v: 3
title: > External References to Values in CBOR Diagnostic Notation (EDN) abbrev: EDN external references docname: draft-ietf-cbor-edn-e-ref-latest date: 2024-06-27 keyword:
- CBOR numbers cat: info stream: IETF
pi: [toc, sortrefs, symrefs, compact, comments]
venue: mail: cbor@ietf.org github: cbor-wg/edn-e-ref latest: https://cbor-wg.github.io/edn-e-ref/draft-ietf-cbor-edn-e-ref.html
ins: C. Bormann
name: Carsten Bormann
org: Universität Bremen TZI
street: Postfach 330440
city: Bremen
code: D-28359
country: Germany
phone: +49-421-218-63921
email: cabo@tzi.org
normative: RFC8949: cbor RFC8610: cddl I-D.ietf-cbor-edn-literals: edn informative: I-D.bormann-cbor-draft-numbers: numbers cddlc: title: CDDL conversion utilities target: https://github.com/cabo/cddlc cbor-diag: title: CBOR diagnostic utilities target: https://github.com/cabo/cbor-diag cbor-diag-e: title: CBOR diagnostic extension e'' target: https://github.com/cabo/cbor-diag-e cbor-diag-ref: title: CBOR diagnostic extension ref'' target: https://github.com/cabo/cbor-diag-ref I-D.bormann-t2trg-deref-id: deref
--- abstract
The Concise Binary Object Representation (CBOR, RFC 8949) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation.
CBOR diagnostic notation (EDN) is widely used to represent CBOR data items in a way that is accessible to humans, for instance for examples in a specification. At the time of writing, EDN did not provide mechanisms for composition of such examples from multiple components or sources. This document uses EDN application extensions to provide two such mechanisms, both of which insert an imported data item into the data item being described in EDN:
The e'' application extension provides a way to import data items,
particularly constant values, from a CDDL model (which itself has ways
to provide composition).
The ref'' application extension provides a way to import data items
that are described in EDN.
--- middle
(Please see abstract.) {{-cbor}} {{-edn}}
See {{-numbers}} for a more general discussion of working with assigned numbers during development of a specification.
{:unnumbered}
In diagnostic notation examples used during development of earlier drafts, authors often used text strings in place of constants they need, even though they actually mean a placeholder for a later, to-be-registered integer.
One example from a recent draft would be:
{
"group_mode" : true,
"gp_enc_alg" : 10,
"hkdf" : 5
}
{: #fig-incorrect pre="dedent" title="Misleading usage of text strings as stand-in for registered constants"}
Not only is the reader misled by seeing text strings in places that are actually intended to be small integers, there are also small integers that are not explained at all (here: 10, 5). The usefulness of this example is greatly reduced. Examples constructed in this are not actually machine-readable -- they seem to be, but they mean the wrong thing in several places without any warning that this is so.
{:unnumbered}
In many cases, the constants needed to clean up this example are already available in a CDDL model, or could be easily made available in this way.
If such a CDDL model can be identified, the EDN application extension
e'constant-name' can be used to reference a constant defined by that
model under the name constant-name.
(Hint: memorize the e as external constant, or enum.)
For the example in {{fig-incorrect}}, such a CDDL model could have at least the content shown in {{fig-cddl}}:
group_mode = 33
gp_enc_alg = 34
hkdf = 31
HMAC-256-256 = 5
AES-CCM-16-64-128 = 10
{: #fig-cddl title="CDDL model defining constants for e''"}
Note that such a model can have other, unrelated CDDL rules that
define more complex data items; only the ones used in an e''
construct need to be constant values.
Using the CDDL model in {{fig-cddl}}, the example in {{fig-incorrect}} can be notated as:
{
e'group_mode' : true,
e'gp_enc_alg' : e'HMAC-256-256',
e'hkdf' : e'AES-CCM-16-64-128'
}
{: #fig-using-e pre="dedent" title="Example updated to use e'constantname' for registered constants"}
This example is equivalent to notating {33: true, 34: 10, 31: 5},
which expresses the concise 10-byte data item that will actually be
interchanged for this example.
Note that the application-oriented literal does not itself define where the CDDL definitions it uses come from. This information needs to come from the context of the example.
{:unnumbered removeinrfc}
The e'' application extension is now implemented in the cbor-diag
tools {{cbor-diag}}, by the cbor-diag-e gem {{cbor-diag-e}}, which can
be installed as:
gem install cbor-diag-e cddlc
(cbor-diag-e uses cddlc {{cddlc}} internally, so it must be in PATH.)
The provided
Use of this extension has two prerequisites:
-
Opt-in to the application extension
e, which in thecbor-diagtools such asdiag2x.rbis done using the-acommand line flag, here:-ae. -
Identification of the CDDL model to be used, which will give the actual values for the constants.
This can be a complete CDDL model for the application, no need to limit it just to constant definitions. (Where the constant values need to be obtained by registration at the time of completion of the document using the examples, the CDDL model can be set up with TBD values of the constants to be assigned, and once they are, the necessary updates are all in one place.)
Assuming that the example in {{fig-using-e}} is in a file called
gmadmin.diag, and that the CDDL model that includes the constants
defined in {{fig-cddl}} is in gmadmin.cddl, the following commands can
be used to translate the `e'`` constants into their actual values:
$ export CBOR_DIAG_CDDL=gmadmin.cddl
$ diag2diag.rb -ae gmadmin.diag
{33: true, 34: 10, 31: 5}{:unnumbered removeinrfc}
The need for this application is there now, while ratification of the present specification might take a year or so. Until then, each document using this scheme can simply use boilerplate such as:
{:quote}
In the CBOR diagnostic notation used in this document, constructs of
the form e’somename' are replaced by the value assigned to
somename in CDDL in figure 0815.
E.g., {e'group_mode': "bar"} stands for {33: "bar"}.
(Choose 0815, group_mode and 33 along the lines of the figure you include with the CDDL definitions needed.)
{:unnumbered}
Examples using CBOR diagnostic notation can get large. One way to manage the size of an example is to make it incomplete. This reduces the usefulness of the example for machine-processing. It can also be misleading, unless the elision is made explicit (see {{Section 3.2 of -edn}}).
{:unnumbered}
In a set of CBOR examples, recurring subtrees can often be identified, the details of which do not need to be repeated in each example.
By enabling examples to reference these subtrees from a separate piece of EDN, each example can focus on what is specific for them.
The ref'' application-oriented literal enables composition by
standing for a CBOR data item from a separate EDN instance that is
referenced using its text as an identifier.
So, for example, if 123.diag is a file containing
[1, 2, 3]
the result of the EDN
[4711.0, true, ref’123.diag’]
is
[4711.0, true, [1, 2, 3]]
The text of the literal can be one of two kinds of identifiers:
-
a file name to be interpreted in the context of the referencing example, as shown above, or
-
a URI that references the EDN to be embedded, as in
[4711.0, true, ref'http://tzi.de/~cabo/123.diag']
{:aside}
(ISSUE: We could use upper-case REF to unambiguously identify one of he two; the current implementation however just tries to parse the literal text as a URI and, if that fails, interprets it as a file name.)
Note that a ref'' application-oriented literal can only be used for
a single CBOR data item; the extension point provided by EDN does not
work for splicing in CBOR sequences.
{:unnumbered removeinrfc}
The ref'' application extension is now implemented in the cbor-diag
tools {{cbor-diag}}, by the cbor-diag-ref gem, which can be installed as:
gem install cbor-diag-ref
For using the application extension, the cbor-diag tools such as
diag2x.rb need to be informed by the -a command line flag,
here: -aref.
For experimenting with the implementation, the web resource
http://tzi.de/~cabo/123.diag contains [1, 2, 3].
This example enables usage as in:
$ echo "[4711.0, true, ref'http://tzi.de/~cabo/123.diag']" >my.diag
$ diag2diag.rb -aref my.diag
[4711.0, true, [1, 2, 3]]
$ echo "[4, 5, 6]" >456.diag
$ echo "[4711.0, true, ref'456.diag']" >my.diag
$ diag2diag.rb -aref my.diag
[4711.0, true, [4, 5, 6]]If a referenced EDN file parses as a CBOR sequence this is currently treated as an error.
{:unnumbered removeinrfc}
Documents that want to use the application extension ref'' now can
use boilerplate similar to that given above for e''.
IANA is requested to make the following two assignments in the CBOR Diagnostic Notation Application-extension Identifiers registry [IANA.cbor-diagnostic-notation]:
| Application-extension Identifier | Description |
|---|---|
| e | import value from external CDDL |
| ref | import value from external EDN |
| {: #tab-iana title="Additions to Application-extension Identifier Registry"} |
All entries the Change Controller "IETF" and the Reference "RFC-XXXX".
The security considerations of {{-cddl}}, {{-cbor}}, and {{-deref}} apply.
The proof of concept implementations described do not do any sanitizing of URLs or file names at all. Upcoming versions of the present document will need to define the right restrictions for external references like this.
--- back
{: numbered="no"}
TBD
Footnotes
-
RFC Editor: please replace RFC-XXXX with the RFC number of this RFC, [IANA.cbor-diagnostic-notation] with a reference to the new registry group, and remove this note. ↩