Internet DRAFT - draft-bormann-core-media-content-type-format
draft-bormann-core-media-content-type-format
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Standards Track H. Birkholz
Expires: 26 August 2021 Fraunhofer SIT
22 February 2021
On Media-Types, Content-Types, and related terminology
draft-bormann-core-media-content-type-format-04
Abstract
There is a lot of confusion about media-types, content-types, and
related terminology.
This memo is an attempt at clearing it up, so we can use consistent
terminology in CoRE and related specifications. It also defines some
ABNF that can be used in these specifications.
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
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This Internet-Draft will expire on 26 August 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Media-Type . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Content-Type . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Content-Coding . . . . . . . . . . . . . . . . . . . . . . . 5
5. Content-Format . . . . . . . . . . . . . . . . . . . . . . . 5
6. Remaining ABNF . . . . . . . . . . . . . . . . . . . . . . . 6
7. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Suggested usage . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.2. SenML . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.3. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
11. Security Considerations . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
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.
[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).
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| type-name = reg-name
| subtype-name = reg-name
|
| reg-name = 1*127reg-name-chars
| reg-name-chars = ALPHA / DIGIT / "!" /
| "#" / "$" / "&" / "." /
| "+" / "-" / "^" / "_"
|
| Figure 1: ABNF for type and subtype, cited from RFC 4288
[RFC6838], obsoleting [RFC4288], restricts the first character of a
reg-name to alphanumeric. It contains the otherwise semantically
equivalent ABNF shown in Figure 2, however adding prose comments that
further limit the use of "." and "+".
type-name = restricted-name
subtype-name = restricted-name
restricted-name = restricted-name-first *126restricted-name-chars
restricted-name-first = ALPHA / DIGIT
restricted-name-chars = ALPHA / DIGIT / "!" / "#" /
"$" / "&" / "-" / "^" / "_"
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
Figure 2: ABNF for type and subtype, as defined from RFC 6838
2. Media-Type
Today, the term "media type" is now generally used for a registered
combination of a type-name and a subtype-name, as well as for the
specification that defines the semantics of this combination. We
further disambiguate by calling the former a _media type name_. An
ABNF definition of "Media-Type-Name":
Media-Type-Name = type-name "/" subtype-name
Figure 3: Definition of Media-Type-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.
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(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 defined
by the media type specification to be mandatory. In HTTP and many
other protocols, media-type-names and parameters are then used
together in a "Content-Type" header field. HTTP [RFC7231] uses the
ABNF in Figure 4:
| Content-Type = media-type
| media-type = type "/" subtype *( OWS ";" OWS parameter )
| type = token
| subtype = token
| token = 1*tchar
| tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
| / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
| / DIGIT / ALPHA
| OWS = *( SP / HTAB )
|
| Figure 4: Content-Type ABNF from RFC 7231
In the ABNF as established by [RFC2616], parts of which became
[RFC7231], the rule name media-type is used for a Media-Type-Name
with parameters attached. We don't follow this inclusive use of
media-type; note that [RFC2616] was quite confused about this term by
claiming (Section 3.7 of [RFC2616]):
Media-type values are registered with the Internet Assigned Number
Authority (IANA [19]).
This clearly reverts to the understanding of Media-Type-Name we use.
In order to resolve some of this confusion, we define as a separate
term:
Content-Type: A Media-Type-Name, optionally associated with
parameters (separated from the media type name and from each other
by a semicolon).
Removing the legacy HTAB characters now shunned in polite
conversation, as well as some other cobwebs, we define the
conventional textual representation of a Content-Type with the ABNF
in Figure 5:
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Content-Type = Media-Type-Name *( *SP ";" *SP parameter )
parameter = token "=" ( token / quoted-string )
token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
quoted-string = %x22 *qdtext %x22
qdtext = SP / %x21 / %x23-5B / %x5D-7E
Figure 5: Definition of Content-Type
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
Section 3.5 of [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
Figure 6: Definition of content-coding as in RFC 2616
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, in Section 1 of [RFC7252], defines a Content-Format as the
combination of a Content-Type and a Content-Coding, identified by a
numeric identifier defined in 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 specifications).
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Content-Format: the combination of a Content-Type and a Content-
Coding, identified by a numeric identifier defined by the "CoAP
Content-Formats" subregistry of [IANA.core-parameters].
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 = "0" / (POS-DIGIT *DIGIT)
Content-Format-String = Content-Type ["@" content-coding]
Figure 7: Definition of Content-Format/-String
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. Remaining ABNF
This specification uses the ABNF given in Figure 8, as originally
defined in [RFC5234] and [RFC8866]:
DIGIT = %x30-39 ; 0 – 9
POS-DIGIT = %x31-39 ; 1 – 9
ALPHA = %x41-5A / %x61-7A ; A – Z / a – z
SP = %x20
Figure 8: Commonly Used ABNF Definitions
7. 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.
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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.
8. Discussion
The ABNF given here is provisional and may need some more cleanup,
such as unifying the various forms of reg-name, token, etc.
(ABNF just shown for illustration is centered, in a blockquote, and
tagged with "<artwork type="abnf;old"...>" in the XML, while the
normative ABNF of this memo is left-aligned and tagged with
"<sourcecode type="abnf"...>".)
The XPath expression "//sourcecode[@type='abnf']/text()" can be used
on the XML form of this specification to extract the ABNF defined
here.
We need to discuss case-insensitivity at some point, which is usually
rather insensitive.
9. Suggested usage
9.1. COSE
Section 3.1 of [RFC8152] defines a common COSE header parameter
(number 3) called "content type" in the description, to indicate the
type of the data in the payload or ciphertext fields.
This header parameter can either be an unsigned integer, indicating a
CoRE Content-Format number, or a text string. The latter alternative
is only defined in general terms. It points to Section 4.2 of
[RFC6838] for 'text values following the syntax of "<type-
name>/<subtype-name>"...', but also discusses the use of parameters
and subparameters; no ABNF or similar detail specification is
provided. The text does not discuss the use of Content-Coding in the
text string form, probably because nothing like the present document
existed at the time, creating a weird gap compared with numeric
Content-Format values. (The text only has trivial changes in its
updated version in Section 3.1 of
[I-D.ietf-cose-rfc8152bis-struct-15].)
The present specification suggests using the production "Content-
Format-String" as a more formal definition of the text string that
can go into the "content type" (number 3) common header parameter in
COSE.
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9.2. SenML
As discussed above, Section 3 of [I-D.keranen-core-senml-data-ct]
makes use of the present specification.
9.3. ...
(to be filled in along further use cases)
10. IANA Considerations
While this memo talks a lot about IANA registries, it does not
require any action from IANA.
11. Security Considerations
Confusion about terminology may, in the worst case, cause security
problems, as can loosely defined syntax elements of a specification.
No other security considerations are known to be raised by the
present specification.
12. References
12.1. Normative References
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<http://www.iana.org/assignments/core-parameters>.
[IANA.http-parameters]
IANA, "Hypertext Transfer Protocol (HTTP) Parameters",
<http://www.iana.org/assignments/http-parameters>.
[IANA.media-types]
IANA, "Media Types",
<http://www.iana.org/assignments/media-types>.
[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>.
[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>.
12.2. Informative References
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[I-D.ietf-cose-rfc8152bis-struct-15]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[I-D.keranen-core-senml-data-ct]
Keranen, A. and C. Bormann, "SenML Data Value Content-
Format Indication", Work in Progress, Internet-Draft,
draft-keranen-core-senml-data-ct-02, 8 July 2019,
<https://www.ietf.org/archive/id/draft-keranen-core-senml-
data-ct-02.txt>.
[RFC1521] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
Mail Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies",
RFC 1521, DOI 10.17487/RFC1521, September 1993,
<https://www.rfc-editor.org/info/rfc1521>.
[RFC1590] Postel, J., "Media Type Registration Procedure", RFC 1590,
DOI 10.17487/RFC1590, March 1994,
<https://www.rfc-editor.org/info/rfc1590>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", RFC 4288, DOI 10.17487/RFC4288,
December 2005, <https://www.rfc-editor.org/info/rfc4288>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[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>.
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[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
Acknowledgements
Matthias Kovatsch forced the authors to make up their minds about
this. Ari Keränen forced them 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 reviving this draft after
a year of dormancy.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
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