Re: [Cbor] Private tag numbers

[Anders Rundgren <[e-mail]>]

On 2023-04-21 18:53, Tony Putman wrote:
> Hi all,
> 
> I'm defining a data schema for purely internal use (no interoperability needed) and encoding it in CBOR. There are points where we may want to vary/extend the contents with new data, and I thought the natural way to do this would be to define different tags to hold the data. I expected to find a set of tag numbers defined in IANA for private use, but there don't appear to be any.
> 
> - Is this a misuse of tags?  And if so, what would be the more natural way to specify this?
> 
> - Is this a general problem that people face and would it be worth defining a set of private tags which are known to be context-sensitive and not interoperable?
> 
Hi Tony,

Personally I find the tag system less useful if you have a large bunch of ever changing objects. Obviously reusing tag numbers for revised objects isn't a good idea.

One way to address this issue would be to register a *single* master tag, and then use locally defined sub-tags to specify the actual object id:

   78564([2, {
     "hi": "there"
   }])

Where 78564 would be your master (registered) tag, and 2 the locally defined id. The overhead in the sample is just two bytes compared to specifying a unique tag for each object.


If saving bytes is not your main concern, you may find a concept borrowed from the XML world useful:
https://www.ietf.org/archive/id/draft-rundgren-cotx-04.html
With public repositories like GitHub, anybody can enjoy a robust and project specific namespace.

Example: https://fido-web-pay.github.io/specification/crypto.html#5

Anders

> Thoughts/comments on this will be gratefully received.
> 
> Tony
> 
> 
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