Internet-Draft New Top Level Media Types July 2022
Dürst Expires 11 January 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.

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

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This Internet-Draft will expire on 11 January 2023.

Table of Contents

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 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.

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.

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.

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".

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.


RFC Editor, please remove this section before publication.

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.


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

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, , <>.
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