Internet Engineering Task Force (IETF) M. 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Trust Legal Provisions and are provided without warranty as described
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 is 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 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), 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 [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, a change from, for different types (in all cases,
properly registered) are strongly recommended. For example, 'x-
scheme-handler/http' should be changed to something like
'application/scheme-handler;
scheme=http' or 'inode/directory' scheme=http'. The type 'inode/
directory' should be changed to 'multipart/inode-directory' or
'application/inode-directory (in all cases, properly registered) is
strongly recommended.
'application/inode-directory.
The document currently defining that previously defined the requirements for new top-level top-
level media types is was RFC 6838 [RFC6838]. Of particular relevance to
the work in this 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 types 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"
(https://www.iana.org/assignments/top-level-media-types) registry and
populated it with the values in Table 1. IANA also added a pointer
to this registry from the "Media Types"
(https://www.iana.org/assignments/media-types) registry group.
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 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 | - |
| | [RFC9695] | 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
IANA has also 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>.
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 were 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,
<http://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, December
2024, <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/