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Internet-Draft 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. It is now time to discuss thawing some of the concepts discussed here. A number of additional proposals have been added.

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 . It is now time to discuss thawing some of the concepts discussed here. A number of additional proposals have been added. 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

Tag-oriented Literals 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. Based on the CoRAL work , the proposal "Application-Oriented Literals in CBOR Extended Diagnostic Notation" 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.

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.

Clarifications A number of errata reports have been made around some details of text string and byte string literal syntax: and . These need to be addressed by re-examining the details of these literal syntaxes. Also, needs to be applied.

Err6527 The ABNF used in for the content of text string literals is rather permissive: This allows almost any non-C0 character to be escaped by a backslash, but critically misses out on the \uXXXX and \uHHHH\uLLLL forms that JSON allows to specify characters in hex. Both can be solved by updating the SESC production to: Now that SESC is more restrictively formulated, this also requires an update to the BCHAR production used in the ABNF syntax for byte string literals: The updated version explicit allows \', which is no longer allowed in the updated SESC:

Err6543 The ABNF used in for the content of byte string literals lumps together byte strings notated as text with byte strings notated in base16 (hex) or base64 (but see also updated BCHAR production above): Errata report 6543 proposes to handle the two cases in separate productions (where, with an updated SESC, BCHAR obviously needs to be updated as above): This potentially causes a subtle change, which is hidden in the WS production: This allows any non-C0 character in a comment, so this fragment becomes possible: The current text is not unambiguously saying whether the three apostrophes need to be escaped with a \ or not, as in: ... which would be supported by the existing ABNF in .

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 has not been formally standardized yet . More generally, something akin to what Schematron is to Relax-NG may be needed.

Module superstructure CDDL rules could be packaged as modules and referenced from other modules. There could be some control of namespace pollution, as well as unambiguous referencing ("versioning"). This is probably best achieved by a pragma-like syntax which could be carried in CDDL comments, leaving each module to be valid CDDL (if missing some rule definitions to be imported).

Namespacing A convention for mapping CDDL-internal names to external ones could be developed, possibly steered by some pragma-like constructs. External names would likely be URI-based, with some conventions as they are used in RDF or Curies. Internal names might look similar to XML QNames. Note that the identifier character set for CDDL deliberately includes $ and @, which could be used in such a convention.

Cross-universe references Often, a CDDL specfication 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 .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. 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.

Potential examples This section shows some examples that illustrate potential syntaxes and semantics to be examined. One of the potential objectives here is to keep documents that make use of this extension generally valid as CDDL 1.0 documents, albeit possibly with a need to add further CDDL 1.0 rules to obtain a complete specification.

How name spaces might look like Implicit namespacing might be provided by using a document reference as a namespace tag: Note that this example establishes a namespace for the prelude (RFC8610.int); maybe it is worth to do that more explicitly.

Explicitly interacting with namespaces New syntax for explicitly interacting with namespaces might be but into RFC 8610 comments, with a specific prefix (and possibly starting left-aligned). Prefixes proposed include ;;< and ;#; the below will use ;# even though that probably could pose too many conflicts; it also might be too inconspicuous. Besides an implicit import such as there also could be an explicit import syntax: Such an explicit syntax might also be able to provide additional parameters such as in the IANA examples above.

Document references A convention for establishing RFC references might be easy to establish, but at least Internet-Draft references and IANA registry references should also supported. It is probably worth to add some indirection here, as names of Internet-Drafts might change (including by becoming RFCs).

Add retroactive exporting to RFCs Existing RFCs with CDDL in them could presume an export ...all... as RFCnnnn (Possibly also per-section exports as in RFC8610.D for the prelude?) Namespace tags for those exports need to be reserved so they cannot be occupied by explicit exporting. New specifications (including RFCs) can "include"/"import" from these namespaces, and maybe "export" their own rules in a more considered way.

Operations

• "export":
  1. prefix: add a namespace name to "local" rulenames:
  2. make that namespace available to other specs
• "import": include (prefixed) definitions from a source
  1. use as is: RFC9090.oid
  2. unprefix: oid
  Example: prelude processing -- include+unprefix from Appendix D of RFC8610.
• "include": find files, turn into namespaces to import from

To be discussed How to find the document that exports a namespace (IANA? Use by other SDOs? Internal use in an org? How to transition between these states?) Multiple documents exporting into one namespace (Immutable RFC9090 namespace vs. "OID"-namespace? Who manages mutable namespaces?) Updates, revisions, versions, semver: 2.2 ; twiddle-wakka: this version or higher ]]>

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.

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 "Regular expressions" (regexps) are a set of related, widely implemented pattern languages used in data modeling formats and query languages that is available in many dialects. This specification defines an interoperable flavor of regexps, I-Regexp. The present version -01 of this document is a slight update of the original trial balloon, meant to determine whether this approach is useful for the JSONPath WG. n.d. n.d. ARM The Constrained RESTful Application Language (CoRAL) defines a data model and interaction model as well as a compact serialization formats for the description of typed connections between resources on the Web ("links"), possible operations on such resources ("forms"), and simple resource metadata. Universität Bremen TZI The Concise Binary Object Representation, CBOR (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 the use of CBOR diagnostic notation with CoRAL and Constrained Resource Identifiers (draft-ietf-core-coral, draft-ietf-core-href). 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.