Internet-Draft New Top-level Media Types November 2022
Dürst Expires 5 May 2023 [Page]
6838 (if approved)
Intended Status:
Best Current Practice
M.J. Dürst
Aoyama Gakuin University

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.

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This Internet-Draft will expire on 5 May 2023.

Table of Contents

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

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

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 included.

2.2. Additional Considerations

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.

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 document.

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.

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.

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 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 recommened.

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 types.

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

ISSUE: Is there a need for a formal registry of top-level types. Such a registry would contain pointers to the definitions of the top-level types, and pointers to the list of subtypes. As a concrete reason for the need, the author of this document had difficulties 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.


RFC Editor, please remove this section before publication.

Changes from draft-ietf-mediaman-toplevel-00 to draft-ietf-mediaman-toplevel-01

  • 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-toplevel-01

  • Add reference to RFC 2077 [RFC2077] for definition of 'model' type.
  • 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 Reinholdtsen.


Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <>.

Informative References

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, , <>.
Muthusamy, Y. K. and C. Ullrich, "The 'haptics' Top-level Media Type", RFC XXXX, <>.
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, , <>.
Borenstein, N. and M. Linimon, "The Extension of MIME Content-Types to a New Medium", RFC 1437, DOI 10.17487/RFC1437, , <>.
Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, , <>.
Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", RFC 2048, DOI 10.17487/RFC2048, , <>.
Nelson, S., Parks, C., and Mitra., "The Model Primary Content Type for Multipurpose Internet Mail Extensions", RFC 2077, DOI 10.17487/RFC2077, , <>.
Taylor, T., "Example Media Types for Use in Documentation", RFC 4735, DOI 10.17487/RFC4735, , <>.
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, , <>.
Lilley, C., "The "font" Top-Level Media Type", RFC 8081, DOI 10.17487/RFC8081, , <>.

Author's Address

Martin J. Dürst
Aoyama Gakuin University
Fuchinobe 5-10-1, Chuo-ku, Sagamihara, Kanagawa