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Guidelines for the Definition of New Top-Level Media Types

Document Type Active Internet-Draft (mediaman WG)
Author Martin J. Dürst
Last updated 2024-07-08 (Latest revision 2024-07-05)
Replaces draft-duerst-mediaman-toplevel
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Jun 2023
Publish any specific criteria or guidance for handling registration of future top-level media types, either as an RFC or a wiki page.
Document shepherd Harald T. Alvestrand
Shepherd write-up Show Last changed 2023-08-18
IESG IESG state RFC Ed Queue
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Murray Kucherawy
Send notices to
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
RFC Editor RFC Editor state EDIT
MEDIAMAN                                                      M.J. Dürst
Internet-Draft                                  Aoyama Gakuin University
Updates: 6838 (if approved)                                  5 July 2024
Intended status: Best Current Practice                                  
Expires: 6 January 2025

       Guidelines for the Definition of New Top-Level Media Types


   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

   [RFC Editor, please remove this paragraph.]  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 6 January 2025.

Copyright Notice

   Copyright (c) 2024 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.

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   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 and Criteria for the Registration of New Top-Level Media
           Types . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Required Criteria . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Additional Considerations . . . . . . . . . . . . . . . .   4
     2.3.  Negative Criteria . . . . . . . . . . . . . . . . . . . .   6
   3.  Top-Level Media Type History  . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Registration of Top-level Media Types . . . . . . . . . .   8
     4.2.  Initialization of the Registry of Top-level Media
           Types . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     Changes from draft-ietf-mediaman-toplevel-01 Onwards  . . . . .  11
     Changes from draft-ietf-mediaman-toplevel-00 to
             draft-ietf-mediaman-toplevel-00 . . . . . . . . . . . .  11
     Changes from draft-duerst-mediaman-toplevel-00 to
             draft-ietf-mediaman-toplevel-01 . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     Normative References  . . . . . . . . . . . . . . . . . . . . .  12
     Informative References  . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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.'.  RFC 6838 [RFC6838], Section 4.2.7 only summarily gave
   criteria for defining additional top-level types.  This document
   provides more detailed criteria for defining additional top-level
   types.  It therefore updates RFC 6838 [RFC6838].

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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.  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",
   "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
   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

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   *  Every new top-level type MUST be defined in a Standards Track RFC
      (see RFC 8126, Section 4.9 [RFC8126]).  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 MUST 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 appropriate 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

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

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

2.2.  Additional Considerations

   *  Existing wide use of an unregistered 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 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 (draft-ietf-mediaman-haptics

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   *  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 to show the
      need for the new top-level type, will allow checking the
      appropriateness of the definition of the new top-level type, will
      avoid separate work for registering an initial slate of subtypes,
      and will 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 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 these 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.

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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 (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), 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.
      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 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

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   |  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 an April Fools RFC, 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

   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 which people
   were using in preference to official, registered types.

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

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   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

   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

4.1.  Registration of Top-level Media Types

   Registrations of new top-level types follow the "Standards Action"
   policy (see RFC 8126, Section 4.9 [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.  They have to contain a "IANA Considerations" section
   requesting addition to the registry of top-level media types, and
   have to document security considerations for the top-level types they

   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

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

   IANA is requested to create and populate a registry of top-level
   media types, This should be done by expanding the "Registries
   included below" section of
   types/media-types.xhtml (assuming this is compatible with IANA
   infrastructure; if not, then there should be at least a pointer from
   that page to this new registry).

   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:

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    | name        | Defining RFC | Registry of    | Comments          |
    |             |              | Subtypes       |                   |
    | application | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | audio       | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | example     | RFC 4735     | -              | no registrations, |
    |             |              |                | for examples only |
    | font        | RFC 8081     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | haptics     | RFC          | [pointer to be | -                 |
    |             | [HAPTICS]    | added by IANA] |                   |
    | image       | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | message     | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | model       | RFC 2077     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | multipart   | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |
    | text        | RFC 2046     | [pointer to be | requires CRLF for |
    |             |              | added by IANA] | newlines          |
    | video       | RFC 2046     | [pointer to be | -                 |
    |             |              | added by IANA] |                   |

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

   IANA is also requested to add pointers to this document and to the
   new registry in the application page at

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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 Onwards

   *  See

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

   *  In the Introduction, add a Background section.

   *  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).

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   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 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.


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,

   [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,

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

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,

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   [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, <>.

   [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,

Author's Address

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

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