Guidelines for the Definition of New Top-Level Media Types

Internet Engineering Task Force (IETF)                        M.J. Dürst
Request for Comments: 9694                      Aoyama Gakuin University
BCP: 13                                                       March 2025
Updates: 6838
Category: Best Current Practice
ISSN: 2070-1721


       Guidelines for the Definition of New Top-Level Media Types

Abstract

   This document defines best practices for defining new top-level media
   types.  It also introduces a registry for top-level media types, and
   contains a short history of top-level media types.  It updates RFC
   6838.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9694.

Copyright Notice

   Copyright (c) 2025 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Background
     1.2.  Requirements Language
   2.  Rules and Criteria for the Registration of New Top-Level Media
           Types
     2.1.  Required Criteria
     2.2.  Additional Considerations
     2.3.  Negative Criteria
   3.  Top-Level Media Type History
   4.  IANA Considerations
     4.1.  Registration of Top-level Media Types
     4.2.  Initialization of the Registry of Top-Level Media Types
   5.  Security Considerations
   Acknowledgements
   References
     Normative References
     Informative References
   Author's Address

1.  Introduction

   This document defines best practices for defining new top-level media
   types.  Top-level media types ('top-level types' for short) appear to
   the left of the slash in a media type, examples being 'text/...',
   'application/...', 'image/...', and so on.  Please note that top-
   level types are different from trees (standards tree, vendor tree,
   personal tree), which (except for the standards tree) are indicated
   immediately to the right of the slash with a prefix of '.../vnd.' or
   '.../prs.'.  Section 4.2.7 of RFC 6838 [RFC6838] only summarily gives
   criteria for defining additional top-level media types.  This
   document provides more detailed criteria for defining additional top-
   level media types.  It therefore updates RFC 6838 [RFC6838].

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.  Please note that assumptions about the
   correctness of a media type must be made carefully, as it could be
   under the control of an attacker.

   In some older scenarios, it may also have been 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 smartphones 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 and Criteria 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 [RFC6838].

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
   included.

   *  Every new top-level type MUST be defined in a Standards Track RFC
      (see Section 4.9 of RFC 8126 [RFC8126]).  This will ensure 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 MUST be defined clearly.  Clear criteria are expected
      to help expert reviewers evaluate whether or not a subtype belongs
      below the new type, and whether the registration template for a
      subtype contains the appropriate information.  Criteria that
      cannot be defined clearly are a strong indication that whatever is
      being talked about is not suitable as a top-level type.

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

   *  At a minimum, one subtype MUST be described.  A top-level type
      without any subtypes serves no purpose.  Please note that the
      'example' top-level describes the subtype 'example'.

2.2.  Additional Considerations

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

   *  On the other hand, the use of unregistered top-level types is
      highly discouraged.

   *  Use of an IETF WG to define a new top-level type is not 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 (RFC 9695 [RFC9695]).

   *  The document defining the new top-level type should include
      initial registrations of actual subtypes.  The exception may be a
      top-level type similar to 'example'.  This will help show the need
      for the new top-level type, allow checking the appropriateness of
      the definition of the new top-level type, avoid separate work for
      registering an initial slate of subtypes, and provide examples of
      what is considered a valid subtype for future subtype
      registrations.

   *  The registration and actual use of a certain number of subtypes
      under the new top-level type should be expected.  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.

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

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

   *  Top-level types can help humans with understanding 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 types
      'image', 'audio', and 'video' are also often handled generically.
      Documents with these top-level 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; it
   identifies 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 nnnnn is an OID (Object Identifier) designating a
      specific media format.

   *  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 (Resource Description Framework) [RDF], there is
      very limited merit in a reverse mapping, 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.
      If a mapping is needed, the recommended solution 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 6648 [RFC6648].

3.  Top-Level Media Type History

   This section briefly describes the history of top-level types.  The
   emphasis is on the aspects of the history that are relevant to the
   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
   following:

   |  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.

   RFC 1437 [RFC1437] defined the first additional top-level type;
   however, it was not registered because RFC 1437 is an April Fools RFC
   that was published purely for entertainment purposes.

   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
   1997.

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

   The 'font' top-level 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 that people
   were using in preference to official, registered types.

   RFC 9695 [RFC9695] defines a new 'haptics' top-level type.  RFC 9695
   and this document were developed in parallel, and RFC 9695 was used
   to cross-check the considerations and procedures defined in this
   document.

   The "Chemical file format" Wikipedia page [CHEMICAL] 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].  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, and is not intended for exchange.  If exchange
   or standardization are desired, different types (in all cases,
   properly registered) are strongly recommended.  As an example, 'x-
   scheme-handler/http' should be changed to something like
   'application/scheme-handler; scheme=http'.  As another example, the
   type 'inode/directory' should be changed to 'multipart/inode-
   directory' or 'application/inode-directory.

   The document that previously defined the requirements for new top-
   level media types was RFC 6838 [RFC6838].  Of particular relevance to
   the work in the current document are Sections 4.2.5 (Application
   Media Types) and 4.2.7 (Additional Top-Level Types) of [RFC6838].
   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 types.

4.  IANA Considerations

4.1.  Registration of Top-level Media Types

   Registrations of new top-level types follow the "Standards Action"
   policy (see Section 4.9 of RFC 8126 [RFC8126]).

   Registrations of new top-level types have to provide the name of the
   top-level type, the defining specification (RFC, or the respective
   draft during the approval process), and, if applicable, some
   comments.  The defining specification has to contain an "IANA
   Considerations" section requesting addition to the registry of top-
   level media types, and has to document security considerations for
   the top-level type they register.

   The comments field is empty or contains short comments about the
   usage of the type.  Comments can be added or updated by the experts
   for subtype registrations under the respective top-level type, and by
   IANA itself.

   There should be at least one subtype, except for registrations that
   are for demonstration purposes only (e.g. the example top-level
   type).

4.2.  Initialization of the Registry of Top-Level Media Types

   IANA has created the "Top-Level Media Types" registry
   (https://www.iana.org/assignments/top-level-media-types) and
   populated it with the values in Table 1.  IANA also added a pointer
   to this registry from the "Media Types" registry group
   (https://www.iana.org/assignments/media-types), and they added
   pointers to this document and to the "Top-Level Media Types" registry
   in the application for a media type at <https://www.iana.org/form/
   media-types>.

   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 initial state of the registry is as follows:

     +=============+==============+==============+===================+
     | Name        | Defining RFC | Registry of  | Comments          |
     |             |              | Subtypes     |                   |
     +=============+==============+==============+===================+
     | application | [RFC2046]    | [Application | -                 |
     |             |              | Media Types] |                   |
     +-------------+--------------+--------------+-------------------+
     | audio       | [RFC2046]    | [Audio Media | -                 |
     |             |              | Types]       |                   |
     +-------------+--------------+--------------+-------------------+
     | example     | [RFC4735]    | [Example     | no registrations, |
     |             |              | Media Types] | for examples only |
     +-------------+--------------+--------------+-------------------+
     | font        | [RFC8081]    | [Font Media  | -                 |
     |             |              | Types]       |                   |
     +-------------+--------------+--------------+-------------------+
     | haptics     | [RFC9695]    | [Haptics     | -                 |
     |             |              | Media Types] |                   |
     +-------------+--------------+--------------+-------------------+
     | image       | [RFC2046]    | [Image Media | -                 |
     |             |              | Types]       |                   |
     +-------------+--------------+--------------+-------------------+
     | message     | [RFC2046]    | [Message     | -                 |
     |             |              | Media Types] |                   |
     +-------------+--------------+--------------+-------------------+
     | model       | [RFC2077]    | [Model Media | -                 |
     |             |              | Types]       |                   |
     +-------------+--------------+--------------+-------------------+
     | multipart   | [RFC2046]    | [Multipart   | -                 |
     |             |              | Media Types] |                   |
     +-------------+--------------+--------------+-------------------+
     | text        | [RFC2046]    | [Text Media  | requires CRLF for |
     |             |              | Types]       | newlines          |
     +-------------+--------------+--------------+-------------------+
     | video       | [RFC2046]    | [Video Media | -                 |
     |             |              | Types]       |                   |
     +-------------+--------------+--------------+-------------------+

     Table 1: Initial Values for the Registry of Top-level Media Types

5.  Security Considerations

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

Acknowledgements

   Continuous encouragement for writing this document 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 types.  Additional information and comments
   were received from Chris Lilley, Graham Kline, Henry S. Rzepa,
   Francesca Palombini, Zaheduzzaman Sarker, Amanda Baber, Paul Wouters,
   Roman Danyliw, John Scudder, Radia Perlman, Lars Eggert, and Antoine
   Fressancourt.  Inspiration for negative criteria or examples was
   provided by Phillip Hallam-Baker, Donald E. Eastlake 3rd, Petter
   Reinholdtsen, and Christian Heller.

References

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Informative References

   [CHEMICAL] Wikipedia, "Chemical file format", 19 July 2024,
              <https://en.wikipedia.org/w/
              index.php?title=Chemical_file_format&oldid=1235421631>.

   [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",
              Journal of Chemical Information Computer Science, vol. 38,
              no. 6, pp. 976-982, DOI 10.1021/ci9803233, 14 August 1998,
              <https://pubs.acs.org/doi/10.1021/ci9803233>.

   [RDF]      Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
              Concepts and Abstract Syntax", W3C Recommendation, 25
              February 2014,
              <https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/>.

   [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,
              <https://www.rfc-editor.org/info/rfc1341>.

   [RFC1437]  Borenstein, N. and M. Linimon, "The Extension of MIME
              Content-Types to a New Medium", RFC 1437,
              DOI 10.17487/RFC1437, April 1993,
              <https://www.rfc-editor.org/info/rfc1437>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [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, <https://www.rfc-editor.org/info/rfc2048>.

   [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,
              <https://www.rfc-editor.org/info/rfc2077>.

   [RFC4735]  Taylor, T., "Example Media Types for Use in
              Documentation", RFC 4735, DOI 10.17487/RFC4735, October
              2006, <https://www.rfc-editor.org/info/rfc4735>.

   [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,
              <https://www.rfc-editor.org/info/rfc6648>.

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,
              <https://www.rfc-editor.org/info/rfc8081>.

   [RFC9695]  Muthusamy, Y. K. and C. Ullrich, "The 'haptics' Top-Level
              Media Type", RFC 9695, DOI 10.17487/RFC9695, March 2025,
              <https://www.rfc-editor.org/info/rfc9695>.

Author's Address

   Martin J. Dürst
   Aoyama Gakuin University
   Fuchinobe 5-10-1, Chuo-ku, Sagamihara, Kanagawa
   252-5258
   Japan
   Phone: +81 42 759 6329
   Email: duerst@it.aoyama.ac.jp
   URI:   https://www.sw.it.aoyama.ac.jp/Dürst/