v6ops                                                  J. Palet Martinez
Internet-Draft                                         Consulintel, S.L.
Intended status: Best Current Practice                   October 8, 2017
Expires: April 11, 2018

           464XLAT Deployment Guidelines in Operator Networks


   This document describes how 464XLAT ([RFC6877]) can be deployed in an
   IPv6 operator network and the issues to be considered.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 11, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  DNSSEC Considerations . . . . . . . . . . . . . . . . . . . .   3
     2.1.  DNSSEC validator aware of DNS64 . . . . . . . . . . . . .   4
     2.2.  Stub validator  . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  CLAT with DNS proxy and validator . . . . . . . . . . . .   4
     2.4.  ACL of clients  . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Mapping-out IPv4 addresses  . . . . . . . . . . . . . . .   4
   3.  Using 464XLAT with/without DNS64  . . . . . . . . . . . . . .   5
   4.  DNS64 and Reverse Mapping Considerations  . . . . . . . . . .   5
   5.  CLAT Translation Considerations . . . . . . . . . . . . . . .   6
   6.  Summary of deployment recommendations for 464XLAT . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   464XLAT ([RFC6877]) describes an architecture that provides IPv4
   connectivity across a network, or part of it, when it is only
   natively transporting IPv6.

   In order to do that, 464XLAT ([RFC6877]) relies on the combination of
   existing protocols:

   1.  The customer-side translator (CLAT) is a stateless IPv4 to IPv6
       translator (NAT46) ([RFC7915]) implemented in the end-user device
       or CE, located at the "customer" edge of the network.

   2.  The provider-side translator (PLAT) is a stateful NAT64
       ([RFC6146]), implemented typically at the opposite edge of the
       operator network, that provides access to both IPv4 and IPv6

   3.  Optionally, DNS64 ([RFC6147]), implemented as part of the PLAT
       allows 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 RR).

   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 initially in IPv6 cellular
   networks, so providing an IPv6-only access network, the end-user

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   device applications can access IPv4-only end-networks/applications,
   despite those applications or devices use literal IPv4 addresses or
   non-IPv6 compliant APIs.

   In addition to that, in the same example of the cellular network
   above, if the User Equipment (UE) provides tethering, other devices
   behind it will be presented with a traditional NAT44, in addition to
   the native IPv6 support.

   Furthermore, 464XLAT ([RFC6877]) can be used in non-cellular IPv6
   wired (xDSL, DOCSIS, FTTH, Ethernet, ...) and wireless (WiFi) network
   architectures, by implementing the CLAT functionality at the CE.

   The remaining sections of this document, despite of any specific
   examples being used, are applicable to any operator network
   architecture, and introduces possible issues and general deployment
   guidelines to be considered when deploying 464XLAT ([RFC6877]) in an
   IPv6 network.

2.  DNSSEC Considerations

   As indicated in Section 8 of [RFC6147] (DNS64, Security
   Considerations), because DNS64 modifies DNS answers and DNSSEC is
   designed to detect such modifications, DNS64 can break DNSSEC.

   If a device connected to an IPv6-only WAN queries for a domain name
   in a signed zone, by means of a recursive name server that supports
   DNS64, and the result is a synthesized AAAA record, and 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.  So, the recursive name server actually
   lie to the client device, however in most of the cases, the client
   will not notice it, because generally they don't perform validation
   themselves as instead rely on their recursive name servers.

   If the client device performs DNSSEC validation on the AAAA record,
   it will fail as it is a synthesized record.

   Similarly, if the client querying the recursive name server is
   another name server configured to use it as a forwarder, and is
   performing DNSSEC validation, it will also fail on any synthesized
   AAAA record.

   There are several possible solutions to avoid breaking DNSSEC:

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2.1.  DNSSEC validator aware of DNS64

   In general, DNS servers with DNS64 function, by default, will not
   synthesize AAAA responses if the DNSSEC OK (DO) flag was set in the
   query.  In this case, as only an A record is available, it means that
   the CLAT will take the responsibility, as in the case of literal IPv4
   addresses, to keep that traffic flow end-to-end as IPv4, so DNSSEC is
   not broken.

2.2.  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,
   as the DNS64 recursive server will not synthesize AAAA responses, the
   client could perform the DNSSEC validation with the A record and then
   may query the network for a NAT64 prefix ([RFC7050]) in order to
   synthesize the AAAA ([RFC6052]).  This allows the client device to
   avoid using the CLAT and still use NAT64 even with DNSSEC.

   Some devices/OSs may implement, instead of CLAT, a simliar function
   by using Bump-in-the-Host ([RFC6535]).  In this case, the
   considerations in the above paragraphs are also applicable.

2.3.  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, following the same approach described
   in the precedent section (Stub validator).  So the DNS proxy actually
   lie to the client devices, which in most of the cases will not notice
   it unless they perform validation themselves.  Again, this allow the
   clients devices to avoid using the CLAT and still use NAT64 with

2.4.  ACL of clients

   In cases of dual-stack clients, stub resolvers should send the AAAA
   queries before the A ones.  So such clients, if DNS64 is enabled,
   will never get A records, even for IPv4-only servers, and they may be
   in the path before the NAT64 and accesible by IPv4.  If DNSSEC is
   being used for all those flows, specific addresses or prefixes can be
   left-out the DNS64 synthesis by means of ACLs.

2.5.  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.

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   Even if this is not related to DNSSEC, this "mapping-out" feature is
   actually quite commonly used to ensure that [RFC1918] addresses (for
   example used by LAN servers) are not synthesized to AAAA.

3.  Using 464XLAT with/without DNS64

   In the case the client device is IPv6-only (either because the stack
   is IPv6-only, or because it is connected via an IPv6-only LAN) and
   the 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 among both.  Because DNS64
   is then required, DNSSEC validation will be only possible if the
   recursive name server is validating the negative response from the
   authoritative name server and the client is not performing

   However, when the client device is dual-stack and/or connected in a
   dual-stack LAN by means of a CLAT (or has the built-in CLAT), 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, so will use the IPv6 path and only one translation
       will be done at the NAT64.  This may break DNSSEC, unless
       measures as described in the precedent section 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 avoids the AAAA
       synthesis and consequently will never break DNSSEC.

   When clients in an operator network use DNS from other networks, for
   example manually configured by users, they may support or not DNS64,
   so the considerations in this section will apply as well.

4.  DNS64 and Reverse Mapping Considerations

   When a client device, using a name server configured to perform
   DNS64, tries to reverse-map a synthesized IPv6 address, the name
   server responds with a CNAME record pointing 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-

   This is the expected behaviour, so no issues to be considered
   regarding DNS reverse mapping.

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5.  CLAT Translation Considerations

   As described in Section 6.3 of [RFC6877] (IPv6 Prefix Handling), if
   the CLAT can be configured with a dedicated /64 prefix for the NAT64
   translation, then it will be possible to do a more efficient
   stateless translation.

   However, if this dedicated prefix is not available, the CLAT will
   need to do a stateful translation, for example performing stateful
   NAT44 for all the IPv4 LAN packets, so they appear as coming from a
   single IPv4 address, and then in turn, stateless translated to a
   single IPv6 address.

   The obvious recommended setup, in order to maximize the CLAT
   performance, is to configure the dedicated translation prefix.  This
   can be easily achieved automatically, if the CE or end-user device is
   able to obtain a shorter prefix by means of DHCPv6-PD ([RFC3633]), so
   the CE can use a /64 for that.

   The above recommendation is often not posible for cellular networks,
   when connecting UEs (some broadband cellular use DHCPv6-PD
   ([RFC3633]), but smartphones, in general, not), as they provide a
   single /64 for each PDP context and use /64 prefix sharing
   ([RFC6877]).  So in this case, the UEs typically have a build-in CLAT
   client, which is doing a stateful NAT44 before the stateless NAT46.

6.  Summary of deployment recommendations for 464XLAT

   As indicated in the introduction of this document, operators willing
   to deploy 464XLAT ([RFC6877]), MUST to support, at least, the
   provider-side translator (PLAT).

   In the case it is a non-cellular network and the operator is
   providing the CEs to the customers, or suggesting them some specific
   models, they MUST support the customer-side translator (CLAT).

   If the operator offers DNS services, in order to increase performance
   by reducing the double translation for all the IPv4 traffic, and
   avoid breaking DNSSEC, they MAY support DNS64.  In this case, if the
   DNS service is offering DNSSEC validation, then it MUST be in such
   way that it is aware of the DNS64.  This is considered de simpler and
   safer approach, and MAY be combined as well with the other possible
   solutions described in this document:

   o  Devices running CLAT SHOULD follow the indications in the "Stub
      validator" section recommendation.  However, most of the time,
      this is out of the control of the operator.

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   o  CEs SHOULD include a DNS proxy and validador.  This is relevant if
      the operator is providing the CE or suggesting it to customers.

   o  ACL of clients and Mapping-out IPv4 addresses MAY be considered by
      each operator, depending on their own infrastructure.

   The ideal configuration for CEs supporting CLAT, is that they support
   DHCPv6-PD ([RFC3633]) and internally reserve one /64 for the
   stateless NAT46 translation.  The operator MUST ensure that the
   customers get allocated prefixes shorter than /64 in order to support
   this optimization.  One way or the other, this is not impacting the
   performance of the operator network.

   As indicated in Section 7 of [RFC6877] (Deployment Considerations),
   operators MAY follow those suggestions in order to take advantage of
   traffic engineering.

   In the case of cellular networks, the considerations regarding DNSSEC
   may appear as out-of-scope, because UEs OSs, commonly don't support
   DNSSEC, however applications running on them may do, or it may be an
   OS "built-in" support in the future.  Moreover, if those devices
   offer tethering, other client devices may be doing the validation,
   hence the relevance of a proper DNSSEC support by the operator

   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 APN
   supporting all types of PDP context (IPv4, IPv6, IPv4v6), in such way
   that the network is able to automatically serve all the possible
   combinations of UEs.

   Finally, if the operator choose to secure the NAT64 prefix, it MUST
   follow the advise indicated in Section 3.1.1. of [RFC7050]
   (Validation of Discovered Pref64::/n).

7.  Security Considerations

   This document does not have any new specific security considerations.

8.  IANA Considerations

   This document does not have any new specific IANA considerations.

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9.  Acknowledgements

   The author would like to acknowledge the inputs of TBD ...  Christian
   Huitema inspired working in this document by suggesting that DNS64
   should never be used, during a discussion regarding the deployment of
   CLAT in the IETF network.

10.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,

   [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,

   [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,

   [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,

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,

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   [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,

   [RFC7278]  Byrne, C., Drown, D., and A. Vizdal, "Extending an IPv6
              /64 Prefix from a Third Generation Partnership Project
              (3GPP) Mobile Interface to a LAN Link", RFC 7278,
              DOI 10.17487/RFC7278, June 2014,

   [RFC7915]  Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
              "IP/ICMP Translation Algorithm", RFC 7915,
              DOI 10.17487/RFC7915, June 2016,

Author's Address

   Jordi Palet Martinez
   Consulintel, S.L.
   Molino de la Navata, 75
   La Navata - Galapagar, Madrid  28420

   Email: jordi.palet@consulintel.es
   URI:   http://www.consulintel.es/

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