Map-like data in CBOR and CDDL
draft-bormann-cbor-cddl-map-like-data-00

Network Working Group                                    C. Bormann, Ed.
Internet-Draft                                    Universität Bremen TZI
Intended status: Informational                                  B. Moran
Expires: 6 October 2021                                      Arm Limited
                                                             H. Birkholz
                                                          Fraunhofer SIT
                                                              E. Cormier
                                                            4 April 2021

                     Map-like data in CBOR and CDDL
                draft-bormann-cbor-cddl-map-like-data-00

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.

   Basic CBOR supports unordered maps free of duplicate keys, similar to
   the way JSON defines JSON objects (RFC 8259).  Using the CBOR
   extension point of tags, tags for a selection of variants of maps and
   multimaps have been registered, but gaps remain.  The present
   document defines a consolidated set of CBOR tags for map-like data
   items involving key-value pairs.

   The Concise Data Definition Language (CDDL), standardized in RFC
   8610, is often used to express CBOR data structure specifications.
   It provides "control operators" as its main language extension point.

   The present document defines a number of control operators that
   enable the description of CBOR data structures that make use of the
   newly defined tags or that employ the same underlying structures.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CBOR tags for map-like data items . . . . . . . . . . . . . .   3
     2.1.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Key/Value Type  . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Ordering  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Key Uniqueness  . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Data Item . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.6.  Related Tags (Informative)  . . . . . . . . . . . . . . .   6
       2.6.1.  Tag 259 . . . . . . . . . . . . . . . . . . . . . . .   6
       2.6.2.  Tag 275 . . . . . . . . . . . . . . . . . . . . . . .   6
       2.6.3.  Tag TBD279  . . . . . . . . . . . . . . . . . . . . .   6
       2.6.4.  Tag TDB280  . . . . . . . . . . . . . . . . . . . . .   6
   3.  CDDL Support for Map-Like Data Items  . . . . . . . . . . . .   7
     3.1.  Map notation for map-like data items  . . . . . . . . . .   7
     3.2.  Uniqueness  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  CDDL typenames  . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Tags  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  CDDL control operators  . . . . . . . . . . . . . . . . .  10
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  10
   7.  Security considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Implementation Considerations  . . . . . . . . . . .  11
     A.1.  Programming Language Containers (Informative) . . . . . .  11

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       A.1.1.  ECMAScript  . . . . . . . . . . . . . . . . . . . . .  12
       A.1.2.  Python  . . . . . . . . . . . . . . . . . . . . . . .  12
       A.1.3.  C++ . . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  CDDL Implementation Considerations  . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   (See abstract for now.)

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.

   This specification uses terminology from [RFC8949] and [RFC8610].  In
   particular, with respect to CDDL control operators, "target" refers
   to the left hand side operand, and "controller" to the right hand
   side operand.  The terms "array" and "map" (if unadorned) refer to
   CBOR major type 4 and CBOR major type 5; this is not called out
   explicitly.

2.  CBOR tags for map-like data items

   This document defines a consolidated set of CBOR tags for map-like
   entities involving key-value pairs.  These tags encode the following
   meta-data concerning map-like data items:

   *  the homogeneity of the key and value types,

   *  whether the order of the key-value pairs carries semantic value
      ("ordered") or needs to be ignored ("unordered"),

   *  the uniqueness of the keys, and,

   *  the major type used to encode the key-value pairs.

   Note that the term "ordered" as used in this document is distinct
   from "sorted" -- "ordered" implies that the order in the data item
   interchanged conveys a semantically relevant ordering, while a
   property "sorted" can easily be established after interchange (by,
   simply, sorting), less often needs to be indicated, and is more
   complex to indicate as it may need details about the sorting

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2.1.  Summary

   +===+====+===========+============+=========+=========+=====+=======+
   |Tag|LSBs|Homogeneous|Homogeneous |Ordering |Duplicate|Data |Related|
   |   |    |Value      |Key         |         |Keys     |Item |Tag    |
   |   |    |           |            |         |Allowed  |     |       |
   +===+====+===========+============+=========+=========+=====+=======+
   |128|0000|No         |No          |Unordered|No       |map  |259    |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |129|0001|No         |No          |Unordered|Yes      |array|TDB280*|
   +---+----+-----------+------------+---------+---------+-----+-------+
   |130|0010|No         |No          |Ordered  |No       |array|TBD279*|
   +---+----+-----------+------------+---------+---------+-----+-------+
   |131|0011|No         |No          |Ordered  |Yes      |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |132|0100|No         |Yes         |Unordered|No       |map  |275    |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |133|0101|No         |Yes         |Unordered|Yes      |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |134|0110|No         |Yes         |Ordered  |No       |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |135|0111|No         |Yes         |Ordered  |Yes      |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |136|1000|Yes        |Yes         |Unordered|No       |map  |       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |137|1001|Yes        |Yes         |Unordered|Yes      |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |138|1010|Yes        |Yes         |Ordered  |No       |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+
   |139|1011|Yes        |Yes         |Ordered  |Yes      |array|       |
   +---+----+-----------+------------+---------+---------+-----+-------+

              Table 1: New CBOR tags defined in this document

   *TBD279: https://github.com/Sekenre/cbor-ordered-map-
   spec/blob/master/CBOR_Ordered_Map.md

   *TBD280: https://github.com/ecorm/cbor-tag-multimap

   [The intention of the present document is to obviate the need for
   defining TBD279/TBD280.]

   Appendix A.1 provides information about constructs in a few
   programming languages that are related to the tags being defined.

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2.2.  Key/Value Type

   Bits 2/3 of the tag provides information on the map's key and value
   types:

   *  0b00xx Unspecified: There is no specified type for the map's keys
      and values

   *  0b01xx Homogeneous Key: All keys have the same data type

   *  0b10xx Homogeneous Key/Value: All values have the same data type
      in addition to all keys having the same data type (the types for
      keys and values may be distinct).

   The semantics for homogeneity shall be the same as for RFC8746
   homogeneous arrays (tag 41).  That is, "which CBOR data items
   constitute elements of the same application type is specific to the
   application" (RFC8746, subsection 3.2).

   Maps with arbitrary keys and homogeneous values are considered
   unusual, so they are left out of this specification in order to
   reduce the number of tags allocated (12 instead of 16).

2.3.  Ordering

   Bit 1 of the tag represents the map's ordering semantics:

   *  0: The order of key-value pairs is unspecified

   *  1: Key-value pairs are encoded in the same order in which they
      were inserted

2.4.  Key Uniqueness

   Bit 0 of the tag represents the uniqueness of the map's keys.

   *  0: Keys are unique

   *  1: Keys may be duplicated (i.e. multimaps)

2.5.  Data Item

   All these map-like data items could be represented as a tag with an
   enclosed array of alternating key-value pairs, as in:

   129(["key1", 1, "key2", 2])

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   However, representing the key-value pairs as a CBOR map for those
   cases where this is possible enables generic decoders that are
   oblivious of these tags to represent the data in a more appropriate
   platform type.

   Specifically, the key-value pairs are represented as a map if and
   only if

   *  the ordering is unspecified and

   *  the keys are unique;

   they are represented as an array of alternating keys and values
   otherwise.

2.6.  Related Tags (Informative)

2.6.1.  Tag 259

   Specification: https://github.com/shanewholloway/js-cbor-
   codec/blob/master/docs/CBOR-259-spec--explicit-maps.md

   The above defined tag 128 may be used instead to guide a JavaScript
   decoder into interpreting a CBOR map as a JavaScript Map instead of
   an Object.

2.6.2.  Tag 275

   Specification: https://github.com/ecorm/cbor-tag-text-key-map

   The above defined tag 132 may be used instead to guide a decoder into
   interpreting a CBOR map as a JavaScript-like Object having only text
   string keys.  The decoder would have to verify the first key to
   establish that the map has homogeneous text string keys.

2.6.3.  Tag TBD279

   Draft specification: https://github.com/Sekenre/cbor-ordered-map-
   spec/blob/master/CBOR_Ordered_Map.md

   The above defined tag 130 may be used instead to encode map-like data
   items where the order of the key-value pairs is semantically
   significant.

2.6.4.  Tag TDB280

   Draft specification: https://github.com/ecorm/cbor-tag-multimap

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   The above defined tag 129 may be used instead to encode a multimap as
   an array of key-value pairs.

3.  CDDL Support for Map-Like Data Items

   The Concise Data Definition Language (CDDL), standardized in RFC
   8610, provides "control operators" as its main language extension
   point.

   The present document defines a number of control operators that
   enable the use of group notation (enclosed in a CDDL map) to specify
   any of the above map-like data structures:

                   +=========+========================+
                   | Name    | Purpose                |
                   +=========+========================+
                   | .omm    | Ordered (Multi-)Map    |
                   +---------+------------------------+
                   | .umm    | Unordered (Multi-)Map  |
                   +---------+------------------------+
                   | .unique | Uniqueness requirement |
                   +---------+------------------------+

                      Table 2: New control operators
                             in this document

3.1.  Map notation for map-like data items

   [needs better examples]

   CDDL already can describe both arrays of alternating keys and values
   and maps (unordered and with unique keys).  The two control operators
   ".omm" and ".umm" introduced in this section enable the use of CDDL
   map notation for map-like types beyond actual maps, increasing
   readability and possibly even reusability.

   In a simple example that provides an unordered collection of zero or
   more home addresses and zero or more work addresses, each labeled as
   such, we use traditional map notation to describe that collection:

   [* (text, any)] .umm {
     * home: address
     * work: address
     $$more-addresses
   }

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   The ".omm" and ".umm" control operators convert a group definition
   enclosed into a CDDL map given as a controller type into an array
   type given as the target type.  The controller type given is
   unwrapped (Section 3.7 of [RFC8610]) into a group.  Keys and values
   of the entries in that group are then alternatingly matched as
   elements in the target array.  Note that both target and controller
   type can contribute to the shaping of the data; declaring the key
   type as "text" limits what can be added to the "$$more-addresses"
   socket.

   ".omm" and ".umm" differ in the semantics of the array type created:
   ".omm" defines an ordered (multi)map, i.e., the order of the key/
   value element pairs in the array matters, while ".umm" defines an
   unordered (multi)map, i.e., data items that present the same set of
   key/value pairs in different orders are equivalent.

   Note that the ability to specify specific ("homogeneous") types is
   provided by the ability to specify the target type, as in the example
   above.

   Note that there is not strictly a need to define a control operator
   for building unordered maps with non-duplicate keys, as existing CBOR
   maps already fill this role, however the use of a map type as the
   target is allowed for symmetry (implying uniqueness of the keys),
   allowing the following:

   {* text => any} .umm {
     ? home: address
     ? work: address
     $$more-addresses
   }

3.2.  Uniqueness

   The ".unique" control annotates the target as requiring uniqueness,
   within the enclosing container(*), of its value, among the other data
   items in that enclosing container that are also marked ".unique",
   with the same controller.

   E.g.,

   feature-set = [* feature .unique "set"]
   ordered-pairs-with-unique-keys-and-values =
     [* (any .unique "key", any .unique "value") ]

   defines a "feature-set" as an array of zero or more "feature" values
   that need to be all different (as they are unique in "set"), and
   "ordered-map-with-unique-keys-and-values" as an array of zero pairs

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   of keys and values, where the keys need to be unique among themselves
   and the values need to be unique among themselves (the latter example
   could employ an ".omm" or ".umm" operator to further restrict what
   can be in these keys and values).

      Discussion: (*) while it is probably not a big problem to define
      what exactly the "enclosing" container is, it may be useful to
      actually define a larger scope of the uniqueness.  CDDL currently
      does not have a way to establish and point to such a larger scope;
      we might define one ad hoc here or leave that for later extension.

4.  CDDL typenames

   For the use with CDDL [RFC8610], the typenames defined in Figure 1
   are recommended unless there is a need for more specific shaping of
   the data.

   anymap = {* any => any}

   tbd128 = #6.128(anymap)
   tbd129 = #6.129([* (any, any)] .umm anymap)
   tbd130 = #6.130([* ((any .unique "mm"), any)] .omm anymap)
   tbd131 = #6.131([* (any, any)] .omm anymap)
   tbd132<k> = #6.132({* k => any})
   tbd133<k> = #6.133([* (k, any)] .umm anymap)
   tbd134<k> = #6.134([* ((k .unique "mm"), any)] .omm anymap)
   tbd135<k> = #6.135([* (k, any)] .omm anymap)
   tbd136<k,v> = #6.136({* k => v})
   tbd137<k,v> = #6.137([* (k, v)] .umm anymap)
   tbd139<k,v> = #6.138([* ((k .unique "mm"), v)] .omm anymap)
   tbd139<k,v> = #6.139([* (k, v)] .omm anymap)

                  Figure 1: Recommended typenames for CDDL

   [fill in better names for tbdnnn]

   Note that there is no need to call out the uniqueness of the keys
   explicitly in tbd128, tbd132, or tbd136, as the use of maps as a
   representation format already provides that key uniqueness.

5.  IANA Considerations

5.1.  Tags

   IANA is requested to allocate the tags of Table 1 in the CBOR tags
   registry [IANA.cbor-tags], using this document as the specification
   reference.

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   The allocations are requested to be assigned from the "specification
   required" space (24..255).

5.2.  CDDL control operators

   This document requests IANA to register the contents of Table 3 into
   the CDDL Control Operators registry [IANA.cddl]:

                          +=========+===========+
                          | Name    | Reference |
                          +=========+===========+
                          | .omm    | [RFCthis] |
                          +---------+-----------+
                          | .umm    | [RFCthis] |
                          +---------+-----------+
                          | .unique | [RFCthis] |
                          +---------+-----------+

                                  Table 3

6.  Implementation Status

   TBD

7.  Security considerations

   The security considerations of [RFC8610] apply.

8.  Normative References

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

   [IANA.cddl]
              IANA, "Concise Data Definition Language (CDDL)",
              <http://www.iana.org/assignments/cddl>.

   [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>.

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   [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>.

Appendix A.  Implementation Considerations

   This non-normative appendix provides information about the use on
   implementations of the tags and control operators defined.

A.1.  Programming Language Containers (Informative)

   The following subsections describe how the tags in this document
   relate to various programming language containers.  Containers that
   are not part of the programming language or its standard libraries
   are not considered here.

   The _Encoding Tag_ column in the following tables provide the
   recommended tag that best represents the given container type.  For
   example, it's possible to use tag 132 for encoding an ECMAScript
   "Map" if all keys happen to be of the same type, however tag 128 is
   more general and applies to any "Map".  When encoding an ECMAScript
   "Object", tag 128 would be technically correct but is too general;
   tag 132 best presents the fact that an "Object" has text keys only.

   The _Decodable Tags_ column in the following tables, are for data
   items can be decoded into the destination container without having to
   inspect the following:

   *  the uniqueness of the keys,

   *  the ordering of the keys, and,

   *  the data types of *every* keys/value pair.

   It may however be necessary to inspect the data types of the *first*
   key-value pair in the case of tags representing homogeneous keys/
   values.

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A.1.1.  ECMAScript

           +==================+==============+================+
           | Container        | Encoding Tag | Decodable Tags |
           +==================+==============+================+
           | Object           | 132          | 132, 136       |
           +------------------+--------------+----------------+
           | Map              | 128          | 128, 132, 136  |
           +------------------+--------------+----------------+
           | "Array" of pairs | 131          | All            |
           +------------------+--------------+----------------+

                                 Table 4

A.1.2.  Python

          +====================+==============+================+
          | Container          | Encoding Tag | Decodable Tags |
          +====================+==============+================+
          | TypedDict          | 136          | 136            |
          +--------------------+--------------+----------------+
          | namedtuple         | 132          | 132, 136       |
          +--------------------+--------------+----------------+
          | dict               | 128          | 128, 132, 136  |
          +--------------------+--------------+----------------+
          | OrderedDict        | 130          | 130, 134, 138  |
          +--------------------+--------------+----------------+
          | "list" of 2-tuples | 131          | All            |
          +--------------------+--------------+----------------+

                                 Table 5

A.1.3.  C++

         +======================+==============+================+
         | Container(s)         | Encoding Tag | Decodable Tags |
         +======================+==============+================+
         | Map<K, T>            | 136          | 136            |
         +----------------------+--------------+----------------+
         | Map<K, D>            | 132          | 132, 136       |
         +----------------------+--------------+----------------+
         | Map<D, D>            | 128          | 128, 132, 136  |
         +----------------------+--------------+----------------+
         | MultiMap<K, T>       | 137          | 137            |
         +----------------------+--------------+----------------+
         | MultiMap<K, D>       | 133          | 133            |
         +----------------------+--------------+----------------+
         | MultiMap<D, D>       | 129          | 128, 129       |

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         +----------------------+--------------+----------------+
         | Sequence<Pair<K, T>> | 139          | [136, 139]     |
         +----------------------+--------------+----------------+
         | Sequence<Pair<K, D>> | 135          | [132, 139]     |
         +----------------------+--------------+----------------+
         | Sequence<Pair<D, D>> | 131          | All            |
         +----------------------+--------------+----------------+

                                 Table 6

   Legend:

   *  "K": Static key type

   *  "T": Static value type

   *  "D": Suitable dynamic type, such as "std::any" or "std::variant"

   *  "Map": "std::map" or "std::unordered_map"

   *  "MultiMap": "std::multimap" or "std::unordered_multimap"

   *  "Sequence": Sequence container that maintains order (e.g.
      "std::vector")

   *  "Pair": Object containing a key and a value, such as "std::pair",
      or "std::tuple".

   Note that a C++ "std::map" stores its key-value pairs in a sorted
   fashion, and does *not* preserve insertion order in the same manner
   as Python's "OrderedDict".

A.2.  CDDL Implementation Considerations

   TBD

Acknowledgements

   The CBOR tags defined in this document were developed by Emile
   Cormier under the sponsorship of Duc Luong, based on discussions with
   Kio Smallwood and Joe Hildebrand.  The CDDL control operators defined
   in this document were developed by Carsten Bormann, Brendan Moran,
   and Henk Birkholz.

Contributors

   Kio Smallwood

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Internet-Draft       Map-like data in CBOR and CDDL           April 2021

   Joe Hildebrand

Authors' Addresses

   Carsten Bormann (editor)
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

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

   Brendan Moran
   Arm Limited

   Email: Brendan.Moran@arm.com

   Henk Birkholz
   Fraunhofer SIT

   Email: henk.birkholz@sit.fraunhofer.de

   Emile Cormier

   Email: emile.cormier.jr@gmail.com

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