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[Cbor] Re: Data model versus serialization (was Re: Scrapping the CDE "Application Profile" concept)

"lgl island-resort.com" <lgl@island-resort.com> Tue, 20 August 2024 19:46 UTC

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From: "lgl island-resort.com" <lgl@island-resort.com>
To: Anders Rundgren <anders.rundgren.net@gmail.com>
Thread-Topic: Data model versus serialization (was Re: [Cbor] Scrapping the CDE "Application Profile" concept)
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Date: Tue, 20 Aug 2024 19:45:59 +0000
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Subject: [Cbor] Re: Data model versus serialization (was Re: Scrapping the CDE "Application Profile" concept)
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On Aug 19, 2024, at 12:56 PM, Anders Rundgren <anders.rundgren.net@gmail.com> wrote:

On 2024-08-19 19:51, lgl island-resort.com<http://island-resort.com/> wrote:
Trying for a bit of clarity on data model versus serialization...
== Data model ==
- Fundamentally, the data types supported — the CBOR generic data model (plus extended data model)
- Most protocol designs are in terms of data types (not serialization)
== Serialization ==
- Says how to encode a particular data type
- A particular serialization is chosen for message size and implementation constraints
- Also, specific serialization is needed for deterministic encoding
- Serialization choices don't narrow or otherwise change the CBOR data model
- Actual implementations of a protocol must agree on serialization to achieve interoperation
Seems to me that JSON has no such split at all. ASN.1 has a little one with DER/BER. CBOR is quite different in having this separation. Necessarily so to support very constrained environments.
Preferred Serialization, CDE and Basic/CIE are primarily serialization specifications (however, they stray into data model with the reduction of big numbers and integers (RFC 8949 3.4.3)).
dCBOR is primarily about the data model. It specifies no new serialization, just re uses CDE. Its main contribution is reduction of floats and integers and exclusion of a few data types.
Does this sound right?

I don't bother too much about the terminology, more about the consequences.

Understanding the difference between serialization and data model was important for me. Made it clear that serialization needs to support the whole data model, including NaN payloads. Made it also clear why we’re spending huge efforts here — other serialization schemes don’t have much serialization variability.


 Carsten talks about mapping.  Numeric reduction is mapping?

Application Profiles are described as exclusions and reductions.

Disallowing “undef” is an exclusion

Folding floats and ints together is a reduction. Similarly folding ints and big nums. Should dCBOR reduce “undef” to “null”?

Makes sense to me as a general way to characterize what an Application Profile does (but still not a fan of Application Profiles).

LL

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