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Bormann 3 Internet-Draft Universität Bremen TZI 4 Intended status: Standards Track 22 October 2021 5 Expires: 25 April 2022 7 Additional Control Operators for CDDL 8 draft-ietf-cbor-cddl-control-07 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 25 April 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 . . . . . . . . . . . . . . . . . . . . . . . . . 12 67 8.1. Normative References . . . . . . . . . . . . . . . . . . 12 68 8.2. Informative References . . . . . . . . . . . . . . . . . 12 69 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13 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]. Note also that the data model underlying 112 CDDL provides for text strings as well as byte strings as two 113 separate types, which are then collectively referred to as "strings". 115 The term ABNF in this specification stands for the combination of 116 [RFC5234] and [RFC7405], i.e., the ABNF control operators defined by 117 this document allow use of the case-sensitive extensions defined in 118 [RFC7405]. 120 2. Computed Literals 122 CDDL as defined in [RFC8610] does not have any mechanisms to compute 123 literals. To cover a large part of the use cases, this specification 124 adds three control operators: .plus for numeric addition, .cat for 125 string concatenation, and .det for string concatenation with 126 dedenting of both sides (target and controller). 128 For these operators, as with all control operators, targets and 129 controllers are types. The resulting type is therefore formally a 130 function of the elements of the cross-product of the two types. Not 131 all tools may be able to work with non-unique targets or controllers. 133 2.1. Numeric Addition 135 In many cases in a specification, numbers are needed relative to a 136 base number. The .plus control identifies a number that is 137 constructed by adding the numeric values of the target and of the 138 controller. 140 Target and controller MUST be numeric. If the target is a floating 141 point number and the controller an integer number, or vice versa, the 142 sum is converted into the type of the target; converting from a 143 floating point number to an integer selects its floor (the largest 144 integer less than or equal to the floating point number, i.e., 145 rounding towards negative infinity). 147 interval = ( 148 BASE => int ; lower bound 149 (BASE .plus 1) => int ; upper bound 150 ? (BASE .plus 2) => int ; tolerance 151 ) 153 X = 0 154 Y = 3 155 rect = { 156 interval 157 interval 158 } 160 Figure 1: Example: addition to a base value 162 The example in Figure 1 contains the generic definition of a CDDL 163 group interval that gives a lower and an upper bound and optionally a 164 tolerance. The parameter BASE allows the non-conflicting use of 165 multiple of these interval groups in one map, by assigning different 166 labels to the entries of the interval. rect combines two of these 167 interval groups into a map, one group for the X dimension (using 0, 168 1, and 2 as labels) and one for Y dimension (using 3, 4, and 5 as 169 labels). 171 2.2. String Concatenation 173 It is often useful to be able to compose string literals out of 174 component literals defined in different places in the specification. 176 The .cat control identifies a string that is built from a 177 concatenation of the target and the controller. Target and 178 controller MUST be strings. The result of the operation has the type 179 of the target. The concatenation is performed on the bytes in both 180 strings. If the target is a text string, the result of that 181 concatenation MUST be valid UTF-8. 183 a = "foo" .cat ' 184 bar 185 baz 186 ' 187 ; on a system where the newline is \n, is the same string as: 188 b = "foo\n bar\n baz\n" 190 Figure 2: Example: concatenation of text and byte string 192 The example in Figure 2 builds a text string named a out of 193 concatenating the target text string "foo" and the controller byte 194 string entered in a text form byte string literal. (This particular 195 idiom is useful when the text string contains newlines, which, as 196 shown in the example for b, may be harder to read when entered in the 197 format that the pure CDDL text string notation inherits from JSON.) 199 2.3. String Concatenation with Dedenting 201 Multi-line string literals for various applications, including 202 embedded ABNF (Section 3), need to be set flush left, at least 203 partially. Often, having some indentation in the source code for the 204 literal can promote readability, as in Figure 3. 206 oid = bytes .abnfb ("oid" .det cbor-tags-oid) 207 roid = bytes .abnfb ("roid" .det cbor-tags-oid) 209 cbor-tags-oid = ' 210 oid = 1*arc 211 roid = *arc 212 arc = [nlsb] %x00-7f 213 nlsb = %x81-ff *%x80-ff 214 ' 216 Figure 3: Example: dedenting concatenation 218 The control operator .det works like .cat, except that both arguments 219 (target and controller) are independently _dedented_ before the 220 concatenation takes place. 222 For the first rule in Figure 3, the result is equivalent to Figure 4. 224 oid = bytes .abnfb 'oid 225 oid = 1*arc 226 roid = *arc 227 arc = [nlsb] %x00-7f 228 nlsb = %x81-ff *%x80-ff 229 ' 231 Figure 4: Dedenting example: result of first .det 233 For the purposes of this specification, we define dedenting as: 235 1. determining the smallest amount of left-most blank space (number 236 of leading space characters) present in all the non-blank lines, 237 and 239 2. removing exactly that number of leading space characters from 240 each line. For blank (blank space only or empty) lines, there 241 may be less (or no) leading space characters than this amount, in 242 which case all leading space is removed. 244 (The name .det is a shortcut for "dedenting cat". The maybe more 245 obvious name .dedcat has not been chosen as it is longer and may 246 invoke unpleasant images.) 248 Occasionally, dedenting of only a single item is needed. This can be 249 achieved by using this operator with an empty string, e.g., "" .det 250 rhs or lhs .det "", which can in turn be combined with a .cat: in the 251 construct lhs .cat ("" .det rhs), only rhs is dedented. 253 3. Embedded ABNF 255 Many IETF protocols define allowable values for their text strings in 256 ABNF [RFC5234] [RFC7405]. It is often desirable to define a text 257 string type in CDDL by employing existing ABNF embedded into the CDDL 258 specification. Without specific ABNF support in CDDL, that ABNF 259 would usually need to be translated into a regular expression (if 260 that is even possible). 262 ABNF is added to CDDL in the same way that regular expressions were 263 added: by defining a .abnf control operator. The target is usually 264 text or some restriction on it, the controller is the text of an ABNF 265 specification. 267 There are several small issues, with solutions given here: 269 * ABNF can be used to define byte sequences as well as UTF-8 text 270 strings interpreted as Unicode scalar sequences. This means this 271 specification defines two control operators: .abnfb for ABNF 272 denoting byte sequences and .abnf for denoting sequences of 273 Unicode scalar values (codepoint) represented as UTF-8 text 274 strings. Both control operators can be applied to targets of 275 either string type; the ABNF is applied to sequence of bytes in 276 the string interpreting that as a sequence of bytes (.abnfb) or as 277 a sequence of code points represented as an UTF-8 text string 278 (.abnf). The controller string MUST be a text string. 280 * ABNF defines a list of rules, not a single expression (called 281 "elements" in [RFC5234]). This is resolved by requiring the 282 controller string to be one valid "element", followed by zero or 283 more valid "rule" separated from the element by a newline; so the 284 controller string can be built by preceding a piece of valid ABNF 285 by an "element" that selects from that ABNF and a newline. 287 * For the same reason, ABNF requires newlines; specifying newlines 288 in CDDL text strings is tedious (and leads to essentially 289 unreadable ABNF). The workaround employs the .cat operator 290 introduced in Section 2.2 and the syntax for text in byte strings. 291 As is customary for ABNF, the syntax of ABNF itself (NOT the 292 syntax expressed in ABNF!) is relaxed to allow a single linefeed 293 as a newline: 295 CRLF = %x0A / %x0D.0A 297 * One set of rules provided in an ABNF specification is often used 298 in multiple positions, in particular staples such as DIGIT and 299 ALPHA. (Note that all rules referenced need to be defined in each 300 ABNF operator controller string -- there is no implicit import of 301 [RFC5234] Core ABNF or other rules.) The composition this calls 302 for can be provided by the .cat operator, and/or by .det if there 303 is indentation to be disposed of. 305 These points are combined into an example in Figure 5, which uses 306 ABNF from [RFC3339] to specify one each of the CBOR tags defined in 307 [RFC8943] and [RFC8949]. 309 ; for RFC 8943 310 Tag1004 = #6.1004(text .abnf full-date) 311 ; for RFC 8949 312 Tag0 = #6.0(text .abnf date-time) 314 full-date = "full-date" .cat rfc3339 315 date-time = "date-time" .cat rfc3339 317 ; Note the trick of idiomatically starting with a newline, separating 318 ; off the element in the concatenations above from the rule-list 319 rfc3339 = ' 320 date-fullyear = 4DIGIT 321 date-month = 2DIGIT ; 01-12 322 date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on 323 ; month/year 324 time-hour = 2DIGIT ; 00-23 325 time-minute = 2DIGIT ; 00-59 326 time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec 327 ; rules 328 time-secfrac = "." 1*DIGIT 329 time-numoffset = ("+" / "-") time-hour ":" time-minute 330 time-offset = "Z" / time-numoffset 332 partial-time = time-hour ":" time-minute ":" time-second 333 [time-secfrac] 334 full-date = date-fullyear "-" date-month "-" date-mday 335 full-time = partial-time time-offset 337 date-time = full-date "T" full-time 338 ' .det rfc5234-core 340 rfc5234-core = ' 341 DIGIT = %x30-39 ; 0-9 342 ; abbreviated here 343 ' 345 Figure 5: Example: employing RFC 3339 ABNF for defining CBOR Tags 347 4. Features 349 Commonly, the kind of validation enabled by languages such as CDDL 350 provides a Boolean result: valid, or invalid. 352 In rapidly evolving environments, this is too simplistic. The data 353 models described by a CDDL specification may continually be enhanced 354 by additional features, and it would be useful even for a 355 specification that does not yet describe a specific future feature to 356 identify the extension point the feature can use, accepting such 357 extensions while marking them as such. 359 The .feature control annotates the target as making use of the 360 feature named by the controller. The latter will usually be a 361 string. A tool that validates an instance against that specification 362 may mark the instance as using a feature that is annotated by the 363 specification. 365 More specifically, the tool's diagnostic output might contain the 366 controller (right-hand side) as a feature name, and the target (left- 367 hand side) as a feature detail. However, in some cases, the target 368 has too much detail, and the specification might want to hint the 369 tool that more limited detail is appropriate. In this case, the 370 controller should be an array, with the first element being the 371 feature name (that would otherwise be the entire controller), and the 372 second element being the detail (usually another string), as 373 illustrated in Figure 6. 375 foo = { 376 kind: bar / baz .feature (["foo-extensions", "bazify"]) 377 } 378 bar = ... 379 baz = ... ; complex stuff that doesn't all need to be in the detail 381 Figure 6: Providing explicit detail with .feature 383 Figure 7 shows what could be the definition of a person, with 384 potential extensions beyond name and organization being marked 385 further-person-extension. Extensions that are known at the time this 386 definition is written can be collected into $$person-extensions. 387 However, future extensions would be deemed invalid unless the 388 wildcard at the end of the map is added. These extensions could then 389 be specifically examined by a user or a tool that makes use of the 390 validation result; the label (map key) actually used makes a fine 391 feature detail for the tool's diagnostic output. 393 Leaving out the entire extension point would mean that instances that 394 make use of an extension would be marked as whole-sale invalid, 395 making the entire validation approach much less useful. Leaving the 396 extension point in, but not marking its use as special, would render 397 mistakes such as using the label "organisation" instead of 398 "organization" invisible. 400 person = { 401 ? name: text 402 ? organization: text 403 $$person-extensions 404 * (text .feature "further-person-extension") => any 405 } 407 $$person-extensions //= (? bloodgroup: text) 409 Figure 7: Map extensibility with .feature 411 Figure 8 shows another example where .feature provides for type 412 extensibility. 414 allowed-types = number / text / bool / null 415 / [* number] / [* text] / [* bool] 416 / (any .feature "allowed-type-extension") 418 Figure 8: Type extensibility with .feature 420 A CDDL tool may simply report the set of features being used; the 421 control then only provides information to the process requesting the 422 validation. One could also imagine a tool that takes arguments 423 allowing the tool to accept certain features and reject others 424 (enable/disable). The latter approach could for instance be used for 425 a JSON/CBOR switch, as illustrated in Figure 9, using SenML [RFC8428] 426 as the example data model used with both JSON and CBOR. 428 SenML-Record = { 429 ; ... 430 ? v => number 431 ; ... 432 } 433 v = JC<"v", 2> 434 JC = J .feature "json" / C .feature "cbor" 436 Figure 9: Describing variants with .feature 438 It remains to be seen if the enable/disable approach can lead to new 439 idioms of using CDDL. The language currently has no way to enforce 440 mutually exclusive use of features, as would be needed in this 441 example. 443 5. IANA Considerations 445 This document requests IANA to register the contents of Table 2 into 446 the registry "CDDL Control Operators" of [IANA.cddl]: 448 +==========+===========+ 449 | Name | Reference | 450 +==========+===========+ 451 | .plus | [RFCthis] | 452 +----------+-----------+ 453 | .cat | [RFCthis] | 454 +----------+-----------+ 455 | .det | [RFCthis] | 456 +----------+-----------+ 457 | .abnf | [RFCthis] | 458 +----------+-----------+ 459 | .abnfb | [RFCthis] | 460 +----------+-----------+ 461 | .feature | [RFCthis] | 462 +----------+-----------+ 464 Table 2: New control 465 operators to be 466 registered 468 6. Implementation Status 470 This section is to be removed before publishing as an RFC. 472 An early implementation of the control operator .feature has been 473 available in the CDDL tool described in Appendix F of [RFC8610] since 474 version 0.8.11. The validator warns about each feature being used 475 and provides the set of target values used with the feature. The 476 other control operators defined in this specification are also 477 implemented as of version 0.8.21 and 0.8.26 (double-handed .det). 479 Andrew Weiss' [CDDL-RS] has an ongoing implementation of this draft 480 which is feature-complete except for the ABNF and dedenting support 481 (https://github.com/anweiss/cddl/pull/79 482 (https://github.com/anweiss/cddl/pull/79)). 484 7. Security considerations 486 The security considerations of [RFC8610] apply. 488 While both [RFC5234] and [RFC7405] state that security is truly 489 believed to be irrelevant to the respective document, the use of 490 formal description techniques cannot only simplify, but sometimes 491 also complicate a specification. This can lead to security problems 492 in implementations and in the specification itself. As with CDDL 493 itself, ABNF should be judiciously applied, and overly complex (or 494 "cute") constructions should be avoided. 496 8. References 498 8.1. Normative References 500 [IANA.cddl] 501 IANA, "Concise Data Definition Language (CDDL)", 502 . 504 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 505 Requirement Levels", BCP 14, RFC 2119, 506 DOI 10.17487/RFC2119, March 1997, 507 . 509 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 510 Specifications: ABNF", STD 68, RFC 5234, 511 DOI 10.17487/RFC5234, January 2008, 512 . 514 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 515 RFC 7405, DOI 10.17487/RFC7405, December 2014, 516 . 518 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 519 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 520 May 2017, . 522 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data 523 Definition Language (CDDL): A Notational Convention to 524 Express Concise Binary Object Representation (CBOR) and 525 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, 526 June 2019, . 528 8.2. Informative References 530 [CDDL-RS] Weiss, A., "cddl-rs", n.d., 531 . 533 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 534 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 535 . 537 [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. 538 Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, 539 DOI 10.17487/RFC8428, August 2018, 540 . 542 [RFC8943] Jones, M., Nadalin, A., and J. Richter, "Concise Binary 543 Object Representation (CBOR) Tags for Date", RFC 8943, 544 DOI 10.17487/RFC8943, November 2020, 545 . 547 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object 548 Representation (CBOR)", STD 94, RFC 8949, 549 DOI 10.17487/RFC8949, December 2020, 550 . 552 Acknowledgements 554 Jim Schaad suggested several improvements. The .feature feature was 555 developed out of a discussion with Henk Birkholz. Paul Kyzivat 556 helped isolate the need for .det. 558 .det is an abbreviation for "dedenting cat", but Det is also the name 559 of a German TV Cartoon character created in the 1960s. 561 Author's Address 563 Carsten Bormann 564 Universität Bremen TZI 565 Postfach 330440 566 D-28359 Bremen 567 Germany 569 Phone: +49-421-218-63921 570 Email: cabo@tzi.org