]> Guidelines for the Definition of New Top Level Media Types Aoyama Gakuin University
Fuchinobe 5-10-1, Chuo-ku, Sagamihara Kanagawa 252-5258 Japan +81 42 759 6329 duerst@it.aoyama.ac.jp https://www.sw.it.aoyama.ac.jp/D%C3%BCrst/
Applications and Real-Time MEDIAMAN Media Type, Top Level 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 media-types@ietf.org, the mailing list of the Media Type Maintenance (mediaman) WG.
Introduction This document defines best practices for defining new top-level media types. RFC 6838 very consisely defines the conditions for defining additional top-level media types. This document expands this and therefore updates RFC 6838. 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 media-types@ietf.org. 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.
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.
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 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, but this was purely for entertainment purposes (please check date). RFC 2046 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, published at the same time as RFC 2046, 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 in 1997. RFC 4735 introduced the 'example' top-level type for use in documentation examples. The 'font' top-level media type was defined in RFC 8081, 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. Wikipedia (at https://en.wikipedia.org/wiki/Chemical_file_format) 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. 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.
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. 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 discouraged.
  • Top-level types can help humans with understading 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.
  • 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 media types 'image', 'audio', and 'video' are also often handled generically. Documents with these top-level media 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.
  • Need for clear criteria for what types do and don't fall under the new top-level type.
  • 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.
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 https://www.iana.org/assignments/media-types/media-types.xhtml. 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.
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.
Changelog 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 for definition of 'model' type.
  • Add examples of use of top-level types for dispatch.
  • Remove a stray '>' before the mention of RFC 4735.
  • Change link to chemical/* Wikipedia page.
  • Remove reference in abstract (pointed out by idnits).
Acknowledgements 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 and examples for applications dispatching on top level media types.
References Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Media Type Specifications and Registration Procedures This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. Informative References MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies This document redefines the format of message bodies to allow multi-part textual and non-textual message bodies to be represented and exchanged without loss of information. [STANDARDS-TRACK] The Extension of MIME Content-Types to a New Medium This document defines one particular type of MIME data, the matter- transport/sentient-life-form type. This memo provides information for the Internet community. It does not specify an Internet standard. Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types This second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK] Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures This set of documents, collectively called the Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages. This fourth document, RFC 2048, specifies various IANA registration procedures for some MIME facilities. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Model Primary Content Type for Multipurpose Internet Mail Extensions The purpose of this memo is to propose an update to Internet RFC 2045 to include a new primary content-type to be known as "model". [STANDARDS-TRACK] Example Media Types for Use in Documentation This document is registration for the 'example' media type and 'example' subtypes within the standards tree. The 'example/*' and '*/example' media types are defined for documentation purposes only. [STANDARDS-TRACK] The "font" Top-Level Media Type This memo serves to register and document the "font" top-level media type, under which subtypes for representation formats for fonts may be registered. This document also serves as a registration application for a set of intended subtypes, which are representative of some existing subtypes already in use, and currently registered under the "application" tree by their separate registrations. The 'haptics' Top-level Media Type Immersion Corporation Immersion Corporation Internet MEDIAMAN This memo serves to register and document the 'haptics' top-level media type, under which subtypes for representation formats for haptics may be registered. This document also serves as a registration application for a set of intended subtypes, which are representative of some existing subtypes already in use.