RFC8746: Concise Binary Object Representation (CBOR) Tags for Typed Arrays

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Internet Engineering Task Force (IETF)                   C. Bormann, Ed.
Request for Comments: 8746                        Universität Bremen TZI
Category: Standards Track                                  February 2020
ISSN: 2070-1721

   Concise Binary Object Representation (CBOR) Tags for Typed Arrays


   The Concise Binary Object Representation (CBOR), as defined in 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.

   This document makes use of this extensibility to define a number of
   CBOR tags for typed arrays of numeric data, as well as additional
   tags for multi-dimensional and homogeneous arrays.  It is intended as
   the reference document for the IANA registration of the CBOR tags

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Typed Arrays
     2.1.  Types of Numbers
   3.  Additional Array Tags
     3.1.  Multi-dimensional Array
       3.1.1.  Row-Major Order
       3.1.2.  Column-Major Order
     3.2.  Homogeneous Array
   4.  Discussion
   5.  CDDL Typenames
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Author's Address

1.  Introduction

   The Concise Binary Object Representation (CBOR) [RFC7049] provides
   for the interchange of structured data without a requirement for a
   pre-agreed schema.  [RFC7049] 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 has received
   interest both in the Web graphics community [TypedArray] and in the
   JavaScript specification (see Section 22.2 (https://www.ecma-
   international.org/ecma-262/10.0/index.html#sec-typedarray-objects) of
   [ECMA-ES10]) as well as in corresponding implementations

   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 can also 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 additional tags for multi-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 for
   each individual case.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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
   familiar notation from the programming language C [C] (including
   C++14's 0bnnn binary literals [CPlusPlus]) with the exception of the
   operator "**", which 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 the same type (the term is neutral
   as to whether that is a representation type or an application data
   model type).

   The terms "big endian" and "little endian" are used to indicate a
   most significant byte first (MSB first) representation of integers
   and a least significant byte first (LSB first) representation,

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
   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, and 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 float or unsigned integer, 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 0f and ll are the two least significant bits,
   respectively, of each 4-bit nibble in the byte.)

   In the CBOR representation, the total number of elements in the array
   is not expressed explicitly but is implied from the length of the
   byte string and the length of each representation.  It can be
   computed from the length, in bytes, of the byte string comprising the
   representation of the array by inverting the previous formula:
   "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 (http://www.ecma-international.org/ecma-262/6.0/#sec-
   touint8clamp) of the ES10 JavaScript specification ("ToUint8Clamp")
   [ECMA-ES10]; 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
   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 another for column-major order [RowColMajor].

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 (any one of a CBOR array of major type 4, 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 multi-
   dimensional 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 multi-
   dimensional array tag in conjunction with a basic CBOR array (which,
   with the small numbers chosen for the example, happens to be

   <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 multi-dimensional 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:  The same as tag 40, except 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

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 therefore 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++
   [CPlusPlus] 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.

   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 that 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.

5.  CDDL Typenames

   For use with CDDL [RFC8610], the typenames defined in Figure 6 are

   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

6.  IANA Considerations

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

   The allocations were assigned from the "specification required" space
   (24..255) with the exception of 1040, which was assigned from the
   "first come first served" space (256..).

       |  Tag | Data Item            | Semantics                  |
       |   40 | array of two arrays* | Multi-dimensional Array,   |
       |      |                      | row-major order            |
       |   41 | array                | Homogeneous Array          |
       |   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        |
       | 1040 | array of two arrays* | Multi-dimensional Array,   |
       |      |                      | column-major order         |

                         Table 3: Values for Tags

   *40 or 1040 data item: The second element of the outer array in the
   data item is a native CBOR array (major type 4) or Typed Array (one
   of tag 64..87)

7.  Security Considerations

   The security considerations of [RFC7049] apply; special attention is
   drawn to the second paragraph of Section 8 of [RFC7049].

   The tag for homogeneous arrays makes a promise about its tagged data
   item, which 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 will therefore need to be
   careful about making this distinction in their input validation.

8.  References

8.1.  Normative References

   [C]        International Organization for Standardization,
              "Information technology - Programming languages - C", ISO/
              IEC 9899:2018, Fourth Edition, June 2018.

              International Organization for Standardization,
              "Programming languages - C++", ISO/IEC 14882:2017, Fifth
              Edition, December 2017.

              ECMA International, "ECMAScript 2019 Language
              Specification", Standard ECMA-262 10th Edition, June 2019,

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic",
              IEEE 754-2019, DOI 10.1109/IEEESTD.2019.8766229,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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

              Mozilla Developer Network, "JavaScript typed arrays", June
              2010, <https://developer.mozilla.org/en-

              Wikipedia, "Row- and column-major order", September 2019,

              Vukicevic, V. and K. Russell, "Typed Array Specification",
              February 2011,


   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 draft version.  Francesca Palombini acted as Shepherd, and
   Alexey Melnikov as responsible Area Director.  Elwyn Davies as Gen-
   ART reviewer and IESG members Martin Vigoureux, Adam Roach, Roman
   Danyliw, and Benjamin Kaduk helped in finding further improvements to
   the text; thanks also to the other reviewers.


   The initial draft version of this specification was written by
   Johnathan Roatch <roatch@gmail.com>.  Many thanks for getting this
   ball rolling.

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

Author's Address

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

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