Packed CBOR

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Table of Contents

1. Introduction

(TO DO, expand on text from abstract here; move references here and neuter them in the abstract as per Section 4.3 of [RFC7322].)¶

The specification defines a transformation from a Packed CBOR data item to the original CBOR data item; it does not define an algorithm for an actual packer. Different packers can differ in the amount of effort they invest in arriving at a minimal packed form.¶

Packed CBOR can employ two kinds of optimization:¶

• structure sharing: substructures (data items) that occur repeatedly in the original CBOR data item can be collapsed to a simple reference to a common representation of that data item. The processing required during consumption is limited to following that reference.¶
• prefix sharing: strings that share a prefix can be replaced by a reference to a common prefix plus the rest of the string. The processing required during consumption is similar to following the prefix reference plus that for an indefinite-length string.¶

A specific application protocol that employs Packed CBOR might allow both kinds of optimization or limit the representation to structure sharing only.¶

2. Packed CBOR

Packed CBOR is defined in CDDL [RFC8610] as in Figure 1:¶

Packed-CBOR = #6.6([rump, [*prefix], *shared])
rump = any
prefix = any
shared = any

(This assumes the allocation of tag number 6, which is motivated further below. Note that the semantics of Tag 6 depend on its content: An integer turns the tag into a shared reference, a string into a prefix reference, and an array into a complete Packed CBOR data item as described above.)¶

The original CBOR data item can be reconstructed by recursively replacing shared and prefix references encountered in the rump by their defined values.¶

3. Discussion

This specification uses up a large number of Simple Values and Tags, in particular one of the rare one-byte tags and half of the one-byte simple values. Since the objective is compression, this is warranted if and only if there is consensus that this specific format could be useful for a wide area of applications, while maintaining reasonable simplicity in particular at the side of the consumer.¶

A maliciously crafted Packed CBOR data item might contain a reference loop. A consumer/decompressor MUST protect against that.¶

The current definition does nothing to help with packing CBOR sequences [RFC8742]; maybe it should.¶

Nesting packed CBOR data items is not useful; maybe it should.¶

4. IANA Considerations

In the registry [IANA.cbor-tags], IANA is requested to allocate the tags defined in Table 3.¶

In the registry [IANA.cbor-simple-values], IANA is requested to allocate the simple values defined in Table 4.¶

5. Security Considerations

The security considerations of RFC 7049 apply.¶

Loops in the Packed CBOR can be used as a denial of service attack, see Section 3.¶

As the unpacking is deterministic, packed forms can be used as signing inputs. (Note that if external dictionaries are added to cbor-packed, this requires additional consideration.)¶

Appendix A. Example

The (JSON-compatible) CBOR data structure depicted in Figure 2, 400 bytes of binary CBOR, could lead to a packed CBOR data item depicted in Figure 3, 307 bytes. Note that this example does not lend itself to prefix compression.¶

{ "store": {
    "book": [
      { "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      },
      { "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      },
      { "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      },
      { "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
      }
    ],
    "bicycle": {
      "color": "red",
      "price": 19.95
    }
  }
}

6([{"store": {
      "book": [
        {simple(1): "reference", simple(2): "Nigel Rees",
         simple(3): "Sayings of the Century", simple(0): simple(5)},
        {simple(1): simple(4), simple(2): "Evelyn Waugh",
         simple(3): "Sword of Honour", simple(0): 12.99},
        {simple(1): simple(4), simple(2): "Herman Melville",
         simple(3): "Moby Dick", simple(6): "0-553-21311-3",
         simple(0): simple(5)},
        {simple(1): simple(4), simple(2): "J. R. R. Tolkien",
         simple(3): "The Lord of the Rings",
         simple(6): "0-395-19395-8", simple(0): 22.99}],
      "bicycle": {"color": "red", simple(0): 19.95}}},
   [],
   "price", "category", "author", "title", "fiction", 8.95, "isbn"])
   /  0          1         2         3         4       5      6   /

TBD: Do this for a W3C Thing Description again to get better packing and to exercise prefix compression...¶

Acknowledgements

CBOR packing was originally invented with the rest of CBOR, but did not make it into [RFC7049]. Various attempts to come up with a specification over the years didn't proceed. In 2017, Sebastian Käbisch proposed investigating compact representations of W3C Thing Descriptions, which prompted the author to come up with essentially the present design.¶

Author's Address