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2 Network Working Group C. Bormann
3 Internet-Draft Universität Bremen TZI
4 Intended status: Standards Track 27 September 2021
5 Expires: 31 March 2022
7 Additional Control Operators for CDDL
8 draft-ietf-cbor-cddl-control-06
10 Abstract
12 The Concise Data Definition Language (CDDL), standardized in RFC
13 8610, provides "control operators" as its main language extension
14 point.
16 The present document defines a number of control operators that were
17 not yet ready at the time RFC 8610 was completed: .plus, .cat and
18 .det for the construction of constants, .abnf/.abnfb for including
19 ABNF (RFC 5234/RFC 7405) in CDDL specifications, and .feature for
20 indicating the use of a non-basic feature in an instance.
22 Status of This Memo
24 This Internet-Draft is submitted in full conformance with the
25 provisions of BCP 78 and BCP 79.
27 Internet-Drafts are working documents of the Internet Engineering
28 Task Force (IETF). Note that other groups may also distribute
29 working documents as Internet-Drafts. The list of current Internet-
30 Drafts is at https://datatracker.ietf.org/drafts/current/.
32 Internet-Drafts are draft documents valid for a maximum of six months
33 and may be updated, replaced, or obsoleted by other documents at any
34 time. It is inappropriate to use Internet-Drafts as reference
35 material or to cite them other than as "work in progress."
37 This Internet-Draft will expire on 31 March 2022.
39 Copyright Notice
41 Copyright (c) 2021 IETF Trust and the persons identified as the
42 document authors. All rights reserved.
44 This document is subject to BCP 78 and the IETF Trust's Legal
45 Provisions Relating to IETF Documents (https://trustee.ietf.org/
46 license-info) in effect on the date of publication of this document.
47 Please review these documents carefully, as they describe your rights
48 and restrictions with respect to this document. Code Components
49 extracted from this document must include Simplified BSD License text
50 as described in Section 4.e of the Trust Legal Provisions and are
51 provided without warranty as described in the Simplified BSD License.
53 Table of Contents
55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
56 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
57 2. Computed Literals . . . . . . . . . . . . . . . . . . . . . . 3
58 2.1. Numeric Addition . . . . . . . . . . . . . . . . . . . . 4
59 2.2. String Concatenation . . . . . . . . . . . . . . . . . . 4
60 2.3. String Concatenation with Dedenting . . . . . . . . . . . 5
61 3. Embedded ABNF . . . . . . . . . . . . . . . . . . . . . . . . 6
62 4. Features . . . . . . . . . . . . . . . . . . . . . . . . . . 8
63 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
64 6. Implementation Status . . . . . . . . . . . . . . . . . . . . 11
65 7. Security considerations . . . . . . . . . . . . . . . . . . . 11
66 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
67 8.1. Normative References . . . . . . . . . . . . . . . . . . 11
68 8.2. Informative References . . . . . . . . . . . . . . . . . 12
69 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
70 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
72 1. Introduction
74 The Concise Data Definition Language (CDDL), standardized in
75 [RFC8610], provides "control operators" as its main language
76 extension point (Section 3.8 of [RFC8610]).
78 The present document defines a number of control operators that were
79 not yet ready at the time RFC 8610 was completed:
81 +==========+=================================================+
82 | Name | Purpose |
83 +==========+=================================================+
84 | .plus | Numeric addition |
85 +----------+-------------------------------------------------+
86 | .cat | String Concatenation |
87 +----------+-------------------------------------------------+
88 | .det | String Concatenation, pre-dedenting |
89 +----------+-------------------------------------------------+
90 | .abnf | ABNF in CDDL (text strings) |
91 +----------+-------------------------------------------------+
92 | .abnfb | ABNF in CDDL (byte strings) |
93 +----------+-------------------------------------------------+
94 | .feature | Indicate name of feature used (extension point) |
95 +----------+-------------------------------------------------+
97 Table 1: New control operators in this document
99 1.1. Terminology
101 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
102 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
103 "OPTIONAL" in this document are to be interpreted as described in
104 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
105 capitals, as shown here.
107 This specification uses terminology from [RFC8610]. In particular,
108 with respect to control operators, "target" refers to the left hand
109 side operand, and "controller" to the right hand side operand.
110 "Tool" refers to tools along the lines of that described in
111 Appendix F of [RFC8610].
113 2. Computed Literals
115 CDDL as defined in [RFC8610] does not have any mechanisms to compute
116 literals. To cover a large part of the use cases, this specification
117 adds three control operators: .plus for numeric addition, .cat for
118 string concatenation, and .det for string concatenation with
119 dedenting of both sides (target and controller).
121 For these operators, as with all control operators, targets and
122 controllers are types. The resulting type is therefore formally a
123 function of the elements of the cross-product of the two types. Not
124 all tools may be able to work with non-unique targets or controllers.
126 2.1. Numeric Addition
128 In many cases in a specification, numbers are needed relative to a
129 base number. The .plus control identifies a number that is
130 constructed by adding the numeric values of the target and of the
131 controller.
133 Target and controller MUST be numeric. If the target is a floating
134 point number and the controller an integer number, or vice versa, the
135 sum is converted into the type of the target; converting from a
136 floating point number to an integer selects its floor (the largest
137 integer less than or equal to the floating point number).
139 interval = (
140 BASE => int ; lower bound
141 (BASE .plus 1) => int ; upper bound
142 ? (BASE .plus 2) => int ; tolerance
143 )
145 X = 0
146 Y = 3
147 rect = {
148 interval
149 interval
150 }
152 Figure 1: Example: addition to a base value
154 The example in Figure 1 contains the generic definition of a group
155 interval that gives a lower and an upper bound and optionally a
156 tolerance. rect combines two of these groups into a map, one group
157 for the X dimension and one for Y dimension.
159 2.2. String Concatenation
161 It is often useful to be able to compose string literals out of
162 component literals defined in different places in the specification.
164 The .cat control identifies a string that is built from a
165 concatenation of the target and the controller. Target and
166 controller MUST be strings. The result of the operation has the type
167 of the target. The concatenation is performed on the bytes in both
168 strings. If the target is a text string, the result of that
169 concatenation MUST be valid UTF-8.
171 a = "foo" .cat '
172 bar
173 baz
174 '
175 ; on a system where the newline is \n, is the same string as:
176 b = "foo\n bar\n baz\n"
178 Figure 2: Example: concatenation of text and byte string
180 The example in Figure 2 builds a text string named a out of
181 concatenating the target text string "foo" and the controller byte
182 string entered in a text form byte string literal. (This particular
183 idiom is useful when the text string contains newlines, which, as
184 shown in the example for b, may be harder to read when entered in the
185 format that the pure CDDL text string notation inherits from JSON.)
187 2.3. String Concatenation with Dedenting
189 Multi-line string literals for various applications, including
190 embedded ABNF (Section 3), need to be set flush left, at least
191 partially. Often, having some indentation in the source code for the
192 literal can promote readability, as in Figure 3.
194 oid = bytes .abnfb ("oid" .det cbor-tags-oid)
195 roid = bytes .abnfb ("roid" .det cbor-tags-oid)
197 cbor-tags-oid = '
198 oid = 1*arc
199 roid = *arc
200 arc = [nlsb] %x00-7f
201 nlsb = %x81-ff *%x80-ff
202 '
204 Figure 3: Example: dedenting concatenation
206 The control operator .det works like .cat, except that both arguments
207 (target and controller) are independently _dedented_ before the
208 concatenation takes place.
210 For the first rule in Figure 3, the result is equivalent to Figure 4.
212 oid = bytes .abnfb 'oid
213 oid = 1*arc
214 roid = *arc
215 arc = [nlsb] %x00-7f
216 nlsb = %x81-ff *%x80-ff
217 '
218 Figure 4: Dedenting example: result of first .det
220 For the purposes of this specification, we define dedenting as:
222 1. determining the smallest amount of left-most blank space (number
223 of leading space characters) in all the non-blank lines, and
225 2. removing exactly that number of leading space characters from
226 each line. For blank (blank space only or empty) lines, there
227 may be less (or no) leading space characters than this amount, in
228 which case all leading space is removed.
230 (The name .det is a shortcut for "dedenting cat". The maybe more
231 obvious name .dedcat has not been chosen as it is longer and may
232 invoke unpleasant images.)
234 Occasionally, dedenting of only a single item is needed. This can be
235 achieved by using this operator with an empty string, e.g., "" .det
236 rhs or lhs .det "", which can in turn be combined with a .cat: in the
237 construct lhs .cat ("" .det rhs), only rhs is dedented.
239 3. Embedded ABNF
241 Many IETF protocols define allowable values for their text strings in
242 ABNF [RFC5234] [RFC7405]. It is often desirable to define a text
243 string type in CDDL by employing existing ABNF embedded into the CDDL
244 specification. Without specific ABNF support in CDDL, that ABNF
245 would usually need to be translated into a regular expression (if
246 that is even possible).
248 ABNF is added to CDDL in the same way that regular expressions were
249 added: by defining a .abnf control operator. The target is usually
250 text or some restriction on it, the controller is the text of an ABNF
251 specification.
253 There are several small issues, with solutions given here:
255 * ABNF can be used to define byte sequences as well as UTF-8 text
256 strings interpreted as Unicode scalar sequences. This means this
257 specification defines two control operators: .abnfb for ABNF
258 denoting byte sequences and .abnf for denoting sequences of
259 Unicode scalar values (codepoint) represented as UTF-8 text
260 strings. Both control operators can be applied to targets of
261 either string type; the ABNF is applied to sequence of bytes in
262 the string interpreting that as a sequence of bytes (.abnfb) or as
263 a sequence of code points represented as an UTF-8 text string
264 (.abnf). The controller string MUST be a text string.
266 * ABNF defines a list of rules, not a single expression (called
267 "elements" in [RFC5234]). This is resolved by requiring the
268 controller string to be one valid "element", followed by zero or
269 more valid "rule" separated from the element by a newline; so the
270 controller string can be built by preceding a piece of valid ABNF
271 by an "element" that selects from that ABNF and a newline.
273 * For the same reason, ABNF requires newlines; specifying newlines
274 in CDDL text strings is tedious (and leads to essentially
275 unreadable ABNF). The workaround employs the .cat operator
276 introduced in Section 2.2 and the syntax for text in byte strings.
277 As is customary for ABNF, the syntax of ABNF itself (NOT the
278 syntax expressed in ABNF!) is relaxed to allow a single linefeed
279 as a newline:
281 CRLF = %x0A / %x0D.0A
283 * One set of rules provided in an ABNF specification is often used
284 in multiple positions, in particular staples such as DIGIT and
285 ALPHA. (Note that all rules referenced need to be defined in each
286 ABNF operator controller string -- there is no implicit import of
287 [RFC5234] Core ABNF or other rules.) The composition this calls
288 for can be provided by the .cat operator, and/or by .det if there
289 is indentation to be disposed of.
291 These points are combined into an example in Figure 5, which uses
292 ABNF from [RFC3339] to specify one each of the CBOR tags defined in
293 [RFC8943] and [RFC8949].
295 ; for RFC 8943
296 Tag1004 = #6.1004(text .abnf full-date)
297 ; for RFC 8949
298 Tag0 = #6.0(text .abnf date-time)
300 full-date = "full-date" .cat rfc3339
301 date-time = "date-time" .cat rfc3339
303 ; Note the trick of idiomatically starting with a newline, separating
304 ; off the element in the concatenations above from the rule-list
305 rfc3339 = '
306 date-fullyear = 4DIGIT
307 date-month = 2DIGIT ; 01-12
308 date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
309 ; month/year
310 time-hour = 2DIGIT ; 00-23
311 time-minute = 2DIGIT ; 00-59
312 time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec
313 ; rules
314 time-secfrac = "." 1*DIGIT
315 time-numoffset = ("+" / "-") time-hour ":" time-minute
316 time-offset = "Z" / time-numoffset
318 partial-time = time-hour ":" time-minute ":" time-second
319 [time-secfrac]
320 full-date = date-fullyear "-" date-month "-" date-mday
321 full-time = partial-time time-offset
323 date-time = full-date "T" full-time
324 ' .det rfc5234-core
326 rfc5234-core = '
327 DIGIT = %x30-39 ; 0-9
328 ; abbreviated here
329 '
331 Figure 5: Example: employing RFC 3339 ABNF for defining CBOR Tags
333 4. Features
335 Commonly, the kind of validation enabled by languages such as CDDL
336 provides a Boolean result: valid, or invalid.
338 In rapidly evolving environments, this is too simplistic. The data
339 models described by a CDDL specification may continually be enhanced
340 by additional features, and it would be useful even for a
341 specification that does not yet describe a specific future feature to
342 identify the extension point the feature can use, accepting such
343 extensions while marking them as such.
345 The .feature control annotates the target as making use of the
346 feature named by the controller. The latter will usually be a
347 string. A tool that validates an instance against that specification
348 may mark the instance as using a feature that is annotated by the
349 specification.
351 More specifically, the tool's diagnostic output might contain the
352 controller (right hand side) as a feature name, and the target (left
353 hand side) as a feature detail. However, in some cases, the target
354 has too much detail, and the specification might want to hint the
355 tool that more limited detail is appropriate. In this case, the
356 controller should be an array, with the first element being the
357 feature name (that would otherwise be the entire controller), and the
358 second element being the detail (usually another string), as
359 illustrated in Figure 6.
361 foo = {
362 kind: bar / baz .feature (["foo-extensions", "bazify"])
363 }
364 bar = ...
365 baz = ... ; complex stuff that doesn't all need to be in the detail
367 Figure 6: Providing explicit detail with .feature
369 Figure 7 shows what could be the definition of a person, with
370 potential extensions beyond name and organization being marked
371 further-person-extension. Extensions that are known at the time this
372 definition is written can be collected into $$person-extensions.
373 However, future extensions would be deemed invalid unless the
374 wildcard at the end of the map is added. These extensions could then
375 be specifically examined by a user or a tool that makes use of the
376 validation result; the label (map key) actually used makes a fine
377 feature detail for the tool's diagnostic output.
379 Leaving out the entire extension point would mean that instances that
380 make use of an extension would be marked as whole-sale invalid,
381 making the entire validation approach much less useful. Leaving the
382 extension point in, but not marking its use as special, would render
383 mistakes such as using the label organisation instead of organization
384 invisible.
386 person = {
387 ? name: text
388 ? organization: text
389 $$person-extensions
390 * (text .feature "further-person-extension") => any
391 }
393 $$person-extensions //= (? bloodgroup: text)
395 Figure 7: Map extensibility with .feature
397 Figure 8 shows another example where .feature provides for type
398 extensibility.
400 allowed-types = number / text / bool / null
401 / [* number] / [* text] / [* bool]
402 / (any .feature "allowed-type-extension")
404 Figure 8: Type extensibility with .feature
406 A CDDL tool may simply report the set of features being used; the
407 control then only provides information to the process requesting the
408 validation. One could also imagine a tool that takes arguments
409 allowing the tool to accept certain features and reject others
410 (enable/disable). The latter approach could for instance be used for
411 a JSON/CBOR switch, as illustrated in Figure 9.
413 SenML-Record = {
414 ; ...
415 ? v => number
416 ; ...
417 }
418 v = JC<"v", 2>
419 JC = J .feature "json" / C .feature "cbor"
421 Figure 9: Describing variants with .feature
423 It remains to be seen if the enable/disable approach can lead to new
424 idioms of using CDDL. The language currently has no way to enforce
425 mutually exclusive use of features, as would be needed in this
426 example.
428 5. IANA Considerations
430 This document requests IANA to register the contents of Table 2 into
431 the registry "CDDL Control Operators" of [IANA.cddl]:
433 +==========+===========+
434 | Name | Reference |
435 +==========+===========+
436 | .plus | [RFCthis] |
437 +----------+-----------+
438 | .cat | [RFCthis] |
439 +----------+-----------+
440 | .det | [RFCthis] |
441 +----------+-----------+
442 | .abnf | [RFCthis] |
443 +----------+-----------+
444 | .abnfb | [RFCthis] |
445 +----------+-----------+
446 | .feature | [RFCthis] |
447 +----------+-----------+
449 Table 2: New control
450 operators to be
451 registered
453 6. Implementation Status
455 This section is to be removed before publishing as an RFC.
457 An early implementation of the control operator .feature has been
458 available in the CDDL tool described in Appendix F of [RFC8610] since
459 version 0.8.11. The validator warns about each feature being used
460 and provides the set of target values used with the feature. The
461 other control operators defined in this specification are also
462 implemented as of version 0.8.21 and 0.8.26 (double-handed .det).
464 Andrew Weiss' [CDDL-RS] has an ongoing implementation of this draft
465 which is feature-complete except for the ABNF and dedenting support
466 (https://github.com/anweiss/cddl/pull/79
467 (https://github.com/anweiss/cddl/pull/79)).
469 7. Security considerations
471 The security considerations of [RFC8610] apply.
473 8. References
475 8.1. Normative References
477 [IANA.cddl]
478 IANA, "Concise Data Definition Language (CDDL)",
479 .
481 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
482 Requirement Levels", BCP 14, RFC 2119,
483 DOI 10.17487/RFC2119, March 1997,
484 .
486 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
487 Specifications: ABNF", STD 68, RFC 5234,
488 DOI 10.17487/RFC5234, January 2008,
489 .
491 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
492 RFC 7405, DOI 10.17487/RFC7405, December 2014,
493 .
495 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
496 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
497 May 2017, .
499 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
500 Definition Language (CDDL): A Notational Convention to
501 Express Concise Binary Object Representation (CBOR) and
502 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
503 June 2019, .
505 8.2. Informative References
507 [CDDL-RS] Weiss, A., "cddl-rs", n.d.,
508 .
510 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
511 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
512 .
514 [RFC8943] Jones, M., Nadalin, A., and J. Richter, "Concise Binary
515 Object Representation (CBOR) Tags for Date", RFC 8943,
516 DOI 10.17487/RFC8943, November 2020,
517 .
519 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
520 Representation (CBOR)", STD 94, RFC 8949,
521 DOI 10.17487/RFC8949, December 2020,
522 .
524 Acknowledgements
526 Jim Schaad suggested several improvements. The .feature feature was
527 developed out of a discussion with Henk Birkholz. Paul Kyzivat
528 helped isolate the need for .det.
530 .det is an abbreviation for "dedenting cat", but Det is also the name
531 of a German TV Cartoon character created in the 1960s.
533 Author's Address
535 Carsten Bormann
536 Universität Bremen TZI
537 Postfach 330440
538 D-28359 Bremen
539 Germany
541 Phone: +49-421-218-63921
542 Email: cabo@tzi.org