CDDL 2.0 -- a draft plan
draft-bormann-cbor-cddl-2-draft-02
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| Author | Carsten Bormann | ||
| Last updated | 2023-03-10 | ||
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draft-bormann-cbor-cddl-2-draft-02
CBOR Working group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 10 March 2023
Expires: 11 September 2023
CDDL 2.0 — a draft plan
draft-bormann-cbor-cddl-2-draft-02
Abstract
The Concise Data Definition Language (CDDL) today is defined by
RFC 8610 and RFC 9165. 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
based on draft-bormann-cbor-cddl-freezer, but is more selective in
what potential features it takes up and more detailed in their
discussion. It is intended to serve as a basis for prototypical
implementations of CDDL 2.0. What specific documents spawn from the
present one or whether this document is evolved into a single
CDDL 2.0 specification.
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
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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.
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. CDDL 1.1 + 2 plan (standards track) . . . . . . . . . . . 3
1.2. Other documents . . . . . . . . . . . . . . . . . . . . . 3
2. Mending syntax deficits . . . . . . . . . . . . . . . . . . . 4
2.1. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 4
3. Processing model: Beyond Validation . . . . . . . . . . . . . 4
4. Module superstructure . . . . . . . . . . . . . . . . . . . . 5
4.1. Cross-universe references . . . . . . . . . . . . . . . . 5
4.2. ABNF is a lot like CDDL . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security considerations . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Fridge . . . . . . . . . . . . . . . . . . . . . . . 7
A.1. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 7
A.2. Cross-universe references . . . . . . . . . . . . . . . . 8
A.2.1. IANA references . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
(Please see abstract.)
Note that the existing extension point can be exercised for new
features in parallel to the work described here. One such draft,
[I-D.bormann-cbor-cddl-more-control], is planned to form the first
set of specifications going forward from the CDDL-2 project together
with [I-D.bormann-cbor-update-8610-grammar].
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)
* Done before *IETF 117*: CDDL 1.1:
[I-D.bormann-cbor-update-8610-grammar], _Grammar_ fixes: Empty
files (enabling CDDL 2), non-literal tags, errata fixes
(implemented)
* Done before *IETF 117*: Parallel to CDDL 1.1: More _control_
operators [I-D.bormann-cbor-cddl-more-control]: Additional control
operators, another iteration like RFC 9165 (implemented)
* Done before *IETF 118*: CDDL 2.0: [I-D.bormann-cbor-cddl-modules]
(import/include implemented; potentially further directives to be
added)
* Done *2024*: CDDL 2.5: Section 3 of the present document
("_annotations_", plus some functionality enabled by that). The
requirements are clear, 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, Mid-2023): Section 5 of
[I-D.bormann-cbor-cddl-freezer]: CDDL-in-JSON format(s) for
interchange of CDDL model information between tools.
* (Informational, with [I-D.bormann-cbor-cddl-modules]):
[I-D.bormann-cbor-rfc-cddl-models] (builds standard collection of
referenceable models).
* (BCP?): [I-D.bormann-cbor-draft-numbers] (BCP for handling
assigned numbers during draft stage)
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More explorative at this point:
* (Standards-Track?) The remaining Section 2 of this document:
application-oriented literals in CDDL; develop with
[I-D.bormann-cbor-edn-literals].
* (Informational or Standards-Track?): [I-D.bormann-cbor-cddl-csv]
(using CDDL to model CSV documents).
2. Mending syntax deficits
The previous content of this section formed the basis for
[I-D.bormann-cbor-update-8610-grammar], 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).
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]).
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.
At this time, existing experimental implementations do not lead to a
clear choice for what processing model enhancements should be in
CDDL 2.0. 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.bormann-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
(Insert new registry for application specific literals here, if
adopted.)
6. Security considerations
The security considerations of [RFC8610] apply.
7. References
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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
[I-D.bormann-cbor-cddl-csv]
Bormann, C., "Using CDDL for CSVs", Work in Progress,
Internet-Draft, draft-bormann-cbor-cddl-csv-02, 27
February 2023, <https://datatracker.ietf.org/doc/html/
draft-bormann-cbor-cddl-csv-02>.
[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-10, 24 October
2022, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-cddl-freezer-10>.
[I-D.bormann-cbor-cddl-modules]
Bormann, C., "CDDL Module Structure", Work in Progress,
Internet-Draft, draft-bormann-cbor-cddl-modules-00, 10
March 2023, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-cddl-modules-00>.
[I-D.bormann-cbor-cddl-more-control]
Bormann, C., "More Control Operators for CDDL", Work in
Progress, Internet-Draft, draft-bormann-cbor-cddl-more-
control-01, 9 March 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
cddl-more-control-01>.
[I-D.bormann-cbor-draft-numbers]
Bormann, C., "Managing CBOR numbers in Internet-Drafts",
Work in Progress, Internet-Draft, draft-bormann-cbor-
draft-numbers-00, 22 February 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
draft-numbers-00>.
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[I-D.bormann-cbor-edn-literals]
Bormann, C., "Application-Oriented Literals in CBOR
Extended Diagnostic Notation", Work in Progress, Internet-
Draft, draft-bormann-cbor-edn-literals-01, 24 October
2022, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-edn-literals-01>.
[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-01, 25 February 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
rfc-cddl-models-01>.
[I-D.bormann-cbor-update-8610-grammar]
Bormann, C., "Updates to the CDDL grammar of RFC 8610",
Work in Progress, Internet-Draft, draft-bormann-cbor-
update-8610-grammar-00, 9 March 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
update-8610-grammar-00>.
[PSVI] "Use Cases for XML Schema PSVI API", 24 June 2002,
<https://www.w3.org/XML/2002/05/psvi-use-cases>.
[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, 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 their
serialization in CBOR. There is currently no way to add such
syntaxes, no defined extension point either.
The proposal "Application-Oriented Literals in CBOR Extended
Diagnostic Notation" [I-D.bormann-cbor-edn-literals] defines
application-oriented literals, e.g., of the form
dt'2019-07-21T19:53Z'
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for datetime items. With additional considerations for unambiguous
syntax, a similar literal form could be included in CDDL.
This proposal opens a name space for the prefix that indicates an
application specific literal. A registry could be provided to make
this name space a genuine extension point. (This is currently the
production bsqual in Appendix B of [RFC8610].)
The syntax provided in [I-D.bormann-cbor-edn-literals] does not
enable the use of CDDL types — it has the same flaw that is being
fixed for tag numbers in Section 3.2 of
[I-D.bormann-cbor-update-8610-grammar].
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 .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. 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
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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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