| Internet-Draft | CDDL grammar updates | December 2023 |
| Bormann | Expires 17 June 2024 | [Page] |
The Concise Data Definition Language (CDDL), as defined in RFC 8610 and RFC 9165, provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON.¶
The present document updates RFC 8610 by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there.¶
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/update-8610-grammar/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-cbor-update-8610-grammar/.¶
Discussion of this document takes place on the CBOR 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/update-8610-grammar.¶
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 17 June 2024.¶
Copyright (c) 2023 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 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.¶
The Concise Data Definition Language (CDDL), as defined in [RFC8610] and [RFC9165], provides an easy and unambiguous way to express structures for protocol messages and data formats that are represented in CBOR or JSON.¶
The present document updates [RFC8610] by addressing errata and making other small fixes for the ABNF grammar defined for CDDL there.¶
errata fix¶
A number of errata reports have been made around some details of text string and byte string literal syntax: [Err6527] and [Err6543]. These are being addressed in this section, updating details of the ABNF for these literal syntaxes. Also, [Err6526] needs to be applied (backslashes have been lost during RFC processing in some text explaining backslash escaping).¶
The ABNF used in [RFC8610] for the content of text string literals is rather permissive:¶
; RFC 8610 ABNF: 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 (which should be
applying here according to Bullet 6 of Section 3.1 of [RFC8610]).
(Note that we import from JSON the unwieldy \uHHHH\uLLLL syntax,
which represents Unicode code points beyond U-FFFF by making them look
like UTF-16 surrogate pairs; CDDL text strings are not using UTF-16 or
surrogates.)¶
Both can be solved by updating the SESC production to:¶
; new rules collectively defining SESC:
SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\
%x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
(%x75 hexchar) ) ; \uXXXX
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
(Notes:
In ABNF, strings such as "A", "B" etc. are case-insensitive, as is
intended here.
We could have written %x62 as %s"b", but didn't, in order to
maximize ABNF tool compatibility.)¶
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:¶
; RFC 8610 ABNF: bytes = [bsqual] %x27 *BCHAR %x27 BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / CRLF bsqual = "h" / "b64"
In BCHAR, the updated version explicitly allows \', which is no
longer allowed in the updated SESC:¶
; new rule for BCHAR: BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF
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):¶
; RFC 8610 ABNF: 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):¶
; Err6543 proposal:
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:¶
; RFC 8610 ABNF: 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].¶
note that the HTML rendering of the heading is butchered by xml2rfc, as noted in https://github.com/ietf-tools/xml2rfc/issues/683; we except this to have been fixed before this document is published¶
This document takes the simpler approach of leaving the processing of
the content of the byte string literal to a semantic step after
processing the syntax of the bytes/BCHAR rules as updated by
Figure 2 and Figure 4.¶
The rules in Figure 7 are therefore applied to the result of this
processing where bsqual is given as h or b64.¶
Note that this approach also works well with the use of byte strings
in Section 3 of [RFC9165].
It does require some care when copy-pasting into CDDL models from ABNF
that contains single quotes (which may also hide as apostrophes
in comments); these need to be escaped or possibly replaced by %x27.¶
Finally, our approach would lend support to extending bsqual in CDDL
similar to the way this is done for CBOR diagnostic notation in [I-D.ietf-cbor-edn-literals].¶
The two subsections in this section specify two small changes to the grammar that are intended to enable certain kinds of specifications.¶
backward (not forward)¶
[RFC8610] requires a CDDL file to have at least one rule.¶
; RFC 8610 ABNF: cddl = S 1*(rule S)
This makes sense when the file has to stand alone, as a CDDL data model needs to have at least one rule to provide an entry point (start rule).¶
With CDDL modules [I-D.ietf-cbor-cddl-modules], CDDL files can also include directives, and these might be the source of all the rules that ultimately make up the module created by the file. Any other rule content in the file has to be available for directive processing, making the requirement for at least one rule cumbersome.¶
Therefore, we extend the grammar as in Figure 9 and make the existence of at least one rule a semantic constraint, to be fulfilled after processing of all directives.¶
; new top-level rule: cddl = S *(rule S)
backward (not forward)¶
The existing ABNF syntax for expressing tags in CDDL is:¶
; extracted from RFC 8610 ABNF:
type2 /= "#" "6" ["." uint] "(" S type S ")"
This means tag numbers can only be given as literal numbers (uints). Some specifications operate on ranges of tag numbers, e.g., [RFC9277] has a range of tag numbers 1668546817 (0x63740101) to 1668612095 (0x6374FFFF) to tag specific content formats. This can currently not be expressed in CDDL.¶
This update extends this to:¶
; new rules collectively defining the tagged case:
type2 /= "#" "6" ["." tag-number] "(" S type S ")"
tag-number = uint / ("<" type ">")
So the above range can be expressed in a CDDL fragment such as:¶
ct-tag<content> = #6.<ct-tag-number>(content) ct-tag-number = 1668546817..1668612095 ; or use 0x63740101..0x6374FFFF¶
Note that this syntax reuses the angle bracket syntax for generics;
this reuse is innocuous as a generic parameter/argument only ever
occurs after a rule name (id), while it occurs after . here.
(Whether there is potential for human confusion can be debated; the
above example deliberately uses generics as well.)¶
The grammar fixes and updates in this document are not believed to create additional security considerations. The security considerations in Section 5 of [RFC8610] do apply, and specifically the potential for confusion is increased in an environment that uses a combination of CDDL tools some of which have been updated and some of which have not been, in particular based on Section 2.¶
This document has no IANA actions.¶
This appendix provides the full ABNF from [RFC8610] with the updates applied in the present document.¶
cddl = S *(rule S)
rule = typename [genericparm] S assignt S type
/ groupname [genericparm] S assigng S grpent
typename = id
groupname = id
assignt = "=" / "/="
assigng = "=" / "//="
genericparm = "<" S id S *("," S id S ) ">"
genericarg = "<" S type1 S *("," S type1 S ) ">"
type = type1 *(S "/" S type1)
type1 = type2 [S (rangeop / ctlop) S type2]
; space may be needed before the operator if type2 ends in a name
type2 = value
/ typename [genericarg]
/ "(" S type S ")"
/ "{" S group S "}"
/ "[" S group S "]"
/ "~" S typename [genericarg]
/ "&" S "(" S group S ")"
/ "&" S groupname [genericarg]
/ "#" "6" ["." tag-number] "(" S type S ")"
/ "#" DIGIT ["." uint] ; major/ai
/ "#" ; any
tag-number = uint / ("<" type ">")
rangeop = "..." / ".."
ctlop = "." id
group = grpchoice *(S "//" S grpchoice)
grpchoice = *(grpent optcom)
grpent = [occur S] [memberkey S] type
/ [occur S] groupname [genericarg] ; preempted by above
/ [occur S] "(" S group S ")"
memberkey = type1 S ["^" S] "=>"
/ bareword S ":"
/ value S ":"
bareword = id
optcom = S ["," S]
occur = [uint] "*" [uint]
/ "+"
/ "?"
uint = DIGIT1 *DIGIT
/ "0x" 1*HEXDIG
/ "0b" 1*BINDIG
/ "0"
value = number
/ text
/ bytes
int = ["-"] uint
; This is a float if it has fraction or exponent; int otherwise
number = hexfloat / (int ["." fraction] ["e" exponent ])
hexfloat = ["-"] "0x" 1*HEXDIG ["." 1*HEXDIG] "p" exponent
fraction = 1*DIGIT
exponent = ["+"/"-"] 1*DIGIT
text = %x22 *SCHAR %x22
SCHAR = %x20-21 / %x23-5B / %x5D-7E / %x80-10FFFD / SESC
SESC = "\" ( %x22 / "/" / "\" / ; \" \/ \\
%x62 / %x66 / %x6E / %x72 / %x74 / ; \b \f \n \r \t
(%x75 hexchar) ) ; \uXXXX
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
bytes = [bsqual] %x27 *BCHAR %x27
BCHAR = %x20-26 / %x28-5B / %x5D-10FFFD / SESC / "\'" / CRLF
bsqual = "h" / "b64"
id = EALPHA *(*("-" / ".") (EALPHA / DIGIT))
ALPHA = %x41-5A / %x61-7A
EALPHA = ALPHA / "@" / "_" / "$"
DIGIT = %x30-39
DIGIT1 = %x31-39
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
BINDIG = %x30-31
S = *WS
WS = SP / NL
SP = %x20
NL = COMMENT / CRLF
COMMENT = ";" *PCHAR CRLF
PCHAR = %x20-7E / %x80-10FFFD
CRLF = %x0A / %x0D.0A
TODO acknowledge.¶