CBOR Extended Diagnostic Notation (EDN)

CBOR Extended Diagnostic Notation (EDN) Universität Bremen TZI

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Internet-Draft The Concise Binary Object Representation (CBOR) (STD 94, RFC 8949) 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, CBOR from the outset (RFC 7049) defined a text-based "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. RFC 8610 extended this into what is known as Extended Diagnostic Notation (EDN). This document consolidates the definition of EDN, sets forth a further step of its evolution, and is intended to serve as a single reference target in specifications that use EDN. It specifies an extension point for adding application-oriented extensions to the diagnostic notation. It then defines two such extensions that enhance EDN with text representations of epoch-based date/times and of IP addresses and prefixes (RFC 9164). A few further additions close some gaps in usability. The document modifies one extension originally specified in Appendix G.4 of RFC 8610 to enable further increasing usability. To facilitate tool interoperation, this document specifies a formal ABNF grammar, and it adds media types. The present revision -12 reflects the branch "roll-up" in the repository, an attempt to contain the entire specification of EDN in this document, instead of describing updates to the existing documents RFC 8949 and RFC 8610. While the WG hasn't taken a decision to follow this updated editorial approach, the feedback has been sufficiently positive that the author believes it is not misleading to make this revision available as the current WG Internet-Draft as well.
That said, this is still a snapshot. The editorial work on the branch "roll-up" is not complete. Content will continue to move between sections. The exact reflection of this document being a replacement for both Section 8 of RFC 8949 and Appendix G of RFC 8610 needs to be recorded in the metadata and in abstract and introduction. 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 The Concise Binary Object Representation (CBOR) (STD 94, RFC 8949) 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, CBOR from the outset (, now Section of RFC 8949 ) defined a text-based "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. extended this into what is known as Extended Diagnostic Notation (EDN). 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 consolidates the definition of EDN, sets forth a further step of its evolution, and is intended to serve as a single reference target in specifications that use EDN. It specifies an extension point for adding application-oriented extensions to the diagnostic notation. It then defines two such extensions that enhance EDN with text representations of epoch-based date/times and of IP addresses and prefixes . A few further additions close some gaps in usability. The document modifies one extension originally specified in to enable further increasing usability. To facilitate tool interoperation, this document specifies a formal ABNF grammar. (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 defined here.) 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.

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. 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 EDN, as 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 , as used for instance for some of the examples in ), and
cddl (which is used for the Concise Data Definition Language, CDDL, see below).

Note that EDN is not meant to be the only text-based representation of CBOR data items. For instance, 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 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 EDN does not provide such as anchor/alias data sharing, at a cost of higher implementation and learning complexity.

Structure of This Document of this document has been built from Section of RFC 8949 and . The latter provided a number of useful extensions to the diagnostic notation originally defined in . Section of RFC 8949 and have collectively been called "Extended Diagnostic Notation" (EDN), giving the present document its name. After introductory material, introduces the concept of application-oriented extension literals and defines the "dt" and "ip" extensions. defines mechanisms for dealing with unknown application-oriented literals and deliberately elided information. gives the formal syntax of EDN in ABNF, with explanations for some features of and additions to this syntax, as an overall grammar () and specific grammars for the content of app-string and byte-string literals (). This is followed by the conventional sections for (), (), and References (, ). An informational comparison of EDN with CDDL follows in .

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" (Concise Data Definition Language) refers to the data definition language defined in and its registered extensions (such as those in ), as well as . Additional information about the relationship between the two languages EDN and CDDL is captured in . 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 common 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.

Overview over CBOR Extended Diagnostic Notation (EDN) 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 EDN, now provides ABNF definitions. EDN is a true superset of JSON as it is defined in in conjunction with (that is, any interoperable JSON text also is an EDN text), extending it both to cover the greater expressiveness of CBOR and to increase its usability. EDN borrows the JSON syntax for numbers (integer and floating-point, ), certain simple values (), UTF-8 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). As EDN is used for truly diagnostic purposes, its implementations MAY support generation and possibly ingestion of EDN for CBOR data items that are well-formed but not valid. It is RECOMMENDED that an implementation enables such usage only explicitly by an API flag. Validity of CBOR data items is discussed in Section of RFC 8949 , with basic validity discussed in Section of RFC 8949 , and tag validity discussed in Section of RFC 8949 . Tag validity is more likely a subject for individual application-oriented extensions, while the two cases of basic validity (for text strings and for maps) are addressed below under the heading of validity. The rest of this section provides an overview over specific features of EDN, 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 .

Comments For presentation to humans, EDN text may benefit from comments. JSON famously does not provide for comments, and the original diagnostic notation in inherited this property. EDN now 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 ("/"): In a position that allows blank space, any text within and including a pair of slashes is considered blank space (and thus effectively a comment).
end-of-line comments, delimited by "#" and an end of line (LINE FEED, U+000A): In a position that allows blank space, any text within and including a pair of a "#" and 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: or, combining the use of inline and end-of-line comments:

Encoding Indicators TODO: align this with Sometimes it is useful to indicate in the diagnostic notation which of several alternative representations were actually used; for example, a data item written >1.5< by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float. The convention for encoding indicators is that anything starting with an underscore and all following characters that are alphanumeric or underscore is an encoding indicator, and can be ignored by anyone not interested in this information. For example, _ or _3. Encoding indicators are always optional. An underscore followed by a decimal digit n indicates that the preceding item (or, for arrays and maps, the item starting with the preceding bracket or brace) was encoded with an additional information value of 24+n. For example, 1.5_1 is a half-precision floating-point number, while 1.5_3 is encoded as double precision. (Note that the encoding indicator "_" is thus an abbreviation of the full form "_7", which is not used.) Encoding Indicators are discussed further below for indefinite length strings, and for arrays and maps.

Numbers In addition to JSON's decimal number literals, EDN provides hexadecimal, octal, and binary number literals in the usual C-language notation (octal with 0o prefix present only). The following are equivalent: As are: 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). A leading "+" sign is a no-op, and a leading "-" sign inverts the sign of the number. So 0, 000, +0 all represent the same integer zero, as does -0; 1, 001, +1 and +0001 all stand for the same integer one, and -1 and -0001 both designate the same integer minus one. Using a decimal point (.) and/or an exponent (e for decimal, p for hexadecimal) turns the number into a floating point number (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. The non-finite floating-point numbers 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). See for additional details of the EDN number syntax. (Note that literals for further number formats, e.g., for representing rational numbers as fractions, or for NaNs with non-zero payloads, can be added as application-oriented literals. Background information beyond that in about the representation of numbers in CBOR can be found in the informational document .)

Strings CBOR distinguishes two kinds of strings: text strings (the bytes in the string constitute UTF-8 text, major type 3), and byte strings (CBOR does not further characterize the bytes that constitute the string, major type 2). EDN notates text strings in a form compatible to that of notating text strings in JSON, with a number of usability extensions. In JSON, no control characters are allowed to occur directly in text string literals; if needed, they can be specified using escapes such as \t or \r. In EDN, 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 EDN text are not copied into the resulting string (see ). No other control characters can occur in a string literal, and the handling of escaped characters (\r etc.) is as in JSON. JSON's escape scheme for characters that are not on Unicode's basic multilingual plane (BMP) is cumbersome. EDN 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): EDN adds a number of ways to notate byte strings, some of which provide detailed access to the bits within those bytes (see ). However, quite often, byte strings carry bytes that can be meaningfully notated as UTF-8 text. Analogously to text string literals delimited by double quotes, EDN allows the use of single quotes (without a prefix) to express byte string literals with UTF-8 text; for instance, the following are equivalent: 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. (See for details.)

Encoding Indicators of Strings The detailed chunk structure of byte and text strings encoded with indefinite length can be notated in the form (_ h'0123', h'4567') and (_ "foo", "bar"). However, for an indefinite-length string with no chunks inside, (_ ) would be ambiguous as to whether a byte string (encoded 0x5fff) or a text string (encoded 0x7fff) 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., to preserve the chunk structure.

Base-Encoded Byte String Literals Besides the unprefixed byte string literals that are analogous to JSON text string literals, EDN provides base-encoded byte string literals. These are notated in one of the base encodings , without padding, enclosed in a single-quoted string literal, prefixed by >h< for base16 or >b64< for base64 or base64url (the actual encodings of the latter do not 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'. (Note that Section of RFC 8949 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 application-oriented literals.) Examples often benefit from some blank space (spaces, line breaks) in byte strings. In EDN, blank space is ignored in prefixed byte strings; for instance, the following are equivalent: Note that the internal syntax of prefixed single-quote literals such as h'' and b64'' can allow comments as blank space (see ). Since slash characters are allowed in b64'', only inline comments are available in b64 string literals.

Embedded CBOR and CBOR Sequences in Byte Strings Where a byte string is to carry an embedded CBOR-encoded item, or more generally a sequence of zero or more such items, the diagnostic notation for these zero or more CBOR data items, separated by commas, can be enclosed in << and >> to notate the byte string resulting from encoding the data items and concatenating the result. For instance, each pair of columns in the following are equivalent: > h'01' <<1, 2>> h'0102' <<"hello", null>> h'65 68656c6c6f f6' <<>> h'' ]]>

Validity of Text Strings TODO: Add general text about validity of text strings, also addressing the semantics of concatenation (+). This could move up some text from and following.

Arrays and Maps EDN borrows the JSON syntax for arrays and maps. (Maps are called objects in JSON.) For maps, EDN extends the JSON syntax by allowing any data item in the map key position (before the colon). 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 and .) The separator commas are now optional in the places where EDN syntax allows commas. (Stylistically, leaving out the commas is more idiomatic when they occur at line breaks.) In addition, 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. In summary, the following eight examples are all equivalent: as are

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].

Validity of Maps As discussed at the start of , EDN implementations MAY support generation and possibly ingestion of EDN for CBOR data items that are well-formed but not valid. For maps, this is relevant for map keys that occur more than once, as in:

Tags A tag is written as a decimal unsigned integer 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 as the following: 1(1363896240)

Simple values EDN uses JSON syntax for the simple values True (>true<), False (>false<), and Null (>null<). Undefined is written >undefined< as in JavaScript. Other simple values are given as "simple()" with the appropriate integer in the parentheses. For example, >simple(42)< indicates major type 7, value 42.

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 the "Interface format" defined in , an address combined with an optional prefix length and an optional zone identifier. This can be represented as in 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 ().

Implementation note: Typically, the ultimate applications will fail if they encounter tags unknown to them, which the ones defined in this section likely are. Where chains of tools are involved in processing EDN, it may be useful to fail earlier than at the ultimate receiver in the chain unless specific processing options (e.g., command line flags) are given that indicate which of these stand-ins are expected at this stage in the chain.

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"]). If a stage of ingestion is not prepared to handle the Unresolved Application-Extension Tag, this is an error and processing has to stop, as if this stage had been ingesting an unknown or unimplemented application-extension literal itself. 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

null (indicating a data item entirely replaced by an ellipsis), or it is
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" combines string concatenation via the + operator () with 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.

ABNF Definitions This section 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 section are intended to be useful in a Parsing Expression Grammar (PEG) parser interpretation (see for an introduction into PEG).

Overall ABNF Definition for Extended Diagnostic Notation This subsection 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.

Overall ABNF Definition of CBOR EDN >" array = "[" spec S [item S *(OC item S) OC] "]" map = "{" spec S [kp S *(OC 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 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 *(OC 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 = "{" (1*"0" [ hexscalar ] / hexscalar) "}" / 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 hexscalar = "10" 4HEXDIG / HEXDIG1 4HEXDIG / non-surrogate / 1*3HEXDIG ; 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" HEXDIG1 = DIGIT1 / "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:

As mentioned in the terminology (), 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.
Unicode CARRIAGE RETURN (U+000D, often seen escaped as "\r" in many programming languages) that exist in the input (unescaped) are ignored as if they were not in the input wherever they appear. This is most important when they are found in (text or byte) string contexts (see the "unescaped" ABNF rule). On some platforms, a carriage return is always added in front of a LINE FEED (U+000A, also often seen escaped as "\n" in many programming languages), but on other platforms, carriage returns are not used at line breaks. The intent behind ignoring unescaped carriage returns is to ensure that input generated or processed on either of these kinds of platforms will generate the same bytes in the CBOR data items created from that input. (Platforms that use just a CARRIAGE RETURN to signify an end of line are no longer relevant and the files they produce are out of scope for this document.) If a carriage return is needed in the CBOR data item, it can be added explicitly using the escaped form \r.
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 now 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.
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 , Section 6.4.4.2 of , or Section 5.13.4 of ; floating-suffix/floating-point-suffix from the latter two is not used here).
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 in preferred serialization (unless modified by an encoding indicator, which then needs to be _1, _2, or _3). For this, the number range needs to fit into an 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.
spec stands for an encoding indicator. (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 of RFC 8949 .) 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.
Extended diagnostic notation allows a (text or byte) string to be built up from multiple (text or byte) string literals, separated by a + operator; these are then concatenated into a single string.

string, string1e, string1, and ellipsis realize: (1) the representation of strings in this form split up into multiple chunks, and (2) the use of ellipses to represent elisions (). Note that the syntax defined here for concatenation of components uses an explicit + operator between the components to be concatenated ( used simple juxtaposition, which was not widely implemented and got in the way of making the use of commas optional in other places via the rule OC). Text strings and byte strings do not mix within such a concatenation, except that 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. The following four text string values (adapted from by updating to explicit + operators) are equivalent: Similarly, the following byte string values are equivalent: The semantic processing of these constructs is governed by the following rules:

A single ... is a general ellipsis, which by itself can stand for any data item. Multiple adjacent concatenated ellipses are equivalent to a single ellipsis.
An ellipsis can be concatenated (on one or both sides) with string chunks (string1); 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).
If there is no ellipsis in the concatenated list, the result of processing the list will always be a single item.
The bytes in the concatenated sequence of string chunks are simply joined together, proceeding from left to right. If the left hand side of a concatenation is a text string, the joining operation results in a text string, and that result needs to be valid UTF-8. If the left hand side is a byte string, the right hand side also needs to be a byte string.
Some of the strings may be app-strings. If the result type of the app-string is an actual (text or byte) string, joining of those string chunks occurs as with chunks directly notated as string literals; 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 subsection 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 ABNF 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

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. This description may employ an abbreviation of the form ai=nn, where nn is the numeric value of the field additional information, the low-order 5 bits of the initial byte (see Section of RFC 8949 ).
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:

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..9999, according to the procedure "IETF Review or IESG Approval", preferably a number less than 1000.

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. The EDN specification provides two explicit extension points, application-extension identifiers () and encoding indicators (). Extensions introduced this way can have their own security considerations (see, e.g., ). When implementing tools that support the use of EDN extensions, the implementer needs to be careful not to inadvertently introduce a vector for an attacker 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 (). For instance, 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 controlling such an extension.

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 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 IEEE Standard for Floating-Point Arithmetic IEEE Information technology — Programming languages — C International Organization for Standardization The text of the standard is also available via https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf Fourth Edition Programming languages — C++ International Organization for Standardization The text of the standard is also available via https://isocpp.org/files/papers/N4860.pdf Sixth Edition 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 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) 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. 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] Concise Problem Details for Constrained Application Protocol (CoAP) APIs This document defines a concise "problem detail" as a way to carry machine-readable details of errors in a Representational State Transfer (REST) response to avoid the need to define new error response formats for REST APIs for constrained environments. The format is inspired by, but intended to be more concise than, the problem details for HTTP APIs defined in RFC 7807. 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. The I-JSON Message Format I-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results. On Numbers in CBOR Universität Bremen TZI The Concise Binary Object Representation (CBOR), as defined in STD 94 (RFC 8949), is a data representation format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. Among the kinds of data that a data representation format needs to be able to carry, numbers have a prominent role, but also have inherent complexity that needs attention from protocol designers and implementers of CBOR libraries and of the applications that use them. This document gives an overview over number formats available in CBOR and some notable CBOR tags registered, and it attempts to provide information about opportunities and potential pitfalls of these number formats. // This is a rather drafty initial revision, pieced together from // various components, so it has a higher level of redundancy than // ultimately desired. 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. External References to Values in CBOR Diagnostic Notation (EDN) Universität Bremen TZI The Concise Binary Object Representation (CBOR, RFC 8949) 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. CBOR diagnostic notation (EDN) is widely used to represent CBOR data items in a way that is accessible to humans, for instance for examples in a specification. At the time of writing, EDN did not provide mechanisms for composition of such examples from multiple components or sources. This document uses EDN application extensions to provide two such mechanisms, both of which insert an imported data item into the data item being described in EDN: The e'' application extension provides a way to import data items, particularly constant values, from a CDDL model (which itself has ways to provide composition). The ref'' application extension provides a way to import data items that are described in EDN. The "dereferenceable identifier" pattern Universität Bremen TZI In a protocol or an application environment, it is often important to be able to create unambiguous identifiers for some meaning (concept or some entity). Due to the simplicity of creating URIs, these have become popular for this purpose. Beyond the purpose of identifiers to be uniquely associated with a meaning, some of these URIs are in principle _dereferenceable_, so something can be placed that can be retrieved when encountering such a URI. // The present revision -04 includes a few clarifications. YAML Media Type This document registers the application/yaml media type and the +yaml structured syntax suffix with IANA. Both identify document components that are serialized according to the YAML specification. YAML Ain't Markup Language (YAML™) Version 1.2 Revision 1.2.2

EDN and CDDL This appendix is for information. 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 breaks 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:

CDDL:

COSE_Sign_Tagged = #6.98(COSE_Sign)
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:

>, # == h'a10126' ... # rest elided here ]) ]]>

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. (TBD)