Internet-Draft CDDL control operators July 2024
Bormann Expires 22 January 2025 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-cbor-cddl-more-control-06
Published:
Intended Status:
Standards Track
Expires:
Author:
C. Bormann
Universität Bremen TZI

More Control Operators for CDDL

Abstract

The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. RFCs have added to this extension point both in an application-specific and a more general way.

The present document defines a number of additional generally applicable control operators for text conversion (Bytes, Integers, JSON, Printf-style formatting) and for an operation on text.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at https://cbor-wg.github.io/cddl-more-control/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-cbor-cddl-more-control/.

Discussion of this document takes place on the Concise Binary Object Representation (CBOR) Maintenance and Extensions Working Group mailing list (mailto:cbor@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at https://www.ietf.org/mailman/listinfo/cbor/.

Source for this draft and an issue tracker can be found at https://github.com/cbor-wg/cddl-more-control.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on 22 January 2025.

1. Introduction

The Concise Data Definition Language (CDDL), standardized in [RFC8610], provides "control operators" as its main language extension point (Section 3.8 of [RFC8610]). RFCs have added to this extension point both in an application-specific [RFC9090] and a more general [RFC9165] way.

The present document defines a number of additional generally applicable control operators:

Table 1: New control operators in this document,
t = target type (left-hand side), c = controller type (right-hand side)
Name t c Purpose
.b64u, .b64c text bytes Base64 representation of byte strings
.b64u-sloppy, .b64c-sloppy text bytes (sloppy-tolerant variants of the above)
.hex, .hexlc, .hexuc text bytes Base16 representation of byte strings
.b32, .h32 text bytes Base32 representation of byte strings
.b45 text bytes Base45 representation of byte strings
.decimal text int Text representation of integer numbers
.printf text array Printf-formatted text representation of data items
.json text any Text representation of JSON values
.join text or bytes array Build text or byte string from array of components

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. Text Conversion

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:

Table 2: Control Operators for Text Conversion of byte strings
name meaning reference
.b64u Base64URL, no padding Section 5 of [RFC4648]
.b64u-sloppy Base64URL, no padding, sloppy Section 5 of [RFC4648]
.b64c Base64 classic, padding Section 4 of [RFC4648]
.b64c-sloppy Base64 classic, padding, sloppy Section 4 of [RFC4648]
.b32 Base32, no padding Section 6 of [RFC4648]
.h32 Base32/hex alphabet, no padding Section 7 of [RFC4648]
.hex Base16 (hex), either case Section 8 of [RFC4648]
.hexlc Base16 (hex), lower case Section 8 of [RFC4648]
.hexuc Base16 (hex), upper case Section 8 of [RFC4648]
.b45 Base45 [RFC9285]

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 do validate the mandates of their base documents. Note that this also means that .b64u and .b64c only accept the alphabets defined for each of them, respectively; this is maybe worth pointing out here explicitly as CDDL's "b64" literal prefix simply accepts either alphabet and this behavior is different from that of these control operators.

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

Table 3: Control Operator for Text Conversion of Integers
name meaning reference
.decimal Decimal Integer ---

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

Table 4: Control Operator for Printf-formatting of data item(s)
name meaning reference
.printf Printf-formatting of data item(s) ---

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 into text strings, the JSON value for which can easily be defined in CDDL. This is supported by a control operator similar to .cbor in Section 3.8.4 of [RFC8610].

Table 5: Control Operator for Text Conversion of JSON values
name meaning reference
.json JSON [STD90]
embedded-claims = text .json claims
claims = {iss: text, exp: text}

Note that a .jsonseq is not provided for [RFC7464], as no use case for inclusion in CDDL is known yet.

There is no way to constrain the use of blank space in data items to be validated; variants (e.g, not providing for any blank space) could be defined.

3. Text Processing

3.1. Join

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

Table 6: Control Operator for Text Generation from Arrays
name meaning reference
.join concatenate elements of 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
Figure 1: Using the .join operator to build dotted-decimal IPv4 addresses

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 MUST be valid UTF-8.

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.

4. IANA Considerations

RFC Editor: please replace RFC-XXXX with the RFC number of this RFC and remove this note.

This document requests IANA to register the contents of Table 7 into the registry "CDDL Control Operators" of [IANA.cddl]:

Table 7: New control operators to be registered
Name Reference
.b64u [RFC-XXXX]
.b64u-sloppy [RFC-XXXX]
.b64c [RFC-XXXX]
.b64c-sloppy [RFC-XXXX]
.b45 [RFC-XXXX]
.b32 [RFC-XXXX]
.h32 [RFC-XXXX]
.hex [RFC-XXXX]
.hexlc [RFC-XXXX]
.hexuc [RFC-XXXX]
.decimal [RFC-XXXX]
.printf [RFC-XXXX]
.json [RFC-XXXX]
.join [RFC-XXXX]

5. Implementation Status

This section is to be removed before publishing as an RFC.

In the CDDL tool described in Appendix F of [RFC8610], the control operators defined in the present revision of this specification are implemented as of version 0.10.4.

6. Security considerations

The security considerations of [RFC8610] apply.

7. References

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, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[C]
International Organization for Standardization, "Information technology — Programming languages — C", Fourth Edition, ISO/IEC 9899:2018, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc7464>.
[RFC7951]
Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, , <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, , <https://www.rfc-editor.org/rfc/rfc9090>.

Acknowledgements

Henk Birkholz suggested the need for many of the control operators defined here.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany