Common CBOR Deterministic Encoding and Application Profiles

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Table of Contents

1. Introduction

CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in its Section 4.2, providing some flexibility for application specific decisions. To facilitate Deterministic Encoding to be offered as a selectable feature of generic encoders, the present document discusses a Common CBOR Deterministic Encoding Profile that can be shared by a large set of applications with potentially diverging detailed requirements. The concept of Application Profiles is layered on top of the Common CBOR Deterministic Encoding Profile and can address those more application specific requirements. The document defines the application profile "Gordian dCBOR" as an example of an application profile built on the Common CBOR Deterministic Encoding Profile.¶

2. Common CBOR Deterministic Encoding Profile

Section 4.2.1 of [STD94] defines Core Deterministic Encoding Requirements for CBOR. It mandates to keep with what are only recommendations for Preferred Encoding for regular CBOR encoders. It adds mandates for definite-length encoding and for a map ordering based on lexicographic ordering of the (deterministically) encoded map keys.¶

Note that this specific set of requirements is elective — in principle, other variants of deterministic encoding can be defined (and have been, now being phased out slowly, as detailed in Section 4.2.3 of [STD94]), or (as many applications of CBOR do today) deterministic encoding is not used at all.¶

The core requirements are designed, however, to provide well-understood and easy to implement rules while maximizing coverage, i.e., the subset of CBOR data items that are fully specified by these rules, and also placing minimal burden on implementations.¶

Section 4.2.2 of [STD94] picks up on the interaction of extensibility (CBOR tags) and deterministic encoding. CBOR itself uses some tags to increase the range of its basic generic data types, e.g., tag 2/3 extend the range of basic major types 0/1 in a seamless way. Section 4.2.2 of [STD94] recommends handling this transition the same way as with the transition between different integer representation lengths in the basic generic data model, i.e., by mandating the Preferred Encoding (Section 3.4.3 of [STD94]).¶

1. The Common CBOR Deterministic Encoding Profile turns this recommendation into a mandate: Integers that can be represented by basic major type 0 and 1 are encoded using the deterministic encoding defined for them, and integers outside this range are encoded using the preferred serialization (Section 3.4.3 of [STD94]) of tag 2 and 3 (i.e., no leading zeros).¶

Most tags capture more specific application semantics and therefore may be harder to define a deterministic encoding for. While the deterministic encoding of their tag internals is often covered by the Core Deterministic Encoding Requirements, the mapping of diverging platform application data types on the tag contents may be hard to do in a deterministic way; see Section 3.2 of [I-D.bormann-cbor-det] for more explanation as well as examples. As the Common CBOR Deterministic Encoding Profile would continually need to address additional issues raised by the registration of new tags, this specification RECOMMENDS that new tag registrations address deterministic encoding in the context of this Profile.¶

A particularly difficult field to obtain deterministic encoding for is floating point numbers, partially because they themselves are often obtained from processes that are not entirely deterministic between platforms. See Section 3.2.2 of [I-D.bormann-cbor-det] for more details. Section 4.2.2 of [STD94] presents a number of choices, which need to be made to obtain a Common CBOR Deterministic Encoding Profile. Specifically (in the order of the bullet list at the end of Section 4.2.2 of [STD94]):¶

2. Besides the mandated use of preferred encoding, there is no further specific action for the two different zero values, e.g., an encoder that is asked by an application to represent a negative floating point zero will generate 0xf98000.¶
3. There is no attempt to mix integers and floating point numbers, i.e., all floating point values are encoded as the preferred floating-point representation that accurately represents the value, independent of whether the floating point value is, mathematically, an integral value (choice 2 of the second bullet).¶
4. There is no special handling of NaN values, except that the preferred encoding rules also apply to NaNs with payloads, using the canonical encoding of NaNs as defined in [IEEE754]. Typically, most applications that employ NaNs in their storage and communication interfaces will only use the NaN with payload 0, which encodes as 0xf97e00.¶
5. There is no special handling of subnormal values.¶
6. The Common CBOR Deterministic Encoding Profile does not presume equivalence of floating point values with other representation (e.g., tag 4/5) with basic floating point values.¶

The main intent here is to preserve the basic generic data model, so application profiles can make their own decisions. For an example of that, see Section 3.1.3.¶

While [I-D.rundgren-deterministic-cbor] is a relatively terse document that is not always easy to interpret, to this author its intent appears to be aligned with that of the Common CBOR Deterministic Encoding Profile defined here.¶

3. Application Profiles

The dCBOR Application Profile specifies the use of Deterministic Encoding as defined in Section 4.2 of [STD94] (see also [I-D.bormann-cbor-det] for more information) together with some application-level rules. As an example, the rules for Gordian dCBOR [I-D.mcnally-deterministic-cbor] are specified in this section.¶

The application-level rules specified by an Application Profile are based on the same Common CBOR Deterministic Encoding Profile; they do not "fork" CBOR.¶

An Application Profile implementation produces well-formed, deterministically encoded CBOR according to [STD94], and existing generic CBOR decoders will therefore be able to decode it, including those that check for Deterministic Encoding. Similarly, generic CBOR encoders will be able to produce valid CBOR that can be processed by Application Profile implementations, if handed Application Profile conforming data model level information from an application.¶

Please note that the separation between standard CBOR processing and the processing required by the Application Profile is a conceptual one: Both Application Profile processing and standard CBOR processing can be combined into a special dCBOR/CBOR encoder/decoder.¶

An Application Profile is intended to be used in conjunction with an application, which typically will use a subset of the CBOR generic data model, which in turn influences which subset of the application profile is used. As a result, an Application Profile places no direct requirement on what subset of CBOR is implemented. For instance, while the dCBOR application profile defines rules for the processing of floating point values, there is no requirement that dCBOR implementations support floating point numbers (or any other kind of number, such as arbitrary precision integers or 64-bit negative integers) when they are used with applications that do not use them.¶

4. CDDL support

[RFC8610] defines control operators to indicate that the contents of a byte string carries a CBOR-encoded data item (.cbor) or a sequence of CBOR-encoded data items (.cborseq).¶

CDDL specifications may want to specify that the data items should be encoded in Common CBOR Deterministic Encoding, or with the dCBOR application profile applied as well. This specification adds four CDDL control operators that can be used for this.¶

The control operators .cde and .cdeseq are exactly like .cbor and .cborseq except that they also require the encoded data item(s) to be in Common CBOR Deterministic Encoding.¶

The control operators .dcbor and .dcborseq are exactly like .cde and .cdeseq except that they also require the encoded data item(s) to conform to the dCBOR application profile.¶

For example, the normative comment in Section 3 of [I-D.draft-mcnally-envelope-03]:¶

leaf = #6.24(bytes)  ; MUST be dCBOR

...can now be formalized as:¶

leaf = #6.24(bytes .dcbor any)

More importantly, if the encoded data item also needs to have a specific structure, this can be expressed by the right hand side (instead of using the most general CDDL type any here).¶

(Note that the ...seq control operators do not enable specifying different deterministic encoding requirements for the elements of the sequence. If a use case for such a feature becomes known, it could be added.)¶

5. Implementation Status

This section is to be removed before publishing as an RFC.¶

(Boilerplate as per Section 2.1 of [RFC7942]:)¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶

6. Security Considerations

TODO Security¶

7. IANA Considerations

RFC Editor: please replace RFCXXXX with the RFC number of this RFC and remove this note.¶

This document requests IANA to register the contents of Table 1 into the registry "CDDL Control Operators" of [IANA.cddl]:¶

Acknowledgments

Section 3.1 of this document is based on the work of Wolf McNally and Christopher Allen as documented in [I-D.mcnally-deterministic-cbor] and discussed in 2023 in the CBOR working group.¶

Contributors

Author's Address