Packed CBOR
draft-ietf-cbor-packed-03

Network Working Group                                         C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                            13 August 2021
Expires: 14 February 2022

                              Packed CBOR
                       draft-ietf-cbor-packed-03

Abstract

   The Concise Binary Object Representation (CBOR, RFC 8949) 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) can work well for CBOR encoded data items, 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 a working-group draft of the CBOR working group of the IETF,
   https://datatracker.ietf.org/wg/cbor/about/
   (https://datatracker.ietf.org/wg/cbor/about/).  Discussion takes
   places on the github repository https://github.com/cbor-wg/cbor-
   packed (https://github.com/cbor-wg/cbor-packed) and on the CBOR WG
   mailing list, https://www.ietf.org/mailman/listinfo/cbor
   (https://www.ietf.org/mailman/listinfo/cbor).

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 February 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Packed CBOR . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Packing Tables  . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Referencing Shared Items  . . . . . . . . . . . . . . . .   4
     2.3.  Referencing Affix Items . . . . . . . . . . . . . . . . .   5
     2.4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Table Setup . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Basic Packed CBOR . . . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

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 employ two kinds of optimization:

   *  item 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.

   *  affix sharing: data items (strings, containers) that share a
      prefix or suffix (affix) can be replaced by a reference to a
      common affix plus the rest of the data item.  For strings, the
      processing required during consumption is similar to following the
      affix 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 item
   sharing only.

   Packed CBOR is defined in two parts: Referencing packing tables
   (Section 2) and setting up packing tables (Section 3).

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Packed reference:  A shared item reference or an affix reference

   Shared item reference:  A reference to a shared item as defined in
      Section 2.2

   Affix reference:  A reference that combines an affix item as defined
      in Section 2.3.

   Affix:  Prefix or suffix.

   Packing tables:  The triple of a shared item table, a prefix table,
      and a suffix table.

   Expansion:  The result of applying a packed reference in the context

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      of given Packing tables.

   The definitions of [RFC8949] 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, in the plain text form of this document, the operator
   "^" stands for exponentiation, and, in the HTML and PDF versions,
   subtraction and negation are rendered as a hyphen ("-", as are
   various dashes).

2.  Packed CBOR

   This section describes the packing tables, their structure, and how
   they are referenced.

2.1.  Packing Tables

   At any point within a data item making use of Packed CBOR, there is a
   Current Set of packing tables that applies.

   There are three packing tables in a Current Set:

   *  Shared item table

   *  Prefix table

   *  Suffix table

   Without any table setup, all these tables are empty arrays.
   Table setup can cause these arrays to be non-empty, where the
   elements are (potentially themselves packed) data items.  Each of the
   tables is indexed by an unsigned integer (starting from 0), which may
   be computed from information in tags and their content as well as
   from CBOR simple values.

2.2.  Referencing Shared Items

   Shared items are stored in the shared item table of the Current Set.

   The shared data items are referenced by using the reference 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.

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               +===========================+==============+
               | reference                 | table index  |
               +===========================+==============+
               | Simple value 0-15         | 0-15         |
               +---------------------------+--------------+
               | Tag 6(unsigned integer N) | 16 + 2*N     |
               +---------------------------+--------------+
               | Tag 6(negative integer N) | 16 - 2*N - 1 |
               +---------------------------+--------------+

                    Table 1: Referencing Shared Values

   As examples in CBOR diagnostic notation (Section 8 of [RFC8949]), the
   first 22 elements of the shared item table are referenced by
   "simple(0)", "simple(1)", ... "simple(15)", "6(0)", "6(-1)", "6(1)",
   "6(-2)", "6(2)", "6(-3)".  (The alternation between unsigned and
   negative integers for even/odd table index values makes systematic
   use of shorter integer encodings first.)

   Taking into account the encoding of these referring data items, 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 negative) values, there is no practical limit to
   how many shared items might be used in a Packed CBOR item.

   Note that the semantics of Tag 6 depend on its content: An integer
   turns the tag into a shared item reference, a string or container
   (map or array) into a prefix reference (see Table 2).

2.3.  Referencing Affix Items

   Prefix items are stored in the prefix table of the Current Set;
   suffix items are stored in the suffix table of the Current Set. We
   collectively call these items affix items; when referencing, which of
   the tables is actually used depends on whether a prefix or a suffix
   reference was used.

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          +===================================+================+
          | prefix reference                  |    table index |
          +===================================+================+
          | Tag 6(suffix)                     |              0 |
          +-----------------------------------+----------------+
          | Tag 225-255(suffix)               |           1-31 |
          +-----------------------------------+----------------+
          | Tag 28704-32767(suffix)           |        32-4095 |
          +-----------------------------------+----------------+
          | Tag 1879052288-2147483647(suffix) | 4096-268435455 |
          +-----------------------------------+----------------+

                    Table 2: Referencing Prefix Values

           +===================================+===============+
           | suffix reference                  |   table index |
           +===================================+===============+
           | Tag 216-223(prefix)               |           0-7 |
           +-----------------------------------+---------------+
           | Tag 27647-28671(prefix)           |        8-1023 |
           +-----------------------------------+---------------+
           | Tag 1811940352-1879048191(prefix) | 1024-67108863 |
           +-----------------------------------+---------------+

                     Table 3: Referencing Suffix Values

   Affix data items are referenced by using the data items in Table 2
   and Table 3.  Each of these implies the table used (prefix or
   suffix), a table index (an unsigned integer) and contains a "rump
   item".  When reconstructing the original data item, such a reference
   is replaced by a data item constructed from the referenced affix data
   item (affix, which might need to be recursively unpacked first)
   "concatenated" with the tag content (rump, again possibly recursively
   unpacked).

   *  For a rump of type array and map, the affix also needs to be an
      array or a map.  For an array, the elements from the prefix are
      prepended, and the elements from a suffix are appended to the rump
      array.  For a map, the entries in the affix are added to those of
      the rump; prefix and suffix references differ in how entries with
      identical keys are combined: for prefix references, an entry in
      the rump with the same key as an entry in the affix overrides the
      one in the affix, while for suffix references, an entry in the
      affix overrides an entry in the rump that has the same key.

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      |  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.

   *  For a rump of one of the string types, the affix also needs to be
      one of the string types; the bytes of the strings are concatenated
      as specified (prefix + rump, rump + suffix).  The result of the
      concatenation gets the type of the rump; this way a single affix
      can be used to build both byte and text strings, depending on what
      type of rump is being used.

   As a contrived (but short) example, if the prefix table is
   "["foobar", "foob", "fo"]", the following prefix references will all
   unpack to ""foobart"": "6("t")", "224("art")", "225("obart")" (the
   last example is not an optimization).

   Taking into account the encoding, there is one single-byte prefix
   reference, 31 (2^5-2^0) two-byte references, 4064 (2^12-2^5) three-
   byte references, and 26843160 (2^28-2^12) five-byte references for
   prefixes. 268435455 (2^28) 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.  The numbers for suffix
   references are one quarter of those, except that there is no single-
   byte reference and 8 two-byte references.

      |  Rationale: Experience suggests that prefix packing might be
      |  more likely than suffix packing.  Also for this reason, there
      |  is no intent to spend a 1+0 tag value for suffix matching.

2.4.  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.

      |  Different strategies for decoding/consuming Packed CBOR are
      |  available.
      |  For example:
      |  

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      |     *  the decoder can decode and unpack the packed item,
      |        presenting an unpacked data item to the application.  In
      |        this case, the onus of dealing with loops is on the
      |        decoder.  (This strategy generally has the highest memory
      |        consumption, but also the simplest interface to the
      |        application.)  Besides avoiding getting stuck in a
      |        reference loop, the decoder will need to control its
      |        resource allocation, as data items can "blow up" during
      |        unpacking.
      |  
      |     *  the decoder can be oblivious of Packed CBOR.  In this
      |        case, the onus of dealing with loops is on the
      |        application, as is the entire onus of dealing with Packed
      |        CBOR.
      |  
      |     *  hybrid models are possible, for instance: The decoder
      |        builds a data item tree directly from the Packed CBOR as
      |        if it were oblivious, but also provides accessors that
      |        hide (resolve) the packing.  In this specific case, the
      |        onus of dealing with loops is on the accessors.
      |  
      |  In general, loop detection can be handled in a similar way in
      |  which loops of symbolic links are handled in a file system: A
      |  system wide limit (often 31 or 40 indirections for symbolic
      |  links) is applied to any reference chase.

      |  ISSUE: The present specification does nothing to help with the
      |  packing of CBOR sequences [RFC8742]; maybe it should.

3.  Table Setup

   The packing references described in Section 2 assume that packing
   tables have been set up.

   By default, all three tables are empty (zero-length arrays).

   Table setup can happen in one of two ways:

   *  By the application environment, e.g., a media type.  These can
      define tables that amount to a static dictionary that can be used
      in a CBOR data item for this application environment.  Note that,
      without this information, a data item that uses such a static
      dictionary can be decoded at the CBOR level, but not fully
      unpacked.  The table setup mechanisms provided by this document
      are defined in such a way that an unpacker can at least recognize
      if this is the case.

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   *  By one or more tags enclosing the packed content.  These can be
      defined to add to the packing tables that already apply to the
      tag.  Usually, the semantics of the tag will be to prepend items
      to one of the tables.  Note that it may be useful to leave a
      particular efficiency tier alone and only prepend to a higher
      tier; e.g., a tag could insert shared items at table index 16 and
      shift anything that was already there further down in the array
      while leaving index 0 to 15 alone.  Explicit additions by tag can
      combine with application-environment supplied tables that apply to
      the entire CBOR data item.

   For table setup, the present specification only defines a single tag,
   which operates by prepending to the (by default empty) tables.

      |  We could also define a tag for dictionary referencing (or
      |  include that in the basic packed CBOR), but the desirable
      |  details are likely to vary considerably between applications.
      |  A URI-based reference would be easy to add, but might be too
      |  inefficient when used in the likely combination with an "ni:"
      |  URI [RFC6920].

3.1.  Basic Packed CBOR

   A predefined tag for packing table setup is defined in CDDL [RFC8610]
   as in Figure 1:

   Basic-Packed-CBOR = #6.51([[*shared-item], [*prefix-item],
                              [*suffix-item], rump])
   rump = any
   prefix-item = any
   suffix-item = any
   shared-item = any

                       Figure 1: Packed CBOR in CDDL

   (This assumes the allocation of tag number 51 for this tag.)

   The arrays given as the first, second, and third element of the
   content of the tag 51 are prepended to the tables for shared items,
   prefixes, and suffixes that apply to the entire tag (by default empty
   tables).

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

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   Packed item references in the newly constructed (low-numbered) parts
   of the table need to be interpreted in the number space of that table
   (which includes the, now higher-numbered inherited parts), while
   references in any existing, inherited (higher-numbered) part continue
   to use the (more limited) number space of the inherited table.

4.  IANA Considerations

   In the registry "CBOR Tags" [IANA.cbor-tags], IANA is requested to
   allocate the tags defined in Table 4.

   +=======================+========+=========+========================+
   |                   Tag |Data    |Semantics| Reference              |
   |                       |Item    |         |                        |
   +=======================+========+=========+========================+
   |                     6 |integer |Packed   | draft-ietf-cbor-packed |
   |                       |(for    |CBOR:    |                        |
   |                       |shared);|shared/  |                        |
   |                       |text    |prefix   |                        |
   |                       |string, |         |                        |
   |                       |byte    |         |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map (for|         |                        |
   |                       |prefix) |         |                        |
   +-----------------------+--------+---------+------------------------+
   |               225-255 |text    |Packed   | draft-ietf-cbor-packed |
   |                       |string, |CBOR:    |                        |
   |                       |byte    |prefix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+
   |           28704-32767 |text    |Packed   | draft-ietf-cbor-packed |
   |                       |string, |CBOR:    |                        |
   |                       |byte    |prefix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+
   | 1879052288-2147483647 |text    |Packed   | draft-ietf-cbor-packed |
   |                       |string, |CBOR:    |                        |
   |                       |byte    |prefix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+
   |               216-223 |text    |Packed   | draft-ietf-cbor-packed |

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   |                       |string, |CBOR:    |                        |
   |                       |byte    |suffix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+
   |           27647-28671 |text    |Packed   | draft-ietf-cbor-packed |
   |                       |string, |CBOR:    |                        |
   |                       |byte    |suffix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+
   | 1811940352-1879048191 |text    |Packed   | draft-ietf-cbor-packed |
   |                       |string, |CBOR:    |                        |
   |                       |byte    |suffix   |                        |
   |                       |string, |         |                        |
   |                       |array,  |         |                        |
   |                       |map     |         |                        |
   +-----------------------+--------+---------+------------------------+

                      Table 4: Values for Tag Numbers

   In the registry "CBOR Simple Values" [IANA.cbor-simple-values], IANA
   is requested to allocate the simple values defined in Table 5.

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

                          Table 5: Simple Values

5.  Security Considerations

   The security considerations of [RFC8949] apply.

   Loops in the Packed CBOR can be used as a denial of service attack,
   see Section 2.4.

   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.)

6.  References

6.1.  Normative References

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   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

6.2.  Informative References

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [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>.

   [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.  Examples

   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, ~309 bytes.  Note that this particular 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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   51(["price", "category", "author", "title", "fiction", 8.95, "isbn"],
      /  0          1         2         3         4       5      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}}}])

                  Figure 3: Example packed CBOR data item

   The (JSON-compatible) CBOR data structure below has been packed with
   shared item and (partial) prefix compression only.

   {
     "name": "MyLED",
     "interactions": [
       {
         "links": [
           {
             "href":
              "http://192.168.1.103:8445/wot/thing/MyLED/rgbValueRed",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "number"
           }
         },
         "name": "rgbValueRed",
         "writable": true,
         "@type": [
           "Property"
         ]
       },
       {
         "links": [
           {
             "href":

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              "http://192.168.1.103:8445/wot/thing/MyLED/rgbValueGreen",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "number"
           }
         },
         "name": "rgbValueGreen",
         "writable": true,
         "@type": [
           "Property"
         ]
       },
       {
         "links": [
           {
             "href":
              "http://192.168.1.103:8445/wot/thing/MyLED/rgbValueBlue",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "number"
           }
         },
         "name": "rgbValueBlue",
         "writable": true,
         "@type": [
           "Property"
         ]
       },
       {
         "links": [
           {
             "href":
              "http://192.168.1.103:8445/wot/thing/MyLED/rgbValueWhite",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "number"
           }
         },
         "name": "rgbValueWhite",

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         "writable": true,
         "@type": [
           "Property"
         ]
       },
       {
         "links": [
           {
             "href":
              "http://192.168.1.103:8445/wot/thing/MyLED/ledOnOff",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "boolean"
           }
         },
         "name": "ledOnOff",
         "writable": true,
         "@type": [
           "Property"
         ]
       },
       {
         "links": [
           {
             "href":
   "http://192.168.1.103:8445/wot/thing/MyLED/colorTemperatureChanged",
             "mediaType": "application/json"
           }
         ],
         "outputData": {
           "valueType": {
             "type": "number"
           }
         },
         "name": "colorTemperatureChanged",
         "@type": [
           "Event"
         ]
       }
     ],
     "@type": "Lamp",
     "id": "0",
     "base": "http://192.168.1.103:8445/wot/thing",
     "@context":
      "http://192.168.1.102:8444/wot/w3c-wot-td-context.jsonld"

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   }

                 Figure 4: Example original CBOR data item

   51([/shared/["name", "@type", "links", "href", "mediaType",
               /  0       1       2        3         4 /
       "application/json", "outputData", {"valueType": {"type":
            /  5               6               7 /
       "number"}}, ["Property"], "writable", "valueType", "type"],
                  /   8            9           10           11 /
      /prefix/ ["http://192.168.1.10", 6("3:8445/wot/thing"),
                 / 6                        225 /
      225("/MyLED/"), 226("rgbValue"), "rgbValue",
        / 226             227           228     /
      {simple(6): simple(7), simple(9): true, simple(1): simple(8)}],
        / 229 /
      /suffix/ [],
      /rump/ {simple(0): "MyLED",
              "interactions": [
      229({simple(2): [{simple(3): 227("Red"), simple(4): simple(5)}],
       simple(0): 228("Red")}),
      229({simple(2): [{simple(3): 227("Green"), simple(4): simple(5)}],
       simple(0): 228("Green")}),
      229({simple(2): [{simple(3): 227("Blue"), simple(4): simple(5)}],
       simple(0): 228("Blue")}),
      229({simple(2): [{simple(3): 227("White"), simple(4): simple(5)}],
       simple(0): "rgbValueWhite"}),
      {simple(2): [{simple(3): 226("ledOnOff"), simple(4): simple(5)}],
       simple(6): {simple(10): {simple(11): "boolean"}}, simple(0):
       "ledOnOff", simple(9): true, simple(1): simple(8)},
      {simple(2): [{simple(3): 226("colorTemperatureChanged"),
       simple(4): simple(5)}], simple(6): simple(7), simple(0):
       "colorTemperatureChanged", simple(1): ["Event"]}],
        simple(1): "Lamp", "id": "0", "base": 225(""),
        "@context": 6("2:8444/wot/w3c-wot-td-context.jsonld")}])

                  Figure 5: Example packed CBOR data item

Acknowledgements

   CBOR packing was originally invented with the rest of CBOR, but did
   not make it into [RFC7049], the predecessor of [RFC8949].  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.

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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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