Internet-Draft | CDDL control operators | June 2024 |
Bormann | Expires 14 December 2024 | [Page] |
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.¶
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.¶
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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:¶
Name | Purpose |
---|---|
.b64u , .b64c
|
Base64 representation of byte strings |
.b64u-sloppy , .b64c-sloppy
|
(sloppy-tolerant variants of the above) |
.hex , .hexlc , .hexuc
|
Base16 representation of byte strings |
.b32 , .h32
|
Base32 representation of byte strings |
.b45
|
Base45 representation of byte strings |
.decimal
|
Text representation of integer numbers |
.printf
|
Printf-formatted text representation of data items |
.json
|
Text representation of JSON values |
.join
|
Building text from array of components |
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 BCP 14 [BCP14] 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".¶
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:¶
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).¶
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.¶
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"
.¶
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].¶
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.¶
Often, text strings need to be constructed out of parts that can best be modeled as an array.¶
name | meaning | reference |
---|---|---|
.join
|
concatenate elements of an array | --- |
For example, an IPv4 address in dotted-decimal might be modeled as in Figure 1.¶
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.¶
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]:¶
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] |
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.¶
Henk Birkholz suggested the need for many of the control operators defined here.¶