely affected.
d. Users that modify the DNS in the CE. This is less common. In
this case, this optimization is not adversely affected, because
it doesn't depend on the operator DNS, it works only based on the
internal CE interaction between the NAT46/CLAT and the stub/proxy
resolver. Note that it may be affected if the operator offers
different "DNS views" or "split DNS", however this is not related
to this optimization and will anyway impact in the other possible
operator optimizations (i.e. CDN/cache features).
e. Combinations of the above ones. No further impact, than the one
already described, is observed.
5.2.11. False detection of a dual-stack host as IPv4-only
If a dual-stack host is issuing the A query using IPv4 transport, and
the AAAA query using IPv6 transport, or in the other way around, or
using different IPv4 addresses for the A and AAAA queries, the EAMT
entry will be created. However, this EAMT entry may not be used by
dual-stack devices or applications, because those devices or
applications should prefer IPv6. If the host is preferring IPv4 for
connecting to the CDN/cache or IPv6-enabled service, it will be
actually using the NAT46/CLAT, including the EAMT entry and
consequently IPv6, so this mechanism will be correcting an
undesirable behavior. This is a special case, which actually seems
to be an incoherent host or application implementation.
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Afterwards, if other IPv4-only devices or applications subsequently
need to connect to the same IPv6-enabled service, they will take
advantage of the already existing EAMT entry, and consequently use
the IPv6-optimised path.
5.2.12. Behavior in presence of Happy Eyeballs
Happy Eyeballs [RFC8305] is only enabled in dual-stack hosts.
Consequently, it is not affected by this optimization because both,
the A and the AAAA queries should be issued by the host as soon after
one another as possible. In summary, the host should not be detected
as IPv4-only, following Section 5.2.1.
Nevertheless, if the same NAT46/CLAT/CE is serving IPv4-only hosts
and dual-stack hosts and both of them are using the same
destinations, an EAMT entry may have been previously created for that
destination. Consequently, if Happy Eyeballs triggers a fallback to
IPv4, it will be actually using the relevant EAMT entry towards the
IPv6 destination. This has the disadvantage that the IPv4-IPv6-IPv4
translation path can't be used by Happy Eyeballs-enabled
applications, so avoiding a real IPv4-fallback and making IPv6 the
only available protocol.
This is the natural and expected path for IPv6-only networks, so
actually it may be considered as a good thing, in the sense that an
operator is interested in knowing as soon as possible, if the
IPv6-only network is not performing correctly.
Note that when using 464XLAT, the WAN link of the NAT46/CLAT/CE is
IPv6-only. So even if Happy Eyeballs is present, IPv4 is expected to
be slower than native IPv6 itself due to delays added by the
NAT46+NAT64 translations. This optimization reduces those delays by
eliminating the second translation (NAT64).
However, there may be cases where this may be understood as
problematic. The possible reasons why Happy Eyeballs may trigger an
IPv4 fallback, in the case of IPv6-only access networks with IPv4aaS,
in general, can be classified as:
1. Failure at the CE or customer LANs. It may happen that the CE or
other devices in the customer LANs are showing erratic behaviors
or malfunctions. It is difficult to believe that this happens
only with IPv6, and if that's the case Happy Eyeballs will not
resolve the issue, because IPv4 is provided as a service on top
of IPv6.
2. Complete failure of the IPv6-only link or IPv6-only operator's
infrastructure (up to the NAT64). In this case, IPv4 will not
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work for that subscriber. Happy Eyeballs will not resolve the
issue, and instead will only be adding some extra delay (the
attempt to fallback to IPv4 before timing-out).
3. Complete failure of both IPv4 and IPv6 links behind the
operator's NAT64 towards the destination. In this case,
typically both, IPv4 and IPv6 will fail (in many cases, they are
dual-stack links, not different links). Again, Happy Eyeballs
will also fail to resolve the issue.
4. Complete failure only in the IPv6 links behind the operator's
NAT64 towards the destination. This is less frequent, bus still
miss-configured AAAA RRs, or diverse paths for IPv4 and IPv6
together with outages or IPv6-only routing issues, could generate
this problem. In this case, Happy Eyeballs could resolve the
issue, however, the optimization will disallow it.
5. Partial failure: Slower IPv6 vs IPv4 path end-to-end. In
general, the added delay of the IPv4 translations and NAT across
the path, increases the chances that IPv4 is faster than IPv6.
However, it may happen that there is some IPv6 specific link
congestion or packet dropping, that generates the reverse
situation, so IPv4 becomes faster than IPv6. Because the
optimization, the end-to-end path is forced to be IPv6, so Happy
Eyeballs will not be able to offer any significative advantage in
resolving the issue.
In summary, the optimization may be hindering the Happy Eyeballs
assistance, only in the last two cases. In one of the cases (partial
failure: slower IPv6 vs IPv4 path end-to-end), just don't help to
make IPv6 faster. In the other case (complete failure only in the
IPv6 links behind the operator's NAT64 towards the destination), it
will completely fail. However, it should be observed that in both
cases, the problem will also impact other operators (even if not
using the optimization), and especially those using only NAT64+DNS64
instead of 464XLAT, or even more, any IPv6-only hosts or applications
in any operator network across the entire Internet. It looks like it
is very important to make sure that, as IPv6 is more prevalent, there
is a better monitoring and failures are detected ASAP, instead of
being hidden by Happy Eyeballs, specially in IPv6-only networks, so
it seems an acceptable trade-off. It should be noticed also that in
IPv6-only with IPv4aaS, the chances of troubles in the IPv4 paths
seem to be higher than in the IPv6, as there are more translations,
more devices, more delays, while the optimization will precisely
reduce them.
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5.2.13. Troubleshooting Implications
When there is a need to troubleshoot IPv4 from the CE LANs, it may
happen that there is an EAMT entry forcing the flow to a given
destination(s) to use IPv6, which will distort the results.
This can be avoided, using a CLI/GUI or provisioning procedure, to
either completely disable the optimization during the
troubleshooting, or create specific static EAMT entries, using the
Valid/Invalid and Auto/Static flags, as described in Section 5.2.3.
Consequently, the CE MUST allow both, disabling the optimization and
the setup of manual/static EAMT entries.
5.3. Approach 3: NAT46/CLAT-provider-EAM-based Solution
Instead of using the DNS proxy/stub resolver to create the EAMT
entries, the operator may push this table (or parts of it) into the
CE/NAT46/CLAT, by using configuration/management mechanisms.
This solution has the advantage of not being affected by any DNS
changes from the user (the EAMT is created by the operator) and
ensures a complete control from the operator. However, it may impact
the cases of devices with a DNS configured by the vendor.
In general, most of the considerations from the previous approach
will apply.
One more advantage of this solution is that the EAMT pairs doesn't
need to match the "real" IPv4/IPv6 addresses available in the A/AAAA
records, as shown in the next example.
www.example.com A 192.0.2.1
AAAA 2001:db8::a:b:c:d
EAMT pulled/pushed entry 192.0.2.1 2001:db8::f:e:d:c
NAT46/CLAT translated to 2001:db8::f:e:d:c
CDN IPv6 interface already is 2001:db8::f:e:d:c
Operator already has a specific route to 2001:db8::f:e:d:c
EAMT may contain TTLs which probably are derived from DNS ones, or
alternatively, a global TTL for the full table.
An alternative way to configure the table, is that the CE is actually
pulling the table (or parts of it) from the operator infrastructure.
In this case it will be mandatory that the entries have individual
TTLs, again probably derived from the DNS ones.
The major drawback of this approach is that it requires a new
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protocol, or an extension to existing ones, in order to push or pull
the EAMT, in addition to the possible impact in terms of bandwidth
each time the CEs reboot, or an EAMT must be pushed to all the CEs,
etc.
6. IPv6-only Services become accessible to IPv4-only devices/apps
One of the issues with the IPv6 deployment, is that those services
which become IPv6-only in Internet, aren't reachable by IPv4-only
devices and applications. This means that new content providers must
support dual-stack even for new services, even while IPv4 public
addresses aren't available.
If NAT46/CLAT/DNS-proxy-EAM approach (Section 5.2) is chosen, it also
offers the chance to resolve this issue. This is possible if
IPv6-only services get configured with an A resource record pointing
to a well-known IPv4 address despite they aren't actually connected
to IPv4. This is out of scope for this document, as it will require
further work and a reservation by IANA, This will mean that those
services will work fine if there is a NAT46/CLAT supporting the
optimization. This A RR has no negative impact if the NAT46/CLAT
doesn't exist, or it is not optimized, because is not reachable via
IPv4-only, so is not a different situation compared with not having
an A RR.
The result of this is equivalent to the approach taken by MAP-T
(Section 12.3 of [RFC7599]). However, it has the advantage that the
MAP-T approach is restricted to services in the MAP-T domain.
In fact, it may become an incentive for the IPv6 deployment in
Internet services, as it provides the option to use a well-known IPv4
address (maybe anycast) for the "non-valid" A RR, that points, in
case of port 80/443 to a web page or service that returns a warning
such as "This service is only available if the network is properly
connected to Internet with IPv6".
7. Conclusions
NAT46/CLAT/DNS-proxy-EAM approach (Section 5.2) seems the right
solution for optimizing the access to dual-stack services, whether
they are located inside or outside the ISP. It is also the only
approach which has no additional requirements for the network
operators (both ISPs and CDN/cache operators).
Having this type of optimization facilitates and increases the usage
of IPv6, even for IPv4-only devices and applications, at the same
time that decreases the use of the NAT64.
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SIIT already has a SHOULD for EAM support, so it is not a high
additional burden the support required for existing implementations
to be updated for this optimization.
8. Security Considerations
This document does not have any new specific security considerations,
besides the ones already known for DNS64.
Spoofed DNS responses could generate incorrect EAMT entries.
However, this seems not different than if the optimization is not in
place and the spoofed DNS responses are cached by the CE DNS proxy/
stub resolver or even by hosts in the CE LANs. It very much depends
on how and where the attack is actually performed.
In both cases, 464XLAT and MAP-T, the CE device should contain a DNS
proxy/stub resolver, which is also required for the optimization.
Nevertheless, it is common that the user change DNS settings. If it
happens, in the case of MAP-T, the port-set is restricted for an
efficient public IPv4 address sharing, so the entropy of the source
ports is significantly lowered. In this case, theoretically MAP-T is
less resilient against cache poisoning ([RFC5452]) compared with
464XLAT. However, an efficient cache poisoning attack requires that
the subscriber operates its own caching DNS server and the attack is
performed in the service provider network, so the chances of a
successful exploitation of this vulnerability are low.
9. IANA Considerations
This document does not have any new specific IANA considerations.
10. Acknowledgements
The authors would like to acknowledge the inputs of Erik Nygren, Fred
Baker, Martin Hunek, Chongfeng Xie, Fernando Gont, Fernando Frediani
and Jen Linkova.
11. References
11.1. Normative References
[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>.
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[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", STD 88,
RFC 3596, DOI 10.17487/RFC3596, October 2003,
<https://www.rfc-editor.org/info/rfc3596>.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC5452, January 2009,
<https://www.rfc-editor.org/info/rfc5452>.
[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>.
[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>.
[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>.
[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>.
[RFC7599] Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
and T. Murakami, "Mapping of Address and Port using
Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
2015, <https://www.rfc-editor.org/info/rfc7599>.
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[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>.
[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>.
[RFC8781] Colitti, L. and J. Linkova, "Discovering PREF64 in Router
Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
2020, <https://www.rfc-editor.org/info/rfc8781>.
11.2. Informative References
[I-D.huitema-dprive-dnsoquic]
Huitema, C., Mankin, A., and S. Dickinson, "Specification
of DNS over Dedicated QUIC Connections", draft-huitema-
dprive-dnsoquic-00 (work in progress), March 2020.
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
DOI 10.17487/RFC1794, April 1995,
<https://www.rfc-editor.org/info/rfc1794>.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737,
DOI 10.17487/RFC5737, January 2010,
<https://www.rfc-editor.org/info/rfc5737>.
[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>.
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[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>.
[RFC8683] Palet Martinez, J., "Additional Deployment Guidelines for
NAT64/464XLAT in Operator and Enterprise Networks",
RFC 8683, DOI 10.17487/RFC8683, November 2019,
<https://www.rfc-editor.org/info/rfc8683>.
Authors' Addresses
Jordi Palet Martinez
The IPv6 Company
Molino de la Navata, 75
La Navata - Galapagar, Madrid 28420
Spain
Email: jordi.palet@theipv6company.com
URI: http://www.theipv6company.com/
Alejandro D'Egidio
Telecentro
Argentina
Email: adegidio@telecentro.net.ar
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