Re: [Cbor] Packed CBOR review

Brendan Moran <Brendan.Moran@arm.com> Thu, 17 June 2021 09:08 UTC

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From: Brendan Moran <Brendan.Moran@arm.com>
To: Carsten Bormann <cabo@tzi.org>
CC: "cbor@ietf.org" <cbor@ietf.org>
Thread-Topic: [Cbor] Packed CBOR review
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Date: Thu, 17 Jun 2021 09:08:18 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/cbor/1OVZSftmnsWvqj19imXm3Is3nMs>
Subject: Re: [Cbor] Packed CBOR review
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> On 16 Jun 2021, at 14:07, Carsten Bormann <cabo@tzi.org> wrote:
>
> On 2021-06-16, at 11:21, Brendan Moran <Brendan.Moran@arm.com> wrote:
>>
>> I would also propose adding another element to the table: an identifier for a pre-shared reference table:
>
> I’m wondering what the allocation mechanics for such an identifier would be.

We can have our cake and eat it too. I don’t see a reason not to support all of these choices.

>
> Let me go through a few cases and explain how I would handle it.
>
> * IANA
>
> Pro: can be very compact.
> Con (pro as well): high threshold, limited set.
>
> Solution: allocate a tag for the identifier, either wrapping a tag 51 or replacing it (in particular if not all tables need to be advanced).
>
> Task: We probably should define how to define such a tag.
>
> * Context
>
> Pro: can be embedded nicely.
> Con: Wherever the context comes from (e.g., referencing standard) needs to make sure the reference is unambiguous.
>
> Task: Not sure what we can do here, except maybe giving an example.
>
> * URI
>
> Pro: Free for all.
> Con: Hard to control.
>
> There will need to be some security and assurance of non-mutability on the data that URI describes.
> Preferred URI scheme would be ni:, but see next.
>
> Task: Define URI import tag; shape of data structure that should be used (almost, but not entirely, like tag 51).
>
> * Hash
>
> Pro: No-brainer
> Con: Can’t find the dictionary unless I already know it.
> Hashes are large (at least when secure).
>
> Task:
> Define Hash import tag, shape of data structure that is being hashed (see above).
>

If we simply use a different type in each case, the encoding is unambiguous:

Basic-Packed-CBOR = #6.51([ ?dictionary-reference, [*shared], [*prefix], [*suffix], rump])
rump = any
prefix = any
suffix = any
shared = any

dictionary-reference /= standard-reference
dictionary-reference /= uri-reference
dictionary-reference /= hash-reference
dictionary-reference /= context-reference

standard-reference = uint
uri-reference = tstr
hash-reference = bstr
context-reference = null


> Apparently, in all cases it would be good to have a single data structure shape pre-defined for pre-populating the tables (“dictionary”).

I can see two possibilities here:

1) just use the existing structure:

Packed-CBOR-Dictionary = [ ?reference, [*shared], [*prefix], [*suffix]]

2) eliminate any distinction between the different tables; leave that to the packing table.

Packed-CBOR-Dictionary = [?reference, [*fragment]]

For simplicity in the parser, I’d be inclined to go with option 1 and just make the Packed-CBOR-Dictionary a packing table with no rump.


Best Regards,
Brendan
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