Concise Binary Object Representation (CBOR) Tags for Typed Arrays
draft-ietf-cbor-array-tags-08

Network Working Group                                    C. Bormann, Ed.
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                           August 14, 2019
Expires: February 15, 2020


   Concise Binary Object Representation (CBOR) Tags for Typed Arrays
                     draft-ietf-cbor-array-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
   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 February 15, 2020.

Copyright Notice

   Copyright (c) 2019 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



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   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 . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Multi-dimensional Array . . . . . . . . . . . . . . . . .   5
       3.1.1.  Row-major Order . . . . . . . . . . . . . . . . . . .   6
       3.1.2.  Column-Major order  . . . . . . . . . . . . . . . . .   7
     3.2.  Homogeneous Array . . . . . . . . . . . . . . . . . . . .   8
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  CDDL typenames  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

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 also can save space
   overhead with encoding a type for each element of an array.

   This document defines a number of interrelated CBOR tags that cover
   these typed arrays, as well as two additional tags for multi-



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   dimensional and homogeneous arrays.  It is intended as the reference
   document for the IANA registration of the tags defined.

   Note that an application that generates CBOR with these tags has
   considerable freedom in choosing variants, e.g., with respect to
   endianness, embedded type (signed vs. unsigned), and number of bits
   per element, or whether a tag defined in this specification is used
   at all instead of more basic CBOR.  In contrast to representation
   variants of single CBOR numbers, there is no representation that
   could be identified as "preferred".  If deterministic encoding is
   desired in a CBOR-based protocol making use of these tags, the
   protocol has to define which of the encoding variants are used in
   which case.

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.

   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.

   The term "array" is used in a general sense in this document, unless
   further specified.  The term "classical CBOR array" describes an
   array represented with CBOR major type 4.  A "homogeneous array" is
   an array of elements that are all of the same type (the term is
   neutral whether that is a representation type or an application data
   model type).

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 (two's complement, sint), and IEEE 754 binary




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   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 [IEEE754].

   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.

   The value is split up into 5 bit fields: 0b010_f_s_e_ll, as detailed
   in Table 2.

     +-------+-------------------------------------------------------+
     | Field | Use                                                   |
     +-------+-------------------------------------------------------+
     | 0b010 | the constant bits 0, 1, 0                             |
     | 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





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   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 0f and ll are the two least significant bits,
   respectively, 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 from the length of the byte string
   in bytes: "bytelength >> (f + ll)".

   For the uint8/sint8 values, the endianness is redundant.  Only the
   tag for the big endian variant is used and assigned as such.  The Tag
   that would signify the little endian variant of sint8 MUST NOT be
   used, its tag number is marked as reserved.  As a special case, the
   Tag that would signify the little endian variant of uint8 is instead
   assigned to signify that the numbers in the array are using clamped
   conversion from integers, as described in more detail in
   Section 7.1.11 ("ToUint8Clamp") of the ES6 JavaScript specification
   [TypedArrayES6]; the assumption here is that a program-internal
   representation of this array after decoding would be marked this way
   for further processing, providing "roundtripping" of JavaScript typed
   arrays through CBOR.

   IEEE 754 binary floating numbers are always signed.  Therefore, for
   the float variants ("f" == 1), there is no need to distinguish
   between signed and unsigned variants; the "s" bit is always zero.
   The Tag numbers where "s" would be one (which would have Tag values
   88 to 95) remain free to use by other specifications.

3.  Additional Array Tags

   This specification defines three additional array tags.  The Multi-
   dimensional Array tags 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 as a signal by an application to identify a
   classical CBOR array as a homogeneous array, even when a Typed Array
   does not apply.

3.1.  Multi-dimensional Array

   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



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   of the multi-dimensional array is known to be the same type as that
   of the Typed Array.

   Two tags are defined by this document, one for elements arranged in
   row-major order, and one for column-major order.

3.1.1.  Row-major Order

   Tag:  40

   Data Item:  array (major type 4) of two arrays, one array (major type
      4) of dimensions, which are unsigned integers distinct from zero,
      and one array (either a CBOR array of major type 4, or a Typed
      Array, or a Homogeneous Array) of elements

   Data in the second array consists of consecutive values where the
   last dimension is considered contiguous (row-major order).

   Figure 1 shows a declaration of a two-dimensional array in the C
   language, a representation of that in CBOR using both a
   multidimensional array tag and a typed array tag.

   uint16_t a[2][3] = {
     {2, 4, 8},   /* row 0 */
     {4, 16, 256},
   };

   <Tag 40> # multi-dimensional array tag
      82       # array(2)
        82      # array(2)
          02     # unsigned(2) 1st Dimension
          03     # unsigned(3) 2nd Dimension
        <Tag 65> # uint16 array
          4c     # byte string(12)
            0002 # unsigned(2)
            0004 # unsigned(4)
            0008 # unsigned(8)
            0004 # unsigned(4)
            0010 # unsigned(16)
            0100 # unsigned(256)

              Figure 1: Multi-dimensional array in C and CBOR

   Figure 2 shows the same two-dimensional array using the
   multidimensional array tag in conjunction with a basic CBOR array
   (which, with the small numbers chosen for the example, happens to be
   shorter).




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   <Tag 40> # multi-dimensional array tag
      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)
          08      # unsigned(8)
          04      # unsigned(4)
          10      # unsigned(16)
          19 0100 # unsigned(256)

         Figure 2: Multi-dimensional array using basic CBOR array

3.1.2.  Column-Major order

   The multidimensional arrays specified in the previous sub-subsection
   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 in this subsection under
   the tag 1040, as illustrated in Figure 3.

   Tag:  1040

   Data Item:  as with tag 40, except that the data in the second array
      consists of consecutive values where the first dimension is
      considered contiguous (column-major order).

   <Tag 1040> # 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







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3.2.  Homogeneous Array

   Tag:  41

   Data Item:  array (major type 4)

   This tag identifies the classical CBOR array (a one-dimensional
   array) tagged by it as a homogeneous array, that is, it has elements
   that are all of the same application model data type.  The element
   type of the array is thus determined by the application model data
   type of the first array element.

   This can be used in application data models that apply specific
   semantics to homogeneous arrays.  Also, in certain cases,
   implementations in strongly typed languages may be able to create
   native homogeneous arrays of specific types instead of ordered lists
   while decoding.  Which CBOR data items constitute elements of the
   same application type is specific to the application.

   Figure 4 shows an example for a homogeneous array of booleans in C++
   and CBOR.

   bool boolArray[2] = { true, false };

   <Tag 41>  # Homogeneous Array Tag
      82           #array(2)
         F5        # true
         F4        # false

                Figure 4: Homogeneous array in C++ and CBOR

   Figure 5 extends the example with a more complex structure.



















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   typedef struct {
     bool active;
     int value;
   } foo;
   foo myArray[2] = { {true, 3}, {true, -4} };

   <Tag 41>
       82  # array(2)
          82  #  array(2)
                F5  # true
                03  # 3
          82 # array(2)
                F5  # true
                23  # -4

                Figure 5: 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.

   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 usually be
   redundant and is therefore not provided by the present specification.















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5.  CDDL typenames

   For the use with CDDL [RFC8610], the typenames defined in Figure 6
   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> = #6.41(array)
   multi-dim<dim, array> = #6.40([dim, array])
   multi-dim-column-major<dim, array> = #6.1040([dim, array])

                 Figure 6: Recommended typenames for CDDL

















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6.  IANA Considerations

   IANA has allocated the tags in Table 3, with the present document as
   the specification reference.  (The reserved value is reserved for a
   future revision of typed array tags.)

   The allocations came out of the "specification required" space
   (24..255), with the exception of 1040, which came out of the "first
   come first served" space (256..).










































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   +------+-------------------+----------------------------------------+
   |  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                            |
   |   40 | array of two      | Multi-dimensional Array, row-major     |
   |      | arrays*           | order                                  |
   | 1040 | array of two      | Multi-dimensional Array, column-major  |
   |      | arrays*           | order                                  |
   |   41 | array             | Homogeneous Array                      |
   +------+-------------------+----------------------------------------+

                         Table 3: Values for Tags

   *) 40 or 1040 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)




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7.  Security Considerations

   The security considerations of RFC 7049 apply; special attention is
   drawn to the second paragraph of Section 8 of RFC 7049.

   The Tag for homogeneous arrays makes a promise about its tagged data
   item that a maliciously constructed CBOR input can then choose to
   ignore.  As always, the decoder therefore has to ensure that it is
   not driven into an undefined state by array elements that do not
   fulfill the promise and that it does continue to fulfill its API
   contract in this case as well.

   As with all formats that are used for data interchange, an attacker
   may have control over the shape of the data delivered as input to the
   application, which therefore needs to validate that shape before it
   makes it the basis of its further processing.  One unique aspect that
   typed arrays add to this is that an attacker might substitute a
   Uint8ClampedArray for where the application expects a Uint8Array, or
   vice versa, potentially leading to very different (and unexpected)
   processing semantics of the in-memory data structures constructed.
   Applications that could be affected by this therefore will need to be
   careful about making this distinction in their input validation.





























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8.  References

8.1.  Normative References

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
              Std 754-2008.

   [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/info/rfc2119>.

   [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>.

   [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/info/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/info/rfc8610>.

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.

   [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>.

Contributors

   The initial draft for this specification was written by Johnathan
   Roatch (roatch@gmail.com).  Many thanks for getting this ball
   rolling.



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   Glenn Engel suggested the tags for multi-dimensional arrays and
   homogeneous arrays.

Acknowledgements

   Jim Schaad provided helpful comments and reminded us that column-
   major order still is in use.  Jeffrey Yaskin helped improve the
   definition of homogeneous arrays.  IANA helped correct an error in a
   previous version.

Author's Address

   Carsten Bormann (editor)
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org































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