Re: [Cbor] Packed CBOR review

Henk Birkholz <henk.birkholz@sit.fraunhofer.de> Thu, 17 June 2021 09:28 UTC

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To: Brendan Moran <Brendan.Moran@arm.com>, Carsten Bormann <cabo@tzi.org>
CC: "cbor@ietf.org" <cbor@ietf.org>
References: <8713C3AB-71C0-4EC0-8977-15F80EC11309@arm.com> <212CE7EA-73BC-47BF-B192-D2D523F4A376@tzi.org> <33C84949-0F9C-432C-9C94-DE2C9EE17976@arm.com>
From: Henk Birkholz <henk.birkholz@sit.fraunhofer.de>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/cbor/Qo6e32uGVap2Yv6Jx97mg66c4c0>
Subject: Re: [Cbor] Packed CBOR review
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Hi Brendan,

quick reply with an upfront, overall +1. Small comments in-line

On 17.06.21 11:08, Brendan Moran wrote:
> 
>> 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.

That sounds absolutely reasonable to me!

> 
>>
>> 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.
Henk
>>
>> 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
> 

Semantics look fine, not sure of the final structure though.

> 
>> 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.

We are really optimizing for bytes here, right? That should be 
significant decision factor. Then again, a tad bit more hierarchy here 
might provide great benefits?

> 
> 
> Best Regards,
> Brendan
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Viele Grüße,

Henk

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