More Control Operators for CDDL

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all capitals, as shown here.¶

Regular expressions mentioned in the text are as defined in [RFC9485].¶

This specification uses terminology from [RFC8610]. In particular, with respect to control operators, "target" refers to the left-hand side operand, and "controller" to the right-hand side operand. "Tool" refers to tools along the lines of that described in Appendix F of [RFC8610]. Note also that the data model underlying CDDL provides for text strings as well as byte strings as two separate types, which are then collectively referred to as "strings".¶

2.1. Byte Strings: Base16 (Hex), Base32, Base45, Base64

A CDDL model often defines data that are byte strings in essence but need to be transported in various encoded forms, such as base64 or hex. This section defines a number of control operators to model these conversions.¶

The control operators generally are of a form that could be used like this:¶

signature-for-json = text .b64u signature
signature = bytes .cbor COSE_Sign1

The specification of these control operators is complicated by the large number of transformations in use. Inspired by Section 8 of RFC 8949 [STD94], we use representations defined in [RFC4648] with the following names:¶

Note that this specification is somewhat opinionated here: It does not provide base64url, base32 or base32hex encoding with padding, or base64 classic without padding. Experience indicates that these combinations only ever occur in error, so the usability of CDDL is increased by not providing them in the first place. Also, adding "c" makes sure that any decision for classic base64 is actively taken.¶

These control operators are "strict" in their matching, i.e., they only match base encodings that conform to the mandates of their defining documents. Note that this also means that .b64u and .b64c only match text strings composed of the set of characters defined for each of them, respectively. (This is maybe worth pointing out here explicitly as this contrasts with the "b64" literal prefix that can be used to notate byte strings in CDDL source code, which simply accepts characters from either alphabet. This behavior is different from the matching behavior of the four base64 control operators defined here.)¶

The additional designation "sloppy" indicates that the text string is not validated for any additional bits being zero, in variance to what is specified in the paragraph behind table 1 in Section 4 of [RFC4648]. Note that the present specification is opinionated again in not specifying a sloppy variant of base32 or base32/hex, as no legacy use of sloppy base32(/hex) was known at the time of writing. Base45 is known to be suboptimal for use in environments with limited data transparency (such as URLs), but is included because of its close relationship to QR codes and its wide use in health informatics (note that base45 is strongly specified not to allow sloppy forms of encoding).¶

2.2. Numbers

The control operator .decimal allows the modeling of text strings that carry numeric information in decimal form, such as in the uint64/int64 formats of YANG-JSON [RFC7951].¶

yang-json-sid = text .decimal (0..9223372036854775807)

Again, the specification is opinionated by only providing integer numbers without leading zeros, i.e., the decimal numbers match the regular expression 0|-?[1-9][0-9]* (of course, further restricted by the control type). See the next section for more flexibility, and for octal, hexadecimal, or binary conversions.¶

2.3. Printf-style Formatting

The control operator .printf allows the modeling of text strings that carry various formatted information, as long as the format can be represented in Printf-style formatting strings as they are used in the C language (see Section 7.21.6.1 of [C]).¶

The controller (right-hand side) of the .printf control is an array of one Printf-style format string and zero or more data items that fit the individual conversion specifications in the format string. The construct matches a text string representing the textual output of an equivalent C-language printf function call that is given the format string and the data items following it in the array.¶

From the printf specification in the C language, length modifiers (paragraph 7) are not used and MUST NOT be included in the format string. The 's' conversion specifier (paragraph 8) is used to interpolate a text string in UTF-8 form. The 'c' conversion specifier (paragraph 8) represents a single Unicode scalar value as a UTF-8 character. The 'p' and 'n' conversion specifiers (paragraph 8) are not used and MUST NOT be included in the format string.¶

In the following example, my_alg_19 matches the text string "0x0013":¶

my_alg_19 = hexlabel<19>
hexlabel<K> = text .printf (["0x%04x", K])

The data items in the controller array do not need to be literals, as for example in:¶

any_alg = hexlabel<1..20>
hexlabel<K> = text .printf (["0x%04x", K])

Here, any_alg matches the text strings "0x0013" or "0x0001" but not "0x1234".¶

2.4. JSON Values

Some applications store complete JSON texts [STD90] into text strings, the JSON value for which can easily be defined in CDDL by using the default JSON-to-CBOR conversion rules provided by Section 6.2 of RFC 8949 [STD94]. This is supported by a control operator similar to .cbor as defined in Section 3.8.4 of [RFC8610].¶

embedded-claims = text .json claims
claims = {iss: text, exp: text}

Notes:¶

• JSON has known interoperability problems [RFC7493]. While Section 4 of [RFC7493] probably is not relevant to this specification, Section 2 of [RFC7493] provides requirements that need to be followed to make use of the generic data model underlying CDDL. Note that the intention of Section 2.2 of [RFC7493] is directly supported by Section 6.2 of RFC 8949 [STD94]. The recommendation to use text strings for representing numbers outside JSON's interoperable range is a requirement on the application data model and therefore needs to be reflected on the right-hand side of the .json control operator.¶

• This control operator provides no way to constrain the use of blank space or other serialization variants in the JSON representation of the data items; restrictions on the serialization to specific variants (e.g, not providing for the addition of any insignificant blank space, prescribing an order in which map entries are serialized) could be defined in future control operators.¶

• A .jsonseq is not provided in this document for [RFC7464], as no use case for inclusion in CDDL is known at the time of writing; again, future control operators could address this use case.¶

3.1. Join

Often, text strings need to be constructed out of parts that can best be modeled as an array.¶

For example, an IPv4 address in dotted-decimal might be modeled as in Figure 1.¶

legacy-ip-address = text .join legacy-ip-address-elements
legacy-ip-address-elements = [bytetext, ".", bytetext, ".",
                              bytetext, ".", bytetext]
bytetext = text .decimal byte
byte = 0..255

The elements of the controller array need to be strings (text or byte strings). The control operator matches a data item if that data item is also a string, built by concatenating the strings in the array. The result of this concatenation is of the same kind of string (text or bytes) as the first element of the array. (If there is no element in the array, the .join construct matches either kind of empty string, obviously further constrained by the control operator target.) The concatenation is performed on the sequences of bytes in the strings. If the result of the concatenation is a text string, the resulting sequence of bytes only matches the target data item if that result is a valid text string (i.e., valid UTF-8; note that in contrast to the algorithm used in Section 3.2.3 of RFC 8949 [STD94] there is no need that all individual byte sequences going into the concatenation constitute valid text strings).¶

Note that this control operator is hard to validate in the most general case, as this would require full parser functionality. Simple implementation strategies will use array elements with constant values as guideposts ("markers", such as the "." in Figure 1) for isolating the variable elements that need further validation at the CDDL data model level. It is therefore recommended to limit the use of .join to simple arrangements where the array elements are laid out explicitly and there are no adjacent variable elements without intervening constant values, and where these constant values do not occur within the text described by the variable elements.
If more complex parsing functionality is required, the ABNF control operators (see Section 3 of [RFC9165]) may be useful; however, these cannot reach back into CDDL-specified elements like .join can do.¶

7.1. Normative References

[BCP14]

Best Current Practice 14, <https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[C]

International Organization for Standardization, "Information technology — Programming languages — C", Fourth Edition, ISO/IEC 9899:2018, June 2018, <https://www.iso.org/standard/74528.html>.
Technically equivalent specification text is available at https://web.archive.org/web/20181230041359if_/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf

[IANA.cddl]

IANA, "Concise Data Definition Language (CDDL)", <https://www.iana.org/assignments/cddl>.

[RFC4648]

Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/rfc/rfc4648>.

[RFC8610]

Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

[RFC9165]

Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, December 2021, <https://www.rfc-editor.org/rfc/rfc9165>.

[RFC9285]

Fältström, P., Ljunggren, F., and D.W. van Gulik, "The Base45 Data Encoding", RFC 9285, DOI 10.17487/RFC9285, August 2022, <https://www.rfc-editor.org/rfc/rfc9285>.

[RFC9485]

Bormann, C. and T. Bray, "I-Regexp: An Interoperable Regular Expression Format", RFC 9485, DOI 10.17487/RFC9485, October 2023, <https://www.rfc-editor.org/rfc/rfc9485>.

[STD90]

Internet Standard 90, <https://www.rfc-editor.org/info/std90>.
At the time of writing, this STD comprises the following:

Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/info/rfc8259>.

[STD94]

Internet Standard 94, <https://www.rfc-editor.org/info/std94>.
At the time of writing, this STD comprises the following:

Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, <https://www.rfc-editor.org/info/rfc8949>.

7.2. Informative References

[RFC7464]

Williams, N., "JavaScript Object Notation (JSON) Text Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015, <https://www.rfc-editor.org/rfc/rfc7464>.

[RFC7493]

Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, <https://www.rfc-editor.org/rfc/rfc7493>.

[RFC7951]

Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, August 2016, <https://www.rfc-editor.org/rfc/rfc7951>.

[RFC9090]

Bormann, C., "Concise Binary Object Representation (CBOR) Tags for Object Identifiers", RFC 9090, DOI 10.17487/RFC9090, July 2021, <https://www.rfc-editor.org/rfc/rfc9090>.