Network Working Group J. Roatch
Internet-Draft
Intended status: Informational C. Bormann
Expires: September 10, 2015 Universitaet Bremen TZI
March 09, 2015
Concise Binary Object Representation (CBOR) Tags for Typed Arrays
draft-jroatch-cbor-tags-02
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. It is intended
as the reference document for the IANA registration of the CBOR tags
defined.
Status of This Memo
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Copyright Notice
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document authors. All rights reserved.
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. Typed Arrays . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Types of numbers . . . . . . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. CDDL typenames . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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] and 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. It is intended as the reference document for the
IANA registration of the tags defined.
1.1. Terminology
The term "byte" is used in its now customary sense as a synonym for
"octet".
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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.
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 64 to 87 (0x40 to 0x57).
The value is split up into 5 bit fields: 0b010_f_s_e_ll, as detailed
in Table 2.
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+-------+-------------------------------------------------------+
| Field | Use |
+-------+-------------------------------------------------------+
| 0b010 | a constant '010' |
| 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 defined in
Section 7.1 of [TypedArrayUpdate].
3. 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.
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4. CDDL typenames
For the use with CDDL [I-D.greevenbosch-appsawg-cbor-cddl], the
typenames defined in Figure 1 are recommended:
ta-uint8 = #6.64(bstr)
ta-uint16be = #6.65(bstr)
ta-uint32be = #6.66(bstr)
ta-uint64be = #6.67(bstr)
ta-uint8-clamped = #6.68(bstr)
ta-uint16le = #6.69(bstr)
ta-uint32le = #6.70(bstr)
ta-uint64le = #6.71(bstr)
ta-sint8 = #6.72(bstr)
ta-sint16be = #6.73(bstr)
ta-sint32be = #6.74(bstr)
ta-sint64be = #6.75(bstr)
; reserved: #6.76(bstr)
ta-sint16le = #6.77(bstr)
ta-sint32le = #6.78(bstr)
ta-sint64le = #6.79(bstr)
ta-float16be = #6.80(bstr)
ta-float32be = #6.81(bstr)
ta-float64be = #6.82(bstr)
ta-float128be = #6.83(bstr)
ta-float16le = #6.84(bstr)
ta-float32le = #6.85(bstr)
ta-float64le = #6.86(bstr)
ta-float128le = #6.87(bstr)
Figure 1: Recommended typenames for CDDL
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5. IANA Considerations
IANA is requested to reserve the tags in Table 3, with the present
document as the specification reference.
+-----+-------------+-----------------------------------------------+
| Tag | Data Item | Semantics |
+-----+-------------+-----------------------------------------------+
| 64 | byte string | uint8 Typed Array |
| 65 | byte string | uint16, big endian, Typed Array |
| 66 | byte string | uint32, big endian, Typed Array |
| 67 | byte string | uint64, big endian, Typed Array |
| 68 | byte string | uint8 Typed Array, clamped arithmetic |
| 69 | byte string | uint16, little endian, Typed Array |
| 70 | byte string | uint32, little endian, Typed Array |
| 71 | byte string | uint64, little endian, Typed Array |
| 72 | byte string | sint8 Typed Array |
| 73 | byte string | sint16, big endian, Typed Array |
| 74 | byte string | sint32, big endian, Typed Array |
| 75 | byte string | sint64, big endian, Typed Array |
| 76 | byte string | (reserved) |
| 77 | byte string | sint16, little endian, Typed Array |
| 78 | byte string | sint32, little endian, Typed Array |
| 79 | byte string | sint64, little endian, Typed Array |
| 80 | byte string | IEEE 754 binary16, big endian, Typed Array |
| 81 | byte string | IEEE 754 binary32, big endian, Typed Array |
| 82 | byte string | IEEE 754 binary64, big endian, Typed Array |
| 83 | byte string | IEEE 754 binary128, big endian, Typed Array |
| 84 | byte string | IEEE 754 binary16, little endian, Typed Array |
| 85 | byte string | IEEE 754 binary32, little endian, Typed Array |
| 86 | byte string | IEEE 754 binary64, little endian, Typed Array |
| 87 | byte string | IEEE 754 binary128, little endian, Typed |
| | | Array |
+-----+-------------+-----------------------------------------------+
Table 3: Values for Tags
6. 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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7. References
7.1. Normative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Greevenbosch, B., Sun, R., and C. Vigano, "CBOR data
definition language: a notational convention to express
CBOR data structures.", draft-greevenbosch-appsawg-cbor-
cddl-04 (work in progress), December 2014.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
7.2. 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: Draft ECMA-262 6th Edition,
ECMAScript 2015 Language Specification)", n.d.,
<http://people.mozilla.org/~jorendorff/
es6-draft.html#sec-typedarray-objects>.
[TypedArrayUpdate]
Herman, D. and K. Russell, "Typed Array Specification",
July 2013,
<https://www.khronos.org/registry/typedarray/specs/
latest/>.
Authors' Addresses
Johnathan Roatch
Email: jroatch@gmail.com
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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