Network Working Group M. Nottingham
Internet-Draft January 2003
Expires: July 2, 2003
The 'dns' Media Type Registration Tree
draft-nottingham-dns-media-tree-00
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
types.
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
spaces.
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 example.com wished to
register a textual media type with the name 'foo' in this fashion,
its media type might be:
text/dns.com.example.foo
XML-based formats SHOULD be conformant with [RFC3023], e.g.:
application/dns.com.example.foo+xml
If example.com 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:
application/dns.com.example.au.bar
whilst a completely separate application format, also identified as
'bar' and minted by the U.S. division might be distinguished as:
application/dns.com.example.us.bar
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/dns.com.example.foo; doc="http://www.example.com/formats/
foo.html"
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'
attribute.
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.
References
[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
EMail: mailto:mnot@pobox.com
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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