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
and restrictions with respect to this document. Code Components
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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