Internet DRAFT - draft-nottingham-dns-media-tree


Network Working Group                                      M. Nottingham
Internet-Draft                                              January 2003
Expires: July 2, 2003

                 The 'dns' Media Type Registration Tree

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   This document specifies the 'dns' media subtype registration tree,
   which is intended to ease the deployment of new Internet applications
   and their associated media types without the need for coordination
   with a central registry.

1. Requirements notation

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

2. Introduction

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   The registration proceedures for Internet media types [RFC2048] allow
   for the specification of media type trees, which allow for faceted
   names to be described in order to increase the efficiency and
   flexibility of the registration process. They also allows for the
   creation of new registration trees, with the advice and consent of
   the IESG.

   This specification describes one such tree.

3. Motivation

   The Internet media type system was designed to encourage certain
   properties, and arguably has been quite successful because of its
   approach. In particular, IANA acts as a central registry to bring
   coordination and convenience, and various levels of community review
   are required before a media type may be registered, to assure some
   level of quality in and appropriate application of media types.

   However, the arrival of the Web, and in particular XML, has changed
   the conditions under which formats are created and used. XML allows
   the creation of business-specific document and protocol formats by
   end users. Often, these parties are unfamiliar with IETF and IANA
   process for registration of media types, and do not have a
   requirement for recognition by a centralized registry. As such, the
   cost of media type registration is not justified in the view of some
   parties that are minting new formats.

   As a result, many formats are created without corresponding media
   types (often under the umbrella of 'text/xml' or 'application/xml').
   Such formats are not first-class citizens on the Internet or the Web;
   one cannot content negotiate for them, for example, and one cannot
   use existing software dispatch mechanisms in MIME software to
   accommodate them.

   It should be noted that these undesirable effects disproportionately
   affect those who wish to use a format in ways that may not have been
   forseen by its creators. As such, the registration system indirectly
   discourages the wide use of those mechanisms that leverage media

   Current registration proceedures do allow for some flexibility to
   accommodate vendor-specific formats (the .vnd tree), "vanity" formats
   (the .prs tree) and ad hoc, experimental formats (the x. tree and its
   predecessor, the 'x-' convention). Unfortunately, these mechanisms do
   not address the problems described here; the x- convention is too
   brittle for most uses (and indeed its use has been discouraged for
   some time), and the .vnd and .prs trees are still based on a
   centralized registry.

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   This specification proposes that the widely recognized DNS
   infrastructure be leveraged to act as a distributed registry, to
   avoid the possibility of collision, whilst removing the need for an
   additional centralized registry. The approach is similar to URI
   schemes that also leverage the DNS to provide locally-managed name

4. The 'dns' Tree

   The 'dns' media subtype tree is intended to be used to identify
   proprietary, ad hoc, experimental, or limited deployment formats.

   It follows the conventions of faceted name trees as specified by
   [RFC2048], and is distinguished by the leading facet 'dns.'. This
   facet MUST be followed by one or more dot-delimited facets that are
   derived from a domain name, in reverse order. Finally, those facets
   MUST be followed by on or more facets that indicate the format's
   identity within that name space.

   Media types using this tree MUST be minted with the knowledge and
   permission of the authority responsible for the corresponding
   Internet domain name. The domain name used MUST be registered in the
   Internet Registry, as delegated by IANA (see [RFC1591]).

   For example, if the entity responsible for wished to
   register a textual media type with the name 'foo' in this fashion,
   its media type might be:


   XML-based formats SHOULD be conformant with [RFC3023], e.g.:


   If were a multinational concern, it may wish to delegate
   authority for minting new types to regional departments. It could do
   so by mandating an additional facet; an application media type minted
   by the Australian division might be:


   whilst a completely separate application format, also identified as
   'bar' and minted by the U.S. division might be distinguished as:


4.1 The 'doc' Attribute

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   Media types in the dns tree MAY use the 'doc' attribute, which
   indicates a URL [RFC2396] that can be used to locate documentation of
   the identified format.

   For example:

   application/; doc="

   The 'doc' attribute is only informative, and MUST NOT be interpreted
   to alter the nature of the format identified; i.e., a media type with
   a 'doc' attribute of "foo" MUST be considered equivalent to the same
   media type with a 'doc' attribute of "bar", or one without a 'doc'

4.2 Format-Specific Attributes

   Formats using this dns tree MAY designate their own attributes, which
   SHOULD be documented at or referenced from the URL specified in the
   'doc' attribute, if present.

5. IANA Considerations

   Implementation of the dns tree does not require IANA coordination.
   Any media type conformant with this specification is considered to be
   registered with IANA.

6. Security Considerations

6.1 Change of Ownership

   Over time, domain names may change ownership. Without proper care,
   media types created by a domain name's previous owner might collide
   with those created by the new owner.

   As a result, when domains which have been used in the registration of
   media types in the dns tree change hands, the previous owner SHOULD
   take care to communicate existing media types to the new owner, and
   the new owner SHOULD take care to avoid collisions. Previous owners
   MAY publish a transition plan to a new domain, if doing so is judged
   to cause minimal disruption.

6.2 Unauthorized Registration

   Media types using the dns tree have no enforced relationship to the
   domain names that they are based upon; the use of domain names is
   only a convention to assure proper name spacing. Implementations
   SHOULD NOT make any assumptions about this relationship, especially

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   regarding security issues.


   [RFC1591]  Postel, J., "Domain Name System Structure and Delegation",
              RFC 1591, March 1994.

   [RFC2048]  Freed, N., Klensin, J. and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

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

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC3023]  Murata, M., St. Laurent, S. and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

Author's Address

   Mark Nottingham


Appendix A. Acknowledgements

   The following people gave input and feedback during the creation of
   this document; Mark Baker, Don Box, Yaron Goland, Ted Hardie, John
   Kemp, Graham Klyne, and David Orchard. All errors are the
   responsibility of the author.

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