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Guidelines for the Definition of New Top-Level Media Types

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Author Martin J. Dürst
Last updated 2023-03-11
Replaces draft-duerst-mediaman-toplevel
RFC stream Internet Engineering Task Force (IETF)
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Responsible AD Murray Kucherawy
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MEDIAMAN                                                      M.J. Dürst
Internet-Draft                                  Aoyama Gakuin University
Updates: 6838 (if approved)                                11 March 2023
Intended status: Best Current Practice                                  
Expires: 12 September 2023

       Guidelines for the Definition of New Top-Level Media Types


   The goal of this document is to identify best practices for defining
   new top-level media types.  It updates RFC 6838, when approved.
   Comments and discussion about this document should be directed to, the mailing list of the Media Type Maintenance
   (mediaman) WG.  Alternatively, issues can be raised on github at

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 12 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Rules for the Registration of New Top-Level Media Types . . .   3
     2.1.  Required Criteria . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Additional Considerations . . . . . . . . . . . . . . . .   4
     2.3.  Negative Criteria . . . . . . . . . . . . . . . . . . . .   5
   3.  Top-Level Media Type History  . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     Changes from draft-ietf-mediaman-toplevel-01 to
             draft-ietf-mediaman-toplevel-02 . . . . . . . . . . . .   8
     Changes from draft-ietf-mediaman-toplevel-00 to
             draft-ietf-mediaman-toplevel-01 . . . . . . . . . . . .   8
     Changes from draft-duerst-mediaman-toplevel-00 to
             draft-ietf-mediaman-toplevel-01 . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     Normative References  . . . . . . . . . . . . . . . . . . . . .   9
     Informative References  . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document defines best practices for defining new top-level media
   types.  RFC 6838 (Media Type Specifications and Registration
   Procedures) only summarily gave criteria for defining additional top-
   level media types.  This document provides more detailled criteria
   for defining additional top-level media types.  It therefore updates
   RFC 6838 (Media Type Specifications and Registration Procedures).

   This document has been adopted by the Media Type Maintenance
   (mediaman) IETF WG.  Comments and discussion about this document
   should be directed to the WG's mailing list at
   Alternatively, issues can be raised on github at

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1.1.  Background

   New top-level types are rare enough and different enough from each
   other that each application needs to be evaluated separately.  The
   main protocol extension point for media types are subtypes below each
   of the main types.  For formats that do not fit below any other top-
   level type, the 'application' top-level type can always be used.

   The main function of media types and subtypes is the dispatch of data
   formats to application code.  In most cases, this requires and is
   done using the full type (i.e. including the subtype, and often some
   parameters).  The top-level type can occasionally serve as a fallback
   for the tentative dispatch to applications handling a very wide range
   of related formats.

   In some older scenarios, it may also be possible to identify a device
   (e.g. a phone for audio messages, a printer or fax device for images,
   a video recorder for videos, a computer for 'application' subtypes).
   However, the current hardware landscape, where computers and smart
   phones can handle a very wide variety of media, makes such a scenario
   look somewhat far-fetched.

   The top-level type can be used for user-directed information.
   Besides direct inspection of the type string by the user, this
   includes using different types of default icons for different top-
   level types.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Rules for the Registration of New Top-Level Media Types

   This section describes the rules and criteria for new top-level media
   types, including criteria already defined in RFC 6838 (Media Type
   Specifications and Registration Procedures).  Further work is needed
   to distinguish between required and optional criteria.

2.1.  Required Criteria

   The following is the list of required criteria for the definition of
   a new top-level type.  Motivations for the requirements are also

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   *  Every new top-level type MUST be defined in a Standards Track RFC.
      This will make sure there is sufficient community interest,
      review, and consensus appropriate for a new top-level type.

   *  The IANA Considerations section of an RFC defining a new top-level
      type MUST request that IANA add this new top-level type to the
      registry of top-level types.

   *  The criteria for what types do and do not fall under the new top-
      level type need to be defined clearly.  Clear criteria are
      expected to help expert reviewers to evaluate whether a subtype
      belongs below the new type or not, and whether the registration
      template for a subtype contains the approriate information.  If
      the criteria cannot be defined clearly, this is a strong
      indication that whatever is being talked about is not suitable as
      a top-level type.

   *  The document defining the new top-level type MUST include initial
      registrations of actual subtypes.  This will help to show the need
      for the new top-level type, will allow to check the
      appropriateness of the definition of the new top-level type, and
      will avoid separate work for registering an initial slate of

   *  The registration and actual use of a certain number of subtypes
      under the new top-level type SHOULD be expected.  At a minimum,
      one actual subtype SHOULD exist.  But the existence of a single
      subtype SHOULD not be enough; it SHOULD be clear that new similar
      types may appear in the future.  Otherwise, the creation of a new
      top-level type is most probably not justified.

   *  Any RFC defining a new top-level type MUST clearly document the
      security considerations applying to all or a significant subset of

   *  The proposers of the new top-level type and the wider community
      should be willing to commit to emitting and consuming the new top-
      level type in environments that they control.

2.2.  Additional Considerations

   *  Existing wide use of an undefined top-level type may be an
      indication of a need, and therefore an argument for formally
      defining this new top-level type.

   *  On the other hand, the use of undefined top-level types is highly

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   *  Use of an IETF WG to define a new top-level type is not be needed,
      but may be advisable in some cases.  There are examples of new
      top-level type definitions without a WG (RFC 2077 [RFC2077]), with
      a short, dedicated WG (RFC 8081 [RFC8081]), and with a WG that
      included other related work (draft-ietf-mediaman-haptics

   *  Desirability for common parameters: The fact that a group of
      (potential) types have (mostly) common parameters may be an
      indication that these belong under a common new top-level type.

   *  Top-level types can help humans with understading and debugging.
      Therefore, evaluating how a new top-level type helps humans
      understand types may be crucial.  But as often with humans,
      opinions may widely differ.

   *  Common restrictions may apply to all subtypes of a top-level type.
      Examples are the restriction to CRLF line endings for subtypes of
      type 'text' (at least in the context of electronic mail), or on
      subtypes of type 'multipart'.

   *  Top-level types are also used frequently in dispatching code.  For
      example "multipart/*" is frequently handled as multipart/mixed,
      without understanding of a specific subtype.  The top-level media
      types 'image', 'audio', and 'video' are also often handled
      generically.  Documents with these top-level media types can be
      passed to applications handling a wide variety of image, audio, or
      video formats.  HTML generating applications can select different
      HTML elements (e.g. <img> or <audio>) for including data of
      different top-level types.  Applications can select different
      icons to represent unknown types in different top-level types.

2.3.  Negative Criteria

   This subsection lists negative criteria for top-level types,
   identifying criteria that are explicitly not reasons for a top-level
   type registration.

   *  A top-level type is not a pointer into another registration space
      that offers duplicate registrations for existing media types.
      Example: a top-level type of 'oid', leading to types of the form
      oid/nnnnn, where nnn is an OID designating a specific media

   *  A top-level type MUST NOT be defined for the mapping of other
      protocol elements to media types.  For example, while there may be
      some merit to a mapping from media types to URIs, e.g. in the
      context of RDF, there is very limited merit in a reverse mapping,

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      and even less merit in creating a top-level type for such a
      mapping.  The same applies to other protocol elements such as file
      extensions or URI schemes.  The recommended solution in case a
      mapping is needed is to choose a single type/subtype and put the
      additional information in an appropriately named parameter.  As an
      example, information on a file extension '.dcat' can be encoded as
      'application/octet-string; filename=foo.dcat'.

   *  Media types are not a general type system.  A top-level type MUST
      NOT be defined if its main or only purpose is to map other type
      systems, e.g. in programming languages or ontologies.

   *  A new top-level type SHOULD NOT generate aliases for existing
      widely used types or subtypes.

   *  Top-level types with an "X-" prefix cannot be registered, and
      SHOULD NOT be used.  This is in line with RFC [RFC6648].

3.  Top-Level Media Type History

   This section shortly describes the history of top-level media types.
   The emphasis is on the aspects of the history that are relevant to
   the (rather rare) adoption of new top-level types when writing this

   RFC 1341 [RFC1341] first defined the structuring of content types
   into (top-level) type and subtype, and introduced the 'text',
   'multipart', 'message', 'image', 'audio', 'video', and 'application'
   top-level types.  That specification also allowed top-level types
   starting with 'X-'.  With respect to new top-level types, it said the

   |  An initial set of seven Content-Types is defined by this document.
   |  This set of top-level names is intended to be substantially
   |  complete.  It is expected that additions to the larger set of
   |  supported types can generally be accomplished by the creation of
   |  new subtypes of these initial types.  In the future, more top-
   |  level types may be defined only by an extension to this standard.
   |  If another primary type is to be used for any reason, it must be
   |  given a name starting with "X-" to indicate its non-standard
   |  status and to avoid a potential conflict with a future official
   |  name.

   The first time an additional top-level type was defined was in RFC
   1437 [RFC1437], but this was purely for entertainment purposes
   (please check date).

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   RFC 2046 [RFC2046] discouraged the use of "X-" for (new) top-level
   types, with the following words:

   |  In general, the use of "X-" top-level types is strongly
   |  discouraged.  Implementors should invent subtypes of the existing
   |  types whenever possible.  In many cases, a subtype of
   |  "application" will be more appropriate than a new top-level type.

   RFC 2048 [RFC2048], published at the same time as RFC 2046 [RFC2046],
   defined requirements for the definition of new top-level types:

   |  In some cases a new media type may not "fit" under any currently
   |  defined top-level content type.  Such cases are expected to be
   |  quite rare.  However, if such a case arises a new top-level type
   |  can be defined to accommodate it.  Such a definition must be done
   |  via standards-track RFC; no other mechanism can be used to define
   |  additional top-level content types.

   The 'model' top-level type was introduced by RFC 2077 [RFC2077] in

   RFC 4735 [RFC4735] introduced the 'example' top-level type for use in
   documentation examples.

   The 'font' top-level media type was defined in RFC 8081 [RFC8081], a
   work of the 'justfont' IETF WG, in 2017.  This was formalizing the
   widespread use of the unofficial 'font' top level type which people
   were using in preference to official, registered types.

   There is ongoing work on defining a new 'haptics' top-level media
   type in draft-ietf-mediaman-haptics [HAPTICS].

   A Wikipedia (at
   reports the unofficial use of a 'chemical' top-level type.  This top-
   level type was proposed by Peter Murray-Rust and Henry Rzepa at a
   workshop at the First WWW conference in May 1994 CHEMIME [CHEMIME].
   It is in widespread use, but remains unregistered.

   Some Linux desktop logic uses what looks like a top-level type of 'x-
   scheme-handler' to map URI schemes to applications.  In addition, the
   type 'inode/directory' is used.  However, this is a purely local,
   system-specific use, not intended for exchange.  If exchange or
   standardization are desired, a change from e.g. 'x-scheme-handler/
   http' to something like 'application/scheme-handler; scheme=http' or
   'inode/directory' to 'multipart/inode-directory' or 'application/
   inode-directory (in all cases, properly registered) is strongly

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   The document currently defining the requirements for new top-level
   media types is RFC 6838 [RFC6838].  Of particular relevance to the
   work in this document are Section 4.2.5 (Application Media Types) and
   Section 4.2.7 (Additional Top-Level Types).  These two sections are
   not strictly aligned, because the first says that anything that
   doesn't go under a more specific type can go under the 'application'
   top-level type, while the later section allows for new top-level

4.  IANA Considerations

   IANA is requested to create and populate a registry of top-level
   media types, This can be done by expanding the "Registries included
   below" section of
   types.xhtml if that is compatible with IANAs infrastructure.

   For each top-level media type, the registry contains the name of the
   type, a pointer to the RFC defining the type, a pointer to IANA's
   registry of subtypes for that type, and a comment field.

   The comment field is empty or contains short comments about the usage
   of the type.  As an example, the comment field for the 'text' type
   may contain a comment saying "requires CRLF for newlines".  As
   another example, the comment field for the 'example' type may say "no
   registrations, for examples only".  Comments can be added or updated
   by the experts for subtype registrations and by IANA itself.

5.  Security Considerations

   This document as such is not expected to introduce any security
   issues.  The security issues of introducing a new top-level media
   type MUST be evaluated and documented carefully.


   RFC Editor, please remove this section before publication.

Changes from draft-ietf-mediaman-toplevel-01 to draft-ietf-mediaman-

   *  See

Changes from draft-ietf-mediaman-toplevel-00 to draft-ietf-mediaman-

   *  In the Introduction, add a Background section.

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   *  Reorganized so that criteria come first, and split criteria
      section into various subsections.

   *  Add reasons to criteria.

   *  Fixes to status and related text pieces.

   *  Cosmetic fixes, in particular getting rid of 'references in your
      face' (e.g.  "RFC ABCD [RFC ABCD]") little by little.

Changes from draft-duerst-mediaman-toplevel-00 to draft-ietf-mediaman-

   *  Add reference to RFC 2077 [RFC2077] for definition of 'model'

   *  Add examples of use of top-level types for dispatch.

   *  Remove a stray '>' before the mention of RFC 4735 [RFC4735].

   *  Change link to chemical/* Wikipedia page.

   *  Remove reference in abstract (pointed out by idnits).


   Continuous encouragement for writing this draft came from Harald
   Alvestrand.  Further encouragement was provided by Murray S.
   Kucherawy.  Both Harald and Murray also provided ideas for actual
   text.  Without them, this memo would never have reached even the
   first draft stage.  Alexey Melnikov provided the difficult to find
   pointer to RFC 2077 [RFC2077] and examples for applications
   dispatching on top-level media types.  Additional information and
   comments were received from Chris Lilley, Graham Kline, and Henry S.
   Rzepa.  Inspiration for negative criteria or examples was provided by
   Phillip Hallam-Baker, Donald E.  Eastlake 3rd, and Petter


Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

Informative References

   [CHEMIME]  Rzepa, H.S., Murray-Rust, P., and B. Whitaker, "The
              Application of Chemical Multipurpose Internet Mail
              Extensions (Chemical MIME) Internet Standards to
              Electronic Mail and World Wide Web Information Exchange",
              DOI 10.1021/ci9803233, 14 August 1998,

   [HAPTICS]  Muthusamy, Y. K. and C. Ullrich, "The 'haptics' Top-level
              Media Type", RFC XXXX, <

   [RFC1341]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
              Mail Extensions): Mechanisms for Specifying and Describing
              the Format of Internet Message Bodies", RFC 1341,
              DOI 10.17487/RFC1341, June 1992,

   [RFC1437]  Borenstein, N. and M. Linimon, "The Extension of MIME
              Content-Types to a New Medium", RFC 1437,
              DOI 10.17487/RFC1437, April 1993,

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", RFC 2048, DOI 10.17487/RFC2048, November
              1996, <>.

   [RFC2077]  Nelson, S., Parks, C., and Mitra., "The Model Primary
              Content Type for Multipurpose Internet Mail Extensions",
              RFC 2077, DOI 10.17487/RFC2077, January 1997,

   [RFC4735]  Taylor, T., "Example Media Types for Use in
              Documentation", RFC 4735, DOI 10.17487/RFC4735, October
              2006, <>.

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   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648,
              DOI 10.17487/RFC6648, June 2012,

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

Author's Address

   Martin J. Dürst
   Aoyama Gakuin University
   Fuchinobe 5-10-1, Chuo-ku, Sagamihara, Kanagawa
   Phone: +81 42 759 6329

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