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On Interoperation of Labels Between mDNS and DNS

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This is an older version of an Internet-Draft that was ultimately published as RFC 8222.
Author Andrew Sullivan
Last updated 2015-07-04
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DNSSD                                                        A. Sullivan
Internet-Draft                                                       Dyn
Intended status: Informational                              July 4, 2015
Expires: January 5, 2016

            On Interoperation of Labels Between mDNS and DNS


   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 5, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( in effect on the date of

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   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  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change History . . . . . . . . . . . . . . . . . . .  10
     A.1.  draft-ietf-dnssd-mdns-dns-interop-01  . . . . . . . . . .  10
     A.2.  draft-ietf-dnssd-mdns-dns-interop-00  . . . . . . . . . .  10
     A.3.  draft-sullivan-dnssd-mdns-dns-interop-01  . . . . . . . .  10
     A.4.  draft-sullivan-dnssd-mdns-dns-interop-00  . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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]).  Conventional use of
   the DNS generally follows the host name rules [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.  Nevertheless, there is a widespread belief that in many
   circumstances domain names cannot be used in the DNS unless they
   cleave to the LDH rule.

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   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 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 DNS-SD is
   to be used in an environment where multiple resolution systems (such
   as mDNS and DNS) are to be queried for services, then some parts of
   the domain names to be queried will need to be compatible with the
   rules and conventions for all the relevant technologies.

   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.  Domain names often might as easily be entered as direct user
   input as by any other method.  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
   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.

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   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 other technologies.  It therefore includes names that might not
   actually exist anywhere.  In addition, what follows depends on the
   idea that not every domain name may be looked up in the DNS.  For
   instance, names ending in "local." (in the presentation format) are
   not ordinarily looked up in the DNS, but instead by querying 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>.  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.

   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

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   will perform the U-label/A-label transformation automatically, saving
   applications from these details.  If this were the main problem,
   however, 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 need to apply to names that might need to
   interoperate with names in the public DNS, and because other
   resolution mechanisms (such as mDNS) 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

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   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 result in unnecessary traffic to the public DNS (or
   both).  In particular, the <Domain> part of a Service Instance Name
   is unlikely to be found in its UTF-8 form in the public DNS tree for
   zones that are using IDNA2008.  By contrast, for example, mDNS
   normally uses UTF-8.

   U-labels cannot contain upper case letters.  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 reliable and predictable (to users) convention for
   case folding even in the presence of diacritics.

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 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 that is exclusively mDNS-based.  Therefore, this
   strategy is rejected.

   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

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   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.

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, it should be noted
   that underscore labels are never subject to IDNA processing (they're
   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 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 implementations ought
   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.

   One might argue against this restriction on either of two grounds:

   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.

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   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.

5.  Acknowledgements

   The author gratefully acknowledges the insights of Stuart Cheshire,
   Kerry Lynn, and Dave Thaler.  Kerry Lynn deserves special gratitude
   for his energy and persistence in pressing unanswered questions.

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.  Therefore, it has no implications for

8.  Informative References

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-dns-terminology-03 (work in
              progress), June 2015.

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   [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.

   [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.

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   [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.

Appendix A.  Change History

A.1.  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.2.  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.3.  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

   o  Provided background on why the Domain portion should be treated

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

   Initial version.

Author's Address

   Andrew Sullivan
   150 Dow St.
   Manchester, NH  03101


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