]> CBOR Extended Diagnostic Notation (EDN): Application-Oriented Literals, ABNF, and Media Type Universität Bremen TZI
Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org
Internet-Draft The Concise Binary Object Representation, CBOR (STD 94, 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 IP addresses and prefixes (RFC 9164). A few further additions close some gaps in usability. To facilitate tool interoperation, this document specifies a formal ABNF definition for extended diagnostic notation (EDN) that accommodates application-oriented literals. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the cbor Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .
Introduction For the Concise Binary Object Representation, CBOR, Section of RFC 8949 in conjunction with defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. Diagnostic notation syntax 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 in 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 IP addresses and prefixes . A few further additions close some gaps in usability. To facilitate tool interoperation, this document specifies a formal ABNF definition for extended diagnostic notation (EDN) that accommodates application-oriented literals. (See for an overall ABNF grammar as well as the ABNF definitions in for grammars for both the byte string presentations predefined in and the application-extensions). 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.
Terminology Section of RFC 8949 defines the original CBOR diagnostic notation, and 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 that enables representing CBOR sequences was added in . 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 of RFC 8949 as well as the extensions from , , 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 as extended in , where the "characters" of Section of RFC 5234 are Unicode scalar values. The term "CDDL" refers to the data definition language defined in and its registered extensions (such as those in ), as well as . The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
(Non-)Objectives of this Document Section of RFC 8949 states the objective of defining a human-readable diagnostic notation with CBOR. In particular, it states:
All actual interchange always happens in the binary format.
One important application of EDN is the notation of CBOR data for humans: in specifications, on whiteboards, and for entering test data. A number of features, such as comments in string literals, are mainly useful for people-to-people communication via EDN. Programs also often output EDN for diagnostic purposes, such as in error messages or to enable comparison (including generation of diffs via tools) with test data. 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 of RFC 8949 ). However, there are even more representation variants in EDN 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 h''; an EDN 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 EDN, and there is no expectation for "roundtripping" from EDN to CBOR and back, i.e., for an ability to convert EDN to binary CBOR and back to EDN while achieving exactly the same result as the original input EDN — the original EDN possibly was created by humans or by a different EDN generator. However, there is a certain expectation that EDN generators can be configured to some basic output format, which:
  • looks like JSON where that is possible;
  • inserts encoding indicators only where the binary form differs from preferred encoding;
  • 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 :.
Additional features such as ensuring deterministic map ordering (Section of RFC 8949 ) 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 application-oriented literals or specific string representations such as embedded CBOR or b64'' in the appropriate places.
Application-Oriented Extension Literals This document extends the syntax used in diagnostic notation for byte string literals to also be available for application-oriented extensions. As per Section of RFC 8949 , the diagnostic notation can notate byte strings in a number of 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 that were adapted from JSON strings are applied equivalently for application-oriented extensions, e.g., within the quoted string \\ stands 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 (). 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. An application-extension (such as dt) MAY also define the meaning of a variant of the application-extension identifier where each lower-case character is replaced by its upper-case counterpart (such as DT), for building an application-oriented extension literal using that all-uppercase variant as the prefix of a single-quoted string. As a convention for such definitions, using the all-uppercase variant implies making use of a tag appropriate for this application-oriented extension (such as tag number 1 for DT). Examples for application-oriented extensions to CBOR diagnostic notation can be found in the following sections.
The "dt" Extension The application-extension identifier "dt" is used to notate a date/time literal that can be used as an Epoch-Based Date/Time as per Section of RFC 8949 . The text of the literal is a Standard Date/Time String as per Section of RFC 8949 . 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 ), the value is a floating-point number; the value is an integer number otherwise. In the all-upper-case variant of the app-prefix, the value is enclosed in a tag number 1. As an example, the CBOR diagnostic notation is equivalent to See for an ABNF definition for the content of dt literals.
The "ip" Extension The application-extension identifier "ip" is used to notate an IP address literal that can be used as an IP address as per . The text of the literal is an IPv4address or IPv6address as per . With the lower-case app-string ip, the value of the literal is a byte string representing the binary IP address. With the upper-case app-string 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. As an additional case, the upper-case app-string IP'' can be used with a prefix such as 2001:db8::/56 or 192.0.2.0/24, with the equivalent tag as its value. (Note that representations of address prefixes need to implement the truncation of the address byte string as described in ; see example below.) For completeness, the lower-case 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 there is no direct representation of an address combined with a prefix length; this can be represented as 52([ip'192.0.2.42',24]), if needed. Examples: the CBOR diagnostic notation is equivalent to See for an ABNF definition for the content of ip literals.
Stand-in Representations in Binary CBOR In some cases, an EDN consumer cannot construct actual CBOR items that represent the CBOR data intended for eventual interchange. This document defines stand-in representation for two such cases:
  • The EDN consumer does not know (or does not implement) an application-extension identifier used in the EDN document () but wants to preserve the information for a later processor.
  • The generator of some EDN intended for human consumption (such as in a specification document) may not want to include parts of the final data item, destructively replacing complete subtrees or possibly just parts of a lengthy string by elisions ().
Handling unknown application-extension identifiers When ingesting CBOR diagnostic notation, any application-oriented extension literals are usually decoded and transformed into the corresponding data item during ingestion. If an application-extension is not known or not implemented by the ingesting process, this is usually an error and processing has to stop. However, in certain cases, it can be desirable to exceptionally carry an uninterpreted application-oriented extension literal in an ingested data item, allowing to postpone its decoding to a specific later stage of ingestion. This specification defines a CBOR Tag for this purpose: The Diagnostic Notation Unresolved Application-Extension Tag, tag number CPA999 (). The content of this tag is an array of two text strings: The application-extension identifier, and the (escape-processed) content of the single-quoted string. For example, dt'1969-07-21T02:56:16Z' can be provisionally represented as /CPA/ 999(["dt", "1969-07-21T02:56:16Z"]). RFC-Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. For each usage of the term "CPA", please remove the prefix "CPA" from the indicated value and replace the residue with the value assigned by IANA; perform an analogous substitution for all other occurrences of the prefix "CPA" in the document. Finally, please remove this note.
Handling information deliberately elided from an EDN document When using EDN for exposition in a document or on a whiteboard, it is often useful to be able to leave out parts of an EDN document that are not of interest at that point of the exposition. To facilitate this, this specification supports the use of an ellipsis (notated as three or more dots in a row, as in ...) to indicate parts of an EDN document that have been elided (and therefore cannot be reconstructed). Upon ingesting EDN as a representation of a CBOR data item for further processing, the occurrence of an ellipsis usually is an error and processing has to stop. However, it is useful to be able to process EDN documents with ellipses in the automation scripts for the documents using them. This specification defines a CBOR Tag that can be used in the ingestion for this purpose: The Diagnostic Notation Ellipsis Tag, tag number CPA888 (). The content of this tag either is
  1. null (indicating a data item entirely replaced by an ellipsis), or it is
  2. an array, the elements of which are alternating between fragments of a string and the actual elisions, represented as ellipses carrying a null as content.
Elisions can stand in for entire subtrees, e.g. in: A single ellipsis (or key/value pair of ellipses) can imply eliding multiple elements in an array (members in a map); if more detailed control is required, a data definition language such as CDDL can be employed. (Note that the stand-in form defined here does not allow multiple key/value pairs with an ellipsis as a key: the CBOR data item would not be valid.) Subtree elisions can be represented in a CBOR data item by using /CPA/888(null) as the stand-in: Elisions also can be used as part of a (text or byte) string: The example "contract" uses string concatenation as per , extending that by allowing ellipses; while the example "signature" uses special syntax that allows the use of ellipses between the bytes notated inside h'' literals. String elisions can be represented in a CBOR data item by a stand-in that wraps an array of string fragments alternating with ellipsis indicators: Note that the use of elisions is different from "commenting out" EDN text, e.g. The consumer of this EDN will ignore the comments and therefore will have no idea after ingestion that some information has been elided; validation steps may then simply fail instead of being informed about the elisions.
IANA Considerations RFC Editor: please replace RFC-XXXX with the RFC number of this RFC, [IANA.cbor-diagnostic-notation] with a reference to the new registry group, and remove this note.
CBOR Diagnostic Notation Application-extension Identifiers Registry IANA is requested to create an "Application-Extension Identifiers" registry in a new "CBOR Diagnostic Notation" registry group [IANA.cbor-diagnostic-notation], with the policy "expert review" (Section of RFC 8126 ). The experts are instructed to be frugal in the allocation of application-extension identifiers that are suggestive of generally applicable semantics, keeping them in reserve for application-extensions that are likely to enjoy wide use and can make good use of their conciseness. The expert is also instructed to direct the registrant to provide a specification (Section of RFC 8126 ), but can make exceptions, for instance when a specification is not available at the time of registration but is likely forthcoming. If the expert becomes aware of application-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:
Application-Extension Identifier:
a lower case ASCII string that starts with a letter and can contain letters and digits after that ([a-z][a-z0-9]*). No other entry in the registry can have the same application-extension identifier.
Description:
a brief description
Change Controller:
(see Section of RFC 8126 )
Reference:
a reference document that provides a description of the application-extension identifier
The initial content of the registry is shown in ; all initial entries have the Change Controller "IETF". Initial Content of Application-extension Identifier Registry
Application-extension Identifier Description Reference
h Reserved RFC8949
b32 Reserved RFC8949
h32 Reserved RFC8949
b64 Reserved RFC8949
dt Date/Time RFC-XXXX
ip IP Address/Prefix RFC-XXXX
Encoding Indicators IANA is requested to create an "Encoding Indicators" registry in the newly created "CBOR Diagnostic Notation" registry group [IANA.cbor-diagnostic-notation], with the policy "specification required" (Section of RFC 8126 ). The experts are instructed to be frugal in the allocation of encoding indicators that are suggestive of generally applicable semantics, keeping them in reserve for encoding indicator registrations that are likely to enjoy wide use and can make good use of their conciseness. If the expert becomes aware of encoding indicators that are deployed and in use, they may also solicit a specification and initiate a registration on their own if they deem such a registration can avert potential future collisions. Each entry in the registry must include:
Encoding Indicator:
an ASCII string that starts with an underscore letter and can contain zero or more underscores, letters and digits after that (_[_A-Za-z0-9]*). No other entry in the registry can have the same Encoding Indicator.
Description:
a brief description
Change Controller:
(see Section of RFC 8126 )
Reference:
a reference document that provides a description of the application-extension identifier
The initial content of the registry is shown in ; all initial entries have the Change Controller "IETF". Initial Content of Encoding Indicator Registry
Encoding Indicator Description Reference
_ Indefinite Length Encoding (ai=31) RFC8949, 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
Media Type IANA is requested to add the following Media-Type to the "Media Types" registry . New Media Type application/cbor-diagnostic
Name Template Reference
cbor-diagnostic application/cbor-diagnostic RFC-XXXX,
Type name:
application
Subtype name:
cbor-diagnostic
Required parameters:
N/A
Optional parameters:
N/A
Encoding considerations:
binary (UTF-8)
Security considerations:
of RFC XXXX
Interoperability considerations:
none
Published specification:
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):
.diag
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:
CBOR diagnostic notation represents CBOR data items, which are the format intended for actual interchange. The media type application/cbor-diagnostic 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
Content-Format IANA is requested to register a Content-Format number in the sub-registry, within the "Constrained RESTful Environments (CoRE) Parameters" Registry , as follows: New Content-Format
Content-Type Content Coding ID Reference
application/cbor-diagnostic - TBD1 RFC-XXXX
TBD1 is to be assigned from the space 256..999.
Stand-in Tags RFC-Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. For each usage of the term "CPA", please remove the prefix "CPA" from the indicated value and replace the residue with the value assigned by IANA; perform an analogous substitution for all other occurrences of the prefix "CPA" in the document. Finally, please remove this note. In the "CBOR Tags" registry , IANA is requested to assign the tags in from the "specification required" space (suggested assignments: 888 and 999), with the present document as the specification reference. Values for Tags
Tag Data Item Semantics Reference
CPA888 null or array Diagnostic Notation Ellipsis RFC-XXXX
CPA999 array Diagnostic Notation
Unresolved Application-Extension		 RFC-XXXX
Security considerations The security considerations of and apply.
References Normative References Concise Binary Object Representation (CBOR) The Concise Binary Object Representation (CBOR) 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. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Concise Binary Object Representation (CBOR) Sequences This document describes the Concise Binary Object Representation (CBOR) Sequence format and associated media type "application/cbor-seq". A CBOR Sequence consists of any number of encoded CBOR data items, simply concatenated in sequence. Structured syntax suffixes for media types allow other media types to build on them and make it explicit that they are built on an existing media type as their foundation. This specification defines and registers "+cbor-seq" as a structured syntax suffix for CBOR Sequences. Augmented BNF for Syntax Specifications: ABNF Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Case-Sensitive String Support in ABNF This document extends the base definition of ABNF (Augmented Backus-Naur Form) to include a way to specify US-ASCII string literals that are matched in a case-sensitive manner. Date and Time on the Internet: Timestamps This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar. Uniform Resource Identifier (URI): Generic Syntax A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] Concise Binary Object Representation (CBOR) Tags for IPv4 and IPv6 Addresses and Prefixes This specification defines two Concise Binary Object Representation (CBOR) tags for use with IPv6 and IPv4 addresses and prefixes. Concise Binary Object Representation (CBOR) Tags IANA Media Types IANA Constrained RESTful Environments (CoRE) Parameters IANA Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. ASCII format for network interchange Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References The Base16, Base32, and Base64 Data Encodings This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] The JavaScript Object Notation (JSON) Data Interchange Format JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. Additional Control Operators for the Concise Data Definition Language (CDDL) The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed:,, and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. Updates to the CDDL grammar of RFC 8610 Universität Bremen TZI The Concise Data Definition Language (CDDL), as defined in RFC 8610 and RFC 9165, provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON. The present document updates RFC 8610 by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there.
ABNF Definitions This appendix collects grammars in ABNF form ( as extended in ) that serve to define the syntax of EDN and some application-oriented literals. Implementation note: The ABNF definitions in this appendix are intended to be useful in a PEG parser interpretation (see for an introduction into PEG).
Overall ABNF Definition for Extended Diagnostic Notation This appendix provides an overall ABNF definition for the syntax of CBOR extended diagnostic notation. To complete the parsing of an app-string with prefix, say, p, the processed sqstr inside it is further parsed using the ABNF definition specified for the production app-string-p in . For simplicity, the internal parsing for the built-in EDN prefixes is specified in the same way. ABNF definitions for h'' and b64'' are provided in and . However, the prefixes b32'' and h32'' are not in wide use and an ABNF definition in this document could therefore not be based on implementation experience.
>" array = "[" spec S [item S *("," S item S) OC] "]" map = "{" spec S [kp S *("," S kp S) OC] "}" kp = item S ":" S item ; We allow %x09 HT in prose, but not in strings blank = %x09 / %x0A / %x0D / %x20 non-slash = blank / %x21-2e / %x30-D7FF / %xE000-10FFFF non-lf = %x09 / %x0D / %x20-D7FF / %xE000-10FFFF S = *blank *(comment *blank) comment = "/" *non-slash "/" / "#" *non-lf %x0A ; optional trailing comma (ignored) OC = ["," S] ; check semantically that strings are either all text or all bytes ; note that there must be at least one string to distinguish streamstring = "(_" S string S *("," S string S) OC ")" spec = ["_" *wordchar] double-quoted = unescaped / "'" / "\" DQUOTE / "\" escapable single-quoted = unescaped / DQUOTE / "\" "'" / "\" escapable escapable = %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 / "/" ; / slash (solidus) U+002F (JSON!) / "\" ; \ backslash (reverse solidus) U+005C / (%s"u" hexchar) ; uXXXX U+XXXX hexchar = non-surrogate / (high-surrogate "\" %s"u" low-surrogate) non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" ODIGIT 2HEXDIG ) high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG ; 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-D7FF ; skip surrogate code points / %xE000-10FFFF DQUOTE = %x22 ; " double quote DIGIT = %x30-39 ; 0-9 DIGIT1 = %x31-39 ; 1-9 ODIGIT = %x30-37 ; 0-7 BDIGIT = %x30-31 ; 0-1 HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" ; Note: double-quoted strings as in "A" are case-insensitive in ABNF lcalpha = %x61-7A ; a-z lcalnum = lcalpha / DIGIT ucalpha = %x41-5A ; A-Z ucalnum = ucalpha / DIGIT wordchar = "_" / lcalnum / ucalpha ; [_a-z0-9A-Z] ]]>
While an ABNF grammar defines the set of character strings that are considered to be valid EDN by this ABNF, the mapping of these character strings into the generic data model of CBOR is not always obvious. The following additional items should help in the interpretation:
  • 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.
  • basenumber stands for an integer in the usual base 16/hexadecimal ("0x"), base 8/octal ("0o"), or base 2/binary ("0b") notation, unless the optional part containing a "p" is present, in which case it stands for a floating point number in the usual hexadecimal notation (which uses a mantissa in hexadecimal and an exponent in decimal notation).
  • spec stands for an encoding indicator. As per Section of RFC 8949 :
    • an underscore _ on its own stands for indefinite length encoding (ai=31, only available behind the opening brace/bracket for map and array: strings have a special syntax streamstring for indefinite length encoding except for the special cases ''_ and ""_), and
    • _0 to _3 stand for ai=24 to ai=27, respectively.
    Surprisingly, Section of RFC 8949 does not address ai=0 to ai=23 — the assumption seems to be that preferred serialization (Section of RFC 8949 ) 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 to make this explicit:
    • _i ("immediate") stands for encoding with ai=0 to ai=23.
    While no pressing use for further values for encoding indicators comes to mind, this is an extension point for EDN; defines a registry for additional values.
  • string and the rules preceding it in the same block realize both the representation of strings that are split up into multiple chunks (Section of RFC 8949 ) and the use of ellipses to represent elisions (). The semantic processing of these rules is relatively complex:
    • A single ... is a general ellipsis, which can stand for any data item.
    • An ellipsis can be surrounded (on one or both sides) by string chunks, the result is a CBOR tag number CPA888 that contains an array with joined together spans of such chunks plus the ellipses represented by 888(null).
    • A simple sequence of string chunks is simply joined together. In both cases of joining strings, the rules of Section of RFC 8949 need to be followed; in particular, if a text string results from the joining operation, that result needs to be valid UTF-8.
    • Some of the strings may be app-strings. If the type of the app-string is an actual string, joining of chunked strings occurs as with directly notated strings; otherwise the occurrence of more than one app-string or an app-string together with a directly notated string cannot be processed.
ABNF Definitions for app-string Content This appendix provides ABNF definitions for application-oriented extension literals defined in 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 .
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 . This syntax accommodates both lower case and upper case hex digits, as well as blank space (including comments) around each hex digit.
ABNF Definition of Hexadecimal Representation of a Byte String
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 . This syntax allows both the classic () and the URL-safe () alphabet to be used. It accommodates, but does not require base64 padding. Note that inclusion of classic base64 makes it impossible to have in-line comments in b64, as "/" is valid base64-classic.
ABNF definition of Base64 Representation of a Byte String
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 from as summarized in :
ABNF Definition of RFC3339 Representation of a Date/Time
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 , as included in slightly updated form in .
ABNF Definition of Textual Representation of an IP Address
EDN and CDDL 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 called schema). The two languages share some similarities, not the least because they have mutually inspired each other. But they have very different roots:
  • EDN syntax is an extension to JSON syntax . (Any (interoperable) JSON text is also valid EDN.)
  • CDDL syntax is inspired by ABNF's syntax .
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 motivations to always validate formal language instances with tools.) Important differences include:
  • Comment syntax. CDDL inherits ABNF's semicolon-delimited end of line characters, while EDN finds nothing in JSON that could be inherited here. Inspired by JavaScript, EDN simplifies JavaScript's copy of the original C comment syntax to be delimited by single slashes (where line ends are not of interest); it also adds end-of-line comments starting with #.
    EDN:
    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 ) allows specifying the actual tag number separately, while EDN's tag syntax is a simple decimal number and a pair of parentheses.
    EDN:
    98(['', {}, /rest elided here: …/])
    CDDL:
    COSE_Sign_Tagged = #6.98(COSE_Sign)
  • Separator character. Like JSON, EDN requires commas as separators between array elements and map members (EDN also allows, but does not require, a trailing comma before the closing bracket/brace, enabling an easier to maintain "terminator" style of their use). CDDL's comma separators in these 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).
  • 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.
    EDN:
    98([/h'a10126'/ << {/alg/ 1: -7 /ECDSA 256/ } >>, /…/])
    CDDL:
    serialized_map = bytes .cbor header_map
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.