Concise Diagnostic Notation (CDN)
draft-ietf-cbor-edn-literals-28
| Document | Type | Active Internet-Draft (cbor WG) | |
|---|---|---|---|
| Author | Carsten Bormann | ||
| Last updated | 2026-09-29 | ||
| Replaces | draft-bormann-cbor-edn-literals | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Reviews | |||
| Additional resources |
GitHub Repository
Mailing list discussion |
||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Paul E. Hoffman | ||
| Shepherd write-up | Show Last changed 2024-05-03 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Andy Newton | ||
| Send notices to | christian@amsuess.com, paul.hoffman@icann.org | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA expert review state | Expert Reviews OK |
draft-ietf-cbor-edn-literals-28
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Updates: 8610, 8949 (if approved) 29 September 2026
Intended status: Standards Track
Expires: 2 April 2027
Concise Diagnostic Notation (CDN)
draft-ietf-cbor-edn-literals-28
Abstract
This document formalizes and consolidates the definition of the
Concise Diagnostic Notation (CDN) of the Concise Binary Object
Representation (CBOR), addressing implementer experience.
Replacing CDN's previous informal descriptions, it updates RFC 8949,
obsoleting its Section 8, and RFC 8610, obsoleting its Appendix G.
It also specifies registry-based extension points and uses them to
support text representations such as of epoch-based dates/times and
of IP addresses and prefixes.
// (This cref will be removed by the RFC editor:) This revision -28
// attempts to reflect various feature removals that have been
// discussed on the mailing list, as a delta to -27. Note that, with
// the focus on the delta, this text is necessary somewhat
// inconsistent. The chairs decided not to include further editorial
// improvements that could achieve a greater degree of consistency.
// The text may be misleading as some of the explanatory sections no
// longer fully reflect the technical content. The text also does
// not have WG input yet on any renaming decisions (CDN name, b1/t1
// name).
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://cbor-
wg.github.io/edn-literal/. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-ietf-cbor-edn-
literals/.
Discussion of this document takes place on the cbor Working Group
mailing list (mailto:cbor@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at
https://www.ietf.org/mailman/listinfo/cbor/.
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Source for this draft and an issue tracker can be found at
https://github.com/cbor-wg/edn-literal.
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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 April 2027.
Copyright Notice
Copyright (c) 2026 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 publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Structure of This Document . . . . . . . . . . . . . . . 6
1.2. Terminology and Conventions . . . . . . . . . . . . . . . 6
1.3. (Non-)Objectives of this Document . . . . . . . . . . . . 7
1.3.1. For Humans . . . . . . . . . . . . . . . . . . . . . 7
1.3.2. Determinism? . . . . . . . . . . . . . . . . . . . . 7
1.3.3. Basic Output Format . . . . . . . . . . . . . . . . . 8
1.3.4. Evolution . . . . . . . . . . . . . . . . . . . . . . 8
1.3.5. Character Repertoire of Source . . . . . . . . . . . 9
2. Concise Diagnostic Notation (CDN) . . . . . . . . . . . . . . 10
2.1. Comments . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1. Discussion . . . . . . . . . . . . . . . . . . . . . 13
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2.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3. Strings . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1. Double-Quoted String Literals . . . . . . . . . . . . 17
2.3.2. Single-Quoted String Literals . . . . . . . . . . . . 18
2.3.3. Raw String Literals . . . . . . . . . . . . . . . . . 19
2.3.4. CBOR Sequence Literals . . . . . . . . . . . . . . . 21
2.3.5. Validity of Text Strings . . . . . . . . . . . . . . 22
2.4. Arrays and Maps . . . . . . . . . . . . . . . . . . . . . 22
2.4.1. Mandatory Separators, Optional Terminators . . . . . 22
2.4.2. Validity of Maps . . . . . . . . . . . . . . . . . . 23
2.5. Tags . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.6. Simple values . . . . . . . . . . . . . . . . . . . . . . 24
3. Prefixed Literals . . . . . . . . . . . . . . . . . . . . . . 24
3.1. Base-Encoded Byte String Literals: h and b64 . . . . . . 26
3.2. Date and Time: dt . . . . . . . . . . . . . . . . . . . . 27
3.3. IP Addresses and Related Structures: ip . . . . . . . . . 28
3.4. Cryptographic Hash Values: hash . . . . . . . . . . . . . 30
3.5. String Concatenation: b1 and t1 . . . . . . . . . . . . . 31
3.6. Creating Indefinite-length Encoded Strings: ilbs and
ilts . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.7. Floating-Point Values: float . . . . . . . . . . . . . . 32
4. Encoding Indicators . . . . . . . . . . . . . . . . . . . . . 33
4.1. Syntax, Semantics, Examples . . . . . . . . . . . . . . . 34
4.2. Encoding Indicators of Arrays and Maps . . . . . . . . . 37
4.3. Deprecated: Indefinite-length Encoding Indicators for
Strings . . . . . . . . . . . . . . . . . . . . . . . . . 38
5. ABNF Definitions . . . . . . . . . . . . . . . . . . . . . . 38
5.1. Overall ABNF Definition for Concise Diagnostic
Notation . . . . . . . . . . . . . . . . . . . . . . . . 39
5.2. ABNF Definitions for App-Extension Content . . . . . . . 45
5.2.1. h: ABNF Definition of Hexadecimal representation of a
byte string . . . . . . . . . . . . . . . . . . . . . 46
5.2.2. b64: ABNF Definition of Base64 representation of a byte
string . . . . . . . . . . . . . . . . . . . . . . . 47
5.2.3. dt: ABNF Definition of RFC 3339 Representation of a
Date/Time . . . . . . . . . . . . . . . . . . . . . . 48
5.2.4. ip: ABNF Definition of Textual Representation of an IP
Address . . . . . . . . . . . . . . . . . . . . . . . 48
5.3. ABNF Definitions for Integrated Extension Parsers . . . . 49
5.3.1. h'': ABNF Definition of Integrated Parser . . . . . . 51
5.3.2. b64'': ABNF Definition of Integrated Parser . . . . . 51
5.3.3. h``: ABNF Definition of Integrated Parser . . . . . . 52
5.3.4. b64``: ABNF Definition of Integrated Parser . . . . . 52
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
6.1. Concise Diagnostic Notation App-extension Identifiers
Registry . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2. Media Type . . . . . . . . . . . . . . . . . . . . . . . 54
6.3. Content-Format . . . . . . . . . . . . . . . . . . . . . 55
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7. Security considerations . . . . . . . . . . . . . . . . . . . 56
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
8.1. Normative References . . . . . . . . . . . . . . . . . . 56
8.2. Informative References . . . . . . . . . . . . . . . . . 59
Appendix A. CDN and CDDL . . . . . . . . . . . . . . . . . . . . 61
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . 62
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . 63
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 63
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 63
1. Introduction
The Concise Binary Object Representation (CBOR) (RFC8949) [STD94] is
a data format whose design goals include the possibility of extremely
small code size, fairly small message size, and extensibility without
the need for version negotiation. In addition to the binary
interchange format, the original CBOR specification described a text-
based "diagnostic notation" (Section 6 of [RFC7049], now Section 8 of
RFC 8949 [STD94]), in order to facilitate conversation about CBOR
data items without having to resort to binary data. Appendix G of
[RFC8610] extended this into what also became known as Extended
Diagnostic Notation (EDN), often including Section 4.2 of [RFC8742]
and draft revisions of the present document. Diagnostic notation is
now specified by this document, obsoleting all these previous
descriptions, and is known as Concise Diagnostic Notation (CDN).
Diagnostic notation syntax is based on JSON, with extensions for
representing CBOR constructs such as binary data and tags.
The interchange format created by standardizing CDN is not intended
to compete with the actual binary interchange format CBOR, but
enables the use of a shared diagnostic notation in tools for and in
documents about CBOR. However, between tools for CBOR development
and diagnosis, document generation systems, continuous integration
(CI) environments, configuration files, and user interfaces for
viewing and editing for all these, CDN is often "interchanged".
Therefore, CDN deserves a specification that facilitates
interoperability within this domain and reliable translation to and
from CBOR. CDN is not designed or intended for general-purpose use
in protocol elements exchanged between systems engaged in processes
outside those listed here.
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This document consolidates and formalizes the definition of CDN,
providing a formal grammar (see Section 5.1 and Section 5.2), and
incorporating small changes based on implementation experience. It
updates RFC8949 by obsoleting Section 8 of RFC 8949 [STD94], and
[RFC8610] by obsoleting Appendix G of [RFC8610]. It is intended to
serve as the single reference target that can be used in
specifications that use CDN.
It also specifies a registry-based extension point for the diagnostic
notation, for adding application-oriented "prefixed" literal forms.
It uses this registry to add prefixed literal forms that enhance CDN
with text representations of various kinds of data items. Among
others, these include epoch-based date/times, IP addresses and
prefixes [RFC9164], as well cryptographic hash values computed from
byte strings.
In addition, this document registers a media type identifier and a
content-format for CDN. This does not elevate its status as an
interchange format, but recognizes that interaction between tools is
often smoother if media types can be used.
| Examples in RFCs often do not use media type identifiers, but
| special sourcecode type names that are allocated in
| https://www.rfc-editor.org/materials/sourcecode-types.txt
| (https://www.rfc-editor.org/materials/sourcecode-types.txt).
| At the time of writing, this resource lists four sourcecode
| type names that can be used in RFCs for including CBOR data
| items and CBOR-related languages:
|
| * cbor (which is actually not useful, as CBOR is a binary
| format and cannot be used in textual examples in an RFC),
|
| * cbor-diag (which is another name for CDN, as is now
| defined in the present document),
|
| * cbor-pretty (which is a possibly annotated and pretty-
| printed hexdump of an encoded CBOR data item, along the
| lines of the grammar of Section 5.2.1, as used for
| instance for some of the examples in Appendix A.3 of
| [RFC9290]), and
|
| * cddl (which is used for the Concise Data Definition
| Language, CDDL, see Section 1.2 below).
Note that CDN is not meant to be the only text-based representation
of CBOR data items. For instance, [YAML] [RFC9512] is able to
represent most CBOR data items, possibly requiring use of YAML's
extension points. YAML does not provide certain features that can be
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useful with tools and documents needing text-based representations of
CBOR data items (such as embedded CBOR or encoding indicators), but
it does provide a host of other features that CDN does not provide
such as anchor/alias data sharing, at a cost of higher implementation
and learning complexity.
1.1. Structure of This Document
Section 2 of this document defines CDN. After introductory material,
Section 3 further illustrates the concept of prefixed literals by
defining a number of them in app-extensions. Section 4 describes
syntax that can be interpreted by a diagnostic implementation to take
note/take control of which of possibly several encoding variants is
in use for a data item; this syntax always includes an underscore
("_") and therefore is visually easy to ignore. Section 5 gives the
formal syntax of CDN in ABNF. This is followed by the conventional
sections for IANA Considerations (6), Security considerations (7),
and References (8). An informational comparison of CDN with CDDL
follows in Appendix A.
1.2. Terminology and Conventions
The term "ABNF" in this document refers to the language defined in
[STD68] as extended in [RFC7405], where the "characters" of
Section 2.3 of RFC 5234 [STD68] are Unicode scalar values. Where
names for ABNF rules are used in the text, they are shown in
typewriter font (not distinguishable in the plaintext rendition of
this document). Brief snippets of grammar may also be given in the
text as I-Regexp regular expressions [RFC9485].
The term "CDDL" (Concise Data Definition Language) refers to the data
definition language defined in [RFC8610] and its registered
extensions (such as those documented in [RFC9165], [RFC9741], and
[RFC9682]). Additional information about the relationship between
the two languages CDN and CDDL is captured in Appendix A.
Examples sometimes need to be quoted in the text, in particular in
cases where the typewriter font used for example text cannot be
distinguished in the plaintext rendition of this document. ASCII
quotes, however, are already taken: true, "true", 'true', and `true`
are all different literals in CDN and should not be confused.
Therefore, a different quoting convention as in »true« or »"true"« is
used for examples in the text where this is needed to remain
unambiguous.
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Superscript notation denotes exponentiation. For example, 2 to the
power of 64+1 is notated: 2^(64+1). In the plain-text rendition of
this specification, superscript notation is not available and
exponentiation is therefore rendered by the surrogate notation seen
here in the plain-text rendition.
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
[BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all
capitals, as shown here.
1.3. (Non-)Objectives of this Document
Section 8 of RFC 8949 [STD94] states the objective of defining a
common human-readable diagnostic notation with CBOR. In particular,
it states:
| All actual interchange always happens in the binary format.
1.3.1. For Humans
One important application of CDN is the notation of CBOR data for
humans: in specifications, on whiteboards, and for entering test
data. A number of features, such as comments inside prefixed string
literals, are mainly useful for people-to-people communication via
CDN. Programs also often output CDN for diagnostic purposes, such as
in error messages or to enable comparison (including generation of
diffs via tools) with test data.
1.3.2. Determinism?
For comparison with test data, it is often useful if different
implementations generate the same (or similar) output for the same
CBOR data items. This is comparable to the objectives of
deterministic serialization for CBOR data items themselves
(Section 4.2 of RFC 8949 [STD94]). However, there are even more
representation variants in CDN than in binary CBOR, and there is
little point in specifically endorsing a single variant as
"deterministic" when other variants may be more useful for human
understanding, e.g., the << >> notation as opposed to, say,
hexadecimal h'' notation; a CDN generator may have quite a few
options that control what presentation variant is most desirable for
the application that it is being used for.
Because of this, a deterministic representation is not defined for
CDN. More generally speaking, there is no expectation for
"roundtripping": Converting CDN to binary CBOR and back to CDN will
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generally not achieve exactly the same result as the original input
CDN. This possibly was created by humans or by a different CDN
generator and may contain presentation information that is not
represented in the binary CBOR.
1.3.3. Basic Output Format
However, there is a certain expectation that CDN generators can be
configured to some basic output format, which:
* looks like JSON where that is possible;
* inserts encoding indicators, if any, only where the binary form
differs from Preferred Serialization (Section 4.1 of RFC 8949
[STD94]);
* uses hexadecimal representation (h'') for byte strings, not b64''
or embedded CBOR (<<>>);
* does not generate elaborate blank space (newlines, indentation)
for pretty-printing, but does use common blank spaces such as
after , and :.
See Section 1.3.5 for more considerations about the character
repertoire used for CDN source text.
Additional features such as ensuring deterministic map ordering
(Section 4.2 of RFC 8949 [STD94]) on output, or even deviating from
the basic configuration in some systematic way, can further assist in
comparing test data. Information obtained from a CDDL model can help
in choosing prefixed literals or specific string representations such
as embedded CBOR or b64'' in the appropriate places.
1.3.4. Evolution
Diagnostic notation was initially designed for interchange situations
where backward compatibility was considered less critical than in
binary CBOR interchanges. This allowed for quite freely making
extensions in Appendix G of [RFC8610] and Section 4.2 of [RFC8742].
However, with increased interchange between CBOR-related tools, this
unrestricted evolution is less desirable.
The present specification supports a more controlled path of evolving
CDN through two well-defined extension points: one general
(Section 3) and one specific to diagnostic processing of encoding
variants (Section 4).
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The present specification makes two changes to the [RFC8610]
extensions that are not entirely backward compatible. These changes
are detailed in Section 2.1.1 and in the aside at the end of
Section 2.3. Some syntax from the original diagnostic notation is
being deprecated (Section 4.3) and replaced (Section 3.6). These
changes are deemed acceptable now because the updated features were
originally introduced under more permissive conditions. With CDN now
more rigidly defined and focusing evolution on the new extension
points, such changes are no longer foreseen.
1.3.5. Character Repertoire of Source
Similar to JSON, CDN is designed to enable representing all CBOR data
items using a source character repertoire just containing printable
ASCII characters (%x20-7e in ABNF) and newlines. However, if
appropriate, CDN can also make full use of larger Unicode
repertoires.
CDN generators may provide configuration to consistently select
either the unescaped (directly readable) or an escaped (ASCII
equivalent) form of characters in string literals; the latter allows
CDN to be used when the diagnostic value of fully escaped characters
may be desired or in environments where non-ASCII characters may not
enjoy full data transparency. Similar to JSON, CDN is designed to
allow a simple tool to convert any CDN into a fully escaped
(printable ASCII and newlines only) form, as well as to inversely
recover unescaped characters for all escapes where this is possible
or for certain subsets of the characters (such as Unicode categories
L, M, N, P, S, plus Zs or just ASCII space).
Special considerations apply to newlines in the source. On some
platforms, a CARRIAGE RETURN character (U+000D or CR, often seen
escaped as "\r" in many programming languages) is always added in
front of a LINE FEED (U+000A or LF) to represent a newline (which are
then referred to as CRLF). On other platforms, carriage returns are
not used at line breaks at all, so a newline is just an LF.
(Platforms that use just a CARRIAGE RETURN by itself to signify an
end of line are no longer relevant and the files they produce are out
of scope for this document.)
Files are often freely converted between these two newline
representations, including by source code revision control systems.
To ensure that platforms will generate the same bytes in the CBOR
data items created from input in either conversion state, CDN MUST
create the same processing result independent of which newline
representation is used by its input.
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To deal with this variability in platform presentation of newlines,
Unicode CARRIAGE RETURN characters that exist in the input unescaped
are ignored as if they were not in the input wherever they appear.
Specifically, any carriage return characters that may be present in a
CDN (text or byte) string literal are not copied into the resulting
string. If a carriage return is needed in a CBOR string data item,
it can be added explicitly, for instance by using the escaped form \r
in single-quoted or double-quoted strings.
2. Concise Diagnostic Notation (CDN)
CBOR is a binary interchange format. To facilitate documentation and
debugging, and in particular to facilitate communication between
entities cooperating in debugging, this document defines a simple
human-readable diagnostic notation. All actual interchange always
happens in the binary format.
Note that diagnostic notation truly was designed as a diagnostic
format; it originally was not meant to be parsed. Therefore, no
formal definition (as in ABNF) was given in the original documents.
Recognizing that formal grammars can aid interoperation of tools and
usability of documents that employ CDN, Section 5 now provides ABNF
definitions.
CDN is a true superset of JSON as it is defined in [STD90] in
conjunction with [RFC7493] (that is, any interoperable [RFC7493] JSON
text also is a CDN text), extending it both to cover the greater
expressiveness of CBOR and to increase its usability.
CDN borrows the JSON syntax for numbers (integer and floating-point,
Section 2.2), certain simple values (Section 2.6), UTF-8 [STD63] text
strings, arrays, and maps (maps are called objects in JSON; the
diagnostic notation extends JSON here by allowing any data item in
the map key position).
CDN provides _literals_ that represent CBOR data items textually.
Many of the forms of literals provided are predefined by this
document, but it also defines an extension point that enables
defining additional _application-oriented extension literals_. These
are also known as _prefixed literals_, as they start with a _prefix_
that identifies the application-oriented extension and possibly a
specific variant of that. Section 3 discusses these in more details
and defines a number of app-extensions that are included with this
specification.
As CDN is used for truly diagnostic purposes, its implementations MAY
support generation and possibly ingestion of CDN for CBOR data items
that are well-formed but not valid. It is RECOMMENDED that an
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implementation enables such usage only explicitly by configuration
(such as an API or CLI flag). Validity of CBOR data items is
discussed in Section 5.3 of RFC 8949 [STD94], with basic validity
discussed in Section 5.3.1 of RFC 8949 [STD94], and tag validity
discussed in Section 5.3.2 of RFC 8949 [STD94]. Tag validity is more
likely a subject for individual app-extensions, while the two cases
of basic validity (for text strings and for maps) are addressed in
Sections 2.3.5 and 2.4.2 under the heading of _validity_.
The rest of this section provides an overview over specific features
of CDN, starting with certain common syntactical features and then
going through kinds of CBOR data items roughly in the order of CBOR
major types. Any additional detailed syntax discussion needed has
been deferred to Section 5.1.
Additional information about implementation and use of CDN is
continuously being collected by the community in [CDN-WIKI].
2.1. Comments
For presentation to humans, CDN text may benefit from comments. JSON
famously does not provide for comments, and the original diagnostic
notation in Section 6 of [RFC7049] inherited this property.
CDN provides two comment syntaxes, which can be used where the syntax
allows blank space (outside of constructs such as numbers, string
literals, etc.):
* inline comments, delimited by slashes ("/") or by C-style "/*" and
"*/":
In a position that allows blank space, each of the following is
considered blank space (and thus effectively a comment):
- any text that starts with a slash followed by a character that
is not a star or a slash, up to another slash, or
- any text that starts with "/*" up to and including the next
following "*/"
* end-of-line comments, delimited by "#" or "//" and an end of line
(LINE FEED, U+000A):
In a position that allows blank space, any text starting with "#"
or "//" and ending with and including the end of the line is
considered blank space (and thus effectively a comment).
Comments can be used to annotate a CBOR structure as in:
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/grasp-message/ [/M_DISCOVERY/ 1, /session-id/ 10584416,
/objective/ [/objective-name/ "opsonize",
/D, N, S/ 7, /loop-count/ 105]]
This reduces to [1, 10584416, ["opsonize", 7, 105]].
Another example, combining the use of inline and end-of-line
comments:
{
/kty/ 1 : 4, # Symmetric
/alg/ 3 : 5, # HMAC 256-256
/k/ -1 : h'6684523ab17337f173500e5728c628547cb37df
e68449c65f885d1b73b49eae1'
}
This reduces to {1: 4, 3: 5, -1:
h'6684523AB17337F173500E5728C628547CB37DFE68449C65F885D1B73B49EAE1'}.
A CDN file used for configuration might look like this (employing
'//' end of line comments throughout and an ornamental C-Style
comment at the start):
/* ### MyApp Configuration
* John Example, 2026-06-09
*/
{
// Top-level config for the app
"appName": "MyApp", // short name shown in UI
"version": "1.2.0",
}
| Note that app-extensions can define their own internal comment
| syntaxes for text inside strings, which may or may not mimic
| the overall comment syntax of CDN. The h'' syntax
| (Section 5.2.1), which the framework for app-extensions was
| designed to include as an instance, provides an equivalent to
| the overall comment syntax inside its text strings. Similarly,
| b64'' (Section 5.2.2) provides a subset of that limited to "#"
| end-of-line comments (the slash character "/" is used in the
| alphabet in classic base64 encoding). None of the other app-
| extensions supplied in this specification provides for such a
| kind of internal comment syntax.
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2.1.1. Discussion
Appendix G.6 of [RFC8610] introduced comments into the diagnostic
notation syntax, limited to inline comments using a bare "/" as the
comment delimiter. It however also hinted at the potential desire to
add end-of-line comments, mentioning both "//" and "#" as start
delimiters.
The present specification adds both, as well as C-style inline
comments ("/*" and "*/" delimiters).
This introduces a backwards-incompatible change, restricting slash-
delimited comments that were allowed by Appendix G.6 of [RFC8610] in
two ways:
* Inline comments no longer can be empty: The construct "//" that
was an empty comment in Appendix G.6 of [RFC8610] is now used
instead to introduce an end-of-line comment. (Note that "//"
still can be used in what is visually "within" a slash-delimited
comment like in the second example below; its first slash actually
ends the current comment and the second slash starts a new one.)
* Enabling the use of C-style inline comments can extend the scope
of what previously were parsed as slash-delimited comments: for
instance, "/*foo/" was a complete comment in Appendix G.6 of
[RFC8610] and now is the beginning of a C-style comment that goes
on up to a "*/".
As an example for what is enabled by this change, the introduction of
C-style inline comments enables a comment explaining a COSE algorithm
identifier, as in
4 /* HMAC 256/64 */
instead of the previously conventional, but often less familiar
4 / HMAC 256//64 /
2.2. Numbers
In addition to JSON's decimal number literals, CDN provides
hexadecimal, octal, and binary number literals in the usual
C-language notation (0x, 0o prefix only, and 0b, respectively).
CBOR distinguishes two basic kinds of numbers: integers and floating
point values. Numbers composed only of digits (of the respective
base) are interpreted as CBOR integers (major type 0/1, or where the
number cannot be represented in this way, major type 6 with tag 2/3).
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A leading "+" sign is a no-op, and a leading "-" sign inverts the
sign of the number. So 0 and +0 represent the same integer zero, as
does -0. Similarly, 1 and +1 stand for the same positive integer
one, and -1 designates the negative integer minus one.
| In addition to being a superset of JSON decimal numbers, the
| grammar for decimal (base-10) numbers is inspired by similar
| grammars, such as those for decimal and double in Sections
| 3.3.3 and 3.3.5 of [XSD2]. Such grammars typically allow
| removing as well as adding insignificant zero digits from/to a
| number representation that would be well-formed in JSON.
| However, there is one pitfall: the original C language
| convention for indicating octal numbers simply precedes the
| octal number with a leading zero digit. When copying data from
| and to data sources that use this convention, there would be
| ambiguity (e.g., in C, 030 is the number 24 decimal, but in
| other languages the number 30 decimal); in effect, the
| semantics of numbers with leading zeros often silently differ.
| Therefore, the CDN grammar restricts the grammar to not allow
| any leading zeros in the integer part, except for a single
| digit zero.
Using a decimal point (.) and/or an exponent (e for decimal, p for
hexadecimal) turns the number into a floating point number (part of
major type 7) instead, irrespective of whether it is an integral
number mathematically. Note that, in floating point numbers, 0.0 is
not the same number as -0.0, even if they are mathematically equal.
In Table 1, all the items on a row are the same number (also shown in
CBOR, hexadecimally), but they are distinct from items in a different
row.
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+========================================+===================+
| CDN | CBOR hex |
+========================================+===================+
| 4711, 0x1267, 0o11147, 0b1001001100111 | 19 1267 # uint |
+----------------------------------------+-------------------+
| 1.5, 0.15e1, 15e-1, 0x1.8p0, 0x18p-4 | F9 3E00 # float16 |
+----------------------------------------+-------------------+
| 0, +0, -0 | 00 # uint |
+----------------------------------------+-------------------+
| 0.0, +0.0 | F9 0000 # float16 |
+----------------------------------------+-------------------+
| -0.0 | F9 8000 # float16 |
+----------------------------------------+-------------------+
| Infinity | F9 7C00 # float16 |
+----------------------------------------+-------------------+
| -Infinity | F9 FC00 # float16 |
+----------------------------------------+-------------------+
| NaN | F9 7E00 # float16 |
+----------------------------------------+-------------------+
Table 1: Example Sets of Equivalent Notations for Some Numbers
The non-finite floating-point values Infinity, -Infinity, and NaN are
written exactly as in this sentence (this is also a way they can be
written in JavaScript, although JSON does not allow them). NaN in
CDN stands for the NaN value with a zero sign bit and an all-zero
significand except for a set quiet bit; this is represented as F9 7E
00 in CBOR Preferred Serialization.
| Table 2 shows how the floating point numbers 1.1 and 1.5 as
| well as these three non-finite values are encoded, both in
| preferred serialization (Section 4.1 of RFC 8949 [STD94]) and
| when encoding indicators (please see Section 4) are given.
|
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| +========================+=====================+
| | CDN | CBOR hex |
| +========================+=====================+
| | 1.1 | fb 3ff199999999999a |
| +------------------------+---------------------+
| | 1.1_1, 1.1_2 | (error) |
| +------------------------+---------------------+
| | 1.1_3 | fb 3ff199999999999a |
| +------------------------+---------------------+
| | 1.5, 1.5_1 | f9 3e00 |
| +------------------------+---------------------+
| | 1.5_2 | fa 3fc00000 |
| +------------------------+---------------------+
| | 1.5_3 | fb 3ff8000000000000 |
| +------------------------+---------------------+
| | Infinity, Infinity_1 | f9 7c00 |
| +------------------------+---------------------+
| | Infinity_2 | fa 7f800000 |
| +------------------------+---------------------+
| | Infinity_3 | fb 7ff0000000000000 |
| +------------------------+---------------------+
| | -Infinity, -Infinity_1 | f9 fc00 |
| +------------------------+---------------------+
| | -Infinity_2 | fa ff800000 |
| +------------------------+---------------------+
| | -Infinity_3 | fb fff0000000000000 |
| +------------------------+---------------------+
| | NaN, NaN_1 | f9 7e00 |
| +------------------------+---------------------+
| | NaN_2 | fa 7fc00000 |
| +------------------------+---------------------+
| | NaN_3 | fb 7ff8000000000000 |
| +------------------------+---------------------+
|
| Table 2: Encoding indicators on floating
| point values
See items 3 to 5 in the bullet list at the end of Section 5.1 for
additional details of the CDN number syntax.
(Note that literals for further number formats, e.g., for
representing rational numbers as fractions, or for other NaN values
than the one called NaN, can be added as app-extensions. Background
information beyond that in [STD94] about the representation of
numbers in CBOR can be found in the informational document
[I-D.bormann-cbor-numbers].)
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2.3. Strings
CBOR distinguishes two kinds of strings: text strings (the bytes in
the string constitute UTF-8 [STD63] text, major type 3), and byte
strings (CBOR does not further characterize the bytes that constitute
the string, major type 2).
(UTF-8) text strings can be directly represented (unprefixed) in CDN
either as double-quoted (Section 2.3.1) or as raw strings
(Section 2.3.3), while byte strings can be represented as single-
quoted strings (Section 2.3.2). The latter is useful for byte
strings carrying bytes that can be meaningfully notated as UTF-8
text.
Many strings are best notated as prefixed literals, which may provide
detailed access to the bits within those bytes (see Section 3.1).
Using an app-extension prefix, prefixed literals can be constructed
out of single-quoted strings and raw strings, as well as sequence
literals (cf. Section 3).
| Before prefixed literals were turned into a general extension
| point for diagnostic notation, Appendix G.4 of [RFC8610] added
| a syntax for concatenating strings by just juxtaposing them.
| This syntax was not widely implemented and is problematic in
| the presence of optional commas; it is now entirely removed
| from CDN and replaced by app-extensions such as Section 3.5.
2.3.1. Double-Quoted String Literals
CDN enables notating text strings in a form compatible to that of
notating text strings in JSON (i.e., as a double-quoted string
literal), with a number of usability enhancements. JSON allows no
control characters in text-string literals; if needed, they can be
specified using escapes such as \t or \r. This also applies to CDN,
and all escaping rules apply as in JSON, with a single exception: In
CDN, string literals additionally can contain newlines (LINEFEED
U+000A), which are copied into the resulting string like other
characters in the string literal. To deal with variability in
platform presentation of newlines, any carriage return characters
(U+000D) that may be present in the CDN string literal are not copied
into the resulting string (see Section 1.3.5).
JSON's escape scheme for characters that are not on Unicode's basic
multilingual plane (BMP) is cumbersome (see Section 7 of RFC 8259
[STD90]). CDN keeps it, but also adds the syntax \u{NNN} where NNN
is the Unicode scalar value as a hexadecimal number. This means the
following are equivalent (the first o is escaped as \u{6f} for no
particular reason):
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"D\u{6f}mino's \u{1F073} + \u{2318}" # \u{}-escape 3 chars
"D\u006Fmino's \uD83C\uDC73 + \u2318" # escape JSON-like
"Domino's 🁳 + ⌘" # unescaped
2.3.2. Single-Quoted String Literals
Analogously to text-string literals delimited by double quotes, CDN
allows the use of single quotes (without a prefix) to express byte-
string literals with UTF-8 text; for instance, the following are
equivalent:
'hello world'
h'68656c6c6f20776f726c64'
The escaping rules of JSON strings are applied equivalently for text-
based byte-string literals, e.g., \\ stands for a single backslash
and \' stands for a single quote. However, to facilitate parsing, in
single-quoted strings CDN excludes certain escaping mechanisms
available for double-quoted strings:
* \/ is an escape in JSON that is available for double-quoted CDN
text strings as well to ensure all JSON texts are CDN literals.
Since CDN's single-quoted strings do not occur in JSON, this
legacy compatibility feature is not available for them.
* \u-based escapes are not available for characters in the range
from U+0020 through U+007E (essentially, printable ASCII).
All other escaping mechanisms that are available in double-quoted
string literals are available in single-quoted string literals.
Single-quoted string literals can occur unprefixed and stand for the
byte string that encodes its text string value (the "content"), or be
prefixed by what looks like an app-extension prefix (see Section 3).
In a prefixed string literal, the text content of the single-quoted
string literal is not used directly as a byte string, but is further
processed in a way that is defined by the meaning given to the
prefix. Depending on the prefix, the result of that processing can,
but often is not, a byte string value.
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Prefixed string literals (whether single-quoted after the prefix or a
raw string (Section 2.3.3)) are used for prefixed literals (see
Section 3, such as base-encoded byte string literals (see
Section 3.1). (Additional kinds of base-encoded string literals can
be defined as prefixed literals by registering their prefixes; there
is no fundamental difference between the original two predefined
base-encoded string literal prefixes (Section 3.1: h, b64) and any
such potential future extension literal prefixes; for simplicity of
expression, both cases are referred to as "prefixed literals".)
2.3.3. Raw String Literals
Both double-quoted and single-quoted string literals handle
backslashes in a special way. For string data items that employ
backslashes themselves, possibly with additional layers of processing
giving this "escaping" mechanism specific application semantics, this
can lead to an exponential duplication of backslashes that has
informally been described as "quoting hell".
CDN therefore also allows text strings to be notated as raw string
literals, which do not perform any special processing on backslashes,
i.e., treat them as raw string content like any other characters.
Instead, data transparency is provided by enclosing the entire string
content in starting and ending delimiters built as a sequence of one
or more backquote (»`«, U+0060 GRAVE ACCENT) characters.
For example, the string content »[^ \t\n\r"'`]«, an I-Regexp
character class that excludes blank space and quoting characters, can
be notated as:
``[^ \t\n\r"'`]``
instead of
"[^ \\t\\n\\r\"'`]"
By using more backquotes for each of the outer delimiters than the
longest sequence of backquotes that can be found in the string,
internal backquotes do not prematurely end the string literal. An
example for a raw string that contains a double backquote and
therefore is notated starting and ending with a triple backquote:
```To emulate typographic quotes, sometimes double backward and
forward single quotes are used, as in ``text.''
```
This mechanism is easy to use for the large majority of cases.
However, without additional rules:
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* raw strings could not be used for empty string data items, which
therefore need to be notated using double- or single-quoted
strings. (Obviously, there is no need to escape the content of
empty strings, so this should not be a problem.)
* raw strings could not be used for string data items that start or
end with backquotes, as these would amalgamate with the start and
end delimiters.
To address these cases (predominantly the latter), two additional
rules are added to perform after processing the backquotes used as
delimiters:
* any single newline (LF or CRLF, see Section 1.3.5) at the start of
the inner string is removed to yield the string content. As a
result:
```a```
can also be expressed as
```
a```
In addition to enabling leading backquotes in raw strings, this
can be very useful for documentation strings etc.
This rule also allows notating »``text''« as:
```
``text''```
* if the first rule does not apply, but the inner string starts with
a space character as well as ends with one, exactly one single
space character starting the inner string together with exactly
one single space character ending the inner string are removed to
yield the string content.
This allows notating »a = ``foo``« as:
``` a = ``foo`` ```
If neither of these rules apply, the inner string between the raw
delimiters is used as the raw string unchanged.
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(The examples given here are minimal in that they show how the
additional rules work; more complex examples would be necessary to
provide additional motivation why this is a good way to handle the
various cases.)
2.3.4. CBOR Sequence Literals
In diagnostic notation, a sequence of zero or more CBOR data item
literals can be enclosed in << and >> and separated by comma or blank
space, optionally prefixed by an app-extension prefix; this
specification speaks of _sequence literals_. CDN mainly deals with
individual data items, not with CBOR sequences [RFC8742], so the CBOR
sequence represented by the sequence literal needs to be further
processed to obtain the value of the literal.
Prefixed sequence literals refer to the app-extension (see Section 3)
identified by the prefix and apply the extension to its sequence
content, resulting in a single data item. This data item may be a
string or not (always), depending on the definition of the app-
extension.
An unprefixed sequence literal applies CBOR encoding to the data
items in its content, taken as a CBOR sequence. The value of the
literal thus is a byte string with the encoded content; this is
commonly referred to as _embedded CBOR_. For instance, each pair of
columns in the following are equivalent:
<<1>> h'01'
<<1, 2>> h'0102'
<<"hello", null>> h'65 68656c6c6f f6'
<<>> h''
A diagnostic implementation is expected to honor encoding indicators
(please see Section 4) on the individual items in the supplied
sequence before assembling them into an encoded CBOR sequence. For
instance, each pair of columns in the following are equivalent:
<<1_1>> h'190001'
<<1_0, 2_2>> h'1801 1a00000002'
<<"hello"_0, null>> h'7805 68656c6c6f f6'
For prefixed sequence literals, the processing of arguments that use
specific encoding variants can be defined by the app-extension being
used. See Section 3.6 for an example of where this is done.
Encoding indicators on the arguments are ignored if the app-extension
does not define special handling of encoding variants.
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2.3.5. Validity of Text Strings
To be valid CBOR, Section 5.3.1 of RFC 8949 [STD94] requires that
text strings are byte sequences in UTF-8 [STD63] form. CDN provides
several ways to construct such byte strings (in particular, see also
Section 3.5). These mechanisms might operate on subsequences that do
not themselves constitute UTF-8, e.g., by building larger sequences
out of concatenating the subsequences; for validity of a text string
resulting from these mechanisms it is only of importance that the
result is UTF-8. Double-quoted, single-quoted, and raw string
literals have been defined such that they lead to byte sequences that
are UTF-8: the source language of CDN is UTF-8, and all escaping
mechanisms lead only to adding further UTF-8 characters. Only app-
extensions (invoked in prefixed literals) can generate non-UTF-8 byte
sequences.
As discussed at the start of Section 2, CDN implementations MAY
support generation and possibly ingestion of CDN for CBOR data items
that are well-formed but not valid; when this is enabled, such
implementations MAY relax the requirement on text strings to be valid
UTF-8.
CBOR has no requirements for its text strings except for conformance
to [STD63]. The same applies to CDN and its source language. No
additional Unicode processing or validation such as normalization or
checking whether a scalar value is actually assigned is foreseen by
CDN, particularly not any processing that is dependent on a specific
Unicode version. Such processing, if offered, MUST NOT get in the
way of processing the data item represented in CDN (i.e., it may be
appropriate to issue warnings but not to error out or to generate
output that does not match the input at the UTF-8 level).
2.4. Arrays and Maps
CDN borrows the JSON syntax for arrays and maps. (Maps are called
objects in JSON.)
For maps, CDN extends the JSON syntax by allowing any data item in
the map key position (before the colon).
2.4.1. Mandatory Separators, Optional Terminators
JSON requires the use of a comma as a separator character between the
elements of an array as well as between the members (key/value pairs)
of a map. (These commas also were required in the original
diagnostic notation defined in [STD94] and [RFC8610].) The separator
commas are now optional in the places where CDN syntax allows commas;
however, where no comma is used in a separator position, there must
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be blank space (composed of at least one space, newline, and/or
comment) instead. (Stylistically, leaving out the commas is more
idiomatic when they occur at line breaks, which provide the blank
space.)
In addition, CDN also allows, but does not require, a trailing comma
before the closing bracket/brace, enabling an easier to maintain
"terminator" style of their use.
In summary, the following eight examples are all equivalent:
[1, 2, 3]
[1, 2, 3,]
[1 2 3]
[1 2 3,]
[1 2, 3]
[1 2, 3,]
[1, 2 3]
[1, 2 3,]
as are
{1: "n", "x": "a"}
{1: "n", "x": "a",}
{1: "n" "x": "a"}
# etc.
As a comma and/or blank/comment is mandatory in a separator position,
»[11]« is unambiguously an array with a single element (the integer
11), different from »[1 1]« or »[1,1]«. As this is a general rule,
»[[] []]« or »[[],[]]« are well-formed CDN, while »[[][]]« is not.
| CDDL's comma separators in the equivalent contexts (CDDL
| groups) are entirely optional (and actually are terminators,
| which together with their optionality allows them to be used
| like separators as well, or even not at all). In summary,
| comma use is now aligned between CDN and CDDL, in a fully
| backward compatible way. (CDDL does allow the stylistically
| questionable »a = [[][]]«, though.)
2.4.2. Validity of Maps
As discussed at the start of Section 2, CDN implementations MAY
support generation and possibly ingestion of CDN for CBOR data items
that are well-formed but not valid (Section 5.3 of RFC 8949 [STD94]).
For maps, this is relevant for map keys that occur more than once, as
in this CDN that is not representing a valid CBOR data item:
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{1: "to", 1: "from"}
2.5. Tags
A tag is written as a decimal unsigned integer (no leading zeros
except for the actual tag number zero, i.e., 0|[1-9][0-9]*) for the
tag number, followed by the tag content in parentheses; for instance,
a date in the format specified by RFC 3339 (ISO 8601) could be
notated as:
0("2013-03-21T20:04:00Z")
or the equivalent epoch-based time:
1(1363896240)
The tag number can be followed by an encoding indicator giving the
encoding of the tag head. For example, a diagnostic implementation
encodes:
1_1(1363896240)
...(assuming Preferred Serialization for the tag content) as:
d9 0001 # tag(1)
1a 514b67b0 # unsigned(1363896240)
2.6. Simple values
CDN uses JSON syntax for the simple values True (»true«), False
(»false«), and Null (»null«). Undefined is written »undefined« as in
JavaScript.
These and all other simple values can be given as "simple()" with the
appropriate decimal unsigned integer (0|[1-9][0-9]*) in the
parentheses. For example, »simple(42)« indicates major type 7, value
42, and »simple(20)« indicates »false«.
3. Prefixed Literals
After a short overview of prefixed literals in general, this section
defines a number of app-extensions included with this specification.
Prefixed literals start with a _prefix_ that identifies the app-
extension and possibly a specific variant of that, immediately
followed by a sequence literal (Section 2.3.4) or a single-quoted or
raw string literal (Section 2.3).
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The string-based forms use their string literal as a shorthand form
for a sequence literal representing a sequence with exactly that one
text string data item, e.g., b64`Zm9v` is a shorthand for
b64<<"Zm9v">> or b64<<`Zm9v`>> (this specific example obviously
depends on Zm9v being allowed and meaning the same within the
different forms of string literals used in the example).
| This notation is generalized from Section 8 of RFC 8949
| [STD94], which provides for notating byte strings in a number
| of [RFC4648] base encodings, where the encoded text is enclosed
| in single quotes, prefixed by a prefix (»h« for base16, »b32«
| for base32, »h32« for base32hex, »b64« for base64 or
| base64url).
|
| This syntax can be thought to establish a name space, with the
| names "h", "b32", "h32", and "b64" taken, but other names being
| unallocated. The present specification allows registering
| additional names for this namespace, which it calls _app-
| extension identifiers_.
More precisely, an _app-extension identifier_ is a registered name
consisting of a lowercase ASCII letter ([a-z]) and zero or more
additional ASCII characters that are either lowercase letters,
digits, or hyphens ([a-z0-9-]). »false«, »true«, »null«, and
»undefined« cannot be used as such identifiers and are reserved.
app-extension identifiers are registered in the "App-Extension
Identifiers" registry (Section 6.1).
An app-extension (such as dt) MAY also define the meaning of one
additional prefix derived from its app-extension identifier by
replacing each lowercase character by its uppercase counterpart (such
as DT). As a convention, using the all-uppercase variant implies
making use of a CBOR tag appropriate for this app-extension (such as
tag number 1 for DT, where in contrast the prefix dt stands for the
unwrapped tag content).
In summary, an app-extension identifier gives rise to one or two
prefixes, one that is lexically identical to the identifier (i.e.,
all lowercase), and potentially another one that is an all-uppercase
variation of it. In addition to specifying which of these two
variations exhibits which specific semantics, the app-extension
specifies what input the extension takes.
When the prefix is used immediately in front of a single-quoted or a
raw string, the input takes the form of a single text string CBOR
data item (this is useful only if the app-extension is designed to
receive a text string as input). When used immediately in front of a
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sequence literal, the input is a CBOR sequence of elements of the
sequence literal as input. (For a single parameter, this is
equivalent to receiving a single CBOR data item as the argument.)
The app-extension can provide behavior that depends on the number of
items supplied as input to it and their data types; it cannot
distinguish between its prefix being used with a single-quoted
string, a raw string, or a CBOR sequence composed of a single text
string data item (as illustrated for instance in Tables 3, 4, and 5).
This specification defines a number of generally applicable app-
extensions (Section 3), both to motivate making these extensions
generally available, and to illustrate the concept.
Of these, the app-extensions h, b64, t1, b1, dt and ip are mandatory
to implement. (As mentioned, for simplicity we use the term "app-
extensions" for the mechanism discussed in this section even if it is
used to describe a part of base CDN.)
3.1. Base-Encoded Byte String Literals: h and b64
Besides the unprefixed byte string literals that are analogous to
JSON text string literals, CDN provides prefixed literals that can
represent byte strings by base-encoding them, typically notated as
prefixed string literals. The app-extension identifier selects one
of the base encodings [RFC4648], without padding. Most often, the
base encoding is enclosed in a single-quoted or raw string literal,
prefixed by »h« for base16 or »b64« for base64 or base64url (the
actual encodings of the latter two have the same meaning where they
overlap, so the string remains unambiguous). For example, the byte
string consisting of the four bytes 12 34 56 78 (given in hexadecimal
here) could be written h'12345678' or b64'EjRWeA' when using single-
quoted string literals, or h`12345678` or b64`EjRWeA` when using raw
string literals.
| (Note that Section 8 of RFC 8949 [STD94] also mentions »b32«
| for base32 and »h32« for base32hex. This has not been
| implemented widely and therefore is not directly included in
| this specification. These and further byte string formats now
| can easily be added back as prefixed literals.)
Examples often benefit from some blank space (spaces, line breaks) in
byte string literals. In the base-encoded byte string literals,
blank space is ignored in the input; for instance, the following are
equivalent:
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h'48656c6c6f20776f726c64'
h'48 65 6c 6c 6f 20 77 6f 72 6c 64'
h'4 86 56c 6c6f
20776 f726c64'
The internal syntax of prefixed single-quote literals such as h'' and
b64'' also allow comments as blank space (see Section 2.1).
h'68656c6c6f20776f726c64'
h'68 65 6c /doubled l!/ 6c 6f # hello
20 /space/
77 6f 72 6c 64' /world/
Slash characters are part of the base64 classic alphabet (see Table 1
in Section 4 of [RFC4648]), and they therefore need to be in the
b64'' set of characters that contribute to the byte string.
Therefore, only end-of-line comments starting with # are available
inside b64 byte string literals.
b64'/base64 not a comment/ but one follows # comment'
h'FDB6AC 7BAE27A2D69CA2699E9EDFDBBADA2779FA25 968C2C'
These two byte string literals stand for the same byte string; the
deliberately confusing base64 content starts with b64'/bas' which is
the same as h'FDB6AC' and ends with b64'lows' which is the same as
h'968C2C'.
3.2. Date and Time: dt
The app-extension identifier "dt" is used to notate a date/time
literal that can be used as an Epoch-Based Date/Time as per
Section 3.4.2 of RFC 8949 [STD94].
The content of the literal is a single Standard Date/Time String as
per Section 3.4.1 of RFC 8949 [STD94], as a text or byte string.
The value of the literal is a number representing the result of a
conversion of the given Standard Date/Time String to an Epoch-Based
Date/Time. If fractional seconds are given in the text (production
time-secfrac in Figure 5), the value is a floating-point number; the
value is an integer number otherwise. In the all-uppercase variant
of the app-prefix, the value is enclosed in a tag number 1.
Each row of Table 3 shows an example of "dt" notation and equivalent
notation not using a prefixed literal.
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+================================+==============+
| dt literal | plain CDN |
+================================+==============+
| dt'1969-07-21T02:56:16Z' | -14159024 |
+--------------------------------+--------------+
| dt'1969-07-21T02:56:16.0Z' | -14159024.0 |
+--------------------------------+--------------+
| dt'1969-07-21T02:56:16.5Z' | -14159023.5 |
+--------------------------------+--------------+
| dt`1969-07-21T02:56:16.5Z` | -14159023.5 |
+--------------------------------+--------------+
| dt<<'1969-07-21T02:56:16.5Z'>> | -14159023.5 |
+--------------------------------+--------------+
| dt<<"1969-07-21T02:56:16.5Z">> | -14159023.5 |
+--------------------------------+--------------+
| dt<<`1969-07-21T02:56:16.5Z`>> | -14159023.5 |
+--------------------------------+--------------+
| DT'1969-07-21T02:56:16Z' | 1(-14159024) |
+--------------------------------+--------------+
Table 3: dt and DT literals vs. plain CDN
See Section 5.2.3 for an ABNF definition for the text string input of
dt literals.
3.3. IP Addresses and Related Structures: ip
The app-extension identifier "ip" is used to notate an IP address
literal that can be used as an IP address as per Section 3 of
[RFC9164].
The input of the literal is a single text string representing an
IPv4address or IPv6address as per Section 3.2.2 of [RFC3986].
With the lowercase app-string prefix ip, the value of the literal is
a byte string representing the binary IP address. With the uppercase
app-string prefix IP, the literal is such a byte string tagged with
tag number 54, if an IPv6address is used, or tag number 52, if an
IPv4address is used.
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As an additional case, the uppercase app-string prefix IP can be used
with an IP address prefix such as 2001:db8::/56 or 192.0.2.0/24, with
the equivalent tag as its value. (Note that [RFC9164]
representations of address prefixes need to implement the truncation
of the address byte string as described in Section 4.2 of [RFC9164];
see example below.) For completeness, the lowercase variant
ip'2001:db8::/56' or ip'192.0.2.0/24' stands for an unwrapped
[56,h'20010db8'] or [24,h'c00002']; however, in this case the
information on whether an address is IPv4 or IPv6 often needs to come
from the context.
Note that this app-extension provides no direct representation of the
"Interface format" defined in Section 3.1.3 of [RFC9164], an address
combined with an optional prefix length and an optional zone
identifier, and therefore no way to reference a zone identifier at
all. (If needed, this format can be put together by building their
structures explicitly, e.g., an interface format without a zone
identifier can be represented as in 52([ip'192.0.2.42',24]), or an
interface format with zone identifier 42 as in
54([ip'fe80::0202:02ff:ffff:fe03:0303',64,42]).)
Each row of Table 4 shows an example of "ip" notation and equivalent
notation not using a prefixed literal.
+====================+=========================================+
| ip literal | plain CDN |
+====================+=========================================+
| ip'192.0.2.42' | h'c000022a' |
+--------------------+-----------------------------------------+
| ip<<'192.0.2.42'>> | h'c000022a' |
+--------------------+-----------------------------------------+
| IP'192.0.2.42' | 52(h'c000022a') |
+--------------------+-----------------------------------------+
| IP'192.0.2.0/24' | 52([24,h'c00002']) |
+--------------------+-----------------------------------------+
| ip'2001:db8::42' | h'20010db8000000000000000000000042' |
+--------------------+-----------------------------------------+
| IP'2001:db8::42' | 54(h'20010db8000000000000000000000042') |
+--------------------+-----------------------------------------+
| IP'2001:db8::/64' | 54([64,h'20010db8']) |
+--------------------+-----------------------------------------+
Table 4: ip and IP literals vs. plain CDN
See Section 5.2.4 for an ABNF definition for the content of ip
literals.
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3.4. Cryptographic Hash Values: hash
The app-extension identifier "hash" is used to notate the input to a
cryptographic hash function as well as to identify such a hash
function. Its value is a byte string that represents the output of
that hash function.
The input of the literal is a (text or byte) string, optionally
followed by either an integer or a text string that identifies the
hash function in the COSE Algorithms registry of the CBOR Object
Signing and Encryption (COSE) registry group [IANA.cose], either by
the identifier (value: integer or string), or, if no algorithm is
registered with this value, by its name used in the registry. If the
second item is not given, the default algorithm used is -16 ("SHA-
256").
No uppercase variant prefix is defined for the app-extension
identifier "hash".
+===============+====================================+
| hash literal | plain CDN |
+===============+====================================+
| hash<<'foo'>> | h'2C26B46B68FFC68FF99B453C1D304134 |
| | 13422D706483BFA0F98A5E886266E7AE' |
+---------------+------------------------------------+
| hash'foo' | h'2C26B46B68FFC68FF99B453C1D304134 |
| | 13422D706483BFA0F98A5E886266E7AE' |
+---------------+------------------------------------+
| hash<<'foo', | h'2C26B46B68FFC68FF99B453C1D304134 |
| -16>> | 13422D706483BFA0F98A5E886266E7AE' |
+---------------+------------------------------------+
| hash<<'foo', | h'2C26B46B68FFC68FF99B453C1D304134 |
| "SHA-256">> | 13422D706483BFA0F98A5E886266E7AE' |
+---------------+------------------------------------+
| hash<<'foo', | h'F7FBBA6E0636F890E56FBBF3283E524C |
| -44>> | 6FA3204AE298382D624741D0DC663832 |
| | 6E282C41BE5E4254D8820772C5518A2C |
| | 5A8C0C7F7EDA19594A7EB539453E1ED7' |
+---------------+------------------------------------+
| hash<<'foo', | h'F7FBBA6E0636F890E56FBBF3283E524C |
| "SHA-512">> | 6FA3204AE298382D624741D0DC663832 |
| | 6E282C41BE5E4254D8820772C5518A2C |
| | 5A8C0C7F7EDA19594A7EB539453E1ED7' |
+---------------+------------------------------------+
Table 5: hash literals vs. plain CDN
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3.5. String Concatenation: b1 and t1
// This section uses the placeholders t1 and b1 as provisional app-
// extension identifiers, allowing the text to stabilize while the
// actual names are still being decided by the WG.
The "b1" and "t1" app-extensions allow a (byte or text) string to be
built up from multiple (byte or text) string literals; these are then
concatenated into a single string.
The following four text string values (adapted from Appendix G.4 of
[RFC8610]) are equivalent:
"Hello world"
t1<<"Hello ", "world">>
t1<<"Hello", h'20', "world">>
t1<<h'48656c6c6f20776f726c64'>>
Similarly, the following byte string values are equivalent:
'Hello world'
b1<"Hello world">
b1<<'Hello ', 'world'>>
b1<<'Hello ', h'776f726c64'>>
b1<<'Hello', h'20', 'world'>>
b1<<h'48656c6c6f20776f726c64', '', b64''>>
b1<<h'4 86 56c 6c6f', h' 20776 f726c64'>>
As the examples show, text strings and byte strings can mix within
such a concatenation, so that, for instance, byte string literal
notation can be used inside a sequence of concatenated text string
notation literals, to encode characters that may be better
represented in an encoded way.
This is realized by simply joining together the bytes in the sequence
of string arguments to the b1/t1 app-extension, proceeding from left
to right.
For "b1", the joining operation results in a byte string. For "t1",
the joining operation results in a text string, and the result
therefore needs to be valid UTF-8 except for "diagnostic"
implementations that support and are enabled for generation/ingestion
of CDN for CBOR data items that are well-formed but not valid; see
also Section 2.3.5.
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3.6. Creating Indefinite-length Encoded Strings: ilbs and ilts
The ilbs and ilts app-extensions are semantically identical to t1 and
b1 at the data model level, but instead of concatenating the
arguments to a single (byte/text) string data item, they build an
indefinite length string out of the arguments, with one chunk of the
correct major type (byte string/text string for ilbs/ilts,
respectively) created per argument.
A diagnostic implementation would honor encoding indicators on each
of the arguments, creating a chunk with the same encoding. As the
app-extension is already implying indefinite length encoding, there
is no point in applying an encoding indicator to the entire prefixed
literal.
'Hello world' 4b 48656c6c6f20776f726c64
ilbs<<>> 5f ff
ilbs<<"Hello world">> 5f 4b 48656c6c6f20776f726c64 ff
ilbs<<'Hello ', "world">> 5f 46 48656c6c6f20 45 776f726c64 ff
ilbs<<'Hello '_0, 'world'>> 5f 5806 48656c6c6f20 45 776f726c64 ff
There is no way to include ellipses in an indefinite length string.
3.7. Floating-Point Values: float
The "float" app-extension enables the notation of 2-byte, 4-byte, and
8-byte byte strings to express floating point values (mt=7,
ai=25/26/27 respectively) by giving their IEEE 754 representation. A
text string used as an argument is interpreted exactly as a hex
literal (like the h prefix); the result is used as the byte string.
The prefixed literal is interpreted as an encoded data item would be
that prefixes the byte string by a single byte 0xF9 (2 bytes, i.e.,
binary16), 0xFA (4 bytes, i.e., binary32), and 0xFB (8 bytes, i.e.,
binary64), respectively. Byte strings of a different length than 2,
4, or 8 raise an error. Note that the interpretation as an encoded
data item does not create or imply an encoding indicator; that can be
added separately.
Example (tool used: edn-abnf -afloat -e):
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🔧 "[float'fe00', float'fe00'_2, float'47110815']" -tpretty ➔
83 # array(3)
F9 FE00 # primitive(65024)
FA FFC00000 # primitive(4290772992)
FA 47110815 # primitive(1192298517)
🔧 "[float'fe00', float'fe00'_2, float'47110815', 0x1.22102ap+15]" ➔
[float'fe00', float'fe00'_2, 37128.08203125, 37128.08203125]
The purpose of this app-extension is to close a gap in CDN's
[IEEE754] binary64 support: Without this (or a similar) extension
there is no way to represent NaN values different from the one called
out at the end of Section 4.1 of RFC 8949 [STD94]: "(for many
applications, the single NaN encoding 0xf97e00 will suffice)". For
finite floating point numbers, the decimal or hex floating point
representations are preferred.
4. Encoding Indicators
Sometimes it is useful to indicate in the diagnostic notation which
of several alternative CBOR representations are actually used; for
example, a data item written »1.5« by a diagnostic decoder might have
been encoded in CBOR as a half-, single-, or double-precision float.
Encoding indicators are always optional: CDN is usually used to
describe CBOR data items at the data model level. For some
diagnostic purposes, it is useful to represent the choice of a
serialization variation by including encoding indicators.
Implementations of CDN generally do not need to provide this
functionality in full; if they do, they can be called "diagnostic
implementations". To be able to process CDN that contains encoding
indicators, a CDN-consuming implementation MUST accept them (i.e.,
process or ignore the presence or absence of each encoding
indicator). It is RECOMMENDED to provide a warning for each encoding
indicator value that is encountered but not further processed.
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When creating CDN as input for a diagnostic CBOR encoder in order to
obtain specific encoding choices, encoding indicators may be placed
manually or by the software generating the CDN. Where no encoding
indicator is placed, a diagnostic CBOR encoder is expected to
generate Preferred Serialization (Section 4.1 of RFC 8949 [STD94])
with definite-length encoding only. Similarly, when using CDN as
output for a diagnostic CBOR decoder, a basic diagnostic
configuration of the tool is expected to provide encoding indicators
only in places where the CBOR input did not use Preferred
Serialization with definite-length encoding (see also Section 1.3.3).
Diagnostic implementations of CDN that process encoding indicators as
discussed here are expected to document their diagnostic behavior and
the processing options that can be selected.
4.1. Syntax, Semantics, Examples
Encoding indicators (Table 6) start with an underscore, often
followed by alphanumeric characters. For example, _ or _3.
Encoding indicators indicate the specific CBOR encoding that was
used, or is to be used, to represent a specific CBOR data item.
This specification defines six encoding indicators. Additional
encoding indicators could be defined in future standards track
updates to this specification.
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+====================+===================+===========+
| Encoding Indicator | Description | Reference |
+====================+===================+===========+
| _ | Indefinite-Length | RFC8949, |
| | Encoding (ai=31) | RFC-XXXX |
+--------------------+-------------------+-----------+
| _i | ai=0 to ai=23 | RFC-XXXX |
+--------------------+-------------------+-----------+
| _0 | ai=24 | RFC8949, |
| | | RFC-XXXX |
+--------------------+-------------------+-----------+
| _1 | ai=25 | RFC8949, |
| | | RFC-XXXX |
+--------------------+-------------------+-----------+
| _2 | ai=26 | RFC8949, |
| | | RFC-XXXX |
+--------------------+-------------------+-----------+
| _3 | ai=27 | RFC8949, |
| | | RFC-XXXX |
+--------------------+-------------------+-----------+
Table 6: Encoding Indicators Defined by This
Specification
Encoding indicators can be ignored by anyone not interested in this
information.
Encoding indicators are placed immediately to the right of the data
item or of a syntactic feature that can stand for the data item the
encoding of which the encoding indicator is controlling. Table 7
provides examples for data items with definite length encoding
indicators used with various kinds of data items ("mt" = major type,
"ignoring e.i." = example encoding when ignoring the encoding
indicators). Examples for encoding indicators controlling indefinite
length encoding can be found in the context of explanations in
Section 4.2 and Section 4.3.
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+====+==============+====================+===============+
| mt | examples | encoding (in hex) | ignoring e.i. |
+====+==============+====================+===============+
| 0 | 1_1 | 190001 | 01 |
| | 0x4711_3 | 1b0000000000004711 | 194711 |
+----+--------------+--------------------+---------------+
| 1 | -1_1 | 390000 | 20 |
+----+--------------+--------------------+---------------+
| 2 | 'A'_1 | 59000141 | 4141 |
+----+--------------+--------------------+---------------+
| 3 | "A"_1 | 79000161 | 6161 |
+----+--------------+--------------------+---------------+
| 4 | [_1 "bar"] | 99000163626172 | 8163626172 |
+----+--------------+--------------------+---------------+
| 5 | {_1 "bar": | b900016362617201 | a16362617201 |
| | 1} | | |
+----+--------------+--------------------+---------------+
| 6 | 1_1(4711) | d90001191267 | c1191267 |
+----+--------------+--------------------+---------------+
| 7 | 1.5_2 | fa3fc00000 | f93e00 |
| | 0x4711p+03_3 | fb4101c44000000000 | fa480e2200 |
+----+--------------+--------------------+---------------+
Table 7: Examples of Definite Length Encoding
Indicators for Different Data Items
(In the following, an abbreviation of the form ai=nn gives nn as the
numeric value of the field _additional information_, the low-order 5
bits of the initial byte: see Section 3 of RFC 8949 [STD94]. This
field is used in encoding the "argument", i.e., the value, tag, or
length; ai=0 to ai=23 mean that the value of the ai field immediately
_is_ the argument, ai=24 to ai=27 mean that the argument is carried
in 2^(ai-24) (1, 2, 4, or 8) additional bytes, and ai=31 means that
indefinite-length encoding is used.)
An underscore followed by a decimal digit n indicates that the item
was or is to be encoded with an additional information value of
ai=24+n. (The item associated to the encoding indicator may be the
preceding item, or, for arrays and maps, the item starting with the
preceding bracket or brace.) For an example involving floating point
values (Section 3.3 of RFC 8949 [STD94]), 1.5_1 is a half-precision
floating-point number (2^1 = 2 additional bytes or 16 bits), while
1.5_3 is encoded as double precision (2^3 = 8 additional bytes or 64
bits). For a tool consuming CDN in a diagnostic mode, encountering
an encoding indicator that does not provide enough space to correctly
encode the unchanged data item given is an error; there is no
truncation or rounding that would change the data item encoded.
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| Truncation or rounding semantics imply performing changes at
| the data model level, which is outside the scope of encoding
| indicators. Such operations can be provided by app-extensions.
The encoding indicator _ (an underscore on its own) is used to
indicate indefinite-length encoding. Indefinite-length encoding uses
ai=31, which could have been indicated by _7, which is therefore not
used and marked as reserved (as are _4, _5, and _6, which would stand
for ai=28 to ai=30, values currently not in use in CBOR; these
encoding indicators will be available if and when CBOR is extended to
make use of them).
Note that the encoding indicator _ is only available behind the
opening brace/bracket for map and array (Section 4.2): strings
originally had a now deprecated special syntax streamstring for
indefinite-length encoding except for the special cases ''_ and ""_
(Section 4.3).
The encoding indicators _0 to _3 indicate ai=24 to ai=27,
respectively; they therefore stand for 1, 2, 4, and 8 bytes of
additional information (ai) following the initial byte in the head of
the data item.
Section 8.1 of RFC 8949 [STD94] does not address ai=0 to ai=23 — the
assumption seems to have been that Preferred Serialization
(Section 4.1 of RFC 8949 [STD94]) will be used when converting CBOR
diagnostic notation to an encoded CBOR data item, so leaving out the
encoding indicator for a data item with a Preferred Serialization
will implicitly use ai=0 to ai=23 if that is possible. The present
specification allows making this explicit:
_i ("immediate") stands for encoding with ai=0 to ai=23, i.e., it
indicates that the argument is encoded directly in the initial byte
of the CBOR item.
Specific forms of encoding indicators are discussed in further detail
in Section 4.2 for arrays and maps and in Section 4.3 for the
deprecated syntax for indefinite-length strings.
4.2. Encoding Indicators of Arrays and Maps
A single underscore can be written after the opening brace of a map
or the opening bracket of an array to indicate that the data item was
represented in indefinite-length format. For example, [_ 1, 2]
contains an indicator that an indefinite-length representation was
used to represent the data item [1, 2].
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At the same position, encoding indicators for specifying the size of
the array or map head for definite-length format can be used instead,
specifically _i or _0 to _3. For example, [_0 false, true] can be
used to specify the encoding of the array [false, true] as 98 02 f4
f5.
4.3. Deprecated: Indefinite-length Encoding Indicators for Strings
In CBOR, indefinite-length encoded (byte or text) strings are
composed of "chunks" (Section 3.2.3 of RFC 8949 [STD94]).
The original diagnostic notation (Section 6.1 of [RFC7049]) provided
a special syntax streamstring for them, which was retained and
further clarified in Section 8.1 of RFC 8949 [STD94]. This syntax
represents the individual chunks in sequence within parentheses, each
optionally followed by a comma, with an encoding indicator _
immediately after the opening parenthesis: e.g., (_ h'0123', h'4567')
or (_ "foo", "bar"). The overall type (byte string or text string)
of the string is provided by the types of the individual chunks,
which all need to be of the same type (Section 3.2.3 of RFC 8949
[STD94]).
In this syntax, an indefinite-length string with no chunks inside, (_
) would be ambiguous as to whether a byte string (encoded 5f ff) or a
text string (encoded 7f ff) is meant and is therefore not used. The
basic forms ''_ and ""_ can be used instead and are reserved for the
case of no chunks only — not as short forms for the (permitted, but
not really useful) encodings with only empty chunks, which need to be
notated as (_ ''), (_ ""), etc., when it is desired to preserve the
chunk structure.
With this document, the streamstring syntax is now deprecated; new
CDN documents should instead use the ilbs/ilts app-extensions
(Section 3.6) to build indefinite-length encoded strings.
5. ABNF Definitions
This section collects grammars in ABNF form ([STD68] as extended in
[RFC7405]) that serve to define the syntax of CDN and some prefixed
literals.
| Implementation note: The ABNF definitions in this section are
| intended to be useful in a Parsing Expression Grammar (PEG)
| parser interpretation (see Appendix A of [RFC8610] for an
| introduction into PEG).
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5.1. Overall ABNF Definition for Concise Diagnostic Notation
This subsection provides an overall ABNF definition for the syntax of
concise diagnostic notation.
| This ABNF definition treats all single-quoted string literals
| the same, whether they are unprefixed and constitute byte
| string literals, or prefixed and their content subject to
| further processing. The text string value of the single-quoted
| strings that goes into that further processing is described
| using separate ABNF definitions in Section 5.2; as a
| convention, the grammar for the content of an app-string with
| prefix, say, p, is described by an ABNF definition with the
| rule name app-string-p.
|
| As an implementation note, some implementations may want to
| integrate the parsing and processing of app-string content for
| certain app-extensions with the overall grammar. Example
| grammars for such integrated parsers are provided with this
| specification in Section 5.3.
For simplicity, the internal parsing for the built-in CDN prefixes is
specified in the same way. ABNF definitions for h''/h`` and
b64''/b64`` are provided in Section 5.2.1 and Section 5.2.2.
seq = S [item *(MSC item) SOC]
one-item = S item S
item = map / array / tagged ; spec inside
/ (number / string / prefixed) spec
/ simple ; no spec
/ streamstring ; deprecated (spec inside)
number = hexfloat / hexint / octint / binint
/ decnumber / nonfin
sign = "+" / "-"
decnumber = [sign] (("0" / DIGIT1 *DIGIT) ["." *DIGIT] / "." 1*DIGIT)
["e" [sign] 1*DIGIT]
hexfloat = [sign] "0x" (1*HEXDIG ["." *HEXDIG] / "." 1*HEXDIG)
"p" [sign] 1*DIGIT
hexint = [sign] "0x" 1*HEXDIG
octint = [sign] "0o" 1*ODIGIT
binint = [sign] "0b" 1*BDIGIT
nonfin = %s"Infinity"
/ %s"-Infinity"
/ %s"NaN"
string = dqstr / rawstring / sqstr / embedded
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simple = %s"false"
/ %s"true"
/ %s"null"
/ %s"undefined"
/ %s"simple(" S simple-number S ")"
simple-number = "25" %x30-35 ; 250-255
/ "2" %x30-34 DIGIT ; 200-249
/ "1" 2DIGIT ; 100-199
/ %x34-39 DIGIT ; 40-99
/ "3" %x32-39 ; 32-39
;; there are no simple values between 24-31
/ "2" %x30-33 ; 20-23
/ "1" DIGIT ; 10-19
/ DIGIT ; 0-9
uint = "0" / DIGIT1 *DIGIT
tagged = uint spec "(" S item S ")"
prefix = lcalpha *lcldh
/ ucalpha *ucldh
prefixed = prefix (sqstr / rawstring / embedded)
rawstring = startrawdelim
raw-inner
alikerawdelim
rawdelim = 1*"`"
startrawdelim = rawdelim
; width (number of backquotes) distinguishes
; between following alikerawdelim and shortrawdelim
alikerawdelim = rawdelim ; width == previous startrawdelim
shortrawdelim = rawdelim ; width < previous startrawdelim
rawchars = 1*(%x0a / %x0d / %x20-5f / %x61-7e / NONASCII)
raw-inner = 1*(rawchars / shortrawdelim)
dqstr = DQUOTE *double-quoted DQUOTE
sqstr = SQUOTE *single-quoted SQUOTE
embedded = "<<" seq ">>"
array = "[" (specms S item *(MSC item) SOC / spec S) "]"
map = "{" (specms S keyp *(MSC keyp) SOC / spec S) "}"
keyp = item S ":" S item
; We allow %x09 HT in prose, but not in string literals
blank = %x09 / %x0A / %x0D / %x20
lblank = %x0A / %x20 ; Not HT or CR (gone)
non-slash = blank / %x21-2e / %x30-7F / NONASCII
non-slash-star = blank / %x21-29 / %x2b-2e / %x30-7F / NONASCII
non-star = blank / %x21-29 / %x2b-7F / NONASCII
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ends-in-star = *non-star 1*"*"
non-lf = %x09 / %x0D / %x20-7F / NONASCII
eol-comment = "#" / "//"
comment = "/" non-slash-star *non-slash "/"
/ "/*" ends-in-star
*(non-slash-star ends-in-star) "/"
/ eol-comment *non-lf %x0A
; optional space
S = *blank *(comment *blank)
; mandatory space
MS = (blank/comment) S
; mandatory comma and/or space
MSC = ("," S) / (MS ["," S])
; optional comma and/or space
SOC = S ["," S]
streamstring = "(_" MS string *(MSC string) SOC ")"
spec = ["_" [specchar]]
specms = ["_" [specchar] MS]
specchar = %s"i" / %x30-33 / reserved-ei
reserved-ei = 1*wordchar ; reserved for standards track use
double-quoted = unescaped
/ SQUOTE
/ "\" escapable-d
single-quoted = unescaped
/ DQUOTE
/ "\" escapable-s
escapable1 = %s"b" ; BS backspace U+0008
/ %s"f" ; FF form feed U+000C
/ %s"n" ; LF line feed U+000A
/ %s"r" ; CR carriage return U+000D
/ %s"t" ; HT horizontal tab U+0009
/ "\" ; \ backslash (reverse solidus) U+005C
escapable-d = escapable1
/ DQUOTE
/ "/" ; / slash (solidus) U+002F (JSON!)
/ (%s"u" hexchar) ; uXXXX U+XXXX
escapable-s = escapable1
/ SQUOTE
/ (%s"u" hexchar-s) ; uXXXX U+XXXX
hexchar = "{" (1*"0" [ hexscalar ] / hexscalar) "}"
/ non-surrogate
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/ two-surrogate
non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG)
/ ("D" ODIGIT 2HEXDIG )
two-surrogate = high-surrogate "\" %s"u" low-surrogate
high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG
low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG
hexscalar = "10" 4HEXDIG / HEXDIG1 4HEXDIG
/ non-surrogate / 1*3HEXDIG
; single-quote hexchar-s: don't allow 0020..007e
hexchar-s = "{" (1*"0" [ hexscalar-s ] / hexscalar-s) "}"
/ non-surrogate-s
/ two-surrogate
non-surrogate-s = "007F" ; rubout
/ "00" ("0"/"1"/"8"/"9"/HEXDIGA) HEXDIG
/ "0" HEXDIG1 2HEXDIG
/ non-surrogate-1
non-surrogate-1 = ((DIGIT1 / "A"/"B"/"C" / "E"/"F") 3HEXDIG)
/ ("D" ODIGIT 2HEXDIG )
hexscalar-s = "10" 4HEXDIG / HEXDIG1 4HEXDIG
/ non-surrogate-1 / HEXDIG1 2HEXDIG
/ ("1"/"8"/"9"/HEXDIGA) HEXDIG
/ "7F"
/ HEXDIG1
; Note that no other C0 characters are allowed, including %x09 HT
unescaped = %x0A ; new line
/ %x0D ; carriage return -- ignored on input
/ %x20-21
; omit 0x22 "
/ %x23-26
; omit 0x27 '
/ %x28-5B
; omit 0x5C \
/ %x5D-7F
/ NONASCII
newline = [%x0D] %x0A
DQUOTE = %x22 ; " double quote
SQUOTE = "'" ; ' single quote
DIGIT = %x30-39 ; 0-9
DIGIT1 = %x31-39 ; 1-9
ODIGIT = %x30-37 ; 0-7
BDIGIT = %x30-31 ; 0-1
HEXDIGA = "A" / "B" / "C" / "D" / "E" / "F"
; Note: double-quoted strings as in "A" are case-insensitive in ABNF
HEXDIG = DIGIT / HEXDIGA
HEXDIG1 = DIGIT1 / HEXDIGA
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lcalpha = %x61-7A ; a-z
lcldh = lcalpha / DIGIT / "-"
ucalpha = %x41-5A ; A-Z
ucldh = ucalpha / DIGIT / "-"
ALPHA = lcalpha / ucalpha
wordchar = "_" / ALPHA / DIGIT ; [_a-z0-9A-Z]
NONASCII = %x80-D7FF / %xE000-10FFFF
Figure 1: Overall ABNF Definition of CDN
While an ABNF grammar defines the set of character strings that are
considered to be valid CDN by this ABNF, the mapping of these
character strings into the generic data model of CBOR is not always
obvious.
// Further information can be moved up to Section 2 by splitting it
// up into information specific to the ABNF grammar and general
// information.
The following additional items should help in the interpretation:
1. As mentioned in the terminology (Section 1.2), the ABNF terminal
values in this document define Unicode scalar values (characters)
rather than their UTF-8 encoding. For example, the Unicode PLACE
OF INTEREST SIGN (U+2318) would be defined in ABNF as %x2318.
2. See Section 1.3.5 for more considerations about the character
repertoire used for CDN source text and, in particular, the
special handling of newline characters in the source.
3. decnumber stands for an integer in the usual decimal notation,
unless at least one of the optional parts starting with "." and
"e" are present, in which case it stands for a floating point
value in the usual decimal notation. Note that the grammar
allows 3. for 3.0 and .3 for 0.3 (also for hexadecimal floating
point below); implementers are advised that some platform numeric
parsers accept only a subset of the floating point syntax in this
document and may require some preprocessing to use here.
4. hexint, octint, and binint stand for an integer in the usual base
16/hexadecimal ("0x"), base 8/octal ("0o"), or base 2/binary
("0b") notation. hexfloat stands for a floating point number in
the usual hexadecimal notation (which uses a mantissa in
hexadecimal and an exponent in decimal notation, see
Section 5.12.3 of [IEEE754], Section 6.4.4.3 of [C], or
Section 5.13.4 of [Cplusplus]; floating-suffix/floating-point-
suffix from the latter two is not used here).
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5. For hexint, octint, binint, and when decnumber stands for an
integer, the corresponding CBOR data item is represented using
major type 0 or 1 if possible, or using tag 2 or 3 if not. In
the latter case, this specification does not define any encoding
indicators that apply. If fine control over encoding is desired,
this can be expressed by being explicit about the representation
as a tag: E.g., 987654321098765432310, which is equivalent to
2(h'35 8a 75 04 38 f3 80 f5 f6') in its Preferred Serialization,
might be written as 2_3(h'00 00 00 35 8a 75 04 38 f3 80 f5 f6'_1)
if leading zeros need to be added during serialization to obtain
specific sizes for tag head, byte string head, and the overall
byte string.
When decnumber stands for a floating point value, and for
hexfloat and nonfin, a floating point data item with major type 7
is used; diagnostic implementations employ Preferred
Serialization unless the item was modified by an encoding
indicator, which then needs to be _1, _2, or _3. For this, the
number range needs to fit into an [IEEE754] binary64 (or the size
corresponding to the encoding indicator), and the precision will
be adjusted to binary64 before further applying Preferred
Serialization (or to the size corresponding to the encoding
indicator). Tag 4/5 representations are not generated in these
cases. Future app-prefixes could be defined to allow more
control for obtaining a tag 4/5 representation directly from a
hex or decimal floating point literal.
6. spec stands for an encoding indicator. See Section 4 for
details.
7. The ABNF grammar for raw strings is lenient; a parser needs to
implement the ABNF comments on alikerawdelim and shortrawdelim as
well. shortrawdelim only matches sequences of backquotes that
are shorter than startrawdelim. alikerawdelim only matches
sequences of backquotes that are exactly as long as
startrawdelim.
| In a PEG parser that implements predicates, these matching
| rules can for instance be implemented as follows:
|
| startrawdelim = rawdelim&{|(rd)|@rdlen = rd.text_value.length}
| shortrawdelim = rawdelim&{|(rd)|rd.text_value.length < @rdlen}
| alikerawdelim = rawdelim&{|(rd)|rd.text_value.length == @rdlen}
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5.2. ABNF Definitions for App-Extension Content
This subsection provides ABNF definitions for the content of prefixed
literals defined in [STD94] and in this specification, where
applicable. These grammars describe the _decoded_ content of the
single-quoted or raw string components that combine with prefixes to
form prefixed literals. Each of these may integrate ABNF rules
defined in Figure 1, which are not always repeated here.
Table 8 summarizes the app-prefix values defined in this document.
+============+===========================+=================+
| app-prefix | content of single-quoted | result type |
| | or raw string | |
+============+===========================+=================+
| h | hexadecimal form of | byte string |
| | binary data | |
+------------+---------------------------+-----------------+
| H | (not used) | |
+------------+---------------------------+-----------------+
| b64 | base64 forms (classic or | byte string |
| | base64url) of binary data | |
+------------+---------------------------+-----------------+
| B64 | (not used) | |
+------------+---------------------------+-----------------+
| dt | RFC 3339 date/time | number (int or |
| | | float) |
+------------+---------------------------+-----------------+
| DT | " | Tag 1 on the |
| | | above |
+------------+---------------------------+-----------------+
| ip | IP address or prefix | byte string, |
| | | array of length |
| | | and byte string |
+------------+---------------------------+-----------------+
| IP | " | Tag 54 (IPv6) |
| | | or 52 (IPv4) on |
| | | the above |
+------------+---------------------------+-----------------+
| hash | string (usually used with | byte string |
| | sequences) | |
+------------+---------------------------+-----------------+
| HASH | (not used) | |
+------------+---------------------------+-----------------+
| t1 | strings (usually used | text string |
| | with sequences) | |
+------------+---------------------------+-----------------+
| T1 | (not used) | |
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+------------+---------------------------+-----------------+
| b1 | strings (usually used | byte string |
| | with sequences) | |
+------------+---------------------------+-----------------+
| B1 | (not used) | |
+------------+---------------------------+-----------------+
| float | floating point value from | floating point |
| | input bytes | value (mt=7) |
+------------+---------------------------+-----------------+
| FLOAT | (not used) | |
+------------+---------------------------+-----------------+
Table 8: App-prefix Values Defined in this Document
Note that implementation platforms may already provide
implementations of grammars used in app-extensions, such as of RFC
3339 for dt'' and of IP address syntax for ip''. CDN-based tools may
want to use these implementation libraries instead of using the
grammars that are provided here as a reference.
For convenience, the common definitions in Figure 2 are not repeated
in the below ABNF grammars.
ALPHA = %x41-5a / %x61-7a
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / HEXDIGA
HEXDIGA = "A" / "B" / "C" / "D" / "E" / "F"
; Note: double-quoted strings as in "A" are case-insensitive in ABNF
lblank = %x0A / %x20 ; Not HT or CR (gone)
non-lf = %x20-7f / NONASCII
NONASCII = %x80-D7FF / %xE000-10FFFF
Figure 2: Common Rules Used in app-extension ABNF grammars
5.2.1. h: ABNF Definition of Hexadecimal representation of a byte
string
The syntax of the content of byte strings represented in hex, such as
h'', h'0815', or h'/head/ 63 /contents/ 66 6f 6f' (another
representation of << "foo" >>), is described by the ABNF in Figure 3.
This syntax accommodates both lowercase and uppercase hex digits, as
well as blank space (including comments) around each hex digit.
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app-string-h = S *(HEXDIG S HEXDIG S / ellipsis S)
[eol-comment *non-lf]
ellipsis = 3*"."
non-slash = lblank / %x21-2e / %x30-7f / NONASCII
non-slash-star = lblank / %x21-29 / %x2b-2e / %x30-7f / NONASCII
non-star = lblank / %x21-29 / %x2b-7f / NONASCII
ends-in-star = *non-star 1*"*"
non-lf = %x20-7f / NONASCII
eol-comment = "#" / "//"
S = *lblank *(comment *lblank)
comment = "/" non-slash-star *non-slash "/"
/ "/*" ends-in-star
*(non-slash-star ends-in-star) "/"
/ eol-comment *non-lf %x0A
Figure 3: ABNF Definition of Hexadecimal Representation of a Byte
String
| The comment syntax provided inside the hex string is intended
| to mimic the overall syntax for comments in CDN (Section 2.1).
| Implementation note: Comments and blank space are also
| described by the following search regexp, which can be used to
| remove them. For display, the regexp is split along the outer
| alternative into four lines, which need to be combined before
| use; \z stands for the end of the string and is notated $ in
| some regexp dialects.
|
| \s|
| /\*(?:[^*]*\*+)(?:[^/*][^*]*\*+)*/|
| /[^/*][^/]*/|
| (?:#|//)[^\n]*(?:\n|\z)
5.2.2. b64: ABNF Definition of Base64 representation of a byte string
The syntax of the content of byte strings represented in base64 is
described by the ABNF in Figure 4.
This syntax allows both the classic (Section 4 of [RFC4648]) and the
URL-safe (Section 5 of [RFC4648]) alphabet to be used. It
accommodates, but does not require base64 padding. Note that
inclusion of classic base64 makes it impossible to have comments
based on slash characters in b64, as "/" is valid base64-classic.
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app-string-b64 = B *(4(b64dig B))
[b64dig B b64dig B ["=" B "=" / b64dig B ["="]] B]
["#" *non-lf]
b64dig = ALPHA / DIGIT / "-" / "_" / "+" / "/"
B = *lblank *(comment *lblank)
comment = "#" *non-lf %x0A
Figure 4: ABNF definition of Base64 Representation of a Byte String
5.2.3. dt: ABNF Definition of RFC 3339 Representation of a Date/Time
The syntax of the content of dt literals can be described by the ABNF
for date-time in Figure 5. This is derived from [RFC3339] as
summarized in Section 3 of [RFC9165].
app-string-dt = date-time
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec
; rules
time-secfrac = "." 1*DIGIT
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-offset = "Z" / time-numoffset
partial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offset
date-time = full-date "T" full-time
Figure 5: ABNF Definition of RFC3339 Representation of a Date/Time
5.2.4. ip: ABNF Definition of Textual Representation of an IP Address
The syntax of the content of ip literals can be described by the ABNF
for IPv4address and IPv6address in Section 3.2.2 of [RFC3986], as
included in slightly updated form in Figure 6.
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app-string-ip = IPaddress ["/" uint]
IPaddress = IPv4address
/ IPv6address
; ABNF from RFC 3986, re-arranged for PEG compatibility:
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ h16 *1( ":" h16 ) ] "::" 3( h16 ":" ) ls32
/ [ h16 *2( ":" h16 ) ] "::" 2( h16 ":" ) ls32
/ [ h16 *3( ":" h16 ) ] "::" h16 ":" ls32
/ [ h16 *4( ":" h16 ) ] "::" ls32
/ [ h16 *5( ":" h16 ) ] "::" h16
/ [ h16 *6( ":" h16 ) ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = "25" %x30-35 ; 250-255
/ "2" %x30-34 DIGIT ; 200-249
/ "1" 2DIGIT ; 100-199
/ %x31-39 DIGIT ; 10-99
/ DIGIT ; 0-9
DIGIT1 = %x31-39 ; 1-9
uint = "0" / DIGIT1 *DIGIT
Figure 6: ABNF Definition of Textual Representation of an IP Address
5.3. ABNF Definitions for Integrated Extension Parsers
For some applications of CDN, it is an optimization to integrate
parsers for the content of some prefixed string literals into the
main parser, handling both the string literal syntax (e.g., escapes
such as \' and \\) and the syntax of the extension content in one go.
For app-extensions that only use printable ASCII characters (from
U+0020 to U+007E) minus single-quote ' and backslash \, the ABNF such
as that given in Section 5.2 can be directly used as an integrated
parser, after adding some glue ABNF. For instance, for app-string-
dt, add an alternative to prefixed that points to a rule for prefixed
single-quoted string literals (Figure 7).
prefixed = sq-app-string-dt /
prefix (sqstr / rawstring / embedded)
sq-app-string-dt = (%s"dt'"/%s"DT'") app-string-dt "'"
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Figure 7: Glue ABNF for Integrated DT Parser
To facilitate writing integrated ABNF for more complex prefixed
string literals, the ABNF definitions in Figure 8 may be useful and
are used in the rest of this section.
i-HT = %s"\t" / %s"\u" ("0009" / "{" *("0") "9}")
i-LF = %x0a / %s"\n" / %s"\u" ("000A" / "{" *("0") "A}")
i-CR = %x0d / %s"\r" / %s"\u" ("000D" / "{" *("0") "D}")
i-blank = i-LF / i-CR / " "
i-non-lf = i-HT / i-CR / %x20-26 / "\'" / %x28-5b
/ "\\" / %x5d-7f / i-NONASCII
i-NONASCII = NONASCII / %s"\u" ESCGE7F
; hex escaping for U+007F or greater
ESCGE7F = "D" ("8"/"9"/"A"/"B") 2HEXDIG
%s"\u" "D" ("C"/"D"/"E"/"F") 2HEXDIG
/ FOURHEX1 / "0" HEXDIG1 2HEXDIG / "00" TWOHEX1
/ "{" *("0")
("10" 4HEXDIG / HEXDIG1 4HEXDIG
/ FOURHEX1 / HEXDIG1 2HEXDIG / TWOHEX1)
"}"
; xxxx - 0xxx - Dhigh\uDloow
FOURHEX1 = (DIGIT1 / "A"/"B"/"C" / "E"/"F") 3HEXDIG
/ "D" ODIGIT 2HEXDIG
; 00xx - ASCII + 007F
TWOHEX1 = ("8"/"9" / HEXDIGA) HEXDIG / "7F"
Figure 8: ABNF Definitions Useful for Integrated Extension Parsers
Similarly, for integrated parsers for prefixed literals built from
raw strings, the ABNF definitions in Figure 9 can be useful.
alikerawdelim only matches sequences of backquotes that are exactly
as long as a previous startrawdelim.
r-non-lf = %x0D / %x20-5f / %x61-7f / NONASCII / shortrawdelim
Figure 9: ABNF Definitions Useful for Raw String Integrated
Extension Parsers
Four subsections with ABNF for integrated parsers follow, a pair for
h'' and b64'', and a pair for h`` and b64``. There is no expectation
for a new app-extension to supply ABNF for an integrated parser (or
any ABNF at all!), in particular if the parsing function is likely to
be fulfilled by a platform library. If ABNF for the content of a
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single-quoted string is available in an app-extension specification,
ABNF for an integrated parser can be written as a separate activity
or also automatically derived (see also [CDN-WIKI], where more
information about implementing integrated parsers is being
collected).
5.3.1. h'': ABNF Definition of Integrated Parser
With glue ABNF similar to that in Figure 7 and common definitions in
Figures 2 and 8, ABNF such as that shown in Figure 10 can be used as
an integrated parser for h prefixed single-quote strings.
sq-app-string-h = %s"h'" s-app-string-h "'"
s-app-string-h = h-S *(HEXDIG h-S HEXDIG h-S / ellipsis h-S)
[eol-comment *i-non-lf]
h-S = *(i-blank) *(h-comment *(i-blank))
h-non-slash = i-blank / %x21-26 / "\'" / %x28-2e
/ %x30-5b / "\\" / %x5d-7f / i-NONASCII
h-non-slash-star = i-blank / %x21-26 / "\'" / %x28-29 / %x2b-2e
/ %x30-5b / "\\" / %x5d-7f / i-NONASCII
h-non-star = i-blank / %x21-26 / "\'" / %x28-29 / %x2b-5b
/ "\\" / %x5d-7f / i-NONASCII
h-ends-in-star = *h-non-star 1*"*"
h-comment = "/" h-non-slash-star *h-non-slash "/"
/ "/*" h-ends-in-star
*(h-non-slash-star h-ends-in-star) "/"
/ eol-comment *i-non-lf i-LF
Figure 10: ABNF Definition for Integrated Hex Parser
5.3.2. b64'': ABNF Definition of Integrated Parser
With glue ABNF similar to that in Figure 7 and common definitions in
Figures 2 and 8, ABNF such as that shown in Figure 11 can be used as
an integrated parser for b64 prefixed single-quote strings.
sq-app-string-b64 = %s"b64'" s-app-string-b64 "'"
s-app-string-b64 = b64-S *(4(b64dig b64-S))
[b64dig b64-S b64dig b64-S
["=" b64-S "=" / b64dig b64-S ["="]] b64-S]
["#" *i-non-lf]
b64dig = ALPHA / DIGIT / "-" / "_" / "+" / "/"
b64-S = *i-blank *(b64-comment *i-blank)
b64-comment = "#" *i-non-lf %x0A
Figure 11: ABNF Definition for Integrated Base64 Parser
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5.3.3. h``: ABNF Definition of Integrated Parser
With glue ABNF similar to that in Figure 7 and common definitions in
Figures 2, 8 and 9, ABNF such as that shown in Figure 12 can be used
as an integrated parser for h prefixed raw strings.
raw-app-string-h = %s"h" startrawdelim r-app-string-h
r-app-string-h = rh-S *(HEXDIG rh-S HEXDIG rh-S / ellipsis rh-S)
(eol-comment *r-non-lf alikerawdelim / alikerawdelim)
rh-S = *(lblank) *(rh-comment *(lblank))
rh-2 = %x61-7f / NONASCII / shortrawdelim
rh-non-slash = lblank / %x21-2e / %x30-5f / rh-2
rh-non-slash-star = lblank / %x21-29 / %x2b-2e / %x30-5f / rh-2
rh-non-star = lblank / %x21-29 / %x2b-5f / rh-2
rh-ends-in-star = *rh-non-star 1*"*"
rh-comment = "/" rh-non-slash-star *rh-non-slash "/"
/ "/*" rh-ends-in-star
*(rh-non-slash-star rh-ends-in-star) "/"
/ eol-comment *r-non-lf %x0A
Figure 12: ABNF Definition for Integrated Raw String Hex Parser
5.3.4. b64``: ABNF Definition of Integrated Parser
With glue ABNF similar to that in Figure 7, common definitions in
Figures 2, 8 and 9 as well as the rule b64dig from Figure 11, ABNF
such as that shown in Figure 13 can be used as an integrated parser
for b64 prefixed raw strings.
raw-app-string-b64 = %s"b64" startrawdelim r-app-string-b64
r-app-string-b64 = rb64-S *(4(b64dig rb64-S))
[b64dig rb64-S b64dig rb64-S
["=" rb64-S "=" / b64dig rb64-S ["="]] rb64-S]
("#" *r-non-lf alikerawdelim / alikerawdelim)
rb64-S = *lblank *(rb64-comment *lblank)
rb64-comment = "#" *r-non-lf %x0A
Figure 13: ABNF Definition for Integrated Raw String Base64 Parser
6. IANA Considerations
// RFC Editor: please replace RFC-XXXX with the RFC number of this
// RFC, [IANA.concise-diagnostic-notation] with a reference to the
// new registry group, and remove this note.
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6.1. Concise Diagnostic Notation App-extension Identifiers Registry
IANA is requested to create an "App-Extension Identifiers" registry
in a new "Concise Diagnostic Notation" registry group [IANA.concise-
diagnostic-notation], with the policy "expert review" (Section 4.5 of
RFC 8126 [BCP26]).
The experts are instructed to be frugal in the allocation of app-
extension identifiers that are suggestive of generally applicable
semantics, keeping them in reserve for app-extensions that are likely
to enjoy wide use and can make good use of their conciseness. The
experts are also instructed to direct the registrant to provide a
specification (Section 4.6 of RFC 8126 [BCP26]), but can make
exceptions, for instance when a specification is not available at the
time of registration but is likely forthcoming. If the experts
become aware of app-extension identifiers that are deployed and in
use, they may also initiate a registration on their own if they deem
such a registration can avert potential future collisions.
Each entry in the registry must include:
App-Extension Identifier:
a lowercase ASCII [STD80] string that starts with a letter and can
contain letters, digits, and hyphens after that ([a-z][a-z0-9-]*).
No other entry in the registry can have the same app-extension
identifier.
Description:
a brief description
Change Controller:
(see Section 2.3 of RFC 8126 [BCP26])
Reference:
a reference document that provides a description of the app-
extension identifier
The initial content of the registry is shown in Table 9; all initial
entries have the Change Controller "IETF".
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+==========================+===========================+===========+
| app-extension identifier | Description | Reference |
+==========================+===========================+===========+
| h | Reserved | RFC8949 |
+--------------------------+---------------------------+-----------+
| b32 | Reserved | RFC8949 |
+--------------------------+---------------------------+-----------+
| h32 | Reserved | RFC8949 |
+--------------------------+---------------------------+-----------+
| b64 | Reserved | RFC8949 |
+--------------------------+---------------------------+-----------+
| false | Reserved | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| true | Reserved | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| null | Reserved | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| undefined | Reserved | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| pragma | Reserved for future use | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| dt | Date/Time | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| ip | IP Address/Prefix | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| hash | Cryptographic Hash | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| b1 | Byte String Concatenation | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| t1 | Text String Concatenation | RFC-XXXX |
+--------------------------+---------------------------+-----------+
| float | Floating-Point Value | RFC-XXXX |
+--------------------------+---------------------------+-----------+
Table 9: Initial Content of app-extension Identifier Registry
6.2. Media Type
IANA is requested to add the following Media-Type to the "Media
Types" registry [IANA.media-types].
+======+=================+=======================+
| Name | Template | Reference |
+======+=================+=======================+
| cdn | application/cdn | RFC-XXXX, Section 6.2 |
+------+-----------------+-----------------------+
Table 10: New Media Type application/cdn
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Type name: application
Subtype name: cdn
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary (UTF-8)
Security considerations: Section 7 of RFC XXXX
Interoperability considerations: none
Published specification: Section 6.2 of RFC XXXX
Applications that use this media type: Tools interchanging a human-
readable form of CBOR
Fragment identifier considerations: The syntax and semantics of
fragment identifiers is as specified for "application/cbor". (At
publication of RFC XXXX, there is no fragment identification
syntax defined for "application/cbor".)
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): .cdn
Macintosh file type code(s): N/A
Person & email address to contact for further information: CBOR WG
mailing list (cbor@ietf.org), or IETF Applications and Real-Time
Area (art@ietf.org)
Intended usage: LIMITED USE
Restrictions on usage: Concise diagnostic notation represents CBOR
data items, which are the format intended for actual interchange.
The media type application/cdn is intended to be used within
documents about CBOR data items, in diagnostics for human
consumption, and in other representations of CBOR data items that
are necessarily text-based such as in configuration files or other
data edited by humans, often under source-code control.
Author/Change controller: IETF
Provisional registration: no
6.3. Content-Format
IANA is requested to register a Content-Format number in the "CoAP
Content-Formats" sub-registry, within the "Constrained RESTful
Environments (CoRE) Parameters" Registry [IANA.core-parameters], as
follows:
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+=================+================+======+===========+
| Content-Type | Content Coding | ID | Reference |
+=================+================+======+===========+
| application/cdn | - | TBD1 | RFC-XXXX |
+-----------------+----------------+------+-----------+
Table 11: New Content-Format for application/cdn
TBD1 is to be assigned from the space 256..9999, according to the
procedure "IETF Review or IESG Approval", preferably a number less
than 1000.
7. Security considerations
The security considerations of [STD94] apply, including by applying
the considerations about the CBOR format to the CDN format in an
analogous sense. Security considerations documented in [RFC8610] for
the CDDL language often are also applicable to the CDN language in an
analogous sense.
The CDN specification defines an explicit extension point: app-
extension identifiers (Section 6.1). Extensions introduced through
these can have their own security considerations, which need to be
considered in the specification for the extension (see, e.g.,
Section 5 of [I-D.ietf-cbor-edn-e-ref]).
Implementers of tools that support the use of CDN extensions need to
avoid inadvertently introducing a vector that allows attackers to
invoke extensions not planned for by the tool operator, who might not
have considered security considerations of specific extensions such
as those posed by their use of dereferenceable identifiers (Section 6
of [I-D.bormann-t2trg-deref-id]).
Tools might require explicitly enabling the use of each extension
that is not on an allowlist. (This task can possibly be made less
onerous by combining it with a mechanism for supplying any parameters
that control such an extension.)
Tools that process app-extensions need to be configured out of band
to enable processing each specific app-extension only if that is
desired. An allowlist built out of the mandatory-to-implement
application extensions may be an exception.
8. References
8.1. Normative References
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[BCP14] Best Current Practice 14,
<https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:
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>.
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>.
[BCP26] Best Current Practice 26,
<https://www.rfc-editor.org/info/bcp26>.
At the time of writing, this BCP comprises the following:
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>.
[C] International Organization for Standardization,
"Information technology — Programming languages — C",
Edition 5, ISO/IEC 9899:2024, October 2024,
<https://www.iso.org/standard/82075.html>. The standard
is widely known as C23. Its technical content is also
available via
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf
(https://www.open-std.org/jtc1/sc22/wg14/www/docs/
n3220.pdf).
[Cplusplus]
International Organization for Standardization,
"Programming languages — C++", Edition 7, ISO/
IEC 14882:2024, October 2024,
<https://www.iso.org/standard/83626.html>. The standard
is widely known as C++23. Its technical content is also
available via https://open-
std.org/jtc1/sc22/wg21/docs/papers/2023/n4950.pdf
(https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/
n4950.pdf).
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<https://www.iana.org/assignments/core-parameters>.
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[IANA.cose]
IANA, "CBOR Object Signing and Encryption (COSE)",
<https://www.iana.org/assignments/cose>.
[IANA.media-types]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
<https://ieeexplore.ieee.org/document/8766229>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/rfc/rfc8742>.
[RFC9164] Richardson, M. and C. Bormann, "Concise Binary Object
Representation (CBOR) Tags for IPv4 and IPv6 Addresses and
Prefixes", RFC 9164, DOI 10.17487/RFC9164, December 2021,
<https://www.rfc-editor.org/rfc/rfc9164>.
[RFC9485] Bormann, C. and T. Bray, "I-Regexp: An Interoperable
Regular Expression Format", RFC 9485,
DOI 10.17487/RFC9485, October 2023,
<https://www.rfc-editor.org/rfc/rfc9485>.
[STD63] Internet Standard 63,
<https://www.rfc-editor.org/info/std63>.
At the time of writing, this STD comprises the following:
Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
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[STD68] Internet Standard 68,
<https://www.rfc-editor.org/info/std68>.
At the time of writing, this STD comprises the following:
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>.
[STD80] Internet Standard 80,
<https://www.rfc-editor.org/info/std80>.
At the time of writing, this STD comprises the following:
Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC20, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[STD94] Internet Standard 94,
<https://www.rfc-editor.org/info/std94>.
At the time of writing, this STD comprises the following:
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
8.2. Informative References
[CDN-WIKI] "CDN Wiki", n.d., <https://github.com/cbor-wg/edn/wiki>.
[I-D.bormann-cbor-numbers]
Bormann, C., "On Numbers in CBOR", Work in Progress,
Internet-Draft, draft-bormann-cbor-numbers-03, 1 March
2026, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-numbers-03>.
[I-D.bormann-t2trg-deref-id]
Bormann, C. and C. Amsüss, "The "dereferenceable
identifier" pattern", Work in Progress, Internet-Draft,
draft-bormann-t2trg-deref-id-07, 24 February 2026,
<https://datatracker.ietf.org/doc/html/draft-bormann-
t2trg-deref-id-07>.
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[I-D.ietf-cbor-edn-e-ref]
Bormann, C., "External References to Values in CBOR
Diagnostic Notation (EDN)", Work in Progress, Internet-
Draft, draft-ietf-cbor-edn-e-ref-03, 1 March 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-e-ref-03>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/rfc/rfc7049>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/rfc/rfc7493>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC9165] Bormann, C., "Additional Control Operators for the Concise
Data Definition Language (CDDL)", RFC 9165,
DOI 10.17487/RFC9165, December 2021,
<https://www.rfc-editor.org/rfc/rfc9165>.
[RFC9290] Fossati, T. and C. Bormann, "Concise Problem Details for
Constrained Application Protocol (CoAP) APIs", RFC 9290,
DOI 10.17487/RFC9290, October 2022,
<https://www.rfc-editor.org/rfc/rfc9290>.
[RFC9512] Polli, R., Wilde, E., and E. Aro, "YAML Media Type",
RFC 9512, DOI 10.17487/RFC9512, February 2024,
<https://www.rfc-editor.org/rfc/rfc9512>.
[RFC9682] Bormann, C., "Updates to the Concise Data Definition
Language (CDDL) Grammar", RFC 9682, DOI 10.17487/RFC9682,
November 2024, <https://www.rfc-editor.org/rfc/rfc9682>.
[RFC9741] Bormann, C., "Concise Data Definition Language (CDDL):
Additional Control Operators for the Conversion and
Processing of Text", RFC 9741, DOI 10.17487/RFC9741, March
2025, <https://www.rfc-editor.org/rfc/rfc9741>.
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[STD90] Internet Standard 90,
<https://www.rfc-editor.org/info/std90>.
At the time of writing, this STD comprises the following:
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[XSD2] Malhotra, A., Ed., Peterson, D., Ed., Thompson, H., Ed.,
Sperberg-McQueen, M., Ed., Biron, P. V., Ed., and S. Gao,
Ed., "W3C XML Schema Definition Language (XSD) 1.1 Part 2:
Datatypes", W3C REC REC-xmlschema11-2-20120405, W3C REC-
xmlschema11-2-20120405, 5 April 2012,
<https://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/>.
[YAML] Ben-Kiki, O., Evans, C., and I. döt Net, "YAML Ain't
Markup Language (YAML™) Version 1.2", Revision 1.2.2, 1
October 2021, <https://yaml.org/spec/1.2.2/>.
Appendix A. CDN and CDDL
This appendix is for information.
CDN was designed as a language to provide a human-readable
representation of an instance, i.e., a single CBOR data item or CBOR
sequence. CDDL was designed as a language to describe an (often
large) set of such instances (which itself constitutes a language),
in the form of a _data definition_ or _grammar_ (or sometimes called
_schema_).
The two languages share some similarities, not the least because they
have mutually inspired each other. But they have very different
roots:
* CDN syntax is an extension to JSON syntax [STD90].
(Any (interoperable) JSON text is also valid CDN.)
* CDDL syntax is inspired by ABNF's syntax [STD68].
For engineers that are using both CDN and CDDL, it is easy to write
"CDDLisms" or "CDNisms" into their drafts that are meant to be in the
other language. (This is one more of the many motivations to always
validate formal language instances with tools.)
Important differences include:
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* Comment syntax. CDDL inherits ABNF's semicolon-delimited end of
line characters, while CDN finds nothing in JSON that could be
inherited here. Inspired by JavaScript, CDN simplifies
JavaScript's copy of the original C comment syntax to be delimited
by single slashes (where line breaks are not of interest); it also
adds traditional C-style inline comments (/* ... */) and end-of-
line comments that start with # or //.
CDN:
{ / alg / 1: -7 / ECDSA 256 / }
{ 1: # alg
-7 # ECDSA 256
}
CDDL: ? 1 => int / tstr, ; algorithm identifier
* Syntax for tags. CDDL's tag syntax is part of the system for
referring to CBOR's fundamentals (the major type 6, in this case)
and (with [RFC9682]) allows specifying the actual tag number
separately, while CDN's tag syntax is a simple decimal number and
a pair of parentheses.
CDN:
98([h'', # empty encoded protected header
{}, # empty unprotected header
])
CDDL: COSE_Sign_Tagged = #6.98(COSE_Sign)
* Embedded CBOR. CDN has a special syntax to describe the content
of byte strings that are encoded CBOR data items. CDDL can
specify these with a control operator, which looks very different.
CDN:
98([<< {/alg/ 1: -7 /ECDSA 256/} >>, # == h'a10126'
])
CDDL: serialized_map = bytes .cbor header_map
List of Figures
Figure 1: Overall ABNF Definition of CDN
Figure 2: Common Rules Used in app-extension ABNF grammars
Figure 3: ABNF Definition of Hexadecimal Representation of a Byte
String
Figure 4: ABNF definition of Base64 Representation of a Byte String
Figure 5: ABNF Definition of RFC3339 Representation of a Date/Time
Figure 6: ABNF Definition of Textual Representation of an IP Address
Figure 7: Glue ABNF for Integrated DT Parser
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Figure 8: ABNF Definitions Useful for Integrated Extension Parsers
Figure 9: ABNF Definitions Useful for Raw String Integrated
Extension Parsers
Figure 10: ABNF Definition for Integrated Hex Parser
Figure 11: ABNF Definition for Integrated Base64 Parser
Figure 12: ABNF Definition for Integrated Raw String Hex Parser
Figure 13: ABNF Definition for Integrated Raw String Base64 Parser
List of Tables
Table 1: Example Sets of Equivalent Notations for Some Numbers
Table 2: Encoding indicators on floating point values
Table 3: dt and DT literals vs. plain CDN
Table 4: ip and IP literals vs. plain CDN
Table 5: hash literals vs. plain CDN
Table 6: Encoding Indicators Defined by This Specification
Table 7: Examples of Definite Length Encoding Indicators for
Different Data Items
Table 8: App-prefix Values Defined in this Document
Table 9: Initial Content of app-extension Identifier Registry
Table 10: New Media Type application/cdn
Table 11: New Content-Format for application/cdn
Acknowledgements
The concept of application-oriented extensions to diagnostic
notation, as well as the definition for the "dt" extension, were
inspired by the CoRAL work by Klaus Hartke.
(TBD)
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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