Behave WG                                                  T. Savolainen
Internet-Draft                                                     Nokia
Intended status: Standards Track                             J. Korhonen
Expires: January 11, 2012                         Nokia Siemens Networks
                                                           July 10, 2011

  Discovery of a Network-Specific NAT64 Prefix using a Well-Known Name


   This document describes a method for detecting presence of DNS64 and
   for learning IPv6 prefix used for protocol translation on an access
   network without explicit support from the access network.  The method
   depends on existence of a known IPv4-only domain name.  The
   information learned enables applications and hosts to perform local
   IPv6 address synthesis and on dual-stack accesses avoid traversal
   through NAT64.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 11, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements and Terminology  . . . . . . . . . . . . . . . . . 3
     2.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . 3
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Host behavior . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.1.  Connectivity test . . . . . . . . . . . . . . . . . . . . . 5
   4.  Considerations for hosting the IPv4-only well-known name  . . . 6
   5.  DNS(64) entity considerations . . . . . . . . . . . . . . . . . 6
   6.  Exit strategy . . . . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  About IPv4 address for the well-known name  . . . . . . . . 7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   As part of the transition to IPv6 NAT64 [RFC6146] and DNS64 [RFC6147]
   technologies will be utilized by some access networks to provide IPv4
   connectivity for IPv6-only hosts.  The DNS64 utilizes IPv6 address
   synthesis to create local IPv6 presentations of peers having only
   IPv4 addresses, hence allowing DNS-using IPv6-only hosts to
   communicate with IPv4-only peers.

   However, DNS64 cannot serve applications not using DNS, such as those
   receiving IPv4 address literals as referrals.  Such applications
   could nevertheless be able to work through NAT64, provided they are
   able to create locally valid IPv6 presentations of peers' IPv4

   Additionally, DNS64 is not able to do IPv6 address synthesis for
   hosts running validating DNSSEC enabled resolvers, but instead the
   synthetization must be done by the hosts.  In order to perform IPv6
   synthesis hosts have to learn the IPv6 prefix(es) used on the access
   network for protocol translation.

   This document describes a best effort method for advanced
   applications and hosts to learn the information required to perform
   local IPv6 address synthesis.  An example application is a browser
   encountering an IPv4 address literal in an IPv6-only access network.
   Another example is a host running validating security aware DNS

   The knowledge of IPv6 address synthetization taking place may also be
   useful if DNS64 and NAT64 are present in dual-stack enabled access
   network.  In such cases hosts may choose to prefer IPv4 in order to
   avoid traversal through protocol translators.

   The described method is intented for the scenarios where network
   assisted NAT64 and prefix discovery solutions are not available.

2.  Requirements and Terminology

2.1.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.2.  Terminology

   Well-Known IPv4-only Name: a fully qualified domain name well-known
   to have only A record.

   Well-Known IPv4 Address: an IPv4 address that is well-known and
   mapped to the well-known name.

3.  Host behavior

   A host requiring information about presence of NAT64 and the IPv6
   prefix used for protocol translation shall send a DNS query for AAAA
   records of a known IPv4-only fully qualified domain name.  This may
   happen, for example, at the moment the host is configured an IPv6
   address of a DNS server.  This may also happen at the time when first
   DNS query for AAAA record is initiated.  The host may perform this
   check in both IPv6-only and dual-stack access networks.

   When sending AAAA query for the known name a host MUST set "Checking
   Disabled (CD)" bit to zero, as otherwise the DNS64 will not perform
   IPv6 address synthesis hence does not reveal the IPv6 prefix(es) used
   for protocol translation.

   A DNS reply with one or more non-empty AAAA records indicates that
   the access network is utilizing IPv6 address synthesis.  A host MUST
   look through all of the received AAAA records to collect all
   available prefixes.  The prefixes may include Well-Known Prefix or
   one or more Network-Specific Prefixes.  In the case of NSPs the host
   SHALL search for the IPv4 address inside of the received IPv6
   addresses to determine used address format.

   An IPv4 address inside synthesized IPv6 address should be found at
   some of the locations described in [RFC6052].  If the searched IPv4
   address is not found on any of the standard locations the network
   must be using different formatting.  Developers may over time learn
   on IPv6 translated address formats that are extensions or
   alternatives to the standard formats.  Developers MAY at that point
   add additional steps to the described discovery procedures.  The
   additional steps are outside the scope of the present document.

   The host should ensure a 32-bit IPv4 address value is present only
   once in an IPv6 address.  In case another instance of the value is
   found inside the IPv6, the host shall repeat the search with another
   IPv4 address, if possible.

   In the case only one IPv6 prefix was present in the DNS response: a
   host shall use that IPv6 prefix for both local synthetization and for

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   detecting synthesis done by the DNS64 entity on the network.

   In the case multiple IPv6 prefixes were present in the DNS response:
   a host SHOULD use all received prefixes when determining whether
   other received IPv6 addresses are synthetic.  However, for selecting
   prefix for the local IPv6 address synthesis host MUST use the
   following prioritization order, of which purpose is to avoid use of
   prefixes containing suffixes reserved for the future [RFC6052]:

   1.  Use NSP having /96 prefix

   2.  Use WKP prefix

   3.  Use longest available NSP prefix

   In the case of NXDOMAIN or empty AAAA reply: the DNS64 is not
   available on the access network, network filtered the well-known
   query on purpose, or something went wrong in the DNS resolution.  All
   unsuccesful cases result in unavailability of a host to perform local
   IPv6 address synthesis.  The host MAY periodically resend AAAA query
   to check if DNS64 has become available or temporary problem cleared.
   The host MAY perform A query for the well-known name to learn whether
   the service is available at all (see section 6 about Exit Strategy).
   The host MAY also continue monitoring DNS replies with IPv6 addresses
   constructed from WKP, in which case the host MAY use the WKP as if it
   were learned during the query for well-known name.

   To save Internet's resources, if possible, a host should perform
   NAT64 discovery only when needed (e.g. when local synthesis is
   required, cached reply timeouts, new network interface is started,
   and so forth.  Furthermore, the host SHOULD cache the replies it
   receives and honor TTLs.

3.1.  Connectivity test

   After the host has obtained a candidate prefix and format for the
   IPv6 address synthesis it may locally synthesize an IPv6 address, by
   using a publicly routable IPv4 address, and test connectivity with
   the resulting IPv6 address.  The connectivity test may be conducted
   e.g. with ICMPv6 or with a transport layer protocol.

   This connectivity test ensures local address synthetization results
   in functional and protocol translatable IPv6 addresses.

   The host MUST NOT perform connectivity test for the well-known IPv4
   address of the well-known name, but instead to some other destination
   such as host vendor servers.

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4.  Considerations for hosting the IPv4-only well-known name

   The authoritative nameserver for the well-known name shall have DNS
   record TTL set to a long value in order to improve effectiveness of
   DNS caching.  The exact value depends on availability time for the
   used public IPv4 address, but should not be longer than one year.

5.  DNS(64) entity considerations

   DNS(64) servers MUST NOT interfere or perform special procedures for
   the queries related to the well-known name until the time has arrived
   for the exit strategy to be deployed.

6.  Exit strategy

   A day will come when this tool is no longer needed or is replaced by
   some other tool.

   In global scope the exit strategy includes sending NXDOMAIN replies
   by the authoritative nameserver of the well-known name with very long

   In local scope, after network administrators have determined there is
   no longer need for this tool in their network, they may start locally
   serving A and AAAA queries for the well-known name with NXDOMAIN

   A client implementation receiving NXDOMAIN for the AAAA query for the
   well-known name is either not talking to DNS64 or this tool has been
   disabled.  NXDOMAIN response also for the A query for the well-known
   name means this tool has been disabled.

7.  Security Considerations

   No security considerations have been identified.

8.  IANA Considerations

   A well-known name should be defined and a public IPv4 address
   allocated (by IANA?  IETF?  Someone else?).

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8.1.  About IPv4 address for the well-known name

   The global IPv4 address for the well-known, if possible, should be
   chosen so that it is unlikely to appear more than once within an IPv6
   address and also as easy as possible to find from within the
   synthetic IPv6 address.  A global address is required as otherwise
   DNS64 entity will not perform AAAA record synthesis.  The address
   does not have to be routable as no communications are initiated to
   the IPv4 address.

   Allocating two IPv4 addresses would improve the heuristics in cases
   where the primary IPv4 address' bit pattern appears more than once in
   the synthetic IPv6 address (NSP prefix contains the same bit pattern
   as the IPv4 address).

   If no well-known IPv4 address is allocated for this method, the
   heuristic requires sending additional A query to learn the IPv4
   address that is sought inside the received IPv6 address.  Without
   knowing IPv4 address it is impossible to determine address format
   used by DNS64.

9.  Acknowledgements

   Authors would like to thank Andrew Sullivan, Dan Wing, Washam Fan,
   Cameron Byrne, Zhenqiang Li, and Christian Huitema for significant
   improvement ideas and comments.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              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, April 2011.

   [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,
              April 2011.

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Authors' Addresses

   Teemu Savolainen
   Hermiankatu 12 D
   FI-33720 Tampere


   Jouni Korhonen
   Nokia Siemens Networks
   Linnoitustie 6
   FI-02600 Espoo


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