Internet-Draft Managing CBOR numbers in Internet-Drafts March 2023
Bormann Expires 14 September 2023 [Page]
Network Working Group
Intended Status:
C. Bormann
Universität Bremen TZI

Managing CBOR numbers in Internet-Drafts


CBOR-based protocols often make use of numbers allocated in a registry. While developing the protocols, those numbers may not yet be available. This impedes the generation of data models and examples that actually can be used by tools.

This short draft proposes a common way to handle these situations, without any changes to existing tools. Such changes are very well possible in the future, at which time this draft will be updated.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 14 September 2023.

1. Introduction

(Please see abstract.) [RFC8949]

2. The Problem

A CBOR-based protocol might want to define a structure using CDDL [RFC8610][RFC9165], like that in Figure 1 (based on [RFC9290]):

problem-details = {
  ? &(title: -1) => oltext
  ? &(detail: -2) => oltext
  ? &(instance: -3) => ~uri
  ? &(response-code: -4) => uint .size 1
  ? &(base-uri: -5) => ~uri
  ? &(base-lang: -6) => tag38-ltag
  ? &(base-rtl: -7) => tag38-direction
  / ... /
  * (uint .feature "extension") => any
Figure 1: CDDL data model, final form

The key numbers shown in this structure are likely to be intended for allocation in an IANA section.

The key numbers will be used in an example in the specification such as shown in Figure 2.

  / title /         -1: "title of the error",
  / detail /        -2: "detailed information about the error",
  / instance /      -3: "coaps://pd.example/FA317434",
  / response-code / -4: 128, / 4.00 /
  4711: {
     / ... /
Figure 2: CBOR-diag example, final form

However, during development, these numbers are not yet fixed; they are likely to move around as parts of the specification are added or deleted.

This document uses the keys for a map as an example. Other such constructs involving assigned numbers might also require temporary values for exposition in a specification, e.g., CBOR tags. For the sake of keeping this document short, examples for these are not given.

3. The Anti-Pattern

What not to do during development:

problem-details = {
  ? "title" => oltext
  ? "detail" => oltext
  ? "instance" => ~uri
  ? "response-code" => uint .size 1
  ? "base-uri" => ~uri
  ? "base-lang" => tag38-ltag
  ? "base-rtl" => tag38-direction
  / ... /
  * (uint .feature "extension") => any
Figure 3: CDDL data model, muddled form
  "title": "title of the error",
  "detail": "detailed information about the error",
  "instance-code": "coaps://pd.example/FA317434",
  "response-code": 128, / 4.00 /
  4711: {
     / ... /
Figure 4: CBOR-diag example, muddled form

This makes the model and the examples compile/check out even before having allocated the actually desired numbers, but it also leads to several problems:

  • It becomes hard to assess what the storage/transmission cost of these structures will be.
  • What is being checked in the CI (continuous integration) for the document is rather different from the final form.
  • Draft implementations trying to make use of these provisional structures have to cater for text strings, which may not actually be needed in the final form (which might expose specification bugs once numbers are used, too late in the process).
  • The work needed to put in the actual numbers, once allocated, is significant and error-prone.
  • It is not certain the CI system used during development can interact with the RFC editor's way of editing the document for publication.

4. What to do during spec development

To make the transition to a published document easier, the document is instead written with the convention demonstrated in the following:

problem-details = {
  ? &(title-CPA: -1) => oltext
  ? &(detail-CPA: -2) => oltext
  ? &(instance-CPA: -3) => ~uri
  ? &(response-code-CPA: -4) => uint .size 1
  ? &(base-uri-CPA: -5) => ~uri
  ? &(base-lang-CPA: -6) => tag38-ltag
  ? &(base-rtl-CPA: -7) => tag38-direction
  / ... /
  * (uint .feature "extension") => any
Figure 5: CDDL data model, development form

CPA is short for "code point allocation", and is a reliable search key for finding the places that need to be updated after allocation.An earlier concept for this draft used TBD in place of CPA, as do many draft specifications being worked on today. TBD is better recognized than CPA, but also could be misunderstood to mean further work by the spec developer is required. A document submitted for publications should not really have "TBD" in it.

In the IANA section, the table to go into the registry is prepared as follows:

Table 1: IANA table, development form
Key value Name CDDL Type Brief description Reference
CPA-1 title text / tag38 short, human-readable summary of the problem shape RFC XXXX
CPA-2 detail text / tag38 human-readable explanation specific to this occurrence of the problem RFC XXXX
CPA-3 instance ~uri URI reference identifying specific occurrence of the problem RFC XXXX
CPA-4 response-code uint .size 1 CoAP response code RFC XXXX
CPA-5 base-uri ~uri Base URI RFC XXXX
CPA-6 base-lang tag38-ltag Base language tag (see tag38) RFC XXXX
CPA-7 base-rtl tag38-direction Base writing direction (see tag38) RFC XXXX

The provisionally made up key numbers will then be used in an example in the specification such as:

  / title-CPA /         -1: "title of the error",
  / detail-CPA /        -2: "detailed information about the error",
  / instance-CPA /      -3: "coaps://pd.example/FA317434",
  / response-code-CPA / -4: 128, / 4.00 /
  4711: {
     / ... /
Figure 6: CBOR-diag example, development form

A "removeInRFC" note in the draft points the RFC editor to the present document so the RFC editor knows what needs to be done at which point. In the publication process, it is easy to remove the -CPA suffixes and CPA prefixes for the RFC editor while filling in the actual IANA allocated numbers and removing the note.

Note that in Table 1, the first column uses the name "CPA-1" for a value that in the rest of the document is assumed to be "-1" (and indicating a preference by the document author for this number); IANA as well as the designated experts involved are expected by the present document to decode this notation.

4.1. Documents with Significant Generated Content Depending on Assignments

Many documents have examples (which might even involve signatures overf the contents) that depend on the assignments in more than the trivial way shown above, and regenerating them may not be easy for the RFC editor to do.

Therefore, for these documents we need another step involving the authors:

Immediately after allocation, but before the RFC-Editor EDIT step, the authors need to regenerate these examples and other generated content depending on the exact allocations.

In the current process, allocation is usually done after IESG approval, after IANA action, so we would need to halt the EDIT step for this regeneration.

Alternatively, we could be more aggressive in invoking some kind of IANA Early Allocation process, near the end of the IESG review. One way to do this with current tooling and process is to perform a late form of actual IANA "Early" Allocation. Or we could amend [BCP9] and/or [BCP100] in a more fundamental way.

We probably need a indicator in addition to CPA that signifies an example or other text must be regenerated (vs. simply be updated by IANA).

5. IANA Considerations

This document makes no requests of IANA. However, it specifies a procedure that can be followed during draft development that has a specific role for IANA and the interaction between RFC editor and IANA at important points during this development. This procedure is intended to be as little of an onus as possible, but that is the author's assessment only. IANA feedback is therefore requested.

6. Security considerations

The security considerations of [RFC8610] and [RFC8949] apply.

7. References

7.1. Normative References

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, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <>.
Bormann, C., "Additional Control Operators for the Concise Data Definition Language (CDDL)", RFC 9165, DOI 10.17487/RFC9165, , <>.

7.2. Informative References

Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, .
Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, .
Dusseault, L. and R. Sparks, "Guidance on Interoperation and Implementation Reports for Advancement to Draft Standard", BCP 9, RFC 5657, .
Housley, R., Crocker, D., and E. Burger, "Reducing the Standards Track to Two Maturity Levels", BCP 9, RFC 6410, .
Resnick, P., "Retirement of the "Internet Official Protocol Standards" Summary Document", BCP 9, RFC 7100, .
Kolkman, O., Bradner, S., and S. Turner, "Characterization of Proposed Standards", BCP 9, RFC 7127, .
Dawkins, S., "Increasing the Number of Area Directors in an IETF Area", BCP 9, RFC 7475, .
Halpern, J., Ed. and E. Rescorla, Ed., "IETF Stream Documents Require IETF Rough Consensus", BCP 9, RFC 8789, .
Rosen, B., "Responsibility Change for the RFC Series", BCP 9, RFC 9282, .
Fossati, T. and C. Bormann, "Concise Problem Details for Constrained Application Protocol (CoAP) APIs", RFC 9290, DOI 10.17487/RFC9290, , <>.


This document was motivated by the AUTH48 experience for RFC 9200..RFC 9203. Then, Jaime Jiménez made me finally write this document. Marco Tiloca provided useful comments on an early presentation of this idea. Michael Richardson pointed out the issues that led to Section 4.1. Carl Wallace provided further comments shining light on the practical aspects of the proposals.

Author's Address

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen