Network Working Group J. Roatch Internet-Draft Intended status: Informational C. Bormann Expires: January 8, 2017 Universitaet Bremen TZI July 07, 2016 Concise Binary Object Representation (CBOR) Tags for Typed Arrays draft-jroatch-cbor-tags-04 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 http://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 January 8, 2017. Copyright Notice Copyright (c) 2016 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 Roatch & Bormann Expires January 8, 2017 [Page 1] Internet-Draft CBOR tags for typed arrays July 2016 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. Roatch & Bormann Expires January 8, 2017 [Page 2] Internet-Draft CBOR tags for typed arrays July 2016 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. Roatch & Bormann Expires January 8, 2017 [Page 3] Internet-Draft CBOR tags for typed arrays July 2016 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 defined 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. Roatch & Bormann Expires January 8, 2017 [Page 4] Internet-Draft CBOR tags for typed arrays July 2016 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 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}, }; # 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 of the array is thus determined by the application type of the first Roatch & Bormann Expires January 8, 2017 [Page 5] Internet-Draft CBOR tags for typed arrays July 2016 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 }; # 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. Roatch & Bormann Expires January 8, 2017 [Page 6] Internet-Draft CBOR tags for typed arrays July 2016 5. CDDL typenames For the use with CDDL [I-D.greevenbosch-appsawg-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 = #6.TBD41(array) multi-dim = #6.TBD40([dim, array]) Figure 3: Recommended typenames for CDDL Roatch & Bormann Expires January 8, 2017 [Page 7] Internet-Draft CBOR tags for typed arrays July 2016 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 | Multi-dimensional Array | | TBD41 | array* | Homogeneous Array | +-------+-------------+---------------------------------------------+ Table 3: Values for Tags *) TBD41 data item: array (major type 4) or Typed Array (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. Roatch & Bormann Expires January 8, 2017 [Page 8] Internet-Draft CBOR tags for typed arrays July 2016 7. Security Considerations The security considerations of RFC 7049 apply; the tags introduced here are not expected to raise security considerations beyond those. Roatch & Bormann Expires January 8, 2017 [Page 9] Internet-Draft CBOR tags for typed arrays July 2016 8. References 8.1. Normative References [I-D.greevenbosch-appsawg-cbor-cddl] Vigano, C. and H. Birkholz, "CBOR data definition language (CDDL): a notational convention to express CBOR data structures", draft-greevenbosch-appsawg-cbor-cddl-08 (work in progress), March 2016. [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, . 8.2. Informative References [ArrayBuffer] Mozilla Developer Network, "JavaScript typed arrays", 2013, . [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, . [TypedArrayUpdate] Herman, D. and K. Russell, "Typed Array Specification", July 2013, . Contributors Glenn Engel suggested the tags for multi-dimensional arrays and homogeneous arrays. Acknowledgements TBD Roatch & Bormann Expires January 8, 2017 [Page 10] Internet-Draft CBOR tags for typed arrays July 2016 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 Roatch & Bormann Expires January 8, 2017 [Page 11]