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Network Working Group                                          J. Roatch
Internet-Draft
Intended status: Informational                                C. Bormann
Expires: February 9, 2018                        Universitaet Bremen TZI
                                                         August 08, 2017


   Concise Binary Object Representation (CBOR) Tags for Typed Arrays
                     draft-jroatch-cbor-tags-06

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

   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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 9, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  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 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.

     +-------+-------------------------------------------------------+
     | 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 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 = #6.64(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)
   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                             |
   +-------+-------------------+---------------------------------------+
   |    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                           |
   | 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 64..87)

   RFC editor note: Please replace TBD40 and TBD41 by the tag numbers
   allocated by IANA throughout the document and delete this note.

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-00
              (work in progress), July 2017.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://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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