Internet DRAFT - draft-ietf-cbor-edn-literals
draft-ietf-cbor-edn-literals
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 1 February 2024
Expires: 4 August 2024
CBOR Extended Diagnostic Notation (EDN): Application-Oriented Literals,
ABNF, and Media Type
draft-ietf-cbor-edn-literals-08
Abstract
The Concise Binary Object Representation, CBOR (STD 94, RFC 8949),
defines a "diagnostic notation" in order to be able to converse about
CBOR data items without having to resort to binary data.
This document specifies how to add application-oriented extensions to
the diagnostic notation. It then defines two such extensions for
text representations of epoch-based date/times and of IP addresses
and prefixes (RFC 9164).
A few further additions close some gaps in usability. To facilitate
tool interoperation, this document specifies a formal ABNF definition
for extended diagnostic notation (EDN) that accommodates application-
oriented literals.
About This Document
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. (Non-)Objectives of this Document . . . . . . . . . . . . 4
2. Application-Oriented Extension Literals . . . . . . . . . . . 6
2.1. The "dt" Extension . . . . . . . . . . . . . . . . . . . 7
2.2. The "ip" Extension . . . . . . . . . . . . . . . . . . . 7
3. Stand-in Representations in Binary CBOR . . . . . . . . . . . 8
3.1. Handling unknown application-extension identifiers . . . 9
3.2. Handling information deliberately elided from an EDN
document . . . . . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
4.1. CBOR Diagnostic Notation Application-extension Identifiers
Registry . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Encoding Indicators . . . . . . . . . . . . . . . . . . . 13
4.3. Media Type . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Content-Format . . . . . . . . . . . . . . . . . . . . . 15
4.5. Stand-in Tags . . . . . . . . . . . . . . . . . . . . . . 16
5. Security considerations . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Normative References . . . . . . . . . . . . . . . . . . 16
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6.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. ABNF Definitions . . . . . . . . . . . . . . . . . . 19
A.1. Overall ABNF Definition for Extended Diagnostic
Notation . . . . . . . . . . . . . . . . . . . . . . . . 19
A.2. ABNF Definitions for app-string Content . . . . . . . . . 23
A.2.1. h: ABNF Definition of Hexadecimal representation of a
byte string . . . . . . . . . . . . . . . . . . . . . 24
A.2.2. b64: ABNF Definition of Base64 representation of a byte
string . . . . . . . . . . . . . . . . . . . . . . . 24
A.2.3. dt: ABNF Definition of RFC 3339 Representation of a
Date/Time . . . . . . . . . . . . . . . . . . . . . . 25
A.2.4. ip: ABNF Definition of Textual Representation of an IP
Address . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix B. EDN and CDDL . . . . . . . . . . . . . . . . . . . . 26
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
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 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. To facilitate
tool interoperation, this document specifies a formal ABNF definition
for extended diagnostic notation (EDN) that accommodates application-
oriented literals. (See Appendix A.1 for an overall ABNF grammar as
well as the ABNF definitions in Appendix A.2 for grammars for both
the byte string presentations predefined in [STD94] and the
application-extensions).
In addition, this document finally registers a media type identifier
and a content-format for CBOR diagnostic notation. This does not
elevate its status as an interchange format, but recognizes that
interaction between tools is often smoother if media types can be
used.
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1.1. Terminology
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" 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].
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.
1.2. (Non-)Objectives of this Document
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.
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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 :.
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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.
2. Application-Oriented Extension Literals
This document extends the syntax used in diagnostic notation for byte
string literals to also be available for application-oriented
extensions.
As per Section 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 4.1).
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.
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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.
2.1. 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 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 Appendix A.2.3 for an ABNF definition for the content of dt
literals.
2.2. 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 Section 3 of
[RFC9164].
The text of the literal is an IPv4address or IPv6address as per
Section 3.2.2 of [RFC3986].
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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 Appendix A.2.4 for an ABNF definition for the content of ip
literals.
3. 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:
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* 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).
3.1. 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 (Section 4.5). 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"]).
// 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.
3.2. 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.
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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 4.5). The content of this tag either is
1. null (indicating a data item entirely replaced by an ellipsis),
or it is
2. an array, the elements of which are alternating between fragments
of a string and the actual elisions, represented as ellipses
carrying a null as content.
Elisions can stand in for entire subtrees, e.g. in:
[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)
}
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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], 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.
4. 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.
4.1. 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 4.5 of RFC 8126 [BCP26]).
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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:
Application-Extension Identifier:
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.
Description:
a brief description
Change Controller:
(see Section 2.3 of RFC 8126 [BCP26])
Reference:
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".
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+==================================+===================+===========+
| 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 |
+----------------------------------+-------------------+-----------+
Table 1: Initial Content of Application-extension Identifier
Registry
4.2. 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 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:
Encoding Indicator:
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.
Description:
a brief description
Change Controller:
(see Section 2.3 of RFC 8126 [BCP26])
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Reference:
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 | RFC8949, |
| | Encoding (ai=31) | 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 |
+--------------------+-------------------+-----------+
Table 2: Initial Content of Encoding Indicator
Registry
4.3. Media Type
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 4.3 |
+-----------------+-----------------------------+-------------+
Table 3: New Media Type application/cbor-diagnostic
Type name: application
Subtype name: cbor-diagnostic
Required parameters: N/A
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Optional parameters: N/A
Encoding considerations: binary (UTF-8)
Security considerations: Section 5 of RFC XXXX
Interoperability considerations: none
Published specification: Section 4.3 of RFC XXXX
Applications that use this media type: Tools interchanging a human-
readable form of CBOR
Fragment identifier considerations: The syntax and semantics of
fragment identifiers is as specified for "application/cbor". (At
publication of RFC XXXX, there is no fragment identification
syntax defined for "application/cbor".)
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): .diag
Macintosh file type code(s): N/A
Person & email address to contact for further information: CBOR WG
mailing list (cbor@ietf.org), or IETF Applications and Real-Time
Area (art@ietf.org)
Intended usage: LIMITED USE
Restrictions on usage: CBOR diagnostic notation represents CBOR data
items, which are the format intended for actual interchange. The
media type application/cbor-diagnostic is intended to be used
within documents about CBOR data items, in diagnostics for human
consumption, and in other representations of CBOR data items that
are necessarily text-based such as in configuration files or other
data edited by humans, often under source-code control.
Author/Change controller: IETF
Provisional registration: no
4.4. Content-Format
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 |
+-----------------------------+----------------+------+-----------+
Table 4: New Content-Format
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TBD1 is to be assigned from the space 256..999.
4.5. 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.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 | Semantics | Reference |
| | Item | | |
+========+===========+==================================+===========+
| CPA888 | null or | Diagnostic Notation Ellipsis | RFC-XXXX |
| | array | | |
+--------+-----------+----------------------------------+-----------+
| CPA999 | array | Diagnostic Notation | RFC-XXXX |
| | | Unresolved Application-Extension | |
+--------+-----------+----------------------------------+-----------+
Table 5: Values for Tags
5. Security considerations
The security considerations of [STD94] and [RFC8610] apply.
6. References
6.1. Normative References
[BCP26] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, June 2017.
<https://www.rfc-editor.org/info/bcp26>
[C] International Organization for Standardization,
"Information technology — Programming languages — C",
Fourth Edition, ISO/IEC 9899:2018, June 2018,
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<https://www.iso.org/standard/74528.html>. The text of
the standard is also available via
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf
[Cplusplus]
International Organization for Standardization,
"Programming languages — C++", Sixth Edition, ISO/
IEC 14882:2020, December 2020,
<https://www.iso.org/standard/79358.html>. The text of
the standard is also available via
https://isocpp.org/files/papers/N4860.pdf
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags>.
[IANA.core-parameters]
IANA, "Constrained RESTful Environments (CoRE)
Parameters",
<https://www.iana.org/assignments/core-parameters>.
[IANA.media-types]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
<https://ieeexplore.ieee.org/document/8766229>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/rfc/rfc8742>.
[RFC9164] Richardson, M. and C. Bormann, "Concise Binary Object
Representation (CBOR) Tags for IPv4 and IPv6 Addresses and
Prefixes", RFC 9164, DOI 10.17487/RFC9164, December 2021,
<https://www.rfc-editor.org/rfc/rfc9164>.
[STD68] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
<https://www.rfc-editor.org/info/std68>
[STD80] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, October 1969.
<https://www.rfc-editor.org/info/std80>
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, December 2020.
<https://www.rfc-editor.org/info/std94>
6.2. Informative References
[I-D.ietf-cbor-update-8610-grammar]
Bormann, C., "Updates to the CDDL grammar of RFC 8610",
Work in Progress, Internet-Draft, draft-ietf-cbor-update-
8610-grammar-03, 29 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
update-8610-grammar-03>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
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[RFC9165] Bormann, C., "Additional Control Operators for the Concise
Data Definition Language (CDDL)", RFC 9165,
DOI 10.17487/RFC9165, December 2021,
<https://www.rfc-editor.org/rfc/rfc9165>.
[STD90] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, December 2017.
<https://www.rfc-editor.org/info/std90>
Appendix A. ABNF Definitions
This appendix 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 appendix are
intended to be useful in a PEG parser interpretation (see Appendix A
of [RFC8610] for an introduction into PEG).
A.1. Overall ABNF Definition for Extended Diagnostic Notation
This appendix provides an overall ABNF definition for the syntax of
CBOR extended diagnostic notation.
To complete the parsing of an app-string with prefix, say, p, the
processed sqstr inside it is further parsed using the ABNF definition
specified for the production app-string-p in Appendix A.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 Appendix A.2.1 and Appendix A.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 *("," S item S) OC] S
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 string1e)
number = (basenumber / decnumber / infin) spec
sign = "+" / "-"
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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 *("," S item S) OC] "]"
map = "{" spec S [kp S *("," S kp S) OC] "}"
kp = item S ":" S item
; We allow %x09 HT in prose, but not in strings
blank = %x09 / %x0A / %x0D / %x20
non-slash = blank / %x21-2e / %x30-D7FF / %xE000-10FFFF
non-lf = %x09 / %x0D / %x20-D7FF / %xE000-10FFFF
S = *blank *(comment *blank)
comment = "/" *non-slash "/"
/ "#" *non-lf %x0A
; optional trailing comma (ignored)
OC = ["," S]
; check semantically that strings are either all text or all bytes
; note that there must be at least one string to distinguish
streamstring = "(_" S string S *("," S string S) OC ")"
spec = ["_" *wordchar]
double-quoted = unescaped
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/ "'"
/ "\" 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
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lcalpha = %x61-7A ; a-z
lcalnum = lcalpha / DIGIT
ucalpha = %x41-5A ; A-Z
ucalnum = ucalpha / DIGIT
wordchar = "_" / lcalnum / ucalpha ; [_a-z0-9A-Z]
Figure 1
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).
* spec stands for an encoding indicator. 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
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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 4.2
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 (Section G.4 of RFC 8949 [STD94]) and the use of ellipses
to represent elisions (Section 3.2). The semantic processing of
these rules is relatively complex:
- A single ... is a general ellipsis, which can stand for any
data item.
- An ellipsis can be surrounded (on one or both sides) by string
chunks, the result is a CBOR tag number CPA888 that contains an
array with joined together spans of such chunks plus the
ellipses represented by 888(null).
- A simple sequence of string chunks is simply joined together.
In both cases of joining strings, the rules of Section G.4 of
RFC 8949 [STD94] 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.
A.2. ABNF Definitions for app-string Content
This appendix 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 rules defined in Figure 1.
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A.2.1. 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 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
Figure 2: ABNF Definition of Hexadecimal Representation of a Byte
String
A.2.2. 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 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
Figure 3: ABNF definition of Base64 Representation of a Byte String
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A.2.3. 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 [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
Figure 4: ABNF Definition of RFC3339 Representation of a Date/Time
A.2.4. 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 Section 3.2.2 of [RFC3986], as
included in slightly updated form in Figure 5.
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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
Figure 5: ABNF Definition of Textual Representation of an IP Address
Appendix B. EDN and CDDL
EDN was designed as a language to provide a human-readable
representation of an instance, i.e., a single CBOR data item or CBOR
sequence. CDDL was designed as a language to describe an (often
large) set of such instances (which itself constitutes a language),
in the form of a _data definition_ or _grammar_ (or sometimes called
_schema_).
The two languages share some similarities, not the least because they
have mutually inspired each other. But they have very different
roots:
* EDN syntax is an extension to JSON syntax [STD90]. (Any
(interoperable) JSON text is also valid EDN.)
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* CDDL syntax is inspired by ABNF's syntax [STD68].
For engineers that are using both EDN and CDDL, it is easy to write
"CDDLisms" or "EDNisms" into their drafts that are meant to be in the
other language. (This is one more of the many motivations to always
validate formal language instances with tools.)
Important differences include:
* Comment syntax. CDDL inherits ABNF's semicolon-delimited end of
line characters, while EDN finds nothing in JSON that could be
inherited here. Inspired by JavaScript, EDN simplifies
JavaScript's copy of the original C comment syntax to be delimited
by single slashes (where line ends are not of interest); it also
adds end-of-line comments starting with #.
EDN:
{ / alg / 1: -7 / ECDSA 256 / }
,
{ 1: # alg
-7 # ECDSA 256
}
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 [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.
EDN: 98(['', {}, /rest elided here: …/])
CDDL: COSE_Sign_Tagged = #6.98(COSE_Sign)
* Separator character. Like JSON, EDN requires commas as separators
between array elements and map members (EDN also allows, but does
not require, a trailing comma before the closing bracket/brace,
enabling an easier to maintain "terminator" style of their use).
CDDL's comma separators in these contexts (CDDL groups) are
entirely optional (and actually are terminators, which together
with their optionality allows them to be used like separators as
well, or even not at all).
* Embedded CBOR. EDN has a special syntax to describe the content
of byte strings that are encoded CBOR data items. CDDL can
specify these with a control operator, which looks very different.
EDN: 98([/h'a10126'/ << {/alg/ 1: -7 /ECDSA 256/ } >>, /…/])
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CDDL: serialized_map = bytes .cbor header_map
Acknowledgements
The concept of application-oriented extensions to diagnostic
notation, as well as the definition for the "dt" extension, were
inspired by the CoRAL work by Klaus Hartke.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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