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CDDL evolution 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.

Introduction 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 , or the specifications exercising its extension points, such as . Significant parts of this draft have now moved over to . The remaining items in this draft are not directly related to the CDDL 2.0 effort. There is always a danger for a document like this to become a shopping list; the intention is to develop this document further based on the rapidly growing real-world experience with the first CDDL standard.

Base language features

Cuts alludes to a new language feature, cuts, and defines it in a fashion that is rather focused on a single application in the context of maps and generating better diagnostic information about them. The present document is expected to grow a more complete definition of cuts, with the expectation that it will be upwards-compatible to the existing one in , before this possibly becomes a mainline language feature in a future version of CDDL.

Literal syntax

Regular Expression Literals Regular expressions currently are notated as strings in CDDL, with all the string escaping rules applied once. It might be convenient to have a more conventional literal format for regular expressions, possibly also providing a place to add modifiers such as /i. This might also imply text .regexp ..., which with the proposal in then raises the question of how to indicate the regular expression flavor. (With the support for ABNF in , the need for this is reduced. Also, the proliferation of regular expression flavors is hard to address with a single syntax.)

Controls Controls are the main extension point of the CDDL language. It is relatively painless to add controls to CDDL; this mechanism has been exercised in for SDNV and ASN.1 OID related byte strings, and in for more generally applicable controls, including an interface to ABNF . Several further candidates have been identified that aren't quite ready for adoption, of which a few shall be listed here.

Control operator .pcre There are many variants of regular expression languages. defines the .regexp control, which is based on XSD regular expressions. As discussed in that section, the most desirable form of regular expressions in many cases is the family called "Perl-Compatible Regular Expressions" (); however, no formally stable definition of PCRE is available at this time for normatively referencing it from an RFC. The present document defines the control operator .pcre, which is similar to .regexp, but uses PCRE2 regular expressions. More specifically, a .pcre control indicates that the text string given as a target needs to match the PCRE regular expression given as a value in the control type, where that regular expression is anchored on both sides. (If anchoring is not desired for a side, .* needs to be inserted there.) Similarly, .es2018re could be defined for ECMAscript 2018 regular expressions with anchors added. See also , which could be specifically called out via .iregexp (even though .regexp as per would also have the same semantics, except for a wider range of regexps).

Endianness in .bits How useful would it be to have another variant of .bits that counts bits like in RFC box notation? (Or at least per-byte? 32-bit words don't always perfectly mesh with byte strings.)

.bitfield control Provide a way to specify bitfields in byte strings and uints to a higher level of detail than is possible with .bits. Strawman: Note that the group within the controlling array can have choices, enabling the whole power of a context-free grammar (but not much more).

Co-occurrence Constraints While there are no co-occurrence constraints in CDDL, many actual use cases can be addressed by using the fact that a group is a grammar: However, constraints that are not just structural/tree-based but are predicates combining parts of the structure cannot be expressed: As a minimum, this requires the ability to reach over to other parts of the tree in a control. Compare JSON Pointer and JSON Relative Pointer . Stefan Goessner's jsonpath is a JSON variant of XPath that is now undergoing standardization . More generally, something akin to what Schematron is to Relax-NG may be needed.

Alternative Representations For CDDL, alternative representations e.g. in JSON (and thus in YAML) could be defined, similar to the way YANG defines an XML-based serialization called YIN in . One proposal for such a syntax is provided by the cddlc tool , which is reproduced below. This could be written up in more detail and agreed upon. The "prim"-labeled array includes support for non-literal tag numbers ().

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 Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document extends the base definition of ABNF (Augmented Backus-Naur Form) to include a way to specify US-ASCII string literals that are matched in a case-sensitive manner. The Concise Binary Object Representation (CBOR), defined in RFC 8949, 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. This document defines CBOR tags for object identifiers (OIDs) and is the reference document for the IANA registration of the CBOR tags so defined. Self-Delimiting Numeric Values (SDNVs) have recently been introduced as a field type in proposed Delay-Tolerant Networking protocols. SDNVs encode an arbitrary-length non-negative integer or arbitrary- length bitstring with minimum overhead. They are intended to provide protocol flexibility without sacrificing economy and to assist in future-proofing protocols under development. This document describes formats and algorithms for SDNV encoding and decoding, along with notes on implementation and usage. This document is a product of the Delay-Tolerant Networking Research Group and has been reviewed by that group. No objections to its publication as an RFC were raised. This document is not an Internet Standards Track specification; it is published for informational purposes. n.d. Universität Bremen TZI Textuality This document specifies I-Regexp, a flavor of regular expressions that is limited in scope with the goal of interoperation across many different regular-expression libraries. n.d. n.d. Universität Bremen TZI 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. JSON Pointer defines a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) document. JSON Pointer is a syntax for specifying locations in a JSON document, starting from the document root. This document defines an extension to the JSON Pointer syntax, allowing relative locations from within the document. YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS-TRACK]

Acknowledgements Many people have asked for CDDL to be completed, soon. These are usually also the people who have brought up observations that led to the proposals discussed here. Sean Leonard has campaigned for a regexp literal syntax.