CBOR Working group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 28 February 2025
Expires: 1 September 2025
CDDL 2.0 and beyond — a draft plan
draft-bormann-cbor-cddl-2-draft-06
Abstract
The Concise Data Definition Language (CDDL) today is defined by
RFC 8610, RFC 9165, RFC 9682, and draft-ietf-cbor-cddl-more-control
(RFC-to-be 9741). RFC 9165 and the latter (as well as some more
application specific specifications such as RFC 9090) have used the
extension point provided in RFC 8610, the control operator.
As CDDL is used in larger projects, feature requirements become known
that cannot be easily mapped into this single extension point.
Hence, there is a need for evolution of the base CDDL specification
itself.
The present document provides a roadmap towards a "CDDL 2.0"; it is
intended to serve as a basis for implementations that evolve with the
concept of CDDL 2.0. It is based on draft-bormann-cbor-cddl-freezer,
but is more selective in what potential features it takes up and more
detailed in their discussion. This document is intended to evolve
over time; it might spawn specific documents and then retire, or it
might eventually be published as a roadmap document.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-cbor-cddl-2-draft/.
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/cddl-2.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. CDDL 1.1 + 2 plan (standards track) . . . . . . . . . . . 3
1.2. Other documents . . . . . . . . . . . . . . . . . . . . . 4
2. Mending syntax deficits . . . . . . . . . . . . . . . . . . . 5
2.1. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 5
3. Processing model: Beyond Validation . . . . . . . . . . . . . 5
3.1. Annotations . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Transformation . . . . . . . . . . . . . . . . . . . . . 6
3.3. Next Steps . . . . . . . . . . . . . . . . . . . . . . . 6
4. Module superstructure . . . . . . . . . . . . . . . . . . . . 6
4.1. Cross-universe references . . . . . . . . . . . . . . . . 6
4.2. ABNF is a lot like CDDL . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
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7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Fridge . . . . . . . . . . . . . . . . . . . . . . . 9
A.1. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 9
A.2. Cross-universe references . . . . . . . . . . . . . . . . 10
A.2.1. IANA references . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
(Please see abstract.)
Note that the existing extension point can be exercised for new
features in parallel to the work described here.
[I-D.ietf-cbor-cddl-more-control] (recently approved, RFC-to-be
9741), forms part of the first set of specifications going forward
from the CDDL-2 project together with [RFC9682].
The rest of this introduction gives a rough overview over what could
be the development plan for CDDL 1.1, 2.0, 2.5.
1.1. CDDL 1.1 + 2 plan (standards track)
This section documents the status in Summer 2024.
CDDL 1.1 milestone (documents technically complete, implemented):
* "CDDL 1.1": [RFC9682], _Grammar_ fixes: Empty files (enabling CDDL
2), non-literal tags, errata fixes. Approved document, in RFC
editor queue (EDIT state) at the time of writing.
* Parallel to CDDL 1.1: More _control_ operators
[I-D.ietf-cbor-cddl-more-control], RFC-to-be 9741: Additional
control operators, another iteration like RFC 9165 before.
CDDL 2.0 work:
* Technically complete before *IETF 119*: CDDL 2.0:
[I-D.ietf-cbor-cddl-modules] (import/include directives,
implemented). Feedback is available from IETF 119, one open
technical issue (sockets); WGLC 1H2025.
* Potentially, further directives to be added. No proposals are
ripe for specification; this work could go into a second document
constituting "CDDL 2.1" so we have the well-understood import/
include available now.
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"CDDL 2.5":
* Being prepared in *1H2025*: CDDL 2.5: Section 3 of the present
document ("_annotations_", plus some functionality enabled by
that). The requirements are reasonably well-understood; the
specific form this takes needs to be worked out. Enables, e.g.,
Section 5 of [I-D.bormann-cbor-cddl-freezer] (co-occurrence).
1.2. Other documents
Not on the main line of development, but important ancillary work:
* (Informational, implemented): Section 6 (alternative
representations) of [I-D.bormann-cbor-cddl-freezer]: CDDL-in-JSON
format(s) for interchange of CDDL model information between tools.
* (Informational, companion to [I-D.ietf-cbor-cddl-modules]):
[I-D.bormann-cbor-rfc-cddl-models] (builds standard collection of
referenceable models).
* (BCP? Informational?): [I-D.bormann-cbor-draft-numbers] (BCP for
handling assigned numbers during draft stage; can stay
informational as the work described is completed and any reference
to the document erased before a specification using it would be
published).
* Application-oriented literal e'' [I-D.ietf-cbor-edn-e-ref] makes
use of [I-D.ietf-cbor-edn-literals] so that diagnostic notation
can refer to named numbers that are specified in CDDL.
Implemented, see [enum-literals] for an introduction.
More explorative at this point:
* (Standards-Track?) The remaining Section 2 of this document:
application-oriented literals in CDDL mirroring the work in
[I-D.ietf-cbor-edn-literals].
* (Informational or Standards-Track?): [I-D.bormann-cbor-cddl-csv]
(using CDDL to model CSV documents).
Important CBOR work that may be reflected in some CDDL extensions:
* Evolving Extended Diagnostic Notation
[I-D.ietf-cbor-edn-literals]. While EDN and CDDL are independent
languages (with EDN rooted in JSON and CDDL in ABNF and Relax-NG),
they are often used together, and developments in one may spawn
parallel work in the other.
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* Common Deterministic Encoding (CDE) [I-D.ietf-cbor-cde] and
related documents. These do define CDDL operators already, which
may be sufficient for initial use; this might be extended once
more experience has been gained.
* Packed CBOR [I-D.ietf-cbor-packed]. CDDL already can be used to
describe the original data item represented in a packed data item.
Requirements for describing the latter have not yet been
collected; there is some relation to transformation (Section 3.2)
that might need to be explored.
2. Mending syntax deficits
The previous content of this section formed the basis for [RFC9682],
except for Section 2.1.
2.1. Tag-oriented Literals
Incomplete, see Appendix A.1.
3. Processing model: Beyond Validation
_Proposal Status_: experiments with implementations ongoing
_Compatibility_: backwards compatible
The basic (implicit) processing model for CDDL 1.0 applies a CDDL
data model to a data item and returns a Boolean that indicates
whether the data item matches that model ("_validation_").
Section 4 of [RFC9165] extends this model with named "_features_". A
validation can indicate which features were used. Validation could
also be parameterized with information about what features are
allowed to be used, enabling variants (see Section 4 of [RFC9165] and
[useful] for examples).
3.1. Annotations
The cddl tool (Appendix F of [RFC8610]) also supports experimental
forms of "annotating" a validated data item with information about
which rules were used to support validation, currently entirely based
on the information that is in a standard CDDL 1.0 data model. This
leads to a more general concept of "_annotation_", where the data
model specification supports "annotating" the validated instance by
optionally supplying information in the model. (The annotated result
is a special case of a "post-schema validation instance" [PSVI], here
one where the data item itself is only augmented, not changed, by the
process.)
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Annotations could in turn provide input to further validation steps,
as is often done with Schematron validation in Relax-NG; with an
appropriate evaluation language this can be used for checking co-
occurrence constraints (Section 5 of
[I-D.bormann-cbor-cddl-freezer]).
3.2. Transformation
Finally, annotations are a first step to _transformation_, i.e.,
describing how a validated data item should be interpreted as a
transformed data item by performing certain computations. This
generally requires even more support from an evaluation language,
simple transformations such as adding in default values may not need
much support though.
3.3. Next Steps
At this time, existing experimental implementations do not lead to a
clear choice for what processing model enhancements should be in
CDDL 2.0 follow-ons. This document proposes to continue the
experimentation and document good approaches.
4. Module superstructure
The previous content of this section formed the basis for
[I-D.ietf-cbor-cddl-modules]. Additional work might be started on
the ideas outlined in the subsections of this section.
4.1. Cross-universe references
See Appendix A.2.
4.2. ABNF is a lot like CDDL
Many of the constructs defined here for CDDL also could be used with
ABNF specifications. ABNF would definitely benefit from a standard
way to import snippets from existing RFCs. Since CDDL contains ABNF
support (Section 3 of [RFC9165]), it would be natural to make some of
the functionality discussed in this section available for ABNF as
well.
5. IANA Considerations
This document makes no requests of IANA.
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6. Security considerations
The security considerations of [RFC8610] apply.
7. References
7.1. Normative References
[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>.
[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>.
7.2. Informative References
[enum-literals]
"[Cbor] Getting diagnostic notation examples in drafts
under control", 26 February 2024,
<https://mailarchive.ietf.org/arch/msg/cbor/
D8h_0Egog89GaRLFNwb1VfKlHI4>.
[EXTRACT-RB]
"extract.rb — extract CDDL from an enum-style IANA
registry", n.d., <https://github.com/cabo/common-
cddl/blob/main/extract.rb>.
[I-D.bormann-cbor-cddl-csv]
Bormann, C. and H. Birkholz, "Using CDDL for CSVs", Work
in Progress, Internet-Draft, draft-bormann-cbor-cddl-csv-
05, 16 June 2024, <https://datatracker.ietf.org/doc/html/
draft-bormann-cbor-cddl-csv-05>.
[I-D.bormann-cbor-cddl-freezer]
Bormann, C., "A feature freezer for the Concise Data
Definition Language (CDDL)", Work in Progress, Internet-
Draft, draft-bormann-cbor-cddl-freezer-14, 28 August 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
cddl-freezer-14>.
[I-D.bormann-cbor-draft-numbers]
Bormann, C., "Managing CBOR codepoints in Internet-
Drafts", Work in Progress, Internet-Draft, draft-bormann-
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cbor-draft-numbers-04, 29 August 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
draft-numbers-04>.
[I-D.bormann-cbor-rfc-cddl-models]
Bormann, C., "CDDL models for some existing RFCs", Work in
Progress, Internet-Draft, draft-bormann-cbor-rfc-cddl-
models-05, 23 February 2025,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
rfc-cddl-models-05>.
[I-D.ietf-cbor-cddl-modules]
Bormann, C. and B. Moran, "CDDL Module Structure", Work in
Progress, Internet-Draft, draft-ietf-cbor-cddl-modules-03,
1 September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-cbor-cddl-modules-03>.
[I-D.ietf-cbor-cddl-more-control]
Bormann, C., "Concise Data Definition Language (CDDL):
Additional Control Operators for the Conversion and
Processing of Text", Work in Progress, Internet-Draft,
draft-ietf-cbor-cddl-more-control-08, 9 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
cddl-more-control-08>.
[I-D.ietf-cbor-cde]
Bormann, C., "CBOR Common Deterministic Encoding (CDE)",
Work in Progress, Internet-Draft, draft-ietf-cbor-cde-08,
10 February 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-cbor-cde-08>.
[I-D.ietf-cbor-edn-e-ref]
Bormann, C., "External References to Values in CBOR
Diagnostic Notation (EDN)", Work in Progress, Internet-
Draft, draft-ietf-cbor-edn-e-ref-01, 29 December 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-e-ref-01>.
[I-D.ietf-cbor-edn-literals]
Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
literals-16, 8 January 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-literals-16>.
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[I-D.ietf-cbor-packed]
Bormann, C. and M. Gütschow, "Packed CBOR", Work in
Progress, Internet-Draft, draft-ietf-cbor-packed-13, 1
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-cbor-packed-13>.
[PSVI] "Use Cases for XML Schema PSVI API", 24 June 2002,
<https://www.w3.org/XML/2002/05/psvi-use-cases>.
[RFC9682] Bormann, C., "Updates to the Concise Data Definition
Language (CDDL) Grammar", RFC 9682, DOI 10.17487/RFC9682,
November 2024, <https://www.rfc-editor.org/rfc/rfc9682>.
[useful] "Useful CDDL", n.d.,
<https://github.com/cbor-wg/cddl/wiki/Useful-CDDL>.
Appendix A. Fridge
This appendix contains sections that may not make it to a 2.0
milestone, but might be part of a followup.
A.1. Tag-oriented Literals
_Proposal Status_: rough idea, porting from EDN
_Compatibility_: backward (not forward)
Some CBOR tags often would be most natural to use in a CDDL spec with
a literal syntax that is tailored to their semantics instead of the
serialization of their tag content in CBOR. There is currently no
way to add such syntaxes, no defined extension point either.
The specification "CBOR Extended Diagnostic Notation (EDN):
Application-Oriented Literals, ABNF, and Media Type"
[I-D.ietf-cbor-edn-literals] defines application-oriented literals,
e.g., of the form
dt'2019-07-21T19:53Z'
for datetime items. With additional considerations for unambiguous
syntax, a similar literal form could be included in CDDL.
This proposal opens a namespace for the prefix that indicates an
application specific literal. A registry could be provided to turn
this namespace into a genuine extension point. (This is currently
the production bsqual in Appendix B of [RFC8610].)
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The syntax provided in [I-D.ietf-cbor-edn-literals] does not enable
the use of named CDDL rules — using it directly in CDDL would have
the same flaw that is being fixed for tag numbers in Section 3.2 of
[RFC9682].
A.2. Cross-universe references
Often, a CDDL specification needs to import from specifications in a
different language or platform.
A.2.1. IANA references
In many cases, CDDL specifications make use of values that are
specified in IANA registries. The proposed .iana control operator
can be used to reference such a set of values.
The reference needs to be able to point to a draft, the registry of
which has not been established yet, as well as to an established IANA
registry.
An example of such a usage might be:
cose-algorithm = int .iana ["cose", "algorithms", "value"]
Unfortunately, the vocabulary employed in IANA registries has not
been designed for machine references. In this case, the potential
values would come from applying the XPath expression
//iana:registry[@id='algorithms']/iana:record/iana:value
to https://www.iana.org/assignments/cose/cose.xml, plus some
filtering on the records returned that only leaves actual
allocations. Section 3.1 of [I-D.bormann-cbor-rfc-cddl-models]
contains an example of a CDDL module that is automatically generated
from those assignments. (The code for this extraction is available
in the document source repository of
[I-D.bormann-cbor-rfc-cddl-models] as [EXTRACT-RB].)
Additional functionality may be needed for filtering with respect to
other columns of the registry record, e.g., <capabilities> in the
case of this example.
Acknowledgements
TBD
Author's Address
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Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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