Internet-Draft | EDN external references | February 2024 |
Bormann | Expires 1 September 2024 | [Page] |
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.¶
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 1 September 2024.¶
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 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.¶
(Please see abstract.) [RFC8949] [I-D.ietf-cbor-edn-literals]¶
See [I-D.bormann-cbor-draft-numbers] for a more general discussion of working with assigned numbers during development of a specification.¶
e''
application extension: importing from CDDL
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:¶
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.¶
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 Figure 1, such a CDDL model could have at least the content shown in Figure 2:¶
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 Figure 2, the example in Figure 1 can be notated as:¶
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.¶
This section is to be removed before publishing as an RFC.¶
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 the cbor-diag
tools such as diag2
x.rb
is done using the -a
command 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 Figure 3 is in a file called
gmadmin.diag
, and that the CDDL model that includes the constants
defined in Figure 2 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}¶
This section is to be removed before publishing as an RFC.¶
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:¶
In the CBOR diagnostic notation used in this document, constructs of the form
e’somename'
are replaced by the value assigned tosomename
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.)¶
ref''
application extension: importing from EDN
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 [I-D.ietf-cbor-edn-literals]).¶
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']¶
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.¶
This section is to be removed before publishing as an RFC.¶
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
diag2
x.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.¶
This section is to be removed before publishing as an RFC.¶
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 |
All entries the Change Controller "IETF" and the Reference "RFC-XXXX".¶
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.¶
The security considerations of [RFC8610], [RFC8949], and [I-D.bormann-t2trg-deref-id] 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.¶