Internet DRAFT - draft-bormann-cbor-packed
draft-bormann-cbor-packed
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 26 July 2020
Expires: 27 January 2021
Packed CBOR
draft-bormann-cbor-packed-01
Abstract
The Concise Binary Object Representation (CBOR, RFC 7049) is a data
format whose design goals include the possibility of extremely small
code size, fairly small message size, and extensibility without the
need for version negotiation.
CBOR does not provide any forms of data compression. CBOR data
items, in particular when generated from legacy data models often
allow considerable gains in compactness when applying data
compression. While traditional data compression techniques such as
DEFLATE (RFC 1951) work well for CBOR, their disadvantage is that the
receiver needs to unpack the compressed form to make use of data.
This specification describes Packed CBOR, a simple transformation of
a CBOR data item into another CBOR data item that is almost as easy
to consume as the original CBOR data item. A separate decompression
step is therefore often not required at the receiver.
Note to Readers
This is an individual submission to the CBOR working group of the
IETF, https://datatracker.ietf.org/wg/cbor/about/
(https://datatracker.ietf.org/wg/cbor/about/). Discussion currently
takes places on the github repository https://github.com/cabo/cbor-
packed (https://github.com/cabo/cbor-packed). If the CBOR WG
believes this is a useful document, discussion is likely to move to
the CBOR WG mailing list and a github repository at the CBOR WG
github organization, https://github.com/cbor-wg (https://github.com/
cbor-wg).
The current version is true work in progress; some of the sections
haven't been filled in yet, and in particular, permission has not
been obtained from tag definition authors to copy over their text.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Bormann Expires 27 January 2021 [Page 1]
Internet-Draft Packed CBOR July 2020
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/.
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 27 January 2021.
Copyright Notice
Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Referencing Shared Items . . . . . . . . . . . . . . . . 4
2.2. Referencing Prefix Items . . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
(TO DO, expand on text from abstract here; move references here and
neuter them in the abstract as per Section 4.3 of [RFC7322].)
Bormann Expires 27 January 2021 [Page 2]
Internet-Draft Packed CBOR July 2020
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:
* structure sharing: substructures (data items) that occur
repeatedly in the original CBOR data item can be collapsed to a
simple reference to a common representation of that data item.
The processing required during consumption is limited to following
that reference.
* prefix sharing: strings that share a prefix can be replaced by a
reference to a common prefix plus the rest of the string. The
processing required during consumption is similar to following the
prefix reference plus that for an indefinite-length string.
A specific application protocol that employs Packed CBOR might allow
both kinds of optimization or limit the representation to structure
sharing only.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The definitions of [I-D.ietf-cbor-7049bis] apply. The term "byte" is
used in its now customary sense as a synonym for "octet". Where bit
arithmetic is explained, this document uses the notation familiar
from the programming language C (including C++14's 0bnnn binary
literals), except that, in the plain text form of this document. the
operator "^" stands for exponentiation.
2. Packed CBOR
Packed CBOR is defined in CDDL [RFC8610] as in Figure 1:
Packed-CBOR = #6.6([rump, [*prefix], *shared])
rump = any
prefix = any
shared = any
Figure 1: Packed CBOR in CDDL
Bormann Expires 27 January 2021 [Page 3]
Internet-Draft Packed CBOR July 2020
(This assumes the allocation of tag number 6, which is motivated
further below. Note that the semantics of Tag 6 depend on its
content: An integer turns the tag into a shared reference, a string
into a prefix reference, and an array into a complete Packed CBOR
data item as described above.)
The original CBOR data item can be reconstructed by recursively
replacing shared and prefix references encountered in the rump by
their defined values.
2.1. Referencing Shared Items
Shared items are stored in the third to last element of the array
used as tag content for tag number 6, numbered starting by 2.
The shared data items are referenced by using the data items in
Table 1. When reconstructing the original data item, such a
reference is replaced by the referenced data item, which is then
recursively unpacked.
+===========================+================+
| reference | element number |
+===========================+================+
| Simple value 0-15 | 2-17 |
+---------------------------+----------------+
| Tag 6(unsigned integer N) | 18 + 2*N |
+---------------------------+----------------+
| Tag 6(negative integer N) | 18 - 2*N - 1 |
+---------------------------+----------------+
Table 1: Referencing Shared Values
Taking into account the encoding, there are 16 one-byte references,
48 two-byte references, 512 three-byte references, 131072 four-byte
references, etc. As integers can grow to very large (or small)
values, there is no practical limit to how many shared items might be
used in a Packed CBOR item.
2.2. Referencing Prefix Items
Shared items are stored in an array that is the second element of the
array used as tag content for tag number 6. This array is indexed
from 0.
Prefix data items are referenced by using the data items in Table 2.
When reconstructing the original data item, such a reference is
replaced by a string constructed from the referenced prefix data item
(prefix, which might need to be recursively unpacked first)
Bormann Expires 27 January 2021 [Page 4]
Internet-Draft Packed CBOR July 2020
concatenated with the tag content (suffix, again possibly recursively
unpacked). The result gets the type of the suffix; this way a single
prefix can be used to build both byte and text strings, depending on
what type of suffix is being used.
+===================================+================+
| reference | element number |
+===================================+================+
| Tag 6(suffix) | 0 |
+-----------------------------------+----------------+
| Tag 224-255(suffix) | 1-32 |
+-----------------------------------+----------------+
| Tag 28672-32767(suffix) | 33-4128 |
+-----------------------------------+----------------+
| Tag 1879048192-2147483647(suffix) | 4129-268439584 |
+-----------------------------------+----------------+
Table 2: Referencing Prefix Values
Taking into account the encoding, there is one one-byte prefix
reference, 32 two-byte references, 4096 three-byte references, and
268435456 five-byte references. 268439585
(2^(28)+2^(12)+2^(5)+2^(0)) is an artificial limit, but should be
high enough that there, again, is no practical limit to how many
prefix items might be used in a Packed CBOR item.
3. Discussion
This specification uses up a large number of Simple Values and Tags,
in particular one of the rare one-byte tags and half of the one-byte
simple values. Since the objective is compression, this is warranted
if and only if there is consensus that this specific format could be
useful for a wide area of applications, while maintaining reasonable
simplicity in particular at the side of the consumer.
A maliciously crafted Packed CBOR data item might contain a reference
loop. A consumer/decompressor MUST protect against that.
The current definition does nothing to help with packing CBOR
sequences [RFC8742]; maybe it should.
Nesting packed CBOR data items is not useful; maybe it should.
4. IANA Considerations
In the registry [IANA.cbor-tags], IANA is requested to allocate the
tags defined in Table 3.
Bormann Expires 27 January 2021 [Page 5]
Internet-Draft Packed CBOR July 2020
+===========+========+================+===========================+++
| Tag | Data | Semantics | Reference |||
| | Item | | |||
+===========+========+================+===========================+++
| 6 | array, | Packed CBOR: | draft-bormann-cbor-packed |||
| |integer,| packed/shared/ | |||
| | text | prefix | |||
| |string, | | |||
| | byte | | |||
| | string | | |||
+-----------+--------+----------------+---------------------------+++
| 224-255 | text | Packed CBOR: | draft-bormann-cbor-packed |||
| | string | prefix | |||
| |or byte | | |||
| | string | | |||
+-----------+--------+----------------+---------------------------+++
|28672-32767| text | Packed CBOR: | draft-bormann-cbor-packed |||
| | string | prefix | |||
| |or byte | | |||
| | string | | |||
+-----------+--------+----------------+---------------------------+++
|1879048192-| text | Packed CBOR: | draft-bormann-cbor-packed |||
| 2147483647| string | prefix | |||
| |or byte | | |||
| | string | | |||
+-----------+--------+----------------+---------------------------+++
Table 3: Values for Tag Numbers
In the registry [IANA.cbor-simple-values], IANA is requested to
allocate the simple values defined in Table 4.
+=======+=====================+===========================+
| Value | Semantics | Reference |
+=======+=====================+===========================+
| 0-15 | Packed CBOR: shared | draft-bormann-cbor-packed |
+-------+---------------------+---------------------------+
Table 4: Simple Values
5. Security Considerations
The security considerations of RFC 7049 apply.
Loops in the Packed CBOR can be used as a denial of service attack,
see Section 3.
Bormann Expires 27 January 2021 [Page 6]
Internet-Draft Packed CBOR July 2020
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
[I-D.ietf-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", Work in Progress, Internet-Draft,
draft-ietf-cbor-7049bis-14, 16 June 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-cbor-
7049bis-14.txt>.
[IANA.cbor-simple-values]
IANA, "Concise Binary Object Representation (CBOR) Simple
Values",
<http://www.iana.org/assignments/cbor-simple-values>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<http://www.iana.org/assignments/cbor-tags>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
6.2. Informative References
[RFC7322] Flanagan, H. and S. Ginoza, "RFC Style Guide", RFC 7322,
DOI 10.17487/RFC7322, September 2014,
<https://www.rfc-editor.org/info/rfc7322>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
Bormann Expires 27 January 2021 [Page 7]
Internet-Draft Packed CBOR July 2020
Appendix A. Example
The (JSON-compatible) CBOR data structure depicted in Figure 2, 400
bytes of binary CBOR, could lead to a packed CBOR data item depicted
in Figure 3, 307 bytes. Note that this example does not lend itself
to prefix compression.
{ "store": {
"book": [
{ "category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
},
{ "category": "fiction",
"author": "Evelyn Waugh",
"title": "Sword of Honour",
"price": 12.99
},
{ "category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
},
{ "category": "fiction",
"author": "J. R. R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
}
],
"bicycle": {
"color": "red",
"price": 19.95
}
}
}
Figure 2: Example original CBOR data item
Bormann Expires 27 January 2021 [Page 8]
Internet-Draft Packed CBOR July 2020
6([{"store": {
"book": [
{simple(1): "reference", simple(2): "Nigel Rees",
simple(3): "Sayings of the Century", simple(0): simple(5)},
{simple(1): simple(4), simple(2): "Evelyn Waugh",
simple(3): "Sword of Honour", simple(0): 12.99},
{simple(1): simple(4), simple(2): "Herman Melville",
simple(3): "Moby Dick", simple(6): "0-553-21311-3",
simple(0): simple(5)},
{simple(1): simple(4), simple(2): "J. R. R. Tolkien",
simple(3): "The Lord of the Rings",
simple(6): "0-395-19395-8", simple(0): 22.99}],
"bicycle": {"color": "red", simple(0): 19.95}}},
[],
"price", "category", "author", "title", "fiction", 8.95, "isbn"])
/ 0 1 2 3 4 5 6 /
Figure 3: Example packed CBOR data item
TBD: Do this for a W3C Thing Description again to get better packing
and to exercise prefix compression...
Acknowledgements
CBOR packing was originally invented with the rest of CBOR, but did
not make it into [RFC7049]. Various attempts to come up with a
specification over the years didn't proceed. In 2017, Sebastian
Käbisch proposed investigating compact representations of W3C Thing
Descriptions, which prompted the author to come up with essentially
the present design.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Bormann Expires 27 January 2021 [Page 9]
