IETF                                                          M.J. Dürst
Internet-Draft                                  Aoyama Gakuin University
Updates: 6838 (if approved)                                 6 March 2022
Intended status: Best Current Practice
Expires: 7 September 2022

       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 [RFC6838], when
   approved.  Comments and discussion about this document should be
   directed to, the mailing list of the Media Type
   Maintenance (mediaman) WG.

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 7 September 2022.

Copyright Notice

   Copyright (c) 2022 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 (
   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.

Dürst                   Expires 7 September 2022                [Page 1]

Internet-Draft          New Top Level Media Types             March 2022

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Top-Level Media Type History  . . . . . . . . . . . . . . . .   2
   3.  Potential Criteria for New Top-Level Media Types  . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     Normative References  . . . . . . . . . . . . . . . . . . . . .   6
     Informative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document defines best practices for defining new top-level media
   types.  RFC 6838 [RFC6838] very consisely defines the conditions for
   defining additional top-level media types.  This document expands
   this and therefore updates RFC 6838 [RFC6838].

   This document is currently a personal draft, but is intended for
   adoption by the Media Type Maintenance (mediaman) IETF WG.  Comments
   and discussion about this document should be directed to that WG's
   mailing list at

   Currently, this document is a collection of information, ideas, and
   text snippets that may be helpful in creating the actual
   specification.  None of the current language is intended to be final.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Top-Level Media Type History

   This section shortly describes the history of top-level media types,
   with a particular emphasis on the (rather rare) adoption of new top-
   level types.

   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

Dürst                   Expires 7 September 2022                [Page 2]

Internet-Draft          New Top Level Media Types             March 2022

   |  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).

   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.

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

   At some point, the 'model' top-level type was introduced.  Any
   pointers to the defining document are greatly appreciated.

   The 'font' top-level media type was defined in RFC 8081 [RFC8081], a
   work of the 'justfont' IETF WG, in 2017.

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

   Wikipedia (at reports the
   unofficial use of a 'chemical' top-level type.

Dürst                   Expires 7 September 2022                [Page 3]

Internet-Draft          New Top Level Media Types             March 2022

   The document currently defining the requirements for new top-level
   media types is RFC 6838 [RFC6838].  Because we are trying to update
   what it says, we are citing the two relevant sections, Section 4.2.5
   and Section 4.2.7, here.

   |  4.2.5.  Application Media Types
   |  The "application" top-level type is to be used for discrete data
   |  that do not fit under any of the other type names, and
   |  particularly for data to be processed by some type of application
   |  program.  This is information that must be processed by an
   |  application before it is viewable or usable by a user.  Expected
   |  uses for the "application" type name include but are not limited
   |  to file transfer, spreadsheets, presentations, scheduling data,
   |  and languages for "active" (computational) material.  (The last,
   |  in particular, can pose security problems that must be understood
   |  by implementors.  The "application/postscript" media type
   |  registration in [RFC2046] provides a good example of how to handle
   |  these issues.)
   |  For example, a meeting scheduler might define a standard
   |  representation for information about proposed meeting dates.  An
   |  intelligent user agent would use this information to conduct a
   |  dialog with the user, and might then send additional material
   |  based on that dialog.  More generally, there have been several
   |  "active" languages developed in which programs in a suitably
   |  specialized language are transported to a remote location and
   |  automatically run in the recipient's environment.  Such
   |  applications may be defined as subtypes of the "application" top-
   |  level type.
   |  The subtype of "application" will often either be the name or
   |  include part of the name of the application for which the data are
   |  intended.  This does not mean, however, that any application
   |  program name may simply be used freely as a subtype of
   |  "application"; the subtype needs to be registered.

   |  4.2.7.  Additional Top-Level Types
   |  In some cases, a new media type may not "fit" under any currently
   |  defined top-level type names.  Such cases are expected to be quite
   |  rare.  However, if such a case does arise, a new type name can be
   |  defined to accommodate it.  Definition of a new top-level type
   |  name MUST be done via a Standards Track RFC; no other mechanism
   |  can be used to define additional type names.

Dürst                   Expires 7 September 2022                [Page 4]

Internet-Draft          New Top Level Media Types             March 2022

   The two sections above are not strictly aligned, because the first
   says 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 types.

3.  Potential Criteria for New Top-Level Media Types

   This section describes potential criteria for new top-level media
   types, including criteria already defined in RFC 6838 [RFC6838].
   Further work is needed to distinguish between required and optional
   criteria.  But it is possible that we end up with just "we didn't
   find any objective criteria for new top-level types, and we will stop
   looking for such criteria".

   *  New top level types are rare enough and different enough that each
      application needs to be evaluated separately.

   *  Need to be documented in a Standards Track RFC.

   *  This Standards Track RFC should include initial registrations of
      actual types.

   *  May (or may not) need an IETF WG for definition.

   *  Existence of a certain number of subtypes that would be grouped
      under the new top-level type.  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.

   *  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

   *  Top-level types mostly help humans; it is unclear to what extent
      top-level types are used by applications directly, as opposed to
      application dispatching and behavior triggered by the type/subtype
      combination.  [More information needed/appreciated here.]
      Therefore, evaluating how a new top-level type helps humans
      understand types may be crucial.  But as often with humans,
      opinions may widely differ.

   *  Need for clear criteria for what types do and don't fall under the
      new top-level type.

Dürst                   Expires 7 September 2022                [Page 5]

Internet-Draft          New Top Level Media Types             March 2022

   *  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.

4.  IANA Considerations

   There is currently no registry of top-level media types, but the list
   of top-level types available for registering subtypes is available at

   There may be a question of whether there is a need for a formal
   registry of top-level types.  Such a registry might contain pointers
   to the definitions of the top-level types.  As a concrete example,
   the author of this document has not yet been able to find the
   definition of the 'model' top level type.

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.


   The initial 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.


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,

   [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

Dürst                   Expires 7 September 2022                [Page 6]

Internet-Draft          New Top Level Media Types             March 2022

   [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, <>.

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

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

Author's Address

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

Dürst                   Expires 7 September 2022                [Page 7]