[Cbor] Packed CBOR review

Brendan Moran <Brendan.Moran@arm.com> Wed, 16 June 2021 09:22 UTC

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From: Brendan Moran <Brendan.Moran@arm.com>
To: "cbor@ietf.org" <cbor@ietf.org>
Thread-Topic: Packed CBOR review
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Date: Wed, 16 Jun 2021 09:21:46 +0000
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Subject: [Cbor] Packed CBOR review
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As promised, here is my review of draft-ietf-cbor-packed.

Overall, I have few comments and I am confident that this draft will improve compactness of cbor structures—at least the ones I care about—with little overhead.

I don’t see any explanation of how to handle hierarchical packing. This would resolve the issue on Page 7.


      |  ISSUE: Not sure that we want to use the efficiencies of
      |  overriding, but having default values supplied out of a
      |  dictionary to be overridden by a rump sounds rather handy.
      |  Note that there is no way to remove a map entry from the table.


My take on this issue is: handle this the same way as hierarchical packing.

I do have a proposal for hierarchical packing. It’s a single, simple rule: References overflow from any table to a parent table. This means that the parser holds a stack of packing tables. Any time it encounters a reference, it attempts to apply that reference to the topmost table in the stack, deducting the size of the table from the reference and traversing the stack to the next table in the stack each time the reference is larger than the number of elements in the table.


Example:

Raw data:
{
    "label1" : "value1",
    "label2" : "value2",
    "label3" : "value3”
}

Compressed:

51([
    [],
    ["label"],
    [],
    51(
        [
            [],
            ["value"],
            [],
            {
               225("1") : 6("1"),
               225("2") : 6("2"),
               225("3") : 6("3")
            }
        ]
    )
])

If the outer table were implicit:

[
    [],
    ["value"],
    [],
    {
       225("1") : 6("1"),
       225("2") : 6("2"),
       225("3") : 6("3")
    }
]


I would also propose adding another element to the table: an identifier for a pre-shared reference table:

[
    “MyTable”,
    [],
    ["value"],
    [],
    {
       225("1") : 6("1"),
       225("2") : 6("2"),
       225("3") : 6("3")
    }
]


Best Regards,
Brendan

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