Network Working Group J. Roatch
Internet-Draft
Intended status: Informational C. Bormann
Expires: August 30, 2018 Universitaet Bremen TZI
February 26, 2018
Concise Binary Object Representation (CBOR) Tags for Typed Arrays
draft-jroatch-cbor-tags-07
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.
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
additional tags for multi-dimensional and homogeneous arrays. It is
intended as the reference document for the IANA registration of the
CBOR tags defined.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 30, 2018.
Copyright Notice
Copyright (c) 2018 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Typed Arrays . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Types of numbers . . . . . . . . . . . . . . . . . . . . 3
3. Additional Array Tags . . . . . . . . . . . . . . . . . . . . 4
3.1. Multi-dimensional Array . . . . . . . . . . . . . . . . . 5
3.2. Homogeneous Array . . . . . . . . . . . . . . . . . . . . 5
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. CDDL typenames . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Concise Binary Object Representation (CBOR, [RFC7049]) provides
for the interchange of structured data without a requirement for a
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
types supported via an IANA registry.
Recently, a simple form of typed arrays of numeric data have received
interest both in the Web graphics community [TypedArray] and in the
JavaScript specification [TypedArrayES6], as well as in corresponding
implementations [ArrayBuffer].
Since these typed arrays may carry significant amounts of data, there
is interest in interchanging them in CBOR without the need of lengthy
conversion of each number in the array.
This document defines a number of interrelated CBOR tags that cover
these typed arrays, as well as two additional tags for multi-
dimensional and homogeneous arrays. It is intended as the reference
document for the IANA registration of the tags defined.
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1.1. Terminology
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 the operator "**" stands for
exponentiation.
2. Typed Arrays
Typed arrays are homogeneous arrays of numbers, all of which are
encoded in a single form of binary representation. The concatenation
of these representations is encoded as a single CBOR byte string
(major type 2), enclosed by a single tag indicating the type and
encoding of all the numbers represented in the byte string.
2.1. Types of numbers
Three classes of numbers are of interest: unsigned integers (uint),
signed integers (twos' complement, sint), and IEEE 754 binary
floating point numbers (which are always signed). For each of these
classes, there are multiple representation lengths in active use:
+-----------+--------+--------+-----------+
| Length ll | uint | sint | float |
+-----------+--------+--------+-----------+
| 0 | uint8 | sint8 | binary16 |
| 1 | uint16 | sint16 | binary32 |
| 2 | uint32 | sint32 | binary64 |
| 3 | uint64 | sint64 | binary128 |
+-----------+--------+--------+-----------+
Table 1: Length values
Here, sintN stands for a signed integer of exactly N bits (for
instance, sint16), and uintN stands for an unsigned integer of
exactly N bits (for instance, uint32). The name binaryN stands for
the number form of the same name defined in IEEE 754.
Since one objective of these tags is to be able to directly ship the
ArrayBuffers underlying the Typed Arrays without re-encoding them,
and these may be either in big endian (network byte order) or in
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
the choice between integer and floating point, the signedness (for
integers), the endianness, and one of the four length values.
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In order to simplify implementation, a range of tags is being
allocated that allows retrieving all this information from the bits
of the tag: Tag values from TBD64 to TBD87.
The value is split up into 5 bit fields: TDB0b010_f_s_e_ll, as
detailed in Table 2.
+----------+-------------------------------------------------------+
| Field | Use |
+----------+-------------------------------------------------------+
| TBD0b010 | a constant such as '010', to be defined |
| f | 0 for integer, 1 for float |
| s | 0 for unsigned integer or float, 1 for signed integer |
| e | 0 for big endian, 1 for little endian |
| ll | A number for the length (Table 1). |
+----------+-------------------------------------------------------+
Table 2: Bit fields in the low 8 bits of the tag
The number of bytes in each array element can then be calculated by
"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.)
In the CBOR representation, the total number of elements in the array
is not expressed explicitly, but implied from the length of the byte
string and the length of each representation. It can be computed
inversely to the previous formula: "bytelength >> (f + ll)".
For the uint8/sint8 values, the endianness is redundant. Only the
big endian variant is used. As a special case, what would be the
little endian variant of uint8 is used to signify that the numbers in
the array are using clamped conversion from integers, as described in
more detail in Section 7.1 of [TypedArrayUpdate].
3. Additional Array Tags
This specification defines two additional array tags. The Multi-
dimensional Array tag can be combined with classical CBOR arrays as
well as with Typed Arrays in order to build multi-dimensional arrays
with constant numbers of elements in the sub-arrays. The Homogeneous
Array tag can be used to facilitate the ingestion of homogeneous
classical CBOR arrays, providing performance advantages even when a
Typed Array does not apply.
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3.1. Multi-dimensional Array
Tag: TBD40
Data Item: array (major type 4) of two arrays, one array (major type
4) of dimensions, and one array (major type 4, a Typed Array, or a
Homogeneous Array) of elements
A multi-dimensional array is represented as a tagged array that
contains two (one-dimensional) arrays. The first array defines the
dimensions of the multi-dimensional array (in the sequence of outer
dimensions towards inner dimensions) while the second array
represents the contents of the multi-dimensional array. If the
second array is itself tagged as a Typed Array then the element type
of the multi-dimensional array is known to be the same type as that
of the Typed Array. Data in the Typed Array byte string consists of
consecutive values where the last dimension is considered contiguous
(row-major order).
uint16_t a[2][3] = {
{0, 1, 2}, /* row 0 */
{3, 4, 5},
};
<Tag TBD40> # multi-dimensional array tag
82 # array(2)
82 # array(2)
02 # unsigned(2) 1st Dimension
03 # unsigned(3) 2nd Dimension
d8 41 # uint16 array
4a # byte string(12)
00 00 # unsigned(0)
00 01 # unsigned(1)
00 02 # unsigned(2)
00 03 # unsigned(3)
00 04 # unsigned(4)
00 05 # unsigned(5)
Figure 1: Multi-dimensional array in C and CBOR
3.2. Homogeneous Array
Tag: TBD41
Data Item: array (major type 4)
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
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of the array is thus determined by the application type of the first
array element. This can be used by implementations in strongly typed
languages while decoding to create native homogeneous arrays of
specific types instead of ordered lists.
Which CBOR data items constitute elements of the same application
type is specific to the application. However, type systems of
programming languages have enough commonality that an application
should be able to create portable homogeneous arrays.
bool boolArray[2] = { true, false };
<Tag TBD41> # Homogeneous Array Tag
82 #array(2)
F5 # true
F4 # false
Figure 2: Homogeneous array in C and CBOR
4. Discussion
Support for both little- and big-endian representation may seem out
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
objective not to require conversion of each array element.
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
of typed arrays in data interchange.
Applying a Homogeneous Array tag to a Typed Array would be redundant
and is therefore not provided by the present specification.
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5. CDDL typenames
For the use with CDDL [I-D.ietf-cbor-cddl], the typenames defined in
Figure 3 are recommended:
ta-uint8 = #6.TBD64(bstr)
ta-uint16be = #6.TBD65(bstr)
ta-uint32be = #6.TBD66(bstr)
ta-uint64be = #6.TBD67(bstr)
ta-uint8-clamped = #6.TBD68(bstr)
ta-uint16le = #6.TBD69(bstr)
ta-uint32le = #6.TBD70(bstr)
ta-uint64le = #6.TBD71(bstr)
ta-sint8 = #6.TBD72(bstr)
ta-sint16be = #6.TBD73(bstr)
ta-sint32be = #6.TBD74(bstr)
ta-sint64be = #6.TBD75(bstr)
; reserved: #6.TBD76(bstr)
ta-sint16le = #6.TBD77(bstr)
ta-sint32le = #6.TBD78(bstr)
ta-sint64le = #6.TBD79(bstr)
ta-float16be = #6.TBD80(bstr)
ta-float32be = #6.TBD81(bstr)
ta-float64be = #6.TBD82(bstr)
ta-float128be = #6.TBD83(bstr)
ta-float16le = #6.TBD84(bstr)
ta-float32le = #6.TBD85(bstr)
ta-float64le = #6.TBD86(bstr)
ta-float128le = #6.TBD87(bstr)
homogeneous<array> = #6.TBD41(array)
multi-dim<dim, array> = #6.TBD40([dim, array])
Figure 3: Recommended typenames for CDDL
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6. IANA Considerations
IANA is requested to allocate the tags in Table 3, with the present
document as the specification reference.
+-------+-------------------+---------------------------------------+
| Tag | Data Item | Semantics |
+-------+-------------------+---------------------------------------+
| TBD64 | byte string | uint8 Typed Array |
| TBD65 | byte string | uint16, big endian, Typed Array |
| TBD66 | byte string | uint32, big endian, Typed Array |
| TBD67 | byte string | uint64, big endian, Typed Array |
| TBD68 | byte string | uint8 Typed Array, clamped arithmetic |
| TBD69 | byte string | uint16, little endian, Typed Array |
| TBD70 | byte string | uint32, little endian, Typed Array |
| TBD71 | byte string | uint64, little endian, Typed Array |
| TBD72 | byte string | sint8 Typed Array |
| TBD73 | byte string | sint16, big endian, Typed Array |
| TBD74 | byte string | sint32, big endian, Typed Array |
| TBD75 | byte string | sint64, big endian, Typed Array |
| TBD76 | byte string | (reserved) |
| TBD77 | byte string | sint16, little endian, Typed Array |
| TBD78 | byte string | sint32, little endian, Typed Array |
| TBD79 | byte string | sint64, little endian, Typed Array |
| TBD80 | byte string | IEEE 754 binary16, big endian, Typed |
| | | Array |
| TBD81 | byte string | IEEE 754 binary32, big endian, Typed |
| | | Array |
| TBD82 | byte string | IEEE 754 binary64, big endian, Typed |
| | | Array |
| TBD83 | byte string | IEEE 754 binary128, big endian, Typed |
| | | Array |
| TBD84 | byte string | IEEE 754 binary16, little endian, |
| | | Typed Array |
| TBD85 | byte string | IEEE 754 binary32, little endian, |
| | | Typed Array |
| TBD86 | byte string | IEEE 754 binary64, little endian, |
| | | Typed Array |
| TBD87 | byte string | IEEE 754 binary128, little endian, |
| | | Typed Array |
| TBD40 | array of two | Multi-dimensional Array |
| | arrays* | |
| TBD41 | array | Homogeneous Array |
+-------+-------------------+---------------------------------------+
Table 3: Values for Tags
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*) TBD40 data item: second element of outer array in data item is
native CBOR array (major type 4) or Typed Array (one of Tag
TBD64..TBD87)
RFC editor note: Please replace TBDnn by the tag numbers allocated by
IANA throughout the document and delete this note. IANA note: To
make the calculations work, TDB64 to TBD87 need to come from a
contiguous range the start of which is divisible by 32.
TO DO: The WG needs to figure out whether it is OK to spend 24 "good"
(1+1 byte) tags for this, whether this all goes to 1+2 byte tags, or
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
The security considerations of RFC 7049 apply; the tags introduced
here are not expected to raise security considerations beyond those.
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8. References
8.1. Normative References
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-02
(work in progress), February 2018.
[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>.
8.2. Informative References
[ArrayBuffer]
Mozilla Developer Network, "JavaScript typed arrays",
2013, <https://developer.mozilla.org/en-
US/docs/Web/JavaScript/Typed_arrays>.
[TypedArray]
Vukicevic, V. and K. Russell, "Typed Array Specification",
February 2011,
<https://www.khronos.org/registry/typedarray/specs/1.0/>.
[TypedArrayES6]
"22.2 TypedArray Objects", in: ECMA-262 6th Edition, The
ECMAScript 2015 Language Specification, June 2015,
<http://www.ecma-international.org/
ecma-262/6.0/#sec-typedarray-objects>.
[TypedArrayUpdate]
Herman, D. and K. Russell, "Typed Array Specification",
July 2013,
<https://www.khronos.org/registry/typedarray/specs/
latest/>.
Contributors
Glenn Engel suggested the tags for multi-dimensional arrays and
homogeneous arrays.
Acknowledgements
TBD
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Authors' Addresses
Johnathan Roatch
Email: jroatch@gmail.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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