RFC9165: Additional Control Operators for the Concise Data Definition Language (CDDL)
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Internet Engineering Task Force (IETF) C. Bormann
Request for Comments: 9165 Universität Bremen TZI
Category: Standards Track December 2021
ISSN: 2070-1721
Additional Control Operators for the Concise Data Definition Language
(CDDL)
Abstract
The Concise Data Definition Language (CDDL), standardized in RFC
8610, provides "control operators" as its main language extension
point.
The present document defines a number of control operators that were
not yet ready at the time RFC 8610 was completed: .plus, .cat, and
.det for the construction of constants; .abnf/.abnfb for including
ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and .feature for
indicating the use of a non-basic feature in an instance.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9165.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction
1.1. Terminology
2. Computed Literals
2.1. Numeric Addition
2.2. String Concatenation
2.3. String Concatenation with Dedenting
3. Embedded ABNF
4. Features
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Author's Address
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]).
The present document defines a number of control operators that were
not yet ready at the time [RFC8610] was completed:
+==========+==================================================+
| Name | Purpose |
+==========+==================================================+
| .plus | Numeric addition |
+----------+--------------------------------------------------+
| .cat | String concatenation |
+----------+--------------------------------------------------+
| .det | String concatenation, pre-dedenting |
+----------+--------------------------------------------------+
| .abnf | ABNF in CDDL (text strings) |
+----------+--------------------------------------------------+
| .abnfb | ABNF in CDDL (byte strings) |
+----------+--------------------------------------------------+
| .feature | Indicates name of feature used (extension point) |
+----------+--------------------------------------------------+
Table 1: New Control Operators in this Document
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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".
The term "ABNF" in this specification stands for the combination of
[RFC5234] and [RFC7405]; i.e., the ABNF control operators defined by
this document allow use of the case-sensitive extensions defined in
[RFC7405].
2. Computed Literals
CDDL as defined in [RFC8610] does not have any mechanisms to compute
literals. To cover a large part of the use cases, this specification
adds three control operators: .plus for numeric addition, .cat for
string concatenation, and .det for string concatenation with
dedenting of both sides (target and controller).
For these operators, as with all control operators, targets and
controllers are types. The resulting type is therefore formally a
function of the elements of the cross-product of the two types. Not
all tools may be able to work with non-unique targets or controllers.
2.1. Numeric Addition
In many cases, numbers are needed relative to a base number in a
specification. The .plus control identifies a number that is
constructed by adding the numeric values of the target and the
controller.
The target and controller both MUST be numeric. If the target is a
floating point number and the controller an integer number, or vice
versa, the sum is converted into the type of the target; converting
from a floating point number to an integer selects its floor (the
largest integer less than or equal to the floating point number,
i.e., rounding towards negative infinity).
interval<BASE> = (
BASE => int ; lower bound
(BASE .plus 1) => int ; upper bound
? (BASE .plus 2) => int ; tolerance
)
X = 0
Y = 3
rect = {
interval<X>
interval<Y>
}
Figure 1: An Example of Addition to a Base Value
The example in Figure 1 contains the generic definition of a CDDL
group interval that gives a lower and upper bound and, optionally, a
tolerance. The parameter BASE allows the non-conflicting use of a
multiple of these interval groups in one map by assigning different
labels to the entries of the interval. The rule rect combines two of
these interval groups into a map, one group for the X dimension
(using 0, 1, and 2 as labels) and one for the Y dimension (using 3,
4, and 5 as labels).
2.2. String Concatenation
It is often useful to be able to compose string literals out of
component literals defined in different places in the specification.
The .cat control identifies a string that is built from a
concatenation of the target and the controller. The target and
controller both MUST be strings. The result of the operation has the
same type as the target. The concatenation is performed on the bytes
in both strings. If the target is a text string, the result of that
concatenation MUST be valid UTF-8.
c = "foo" .cat '
bar
baz
'
; on a system where the newline is \n, is the same string as:
b = "foo\n bar\n baz\n"
Figure 2: An Example of Concatenation of Text and Byte Strings
The example in Figure 2 builds a text string named c from
concatenating the target text string "foo" and the controller byte
string entered in a text form byte string literal. (This particular
idiom is useful when the text string contains newlines, which, as
shown in the example for b, may be harder to read when entered in the
format that the pure CDDL text string notation inherits from JSON.)
2.3. String Concatenation with Dedenting
Multi-line string literals for various applications, including
embedded ABNF (Section 3), need to be set flush left, at least
partially. Often, having some indentation in the source code for the
literal can promote readability, as in Figure 3.
oid = bytes .abnfb ("oid" .det cbor-tags-oid)
roid = bytes .abnfb ("roid" .det cbor-tags-oid)
cbor-tags-oid = '
oid = 1*arc
roid = *arc
arc = [nlsb] %x00-7f
nlsb = %x81-ff *%x80-ff
'
Figure 3: An Example of Dedenting Concatenation
The control operator .det works like .cat, except that both arguments
(target and controller) are independently _dedented_ before the
concatenation takes place.
For the first rule in Figure 3, the result is equivalent to Figure 4.
oid = bytes .abnfb 'oid
oid = 1*arc
roid = *arc
arc = [nlsb] %x00-7f
nlsb = %x81-ff *%x80-ff
'
Figure 4: Dedenting Example: Result of First .det
For the purposes of this specification, we define "dedenting" as:
1. determining the smallest amount of leftmost blank space (number
of leading space characters) present in all the non-blank lines,
and
2. removing exactly that number of leading space characters from
each line. For blank (blank space only or empty) lines, there
may be fewer (or no) leading space characters than this amount,
in which case all leading space is removed.
(The name .det is a shortcut for "dedenting cat". The maybe more
obvious name .dedcat has not been chosen as it is longer and may
invoke unpleasant images.)
Occasionally, dedenting of only a single item is needed. This can be
achieved by using this operator with an empty string, e.g., "" .det
rhs or lhs .det "", which can in turn be combined with a .cat: in the
construct lhs .cat ("" .det rhs), only rhs is dedented.
3. Embedded ABNF
Many IETF protocols define allowable values for their text strings in
ABNF [RFC5234] [RFC7405]. It is often desirable to define a text
string type in CDDL by employing existing ABNF embedded into the CDDL
specification. Without specific ABNF support in CDDL, that ABNF
would usually need to be translated into a regular expression (if
that is even possible).
ABNF is added to CDDL in the same way that regular expressions were
added: by defining a .abnf control operator. The target is usually
text or some restriction on it, and the controller is the text of an
ABNF specification.
There are several small issues; the solutions are given here:
* ABNF can be used to define byte sequences as well as UTF-8 text
strings interpreted as Unicode scalar sequences. This means this
specification defines two control operators: .abnfb for ABNF
denoting byte sequences and .abnf for denoting sequences of
Unicode scalar values (code points) represented as UTF-8 text
strings. Both control operators can be applied to targets of
either string type; the ABNF is applied to the sequence of bytes
in the string and interprets it as a sequence of bytes (.abnfb) or
as a sequence of code points represented as an UTF-8 text string
(.abnf). The controller string MUST be a string. When a byte
string, it MUST be valid UTF-8 and is interpreted as the text
string that has the same sequence of bytes.
* ABNF defines a list of rules, not a single expression (called
"elements" in [RFC5234]). This is resolved by requiring the
controller string to be one valid "element", followed by zero or
more valid "rules" separated from the element by a newline; thus,
the controller string can be built by preceding a piece of valid
ABNF by an "element" that selects from that ABNF and a newline.
* For the same reason, ABNF requires newlines; specifying newlines
in CDDL text strings is tedious (and leads to essentially
unreadable ABNF). The workaround employs the .cat operator
introduced in Section 2.2 and the syntax for text in byte strings.
As is customary for ABNF, the syntax of ABNF itself (_not_ the
syntax expressed in ABNF!) is relaxed to allow a single line feed
as a newline:
CRLF = %x0A / %x0D.0A
* One set of rules provided in an ABNF specification is often used
in multiple positions, particularly staples such as DIGIT and
ALPHA. (Note that all rules referenced need to be defined in each
ABNF operator controller string -- there is no implicit import of
core ABNF rules from [RFC5234] or other rules.) The composition
this calls for can be provided by the .cat operator and/or by .det
if there is indentation to be disposed of.
These points are combined into an example in Figure 5, which uses
ABNF from [RFC3339] to specify one of each of the Concise Binary
Object Representation (CBOR) tags defined in [RFC8943] and [RFC8949].
; for RFC 8943
Tag1004 = #6.1004(text .abnf full-date)
; for RFC 8949
Tag0 = #6.0(text .abnf date-time)
full-date = "full-date" .cat rfc3339
date-time = "date-time" .cat rfc3339
; Note the trick of idiomatically starting with a newline, separating
; off the element in the concatenations above from the rule-list
rfc3339 = '
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec
; rules
time-secfrac = "." 1*DIGIT
time-numoffset = ("+" / "-") time-hour ":" time-minute
time-offset = "Z" / time-numoffset
partial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offset
date-time = full-date "T" full-time
' .det rfc5234-core
rfc5234-core = '
DIGIT = %x30-39 ; 0-9
; abbreviated here
'
Figure 5: An Example of Employing ABNF from RFC 3339 for Defining
CBOR Tags
4. Features
Commonly, the kind of validation enabled by languages such as CDDL
provides a Boolean result: valid or invalid.
In rapidly evolving environments, this is too simplistic. The data
models described by a CDDL specification may continually be enhanced
by additional features, and it would be useful even for a
specification that does not yet describe a specific future feature to
identify the extension point the feature can use and accept such
extensions while marking them as extensions.
The .feature control annotates the target as making use of the
feature named by the controller. The latter will usually be a
string. A tool that validates an instance against that specification
may mark the instance as using a feature that is annotated by the
specification.
More specifically, the tool's diagnostic output might contain the
controller (right-hand side) as a feature name and the target (left-
hand side) as a feature detail. However, in some cases, the target
has too much detail, and the specification might want to hint to the
tool that more limited detail is appropriate. In this case, the
controller should be an array, with the first element being the
feature name (that would otherwise be the entire controller) and the
second element being the detail (usually another string), as
illustrated in Figure 6.
foo = {
kind: bar / baz .feature (["foo-extensions", "bazify"])
}
bar = ...
baz = ... ; complex stuff that doesn't all need to be in the detail
Figure 6: Providing Explicit Detail with .feature
Figure 7 shows what could be the definition of a person, with
potential extensions beyond name and organization being marked
further-person-extension. Extensions that are known at the time this
definition is written can be collected into $$person-extensions.
However, future extensions would be deemed invalid unless the
wildcard at the end of the map is added. These extensions could then
be specifically examined by a user or a tool that makes use of the
validation result; the label (map key) actually used makes a fine
feature detail for the tool's diagnostic output.
Leaving out the entire extension point would mean that instances that
make use of an extension would be marked as wholesale invalid, making
the entire validation approach much less useful. Leaving the
extension point in but not marking its use as special would render
mistakes (such as using the label "organisation" instead of
"organization") invisible.
person = {
? name: text
? organization: text
$$person-extensions
* (text .feature "further-person-extension") => any
}
$$person-extensions //= (? bloodgroup: text)
Figure 7: Map Extensibility with .feature
Figure 8 shows another example where .feature provides for type
extensibility.
allowed-types = number / text / bool / null
/ [* number] / [* text] / [* bool]
/ (any .feature "allowed-type-extension")
Figure 8: Type Extensibility with .feature
A CDDL tool may simply report the set of features being used; the
control then only provides information to the process requesting the
validation. One could also imagine a tool that takes arguments,
allowing the tool to accept certain features and reject others
(enable/disable). The latter approach could, for instance, be used
for a JSON/CBOR switch, as illustrated in Figure 9, using Sensor
Measurement Lists (SenML) [RFC8428] as the example data model used
with both JSON and CBOR.
SenML-Record = {
; ...
? v => number
; ...
}
v = JC<"v", 2>
JC<J,C> = J .feature "json" / C .feature "cbor"
Figure 9: Describing Variants with .feature
It remains to be seen if the enable/disable approach can lead to new
idioms of using CDDL. The language currently has no way to enforce
mutually exclusive use of features, as would be needed in this
example.
5. IANA Considerations
IANA has registered the contents of Table 2 into the "CDDL Control
Operators" registry of [IANA.cddl]:
+==========+===========+
| Name | Reference |
+==========+===========+
| .plus | RFC 9165 |
+----------+-----------+
| .cat | RFC 9165 |
+----------+-----------+
| .det | RFC 9165 |
+----------+-----------+
| .abnf | RFC 9165 |
+----------+-----------+
| .abnfb | RFC 9165 |
+----------+-----------+
| .feature | RFC 9165 |
+----------+-----------+
Table 2: New Control
Operators
6. Security Considerations
The security considerations of [RFC8610] apply.
While both [RFC5234] and [RFC7405] state that security is truly
believed to be irrelevant to the respective document, the use of
formal description techniques cannot only simplify but sometimes also
complicate a specification. This can lead to security problems in
implementations and in the specification itself. As with CDDL
itself, ABNF should be judiciously applied, and overly complex (or
"cute") constructions should be avoided.
7. References
7.1. Normative References
[IANA.cddl]
IANA, "Concise Data Definition Language (CDDL)",
<https://www.iana.org/assignments/cddl>.
[RFC2119] 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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] 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>.
[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/info/rfc8610>.
7.2. Informative References
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
DOI 10.17487/RFC8428, August 2018,
<https://www.rfc-editor.org/info/rfc8428>.
[RFC8943] Jones, M., Nadalin, A., and J. Richter, "Concise Binary
Object Representation (CBOR) Tags for Date", RFC 8943,
DOI 10.17487/RFC8943, November 2020,
<https://www.rfc-editor.org/info/rfc8943>.
[RFC8949] 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>.
Acknowledgements
Jim Schaad suggested several improvements. The .feature feature was
developed out of a discussion with Henk Birkholz. Paul Kyzivat
helped isolate the need for .det.
.det is an abbreviation for "dedenting cat", but Det is also the name
of a German TV cartoon character created in the 1960s.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org