Additional Control Operators for CDDL

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Table of Contents

1. Introduction

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 did not make it into RFC 8610:¶

2. Computed Literals

CDDL as defined in [RFC8610] does not have any mechanisms to compute literals. As an 80 % solution, this specification adds two control operators: .cat for string concatenation, and .plus for numeric addition.¶

a = "foo" .cat '
  bar
  baz
'
; is the same string as:
b = "foo\n  bar\n  baz\n"

interval<BASE> = (
  BASE => int             ; lower bound
  (BASE .plus 1) => int   ; upper bound
  ? (BASE .plus 2) => int ; tolerance
)

X = 0
Y = 3
rect = {
  interval<X>
  interval<Y>
}

3. Embedded ABNF

Many IETF protocols define allowable values for their text strings in ABNF [RFC5234] [RFC7405]. It is often desirable to define a text string type in CDDL by employing existing ABNF embedded into the CDDL specification. Without specific ABNF support in CDDL, that ABNF would usually need to be translated into a regular expression (if that is even possible).¶

ABNF can directly be added to CDDL in the same way that regular expressions were added: by defining a .abnf control operator.¶

There are several small issues, with solutions given here:¶

• ABNF can be used to define byte sequences as well as UTF-8 text strings interpreted as Unicode scalar sequences. This means this specification defines two control operators: .abnfb for ABNF denoting byte sequences and .abnf for denoting sequences of Unicode scalar values (codepoint) represented UTF-8 text strings.¶
• ABNF defines a list of rules, not a single expression (called "elements" in [RFC5234]). This is resolved by requiring the control string to be one "element", followed by zero or more "rule".¶
• For the same reason, ABNF requires newlines; specifying newlines in CDDL text strings is tedious (and leads to essentially unreadable ABNF). The workaround employs the .cat operator introduced in Section 2.1 and the syntax for text in byte strings.¶
• One set of rules provided in an ABNF specification is often used in multiple positions, in particular staples such as DIGIT and ALPHA. The composition this calls for can also be provided by the .cat operator.¶

These points, combined into an example in Figure 3, which uses ABNF from [RFC3339] to specify the CBOR tags defined in [I-D.ietf-cbor-date-tag].¶

; for draft-ietf-cbor-date-tag
Tag1004 = #6.1004(text .abnf full-date)
; for RFC 7049
Tag0 = #6.0(text .abnf date-time)

full-date = "full-date" .cat rfc3339
date-time = "date-time" .cat rfc3339

; Note the trick of idiomatically starting with a newline, separating
;   off the element in the .cat from the rule-list
rfc3339 = '
   date-fullyear   = 4DIGIT
   date-month      = 2DIGIT  ; 01-12
   date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                             ; month/year
   time-hour       = 2DIGIT  ; 00-23
   time-minute     = 2DIGIT  ; 00-59
   time-second     = 2DIGIT  ; 00-58, 00-59, 00-60 based on leap sec
                             ; rules
   time-secfrac    = "." 1*DIGIT
   time-numoffset  = ("+" / "-") time-hour ":" time-minute
   time-offset     = "Z" / time-numoffset

   partial-time    = time-hour ":" time-minute ":" time-second
                     [time-secfrac]
   full-date       = date-fullyear "-" date-month "-" date-mday
   full-time       = partial-time time-offset

   date-time       = full-date "T" full-time
' .cat rfc5234-core

rfc5234-core = '
         DIGIT          =  %x30-39 ; 0-9
; abbreviated here
'

4. Features

Traditionally, the kind of validation enabled by languages such as CDDL provided a Boolean result: valid, or invalid.¶

In rapidly evolving environments, this is too simplistic. The data models described by a CDDL specification may continually be enhanced by additional features, and it would be useful even for a specification that does not yet describe a specific future feature to identify the extension point the feature can use, accepting such extensions while marking them as such.¶

The .feature control annotates the target as making use of the feature named by the controller. The latter will usually be a string. A tool that validates an instance against that specification may mark the instance as using a feature that is annotated by the specification.¶

Figure 4 shows what could be the definition of a person, with potential extensions beyond name and organization being marked further-person-extension. Extensions that are known at the time this definition is known can be collected into $$person-extensions. However, future extensions would be deemed invalid unless the wildcard at the end of the map is added. These extensions could then be specifically examined by a user or a tool that makes use of the validation result.¶

Leaving out the entire extension point would mean that instances that make use of an extension would be marked as whole-sale invalid, making the entire validation approach much less useful. Leaving the extension point in, but not marking its use as special, would render mistakes such as using the label organisation instead of organization invisible.¶

person = {
  ? name: text
  ? organization: text
  $$person-extensions
  * (text .feature "further-person-extension") => any
}

$$person-extensions //= (? bloodgroup: text)

5. IANA Considerations

This document requests IANA to register the contents of Table 2 into the CDDL Control Operators registry [IANA.cddl]:¶

6. Implementation Status

An early implementation of the control operator .feature has been available in the CDDL tool since version 0.8.11. The validator warns about each feature being used and provides the set of target values used with the feature.¶

7. Security considerations

The security considerations of [RFC8610] apply.¶

Acknowledgements

The .feature feature was developed out of a discussion with Henk Birkholz.¶

Author's Address