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Network Working Group R. Bush Request for Comments: 3363 A. Durand Updates: 2673, 2874 B. Fink Category: Informational O. Gudmundsson

                                                               T. Hain
                                                           August 2002
          Representing Internet Protocol version 6 (IPv6)
             Addresses in the Domain Name System (DNS)

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.


 This document clarifies and updates the standards status of RFCs that
 define direct and reverse map of IPv6 addresses in DNS.  This
 document moves the A6 and Bit label specifications to experimental

1. Introduction

 The IETF had begun the process of standardizing two different address
 formats for IPv6 addresses AAAA [RFC1886] and A6 [RFC2874] and both
 are at proposed standard.  This had led to confusion and conflicts on
 which one to deploy.  It is important for deployment that any
 confusion in this area be cleared up, as there is a feeling in the
 community that having more than one choice will lead to delays in the
 deployment of IPv6.  The goal of this document is to clarify the
 This document also discusses issues relating to the usage of Binary
 Labels [RFC 2673] to support the reverse mapping of IPv6 addresses.
 This document is based on extensive technical discussion on various
 relevant working groups mailing lists and a joint DNSEXT and NGTRANS
 meeting at the 51st IETF in August 2001.  This document attempts to
 capture the sense of the discussions and reflect them in this
 document to represent the consensus of the community.

Bush, et. al. Informational [Page 1] RFC 3363 Representation of IPv6 Addresses in DNS August 2002

 The main arguments and the issues are covered in a separate document
 [RFC3364] that reflects the current understanding of the issues.
 This document summarizes the outcome of these discussions.
 The issue of the root of reverse IPv6 address map is outside the
 scope of this document and is covered in a different document

1.1 Standards Action Taken

 This document changes the status of RFCs 2673 and 2874 from Proposed
 Standard to Experimental.

2. IPv6 Addresses: AAAA RR vs A6 RR

 Working group consensus as perceived by the chairs of the DNSEXT and
 NGTRANS working groups is that:
 a) AAAA records are preferable at the moment for production
    deployment of IPv6, and
 b) that A6 records have interesting properties that need to be better
    understood before deployment.
 c) It is not known if the benefits of A6 outweigh the costs and

2.1 Rationale

 There are several potential issues with A6 RRs that stem directly
 from the feature that makes them different from AAAA RRs: the ability
 to build up addresses via chaining.
 Resolving a chain of A6 RRs involves resolving a series of what are
 nearly-independent queries.  Each of these sub-queries takes some
 non-zero amount of time, unless the answer happens to be in the
 resolver's local cache already.  Other things being equal, we expect
 that the time it takes to resolve an N-link chain of A6 RRs will be
 roughly proportional to N.  What data we have suggests that users are
 already impatient with the length of time it takes to resolve A RRs
 in the IPv4 Internet, which suggests that users are not likely to be
 patient with significantly longer delays in the IPv6 Internet, but
 terminating queries prematurely is both a waste of resources and
 another source of user frustration.  Thus, we are forced to conclude
 that indiscriminate use of long A6 chains is likely to lead to
 increased user frustration.

Bush, et. al. Informational [Page 2] RFC 3363 Representation of IPv6 Addresses in DNS August 2002

 The probability of failure during the process of resolving an N-link
 A6 chain also appears to be roughly proportional to N, since each of
 the queries involved in resolving an A6 chain has roughly the same
 probability of failure as a single AAAA query.
 Last, several of the most interesting potential applications for A6
 RRs involve situations where the prefix name field in the A6 RR
 points to a target that is not only outside the DNS zone containing
 the A6 RR, but is administered by a different organization entirely.
 While pointers out of zone are not a problem per se, experience both
 with glue RRs and with PTR RRs in the IN-ADDR.ARPA tree suggests that
 pointers to other organizations are often not maintained properly,
 perhaps because they're less susceptible to automation than pointers
 within a single organization would be.

2.2 Recommended Standard Action

 Based on the perceived consensus, this document recommends that RFC
 1886 stay on standards track and be advanced, while moving RFC 2874
 to Experimental status.

3. Bitlabels in the Reverse DNS Tree

 RFC 2673 defines a new DNS label type.  This was the first new type
 defined since RFC 1035 [RFC1035].  Since the development of 2673 it
 has been learned that deployment of a new type is difficult since DNS
 servers that do not support bitlabels reject queries containing bit
 labels as being malformed.  The community has also indicated that
 this new label type is not needed for mapping reverse addresses.

3.1 Rationale

 The hexadecimal text representation of IPv6 addresses appears to be
 capable of expressing all of the delegation schemes that we expect to
 be used in the DNS reverse tree.

3.2 Recommended Standard Action

 RFC 2673 standard status is to be changed from Proposed to
 Experimental.  Future standardization of these documents is to be
 done by the DNSEXT working group or its successor.

Bush, et. al. Informational [Page 3] RFC 3363 Representation of IPv6 Addresses in DNS August 2002

4. DNAME in IPv6 Reverse Tree

 The issues for DNAME in the reverse mapping tree appears to be
 closely tied to the need to use fragmented A6 in the main tree: if
 one is necessary, so is the other, and if one isn't necessary, the
 other isn't either.  Therefore, in moving RFC 2874 to experimental,
 the intent of this document is that use of DNAME RRs in the reverse
 tree be deprecated.

5. Acknowledgments

 This document is based on input from many members of the various IETF
 working groups involved in this issues.  Special thanks go to the
 people that prepared reading material for the joint DNSEXT and
 NGTRANS working group meeting at the 51st IETF in London, Rob
 Austein, Dan Bernstein, Matt Crawford, Jun-ichiro itojun Hagino,
 Christian Huitema.  Number of other people have made number of
 comments on mailing lists about this issue including Andrew W.
 Barclay, Robert Elz, Johan Ihren, Edward Lewis, Bill Manning, Pekka
 Savola, Paul Vixie.

6. Security Considerations

 As this document specifies a course of action, there are no direct
 security considerations.  There is an indirect security impact of the
 choice, in that the relationship between A6 and DNSSEC is not well
 understood throughout the community, while the choice of AAAA does
 leads to a model for use of DNSSEC in IPv6 networks which parallels
 current IPv4 practice.

7. IANA Considerations


Normative References

 [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
            Specification", STD 13, RFC 1035, November 1987.
 [RFC1886]  Thompson, S. and C. Huitema, "DNS Extensions to support IP
            version 6", RFC 1886, December 1995.
 [RFC2673]  Crawford, M., "Binary Labels in the Domain Name System",
            RFC 2673, August 1999.
 [RFC2874]  Crawford, M. and C. Huitema, "DNS Extensions to Support
            IPv6 Address Aggregation and Renumbering", RFC 2874, July

Bush, et. al. Informational [Page 4] RFC 3363 Representation of IPv6 Addresses in DNS August 2002

 [RFC3152]  Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152
            August 2001.

Informative References

 [RFC3364]  Austein, R., "Tradeoffs in Domain Name System (DNS)
            Support for Internet Protocol version 6 (IPv6)", RFC 3364,
            August 2002.

Editors' Addresses

 Randy Bush
 Alain Durand
 Bob Fink
 Olafur Gudmundsson
 Tony Hain

Bush, et. al. Informational [Page 5] RFC 3363 Representation of IPv6 Addresses in DNS August 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
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 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bush, et. al. Informational [Page 6]

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