CBOR Extended Diagnostic Notation (EDN)

CBOR Extended Diagnostic Notation (EDN) Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

Internet-Draft This document formalizes and consolidates the definition of the Extended Diagnostic Notation (EDN) of the Concise Binary Object Representation (CBOR), addressing implementer experience. Replacing EDN's previous informal descriptions, it updates RFC 8949, obsoleting its Section 8, and RFC 8610, obsoleting its Appendix G. It also specifies and uses registry-based extension points, using one to support text representations of epoch-based dates/times and of IP addresses and prefixes. (This cref will be removed by the RFC editor:)
The present -19 includes the definition of the cri'' application-extension. cri'' was previously defined in draft-ietf-core-href; however the latter document overtook the present document in the approval process. As the definition of cri'' is dependent on the present document (and conversely has essentially no dependency on the technical content of draft-ietf-core-href beyond its mere existence), the text (including IANA considerations) has been moved here. -19 is intended for use at the CBOR WG meeting at IETF 124. 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) (RFC8949) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. In addition to the binary interchange format, the original CBOR specification described a text-based "diagnostic notation" (, now Section of RFC 8949 ), in order to be able to converse about CBOR data items without having to resort to binary data. extended this into what is also 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 EDN in addition to the actual binary interchange format CBOR does not serve to create a competing interchange format, but enables the use of a shared diagnostic notation in tools for and in documents about CBOR. Still, between tools for CBOR development and diagnosis, document generation systems, continuous integration (CI) environments, configuration files, and user interfaces for viewing and editing for all these, EDN is often "interchanged" and therefore merits a specification that facilitates interoperability within this domain as well as reliable translation to and from CBOR. EDN is not designed or intended for general-purpose use in protocol elements exchanged between systems engaged in processes outside those listed here. This document consolidates and formalizes the definition of EDN, providing a formal grammar (see and ), and incorporating small changes based on implementation experience. It updates , obsoleting Section of RFC 8949 , and , obsoleting . It is intended to serve as a single reference target that can be used in specifications that use EDN. It also specifies two registry-based extension points for the diagnostic notation: one for additional encoding indicators, and one for adding application-oriented literal forms. It uses these registries to add encoding indicators for a more complete coverage of encoding variation, and to add application-oriented literal forms that enhance EDN with text representations of epoch-based date/times and of IP addresses and prefixes as well as an application-oriented literal that represents cryptographic hash values computed from byte strings. In addition, this document 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, illustrates the concept of application-oriented extension literals by defining the "dt", "ip", and "hash" 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; as at least some elements of the extended form are now near-universally used, the terms "diagnostic notation" and "EDN" have become synonyms in the context of CBOR. 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. Brief snippets of grammar may be given in the text as I-Regexp regular expressions . 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 () () 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.

For Humans 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 inside prefixed 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.

Determinism? For comparison with test data, it is often useful if different implementations generate the same (or similar) output for the same CBOR data items. This is comparable to the objectives of deterministic serialization for CBOR data items themselves (Section 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.

Basic Output Format 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 :.

EDN generators may provide configuration to consistently select either the unescaped (directly readable) or an escaped (ASCII equivalent) form of characters in string literals; the latter allows EDN to be used when the diagnostic value of fully escaped characters may be desired or in environments where non-ASCII characters may not enjoy full data transparency. Similar to JSON, EDN is designed to allow a simple tool to convert any EDN (including EDN with application extensions unknown to the tool) into fully escaped (printable ASCII and newlines only) form, as well as to inversely recover unescaped characters for all escapes where this is possible or for certain subsets of the characters (such as Unicode categories L, M, N, P, S, plus Zs or just ASCII space). 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 configuration (such as an API or CLI 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 in Sections and 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 .

Application-Oriented Extension Literals EDN provides literals that represent CBOR data items textually. Many of the forms of literals provided are predefined by this document, but it also defines an extension point that enables defining application-oriented extension literals, or extension literals for short. Extension literals start with a prefix that identifies the application-oriented extension, immediately followed by a sequence literal () or a single-quoted string literal (). The latter form uses its single-quoted string literal as a shorthand form for a sequence literal representing a sequence with exactly that one string data item. More precisely, 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, digits, or hyphens ([a-z0-9-]). »false«, »true«, »null«, and »undefined« cannot be used as such identifiers and are reserved. Application-extension identifiers are registered in a registry (). An application-extension (such as dt) MAY also define the meaning of a variant prefix derived from its application-extension identifier by replacing each lower-case character by its upper-case counterpart (such as DT). 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, where dt stands for the unwrapped number). This specification defines a number of generally applicable application-oriented extensions (), both to motivate making these extensions generally available, and to illustrate the concept.

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: This reduces to [1, 10584416, ["opsonize", 7, 105]]. Another example, combining the use of inline and end-of-line comments: This reduces to {1: 4, 3: 5, -1: h'6684523AB17337F173500E5728C628547CB37DFE68449C65F885D1B73B49EAE1'}.

Encoding Indicators 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 immediately 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. Encoding indicators are placed immediately to the right of the data item or of a syntactic feature that can stand for the data item the encoding of which the encoding indicator is controlling. provides examples for encoding indicators used with various kinds of data items. Examples of Encoding Indicators for Different Data Items (mt = major type)

mt	examples
0	`1_1`, `0x4711_3`
1	`-1_1`
2	`'A'_1`
3	`"A"_1`
4	`[_1 "bar"]`
5	`{_1 "bar": 1}`
6	`1_1(4711)`
7	`1.5_2`, `0x4711p+03_3`

(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 . This field is used in encoding the "argument", i.e., the value, tag, or length; ai=0 to ai=23 mean that the value of the ai field immediately is the argument, ai=24 to ai=27 mean that the argument is carried in 2^ai-24 (1, 2, 4, or 8) additional bytes, and ai=31 means that indefinite length encoding is used.) An underscore followed by a decimal digit n indicates that the preceding item (or, for arrays and maps, the item starting with the preceding bracket or brace) was encoded with an additional information value of ai=24+n. For example, 1.5_1 is a half-precision floating-point number (2¹ = 2 additional bytes or 16 bits), while 1.5_3 is encoded as double precision (2³ = 8 additional bytes or 64 bits). The encoding indicator _ is an abbreviation of what would in full form be _7, which is not used. Therefore, an underscore _ on its own stands for indefinite length encoding (ai=31). (Note that this encoding indicator is 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 ""_ ().) The encoding indicators _0 to _3 can be used to indicate ai=24 to ai=27, respectively; they therefore stand for 1, 2, 4, and 8 bytes of additional information (ai) following the initial byte in the head of the data item. (The abbreviation of _7 into _ was discussed above. _4 to _6 are not currently used in CBOR, but will be available if and when CBOR is extended to make use of ai=28 to ai=30.) Surprisingly, Section of RFC 8949 does not address ai=0 to ai=23 — the assumption seems to have been 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 making this explicit: _i ("immediate") stands for encoding with ai=0 to ai=23, i.e., it indicates that the argument is encoded directly in the initial byte of the CBOR item. 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. Encoding Indicators are discussed in further detail in for indefinite length strings and in 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). 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. Similarly, 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. In , all the items on a row are the same number (also shown in CBOR, hexadecimally), but they are distinct from items in a different row. Example Sets of Equivalent Notations for Some Numbers

EDN	CBOR hex
`4711`, `0x1267`, `0o11147`, `0b1001001100111`	`19 1267` # uint
`1.5`, `0.15e1`, `15e-1`, `0x1.8p0`, `0x18p-4`	`F9 3E00` # float16
`0`, `+0`, `-0`	`00` # uint
`0.0`, `+0.0`	`F9 0000` # float16
`-0.0`	`F9 8000` # float16

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).

Text String Literals EDN notates text strings in a form compatible to that of notating text strings in JSON (i.e., as a double-quoted string literal), with a number of usability 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 string literal are not copied into the resulting string (see ). No other control characters can occur directly 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 (see Section of RFC 8259 ). 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):

Byte String Literals 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 ().

Single-Quoted String Literals 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. However, to facilitate parsing, in single-quoted strings EDN excludes certain escaping mechanisms available for double-quoted strings:

\/ is an escape in JSON that is available for EDN text strings as well to ensure all JSON texts are EDN literals. Since EDN's single-quoted strings to not occur in JSON, this legacy compatibility feature is not available for them.
\u-based escapes are not available for characters in the range from U+0020 to U+007e (essentially, printable ASCII).

Single-quoted string literals can occur unprefixed and stand for the byte string that encodes its text string value (the "content"), or be prefixed by what looks like an application-extension prefix (see ). In a prefixed string literal, the text content of the single-quoted string literal is not used directly as a byte string, but is further processed in a way that is defined by the meaning given to the prefix. Depending on the prefix, the result of that processing can, but need not be, a byte string value. Prefixed string literals (which are always single-quoted after the prefix) are used both for base-encoded byte string literals (see ) and for application-oriented extension literals (see , called app-string). (Additional kinds of base-encoded string literals can be defined as application-oriented extension literals by registering their prefixes; there is no fundamental difference between the two predefined base-encoded string literal prefixes (h, b64) and any such potential future extension literal prefixes.)

Encoding Indicators of Strings For indefinite length encoding, strings (byte and text strings) have a special syntax streamstring. This is used (except for the special cases ''_ and ""_ below) to notate their detailed composition into individual "chunks" (Section of RFC 8949 ), by representing the individual chunks in sequence within parentheses, each optionally followed by a comma, with an encoding indicator _ immediately after the opening parenthesis: e.g., (_ h'0123', h'4567') or (_ "foo", "bar"). The overall type (byte string or text string) of the string is provided by the types of the individual chunks, which all need to be of the same type (Section of RFC 8949 ). 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., when it is desired 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 as prefixed string literals that carry one of the base encodings , without padding, i.e., the base encoding is 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'. Examples often benefit from some blank space (spaces, line breaks) in byte strings literals. In certain EDN prefixed byte string literals, blank space is ignored; for instance, the following are equivalent: The internal syntax of prefixed single-quote literals such as h'' and b64'' can also allow comments as blank space (see ). Slash characters are part of the base64 classic alphabet (see Table 1 in ), and they therefore need be in the b64'' set of characters that contribute to the byte string. Therefore, only end-of-line comments are available in b64 byte string literals. These two byte string literals stand for the same byte string; the deliberately confusing base64 content starts with b64'/bas' which is the same as h'FDB6AC' and ends with b64'lows' which is the same as h'968C2C'.

CBOR Sequence Literals In diagnostic notation, a sequence of zero or more CBOR data item literals can be enclosed in << and >>, optionally prefixed by an application-extension prefix; we speak of sequence literals. EDN mainly deals with individual data items, not with CBOR sequences , so the CBOR sequence represented by the sequence literal needs to be further processed to obtain the value of the literal. Prefixed sequence literals refer to the application extension (see ) identified by the prefix and apply the extension to its sequence content, resulting in a single data item. This data item may be a string or may not (always) be, depending on the definition of the application extension. An unprefixed sequence literal applies CBOR encoding to the data items in its content, taken as a CBOR sequence. The value of the literal thus is a byte string with the encoded content; we also speak of embedded CBOR. 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 To be valid CBOR, Section of RFC 8949 requires that text strings are byte sequences in UTF-8 form. EDN provides several ways to construct such byte strings (see for details). These mechanisms might operate on subsequences that do not themselves constitute UTF-8, e.g., by building larger sequences out of concatenating the subsequences; for validity of a text string resulting from these mechanisms it is only of importance that the result is UTF-8. Both double-quoted and single-quoted string literals have been defined such that they lead to byte sequences that are UTF-8: the source language of EDN is UTF-8, and all escaping mechanisms lead only to adding further UTF-8 characters. Only prefixed string literals, other application-extensions, or in certain cases concatenation () can generate non-UTF-8 byte sequences. 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; when this is enabled, such implementations MAY relax the requirement on text strings to be valid UTF-8. Note that neither CBOR about its text strings nor EDN about its source language make any requirements except for conformance to . No additional Unicode processing or validation such as normalization or checking whether a scalar value is actually assigned is foreseen by EDN, particularly not any processing that is dependent on a specific Unicode version. Such processing, if offered, MUST NOT get in the way of processing the data item represented in EDN (i.e., it may be appropriate to issue warnings but not to error out or to generate output that does not match the input at the UTF-8 level).

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]. At the same position, encoding indicators for specifying the size of the array or map head for definite-length format can be used instead, specifically _i or _0 to _3. For example [_0 false, true] can be used to specify the encoding of the array [false, true] as 98 02 f4 f5.

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 (Section of RFC 8949 ). 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) The tag number can be followed by an encoding indicator giving the encoding of the tag head. For example: 1_1(1363896240) (assuming preferred encoding for the tag content) is encoded as

Simple values EDN uses JSON syntax for the simple values True (»true«), False (»false«), and Null (»null«). Undefined is written »undefined« as in JavaScript. These and all other simple values can be given as "simple()" with the appropriate integer in the parentheses. For example, »simple(42)« indicates major type 7, value 42, and »simple(0x14)« indicates »false«, as does »simple(20)« or »simple(0b10100)«.

Application-Oriented Extension Literals This document extends the syntax used in diagnostic notation to also enable application-oriented extensions. This section defines a number of application-oriented extensions.

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 content of the literal is a single Standard Date/Time String as per Section of RFC 8949 , as a text or byte string. The value of the literal is a number representing the result of a conversion of the given Standard Date/Time String to an Epoch-Based Date/Time. If fractional seconds are given in the text (production time-secfrac in ), 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. Each row of shows an example of "dt" notation and equivalent notation not using an application-extension identifier. dt and DT literals vs. plain EDN

dt literal	plain EDN
`dt'1969-07-21T02:56:16Z'`	`-14159024`
`dt'1969-07-21T02:56:16.0Z'`	`-14159024.0`
`dt'1969-07-21T02:56:16.5Z'`	`-14159023.5`
`dt<<'1969-07-21T02:56:16.5Z'>>`	`-14159023.5`
`dt<<"1969-07-21T02:56:16.5Z">>`	`-14159023.5`
`DT'1969-07-21T02:56:16Z'`	`1(-14159024)`

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 content of the literal is a single IPv4address or IPv6address as per , as a text or byte string. With the lower-case app-string prefix ip, the value of the literal is a byte string representing the binary IP address. With the upper-case app-string prefix IP, the literal is such a byte string tagged with tag number 54, if an IPv6address is used, or tag number 52, if an IPv4address is used. As an additional case, the upper-case app-string prefix IP'' can be used with an IP address prefix such as 2001:db8::/56 or 192.0.2.0/24, with the equivalent tag as its value. (Note that 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 this application-extension provides no direct representation of the "Interface format" defined in , an address combined with an optional prefix length and an optional zone identifier, and therefore no way to reference a zone identifier at all. (If needed, this format can be put together by building their structures explicitly, e.g., an interface format without a zone identifier can be represented as in 52([ip'192.0.2.42',24]), or an interface format with zone identifier 42 as in 54([ip'fe80::0202:02ff:ffff:fe03:0303',64,42]).) Each row of shows an example of "ip" notation and equivalent notation not using an application-extension identifier. ip and IP literals vs. plain EDN

ip literal	plain EDN
`ip'192.0.2.42'`	`h'c000022a'`
`ip<<'192.0.2.42'>>`	`h'c000022a'`
`IP'192.0.2.42'`	`52(h'c000022a')`
`IP'192.0.2.0/24'`	`52([24,h'c00002'])`
`ip'2001:db8::42'`	`h'20010db8000000000000000000000042'`
`IP'2001:db8::42'`	`54(h'20010db8000000000000000000000042')`
`IP'2001:db8::/64'`	`54([64,h'20010db8'])`

See for an ABNF definition for the content of ip literals.

The "hash" Extension The application-extension identifier "hash" is used to notate the input to a cryptographic hash function as well as identify such a hash function to obtain a byte string that represents the output of that hash function. The content of the literal is a string, optionally followed by either an integer or a text string that identifies the hash function in the COSE Algorithms registry of the CBOR Object Signing and Encryption (COSE) registry group , either by the identifier (value: integer or string), or, if no algorithm is registered with this value, by its name used in the registry. If the second item is not given, the default algorithm used is -16 ("SHA-256"). No uppercase variant prefix is defined for the application-extension identifier "hash". hash literals vs. plain EDN

hash literal	plain EDN
`hash<<'foo'>>`	h'2C26B46B68FFC68FF99B453C1D304134 13422D706483BFA0F98A5E886266E7AE'
`hash'foo'`	h'2C26B46B68FFC68FF99B453C1D304134 13422D706483BFA0F98A5E886266E7AE'
`hash<<'foo', -16>>`	h'2C26B46B68FFC68FF99B453C1D304134 13422D706483BFA0F98A5E886266E7AE'
`hash<<'foo', "SHA-256">>`	h'2C26B46B68FFC68FF99B453C1D304134 13422D706483BFA0F98A5E886266E7AE'
`hash<<'foo', -44>>`	h'F7FBBA6E0636F890E56FBBF3283E524C 6FA3204AE298382D624741D0DC663832 6E282C41BE5E4254D8820772C5518A2C 5A8C0C7F7EDA19594A7EB539453E1ED7'
`hash<<'foo', "SHA-512">>`	h'F7FBBA6E0636F890E56FBBF3283E524C 6FA3204AE298382D624741D0DC663832 6E282C41BE5E4254D8820772C5518A2C 5A8C0C7F7EDA19594A7EB539453E1ED7'

The "cri" Extension The application-extension identifier "cri" is used to notate an EDN literal for a CRI reference as defined in . The text of the literal is a URI Reference as per or an IRI Reference as per . The value of the literal is a CRI reference that can be converted to the text of the literal using the procedure of . Note that there may be more than one CRI reference that can be converted to the URI/IRI reference given; implementations are expected to favor the simplest variant available and make non-surprising choices otherwise. In the all-upper-case variant of the app-prefix, the value is enclosed in a tag number CPA99. As an example, the CBOR diagnostic notation is equivalent to See for an ABNF definition for the content of cri 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 a text string for the application-extension identifier, and another array:

For app-strings, the second array contains a single item, a text string containing the text notated by the single-quoted string in the app-string.
For app-sequences, the second array contains zero or more items, which represent each item in the sequence contained in the app-sequence.

For example, cri'https://example.com' can be represented as /CPA/ 999(["cri", ["https://example.com"]]), or hash<<"data", -44>> as /CPA/ 999(["hash", ["data", -44]]). 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. 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 >" sqstr = SQUOTE *single-quoted SQUOTE bstr = app-string / sqstr / app-sequence / embedded ; app-string/-sequence could be any type tstr = DQUOTE *double-quoted DQUOTE embedded = "<<" seq ">>" array = "[" (specms S item *(MSC item) SOC / spec S) "]" map = "{" (specms S keyp *(MSC keyp) SOC / spec S) "}" keyp = item S ":" S item ; We allow %x09 HT in prose, but not in strings blank = %x09 / %x0A / %x0D / %x20 non-slash = blank / %x21-2e / %x30-7F / NONASCII non-lf = %x09 / %x0D / %x20-7F / NONASCII comment = "/" *non-slash "/" / "#" *non-lf %x0A ; optional space S = *blank *(comment *blank) ; mandatory space MS = (blank/comment) S ; mandatory comma and/or space MSC = ("," S) / (MS ["," S]) ; optional comma and/or space SOC = S ["," S] ; check semantically that strings are either all text or all bytes ; note that there must be at least one string to distinguish streamstring = "(_" MS string *(MSC string) SOC ")" spec = ["_" *wordchar] specms = ["_" *wordchar MS] double-quoted = unescaped / SQUOTE / "\" escapable-d single-quoted = unescaped / DQUOTE / "\" escapable-s escapable1 = %s"b" ; BS backspace U+0008 / %s"f" ; FF form feed U+000C / %s"n" ; LF line feed U+000A / %s"r" ; CR carriage return U+000D / %s"t" ; HT horizontal tab U+0009 / "\" ; \ backslash (reverse solidus) U+005C escapable-d = escapable1 / DQUOTE / "/" ; / slash (solidus) U+002F (JSON!) / (%s"u" hexchar) ; uXXXX U+XXXX escapable-s = escapable1 / SQUOTE / (%s"u" hexchar-s) ; uXXXX U+XXXX hexchar = "{" (1*"0" [ hexscalar ] / hexscalar) "}" / non-surrogate / two-surrogate non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" ODIGIT 2HEXDIG ) two-surrogate = high-surrogate "\" %s"u" low-surrogate high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG hexscalar = "10" 4HEXDIG / HEXDIG1 4HEXDIG / non-surrogate / 1*3HEXDIG ; single-quote hexchar-s: don't allow 0020..007e hexchar-s = "{" (1*"0" [ hexscalar-s ] / hexscalar-s) "}" / non-surrogate-s / two-surrogate non-surrogate-s = "007F" ; rubout / "00" ("0"/"1"/"8"/"9"/HEXDIGA) HEXDIG / "0" HEXDIG1 2HEXDIG / non-surrogate-1 non-surrogate-1 = ((DIGIT1 / "A"/"B"/"C" / "E"/"F") 3HEXDIG) / ("D" ODIGIT 2HEXDIG ) hexscalar-s = "10" 4HEXDIG / HEXDIG1 4HEXDIG / non-surrogate-1 / HEXDIG1 2HEXDIG / ("1"/"8"/"9"/HEXDIGA) HEXDIG / "7F" / HEXDIG1 ; Note that no other C0 characters are allowed, including %x09 HT unescaped = %x0A ; new line / %x0D ; carriage return -- ignored on input / %x20-21 ; omit 0x22 " / %x23-26 ; omit 0x27 ' / %x28-5B ; omit 0x5C \ / %x5D-7F / NONASCII DQUOTE = %x22 ; " double quote SQUOTE = "'" ; ' single quote DIGIT = %x30-39 ; 0-9 DIGIT1 = %x31-39 ; 1-9 ODIGIT = %x30-37 ; 0-7 BDIGIT = %x30-31 ; 0-1 HEXDIGA = "A" / "B" / "C" / "D" / "E" / "F" ; Note: double-quoted strings as in "A" are case-insensitive in ABNF HEXDIG = DIGIT / HEXDIGA HEXDIG1 = DIGIT1 / HEXDIGA lcalpha = %x61-7A ; a-z lcldh = lcalpha / DIGIT / "-" ucalpha = %x41-5A ; A-Z ucldh = ucalpha / DIGIT / "-" ALPHA = lcalpha / ucalpha wordchar = "_" / ALPHA / DIGIT ; [_a-z0-9A-Z] NONASCII = %x80-D7FF / %xE000-10FFFF ]]> 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 by itself 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 6.4.4.3 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. See for details.
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 except for implementations that support and are enabled for generation/ingestion of EDN for CBOR data items that are well-formed but not valid. 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. (This determination must be made at the time the app-string is interpreted; see for how this may not be immediately during parsing.)

ABNF Definitions for Application Extension Content This subsection provides ABNF definitions for the content of application-oriented extension literals defined in and in this specification, where applicable. These grammars describe the decoded content of the sqstr components that combine with the application-extension identifiers used as prefixes to form application-oriented extension literals. Each of these may integrate ABNF rules defined in , which are not always repeated here. summarizes the app-prefix values defined in this document. App-prefix Values Defined in this Document

app-prefix	content of single-quoted string	result type
h	hexadecimal form of binary data	byte string
H	(not used)
b64	base64 forms (classic or base64url) of binary data	byte string
B64	(not used)
dt	RFC 3339 date/time	number (int or float)
DT	"	Tag 1 on the above
ip	IP address or prefix	byte string, array of length and byte string
IP	"	Tag 54 (IPv6) or 52 (IPv4) on the above

Note that implementation platforms may already provide implementations of grammars used in application-extensions, such as of RFC 3339 for dt'' and of IP address syntax for ip''. EDN-based tools may want to use these implementation libraries instead of using the grammars that are provided here as a reference.

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

cri: ABNF Definition of URI Representation of a CRI It can be expected that implementations of the application-extension identifier "cri" will make use of platform-provided URI implementations, which will include a URI parser. In case such a URI parser is not available or inconvenient to integrate, a grammar of the content of cri literals is provided by the ABNF for URI-reference in Section of RFC 3986 with certain re-arrangements taken from ; these are reproduced in . If the content is not ASCII only (i.e., for IRIs), first apply and apply this grammar to the result.

ABNF Definition of URI Representation of a CRI segment = *pchar segment-nz = 1*pchar segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" ) ; non-zero-length segment without any colon ":" pchar = unreserved / pct-encoded / sub-delims / ":" / "@" query = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" ) pct-encoded = "%" HEXDIG HEXDIG unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" reserved = gen-delims / sub-delims gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@" sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "=" ]]>

ABNF Definitions for Integrated Extension Parsers For some applications of EDN, it is an optimization to integrate parsers for the content of some prefixed single-quoted string literals into the main parser, handling both the string literal syntax (e.g., escapes such as \' and \\) and the syntax of the extension content in one go. For application-extensions that only use printable ASCII characters (from U+0020 to U+007E) minus single-quote ' and backslash \, the ABNF such as that given in can be directly used as an integrated parser, after adding some glue ABNF. For instance, for app-string-dt, add an alternative to bstr that points to a rule for prefixed single-quoted string literals ().

Glue Code for Integrated DT Parser To facilitate writing integrated ABNF for more complex prefixed string literals, the ABNF definitions in may be useful and are used in the rest of this section.

ABNF Definitions Useful for Integrated Extension Parsers Two subsections with ABNF for integrated parsers follow, one for h'', and one for b64''. There is no requirement for a new application-extension to supply ABNF for an integrated parser (or any ABNF at all!), in particular if the parsing function is likely to be fulfilled by a platform library. If ABNF for the content of a single-quoted string is available in an application-extension specification, ABNF for an integrated parser can be written as a separate activity or also automatically derived. At the time of writing, one example for a tool performing such a derivation is available as open-source software .

h: ABNF Definition of Integrated Parser With glue code similar to that in , ABNF such as that shown in can be used as an integrated parser for h'' prefixed single-quote strings.

ABNF Definition for Integrated Hex Parser

b64: ABNF Definition of Integrated Parser With glue code similar to that in , ABNF such as that shown in can be used as an integrated parser for h'' prefixed single-quote strings.

ABNF Definition for Integrated Base64 Parser

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, digits, and hyphens 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
false	Reserved	RFC-XXXX
true	Reserved	RFC-XXXX
null	Reserved	RFC-XXXX
undefined	Reserved	RFC-XXXX
dt	Date/Time	RFC-XXXX
ip	IP Address/Prefix	RFC-XXXX
hash	Cryptographic Hash	RFC-XXXX
cri	Constrained Resource Identifier	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
_4	Reserved (for ai=28)	RFC-XXXX
_5	Reserved (for ai=29)	RFC-XXXX
_6	Reserved (for ai=30)	RFC-XXXX
_7	Reserved (see _)	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 for application/cbor-diagnostic

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] UTF-8, a transformation format of ISO 10646 ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. 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] Internationalized Resource Identifiers (IRIs) This document defines a new protocol element, the Internationalized Resource Identifier (IRI), as a complement of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). A mapping from IRIs to URIs is defined, which means that IRIs can be used instead of URIs, where appropriate, to identify resources. The approach of defining a new protocol element was chosen instead of extending or changing the definition of URIs. This was done in order to allow a clear distinction and to avoid incompatibilities with existing software. Guidelines are provided for the use and deployment of IRIs in various protocols, formats, and software components that currently deal with URIs. 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. I-Regexp: An Interoperable Regular Expression Format This document specifies I-Regexp, a flavor of regular expression that is limited in scope with the goal of interoperation across many different regular expression libraries. CBOR Object Signing and Encryption (COSE) IANA 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 Constrained Resource Identifiers Universität Bremen TZI Fraunhofer SIT The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that represents the URI components in Concise Binary Object Representation (CBOR) rather than as a sequence of characters. This approach simplifies parsing, comparison, and reference resolution in environments with severe limitations on processing power, code size, and memory size. This RFC updates RFC 7595 to add a note on how the "URI Schemes" registry of RFC 7595 cooperates with the "CRI Scheme Numbers" registry created by the present RFC. // (This "cref" paragraph will be removed by the RFC editor:) The // present revision –25 contains a few more tweaks to address follow- // on AD review comments as well as comments from the ARTART review. // It is intended to be ready for IESG evaluation. IEEE Standard for Floating-Point Arithmetic IEEE Information technology — Programming languages — C International Organization for Standardization   The standard is widely known as C23. Its technical content is also available via https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf. Edition 5 Programming languages — C++ International Organization for Standardization   The standard is widely known as C++23. Its technical content is also available via https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/n4950.pdf. Edition 7 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:.plus,.cat, and.det for the construction of constants;.abnf/.abnfb for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and.feature 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. 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 PEG-parsing using ABNF grammars (via treetop) n.d.

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

List of Figures

:
:
:
:
:
:
:
:
:
:

List of Tables

:
:
:
:
:
:
:
:
:
:
:

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)