On Media-Types, Content-Types, and related terminology

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Table of Contents

1. Introduction

[RFC1590] introduced media types and their registration. That document took MIME types from [RFC1521] and gave them a new name. At that time, the term "media type" was often used just for the major type ("text", "audio"), and what we call a media-type now was the combination of a type and a subtype. This lives on in [RFC6838], which does not even have an ABNF [RFC5234] production for media type. [RFC6838]'s predecessor, [RFC4288], supplied the ABNF shown in (Figure 1).¶

type-name = reg-name
subtype-name = reg-name

reg-name = 1*127reg-name-chars
reg-name-chars = ALPHA / DIGIT / "!" /
                 "#" / "$" / "&" / "." /
                 "+" / "-" / "^" / "_"

[RFC4288] contains the semantically equivalent ABNF in Figure 2, which also provides comments that limit the use of "." and "+".¶

type-name = restricted-name
subtype-name = restricted-name

restricted-name = restricted-name-first *126restricted-name-chars
restricted-name-first  = ALPHA / DIGIT
restricted-name-chars  = ALPHA / DIGIT / "!" / "#" /
                         "$" / "&" / "-" / "^" / "_"
restricted-name-chars =/ "." ; Characters before first dot always
                             ; specify a facet name
restricted-name-chars =/ "+" ; Characters after last plus always
                             ; specify a structured syntax suffix

2. Media-Type

However, the term "media type" is now generally used for a registered combination of a type-name and a subtype-name. We further disambiguate by calling this a media type name, as, in ABNF:¶

Media-Type-Name = type-name "/" subtype-name

For the purposes of this memo, we define:¶

Media-Type-Name:

A combination of a type-name and a subtype-name registered in [IANA.media-types], conventionally identified by the two names separated by a slash.¶

(This leaves the term "Media Type" for the actual specification that is registered under the Media-Type-Name.)¶

3. Content-Type

Media types can have parameters [RFC6838], some of which are mandatory. In HTTP and many other protocols, these are then used in a "Content-Type" header field. HTTP [RFC7231] uses:¶

Content-Type = media-type
media-type = type "/" subtype *( OWS ";" OWS parameter )
type       = token
subtype    = token
token          = 1*tchar
tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
               / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
               / DIGIT / ALPHA
OWS        = *( SP / HTAB )

We don't follow this inclusive use established by [RFC2616], parts of which became [RFC7231], namely to use the term media-type for a Media-Type-Name with parameters; note that [RFC2616] was quite confused about this by claiming (Section 3.7):¶

• Media-type values are registered with the Internet Assigned Number Authority (IANA [19]).¶

This clearly reverts to the understanding of Media-Type-Name we use. We instead define as a separate term:¶

Content-Type:

A Media-Type-Name, optionally associated with parameters (separated from the media type name and from each other by a semicolon).¶

Removing the legacy HTAB characters now shunned in polite conversation, as well as some other cobwebs, we define the conventional textual representation of a Content-Type as:¶

Content-Type   = Media-Type-Name *( *SP ";" *SP parameter )
parameter      = token "=" ( token / quoted-string )

token          = 1*tchar
tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
               / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
               / DIGIT / ALPHA
quoted-string  = %x22 *qdtext %x22
qdtext         = SP / %x21 / %x23-5B / %x5D-7E

Note that there is a slight inconsistency between the "token" used here and the "reg-name"/"restricted-name" used above; since media type parameters probably will be defined within the guard rails set by [RFC7231], we need to use HTTP's more comprehensive definition here.¶

4. Content-Coding

[RFC2616] also introduced the term Content-Coding, a registered name for an encoding transformation that has been or can be applied to a representation:¶

content-coding   = token

Confusingly, in HTTP the Content-Coding is then given in a header field called "Content-Encoding"; we NEVER use this term (except when we are in error). Instead we define:¶

Content-Coding:

a registered name for an encoding transformation that has been or can be applied to a representation.¶

Content-Codings are registered in the HTTP Content Coding Registry, a subregistry of [IANA.http-parameters]. We often use the "identity" Content-Coding, which is the identity transformation, and often fail to identify that Content-Coding by name, instead calling it "no Content-Coding".¶

5. Content-Format

CoAP [RFC7252] defines a Content-Format as the combination of a Content-Type and a Content-Coding, identified by a numeric identifier defined by the "CoAP Content-Formats" registry (a subregistry of [IANA.core-parameters]), but in more confusing words (it did not have the benefit of the present memo).¶

Content-Format:

the combination of a Content-Type and a Content-Coding, identified by a numeric identifier defined by the "CoAP Content-Formats" registry.¶

Note that there has not been a conventional string representation of just the combination of a Content-Type and a Content-Coding; Content-Formats so far always are identified by their registered Content-Format numbers. However, there are applications where that is useful [I-D.keranen-core-senml-data-ct], so we define:¶

Content-Format = 1*DIGIT
Content-Format-String   = Content-Type ["@" content-coding]

This allows the use of Content-Format-Strings such as "application/json@deflate" in place of the less self-describing content-format "11050", or other combinations that do not have a content-format number defined yet.¶

Content-Format-Strings MUST NOT explicitly use the content-coding value of "identity" (i.e., if an identity content-coding is desired, the entire optional part including the "@" sign is left out).¶

Note that a quoted string inside a content-type parameter might contain an "@" sign, so the parsing of Content-Format-Strings cannot be done in a too simplistic way.¶

6. Abbreviations

Media type names are sometimes abbreviated as "mt", and Content-Types as "ct". We propose not to use those abbreviations: Where the long form of the values can be used, the long form "Content-Type" can also be used to name them.¶

For historical reasons, both [RFC6690] and [RFC7252] use the abbreviation "ct" for Content-Format (think first and last character).¶

For Content-Coding, the abbreviation "cc" can be used.¶

7. Discussion

The ABNF given here is provisional and needs to be cleaned up: We need to unify the various forms of reg-name, token, etc.¶

(ABNF just shown for illustration is centered and tagged with "abnf;old" in the XML, while the normative ABNF of this memo is left-aligned and tagged with "abnf".)¶

We need to discuss case-insensitivity, which is usually rather insensitive.¶

8. IANA Considerations

While this memo talks a lot about IANA registries, it does not require any action from IANA.¶

9. Security Considerations

Confusion about terminology may, in the worst case, cause security problems. No other security considerations are known to be raised by the present memo.¶

Acknowledgements

Matthias Kovatsch forced the author to make up his mind about this. Ari Keranen forced him to write it up, then, and created a convincing use case of Content-Format-Strings. John Mattsson alerted us to a mistake. Alexey Melnikov suggested to revive this draft after a year of dormancy.¶

Author's Address