Re: [Suit] Packed CBOR

Brendan:

Perhaps it is too early to say, but is this CBOR-based dictionary compression scheme likely to be widely supported in CBOR libraries?  To me, the answer to that question weighs heavily on the preferred way forward.

Russ

> On Jul 30, 2020, at 11:46 AM, Brendan Moran <Brendan.Moran@arm.com> wrote:
> 
> On Monday, Carsten presented Packed CBOR (draft-bormann-cbor-packed-00) at the CBOR working group, which adopted it. This is an extension to the CBOR standard (RFC7049) that enables “packing” of CBOR objects using a CBOR-based dictionary compression scheme.
> 
> If this had been in draft 18 months ago, the SUIT manifest would have used it instead of the “common” block. The common block is a de-facto packing mechanism that would be largely supplanted by packed CBOR. It simplifies many of the schemes that SUIT already uses and makes the manifest more compact besides.
> 
> If we were to adopt it now, this would cause two substantial changes in SUIT:
> 
> 1. Removing several existing SUIT deduplication mechanisms.
> 2. Placing a dependency on draft-ietf-cbor-packed-00.
> 
> Both of these would delay SUIT.
> 
> It would have benefits:
> 
> 1. Simplify the manifest structure (complexity moved to packed cbor)
> 2. Make the manifest smaller
> 
> 
> I see several options ahead of us:
> 1. Make no change, apply packed CBOR as and when it makes sense.
> 2. Make no change now, but plan for a v2 SUIT manifest draft
> 3. Adopt packed CBOR & simplify manifest now.
> 
> Option 1 is somewhat problematic in that it splits the ecosystem we’re trying to create. Option 2 does the same, but provides more benefits. 2 is arguably more detectable, since it’s easier to report manifest v2 support, than to report support for a specific CBOR tag within the SUIT Manifest Processor.
> 
> Option 3 delays SUIT both for updates to SUIT itself, and for the dependency on draft-ietf-cbor-packed-00.
> 
> If it weren’t for the delays, I’d go for Option 3. With the delays, I’m not sure what the right answer is. 1 & 2 fracture the ecosystem of updatable devices into “haves” and “have nots.”
> 
> I plan to discuss this tomorrow at the SUIT meeting.
> 
> 
> Best Regards,
> Brendan
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