|Internet-Draft||CBOR EDN Literals||July 2023|
|Bormann||Expires 24 January 2024||[Page]|
- Network Working Group
- Intended Status:
Application-Oriented Literals in CBOR Extended Diagnostic Notation
The Concise Binary Object Representation, CBOR (RFC 8949), defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data.¶
This document specifies how to add application-oriented extensions to the diagnostic notation. It then defines two such extensions for text representations of epoch-based date/times and of Constrained Resource Identifiers (draft-ietf-core-href).¶
To facilitate tool interoperation, this document also specifies a formal ABNF definition for extended diagnostic notation (EDN) that accommodates application-oriented literals.¶
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:email@example.com), which is archived at https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at https://www.ietf.org/mailman/listinfo/cbor/.¶
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For the Concise Binary Object Representation, CBOR, Section 8 of [RFC8949] in conjunction with Appendix G of [RFC8610] defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. Diagnostic notation is based on JSON, with extensions for representing CBOR constructs such as binary data and tags. (Standardizing this together with the actual interchange format does not serve to create another interchange format, but enables the use of a shared diagnostic notation in tools for and documents about CBOR.)¶
This document specifies how to add application-oriented extensions to the diagnostic notation. It then defines two such extensions for text representations of epoch-based date/times and of Constrained Resource Identifiers [I-D.ietf-core-href].¶
To facilitate tool interoperation, this document also specifies a formal ABNF definition for extended diagnostic notation (EDN) that accommodates application-oriented literals. (See Appendix A.1 for an overall ABNF grammar as well as the ABNF definitions in Appendix A.2 for grammars for both the byte string presentations predefined in [RFC8949] and the application-extensions).¶
Section 8 of [RFC8949] defines the original CBOR diagnostic notation, and Appendix G of [RFC8610] supplies a number of extensions to the diagnostic notation that result in the Extended Diagnostic Notation (EDN). The diagnostic notation extensions include popular features such as embedded CBOR (encoded CBOR data items in byte strings) and comments. A simple diagnostic notation extension for CBOR sequences was added in Section 4.2 of [RFC8742]. As diagnostic notation is not used in the kind of interchange situations where backward compatibility would pose a significant obstacle, there is little point in not using these extensions.¶
Therefore, when we refer to "diagnostic notation", we mean to include the original notation from Section 8 of [RFC8949] as well as the extensions from Appendix G of [RFC8610], Section 4.2 of [RFC8742], and the present document. However, we stick to the abbreviation "EDN" as it has become quite popular and is more sharply distinguishable from other meanings than "DN" would be.¶
In a similar vein, the term "ABNF" in this document refers to the language defined in [RFC5234] as extended in [RFC7405], even if the latter extensions are not currently used in this document. The term "CDDL" refers to the data definition language defined in [RFC8610] and its registered extensions (such as those in [RFC9165]), as well as [I-D.ietf-cbor-update-8610-grammar].¶
This document extends the syntax used in diagnostic notation for byte string literals to also be available for application-oriented extensions.¶
As per Section 8 of [RFC8949], the diagnostic notation can notate byte strings in a number of [RFC4648] base encodings, where the encoded text is enclosed in single quotes, prefixed by an identifier (>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 defines additional names for this namespace,
which we call application-extension identifiers.
For the quoted string, the same rules apply as for byte strings.
In particular, the escaping rules of JSON strings are applied
equivalently for application-oriented extensions, e.g.,
for a single backslash and
\' stands for a single quote.¶
An application-extension identifier is a name consisting of a lower-case ASCII letter (a-z) and zero or more additional ASCII characters that are either lower-case letters or digits (a-z0-9).¶
Application-extension identifiers are registered in a registry (Section 5.1). Prefixing a single-quoted string, an application-extension identifier is used to build an application-oriented extension literal, which stands for a CBOR data item the value of which is derived from the text given in the single-quoted string using a procedure defined in the specification for an application-extension identifier.¶
Examples for application-oriented extensions to CBOR diagnostic notation can be found in the following sections.¶
In addition, this document finally registers a media type identifier and a content-format for CBOR diagnostic notation. 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.¶
The value of the literal is a CRI that can be converted to the text of the literal using the procedure of Section 6.1 of [I-D.ietf-core-href]. Note that there may be more than one CRI that can be converted to the URI/IRI given; implementations are expected to favor the simplest variant available and make non-surprising choices otherwise.¶
As an example, the CBOR diagnostic notation¶
is equivalent to¶
[-4, ["example", "com"], ["bottarga", "shaved"]]¶
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
If fractional seconds are given in the text (production
time-secfrac in Figure 4), the value is a
floating-point number; the value is an integer number otherwise.¶
As an example, the CBOR diagnostic notation¶
is equivalent to¶
|cri||Constrained Resource Identifier||RFCthis|
(Define policy: probably specification required?; detailed template)¶
|cbor-diagnostic||application/cbor-diagnostic||RFC XXXX, Section 5.2|
- Type name:
- Subtype name:
- Required parameters:
- Optional parameters:
- Encoding considerations:
- Security considerations:
- Interoperability considerations:
- Published specification:
- 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:
- Person & email address to contact for further information:
CBOR WG mailing list (firstname.lastname@example.org), or IETF Applications and Real-Time Area (email@example.com)¶
- Intended usage:
- Restrictions on usage:
- Author/Change controller:
- Provisional registration:
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:¶
TBD1 is to be assigned from the space 256..999.¶
Anything else meaningful to say here?¶
- Bormann, C., "Updates to the CDDL grammar of RFC 8610", Work in Progress, Internet-Draft, draft-ietf-cbor-update-8610-grammar-00, , <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-update-8610-grammar-00>.
- Bormann, C. and H. Birkholz, "Constrained Resource Identifiers", Work in Progress, Internet-Draft, draft-ietf-core-href-13, , <https://datatracker.ietf.org/doc/html/draft-ietf-core-href-13>.
- IANA, "Media Types", <https://www.iana.org/assignments/media-types>.
- Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, , <https://www.rfc-editor.org/rfc/rfc3339>.
- Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <https://www.rfc-editor.org/rfc/rfc3986>.
- Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987, , <https://www.rfc-editor.org/rfc/rfc3987>.
- Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <https://www.rfc-editor.org/rfc/rfc5234>.
- Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, , <https://www.rfc-editor.org/rfc/rfc7405>.
- 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, , <https://www.rfc-editor.org/rfc/rfc8610>.
- Bormann, C., "Concise Binary Object Representation (CBOR) Sequences", RFC 8742, DOI 10.17487/RFC8742, , <https://www.rfc-editor.org/rfc/rfc8742>.
- Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
- Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, , <https://www.rfc-editor.org/rfc/rfc9165>.
This appendix provides an overall ABNF definition for the syntax of CBOR extended diagnostic notation.¶
For simplicity, the internal parsing for the built-in EDN prefixes is
specified in the same way.
ABNF definitions for
b64'' are provided in Appendix A.2.1 and
However, the prefixes
h32'' are not in wide use and an
ABNF definition in this document could therefore not be based on
This appendix provides ABNF definitions for application-oriented extension
literals defined in [RFC8949] and in this specification.
These grammars describe the decoded content of the
sqstr components that
combine with the application-extension identifiers to form
application-oriented extension literals.
Each of these may make use of rules defined in Figure 1.¶
The syntax of the content of byte strings represented in hex,
h'/head/ 63 /contents/ 66 6f 6f'
(another representation of
<< "foo" >>), is described by the ABNF in Figure 2.
This syntax accommodates both lower case and upper case hex digits, as
well as blank space (including comments) around each hex digit.¶
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 in b64, as "/" is valid base64-classic.¶
EDN 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 schema).¶
The two languages share some similarities, not the least because they have mutually inspired each other. But they have very different roots:¶
- EDN is an extension to JSON [RFC8259]. (Any (interoperable) JSON text is also valid EDN.)¶
- CDDL is inspired by ABNF's syntax [RFC5234].¶
For engineers that are using both EDN and CDDL, it is easy to write "CDDLisms" or "EDNisms" into their drafts that are meant to be in the other language. (This is one more of the many reasons to always validate formal language instances with tools.)¶
Important differences include:¶
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 [I-D.ietf-cbor-update-8610-grammar]) allows specifying the actual tag number separately, while EDN's tag syntax is a simple decimal number and a pair of parentheses.¶
- Separator character. Like JSON, EDN requires commas as separators between array elements and map members and doesn't allow a trailing comma before the closing bracket/brace. CDDL's comma separators in these contexts (CDDL groups) are optional (and actually are terminators, which together with their optionality allows them to be used like separators as well or even not at all).¶
Embedded CBOR. EDN 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.¶