Internet DRAFT - draft-bormann-cddl-freezer
draft-bormann-cddl-freezer
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational January 27, 2018
Expires: July 31, 2018
A feature freezer for the Concise Data Definition Language (CDDL)
draft-bormann-cddl-freezer-00
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 collects nice-to-have features that did not make it into the
first RFC for CDDL.
Status of This Memo
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This Internet-Draft will expire on July 31, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Literal syntax . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. Computed Literals . . . . . . . . . . . . . . . . . . . . 2
3.2. Tag-oriented Literals . . . . . . . . . . . . . . . . . . 3
3.3. Regular Expression Literals . . . . . . . . . . . . . . . 3
4. Embedded ANBF . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Controls . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5.1. Control operator .pcre . . . . . . . . . . . . . . . . . 4
6. Module superstructure . . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
8. Security considerations . . . . . . . . . . . . . . . . . . . 4
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
9.1. Normative References . . . . . . . . . . . . . . . . . . 4
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
(TO DO: Insert an extended form of the abstract first here, expanding
the reference to [I-D.ietf-cbor-cddl].)
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. Cuts
Section 3.5.3 of [I-D.ietf-cbor-cddl] 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 [I-D.ietf-cbor-cddl], before this possibly
becomes a mainline language feature in a future version of CDDL.
3. Literal syntax
3.1. Computed Literals
CDDL cannot compute.
This is a liability in at least two situations:
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o integers often need to be defined relative to an offset. It might
be desirable to be able to write something like:
base = 16777216
a = base + 1
b = base + 2
o some string literals (in particular, complex regular expressions)
would best be composed from components. This could be done by
adding a concatenation operator (maybe even "+" as used for
addition above), or by adding string interpolation to the string
literal syntax.
3.2. 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.
3.3. 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 5.1 then raises the question of how to indicate the regular
expression flavor.
4. Embedded ANBF
It would often be desirable to define a text string type by employing
ABNF [RFC5234] [RFC7405] embedded into the CDDL specification.
Currently, that ABNF would usually need to be translated into a
regular expression (if that is even possible).
Note that some applications of computed literals for strings could be
covered by such a feature (or partially vice versa).
5. 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.
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5.1. Control operator .pcre
There are many variants of regular expression languages.
Section 3.8.3 of [I-D.ietf-cbor-cddl] 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.)
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).
7. IANA Considerations
This document makes no requests of IANA.
8. Security considerations
The security considerations of [I-D.ietf-cbor-cddl] apply.
9. References
9.1. Normative References
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-00
(work in progress), July 2017.
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9.2. Informative References
[PCRE] "Perl-compatible Regular Expressions (revised API:
PCRE2)", n.d., <http://pcre.org/current/doc/html/>.
[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>.
[XSD2] Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
Second Edition", World Wide Web Consortium Recommendation
REC-xmlschema-2-20041028, October 2004,
<http://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 syntax.
Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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