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Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                              13 July 2020
Expires: 14 January 2021


                              Packed CBOR
                      draft-bormann-cbor-packed-00

Abstract

   The Concise Binary Object Representation (CBOR, RFC 7049) is a data
   format whose design goals include the possibility of extremely small
   code size, fairly small message size, and extensibility without the
   need for version negotiation.

   CBOR does not provide any forms of data compression.  CBOR data
   items, in particular when generated from legacy data models often
   allow considerable gains in compactness when applying data
   compression.  While traditional data compression techniques such as
   DEFLATE (RFC 1951) work well for CBOR, their disadvantage is that the
   receiver needs to unpack the compressed form to make use of data.

   This specification describes Packed CBOR, a simple transformation of
   a CBOR data item into another CBOR data item that is almost as easy
   to consume as the original CBOR data item.  A separate decompression
   step is therefore often not required at the receiver.

Note to Readers

   This is an individual submission to the CBOR working group of the
   IETF, https://datatracker.ietf.org/wg/cbor/about/
   (https://datatracker.ietf.org/wg/cbor/about/).  Discussion currently
   takes places on the github repository https://github.com/cabo/cbor-
   packed (https://github.com/cabo/cbor-packed).  If the CBOR WG
   believes this is a useful document, discussion is likely to move to
   the CBOR WG mailing list and a github repository at the CBOR WG
   github organization, https://github.com/cbor-wg (https://github.com/
   cbor-wg).

   The current version is true work in progress; some of the sections
   haven't been filled in yet, and in particular, permission has not
   been obtained from tag definition authors to copy over their text.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.



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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Packed CBOR . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Referencing Shared Items  . . . . . . . . . . . . . . . .   4
     2.2.  Referencing Prefix Items  . . . . . . . . . . . . . . . .   4
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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].)




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   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 employs 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.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The definitions of [I-D.ietf-cbor-7049bis] apply.  The term "byte" is
   used in its now customary sense as a synonym for "octet".  Where bit
   arithmetic is explained, this document uses the notation familiar
   from the programming language C (including C++14's 0bnnn binary
   literals), except that the operator "**" stands for exponentiation.

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

                       Figure 1: Packed CBOR in CDDL

   (This assumes the allocation of tag number 6, which is motivated
   further below.)




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

2.1.  Referencing Shared Items

   Shared items are stored in the third to last element of the array
   used as tag content for tag number 6, numbered starting by 2.

   The shared data items are referenced by using the data items in
   Table 1.  When reconstructing the original data item, such a
   reference is replaced by the referenced data item, which is then
   recursively unpacked.

              +===========================+================+
              | reference                 | element number |
              +===========================+================+
              | Simple value 0-15         | 2-17           |
              +---------------------------+----------------+
              | Tag 6(unsigned integer N) | 18 + 2*N       |
              +---------------------------+----------------+
              | Tag 6(negative integer N) | 18 - 2*N - 1   |
              +---------------------------+----------------+

                    Table 1: Referencing Shared Values

   Taking into account the encoding, there are 16 one-byte references,
   48 two-byte references, 512 three-byte references, 131072 four-byte
   references, etc.  As integers can grow to very large (or small)
   values, there is no practical limit to how many shared items might be
   used in a Packed CBOR item.

2.2.  Referencing Prefix Items

   Shared items are stored in an array that is the second element of the
   array used as tag content for tag number 6.  This array is indexed
   from 0.

   Prefix data items are referenced by using the data items in Table 2.
   When reconstructing the original data item, such a reference is
   replaced by a string constructed from the referenced prefix data item
   (prefix, which might need to be recursively unpacked first)
   concatenated with the tag content (suffix, again possibly recursively
   unpacked).  The result gets the type of the suffix; this way a single
   prefix can be used to build both byte and text strings, depending on
   what type of suffix is being used.





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          +===================================+================+
          | reference                         | element number |
          +===================================+================+
          | Tag 6(suffix)                     |              0 |
          +-----------------------------------+----------------+
          | Tag 224-255(suffix)               |           1-32 |
          +-----------------------------------+----------------+
          | Tag 28672-32767(suffix)           |        33-4128 |
          +-----------------------------------+----------------+
          | Tag 1879048192-2147483647(suffix) | 4129-268439584 |
          +-----------------------------------+----------------+

                    Table 2: Referencing Prefix Values

   Taking into account the encoding, there is one one-byte prefix
   reference, 32 two-byte references, 4096 three-byte references, and
   268435456 five-byte references.  268439585 (2**28+2**12+2**5+2**0) is
   an artificial limit, but should be high enough that there, again, is
   no practical limit to how many prefix items might be used in a Packed
   CBOR item.

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.

   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.

   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.





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   +===========+========+================+===========================+++
   |       Tag | Data   | Semantics      | Reference                 |||
   |           | Item   |                |                           |||
   +===========+========+================+===========================+++
   |         6 | array, | Packed CBOR:   | draft-bormann-cbor-packed |||
   |           |integer,| packed/shared/ |                           |||
   |           | text   | prefix         |                           |||
   |           |string, |                |                           |||
   |           | byte   |                |                           |||
   |           | string |                |                           |||
   +-----------+--------+----------------+---------------------------+++
   |   224-255 | text   | Packed CBOR:   | draft-bormann-cbor-packed |||
   |           | string | prefix         |                           |||
   |           |or byte |                |                           |||
   |           | string |                |                           |||
   +-----------+--------+----------------+---------------------------+++
   |28672-32767| text   | Packed CBOR:   | draft-bormann-cbor-packed |||
   |           | string | prefix         |                           |||
   |           |or byte |                |                           |||
   |           | string |                |                           |||
   +-----------+--------+----------------+---------------------------+++
   |1879048192-| text   | Packed CBOR:   | draft-bormann-cbor-packed |||
   | 2147483647| string | prefix         |                           |||
   |           |or byte |                |                           |||
   |           | string |                |                           |||
   +-----------+--------+----------------+---------------------------+++

                      Table 3: Values for Tag Numbers

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

        +=======+=====================+===========================+
        | Value | Semantics           | Reference                 |
        +=======+=====================+===========================+
        |  0-15 | Packed CBOR: shared | draft-bormann-cbor-packed |
        +-------+---------------------+---------------------------+

                           Table 4: Simple Values

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.

6.  References



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6.1.  Normative References

   [I-D.ietf-cbor-7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", Work in Progress, Internet-Draft,
              draft-ietf-cbor-7049bis-14, 16 June 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-cbor-
              7049bis-14.txt>.

   [IANA.cbor-simple-values]
              IANA, "Concise Binary Object Representation (CBOR) Simple
              Values",
              <http://www.iana.org/assignments/cbor-simple-values>.

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <http://www.iana.org/assignments/cbor-tags>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

6.2.  Informative References

   [RFC7322]  Flanagan, H. and S. Ginoza, "RFC Style Guide", RFC 7322,
              DOI 10.17487/RFC7322, September 2014,
              <https://www.rfc-editor.org/info/rfc7322>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/info/rfc8742>.









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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
       }
     }
   }

                 Figure 2: Example original CBOR data item











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   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   /

                  Figure 3: Example packed CBOR data item

   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

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org










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