| Internet-Draft | CBOR Extended Diagnostic Notation (EDN) | July 2024 |
| Bormann | Expires 5 January 2025 | [Page] |
The Concise Binary Object Representation, CBOR (STD 94, RFC 8949), defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. RFC 8610 extends this into what is known as Extended Diagnostic Notation (EDN).¶
This document sets forth a further step of evolution of EDN, and it is intended to serve as a single reference target in specifications that use EDN.¶
It specifies how to add application-oriented extensions to the diagnostic notation. It then defines two such extensions for text representations of epoch-based date/times and of IP addresses and prefixes (RFC 9164).¶
A few further additions close some gaps in usability. It modifies one extension specified in Appendix G.4 of RFC 8610 to enable further increasing usability. To facilitate tool interoperation, this document specifies a formal ABNF definition for EDN as defined today, and it adds media types.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://cbor-wg.github.io/edn-literal/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-cbor-edn-literals/.¶
Discussion of this document takes place on the cbor Working Group mailing list (mailto:cbor@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at https://www.ietf.org/mailman/listinfo/cbor/.¶
Source for this draft and an issue tracker can be found at https://github.com/cbor-wg/edn-literal.¶
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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 5 January 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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For the Concise Binary Object Representation (CBOR) Section 8 of RFC 8949 [STD94] in conjunction with Appendix G of [RFC8610] defines a "diagnostic notation" in order to be able to converse about CBOR data items without having to resort to binary data. Diagnostic notation 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 sets forth a further step of evolution of EDN, and it is intended to serve as a single reference target in specifications that use EDN.¶
It specifies how to add application-oriented extensions to the diagnostic notation. It then defines two such extensions for text representations of epoch-based date/times and of IP addresses and prefixes [RFC9164].¶
A few further additions close some gaps in usability. It modifies one extension specified in Appendix G.4 of RFC 8610 to enable further increasing usability. To facilitate tool interoperation, this document specifies a formal ABNF definition for EDN as defined today. (See Section 4.1 for an overall ABNF grammar as well as the ABNF definitions in Section 4.2 for grammars for both the byte string presentations predefined in [STD94] 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.¶
After introductory material, Section 2 introduces the concept of application-oriented extension literals and defines the "dt" and "ip" extensions. Section 3 defines mechanisms for dealing with unknown application-oriented literals and deliberately elided information. Section 4 gives the formal syntax of EDN in ABNF, with explanations for some features of and additions to this syntax, as an overall grammar (Section 4.1) and specific grammars for the content of app-string and byte-string literals (Section 4.2). This is followed by the conventional sections for IANA Considerations (5), Security considerations (6), and References (7.1, 7.2). An informational comparison of EDN with CDDL follows in Appendix A.¶
Section 8 of RFC 8949 [STD94] defines the original CBOR diagnostic notation, and Appendix G of [RFC8610] supplies a number of extensions to the diagnostic notation that result in the Extended Diagnostic Notation (EDN). The diagnostic notation extensions include popular features such as embedded CBOR (encoded CBOR data items in byte strings) and comments. A simple diagnostic notation extension that enables representing CBOR sequences was added in Section 4.2 of [RFC8742]. As diagnostic notation is not used in the kind of interchange situations where backward compatibility would pose a significant obstacle, there is little point in not using these extensions.¶
Therefore, when we refer to "diagnostic notation", we mean to include the original notation from Section 8 of RFC 8949 [STD94] as well as the extensions from Appendix G of [RFC8610], Section 4.2 of [RFC8742], and the present document. However, we stick to the abbreviation "EDN" as it has become quite popular and is more sharply distinguishable from other meanings than "DN" would be.¶
In a similar vein, the term "ABNF" in this document refers to the language defined in [STD68] as extended in [RFC7405], where the "characters" of Section 2.3 of RFC 5234 [STD68] are Unicode scalar values.¶
The term "CDDL" (Concise Data Definition Language) refers to the data definition language defined in [RFC8610] and its registered extensions (such as those in [RFC9165]), as well as [I-D.ietf-cbor-update-8610-grammar]. Additional information about the relationship between the two languages EDN and CDDL is captured in Appendix A.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Section 8 of RFC 8949 [STD94] states the objective of defining a human-readable diagnostic notation with CBOR. In particular, it states:¶
All actual interchange always happens in the binary format.¶
One important application of EDN is the notation of CBOR data for humans: in specifications, on whiteboards, and for entering test data. A number of features, such as comments in string literals, are mainly useful for people-to-people communication via EDN. Programs also often output EDN for diagnostic purposes, such as in error messages or to enable comparison (including generation of diffs via tools) with test data.¶
For comparison with test data, it is often useful if different
implementations generate the same (or similar) output for the same
CBOR data items.
This is comparable to the objectives of deterministic serialization
for CBOR data items themselves (Section 4.2 of RFC 8949 [STD94]).
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 4.2 of RFC 8949 [STD94]) on output, or even deviating from the basic
configuration in some systematic way, can further assist in comparing
test data.
Information obtained from a CDDL model can help in choosing
application-oriented literals or specific string representations such
as embedded CBOR or b64'' in the appropriate places.¶
This document extends the syntax used in diagnostic notation for byte string literals to also be available for application-oriented extensions.¶
As per Section 8 of RFC 8949 [STD94], the diagnostic notation can notate byte strings in a number of [RFC4648] base encodings, where the encoded text is enclosed in single quotes, prefixed by an identifier (»h« for base16, »b32« for base32, »h32« for base32hex, »b64« for base64 or base64url).¶
This syntax can be thought to establish a name space, with the names
"h", "b32", "h32", and "b64" taken, but other names being unallocated.
The present specification defines additional names for this namespace,
which we call application-extension identifiers.
For the quoted string, the same rules apply as for byte strings.
In particular, the escaping rules 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 (Section 5.1).¶
Prefixing a single-quoted string, an application-extension identifier is used to build an application-oriented extension literal, which stands for a CBOR data item the value of which is derived from the text given in the single-quoted string using a procedure defined in the specification for an application-extension identifier.¶
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 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 3.4.2 of RFC 8949 [STD94].¶
The text of the literal is a Standard Date/Time String as per Section 3.4.1 of RFC 8949 [STD94].¶
The value of the literal is a number representing the result of a
conversion of the given Standard Date/Time String to an Epoch-Based
Date/Time.
If fractional seconds are given in the text (production
time-secfrac in Figure 4), 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¶
dt'1969-07-21T02:56:16Z', dt'1969-07-21T02:56:16.5Z', DT'1969-07-21T02:56:16Z'¶
is equivalent to¶
-14159024, -14159023.5, 1(-14159024)¶
See Section 4.2.3 for an ABNF definition for the content of dt literals.¶
The application-extension identifier "ip" is used to notate an IP address literal that can be used as an IP address as per Section 3 of [RFC9164].¶
The text of the literal is an IPv4address or IPv6address as per Section 3.2.2 of [RFC3986].¶
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 [RFC9164] representations of address prefixes need to
implement the truncation of the address byte string as described in
Section 4.2 of [RFC9164]; see example below.)
For completeness, the lower-case variant ip'2001:db8::/56' or ip'192.0.2.0/24' stands for
an unwrapped [56,h'20010db8'] or [24,h'c00002']; however, in this case the information
on whether an address is IPv4 or IPv6 often needs to come from the context.¶
Note that there is no direct representation of an address combined
with a prefix length; this can be represented as
52([ip'192.0.2.42',24]), if needed.¶
Examples: the CBOR diagnostic notation¶
ip'192.0.2.42', IP'192.0.2.42', IP'192.0.2.0/24', ip'2001:db8::42', IP'2001:db8::42', IP'2001:db8::/64'¶
is equivalent to¶
h'c000022a', 52(h'c000022a'), 52([24,h'c00002']), h'20010db8000000000000000000000042', 54(h'20010db8000000000000000000000042'), 54([64,h'20010db8'])¶
See Section 4.2.4 for an ABNF definition for the content of ip literals.¶
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 (Section 3.1) 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 (Section 3.2).¶
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.¶
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 (Section 5.5).
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.¶
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 (Section 5.5). 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:¶
[1, 2, ..., 3]
{ "a": 1,
"b": ...,
...: ...
}
¶
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:¶
[1, 2, 888(null), 3]
{ "a": 1,
"b": 888(null),
888(null): 888(null)
}
¶
Elisions also can be used as part of a (text or byte) string:¶
{ "contract": "Herewith I buy" + ... + "gned: Alice & Bob",
"signature": h'4711...0815',
}
¶
The example "contract" uses string concatenation as per Appendix G.4 of [RFC8610] as updated by Section 4.1, extending that by allowing
ellipses; while the example
"signature" uses special syntax that allows the use of ellipses
between the bytes notated inside h'' literals.¶
String elisions can be represented in a CBOR data item by a stand-in that wraps an array of string fragments alternating with ellipsis indicators:¶
{ "contract": /CPA/888(["Herewith I buy", 888(null),
"gned: Alice & Bob"]),
"signature": 888([h'4711', 888(null), h'0815']),
}
¶
Note that the use of elisions is different from "commenting out" EDN text, e.g.:¶
{ "contract": "Herewith I buy" /.../ "gned: Alice & Bob",
"signature": h'4711/.../0815',
# ...: ...
}
¶
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.¶
This section collects grammars in ABNF form ([STD68] as extended in [RFC7405]) 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 Appendix A of [RFC8610] for an introduction into PEG).¶
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 Section 4.2.¶
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 Section 4.2.1 and
Section 4.2.2.
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.¶
seq = S [item S *(OC item S) OC]
one-item = S item S
item = map / array / tagged
/ number / simple
/ string / streamstring
string1 = (tstr / bstr) spec
string1e = string1 / ellipsis
ellipsis = 3*"." ; "..." or more dots
string = string1e *(S "+" S string1e)
number = (basenumber / decnumber / infin) spec
sign = "+" / "-"
decnumber = [sign] (1*DIGIT ["." *DIGIT] / "." 1*DIGIT)
["e" [sign] 1*DIGIT]
basenumber = [sign] "0" ("x" 1*HEXDIG
[["." *HEXDIG] "p" [sign] 1*DIGIT]
/ "x" "." 1*HEXDIG "p" [sign] 1*DIGIT
/ "o" 1*ODIGIT
/ "b" 1*BDIGIT)
infin = %s"Infinity"
/ %s"-Infinity"
/ %s"NaN"
simple = %s"false"
/ %s"true"
/ %s"null"
/ %s"undefined"
/ %s"simple(" S item S ")"
uint = "0" / DIGIT1 *DIGIT
tagged = uint spec "(" S item S ")"
app-prefix = lcalpha *lcalnum ; including h and b64
/ ucalpha *ucalnum ; tagged variant, if defined
app-string = app-prefix sqstr
sqstr = "'" *single-quoted "'"
bstr = app-string / sqstr / embedded
; app-string could be any type
tstr = DQUOTE *double-quoted DQUOTE
embedded = "<<" seq ">>"
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:¶
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.¶
basenumber stands for an integer in the usual base 16/hexadecimal
("0x"), base 8/octal ("0o"), or base 2/binary ("0b") notation, unless the
optional part containing a "p" is present, in which case it stands
for a floating point number in the usual hexadecimal notation (which
uses a mantissa in hexadecimal and an exponent in decimal notation,
see Section 5.12.3 of [IEEE754], Section 6.4.4.2 of [C], or Section
5.13.4 of [Cplusplus]; floating-suffix/floating-point-suffix from
the latter two is not used here).¶
When decnumber or basenumber 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.¶
Otherwise, and for infin, 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 a 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 3 of RFC 8949 [STD94].)¶
As per Section 8.1 of RFC 8949 [STD94]:¶
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 8.1 of RFC 8949 [STD94] does not address ai=0 to
ai=23 — the assumption seems to be that preferred serialization
(Section 4.1 of RFC 8949 [STD94]) will be used when converting CBOR
diagnostic notation to an encoded CBOR data item, so leaving out the
encoding indicator for a data item with a preferred serialization
will implicitly use ai=0 to ai=23 if that is possible.
The present specification allows 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; Section 5.2 defines a registry for additional values.¶
string and the rules preceding it in the same block realize both
the representation of strings that are split up into multiple chunks
(Appendix G.4 of [RFC8610]) and the use of ellipses to represent elisions
(Section 3.2).¶
Note that the syntax defined here for concatenation of components
uses an explicit + operator between the components to be
concatenated; Appendix G.4 of [RFC8610] used simple juxtaposition,
which got in the way of making the use of commas optional in other
places (OC).
The example equivalent text strings from Appendix G.4 of [RFC8610] now read:¶
"Hello world" "Hello " + "world" "Hello" + h'20' + "world" "" + h'48656c6c6f20776f726c64' + ""¶
Similarly, the following byte string values are equivalent:¶
'Hello world' 'Hello ' + 'world' 'Hello ' + h'776f726c64' 'Hello' + h'20' + 'world' '' + h'48656c6c6f20776f726c64' + '' + b64'' h'4 86 56c 6c6f' + h' 20776 f726c64'¶
The semantic processing of these rules is relatively complex:¶
A single ... is a general ellipsis, which can stand for any data
item.¶
An ellipsis can be 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).¶
A concatenated sequence of string chunks is simply joined together. In both cases of joining strings, the rules of Appendix G.4 of [RFC8610] need to be followed; in particular, if a text string results from the joining operation, that result needs to be valid UTF-8.¶
Some of the strings may be app-strings. If the type of the app-string is an actual string, joining of chunked strings occurs as with directly notated strings; otherwise the occurrence of more than one app-string or an app-string together with a directly notated string cannot be processed.¶
This subsection provides ABNF definitions for application-oriented extension
literals defined in [STD94] 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 Figure 1.¶
The syntax of the content of byte strings represented in hex,
such as h'', h'0815', or h'/head/ 63 /contents/ 66 6f 6f'
(another representation of << "foo" >>), is described by the ABNF in Figure 2.
This syntax accommodates both lower case and upper case hex digits, as
well as blank space (including comments) around each hex digit.¶
app-string-h = S *(HEXDIG S HEXDIG S / ellipsis S)
["#" *non-lf]
ellipsis = 3*"."
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
DIGIT = %x30-39 ; 0-9
blank = %x09 / %x0A / %x0D / %x20
non-slash = blank / %x21-2e / %x30-10FFFF
non-lf = %x09 / %x0D / %x20-D7FF / %xE000-10FFFF
S = *blank *(comment *blank )
comment = "/" *non-slash "/"
/ "#" *non-lf %x0A
The syntax of the content of byte strings represented in base64 is described by the ABNF in Figure 2.¶
This syntax allows both the classic (Section 4 of [RFC4648]) and the URL-safe (Section 5 of [RFC4648]) alphabet to be used. It accommodates, but does not require base64 padding. Note that inclusion of classic base64 makes it impossible to have in-line comments in b64, as "/" is valid base64-classic.¶
app-string-b64 = B *(4(b64dig B))
[b64dig B b64dig B ["=" B "=" / b64dig B ["="]] B]
["#" *inon-lf]
b64dig = ALPHA / DIGIT / "-" / "_" / "+" / "/"
B = *iblank *(icomment *iblank)
iblank = %x0A / %x20 ; Not HT or CR (gone)
icomment = "#" *inon-lf %x0A
inon-lf = %x20-D7FF / %xE000-10FFFF
ALPHA = %x41-5a / %x61-7a
DIGIT = %x30-39
The syntax of the content of dt literals can be described by the
ABNF for date-time from [RFC3339] as summarized in Section 3 of [RFC9165]:¶
app-string-dt = date-time
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec
; rules
time-secfrac = "." 1*DIGIT
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-offset = "Z" / time-numoffset
partial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offset
date-time = full-date "T" full-time
DIGIT = %x30-39 ; 0-9
The syntax of the content of ip literals can be described by the
ABNF for IPv4address and IPv6address in Section 3.2.2 of [RFC3986],
as included in slightly updated form in Figure 5.¶
app-string-ip = IPaddress ["/" uint]
IPaddress = IPv4address
/ IPv6address
; ABNF from RFC 3986, re-arranged for PEG compatibility:
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ h16 *1( ":" h16 ) ] "::" 3( h16 ":" ) ls32
/ [ h16 *2( ":" h16 ) ] "::" 2( h16 ":" ) ls32
/ [ h16 *3( ":" h16 ) ] "::" h16 ":" ls32
/ [ h16 *4( ":" h16 ) ] "::" ls32
/ [ h16 *5( ":" h16 ) ] "::" h16
/ [ h16 *6( ":" h16 ) ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = "25" %x30-35 ; 250-255
/ "2" %x30-34 DIGIT ; 200-249
/ "1" 2DIGIT ; 100-199
/ %x31-39 DIGIT ; 10-99
/ DIGIT ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
DIGIT = %x30-39 ; 0-9
DIGIT1 = %x31-39 ; 1-9
uint = "0" / DIGIT1 *DIGIT
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.¶
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 4.5 of RFC 8126 [BCP26]).¶
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 4.6 of RFC 8126 [BCP26]), but can make exceptions, for instance when a specification is not available at the time of registration but is likely forthcoming. If the 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:¶
a lower case ASCII [STD80] 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.¶
a brief description¶
a reference document that provides a description of the application-extension identifier¶
The initial content of the registry is shown in Table 1; all initial entries have the Change Controller "IETF".¶
| 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 |
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 4.6 of RFC 8126 [BCP26]).¶
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:¶
an ASCII [STD80] 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.¶
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 3 of RFC 8949 [STD94]).¶
a reference document that provides a description of the application-extension identifier¶
The initial content of the registry is shown in Table 2; all initial entries have the Change Controller "IETF".¶
| 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 |
IANA is requested to add the following Media-Type to the "Media Types" registry [IANA.media-types].¶
| Name | Template | Reference |
|---|---|---|
| cbor-diagnostic | application/cbor-diagnostic | RFC-XXXX, Section 5.3 |
application¶
cbor-diagnostic¶
N/A¶
N/A¶
binary (UTF-8)¶
none¶
Section 5.3 of RFC XXXX¶
Tools interchanging a human-readable form of CBOR¶
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".)¶
CBOR WG mailing list (cbor@ietf.org), or IETF Applications and Real-Time Area (art@ietf.org)¶
LIMITED USE¶
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.¶
IETF¶
no¶
IANA is requested to register a Content-Format number in the "CoAP Content-Formats" sub-registry, within the "Constrained RESTful Environments (CoRE) Parameters" Registry [IANA.core-parameters], as follows:¶
| 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.¶
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.cbor-tags], IANA is requested to assign the tags in Table 5 from the "specification required" space (suggested assignments: 888 and 999), with the present document as the specification reference.¶
| Tag | Data Item | Semantics | Reference |
|---|---|---|---|
| CPA888 | null or array | Diagnostic Notation Ellipsis | RFC-XXXX |
| CPA999 | array | Diagnostic Notation Unresolved Application-Extension |
RFC-XXXX |
The security considerations of [STD94] and [RFC8610] apply.¶
The EDN specification provides two explicit extension points, application-extension identifiers (Section 5.1) and encoding indicators (Section 5.2). Extensions introduced this way can have their own security considerations (see, e.g., Section 5 of [I-D.bormann-cbor-e-ref]). 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 (Section 6 of [I-D.bormann-t2trg-deref-id]). 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.¶
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 [STD90]. (Any (interoperable) JSON text is also valid EDN.)¶
For engineers that are using both EDN and CDDL, it is easy to write "CDDLisms" or "EDNisms" into their drafts that are meant to be in the other language. (This is one more of the many motivations to always validate formal language instances with tools.)¶
Important differences include:¶
Comment syntax. CDDL inherits ABNF's semicolon-delimited end of
line characters, while EDN finds nothing in JSON that could be inherited here.
Inspired by JavaScript, EDN simplifies JavaScript's copy of the
original C comment syntax to be delimited by single slashes (where
line ends are not of interest); it also adds end-of-line comments
starting with #.¶
Syntax for tags. CDDL's tag syntax is part of the system for referring to CBOR's fundamentals (the major type 6, in this case) and (with [I-D.ietf-cbor-update-8610-grammar]) allows specifying the actual tag number separately, while EDN's tag syntax is a simple decimal number and a pair of parentheses.¶
Previously, the use of comma as separator character. Move this after embedded CBOR as it actually no longer is a difference. But first check the diff... JSON requires commas as separators between array elements and map members; these commas also were required in the original diagnostic notation defined in [STD94] and the EDN defined in [RFC8610]. They are now optional in the places where EDN syntax allows commas. (EDN also allows, but does not require, a trailing comma before the closing bracket/brace, enabling an easier to maintain "terminator" style of their use). CDDL's comma separators in the equivalent contexts (CDDL groups) are entirely optional (and actually are terminators, which together with their optionality allows them to be used like separators as well, or even not at all). In summary, comma use is now aligned between EDN and CDDL, in a fully backwards compatible way.¶
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.¶
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)¶