A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cbor-cddl-freezer-08
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Carsten Bormann
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2021-06-25
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Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 25 June 2021
Expires: 27 December 2021
A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cbor-cddl-freezer-08
Abstract
In defining the Concise Data Definition Language (CDDL), some
features have turned up that would be nice to have. In the interest
of completing this specification in a timely manner, the present
document was started to collect nice-to-have features that did not
make it into the first RFC for CDDL, RFC 8610.
It is now time to discuss thawing some of the concepts discussed
here. A number of additional proposals have been added.
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/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 December 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Base language features . . . . . . . . . . . . . . . . . . . 3
2.1. Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Literal syntax . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 3
3.2. Regular Expression Literals . . . . . . . . . . . . . . . 3
3.3. Clarifications . . . . . . . . . . . . . . . . . . . . . 4
3.3.1. Err6527 . . . . . . . . . . . . . . . . . . . . . . . 4
3.3.2. Err6543 . . . . . . . . . . . . . . . . . . . . . . . 5
4. Controls . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Control operator .pcre . . . . . . . . . . . . . . . . . 6
4.2. Endianness in .bits . . . . . . . . . . . . . . . . . . . 6
4.3. .bitfield control . . . . . . . . . . . . . . . . . . . . 6
5. Co-occurrence Constraints . . . . . . . . . . . . . . . . . . 7
6. Module superstructure . . . . . . . . . . . . . . . . . . . . 8
6.1. Namespacing . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Cross-universe references . . . . . . . . . . . . . . . . 8
6.2.1. IANA references . . . . . . . . . . . . . . . . . . . 8
7. Alternative Representations . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security considerations . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In defining the Concise Data Definition Language (CDDL), some
features have turned up that would be nice to have. In the interest
of completing this specification in a timely manner, the present
document was started to collect nice-to-have features that did not
make it into the first RFC for CDDL [RFC8610].
It is now time to discuss thawing some of the concepts discussed
here. A number of additional proposals have been added.
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There is always a danger for a document like this to become a
shopping list; the intention is to develop this document further
based on real-world experience with the first CDDL standard.
2. Base language features
2.1. Cuts
Section 3.5.4 of [RFC8610] alludes to a new language feature, _cuts_,
and defines it in a fashion that is rather focused on a single
application in the context of maps and generating better diagnostic
information about them.
The present document is expected to grow a more complete definition
of cuts, with the expectation that it will be upwards-compatible to
the existing one in [RFC8610], before this possibly becomes a
mainline language feature in a future version of CDDL.
3. Literal syntax
3.1. Tag-oriented Literals
Some CBOR tags often would be most natural to use in a CDDL spec with
a literal syntax that is tailored to their semantics instead of their
serialization in CBOR. There is currently no way to add such
syntaxes, no defined extension point either.
The text form of CoRAL [I-D.ietf-core-coral] defines literals of the
form
dt'2019-07-21T19:53Z'
for datetime items. (Similar advances should then probably be made
in diagnostic notation.)
3.2. Regular Expression Literals
Regular expressions currently are notated as strings in CDDL, with
all the string escaping rules applied once. It might be convenient
to have a more conventional literal format for regular expressions,
possibly also providing a place to add modifiers such as "/i". This
might also imply "text .regexp ...", which with the proposal in
Section 4.1 then raises the question of how to indicate the regular
expression flavor.
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3.3. Clarifications
A number of errata reports have been made around some details of text
string and byte string literal syntax: [Err6527] and [Err6543].
These need to be addressed by re-examining the details of these
literal syntaxes. Also, [Err6526] needs to be applied.
3.3.1. Err6527
The ABNF used in [RFC8610] for the content of text string literals is
rather permissive:
text = %x22 *SCHAR %x22
SCHAR = %x20-21 / %x23-5B / %x5D-7E / %x80-10FFFD / SESC
SESC = "\" (%x20-7E / %x80-10FFFD)
This allows almost any non-C0 character to be escaped by a backslash,
but critically misses out on the "\uXXXX" and "\uHHHH\uLLLL" forms
that JSON allows to specify characters in hex. Both can be solved by
updating the SESC production to:
SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\
%x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
(%x75 hexchar) ) ; \u
hexchar = non-surrogate / (high-surrogate "\" %x75 low-surrogate)
non-surrogate = ((DIGIT / "A"/"B"/"C" / "E"/"F") 3HEXDIG) /
("D" %x30-37 2HEXDIG )
high-surrogate = "D" ("8"/"9"/"A"/"B") 2HEXDIG
low-surrogate = "D" ("C"/"D"/"E"/"F") 2HEXDIG
Now that SESC is more restrictively formulated, this also requires an
update to the BCHAR production used in the ABNF syntax for byte
string literals:
bytes = [bsqual] %x27 *BCHAR %x27
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
bsqual = "h" / "b64"
The updated version explicit allows "\'", which is no longer allowed
in the updated SESC:
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF
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3.3.2. Err6543
The ABNF used in [RFC8610] for the content of byte string literals
lumps together byte strings notated as text with byte strings notated
in base16 (hex) or base64 (but see also updated BCHAR production
above):
bytes = [bsqual] %x27 *BCHAR %x27
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
Errata report 6543 proposes to handle the two cases in separate
productions (where, with an updated SESC, BCHAR obviously needs to be
updated as above):
bytes = %x27 *BCHAR %x27
/ bsqual %x27 *QCHAR %x27
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF
QCHAR = DIGIT / ALPHA / "+" / "/" / "-" / "_" / "=" / WS
This potentially causes a subtle change, which is hidden in the WS
production:
WS = SP / NL
SP = %x20
NL = COMMENT / CRLF
COMMENT = ";" *PCHAR CRLF
PCHAR = %x20-7E / %x80-10FFFD
CRLF = %x0A / %x0D.0A
This allows any non-C0 character in a comment, so this fragment
becomes possible:
foo = h'
43424F52 ; 'CBOR'
0A ; LF, but don't use CR!
'
The current text is not unambiguously saying whether the three
apostrophes need to be escaped with a "\" or not, as in:
foo = h'
43424F52 ; \'CBOR\'
0A ; LF, but don\'t use CR!
'
... which would be supported by the existing ABNF in [RFC8610].
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4. Controls
Controls are the main extension point of the CDDL language. It is
relatively painless to add controls to CDDL. Several candidates have
been identified that aren't quite ready for adoption, of which one
shall be listed here.
4.1. Control operator .pcre
There are many variants of regular expression languages.
Section 3.8.3 of [RFC8610] defines the .regexp control, which is
based on XSD [XSD2] regular expressions. As discussed in that
section, the most desirable form of regular expressions in many cases
is the family called "Perl-Compatible Regular Expressions" ([PCRE]);
however, no formally stable definition of PCRE is available at this
time for normatively referencing it from an RFC.
The present document defines the control operator .pcre, which is
similar to .regexp, but uses PCRE2 regular expressions. More
specifically, a ".pcre" control indicates that the text string given
as a target needs to match the PCRE regular expression given as a
value in the control type, where that regular expression is anchored
on both sides. (If anchoring is not desired for a side, ".*" needs
to be inserted there.)
Similarly, ".es2018re" could be defined for ECMAscript 2018 regular
expressions with anchors added.
See also [I-D.draft-bormann-jsonpath-iregexp], which could be
specifically called out via ".iregexp" (even though ".regexp" as per
Section 3.8.3 of [RFC8610] would also have the same semantics).
4.2. Endianness in .bits
How useful would it be to have another variant of .bits that counts
bits like in RFC box notation? (Or at least per-byte? 32-bit words
don't always perfectly mesh with byte strings.)
4.3. .bitfield control
Provide a way to specify bitfields in byte strings and uints to a
higher level of detail than is possible with .bits. Strawman:
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Field = uint .bitfield Fieldbits
Fieldbits = [
flag1: [1, bool],
val: [4, Vals],
flag2: [1, bool],
]
Vals = &(A: 0, B: 1, C: 2, D: 3)
Note that the group within the controlling array can have choices,
enabling the whole power of a context-free grammar (but not much
more).
5. Co-occurrence Constraints
While there are no co-occurrence constraints in CDDL, many actual use
cases can be addressed by using the fact that a group is a grammar:
postal = {
( street: text,
housenumber: text) //
( pobox: text .regexp "[0-9]+" )
}
However, constraints that are not just structural/tree-based but are
predicates combining parts of the structure cannot be expressed:
session = {
timeout: uint,
}
other-session = {
timeout: uint .lt [somehow refer to session.timeout],
}
As a minimum, this requires the ability to reach over to other parts
of the tree in a control. Compare JSON Pointer [RFC6901] and JSON
Relative Pointer [I-D.handrews-relative-json-pointer]. Stefan
Goessner's jsonpath is a JSON variant of XPath that has not been
formally standardized [jsonpath].
More generally, something akin to what Schematron is to Relax-NG may
be needed.
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6. Module superstructure
CDDL rules could be packaged as modules and referenced from other
modules. There could be some control of namespace pollution, as well
as unambiguous referencing ("versioning").
This is probably best achieved by a pragma-like syntax which could be
carried in CDDL comments, leaving each module to be valid CDDL (if
missing some rule definitions to be imported).
6.1. Namespacing
A convention for mapping CDDL-internal names to external ones could
be developed, possibly steered by some pragma-like constructs.
External names would likely be URI-based, with some conventions as
they are used in RDF or Curies. Internal names might look similar to
XML QNames. Note that the identifier character set for CDDL
deliberately includes $ and @, which could be used in such a
convention.
6.2. Cross-universe references
Often, a CDDL specfication needs to import from specifications in a
different language or platform.
6.2.1. IANA references
In many cases, CDDL specifications make use of values that are
specified in IANA registries. The ".iana" control operator can be
used to reference such a set of values.
The reference needs to be able to point to a draft, the registry of
which has not been established yet, as well as to an established IANA
registry.
An example of such a usage might be:
cose-algorithm = int .iana ["cose", "algorithms", "value"]
Unfortunately, the vocabulary employed in IANA registries has not
been designed for machine references. In this case, the potential
values would come from applying the XPath expression
//iana:registry[@id='algorithms']/iana:record/iana:value
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to "https://www.iana.org/assignments/cose/cose.xml", plus some
filtering on the records returned that only leaves actual
allocations. Additional functionality may be needed for filtering
with respect to other columns of the registry record, e.g.,
"<capabilities>" in the case of this example.
7. Alternative Representations
For CDDL, alternative representations e.g. in JSON (and thus in YAML)
could be defined, similar to the way YANG defines an XML-based
serialization called YIN in Section 11 of [RFC6020]. One proposal
for such a syntax is provided by the "cddlc" tool [cddlc]; this could
be written up and agreed upon.
cddlj = ["cddl", +rule]
rule = ["=" / "/=" / "//=", namep, type]
namep = ["name", id] / ["gen", id, +id]
id = text .regexp "[A-Za-z@_$](([-.])*[A-Za-z0-9@_$])*"
op = ".." / "..." /
text .regexp "\\.[A-Za-z@_$](([-.])*[A-Za-z0-9@_$])*"
namea = ["name", id] / ["gen", id, +type]
type = value / namea / ["op", op, type, type] /
["map", group] / ["ary", group] / ["tcho", 2*type] /
["unwrap", namea] / ["enum", group / namea] /
["prim", ?(0..7, ?uint)]
group = ["mem", null/type, type] /
["rep", uint, uint/false, group] /
["seq", 2*group] / ["gcho", 2*group]
value = ["number"/"text"/"bytes", text]
8. IANA Considerations
This document makes no requests of IANA.
9. Security considerations
The security considerations of [RFC8610] apply.
10. References
10.1. Normative References
[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>.
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10.2. Informative References
[cddlc] "CDDL conversion utilities", n.d.,
<https://github.com/cabo/cddlc>.
[Err6526] "Errata Report 6526", RFC 8610,
<https://www.rfc-editor.org/errata/eid6526>.
[Err6527] "Errata Report 6527", RFC 8610,
<https://www.rfc-editor.org/errata/eid6527>.
[Err6543] "Errata Report 6543", RFC 8610,
<https://www.rfc-editor.org/errata/eid6543>.
[I-D.draft-bormann-jsonpath-iregexp]
Bormann, C., "I-Regexp: An Interoperable Regexp Format",
Work in Progress, Internet-Draft, draft-bormann-jsonpath-
iregexp-00, 12 May 2021, <https://www.ietf.org/archive/id/
draft-bormann-jsonpath-iregexp-00.txt>.
[I-D.handrews-relative-json-pointer]
Luff, G. and H. Andrews, "Relative JSON Pointers", Work in
Progress, Internet-Draft, draft-handrews-relative-json-
pointer-02, 18 September 2019,
<https://www.ietf.org/archive/id/draft-handrews-relative-
json-pointer-02.txt>.
[I-D.ietf-core-coral]
Hartke, K., "The Constrained RESTful Application Language
(CoRAL)", Work in Progress, Internet-Draft, draft-ietf-
core-coral-03, 9 March 2020,
<https://www.ietf.org/archive/id/draft-ietf-core-coral-
03.txt>.
[jsonpath] "jsonpath online evaluator", n.d., <https://jsonpath.com>.
[PCRE] "Perl-compatible Regular Expressions (revised API:
PCRE2)", n.d., <http://pcre.org/current/doc/html/>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
"JavaScript Object Notation (JSON) Pointer", RFC 6901,
DOI 10.17487/RFC6901, April 2013,
<https://www.rfc-editor.org/info/rfc6901>.
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[XSD2] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
Second Edition", World Wide Web Consortium Recommendation
REC-xmlschema-2-20041028, 28 October 2004,
<https://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.
Acknowledgements
Many people have asked for CDDL to be completed, soon. These are
usually also the people who have brought up observations that led to
the proposals discussed here. Sean Leonard has campaigned for a
regexp literal syntax.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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