Packed CBOR
draft-ietf-cbor-packed-09
| Document | Type | Active Internet-Draft (cbor WG) | |
|---|---|---|---|
| Author | Carsten Bormann | ||
| Last updated | 2023-07-10 | ||
| Replaces | draft-bormann-cbor-packed | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources |
GitHub Repository
Mailing list discussion |
||
| Stream | WG state | Waiting for WG Chair Go-Ahead | |
| Document shepherd | Barry Leiba | ||
| Shepherd write-up | Show Last changed 2022-05-19 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | barryleiba@computer.org |
draft-ietf-cbor-packed-09
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Standards Track 10 July 2023
Expires: 11 January 2024
Packed CBOR
draft-ietf-cbor-packed-09
Abstract
The Concise Binary Object Representation (CBOR, RFC 8949 == STD 94)
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 decompress the compressed
form to make use of the 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.
// The present version (-09) provides two table setup tags (common,
// split setup) and discusses behavior in case of references to
// unpopulated table entries during unpacking.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-cbor-packed/.
Discussion of this document takes place on the CBOR Working Group
mailing list (mailto:cbor@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/cbor/. Subscribe at
https://www.ietf.org/mailman/listinfo/cbor/.
Source for this draft and an issue tracker can be found at
https://github.com/cbor-wg/cbor-packed.
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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/.
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 11 January 2024.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Packed CBOR . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Packing Tables . . . . . . . . . . . . . . . . . . . . . 5
2.2. Referencing Shared Items . . . . . . . . . . . . . . . . 6
2.3. Referencing Argument Items . . . . . . . . . . . . . . . 7
2.4. Concatenation . . . . . . . . . . . . . . . . . . . . . . 9
2.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 10
3. Table Setup . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. Basic Packed CBOR . . . . . . . . . . . . . . . . . . . . 12
4. Function Tags . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Join Function Tags . . . . . . . . . . . . . . . . . . . 13
5. Tag Validity: Tag Equivalence Principle . . . . . . . . . . . 14
5.1. Tag Equivalence . . . . . . . . . . . . . . . . . . . . . 15
5.2. Tag Equivalence of Packed CBOR Tags . . . . . . . . . . . 16
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6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6.1. CBOR Tags Registry . . . . . . . . . . . . . . . . . . . 16
6.2. CBOR Simple Values Registry . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The Concise Binary Object Representation (CBOR, [STD94]) 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 [RFC1951] can work well for CBOR encoded data items, their
disadvantage is that the receiver needs to decompress the compressed
form to make use of the 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.
This document defines the Packed CBOR format by specifying the
transformation from a Packed CBOR data item to the original CBOR data
item; it does not define an algorithm for a packer. Different
packers can differ in the amount of effort they invest in arriving at
a minimal packed form; often, they simply employ the sharing that is
natural for a specific application.
Packed CBOR can make use of 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.
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* argument sharing: application of a function with two arguments,
one of which is shared. Data items (strings, containers) that
share a prefix or suffix (affix), or more generally data items
that can be constructed from a function taking a shared argument
and a rump data item, can be replaced by a reference to the shared
argument plus a rump data item. For strings and the default
"concatenation" function, the processing required during
consumption is similar to following the argument 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 argument reference.
Shared item reference: A reference to a shared item as defined in
Section 2.2.
Argument reference: A reference that combines a shared argument with
a rump item as defined in Section 2.3.
Affix: Prefix or suffix, used as an argument in an argument
reference employing the default function "concatenation".
Function reference: An argument reference that uses a tag for
argument, rump, or both, causing the application of a function to
reconstruct the data item.
Packing tables: The pair of a shared item table and an argument
table.
Current set: The packing tables in effect at the data item under
consideration.
Expansion: The result of applying a packed reference in the context
of given Packing tables.
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The definitions of [STD94] apply. Specifically: The term "byte" is
used in its now customary sense as a synonym for "octet"; "byte
strings" are CBOR data items carrying a sequence of zero or more
(binary) bytes, while "text strings" are CBOR data items carrying a
sequence of zero or more Unicode code points (more precisely: Unicode
scalar values), encoded in UTF-8 [STD63].
Where arithmetic is explained, this document uses the notation
familiar from the programming language C, except that ".." denotes a
range that includes both ends given, in the HTML and PDF versions,
subtraction and negation are rendered as a hyphen ("-", as are
various dashes), and superscript notation denotes exponentiation.
For example, 2 to the power of 64 is notated: 2^64. In the plain-
text version of this specification, superscript notation is not
available and therefore is rendered by a surrogate notation. That
notation is not optimized for this RFC; it is unfortunately ambiguous
with C's exclusive-or and requires circumspection from the reader of
the plain-text version.
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 two packing tables in a Current Set:
* Shared item table
* Argument table
Without any table setup, these two 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). Such an
index may be derived from information in tags and their content as
well as from CBOR simple values.
Table setup mechanisms (see Section 3) may include all information
needed for table setup within the packed CBOR data item, or they may
refer external information. This information may be immutable, or
may be intended to potentially grow over time. This raises the
question of how a reference to a new item should be handled when the
unpacker uses an older version of the external information.
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If, during unpacking, an index is used that references an item that
is unpopulated in (e.g., outside the size of) the table in use, this
MAY be treated as an error by the unpacker and abort the unpacking.
Alternatively, the unpacker MAY provide the special value
1112(undefined) (the simple value >undefined< as per Section 5.7 of
[STD94], enclosed in the tag 1112) to the application and leave the
error handling to the application. An unpacker SHOULD document which
of these two alternatives has been chosen. CBOR based protocols that
include the use of packed CBOR MAY require that unpacking errors are
tolerated in some positions.
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.
+===========================+==============+
| 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 [STD94]), 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 — "zigzag encoding" — 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 CBOR integers can
grow to very large (or very 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 tag content: An
integer turns the tag into a shared item reference, whereas a string
or container (map or array) turns it into a straight (prefix)
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reference (see Table 2). Note also that the tag content of Tag 6 may
itself be packed, so it may need to be unpacked to make this
determination.
2.3. Referencing Argument Items
The argument table serves as a common table that can be used for
argument references, i.e., for concatenation as well as references
involving a function tag.
When referencing an argument, a distinction is made between straight
and inverted references; if no function tag is involved, a straight
reference combines a prefix out of the argument table with the rump
data item, and an inverted reference combines a rump data item with a
suffix out of the argument table.
+===========================================+=================+
| straight reference | table index |
+===========================================+=================+
| Tag 6(straight rump) | 0 |
+-------------------------------------------+-----------------+
| Tag 224..255(straight rump) | 0..31 |
+-------------------------------------------+-----------------+
| Tag 28704..32767(straight rump) | 32..4095 |
+-------------------------------------------+-----------------+
| Tag 1879052288..2147483647(straight rump) | 4096..268435455 |
+-------------------------------------------+-----------------+
Table 2: Straight Referencing (e.g., Prefix) Arguments
+===========================================+================+
| inverted reference | table index |
+===========================================+================+
| Tag 216..223(inverted rump) | 0..7 |
+-------------------------------------------+----------------+
| Tag 27647..28671(inverted rump) | 8..1023 |
+-------------------------------------------+----------------+
| Tag 1811940352..1879048191(inverted rump) | 1024..67108863 |
+-------------------------------------------+----------------+
Table 3: Inverted Referencing (e.g., Suffix) Arguments
Argument data items are referenced by using the reference data items
in Table 2 and Table 3.
The tag number of the reference indicates a table index (an unsigned
integer) leading to the "argument"; the tag content of the reference
is the "rump item".
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When reconstructing the original data item, such a reference is
replaced by a data item constructed from the argument data item found
in the table (argument, which might need to be recursively unpacked
first) and the rump data item (rump, again possibly recursively
unpacked).
Separate from the tag used as a reference, a tag ("function tag") may
be involved to supply a function to be used in resolving the
reference. It is crucial not to confuse reference tag and, if
present, function tag.
A straight reference uses the argument as the provisional left-hand
side and the rump data item as the right-hand side. An inverted
reference uses the rump data item as the provisional left-hand side
and the argument as the right-hand side.
In both cases, the provisional left-hand side is examined. If it is
a tag ("function tag"), it is "unwrapped": The function tag's tag
number is established as the function to be applied, and the tag
content is kept as the unwrapped left-hand side. If the provisional
left-hand side is not a tag, it is kept as the unwrapped left-hand
side, and the function to be applied is concatenation, as defined
below. The right-hand side is taken as is as the unwrapped right-
hand side.
If a function tag was given, the reference is replaced by the result
of applying the unpacking function to be computed to the left and
right-hand sides. The unpacking function is defined by the
definition of the tag number supplied. If that definition does not
define an unpacking function, the result of the unpacking is not
valid.
If no function tag was given, the reference is replaced by the left-
hand side "concatenated" with the right-hand side, where
concatenation is defined as in Section 2.4.
As a contrived (but short) example, if the argument table is
["foobar", h'666f6f62', "fo"], each of the following straight
(prefix) references will unpack to "foobart": 6("t"), 225("art"),
226("obart") (the byte string h'666f6f62' == 'foob' is concatenated
into a text string, and the last example is not an optimization).
Note that table index 0 of the argument table can be referenced both
with tag 6 and tag 224, however tag 6 with an integer content is used
for shared item references (see Table 1), so to combine index 0 with
an integer rump, tag 224 needs to be used.
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Taking into account the encoding and ignoring the less optimal tag
224, there is one single-byte straight (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 straight
references. 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
inverted (suffix) references are one quarter of those, except that
there is no single-byte reference and 8 two-byte references.
| Rationale: Experience suggests that straight (prefix) packing
| might be more likely than inverted (suffix) packing. Also for
| this reason, there is no intent to spend a 1+0 tag value for
| inverted packing.
2.4. Concatenation
The concatenation function is defined as follows:
* If both left-hand side and right-hand side are arrays, the result
of the concatenation is an array with all elements of the left-
hand-side array followed by the elements of the right-hand side
array.
* If both left-hand side and right-hand side are maps, the result of
the concatenation is a map that is initialized with a copy of the
left-hand-side map, and then filled in with the members of the
right-hand side map, replacing any existing members that have the
same key.
| NOTE: One application of the rule for straight references is to
| supply default values out of a dictionary, which can then be
| overridden by the entries in the map supplied as the rump data
| item. Note that this pattern provides no way to remove a map
| entry from the prefix table entry.
* If both left-hand side and right-hand side are one of the string
types (not necessarily the same), the bytes of the left-hand side
are concatenated with the bytes of the right-hand side. Byte
strings concatenated with text strings need to contain valid UTF-8
data. The result of the concatenation gets the type of the
unwrapped rump data item; this way a single argument table entry
can be used to build both byte and text strings, depending on what
type of rump is being used.
* If one side is one of the string types, and the other side is an
array, the result of the concatenation is equivalent to the
application of the "join" function (Section 4.1) to the string as
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the left-hand side and the array as the right-hand side. The
original right-hand side of the concatenation determines the
string type of the result.
* Other type combinations of left-hand side and right-hand side are
not valid.
2.5. Discussion
This specification uses up a large number of Simple Values and Tags,
in particular one of the rare one-byte tags and two thirds of the
one-byte simple values. Since the objective is compression, this is
warranted only based on a 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:
|
| * 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.
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| NOTE: The present specification does nothing to help with the
| packing of CBOR sequences [RFC8742]; maybe such a specification
| should be added.
3. Table Setup
The packing references described in Section 2 assume that packing
tables have been set up.
By default, both 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.
* By one or more _table-building_ tags enclosing the packed content.
Each tag is usually defined to build an augmented table by adding
to the packing tables that already apply to the tag, and to apply
the resulting augmented table when unpacking the tag content.
Usually, the semantics of the tag will be to prepend items to one
or more of the tables. (The specific behavior of any such tag, in
the presence of a table applying to it, needs to be carefully
specified.)
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.
Packed item references in the newly constructed (low-numbered)
parts of the table are usually 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.
Where external information is used in a table setup mechanism that is
not immutable, care needs to be taken so that, over time, references
to existing table entries stay valid (i.e., the information is only
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extended)), and that a maximum size of this information is given.
This allows an unpacker to recognize references to items that are not
yet defined in the version of the external reference that it uses,
providing backward and possibly limited (degraded) forward
compatibility.
For table setup, the present specification only defines two simple
table-building tags, which operate by prepending to the (by default
empty) tables.
| Additional tags can be defined for dictionary referencing
| (possible combining that with Basic Packed CBOR mechanisms).
| The desirable details are likely to vary considerably between
| applications. A URI-based reference would be easy to define,
| but might be too inefficient when used in the likely
| combination with an ni: URI [RFC6920].
3.1. Basic Packed CBOR
Two tags are predefined by this specification for packing table
setup. They are defined in CDDL [RFC8610] as in Figure 1:
Basic-Packed-CBOR = #6.113([[*shared-and-argument-item], rump])
Split-Basic-Packed-CBOR =
#6.1113([[*shared-item], [*argument-item], rump])
rump = any
shared-and-argument-item = any
argument-item = any
shared-item = any
Figure 1: Packed CBOR in CDDL
(This assumes the allocation of tag numbers 113 ('q') and 1113 for
these tags.)
The array given as the first element of the content of tag 113
("Basic-Packed-CBOR") is prepended to both the tables for shared
items and arguments that apply to the entire tag (by default empty
tables). The arrays given as the first and second element of the
content of the tag 1113 ("Split-Basic-Packed-CBOR") are prepended to
the tables for shared items and arguments that apply to the entire
tag (by default empty tables). As discussed in the introduction to
this section, references in the supplied new arrays use the new
number space (where inherited items are shifted by the new items
given), while the inherited items themselves use the inherited number
space (so their semantics do not change by the mere action of
inheritance).
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The original CBOR data item can be reconstructed by recursively
replacing shared and argument references encountered in the rump by
their expansions.
4. Function Tags
Function tags that occur in an argument or a rump supply the
semantics for reconstructing a data item from their tag content and
the non-dominating rump or argument, respectively. The present
specification defines a pair of function tags.
4.1. Join Function Tags
Tag 106 ('j') defines the "join" unpacking function, based on the
concatenation function (Section 2.4).
The join function expects an item that can be concatenated as its
left-hand side, and an array of such items as its right-hand side.
Joining works by sequentially applying the concatenation function to
the elements of the right-hand-side array, interspersing the left-
hand side as the "joiner".
An example in functional notation: join(", ", ["a", "b", "c"])
returns "a, b, c".
For a right-hand side of one or more elements, the first element
determines the type of the result when text strings and byte strings
are mixed in the argument. For a right-hand side of one element, the
joiner is not used, and that element returned. For a right-hand side
of zero elements, a neutral element is generated based on the type of
the joiner (empty text/byte string for a text/byte string, empty
array for an array, empty map for a map).
For an example, we assume this unpacked data item:
["https://packed.example/foo.html",
"coap:://packed.example/bar.cbor",
"mailto:support@packed.example"]
A packed form of this using straight references could be:
113([[106("packed.example")],
[6(["https://", "/foo.html"]),
6(["coap://", "/bar.cbor"]),
6(["mailto:support@", ""])]
])
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Tag 105 ('i') defines the "ijoin" unpacking function, which is
exactly like that of tag 106, except that the left-hand side and
right-hand side are interchanged ('i').
A packed form of the first example using inverted references and the
ijoin tag could be:
113([["packed.example"],
[216(105(["https://", "/foo.html"]),
216(105(["coap://", "/bar.cbor"]),
216("mailto:support@")]
])
A packed form of an array with many URIs that reference SenML items
from the same place could be:
113([[105(["coaps://[2001::db8::1]/s/", ".senml"])],
[6("temp-freezer"),
6("temp-fridge"),
6("temp-ambient")]
])
5. Tag Validity: Tag Equivalence Principle
In Section 5.3.2 of [STD94], the validity of tags is defined in terms
of type and value of their tag content. The CBOR Tag registry
[IANA.cbor-tags] Section 9.2 of [STD94] allows recording the "data
item" for a registered tag, which is usually an abbreviated
description of the top-level data type allowed for the tag content.
In other words, in the registry, the validity of a tag of a given tag
number is described in terms of the top-level structure of the data
carried in the tag content. The description of a tag might add
further constraints for the data item. But in any case, a tag
definition can only specify validity based on the structure of its
tag content.
In Packed CBOR, a reference tag might be "standing in" for the actual
tag content of an outer tag, or for a structural component of that.
In this case, the formal structure of the outer tag's content before
unpacking usually no longer fulfills the validity conditions of the
outer tag.
The underlying problem is not unique to Packed CBOR. For instance,
[RFC8746] describes tags 64..87 that "stand in" for CBOR arrays (the
native form of which has major type 4). For the other tags defined
in this specification, which require some array structure of the tag
content, a footnote was added:
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| [...] The second element of the outer array in the data item is a
| native CBOR array (major type 4) or Typed Array (one of tag
| 64..87)
The top-down approach to handle the "rendezvous" between the outer
and inner tags does not support extensibility: any further Typed
Array tags being defined do not inherit the exception granted to tag
number 64..87; they would need to formally update all existing tag
definitions that can accept typed arrays or be of limited use with
these existing tags.
Instead, the tag validity mechanism needs to be extended by a bottom-
up component: A tag definition needs to be able to declare that the
tag can "stand in" for, (is, in terms of tag validity, equivalent to)
some structure.
E.g., tag 64..87 could have declared their equivalence to the CBOR
major type 4 arrays they stand in for.
| Note that not all domain extensions to tags can be addressed
| using the equivalence principle: E.g., on a data model level,
| numbers with arbitrary exponents ([ARB-EXP], tags 264 and 265)
| are strictly a superset of CBOR's predefined fractional types,
| tags 4 and 5. They could not simply declare that they are
| equivalent to tags 4 and 5 as a tag requiring a fractional
| value may have no way to handle the extended range of tag 264
| and 265.
5.1. Tag Equivalence
A tag definition MAY declare Tag Equivalence to some existing
structure for the tag, under some conditions defined by the new tag
definition. This, in effect, extends all existing tag definitions
that accept the named structure to accept the newly defined tag under
the conditions given for the Tag Equivalence.
A number of limitations apply to Tag Equivalence, which therefore
should be applied deliberately and sparingly:
* Tag Equivalence is a new concept, which may not be implemented by
an existing generic decoder. A generic decoder not implementing
tag equivalence might raise tag validity errors where Tag
Equivalence says there should be none.
* A CBOR protocol MAY specify the use of Tag Equivalence,
effectively limiting its full use to those generic encoders that
implement it. Existing CBOR protocols that do not address Tag
Equivalence implicitly have a new variant that allows Tag
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Equivalence (e.g., to support Packed CBOR with an existing
protocol). A CBOR protocol that does address Tag Equivalence MAY
be explicit about what kinds of Tag Equivalence it supports (e.g.,
only the reference tags employed by Packed CBOR and certain table
setup tags).
* There is currently no way to express Tag Equivalence in CDDL. For
Packed CBOR, CDDL would typically be used to describe the unpacked
CBOR represented by it; further restricting the Packed CBOR is
likely to lead to interoperability problems. (Note that, by
definition, there is no need to describe Tag Equivalence on the
receptacle side; only for the tag that declares Tag Equivalence.)
* The registry "CBOR Tags" [IANA.cbor-tags] currently does not have
a way to record any equivalence claimed for a tag. A convention
would be to alert to Tag Equivalence in the "Semantics (short
form)" field of the registry.
// Needs to be done for the tag registrations here.
5.2. Tag Equivalence of Packed CBOR Tags
The reference tags in this specification declare their equivalence to
the unpacked shared items or function results they represent.
The table setup tags 113 and 1113 declare its equivalence to the
unpacked CBOR data item represented by it.
6. IANA Considerations
6.1. CBOR Tags Registry
In the registry "CBOR Tags" [IANA.cbor-tags], IANA is requested to
allocate the tags defined in Table 4.
+========================+================+===========+===========+
| Tag | Data Item | Semantics | Reference |
+========================+================+===========+===========+
| 6 | integer (for | Packed | draft- |
| | shared); text | CBOR: | ietf- |
| | string, byte | shared/ | cbor- |
| | string, array, | straight | packed |
| | map, tag (for | | |
| | straight) | | |
+------------------------+----------------+-----------+-----------+
| 105 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | ijoin | cbor- |
| | tag | function | packed |
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+------------------------+----------------+-----------+-----------+
| 106 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | join | cbor- |
| | tag | function | packed |
+------------------------+----------------+-----------+-----------+
| 113 | array (shared- | Packed | draft- |
| | and-argument- | CBOR: | ietf- |
| | items, rump) | table | cbor- |
| | | setup | packed |
+------------------------+----------------+-----------+-----------+
| 224..255 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | straight | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
| 1112 | undefined | Packed | draft- |
| | (0xf7) | CBOR: | ietf- |
| | | reference | cbor- |
| | | error | packed |
+------------------------+----------------+-----------+-----------+
| 1113 | array (shared- | Packed | draft- |
| | items, | CBOR: | ietf- |
| | argument- | table | cbor- |
| | items, rump) | setup | packed |
+------------------------+----------------+-----------+-----------+
| 28704..32767 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | straight | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
| 1879052288..2147483647 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | straight | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
| 216..223 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | inverted | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
| 27647..28671 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
| | array, map, | inverted | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
| 1811940352..1879048191 | text string, | Packed | draft- |
| | byte string, | CBOR: | ietf- |
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| | array, map, | inverted | cbor- |
| | tag | | packed |
+------------------------+----------------+-----------+-----------+
Table 4: Values for Tag Numbers
6.2. CBOR Simple Values Registry
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
7. Security Considerations
The security considerations of [STD94] apply.
Loops in the Packed CBOR can be used as a denial of service attack,
see Section 2.5.
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.)
8. References
8.1. Normative References
[IANA.cbor-simple-values]
IANA, "Concise Binary Object Representation (CBOR) Simple
Values",
<https://www.iana.org/assignments/cbor-simple-values>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://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/rfc/rfc2119>.
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[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/rfc/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/rfc/rfc8610>.
[STD94] 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/rfc/rfc8949>.
8.2. Informative References
[ARB-EXP] Occil, P., "Arbitrary-Exponent Numbers", Specification for
Registration of CBOR Tags 264 and 265,
<http://peteroupc.github.io/CBOR/bigfrac.html>.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<https://www.rfc-editor.org/rfc/rfc1951>.
[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/rfc/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/rfc/rfc7049>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/rfc/rfc8742>.
[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/rfc/rfc8746>.
[STD63] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
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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, so it uses the simple common-table
setup form (tag 113).
{ "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": 8.95
},
{ "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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113([["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 and employs the
split-table setup form (tag 1113).
{
"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": [
{
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"href":
"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"
}
},
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"name": "rgbValueWhite",
"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":
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"http://192.168.1.102:8444/wot/w3c-wot-td-context.jsonld"
}
Figure 4: Example original CBOR data item
1113([/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 /
/argument/ ["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 /
/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 part of the original proposal that turned into CBOR,
but did not make it into [RFC7049], the predecessor of [STD94].
Various attempts to come up with a specification over the years did
not proceed. In 2017, Sebastian Käbisch proposed investigating
compact representations of W3C Thing Descriptions, which prompted the
author to come up with what turned into 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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