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CBOR Working group Internet-Draft 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. This document is intended to evolve over time; it might spawn specific documents and then retire or eventually be published as a roadmap document. About This Document Status information for this document may be found at . Discussion of this document takes place on the cbor Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction (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, , is planned to form the first set of specifications going forward from the CDDL-2 project together with . The rest of this introduction gives a rough overview over what could be the development plan for CDDL 1.1, 2.0, 2.5.

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": , 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 : Additional control operators, another iteration like RFC 9165 before. Passed WGLC, to be submitted to IESG after a final chair check.

CDDL 2.0 work:

• Technically complete before IETF 119: CDDL 2.0: (import/include directives, implemented). Feedback is available from IETF 119, one open technical issue (sockets); WGLC before IETF 121.
• 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.

"CDDL 2.5":

• Being prepared in 2024: CDDL 2.5: 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., (co-occurrence).

Other documents Not on the main line of development, but important ancillary work:

• (Informational, implemented): Section alternative representations of : CDDL-in-JSON format(s) for interchange of CDDL model information between tools.
• (Informational, companion to ): (builds standard collection of referenceable models).
• (BCP? Informational?): (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'' so diagnostic notation can refer to named numbers that are specified in CDDL (makes use of , implemented, see for an introduction).

More explorative at this point:

• (Standards-Track?) The remaining of this document: application-oriented literals in CDDL mirroring the work in .
• (Informational or Standards-Track?): (using CDDL to model CSV documents).

Important CBOR work that may be reflected in some CDDL extensions:

• Evolving Extended Diagnostic Notation . 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.
• Common Deterministic Encoding (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 . 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 that might need to be explored.

Mending syntax deficits The previous content of this section formed the basis for , except for .

Tag-oriented Literals Incomplete, see .

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"). 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 and for examples).

Annotations The cddl tool () 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" , here one where the data item itself is only augmented, not changed, by the process.) 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 ().

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.

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.

Module superstructure The previous content of this section formed the basis for . Additional work might be started on the ideas outlined in the subsections of this section.

Cross-universe references See .

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 (), it would be natural to make some of the functionality discussed in this section available for ABNF as well.

IANA Considerations This document makes no requests of IANA.

Security considerations The security considerations of apply.

References Normative References This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed:,, and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. Informative References Universität Bremen TZI In defining the Concise Data Definition Language (CDDL), some features have turned up that would be nice to have. In the interest of completing this specification in a timely manner, the present document was started to collect nice-to-have features that did not make it into the first RFC for CDDL, RFC 8610, or the specifications exercising its extension points, such as RFC 9165. Significant parts of this draft have now moved over to the CDDL 2.0 project, described in draft-bormann-cbor-cddl-2-draft. The remaining items in this draft are not directly related to the CDDL 2.0 effort. Universität Bremen TZI 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. Universität Bremen TZI The Concise Data Definition Language (CDDL), as defined in RFC 8610 and RFC 9165, provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON. The present document updates RFC 8610 by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there. Universität Bremen TZI The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. RFCs have added to this extension point both in an application-specific and a more general way. The present document defines a number of additional generally applicable control operators for text conversion (Bytes, Integers, JSON, Printf-style formatting) and for an operation on text. Universität Bremen TZI Arm Limited At the time of writing, the Concise Data Definition Language (CDDL) is defined by RFC 8610 and RFC 9165. The latter has used the extension point provided in RFC 8610, the _control operator_. As CDDL is being used in larger projects, the need for features has become known that cannot be easily mapped into this single extension point. The present document defines a backward- and forward-compatible way to add a module structure to CDDL. Universität Bremen TZI A number of CBOR- and JSON-based protocols have been defined before CDDL was standardized or widely used. This short draft records some CDDL definitions for such protocols, which could become part of a library of CDDL definitions available for use in CDDL2 processors. It focuses on CDDL in (almost) published IETF RFCs. Universität Bremen TZI CBOR-based protocols often make use of numbers allocated in a registry. During development of the protocols, those numbers may not yet be available. This impedes the generation of data models and examples that actually can be used by tools. This short draft proposes a common way to handle these situations, without any changes to existing tools. Also, in conjunction with the application-oriented EDN literal "e", a further reduction in editorial processing of CBOR examples around the time of approval can be achieved. Universität Bremen TZI Fraunhofer SIT The Concise Data Definition Language (CDDL), standardized in RFC 8610, is defined to provide data models for data shaped like JSON or CBOR. Another representation format that is quote popular is the CSV (Comma-Separated Values) file as defined by RFC 4180. The present document shows a way how to use CDDL to provide a data model for CSV files. Universität Bremen TZI TUD Dresden University of Technology The Concise Binary Object Representation (CBOR, RFC 8949 == STD 94) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. CBOR does not provide any forms of data compression. CBOR data items, in particular when generated from legacy data models, often allow considerable gains in compactness when applying data compression. While traditional data compression techniques such as DEFLATE (RFC 1951) can work well for CBOR encoded data items, their disadvantage is that the receiver needs to decompress the compressed form to make use of the data. This specification describes Packed CBOR, a simple transformation of a CBOR data item into another CBOR data item that is almost as easy to consume as the original CBOR data item. A separate decompression step is therefore often not required at the receiver. // The present version (-12) updates the IANA "Values for Tag // Numbers" table, sorting it and cleaning up the "Data Item" column. Universität Bremen TZI CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in its Section 4.2, providing some flexibility for application specific decisions. To facilitate Deterministic Encoding to be offered as a selectable feature of generic encoders, the present document defines a CBOR Common Deterministic Encoding (CDE) Profile that can be shared by a large set of applications with potentially diverging detailed requirements. It also defines "Basic Serialization", which stops short of the potentially more onerous requirements that make CDE fully deterministic, while employing most of its reductions of the variability needing to be handled by decoders. n.d.

Fridge This appendix contains sections that may not make it to a 2.0 milestone, but might be part of a followup.

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" 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 .) The syntax provided in 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 .

Cross-universe references Often, a CDDL specification needs to import from specifications in a different language or platform.

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: 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 to https://www.iana.org/assignments/cose/cose.xml, plus some filtering on the records returned that only leaves actual allocations. contains an example of a CDDL module that is automatically generated from those assignments. 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