Internet DRAFT - draft-bormann-cbor-cddl-map-like-data
draft-bormann-cbor-cddl-map-like-data
Network Working Group C. Bormann, Ed.
Internet-Draft Universität Bremen TZI
Intended status: Informational B. Moran
Expires: 3 December 2021 Arm Limited
H. Birkholz
Fraunhofer SIT
E. Cormier
1 June 2021
Map-like data in CBOR and CDDL
draft-bormann-cbor-cddl-map-like-data-01
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 non-ordered 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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 3 December 2021.
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/
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Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . . . 10
5.1. Tags . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. CDDL control operators . . . . . . . . . . . . . . . . . 10
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
7. Security considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Implementation Considerations . . . . . . . . . . . 11
A.1. Programming Language Containers (Informative) . . . . . . 11
A.1.1. ECMAScript . . . . . . . . . . . . . . . . . . . . . 12
A.1.2. Python . . . . . . . . . . . . . . . . . . . . . . . 12
A.1.3. C++ . . . . . . . . . . . . . . . . . . . . . . . . . 13
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A.2. CDDL Implementation Considerations . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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 types of the keys, and of the types of the
values;
* whether the order of the key-value pairs carries semantic value
("ordered") or needs to be ignored ("non-ordered");
* 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 |Non- |No |map |259 |
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|129|0001|No | No |Non- |Yes |array|TDB280*|
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|130|0010|No | No |Ordered |No |array|TBD279*|
+---+----+-----------+-------------+--------+---------+-----+-------+
|131|0011|No | No |Ordered |Yes |array| |
+---+----+-----------+-------------+--------+---------+-----+-------+
|132|0100|No | Yes |Non- |No |map |275 |
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|133|0101|No | Yes |Non- |Yes |array| |
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|134|0110|No | Yes |Ordered |No |array| |
+---+----+-----------+-------------+--------+---------+-----+-------+
|135|0111|No | Yes |Ordered |Yes |array| |
+---+----+-----------+-------------+--------+---------+-----+-------+
|136|1000|Yes | Yes |Non- |No |map | |
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|137|1001|Yes | Yes |Non- |Yes |array| |
| | | | |Ordered | | | |
+---+----+-----------+-------------+--------+---------+-----+-------+
|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.]
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Appendix A.1 provides information about constructs in a few
programming languages that are related to the tags being defined.
2.2. Key/Value Type
Bits 2 and 3 of the tag provide 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" (Section 3.2 of [RFC8746]).
Maps with arbitrary keys and homogeneous values are considered
unusual, so they are left out of this specification so that fewer tag
numbers need to be 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 within the map
* 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:
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129(["key1", 1, "key2", 2])
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;
otherwise, they are represented as an array of alternating keys and
values ("flattened alist", see Figure 1).
FAList<K, V> = [* (K, V)]
Figure 1: CDDL for order-preserving representation of maps
*Issue*: [MAPREP] discusses alternative representations of (ordered
and other) maps. How much of this do we need to address here?
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
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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
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 |
+---------+-------------------------+
| .nomm | Non-Ordered (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 (non-ordered and with unique keys). The two control
operators ".omm" and ".nomm" 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 non-ordered 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:
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[* (text, any)] .nomm {
* home: address
* work: address
$$more-addresses
}
The ".omm" and ".nomm" control operators convert a group definition
enclosed into a CDDL map given as a controller type into an array
type given as the target type. The controller type given is
unwrapped (Section 3.7 of [RFC8610]) into a group. Keys and values
of the entries in that group are then alternatingly matched as
elements in the target array. Note that both target and controller
type can contribute to the shaping of the data; declaring the key
type as "text" limits what can be added to the "$$more-addresses"
socket.
".omm" and ".nomm" 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 ".nomm" defines an
non-ordered (multi)map, i.e., data items that present the same set of
key/value pairs in different orders are equivalent.
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 non-ordered 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} .nomm {
? 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",
under the same label (given as the controller).
E.g.,
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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 under the label
"set"), and "ordered-map-with-unique-keys-and-values" as an array of
zero pairs 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 ".nomm" 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 2
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)] .nomm 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)] .nomm 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)] .nomm anymap)
tbd139<k,v> = #6.138([* ((k .unique "mm"), v)] .omm anymap)
tbd139<k,v> = #6.139([* (k, v)] .omm anymap)
Figure 2: Recommended typenames for CDDL
*Issue*: 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.
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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.
The allocations are requested to be assigned from the "specification
required" space (24..255). The values in the column labeled "Tag" in
Table 1 are suggested as the allocated tag numbers.
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] |
+---------+-----------+
| .nomm | [RFCthis] |
+---------+-----------+
| .unique | [RFCthis] |
+---------+-----------+
Table 3: New control
operators to be
registered
6. Implementation Status
TBD
7. Security considerations
The security considerations of [RFC8610] apply.
8. References
8.1. Normative References
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<http://www.iana.org/assignments/cbor-tags>.
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[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>.
[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>.
[RFC8746] Bormann, C., Ed., "Concise Binary Object Representation
(CBOR) Tags for Typed Arrays", RFC 8746,
DOI 10.17487/RFC8746, February 2020,
<https://www.rfc-editor.org/info/rfc8746>.
[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>.
8.2. Informative References
[MAPREP] Bormann, C., "Re: [Cbor] "ordered hash"", cbor@ietf.org
mailing list message, 30 July 2020,
<https://mailarchive.ietf.org/arch/msg/
cbor/5MuDSyPivZ7JfPhsfwCaW2usFHQ>.
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.
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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.
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 |
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+--------------------+--------------+----------------+
| "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 |
+----------------------+--------------+----------------+
| 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")
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* "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
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
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Email: henk.birkholz@sit.fraunhofer.de
Emile Cormier
Email: emile.cormier.jr@gmail.com
Bormann, et al. Expires 3 December 2021 [Page 15]
