draft-ietf-cbor-array-tags-00.txt   draft-ietf-cbor-array-tags-01.txt 
Network Working Group J. Roatch Network Working Group J. Roatch
Internet-Draft Internet-Draft
Intended status: Informational C. Bormann Intended status: Informational C. Bormann
Expires: April 25, 2019 Universitaet Bremen TZI Expires: August 31, 2019 Universitaet Bremen TZI
October 22, 2018 February 27, 2019
Concise Binary Object Representation (CBOR) Tags for Typed Arrays Concise Binary Object Representation (CBOR) Tags for Typed Arrays
draft-ietf-cbor-array-tags-00 draft-ietf-cbor-array-tags-01
Abstract Abstract
The Concise Binary Object Representation (CBOR, RFC 7049) is a data The Concise Binary Object Representation (CBOR, RFC 7049) is a data
format whose design goals include the possibility of extremely small format whose design goals include the possibility of extremely small
code size, fairly small message size, and extensibility without the code size, fairly small message size, and extensibility without the
need for version negotiation. need for version negotiation.
The present document makes use of this extensibility to define a The present document makes use of this extensibility to define a
number of CBOR tags for typed arrays of numeric data, as well as two number of CBOR tags for typed arrays of numeric data, as well as two
skipping to change at page 1, line 40 skipping to change at page 1, line 40
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019. This Internet-Draft will expire on August 31, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Typed Arrays . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Typed Arrays . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Types of numbers . . . . . . . . . . . . . . . . . . . . 3 2.1. Types of numbers . . . . . . . . . . . . . . . . . . . . 3
3. Additional Array Tags . . . . . . . . . . . . . . . . . . . . 4 3. Additional Array Tags . . . . . . . . . . . . . . . . . . . . 4
3.1. Multi-dimensional Array . . . . . . . . . . . . . . . . . 5 3.1. Multi-dimensional Array . . . . . . . . . . . . . . . . . 5
3.2. Homogeneous Array . . . . . . . . . . . . . . . . . . . . 6 3.2. Homogeneous Array . . . . . . . . . . . . . . . . . . . . 7
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. CDDL typenames . . . . . . . . . . . . . . . . . . . . . . . 8 5. CDDL typenames . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 11 8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . 13
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
The Concise Binary Object Representation (CBOR, [RFC7049]) provides The Concise Binary Object Representation (CBOR, [RFC7049]) provides
for the interchange of structured data without a requirement for a for the interchange of structured data without a requirement for a
pre-agreed schema. RFC 7049 defines a basic set of data types, as pre-agreed schema. RFC 7049 defines a basic set of data types, as
well as a tagging mechanism that enables extending the set of data well as a tagging mechanism that enables extending the set of data
types supported via an IANA registry. types supported via an IANA registry.
Recently, a simple form of typed arrays of numeric data have received Recently, a simple form of typed arrays of numeric data have received
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Typed arrays are homogeneous arrays of numbers, all of which are Typed arrays are homogeneous arrays of numbers, all of which are
encoded in a single form of binary representation. The concatenation encoded in a single form of binary representation. The concatenation
of these representations is encoded as a single CBOR byte string of these representations is encoded as a single CBOR byte string
(major type 2), enclosed by a single tag indicating the type and (major type 2), enclosed by a single tag indicating the type and
encoding of all the numbers represented in the byte string. encoding of all the numbers represented in the byte string.
2.1. Types of numbers 2.1. Types of numbers
Three classes of numbers are of interest: unsigned integers (uint), Three classes of numbers are of interest: unsigned integers (uint),
signed integers (twos' complement, sint), and IEEE 754 binary signed integers (two's complement, sint), and IEEE 754 binary
floating point numbers (which are always signed). For each of these floating point numbers (which are always signed). For each of these
classes, there are multiple representation lengths in active use: classes, there are multiple representation lengths in active use:
+-----------+--------+--------+-----------+ +-----------+--------+--------+-----------+
| Length ll | uint | sint | float | | Length ll | uint | sint | float |
+-----------+--------+--------+-----------+ +-----------+--------+--------+-----------+
| 0 | uint8 | sint8 | binary16 | | 0 | uint8 | sint8 | binary16 |
| 1 | uint16 | sint16 | binary32 | | 1 | uint16 | sint16 | binary32 |
| 2 | uint32 | sint32 | binary64 | | 2 | uint32 | sint32 | binary64 |
| 3 | uint64 | sint64 | binary128 | | 3 | uint64 | sint64 | binary128 |
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little endian form, we need to define tags for both variants. little endian form, we need to define tags for both variants.
In total, this leads to 24 variants. In the tag, we need to express In total, this leads to 24 variants. In the tag, we need to express
the choice between integer and floating point, the signedness (for the choice between integer and floating point, the signedness (for
integers), the endianness, and one of the four length values. integers), the endianness, and one of the four length values.
In order to simplify implementation, a range of tags is being In order to simplify implementation, a range of tags is being
allocated that allows retrieving all this information from the bits allocated that allows retrieving all this information from the bits
of the tag: Tag values from TBD64 to TBD87. of the tag: Tag values from TBD64 to TBD87.
The value is split up into 5 bit fields: TDB0b010_f_s_e_ll, as The value is split up into 5 bit fields: TBD0b010_f_s_e_ll, as
detailed in Table 2. detailed in Table 2.
+----------+-------------------------------------------------------+ +----------+-------------------------------------------------------+
| Field | Use | | Field | Use |
+----------+-------------------------------------------------------+ +----------+-------------------------------------------------------+
| TBD0b010 | a constant such as '010', to be defined | | TBD0b010 | a constant such as '010', to be defined |
| f | 0 for integer, 1 for float | | f | 0 for integer, 1 for float |
| s | 0 for unsigned integer or float, 1 for signed integer | | s | 0 for unsigned integer or float, 1 for signed integer |
| e | 0 for big endian, 1 for little endian | | e | 0 for big endian, 1 for little endian |
| ll | A number for the length (Table 1). | | ll | A number for the length (Table 1). |
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"2**(f + ll)" (or "1 << (f + ll)" in a typical programming language). "2**(f + ll)" (or "1 << (f + ll)" in a typical programming language).
(Notice that f and ll are the lsb of each nibble (4bit) in the byte.) (Notice that f and ll are the lsb of each nibble (4bit) in the byte.)
In the CBOR representation, the total number of elements in the array In the CBOR representation, the total number of elements in the array
is not expressed explicitly, but implied from the length of the byte is not expressed explicitly, but implied from the length of the byte
string and the length of each representation. It can be computed string and the length of each representation. It can be computed
inversely to the previous formula: "bytelength >> (f + ll)". inversely to the previous formula: "bytelength >> (f + ll)".
For the uint8/sint8 values, the endianness is redundant. Only the For the uint8/sint8 values, the endianness is redundant. Only the
big endian variant is used. The little endian variant of sint8 MUST big endian variant is used. The little endian variant of sint8 MUST
NOT be used, its tag is marked as reserved. As a special case, what NOT be used, its tag is marked as reserved. As a special case, the
would be the little endian variant of uint8 is used to signify that tag number that would have been the little endian variant of uint8 is
the numbers in the array are using clamped conversion from integers, used to signify that the numbers in the array are using clamped
as described in more detail in Section 7.1 of [TypedArrayUpdate]. conversion from integers, as described in more detail in Section 7.1
of [TypedArrayUpdate].
3. Additional Array Tags 3. Additional Array Tags
This specification defines two additional array tags. The Multi- This specification defines three additional array tags. The Multi-
dimensional Array tag can be combined with classical CBOR arrays as dimensional Array tags can be combined with classical CBOR arrays as
well as with Typed Arrays in order to build multi-dimensional arrays well as with Typed Arrays in order to build multi-dimensional arrays
with constant numbers of elements in the sub-arrays. The Homogeneous with constant numbers of elements in the sub-arrays. The Homogeneous
Array tag can be used to facilitate the ingestion of homogeneous Array tag can be used to facilitate the ingestion of homogeneous
classical CBOR arrays, providing performance advantages even when a classical CBOR arrays, providing performance advantages even when a
Typed Array does not apply. Typed Array does not apply.
3.1. Multi-dimensional Array 3.1. Multi-dimensional Array
Tag: TBD40 Tag: TBD40
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86 # array(6) 86 # array(6)
02 # unsigned(2) 02 # unsigned(2)
04 # unsigned(4) 04 # unsigned(4)
08 # unsigned(8) 08 # unsigned(8)
04 # unsigned(4) 04 # unsigned(4)
10 # unsigned(16) 10 # unsigned(16)
19 0100 # unsigned(256) 19 0100 # unsigned(256)
Figure 2: Multi-dimensional array using basic CBOR array Figure 2: Multi-dimensional array using basic CBOR array
Note that these arrays are in "row major" order; if a representation Tag: TBD1040
for "column major" order arrays is desired, it can be defined
analogously with a new tag (but the present document does not). Data Item: as with tag TBD40
Note that above arrays are in "row major" order, which is the
preferred order for the purposes of this specification. An analogous
representation that uses "column major" order arrays is provided
under the tag TBD1040, as illustrated in Figure 3.
<Tag TBD1040> # multi-dimensional array tag, column major order
82 # array(2)
82 # array(2)
02 # unsigned(2) 1st Dimension
03 # unsigned(3) 2nd Dimension
86 # array(6)
02 # unsigned(2)
04 # unsigned(4)
04 # unsigned(4)
10 # unsigned(16)
08 # unsigned(8)
19 0100 # unsigned(256)
Figure 3: Multi-dimensional array using basic CBOR array, column
major order
3.2. Homogeneous Array 3.2. Homogeneous Array
Tag: TBD41 Tag: TBD41
Data Item: array (major type 4) Data Item: array (major type 4)
This tag provides a hint to decoders that the array tagged by it has This tag provides a hint to decoders that the array tagged by it has
elements that are all of the same application type. The element type elements that are all of the same application type. The element type
of the array is thus determined by the application type of the first of the array is thus determined by the application type of the first
array element. This can be used by implementations in strongly typed array element. This can be used by implementations in strongly typed
languages while decoding to create native homogeneous arrays of languages while decoding to create native homogeneous arrays of
specific types instead of ordered lists. specific types instead of ordered lists.
Which CBOR data items constitute elements of the same application Which CBOR data items constitute elements of the same application
type is specific to the application. However, type systems of type is specific to the application. However, type systems of
programming languages have enough commonality that an application programming languages have enough commonality that an application
should be able to create portable homogeneous arrays. should be able to create portable homogeneous arrays.
Figure 3 shows an example for a homogeneous array of booleans in C++ Figure 4 shows an example for a homogeneous array of booleans in C++
and CBOR. and CBOR.
bool boolArray[2] = { true, false }; bool boolArray[2] = { true, false };
<Tag TBD41> # Homogeneous Array Tag <Tag TBD41> # Homogeneous Array Tag
82 #array(2) 82 #array(2)
F5 # true F5 # true
F4 # false F4 # false
Figure 3: Homogeneous array in C++ and CBOR Figure 4: Homogeneous array in C++ and CBOR
Figure 4 extends the example with a more complex structure. Figure 5 extends the example with a more complex structure.
typedef struct { typedef struct {
bool active; bool active;
int value; int value;
} foo; } foo;
foo myArray[2] = { {true, 3}, {true, -4} }; foo myArray[2] = { {true, 3}, {true, -4} };
<Tag TBD41> <Tag TBD41>
82 # array(2) 82 # array(2)
82 # array(2) 82 # array(2)
F5 # true F5 # true
03 # 3 03 # 3
82 # array(2) 82 # array(2)
F5 # true F5 # true
23 # -4 23 # -4
Figure 4: Homogeneous array in C++ and CBOR Figure 5: Homogeneous array in C++ and CBOR
4. Discussion 4. Discussion
Support for both little- and big-endian representation may seem out Support for both little- and big-endian representation may seem out
of character with CBOR, which is otherwise fully big endian. This of character with CBOR, which is otherwise fully big endian. This
support is in line with the intended use of the typed arrays and the support is in line with the intended use of the typed arrays and the
objective not to require conversion of each array element. objective not to require conversion of each array element.
This specification allocates a sizable chunk out of the single-byte This specification allocates a sizable chunk out of the single-byte
tag space. This use of code point space is justified by the wide use tag space. This use of code point space is justified by the wide use
of typed arrays in data interchange. of typed arrays in data interchange.
Providing a column-major order variant of the multi-dimensional array
may seem superfluous to some, and useful to others. It is cheap to
define the additional tag so it is available when actually needed.
Allocating it out of a different number space makes the preference
for row-major evident.
Applying a Homogeneous Array tag to a Typed Array would be redundant Applying a Homogeneous Array tag to a Typed Array would be redundant
and is therefore not provided by the present specification. and is therefore not provided by the present specification.
5. CDDL typenames 5. CDDL typenames
For the use with CDDL [I-D.ietf-cbor-cddl], the typenames defined in For the use with CDDL [I-D.ietf-cbor-cddl], the typenames defined in
Figure 5 are recommended: Figure 6 are recommended:
ta-uint8 = #6.TBD64(bstr) ta-uint8 = #6.TBD64(bstr)
ta-uint16be = #6.TBD65(bstr) ta-uint16be = #6.TBD65(bstr)
ta-uint32be = #6.TBD66(bstr) ta-uint32be = #6.TBD66(bstr)
ta-uint64be = #6.TBD67(bstr) ta-uint64be = #6.TBD67(bstr)
ta-uint8-clamped = #6.TBD68(bstr) ta-uint8-clamped = #6.TBD68(bstr)
ta-uint16le = #6.TBD69(bstr) ta-uint16le = #6.TBD69(bstr)
ta-uint32le = #6.TBD70(bstr) ta-uint32le = #6.TBD70(bstr)
ta-uint64le = #6.TBD71(bstr) ta-uint64le = #6.TBD71(bstr)
ta-sint8 = #6.TBD72(bstr) ta-sint8 = #6.TBD72(bstr)
skipping to change at page 8, line 36 skipping to change at page 9, line 36
ta-float16be = #6.TBD80(bstr) ta-float16be = #6.TBD80(bstr)
ta-float32be = #6.TBD81(bstr) ta-float32be = #6.TBD81(bstr)
ta-float64be = #6.TBD82(bstr) ta-float64be = #6.TBD82(bstr)
ta-float128be = #6.TBD83(bstr) ta-float128be = #6.TBD83(bstr)
ta-float16le = #6.TBD84(bstr) ta-float16le = #6.TBD84(bstr)
ta-float32le = #6.TBD85(bstr) ta-float32le = #6.TBD85(bstr)
ta-float64le = #6.TBD86(bstr) ta-float64le = #6.TBD86(bstr)
ta-float128le = #6.TBD87(bstr) ta-float128le = #6.TBD87(bstr)
homogeneous<array> = #6.TBD41(array) homogeneous<array> = #6.TBD41(array)
multi-dim<dim, array> = #6.TBD40([dim, array]) multi-dim<dim, array> = #6.TBD40([dim, array])
multi-dim-column-major<dim, array> = #6.TBD1040([dim, array])
Figure 5: Recommended typenames for CDDL Figure 6: Recommended typenames for CDDL
6. IANA Considerations 6. IANA Considerations
IANA is requested to allocate the tags in Table 3, with the present IANA is requested to allocate the tags in Table 3, with the present
document as the specification reference. (The reserved value is document as the specification reference. (The reserved value is
reserved for a future revision of typed array tags.) reserved for a future revision of typed array tags.)
The allocations come out of the "specification required" space
(24..255), with the exception of TBD1040, which comes out of the
"first come first served" space (256..).
+-------+-------------------+---------------------------------------+ +-------+-------------------+---------------------------------------+
| Tag | Data Item | Semantics | | Tag | Data Item | Semantics |
+-------+-------------------+---------------------------------------+ +-------+-------------------+---------------------------------------+
| TBD64 | byte string | uint8 Typed Array | | TBD64 | byte string | uint8 Typed Array |
| TBD65 | byte string | uint16, big endian, Typed Array | | TBD65 | byte string | uint16, big endian, Typed Array |
| TBD66 | byte string | uint32, big endian, Typed Array | | TBD66 | byte string | uint32, big endian, Typed Array |
| TBD67 | byte string | uint64, big endian, Typed Array | | TBD67 | byte string | uint64, big endian, Typed Array |
| TBD68 | byte string | uint8 Typed Array, clamped arithmetic | | TBD68 | byte string | uint8 Typed Array, clamped arithmetic |
| TBD69 | byte string | uint16, little endian, Typed Array | | TBD69 | byte string | uint16, little endian, Typed Array |
| TBD70 | byte string | uint32, little endian, Typed Array | | TBD70 | byte string | uint32, little endian, Typed Array |
skipping to change at page 10, line 5 skipping to change at page 11, line 47
| | | Typed Array | | | | Typed Array |
| TBD87 | byte string | IEEE 754 binary128, little endian, | | TBD87 | byte string | IEEE 754 binary128, little endian, |
| | | Typed Array | | | | Typed Array |
| TBD40 | array of two | Multi-dimensional Array | | TBD40 | array of two | Multi-dimensional Array |
| | arrays* | | | | arrays* | |
| TBD41 | array | Homogeneous Array | | TBD41 | array | Homogeneous Array |
+-------+-------------------+---------------------------------------+ +-------+-------------------+---------------------------------------+
Table 3: Values for Tags Table 3: Values for Tags
*) TBD40 data item: second element of outer array in data item is *) TBD40 or TBD1040 data item: second element of outer array in data
native CBOR array (major type 4) or Typed Array (one of Tag item is native CBOR array (major type 4) or Typed Array (one of Tag
TBD64..TBD87) TBD64..TBD87)
RFC editor note: Please replace TBDnn by the tag numbers allocated by RFC editor note: Please replace TBDnn by the tag numbers allocated by
IANA throughout the document and delete this note. IANA note: To IANA throughout the document and delete this note; this also applies
make the calculations work, TDB64 to TBD87 need to come from a to the binary representation TBD0b010 in Section 2.1, which becomes
contiguous range the start of which is divisible by 32. 0b010 if the numbers are allocated as proposed. IANA note: To make
the calculations work, TDB64 to TBD87 need to come from a contiguous
TO DO: The WG needs to figure out whether it is OK to spend 24 "good" range the start of which is divisible by 32, which they do if the
(1+1 byte) tags for this, whether this all goes to 1+2 byte tags, or "TBD" is simply removed.
whether maybe the layout of the bits in the tag should change to move
the larger datatypes into the 1+2 range and just the 8-bit ones into
the 1+1 range.
7. Security Considerations 7. Security Considerations
The security considerations of RFC 7049 apply; special attention is The security considerations of RFC 7049 apply; special attention is
drawn to the second paragraph of Section 8 of RFC 7049. The tags drawn to the second paragraph of Section 8 of RFC 7049. The tags
introduced here are not expected to raise security considerations introduced here are not expected to raise security considerations
beyond those. beyond those.
8. References 8. References
8.1. Normative References 8.1. Normative References
[I-D.ietf-cbor-cddl] [I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to definition language (CDDL): a notational convention to
express CBOR and JSON data structures", draft-ietf-cbor- express CBOR and JSON data structures", draft-ietf-cbor-
cddl-05 (work in progress), August 2018. cddl-07 (work in progress), February 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>. October 2013, <https://www.rfc-editor.org/info/rfc7049>.
skipping to change at page 12, line 8 skipping to change at page 14, line 8
[TypedArrayUpdate] [TypedArrayUpdate]
Herman, D. and K. Russell, "Typed Array Specification", Herman, D. and K. Russell, "Typed Array Specification",
July 2013, July 2013,
<https://www.khronos.org/registry/typedarray/specs/ <https://www.khronos.org/registry/typedarray/specs/
latest/>. latest/>.
Contributors Contributors
Glenn Engel suggested the tags for multi-dimensional arrays and Glenn Engel suggested the tags for multi-dimensional arrays and
homogeneous arrays. homogeneous arrays, Jim Schaad reminded us that column-major order
still is in use.
Acknowledgements Acknowledgements
TBD TBD
Authors' Addresses Authors' Addresses
Johnathan Roatch Johnathan Roatch
Email: jroatch@gmail.com Email: jroatch@gmail.com
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