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On Interoperation of Labels Among Conventional DNS and Other Resolution Systems

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8222.
Author Andrew Sullivan
Last updated 2016-07-12 (Latest revision 2016-07-03)
Replaces draft-sullivan-dnssd-mdns-dns-interop
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Suzanne Woolf
IESG IESG state Became RFC 8222 (Informational)
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Send notices to "Suzanne Woolf" <>
DNSSD                                                        A. Sullivan
Internet-Draft                                                       Dyn
Intended status: Informational                              July 3, 2016
Expires: January 4, 2017

On Interoperation of Labels Among Conventional DNS and Other Resolution


   Despite its name, DNS-Based Service Discovery can use naming systems
   other than the Domain Name System when looking for services.
   Moreover, when it uses the DNS, DNS-Based Service Discovery uses the
   full capability of DNS, rather than using a subset of available
   octets.  In order for DNS-SD to be used effectively in environments
   where multiple different name systems and conventions for their
   operation are in use, it is important to attend to differences in the
   underlying technology and operational environment.  This memo
   presents an outline of the requirements for selection of labels for
   conventional DNS and other resolution systems when they are expected
   to interoperate in this manner.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 4, 2017.

Copyright Notice

   Copyright (c) 2016 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

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and terms used in this document . . . . . . .   3
   2.  Why there could be a problem at all . . . . . . . . . . . . .   4
   3.  Requirements for a profile for label interoperation . . . . .   5
   4.  DNS-SD portions . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  The <Instance> Portion of the Service
           Instance Name . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  The <Service> Portion of the Service
           Instance Name . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  The <Domain> Portion of the             Service Instance
           Name  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change History . . . . . . . . . . . . . . . . . . .  11
     A.1.  draft-ietf-dnssd-mdns-dns-interop-03  . . . . . . . . . .  11
     A.2.  draft-ietf-dnssd-mdns-dns-interop-02  . . . . . . . . . .  11
     A.3.  draft-ietf-dnssd-mdns-dns-interop-01  . . . . . . . . . .  11
     A.4.  draft-ietf-dnssd-mdns-dns-interop-00  . . . . . . . . . .  11
     A.5.  draft-sullivan-dnssd-mdns-dns-interop-01  . . . . . . . .  11
     A.6.  draft-sullivan-dnssd-mdns-dns-interop-00  . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   DNS-Based Service Discovery (DNS-SD, [RFC6763]) specifies a mechanism
   for discovering services using queries to the Domain Name System
   (DNS, [RFC1034], [RFC1035]); and to any other system that uses domain
   names, such as Multicast DNS (mDNS, [RFC6762]).  Many applications
   that use the DNS follow "Internet hostname" syntax [RFC0952] for
   labels -- the so-called LDH rule.  That convention is the reason
   behind the development of Internationalized Domain Names for
   Applications (IDNA2008, [RFC5890], [RFC5891], [RFC5892], [RFC5893],
   [RFC5894], [RFC5895]).  It is worth noting that the LDH rule is a
   convention, and not a rule of the DNS; this is made entirely plain by
   [RFC2181], section 11, and discussed further in [RFC6055], section 3.
   Nevertheless, there is a widespread belief that in many circumstances

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   domain names cannot be used in the DNS unless they cleave to the LDH

   At the same time, mDNS requires that labels be encoded in UTF-8, and
   permits a range of characters in labels that are not permitted by
   IDNA2008 or the LDH rule.  For example, mDNS encourages the use of
   spaces and punctuation in mDNS names (see [RFC6763], section 4.1.3).
   It does not restrict which Unicode code points may be used in those
   labels, so long as the code points are UTF-8 in Net-Unicode [RFC5198]

   Users and developers of applications are, of course, frequently
   unconcerned with (not to say oblivious to) the name-resolution
   system(s) in service at any given moment, and are inclined simply to
   use the same domain names in different contexts.  As a result, the
   same domain name might be tried using different name resolution
   technologies.  If a given name will not work across the various
   environments, then user expectations are likely to be best satisfied
   when at least some parts of the domain names to be queried are
   compatible with the rules and conventions for all the relevant
   technologies.  Given the uses of DNS-SD, a choice for such
   compatibility likely lies with the application designer or service

   One approach to interoperability under these circumstances is to use
   a single operational convention (a "profile") for domain names under
   the different naming systems.  This memo assumes such a use profile,
   and attempts to outline what is necessary to make it work without
   specifying any particular technology.  It does assume, however, that
   the global DNS is eventually likely to be implicated.  Given the
   general tendency of all resolution eventually to fall through to the
   DNS, that assumption does not seem controversial.

   It is worth noting that users of DNS-SD do not use the service
   discovery names in the same way that users of other domain names
   might.  In many cases, domain names can be entered as direct user
   input.  But the service discovery context generally assumes users are
   picking a service from a list.  As a result, the sorts of application
   considerations that are appropriate to the general-purpose DNS name,
   and that resulted in the A-label/U-label split (see below) in
   IDNA2008, are not entirely the right approach for DNS-SD.

1.1.  Conventions and terms used in this document

   Wherever appropriate, this memo uses the terminology defined in
   Section 2 of [RFC5890].  In particular, the reader is assumed to be
   familiar with the terms "U-label", "LDH label", and "A-label" from
   that document.  Similarly, the reader is assumed to be familiar with

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   the U+NNNN notation for Unicode code points used in [RFC5890] and
   other documents dealing with Unicode code points.  In the interests
   of brevity and consistency, the definitions are not repeated here.

   Sometimes this memo refers to names in the DNS as though the LDH rule
   and IDNA2008 are strict requirements.  They are not.  DNS labels are,
   in principle, just collections of octets, and therefore in principle
   the LDH rule is not a constraint.  In practice, applications
   sometimes intercept labels that do not conform to the LDH rule and
   apply IDNA and other transformations.

   The DNS, perhaps unfortunately, has produced its own jargon.
   Unfamiliar DNS-related terms in this memo should be found in

   The term "owner name" (common to the DNS vernacular; see above) is
   used here to apply not just to the domain names to be looked up in
   the DNS, but to any name that might be looked up either in the DNS or
   using another technology.  It therefore includes names that might not
   actually exist anywhere.  In addition, what follows depends on the
   idea that not every domain name will be looked up in the DNS.  For
   instance, names ending in "local." (in the presentation format) are
   not ordinarily looked up using DNS, but instead looked up using mDNS.

   DNS-SD specifies three portions of the owner name for a DNS-SD
   resource record.  These are the <Instance> portion, the <Service>
   portion, and the <Domain> portion.  The owner name made of these
   three parts is called the Service Instance Name.  It is worth
   observing that a portion may be more than one label long.  See
   [RFC6763], section 4.1.  Further discussion of the parts is found in
   Section 4.

   Throughout this memo, mDNS is used liberally as the alternative
   resolution mechanism to DNS.  This is for convenience rather than
   rigour: any alternative name resolution to DNS could present the same
   friction with the prevailing operational conventions of the global
   DNS.  It so happens that mDNS is the overwhelmingly successful
   alternative as of this writing, so it is used in order to make the
   issues plainer to the reader.  Other alternative resolution
   mechanisms may in general be read wherever mDNS appears in the text,
   except where details of the mDNS specification appear.

2.  Why there could be a problem at all

   One might reasonably wonder why there is a problem to be solved at
   all.  After all, DNS labels permit any octet whatsoever, and anything
   that can be useful with DNS-SD cannot use any names that are outside
   the protocol strictures of the DNS.

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   The reason for the trouble is twofold.  First, and least troublesome,
   is the possibility of resolvers that are attempting to offer IDNA
   service system-wide.  Given the design of IDNA2008, it is reasonable
   to suppose that on some systems high-level name resolution libraries
   will perform the U-label/A-label transformation automatically, saving
   applications from these details.  But system-level services do not
   always have available to them the resolution context, and may apply
   the transformation in a way that foils rather than helps the
   application.  Of course, if this were the main problem, it would
   presumably be self-correcting; for the right answer would be, "Don't
   use those libraries for DNS-SD," and DNS-SD would not work reliably
   in cases where such libraries were in use.  This would be
   unfortunate; but given that DNS-SD in Internet contexts is as of this
   writing not in ubiquitous use, it should not represent a fatal issue.

   The greater problem is that the "infrastructure" types of DNS service
   -- the root zone, the top-level domains, and so on -- have embraced
   IDNA and refuse registration of raw UTF-8 into their zones.  As of
   this writing there is (perhaps unfortunately) no reliable way to
   discover where these sorts of DNS services end.  Nevertheless, some
   client programs (notably web browsers) have adopted a number of
   different policies about how domain names will be looked up and
   presented to users given the policies of the relevant DNS zone
   operators.  None of these policies permit raw UTF-8.  Since it is
   anticipated that DNS-SD when used with the DNS will be inside domain
   names beneath those kinds of "infrastructure" domains, the
   implications of IDNA2008 must be a consideration.

   For further exploration of issues relating to encoding of domain
   names generally, the reader should consult [RFC6055].

3.  Requirements for a profile for label interoperation

   Any interoperability between DNS (including prevailing operational
   conventions) and other resolution technologies will require
   interoperability across the portions of a DNS-SD Service Instance
   Name that are implicated in regular DNS lookups.  Only some portions
   are implicated.  In any case, if a given portion is implicated, the
   profile will need to apply to all labels in that portion.

   In addition, because DNS-SD Service Instance Names can be used in a
   domain name slot, care must be taken by DNS-SD-aware resolvers to
   handle the different portions as outlined here, so that DNS-SD
   portions that do not use IDNA2008 will not be treated as U-labels and
   will not accidentally undergo IDNA processing.

   Because the profile will apply to names that might appear in the
   public DNS, and because other resolution mechanisms (such as mDNS)

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   could permit labels that IDNA does not, the profile might reduce the
   labels that could be used with those other resolution mechanisms.
   One consequence of this is that some recommendations from [RFC6763]
   will not really be possible to implement using names subject to the
   profile.  In particular, [RFC6763], section 4.1.3 recommends that
   labels always be stored and communicated as UTF-8, even in the DNS.
   Because of the way the public DNS is currently operated (see
   Section 2), the advice to store and transmit labels as UTF-8 in the
   DNS is likely either to encounter problems, or to result in
   unnecessary traffic to the public DNS, or both.  In particular, many
   labels in the <Domain> part of a Service Instance Name are unlikely
   to be found in the UTF-8 form in the public DNS tree for zones that
   are using IDNA2008.  By contrast, for example, mDNS normally uses

   U-labels cannot contain upper case letters (see [RFC5894], sections
   3.1.3 and 4.2).  That restriction extends to ASCII-range upper case
   letters that work fine in LDH-labels.  It may be confusing that the
   character "A" works in the DNS when none of the characters in the
   label has a diacritic, but does not work when there is such a
   diacritic in the label.  Labels in mDNS names (or other resolution
   technologies) may contain upper case characters, so the profile will
   need either to restrict the use of upper case or come up with a
   convention for case folding (even in the presence of diacritics) that
   is reliable and predictable to users.

4.  DNS-SD portions

   Service Instance Names are made up of three portions.

4.1.  The <Instance> Portion of the Service Instance Name

   [RFC6763] is clear that the <Instance> portion of the Service
   Instance Name is intended for presentation to users, and therefore
   virtually any character is permitted in it.  There are two ways that
   a profile might address this portion.

   The first way would be to treat this portion as likely to be
   intercepted by system-wide IDNA-aware (but otherwise context-unaware)
   resolvers, or likely subject to strict IDNA conformance requirements
   for publication in the relevant zone.  In this case, the portion
   would need to be made subject to the profile, thereby curtailing what
   characters may appear in this portion.  This approach permits DNS-SD
   to use any standard system resolver but presents inconsistencies with
   the DNS-SD specification and with DNS-SD use that is exclusively
   mDNS-based.  Therefore, this strategy is rejected.

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   Instead, DNS-SD implementations can intercept the <Instance> portion
   of a Service Instance Name and ensure that those labels are never
   handed to IDNA-aware resolvers that might attempt to convert these
   labels into A-labels.  Under this approach, the DNS-SD <Instance>
   portion works as it always does, but at the cost of using special
   resolution code built into the DNS-SD system.  A practical
   consequence of this is that zone operators need to be prepared not to
   apply the LDH rule to all labels, and may need to make special
   concessions to ensure that the <Instance> portion can contain spaces,
   upper and lower case, and any UTF-8 code point; or else to prepare a
   user interface to handle the exceptions that would otherwise be
   generated.  Automatic conversion to A-labels is not acceptable.

   It is worth noting that this advice is not actually compatible with
   advice in [RFC6055], section 4.  That section appears to assume that
   names are not really composed of subsections, but because [RFC6763]
   specifies portions of names, the advice in this memo is to follow the
   advice of [RFC6055] according to the portion of the domain name,
   rather than for the whole domain name.  As a practical matter, this
   likely means special-purpose name resolution software for DNS-SD.

4.2.  The <Service> Portion of the Service Instance Name

   DNS-SD includes a <Service> component in the Service Instance Name.
   This component is not really user-facing data, but is instead control
   data embedded in the Service Instance Name.  This component includes
   so-called "underscore labels", which are labels prepended with U+005F
   (_).  The underscore label convention was established by DNS SRV
   ([RFC2782]) for identifying metadata inside DNS names.  A system-wide
   resolver (or DNS middlebox) that cannot handle underscore labels will
   not work with DNS-SD at all, so it is safe to suppose that such
   resolvers will not attempt to do special processing on these labels.
   Therefore, the <Service> portion of the Service Instance Name will
   not be subject to the profile.  By the same token, underscore labels
   are never subject to IDNA processing (they are formally
   incompatible), and therefore concerns about IDNA are irrelevant for
   these labels.

4.3.  The <Domain> Portion of the Service Instance Name

   The <Domain> portion of the Service Instance Name forms an integral
   part of the owner name submitted for DNS resolution.  A system-wide
   resolver that is IDNA2008-aware is likely to interpret labels with
   UTF-8 in the owner name as candidates for IDNA2008 processing.  More
   important, operators of internationalized domain names will
   frequently publish such names in the public DNS as A-labels;
   certainly, the top-most labels will always be A-labels.  Therefore,
   these labels will need to be subject to the profile.  DNS-SD

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   implementations ought somehow to identify the <Domain> portion of the
   Service Instance Name and treat it subject to IDNA2008 in case the
   domain is to be queried from the global DNS.  In the event that the
   <Domain> portion of the Service Instance Name fails to resolve, it is
   acceptable to substitute labels with plain UTF-8, starting at the
   lowest label in the DNS tree and working toward the root.  This
   approach differs from the rule for resolution published in [RFC6763],
   because it privileges IDNA2008-compatible labels over UTF-8 labels.
   There is more than one way to achieve such a result, but in terms of
   predictability it is probably best if the lowest-level resolution
   component is able to learn the correct resolution context, so that it
   can perform the correct transformations on the various domain

   One might argue against the above restriction on either of two

   1.  It is possible the names may be in the DNS in UTF-8, and RFC 6763
       already specifies a fallback strategy of progressively attempting
       first the UTF-8 label lookup (it might not be a U-label) and then
       if possible the A-label lookup.

   2.  Zone administrators that wish to support DNS-SD can publish a
       UTF-8 version of the zone along side the A-label version of the

   The first of these is rejected because it represents a potentially
   significant increase in DNS lookup traffic for no value.  It is
   possible for a DNS-SD application to identify the <Domain> portion of
   the Service Instance Name.  The standard way to publish IDNs on the
   Internet uses IDNA.  Therefore, additional lookups should not be
   encouraged.  When [RFC6763] was published, the bulk of IDNs were
   lower in the tree.  Now that there are internationalized labels in
   the root zone, it is desirable to minimize queries to the Internet
   infrastructure if they are sure to be answered in the negative.

   The second reason depends on the idea that it is possible to maintain
   two names in sync with one another.  This is not strictly speaking
   true, although in this case the domain operator could simply create a
   DNAME record [RFC6672] from the UTF-8 name to the IDNA2008 zone.
   This still, however, relies on being able to reach the (UTF-8) name
   in question, and it is unlikely that the UTF-8 version of the zone
   will be delegated from anywhere.  Moreover, in many organizations the
   support for DNS-SD and the support for domain name delegations are
   not performed by the same department, and depending on a co-
   ordination between the two will make the system more fragile, or
   slower, or both.

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   Some resolvers -- particularly those that are used in mixed DNS and
   non-DNS environments -- may be aware of different operational
   conventions in different parts of the DNS tree.  For example, it may
   be possible for implementations to use hints about the boundary of an
   organization's domain name infrastructure, in order to tell (for
   instance) that is part of the Example Organization while
   com. is a large delegation-centric zone on the public Internet.  In
   such cases, the resolution system might reverse its preferences to
   prefer plain UTF-8 labels when resolving names below the boundary
   point in the DNS tree.  The result would be that any lookup past the
   boundary point and closer to the root would use LDH-labels first,
   falling back to UTF-8 only after a failure; but a lookup below the
   boundary point would use UTF-8 labels first, and try other strategies
   only in case of negative answers.  The mechanism to learn such a
   boundary is beyond the scope of this document.

5.  Acknowledgements

   The author gratefully acknowledges the insights of Joe Abley, Stuart
   Cheshire, Paul Hoffman, Kerry Lynn, and Dave Thaler.  Kerry Lynn
   deserves special gratitude for his energy and persistence in pressing
   unanswered questions.  Doug Otis sent many comments about visual

6.  IANA Considerations

   This memo makes no requests of IANA.

7.  Security Considerations

   This memo presents some requirements for future development, but does
   not specify anything.  It makes no additional security-specific
   requirements.  Issues arising due to visual confusability of names
   apply to this case as well as to any other case of internationalized
   names, but interoperation between different resolution systems and
   conventions does not alter the severity of those issues.

8.  Informative References

   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, October 1985.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

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   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, September 2010.

   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
              Encodings for Internationalized Domain Names", RFC 6055,
              February 2011.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, June 2012.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

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   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.

Appendix A.  Change History

   Note to RFC Editor: this section should be removed prior to

A.1.  draft-ietf-dnssd-mdns-dns-interop-03

   o  Additional alteration of title

   o  Attempt to address WGLC comments from Dave Thaler (2016-04-02)

A.2.  draft-ietf-dnssd-mdns-dns-interop-02

   o  Altered the title to make it more generic than mDNS.

   o  Addressed issues raised by Dave Thaler in review on 2015-07-18.

   o  Added a note to Section 7 about visual confusion.  I don't know
      whether this will satisfy Doug Otis but it is the only thing I can
      see that could possibly be relevant.

   o  Added discussion of finding "boundary" in Section 4.3.

A.3.  draft-ietf-dnssd-mdns-dns-interop-01

   Alter text to make clear that the main issue is the way the public
   DNS is currently administered, not system resolvers.  I suppose this
   should have been clear before, but I didn't do that.  Many thanks to
   Kerry Lynn for penetrating questions that illuminated what I'd left

A.4.  draft-ietf-dnssd-mdns-dns-interop-00

   1st WG version

   Add text to make clear that fallback from A-label lookup to UTF-8
   label lookup ok, per WG comments at IETF 91

A.5.  draft-sullivan-dnssd-mdns-dns-interop-01

   o  Decided which portions would be affected

   o  Explained the difference in user interfaces between DNS-SD and
      usual DNS operation

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   o  Provided background on why the Domain portion should be treated

A.6.  draft-sullivan-dnssd-mdns-dns-interop-00

   Initial version.

Author's Address

   Andrew Sullivan
   150 Dow St.
   Manchester, NH  03101


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