Internet DRAFT - draft-bormann-cbor-cddl-freezer
draft-bormann-cbor-cddl-freezer
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 28 February 2024
Expires: 31 August 2024
A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cbor-cddl-freezer-13
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, or the specifications
exercising its extension points, such as RFC 9165.
Significant parts of this draft have now moved over to the CDDL 2.0
project, described in draft-bormann-cbor-cddl-2-draft. The remaining
items in this draft are not directly related to the CDDL 2.0 effort.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 31 August 2024.
Copyright Notice
Copyright (c) 2024 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 (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
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Base language features . . . . . . . . . . . . . . . . . . . 3
2.1. Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Literal syntax . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Regular Expression Literals (WONTFIX) . . . . . . . . . . 3
4. Controls . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Control operator .pcre . . . . . . . . . . . . . . . . . 4
4.2. Endianness in .bits . . . . . . . . . . . . . . . . . . . 4
4.3. .bitfield control . . . . . . . . . . . . . . . . . . . . 4
5. Co-occurrence Constraints . . . . . . . . . . . . . . . . . . 5
6. Alternative Representations . . . . . . . . . . . . . . . . . 6
7. Other target formats . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Security considerations . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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], or the specifications
exercising its extension points, such as [RFC9165].
Significant parts of this draft have now moved over to
[I-D.bormann-cbor-cddl-2-draft], which in turn references
[I-D.ietf-cbor-update-8610-grammar],
[I-D.ietf-cbor-cddl-more-control], [I-D.ietf-cbor-cddl-modules]. The
remaining items in Sections 3 to 4 of this draft are not directly
related to the CDDL 2.0 effort. Section 5 might turn into a part of
CDDL 2.5.
The remaining sections are not proposing to change CDDL, but are
ancillary developments: Section 6 is more interesting for the
ecosystem of tools around CDDL. Section 7 examines extending the
area of application of CDDL beyond CBOR and JSON.
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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 the rapidly growing real-world experience with the first
CDDL standard. Some sections are labeled WONTFIX, reflecting an
assumption that the specific extension objective will not be
addressed or will be addressed in a different way.
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
Literal syntax is also discussed in Appendix A.1 of
[I-D.bormann-cbor-cddl-2-draft], which might provide another approach
to Section 3.1. This appendix is in turn based on ideas in
[I-D.ietf-cbor-edn-literals].
3.1. Regular Expression Literals (WONTFIX)
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.
(With the support for ABNF in [RFC9165], the need for this is
reduced. Also, the proliferation of regular expression flavors is
hard to address with a single syntax.)
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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; this mechanism has been
exercised in [RFC9090] for SDNV [RFC6256] and ASN.1 OID related byte
strings, and in [RFC9165] for more generally applicable controls,
including an interface to ABNF [RFC5234] [RFC7405]. A more recent
collection of additions that is ready for standardization is
specified in [I-D.ietf-cbor-cddl-more-control].
Several further candidates have been identified that aren't quite
ready for adoption, of which a few 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 [RFC9485], 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, except for a wider range of regexps).
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], as well as
Stefan Gössner's jsonpath, a JSON analogue of XPath that has recently
been standardized [RFC9535].
More generally, something akin to what Schematron is to Relax-NG may
be needed.
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6. 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], which is
reproduced below. This could be written up in more detail and agreed
upon. (Since cddlc version 0.1.8, the "mem"-labeled array includes
information about the presence of a cut, see Section 3.5.4 of
[RFC8610].)
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", ?((6, type/uint, ?type) // (0..7, ?uint))]
group = ["mem", bool, null/type, type] /
["rep", uint, uint/false, group] /
["seq", 2*group] / ["gcho", 2*group]
value = ["number"/"text"/"bytes", text]
The "prim"-labeled array includes support for non-literal tag numbers
(Section 3.2 of [I-D.ietf-cbor-update-8610-grammar]).
More recently, a variant of this format has been used for easier
processing. It collects rules in a map (JSON object) and binds
generic parameters to argument positions. This variant will be
described in a further revision of this document.
7. Other target formats
CDDL has originally been designed to describe CBOR and JSON data.
One format of interest is comma-separated values, CSV [RFC4180].
[I-D.bormann-cbor-cddl-csv] is a draft for using CDDL models with
CSV.
8. IANA Considerations
This document makes no requests of IANA.
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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/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>.
10.2. Informative References
[cddlc] "CDDL conversion utilities", n.d.,
<https://github.com/cabo/cddlc>.
[I-D.bormann-cbor-cddl-2-draft]
Bormann, C., "CDDL 2.0 and beyond -- a draft plan", Work
in Progress, Internet-Draft, draft-bormann-cbor-cddl-2-
draft-04, 27 February 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
cddl-2-draft-04>.
[I-D.bormann-cbor-cddl-csv]
Bormann, C. and H. Birkholz, "Using CDDL for CSVs", Work
in Progress, Internet-Draft, draft-bormann-cbor-cddl-csv-
04, 24 December 2023,
<https://datatracker.ietf.org/doc/html/draft-bormann-cbor-
cddl-csv-04>.
[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://datatracker.ietf.org/doc/html/draft-handrews-
relative-json-pointer-02>.
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[I-D.ietf-cbor-cddl-modules]
Bormann, C., "CDDL Module Structure", Work in Progress,
Internet-Draft, draft-ietf-cbor-cddl-modules-01, 18
December 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-cbor-cddl-modules-01>.
[I-D.ietf-cbor-cddl-more-control]
Bormann, C., "More Control Operators for CDDL", Work in
Progress, Internet-Draft, draft-ietf-cbor-cddl-more-
control-03, 26 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
cddl-more-control-03>.
[I-D.ietf-cbor-edn-literals]
Bormann, C., "CBOR Extended Diagnostic Notation (EDN):
Application-Oriented Literals, ABNF, and Media Type", Work
in Progress, Internet-Draft, draft-ietf-cbor-edn-literals-
08, 1 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
edn-literals-08>.
[I-D.ietf-cbor-update-8610-grammar]
Bormann, C., "Updates to the CDDL grammar of RFC 8610",
Work in Progress, Internet-Draft, draft-ietf-cbor-update-
8610-grammar-03, 29 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
update-8610-grammar-03>.
[PCRE] "Perl-compatible Regular Expressions (revised API:
PCRE2)", n.d., <http://pcre.org/current/doc/html/>.
[RFC4180] Shafranovich, Y., "Common Format and MIME Type for Comma-
Separated Values (CSV) Files", RFC 4180,
DOI 10.17487/RFC4180, October 2005,
<https://www.rfc-editor.org/rfc/rfc4180>.
[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/rfc/rfc5234>.
[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/rfc/rfc6020>.
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[RFC6256] Eddy, W. and E. Davies, "Using Self-Delimiting Numeric
Values in Protocols", RFC 6256, DOI 10.17487/RFC6256, May
2011, <https://www.rfc-editor.org/rfc/rfc6256>.
[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/rfc/rfc6901>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[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>.
[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>.
[RFC9535] Gössner, S., Ed., Normington, G., Ed., and C. Bormann,
Ed., "JSONPath: Query Expressions for JSON", RFC 9535,
DOI 10.17487/RFC9535, February 2024,
<https://www.rfc-editor.org/rfc/rfc9535>.
[XSD2] Malhotra, A., Ed. and P. V. Biron, Ed., "XML Schema Part
2: Datatypes Second Edition", W3C REC REC-xmlschema-
2-20041028, W3C REC-xmlschema-2-20041028, 28 October 2004,
<https://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.
Acknowledgements
Before [RFC8610] was finally published, 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
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Email: cabo@tzi.org
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