Concise Binary Object Representation (CBOR)
draft-ietf-cbor-7049bis-06

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Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                              P. Hoffman
Expires: January 3, 2020                                           ICANN
                                                           July 02, 2019

              Concise Binary Object Representation (CBOR)
                     draft-ietf-cbor-7049bis-06

Abstract

   The Concise Binary Object Representation (CBOR) 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.  These design goals make it different from
   earlier binary serializations such as ASN.1 and MessagePack.

   This document obsoletes RFC 7049.

Contributing

   This document is being worked on in the CBOR Working Group.  Please
   contribute on the mailing list there, or in the GitHub repository for
   this draft: https://github.com/cbor-wg/CBORbis

   The charter for the CBOR Working Group says that the WG will update
   RFC 7049 to fix verified errata.  Security issues and clarifications
   may be addressed, but changes to this document will ensure backward
   compatibility for popular deployed codebases.  This document will be
   targeted at becoming an Internet Standard.

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 January 3, 2020.

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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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  CBOR Data Models  . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Extended Generic Data Models  . . . . . . . . . . . . . .   8
     2.2.  Specific Data Models  . . . . . . . . . . . . . . . . . .   8
   3.  Specification of the CBOR Encoding  . . . . . . . . . . . . .   9
     3.1.  Major Types . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Indefinite Lengths for Some Major Types . . . . . . . . .  12
       3.2.1.  The "break" Stop Code . . . . . . . . . . . . . . . .  12
       3.2.2.  Indefinite-Length Arrays and Maps . . . . . . . . . .  12
       3.2.3.  Indefinite-Length Byte Strings and Text Strings . . .  14
     3.3.  Floating-Point Numbers and Values with No Content . . . .  15
     3.4.  Optional Tagging of Items . . . . . . . . . . . . . . . .  17
       3.4.1.  Date and Time . . . . . . . . . . . . . . . . . . . .  19
       3.4.2.  Standard Date/Time String . . . . . . . . . . . . . .  19
       3.4.3.  Epoch-based Date/Time . . . . . . . . . . . . . . . .  19
       3.4.4.  Bignums . . . . . . . . . . . . . . . . . . . . . . .  20
       3.4.5.  Decimal Fractions and Bigfloats . . . . . . . . . . .  21
       3.4.6.  Content Hints . . . . . . . . . . . . . . . . . . . .  22
         3.4.6.1.  Encoded CBOR Data Item  . . . . . . . . . . . . .  22
         3.4.6.2.  Expected Later Encoding for CBOR-to-JSON
                   Converters  . . . . . . . . . . . . . . . . . . .  23
         3.4.6.3.  Encoded Text  . . . . . . . . . . . . . . . . . .  23
       3.4.7.  Self-Described CBOR . . . . . . . . . . . . . . . . .  24
   4.  Serialization Considerations  . . . . . . . . . . . . . . . .  25
     4.1.  Preferred Serialization . . . . . . . . . . . . . . . . .  25
     4.2.  Deterministically Encoded CBOR  . . . . . . . . . . . . .  26
       4.2.1.  Core Deterministic Encoding Requirements  . . . . . .  26
       4.2.2.  Additional Deterministic Encoding Considerations  . .  27
       4.2.3.  Length-first map key ordering . . . . . . . . . . . .  28

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   5.  Creating CBOR-Based Protocols . . . . . . . . . . . . . . . .  29
     5.1.  CBOR in Streaming Applications  . . . . . . . . . . . . .  30
     5.2.  Generic Encoders and Decoders . . . . . . . . . . . . . .  30
     5.3.  Invalid Items . . . . . . . . . . . . . . . . . . . . . .  31
     5.4.  Handling Unknown Simple Values and Tags . . . . . . . . .  32
     5.5.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  32
     5.6.  Specifying Keys for Maps  . . . . . . . . . . . . . . . .  33
       5.6.1.  Equivalence of Keys . . . . . . . . . . . . . . . . .  34
     5.7.  Undefined Values  . . . . . . . . . . . . . . . . . . . .  35
     5.8.  Strict Decoding Mode  . . . . . . . . . . . . . . . . . .  35
   6.  Converting Data between CBOR and JSON . . . . . . . . . . . .  36
     6.1.  Converting from CBOR to JSON  . . . . . . . . . . . . . .  36
     6.2.  Converting from JSON to CBOR  . . . . . . . . . . . . . .  38
   7.  Future Evolution of CBOR  . . . . . . . . . . . . . . . . . .  39
     7.1.  Extension Points  . . . . . . . . . . . . . . . . . . . .  39
     7.2.  Curating the Additional Information Space . . . . . . . .  40
   8.  Diagnostic Notation . . . . . . . . . . . . . . . . . . . . .  40
     8.1.  Encoding Indicators . . . . . . . . . . . . . . . . . . .  41
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
     9.1.  Simple Values Registry  . . . . . . . . . . . . . . . . .  42
     9.2.  Tags Registry . . . . . . . . . . . . . . . . . . . . . .  42
     9.3.  Media Type ("MIME Type")  . . . . . . . . . . . . . . . .  43
     9.4.  CoAP Content-Format . . . . . . . . . . . . . . . . . . .  44
     9.5.  The +cbor Structured Syntax Suffix Registration . . . . .  44
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  45
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  47
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  47
     11.2.  Informative References . . . . . . . . . . . . . . . . .  48
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  50
   Appendix B.  Jump Table . . . . . . . . . . . . . . . . . . . . .  54
   Appendix C.  Pseudocode . . . . . . . . . . . . . . . . . . . . .  57
   Appendix D.  Half-Precision . . . . . . . . . . . . . . . . . . .  59
   Appendix E.  Comparison of Other Binary Formats to CBOR's Design
                Objectives . . . . . . . . . . . . . . . . . . . . .  60
     E.1.  ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . .  61
     E.2.  MessagePack . . . . . . . . . . . . . . . . . . . . . . .  61
     E.3.  BSON  . . . . . . . . . . . . . . . . . . . . . . . . . .  62
     E.4.  MSDTP: RFC 713  . . . . . . . . . . . . . . . . . . . . .  62
     E.5.  Conciseness on the Wire . . . . . . . . . . . . . . . . .  62
   Appendix F.  Changes from RFC 7049  . . . . . . . . . . . . . . .  63
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  64

1.  Introduction

   There are hundreds of standardized formats for binary representation
   of structured data (also known as binary serialization formats).  Of
   those, some are for specific domains of information, while others are

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   generalized for arbitrary data.  In the IETF, probably the best-known
   formats in the latter category are ASN.1's BER and DER [ASN.1].

   The format defined here follows some specific design goals that are
   not well met by current formats.  The underlying data model is an
   extended version of the JSON data model [RFC8259].  It is important
   to note that this is not a proposal that the grammar in RFC 8259 be
   extended in general, since doing so would cause a significant
   backwards incompatibility with already deployed JSON documents.
   Instead, this document simply defines its own data model that starts
   from JSON.

   Appendix E lists some existing binary formats and discusses how well
   they do or do not fit the design objectives of the Concise Binary
   Object Representation (CBOR).

   This document obsoletes [RFC7049].

1.1.  Objectives

   The objectives of CBOR, roughly in decreasing order of importance,
   are:

   1.  The representation must be able to unambiguously encode most
       common data formats used in Internet standards.

       *  It must represent a reasonable set of basic data types and
          structures using binary encoding.  "Reasonable" here is
          largely influenced by the capabilities of JSON, with the major
          addition of binary byte strings.  The structures supported are
          limited to arrays and trees; loops and lattice-style graphs
          are not supported.

       *  There is no requirement that all data formats be uniquely
          encoded; that is, it is acceptable that the number "7" might
          be encoded in multiple different ways.

   2.  The code for an encoder or decoder must be able to be compact in
       order to support systems with very limited memory, processor
       power, and instruction sets.

       *  An encoder and a decoder need to be implementable in a very
          small amount of code (for example, in class 1 constrained
          nodes as defined in [RFC7228]).

       *  The format should use contemporary machine representations of
          data (for example, not requiring binary-to-decimal
          conversion).

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   3.  Data must be able to be decoded without a schema description.

       *  Similar to JSON, encoded data should be self-describing so
          that a generic decoder can be written.

   4.  The serialization must be reasonably compact, but data
       compactness is secondary to code compactness for the encoder and
       decoder.

       *  "Reasonable" here is bounded by JSON as an upper bound in
          size, and by implementation complexity maintaining a lower
          bound.  Using either general compression schemes or extensive
          bit-fiddling violates the complexity goals.

   5.  The format must be applicable to both constrained nodes and high-
       volume applications.

       *  This means it must be reasonably frugal in CPU usage for both
          encoding and decoding.  This is relevant both for constrained
          nodes and for potential usage in applications with a very high
          volume of data.

   6.  The format must support all JSON data types for conversion to and
       from JSON.

       *  It must support a reasonable level of conversion as long as
          the data represented is within the capabilities of JSON.  It
          must be possible to define a unidirectional mapping towards
          JSON for all types of data.

   7.  The format must be extensible, and the extended data must be
       decodable by earlier decoders.

       *  The format is designed for decades of use.

       *  The format must support a form of extensibility that allows
          fallback so that a decoder that does not understand an
          extension can still decode the message.

       *  The format must be able to be extended in the future by later
          IETF standards.

1.2.  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

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   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".  All multi-byte values are encoded in network byte order
   (that is, most significant byte first, also known as "big-endian").

   This specification makes use of the following terminology:

   Data item:  A single piece of CBOR data.  The structure of a data
      item may contain zero, one, or more nested data items.  The term
      is used both for the data item in representation format and for
      the abstract idea that can be derived from that by a decoder.

   Decoder:  A process that decodes a well-formed CBOR data item and
      makes it available to an application.  Formally speaking, a
      decoder contains a parser to break up the input using the syntax
      rules of CBOR, as well as a semantic processor to prepare the data
      in a form suitable to the application.

   Encoder:  A process that generates the representation format of a
      CBOR data item from application information.

   Data Stream:  A sequence of zero or more data items, not further
      assembled into a larger containing data item.  The independent
      data items that make up a data stream are sometimes also referred
      to as "top-level data items".

   Well-formed:  A data item that follows the syntactic structure of
      CBOR.  A well-formed data item uses the initial bytes and the byte
      strings and/or data items that are implied by their values as
      defined in CBOR and does not include following extraneous data.
      CBOR decoders by definition only return contents from well-formed
      data items.

   Valid:  A data item that is well-formed and also follows the semantic
      restrictions that apply to CBOR data items.

   Stream decoder:  A process that decodes a data stream and makes each
      of the data items in the sequence available to an application as
      they are received.

   Where bit arithmetic or data types are explained, this document uses
   the notation familiar from the programming language C, except that
   "**" denotes exponentiation.  Similar to the "0x" notation for
   hexadecimal numbers, numbers in binary notation are prefixed with
   "0b".  Underscores can be added to such a number solely for
   readability, so 0b00100001 (0x21) might be written 0b001_00001 to

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   emphasize the desired interpretation of the bits in the byte; in this
   case, it is split into three bits and five bits.  Encoded CBOR data
   items are sometimes given in the "0x" or "0b" notation; these values
   are first interpreted as numbers as in C and are then interpreted as
   byte strings in network byte order, including any leading zero bytes
   expressed in the notation.

2.  CBOR Data Models

   CBOR is explicit about its generic data model, which defines the set
   of all data items that can be represented in CBOR.  Its basic generic
   data model is extensible by the registration of simple type values
   and tags.  Applications can then subset the resulting extended
   generic data model to build their specific data models.

   Within environments that can represent the data items in the generic
   data model, generic CBOR encoders and decoders can be implemented
   (which usually involves defining additional implementation data types
   for those data items that do not already have a natural
   representation in the environment).  The ability to provide generic
   encoders and decoders is an explicit design goal of CBOR; however
   many applications will provide their own application-specific
   encoders and/or decoders.

   In the basic (un-extended) generic data model, a data item is one of:

   o  an integer in the range -2**64..2**64-1 inclusive

   o  a simple value, identified by a number between 0 and 255, but
      distinct from that number

   o  a floating point value, distinct from an integer, out of the set
      representable by IEEE 754 binary64 (including non-finites)
      [IEEE754]

   o  a sequence of zero or more bytes ("byte string")

   o  a sequence of zero or more Unicode code points ("text string")

   o  a sequence of zero or more data items ("array")

   o  a mapping (mathematical function) from zero or more data items
      ("keys") each to a data item ("values"), ("map")

   o  a tagged data item, comprising a tag (an integer in the range
      0..2**64-1) and a value (a data item)

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   Note that integer and floating-point values are distinct in this
   model, even if they have the same numeric value.

   Also note that serialization variants, such as number of bytes of the
   encoded floating value, or the choice of one of the ways in which an
   integer, the length of a text or byte string, the number of elements
   in an array or pairs in a map, or a tag value, (collectively "the
   argument", see Section 3) can be encoded, are not visible at the
   generic data model level.

2.1.  Extended Generic Data Models

   This basic generic data model comes pre-extended by the registration
   of a number of simple values and tags right in this document, such
   as:

   o  "false", "true", "null", and "undefined" (simple values identified
      by 20..23)

   o  integer and floating point values with a larger range and
      precision than the above (tags 2 to 5)

   o  application data types such as a point in time or an RFC 3339
      date/time string (tags 1, 0)

   Further elements of the extended generic data model can be (and have
   been) defined via the IANA registries created for CBOR.  Even if such
   an extension is unknown to a generic encoder or decoder, data items
   using that extension can be passed to or from the application by
   representing them at the interface to the application within the
   basic generic data model, i.e., as generic values of a simple type or
   generic tagged items.

   In other words, the basic generic data model is stable as defined in
   this document, while the extended generic data model expands by the
   registration of new simple values or tags, but never shrinks.

   While there is a strong expectation that generic encoders and
   decoders can represent "false", "true", and "null" ("undefined" is
   intentionally omitted) in the form appropriate for their programming
   environment, implementation of the data model extensions created by
   tags is truly optional and a matter of implementation quality.

2.2.  Specific Data Models

   The specific data model for a CBOR-based protocol usually subsets the
   extended generic data model and assigns application semantics to the
   data items within this subset and its components.  When documenting

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   such specific data models, where it is desired to specify the types
   of data items, it is preferred to identify the types by the names
   they have in the generic data model ("negative integer", "array")
   instead of by referring to aspects of their CBOR representation
   ("major type 1", "major type 4").

   Specific data models can also specify what values (including values
   of different types) are equivalent for the purposes of map keys and
   encoder freedom.  For example, in the generic data model, a valid map
   MAY have both "0" and "0.0" as keys, and an encoder MUST NOT encode
   "0.0" as an integer (major type 0, Section 3.1).  However, if a
   specific data model declares that floating point and integer
   representations of integral values are equivalent, using both map
   keys "0" and "0.0" in a single map would be considered duplicates and
   so invalid, and an encoder could encode integral-valued floats as
   integers or vice versa, perhaps to save encoded bytes.

3.  Specification of the CBOR Encoding

   A CBOR data item (Section 2) is encoded to or decoded from a byte
   string carrying a well-formed encoded data item as described in this
   section.  The encoding is summarized in Table 5.  An encoder MUST
   produce only well-formed encoded data items.  A decoder MUST NOT
   return a decoded data item when it encounters input that is not a
   well-formed encoded CBOR data item (this does not detract from the
   usefulness of diagnostic and recovery tools that might make available
   some information from a damaged encoded CBOR data item).

   The initial byte of each encoded data item contains both information
   about the major type (the high-order 3 bits, described in
   Section 3.1) and additional information (the low-order 5 bits).  With
   a few exceptions, the additional information's value describes how to
   load an unsigned integer "argument":

   Less than 24:  The argument's value is the value of the additional
      information.

   24, 25, 26, or 27:  The argument's value is held in the following 1,
      2, 4, or 8 bytes, respectively, in network byte order.  For major
      type 7 and additional information value 25, 26, 27, these bytes
      are not used as an integer argument, but as a floating point value
      (see Section 3.3).

   28, 29, 30:  These values are reserved for future additions to the
      CBOR format.  In the present version of CBOR, the encoded item is
      not well-formed.

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   31:  No argument value is derived.  If the major type is 0, 1, or 6,
      the encoded item is not well-formed.  For major types 2 to 5, the
      item's length is indefinite, and for major type 7, the byte does
      not consitute a data item at all but terminates an indefinite
      length item; both are described in Section 3.2.

   The meaning of this argument depends on the major type.  For example,
   in major type 0, the argument is the value of the data item itself
   (and in major type 1 the value of the data item is computed from the
   argument); in major type 2 and 3 it gives the length of the string
   data in bytes that follows; and in major types 4 and 5 it is used to
   determine the number of data items enclosed.

   If the encoded sequence of bytes ends before the end of a data item,
   that item is not well-formed.  If the encoded sequence of bytes still
   has bytes remaining after the outermost encoded item is decoded, that
   encoding is not a single well-formed CBOR item; depending on the
   application, the decoder may either treat the encoding as not well-
   formed or just identify the start of the remaining bytes to the
   application.

   A CBOR decoder implementation can be based on a jump table with all
   256 defined values for the initial byte (Table 5).  A decoder in a
   constrained implementation can instead use the structure of the
   initial byte and following bytes for more compact code (see
   Appendix C for a rough impression of how this could look).

3.1.  Major Types

   The following lists the major types and the additional information
   and other bytes associated with the type.

   Major type 0:  an integer in the range 0..2**64-1 inclusive.  The
      value of the encoded item is the argument itself.  For example,
      the integer 10 is denoted as the one byte 0b000_01010 (major type
      0, additional information 10).  The integer 500 would be
      0b000_11001 (major type 0, additional information 25) followed by
      the two bytes 0x01f4, which is 500 in decimal.

   Major type 1:  a negative integer in the range -2**64..-1 inclusive.
      The value of the item is -1 minus the argument.  For example, the
      integer -500 would be 0b001_11001 (major type 1, additional
      information 25) followed by the two bytes 0x01f3, which is 499 in
      decimal.

   Major type 2:  a byte string.  The number of bytes in the string is
      equal to the argument.  For example, a byte string whose length is
      5 would have an initial byte of 0b010_00101 (major type 2,

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      additional information 5 for the length), followed by 5 bytes of
      binary content.  A byte string whose length is 500 would have 3
      initial bytes of 0b010_11001 (major type 2, additional information
      25 to indicate a two-byte length) followed by the two bytes 0x01f4
      for a length of 500, followed by 500 bytes of binary content.

   Major type 3:  a text string (Section 2), encoded as UTF-8
      ([RFC3629]).  The number of bytes in the string is equal to the
      argument.  A string containing an invalid UTF-8 sequence is well-
      formed but invalid.  This type is provided for systems that need
      to interpret or display human-readable text, and allows the
      differentiation between unstructured bytes and text that has a
      specified repertoire and encoding.  In contrast to formats such as
      JSON, the Unicode characters in this type are never escaped.
      Thus, a newline character (U+000A) is always represented in a
      string as the byte 0x0a, and never as the bytes 0x5c6e (the
      characters "\" and "n") or as 0x5c7530303061 (the characters "\",
      "u", "0", "0", "0", and "a").

   Major type 4:  an array of data items.  Arrays are also called lists,
      sequences, or tuples.  The argument is the number of data items in
      the array.  Items in an array do not need to all be of the same
      type.  For example, an array that contains 10 items of any type
      would have an initial byte of 0b100_01010 (major type of 4,
      additional information of 10 for the length) followed by the 10
      remaining items.

   Major type 5:  a map of pairs of data items.  Maps are also called
      tables, dictionaries, hashes, or objects (in JSON).  A map is
      comprised of pairs of data items, each pair consisting of a key
      that is immediately followed by a value.  The argument is the
      number of _pairs_ of data items in the map.  For example, a map
      that contains 9 pairs would have an initial byte of 0b101_01001
      (major type of 5, additional information of 9 for the number of
      pairs) followed by the 18 remaining items.  The first item is the
      first key, the second item is the first value, the third item is
      the second key, and so on.  A map that has duplicate keys may be
      well-formed, but it is not valid, and thus it causes indeterminate
      decoding; see also Section 5.6.

   Major type 6:  a tagged data item whose tag is the argument and whose
      value is the single following encoded item.  See Section 3.4.

   Major type 7:  floating-point numbers and simple values, as well as
      the "break" stop code.  See Section 3.3.

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   These eight major types lead to a simple table showing which of the
   256 possible values for the initial byte of a data item are used
   (Table 5).

   In major types 6 and 7, many of the possible values are reserved for
   future specification.  See Section 9 for more information on these
   values.

3.2.  Indefinite Lengths for Some Major Types

   Four CBOR items (arrays, maps, byte strings, and text strings) can be
   encoded with an indefinite length using additional information value
   31.  This is useful if the encoding of the item needs to begin before
   the number of items inside the array or map, or the total length of
   the string, is known.  (The application of this is often referred to
   as "streaming" within a data item.)

   Indefinite-length arrays and maps are dealt with differently than
   indefinite-length byte strings and text strings.

3.2.1.  The "break" Stop Code

   The "break" stop code is encoded with major type 7 and additional
   information value 31 (0b111_11111).  It is not itself a data item: it
   is just a syntactic feature to close an indefinite-length item.

   If the "break" stop code appears anywhere where a data item is
   expected, other than directly inside an indefinite-length string,
   array, or map -- for example directly inside a definite-length array
   or map -- the enclosing item is not well-formed.

3.2.2.  Indefinite-Length Arrays and Maps

   Indefinite-length arrays and maps are represented using their major
   type with the additional information value of 31, followed by an
   arbitrary-length sequence of items for an array or key/value pairs
   for a map, followed by the "break" stop code (Section 3.2.1).  In
   other words, indefinite-length arrays and maps look identical to
   other arrays and maps except for beginning with the additional
   information value of 31 and ending with the "break" stop code.

   If the break stop code appears after a key in a map, in place of that
   key's value, the map is not well-formed.

   There is no restriction against nesting indefinite-length array or
   map items.  A "break" only terminates a single item, so nested
   indefinite-length items need exactly as many "break" stop codes as
   there are type bytes starting an indefinite-length item.

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   For example, assume an encoder wants to represent the abstract array
   [1, [2, 3], [4, 5]].  The definite-length encoding would be
   0x8301820203820405:

   83        -- Array of length 3
      01     -- 1
      82     -- Array of length 2
         02  -- 2
         03  -- 3
      82     -- Array of length 2
         04  -- 4
         05  -- 5

   Indefinite-length encoding could be applied independently to each of
   the three arrays encoded in this data item, as required, leading to
   representations such as:

   0x9f018202039f0405ffff
   9F        -- Start indefinite-length array
      01     -- 1
      82     -- Array of length 2
         02  -- 2
         03  -- 3
      9F     -- Start indefinite-length array
         04  -- 4
         05  -- 5
         FF  -- "break" (inner array)
      FF     -- "break" (outer array)

   0x9f01820203820405ff
   9F        -- Start indefinite-length array
      01     -- 1
      82     -- Array of length 2
         02  -- 2
         03  -- 3
      82     -- Array of length 2
         04  -- 4
         05  -- 5
      FF     -- "break"

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   0x83018202039f0405ff
   83        -- Array of length 3
      01     -- 1
      82     -- Array of length 2
         02  -- 2
         03  -- 3
      9F     -- Start indefinite-length array
         04  -- 4
         05  -- 5
         FF  -- "break"

   0x83019f0203ff820405
   83        -- Array of length 3
      01     -- 1
      9F     -- Start indefinite-length array
         02  -- 2
         03  -- 3
         FF  -- "break"
      82     -- Array of length 2
         04  -- 4
         05  -- 5

   An example of an indefinite-length map (that happens to have two key/
   value pairs) might be:

   0xbf6346756ef563416d7421ff
   BF           -- Start indefinite-length map
      63        -- First key, UTF-8 string length 3
         46756e --   "Fun"
      F5        -- First value, true
      63        -- Second key, UTF-8 string length 3
         416d74 --   "Amt"
      21        -- Second value, -2
      FF        -- "break"

3.2.3.  Indefinite-Length Byte Strings and Text Strings

   Indefinite-length strings are represented by a byte containing the
   major type and additional information value of 31, followed by a
   series of byte or text strings ("chunks") that have definite lengths,
   followed by the "break" stop code (Section 3.2.1).  The data item
   represented by the indefinite-length string is the concatenation of
   the chunks.

   If any item between the indefinite-length string indicator
   (0b010_11111 or 0b011_11111) and the "break" stop code is not a
   definite-length string item of the same major type, the string is not
   well-formed.

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   If any definite-length text string inside an indefinite-length text
   string is invalid, the indefinite-length text string is invalid.
   Note that this implies that the bytes of a single UTF-8 character
   cannot be spread between chunks: a new chunk can only be started at a
   character boundary.

   For example, assume the sequence:

   0b010_11111 0b010_00100 0xaabbccdd 0b010_00011 0xeeff99 0b111_11111

   5F              -- Start indefinite-length byte string
      44           -- Byte string of length 4
         aabbccdd  -- Bytes content
      43           -- Byte string of length 3
         eeff99    -- Bytes content
      FF           -- "break"

   After decoding, this results in a single byte string with seven
   bytes: 0xaabbccddeeff99.

3.3.  Floating-Point Numbers and Values with No Content

   Major type 7 is for two types of data: floating-point numbers and
   "simple values" that do not need any content.  Each value of the
   5-bit additional information in the initial byte has its own separate
   meaning, as defined in Table 1.  Like the major types for integers,
   items of this major type do not carry content data; all the
   information is in the initial bytes.

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   +------------+------------------------------------------------------+
   | 5-Bit      | Semantics                                            |
   | Value      |                                                      |
   +------------+------------------------------------------------------+
   | 0..23      | Simple value (value 0..23)                           |
   |            |                                                      |
   | 24         | Simple value (value 32..255 in following byte)       |
   |            |                                                      |
   | 25         | IEEE 754 Half-Precision Float (16 bits follow)       |
   |            |                                                      |
   | 26         | IEEE 754 Single-Precision Float (32 bits follow)     |
   |            |                                                      |
   | 27         | IEEE 754 Double-Precision Float (64 bits follow)     |
   |            |                                                      |
   | 28-30      | Unassigned, not well-formed in the present document  |
   |            |                                                      |
   | 31         | "break" stop code for indefinite-length items        |
   |            | (Section 3.2.1)                                      |
   +------------+------------------------------------------------------+

        Table 1: Values for Additional Information in Major Type 7

   As with all other major types, the 5-bit value 24 signifies a single-
   byte extension: it is followed by an additional byte to represent the
   simple value.  (To minimize confusion, only the values 32 to 255 are
   used.)  This maintains the structure of the initial bytes: as for the
   other major types, the length of these always depends on the
   additional information in the first byte.  Table 2 lists the values
   assigned and available for simple types.

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                       +---------+-----------------+
                       | Value   | Semantics       |
                       +---------+-----------------+
                       | 0..19   | (Unassigned)    |
                       |         |                 |
                       | 20      | False           |
                       |         |                 |
                       | 21      | True            |
                       |         |                 |
                       | 22      | Null            |
                       |         |                 |
                       | 23      | Undefined value |
                       |         |                 |
                       | 24..31  | (Reserved)      |
                       |         |                 |
                       | 32..255 | (Unassigned)    |
                       +---------+-----------------+

                          Table 2: Simple Values

   An encoder MUST NOT issue two-byte sequences that start with 0xf8
   (major type = 7, additional information = 24) and continue with a
   byte less than 0x20 (32 decimal).  Such sequences are not well-
   formed.  (This implies that an encoder cannot encode false, true,
   null, or undefined in two-byte sequences, only the one-byte variants
   of these are well-formed.)

   The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
   IEEE 754 binary floating-point values [IEEE754].  These floating-
   point values are encoded in the additional bytes of the appropriate
   size.  (See Appendix D for some information about 16-bit floating
   point.)

3.4.  Optional Tagging of Items

   In CBOR, a data item can optionally be preceded by a tag to give it
   additional semantics while retaining its structure.  The tag is major
   type 6, and represents an unsigned integer as indicated by the tag's
   argument (Section 3); the (sole) data item is carried as content
   data.  If a tag requires structured data, this structure is encoded
   into the nested data item.  The definition of a tag usually restricts
   what kinds of nested data item or items are valid for this tag.

   For example, assume that a byte string of length 12 is marked with a
   tag to indicate it is a positive bignum (Section 3.4.4).  This would
   be marked as 0b110_00010 (major type 6, additional information 2 for
   the tag) followed by 0b010_01100 (major type 2, additional

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   information of 12 for the length) followed by the 12 bytes of the
   bignum.

   Decoders do not need to understand tags, and thus tags may be of
   little value in applications where the implementation creating a
   particular CBOR data item and the implementation decoding that stream
   know the semantic meaning of each item in the data flow.  Their
   primary purpose in this specification is to define common data types
   such as dates.  A secondary purpose is to allow optional tagging when
   the decoder is a generic CBOR decoder that might be able to benefit
   from hints about the content of items.  Understanding the semantic
   tags is optional for a decoder; it can just jump over the initial
   bytes of the tag and interpret the tagged data item itself.

   A tag always applies to the item that directly follows it.  Thus, if
   tag A is followed by tag B, which is followed by data item C, tag A
   applies to the result of applying tag B on data item C.  That is, a
   tagged item is a data item consisting of a tag and a value.  The
   content of the tagged item is the data item (the value) that is being
   tagged.

   IANA maintains a registry of tag values as described in Section 9.2.
   Table 3 provides a list of values that were defined in [RFC7049],
   with definitions in the rest of this section.  Note that many other
   tags have been defined since the publication of [RFC7049]; see the
   registry described at Section 9.2 for the complete list.

   +-------+-----------+-----------------------------------------------+
   | Tag   | Data Item | Semantics                                     |
   +-------+-----------+-----------------------------------------------+
   | 0     | UTF-8     | Standard date/time string; see Section 3.4.2  |
   |       | string    |                                               |
   |       |           |                                               |
   | 1     | multiple  | Epoch-based date/time; see Section 3.4.3      |
   |       |           |                                               |
   | 2     | byte      | Positive bignum; see Section 3.4.4            |
   |       | string    |                                               |
   |       |           |                                               |
   | 3     | byte      | Negative bignum; see Section 3.4.4            |
   |       | string    |                                               |
   |       |           |                                               |
   | 4     | array     | Decimal fraction; see Section 3.4.5           |
   |       |           |                                               |
   | 5     | array     | Bigfloat; see Section 3.4.5                   |
   |       |           |                                               |
   | 21    | multiple  | Expected conversion to base64url encoding;    |
   |       |           | see Section 3.4.6.2                           |
   |       |           |                                               |

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   | 22    | multiple  | Expected conversion to base64 encoding; see   |
   |       |           | Section 3.4.6.2                               |
   |       |           |                                               |
   | 23    | multiple  | Expected conversion to base16 encoding; see   |
   |       |           | Section 3.4.6.2                               |
   |       |           |                                               |
   | 24    | byte      | Encoded CBOR data item; see Section 3.4.6.1   |
   |       | string    |                                               |
   |       |           |                                               |
   | 32    | UTF-8     | URI; see Section 3.4.6.3                      |
   |       | string    |                                               |
   |       |           |                                               |
   | 33    | UTF-8     | base64url; see Section 3.4.6.3                |
   |       | string    |                                               |
   |       |           |                                               |
   | 34    | UTF-8     | base64; see Section 3.4.6.3                   |
   |       | string    |                                               |
   |       |           |                                               |
   | 35    | UTF-8     | Regular expression; see Section 3.4.6.3       |
   |       | string    |                                               |
   |       |           |                                               |
   | 36    | UTF-8     | MIME message; see Section 3.4.6.3             |
   |       | string    |                                               |
   |       |           |                                               |
   | 55799 | multiple  | Self-described CBOR; see Section 3.4.7        |
   +-------+-----------+-----------------------------------------------+

                         Table 3: Values for Tags

3.4.1.  Date and Time

   Protocols using tag values 0 and 1 extend the generic data model
   (Section 2) with data items representing points in time.

3.4.2.  Standard Date/Time String

   Tag value 0 contains a text string in the standard format described
   by the "date-time" production in [RFC3339], as refined by Section 3.3
   of [RFC4287], representing the point in time described there.  A
   nested item of another type or that doesn't match the [RFC4287]
   format is invalid.

3.4.3.  Epoch-based Date/Time

   Tag value 1 contains a numerical value counting the number of seconds
   from 1970-01-01T00:00Z in UTC time to the represented point in civil
   time.

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   The tagged item MUST be an unsigned or negative integer (major types
   0 and 1), or a floating-point number (major type 7 with additional
   information 25, 26, or 27).  Other contained types are invalid.

   Non-negative values (major type 0 and non-negative floating-point
   numbers) stand for time values on or after 1970-01-01T00:00Z UTC and
   are interpreted according to POSIX [TIME_T].  (POSIX time is also
   known as UNIX Epoch time.  Note that leap seconds are handled
   specially by POSIX time and this results in a 1 second discontinuity
   several times per decade.)  Note that applications that require the
   expression of times beyond early 2106 cannot leave out support of
   64-bit integers for the tagged value.

   Negative values (major type 1 and negative floating-point numbers)
   are interpreted as determined by the application requirements as
   there is no universal standard for UTC count-of-seconds time before
   1970-01-01T00:00Z (this is particularly true for points in time that
   precede discontinuities in national calendars).  The same applies to
   non-finite values.

   To indicate fractional seconds, floating point values can be used
   within Tag 1 instead of integer values.  Note that this generally
   requires binary64 support, as binary16 and binary32 provide non-zero
   fractions of seconds only for a short period of time around early
   1970.  An application that requires Tag 1 support may restrict the
   tagged value to be an integer (or a floating-point value) only.

3.4.4.  Bignums

   Protocols using tag values 2 and 3 extend the generic data model
   (Section 2) with "bignums" representing arbitrarily sized integers.
   In the generic data model, bignum values are not equal to integers
   from the basic data model, but specific data models can define that
   equivalence, and preferred encoding never makes use of bignums that
   also can be expressed as basic integers (see below).

   Bignums are encoded as a byte string data item, which is interpreted
   as an unsigned integer n in network byte order.  Contained items of
   other types are invalid.  For tag value 2, the value of the bignum is
   n.  For tag value 3, the value of the bignum is -1 - n.  The
   preferred encoding of the byte string is to leave out any leading
   zeroes (note that this means the preferred encoding for n = 0 is the
   empty byte string, but see below).  Decoders that understand these
   tags MUST be able to decode bignums that do have leading zeroes.  The
   preferred encoding of an integer that can be represented using major
   type 0 or 1 is to encode it this way instead of as a bignum (which
   means that the empty string never occurs in a bignum when using
   preferred encoding).  Note that this means the non-preferred choice

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   of a bignum representation instead of a basic integer for encoding a
   number is not intended to have application semantics (just as the
   choice of a longer basic integer representation than needed, such as
   0x1800 for 0x00 does not).

   For example, the number 18446744073709551616 (2**64) is represented
   as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major
   type 2, length 9), followed by 0x010000000000000000 (one byte 0x01
   and eight bytes 0x00).  In hexadecimal:

   C2                        -- Tag 2
      49                     -- Byte string of length 9
         010000000000000000  -- Bytes content

3.4.5.  Decimal Fractions and Bigfloats

   Protocols using tag value 4 extend the generic data model with data
   items representing arbitrary-length decimal fractions m*(10*e).
   Protocols using tag value 5 extend the generic data model with data
   items representing arbitrary-length binary fractions m*(2*e).  As
   with bignums, values of different types are not equal in the generic
   data model.

   Decimal fractions combine an integer mantissa with a base-10 scaling
   factor.  They are most useful if an application needs the exact
   representation of a decimal fraction such as 1.1 because there is no
   exact representation for many decimal fractions in binary floating
   point.

   Bigfloats combine an integer mantissa with a base-2 scaling factor.
   They are binary floating-point values that can exceed the range or
   the precision of the three IEEE 754 formats supported by CBOR
   (Section 3.3).  Bigfloats may also be used by constrained
   applications that need some basic binary floating-point capability
   without the need for supporting IEEE 754.

   A decimal fraction or a bigfloat is represented as a tagged array
   that contains exactly two integer numbers: an exponent e and a
   mantissa m.  Decimal fractions (tag 4) use base-10 exponents; the
   value of a decimal fraction data item is m*(10**e).  Bigfloats (tag
   5) use base-2 exponents; the value of a bigfloat data item is
   m*(2**e).  The exponent e MUST be represented in an integer of major
   type 0 or 1, while the mantissa also can be a bignum (Section 3.4.4).
   Contained items with other structures are invalid.

   An example of a decimal fraction is that the number 273.15 could be
   represented as 0b110_00100 (major type of 6 for the tag, additional
   information of 4 for the type of tag), followed by 0b100_00010 (major

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   type of 4 for the array, additional information of 2 for the length
   of the array), followed by 0b001_00001 (major type of 1 for the first
   integer, additional information of 1 for the value of -2), followed
   by 0b000_11001 (major type of 0 for the second integer, additional
   information of 25 for a two-byte value), followed by
   0b0110101010110011 (27315 in two bytes).  In hexadecimal:

   C4             -- Tag 4
      82          -- Array of length 2
         21       -- -2
         19 6ab3  -- 27315

   An example of a bigfloat is that the number 1.5 could be represented
   as 0b110_00101 (major type of 6 for the tag, additional information
   of 5 for the type of tag), followed by 0b100_00010 (major type of 4
   for the array, additional information of 2 for the length of the
   array), followed by 0b001_00000 (major type of 1 for the first
   integer, additional information of 0 for the value of -1), followed
   by 0b000_00011 (major type of 0 for the second integer, additional
   information of 3 for the value of 3).  In hexadecimal:

   C5             -- Tag 5
      82          -- Array of length 2
         20       -- -1
         03       -- 3

   Decimal fractions and bigfloats provide no representation of
   Infinity, -Infinity, or NaN; if these are needed in place of a
   decimal fraction or bigfloat, the IEEE 754 half-precision
   representations from Section 3.3 can be used.  For constrained
   applications, where there is a choice between representing a specific
   number as an integer and as a decimal fraction or bigfloat (such as
   when the exponent is small and non-negative), there is a quality-of-
   implementation expectation that the integer representation is used
   directly.

3.4.6.  Content Hints

   The tags in this section are for content hints that might be used by
   generic CBOR processors.  These content hints do not extend the
   generic data model.

3.4.6.1.  Encoded CBOR Data Item

   Sometimes it is beneficial to carry an embedded CBOR data item that
   is not meant to be decoded immediately at the time the enclosing data
   item is being decoded.  Tag 24 (CBOR data item) can be used to tag
   the embedded byte string as a data item encoded in CBOR format.

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   Contained items that aren't byte strings are invalid.  Any contained
   byte string is valid, even if it encodes an invalid or ill-formed
   CBOR item.

3.4.6.2.  Expected Later Encoding for CBOR-to-JSON Converters

   Tags 21 to 23 indicate that a byte string might require a specific
   encoding when interoperating with a text-based representation.  These
   tags are useful when an encoder knows that the byte string data it is
   writing is likely to be later converted to a particular JSON-based
   usage.  That usage specifies that some strings are encoded as base64,
   base64url, and so on.  The encoder uses byte strings instead of doing
   the encoding itself to reduce the message size, to reduce the code
   size of the encoder, or both.  The encoder does not know whether or
   not the converter will be generic, and therefore wants to say what it
   believes is the proper way to convert binary strings to JSON.

   The data item tagged can be a byte string or any other data item.  In
   the latter case, the tag applies to all of the byte string data items
   contained in the data item, except for those contained in a nested
   data item tagged with an expected conversion.

   These three tag types suggest conversions to three of the base data
   encodings defined in [RFC4648].  For base64url encoding (tag 21),
   padding is not used (see Section 3.2 of RFC 4648); that is, all
   trailing equals signs ("=") are removed from the encoded string.  For
   base64 encoding (tag 22), padding is used as defined in RFC 4648.
   For both base64url and base64, padding bits are set to zero (see
   Section 3.5 of RFC 4648), and encoding is performed without the
   inclusion of any line breaks, whitespace, or other additional
   characters.  Note that, for all three tags, the encoding of the empty
   byte string is the empty text string.

3.4.6.3.  Encoded Text

   Some text strings hold data that have formats widely used on the
   Internet, and sometimes those formats can be validated and presented
   to the application in appropriate form by the decoder.  There are
   tags for some of these formats.  As with tags 21 to 23, if these tags
   are applied to an item other than a text string, they apply to all
   text string data items it contains.

   o  Tag 32 is for URIs, as defined in [RFC3986].  If the text string
      doesn't match the "URI-reference" production, the string is
      invalid.

   o  Tags 33 and 34 are for base64url- and base64-encoded text strings,
      as defined in [RFC4648].  If any of:

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      *  the encoded text string contains non-alphabet characters or
         only 1 character in the last block of 4, or

      *  the padding bits in a 2- or 3-character block are not 0, or

      *  the base64 encoding has the wrong number of padding characters,
         or

      *  the base64url encoding has padding characters,

      the string is invalid.

   o  Tag 35 is for regular expressions that are roughly in Perl
      Compatible Regular Expressions (PCRE/PCRE2) form [PCRE] or a
      version of the JavaScript regular expression syntax [ECMA262].
      (Note that more specific identification may be necessary if the
      actual version of the specification underlying the regular
      expression, or more than just the text of the regular expression
      itself, need to be conveyed.)  Any contained string value is
      valid.

   o  Tag 36 is for MIME messages (including all headers), as defined in
      [RFC2045].  A text string that isn't a valid MIME message is
      invalid.

   Note that tags 33 and 34 differ from 21 and 22 in that the data is
   transported in base-encoded form for the former and in raw byte
   string form for the latter.

3.4.7.  Self-Described CBOR

   In many applications, it will be clear from the context that CBOR is
   being employed for encoding a data item.  For instance, a specific
   protocol might specify the use of CBOR, or a media type is indicated
   that specifies its use.  However, there may be applications where
   such context information is not available, such as when CBOR data is
   stored in a file that does not have disambiguating metadata.  Here,
   it may help to have some distinguishing characteristics for the data
   itself.

   Tag 55799 is defined for this purpose.  It does not impart any
   special semantics on the data item that follows; that is, the
   semantics of a data item tagged with tag 55799 is exactly identical
   to the semantics of the data item itself.

   The serialization of this tag is 0xd9d9f7, which does not appear to
   be in use as a distinguishing mark for any frequently used file
   types.  In particular, 0xd9d9f7 is not a valid start of a Unicode

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   text in any Unicode encoding if it is followed by a valid CBOR data
   item.

   For instance, a decoder might be able to decode both CBOR and JSON.
   Such a decoder would need to mechanically distinguish the two
   formats.  An easy way for an encoder to help the decoder would be to
   tag the entire CBOR item with tag 55799, the serialization of which
   will never be found at the beginning of a JSON text.

4.  Serialization Considerations

4.1.  Preferred Serialization

   For some values at the data model level, CBOR provides multiple
   serializations.  For many applications, it is desirable that an
   encoder always chooses a preferred serialization; however, the
   present specification does not put the burden of enforcing this
   preference on either encoder or decoder.

   Some constrained decoders may be limited in their ability to decode
   non-preferred serializations: For example, if only integers below
   1_000_000_000 are expected in an application, the decoder may leave
   out the code that would be needed to decode 64-bit arguments in
   integers.  An encoder that always uses preferred serialization
   ("preferred encoder") interoperates with this decoder for the numbers
   that can occur in this application.  More generally speaking, it
   therefore can be said that a preferred encoder is more universally
   interoperable (and also less wasteful) than one that, say, always
   uses 64-bit integers.

   Similarly, a constrained encoder may be limited in the variety of
   representation variants it supports in such a way that it does not
   emit preferred serializations ("variant encoder"): Say, it could be
   designed to always use the 32-bit variant for an integer that it
   encodes even if a short representation is available (again, assuming
   that there is no application need for integers that can only be
   represented with the 64-bit variant).  A decoder that does not rely
   on only ever receiving preferred serializations ("variation-tolerant
   decoder") can there be said to be more universally interoperable (it
   might very well optimize for the case of receiving preferred
   serializations, though).  Full implementations of CBOR decoders are
   by definition variation-tolerant; the distinction is only relevant if
   a constrained implementation of a CBOR decoder meets a variant
   encoder.

   The preferred serialization always uses the shortest form of
   representing the argument (Section 3)); it also uses the shortest
   floating point encoding that preserves the value being encoded (see

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   Section 5.5).  Definite length encoding is preferred whenever the
   length is known at the time the serialization of the item starts.

4.2.  Deterministically Encoded CBOR

   Some protocols may want encoders to only emit CBOR in a particular
   deterministic format; those protocols might also have the decoders
   check that their input is in that deterministic format.  Those
   protocols are free to define what they mean by a "deterministic
   format" and what encoders and decoders are expected to do.  This
   section defines a set of restrictions that can serve as the base of
   such a deterministic format.

4.2.1.  Core Deterministic Encoding Requirements

   A CBOR encoding satisfies the "core deterministic encoding
   requirements" if it satisfies the following restrictions:

   o  Arguments (see Section 3) for integers, lengths in major types 2
      through 5, and tags MUST be as short as possible.  In particular:

      *  0 to 23 and -1 to -24 MUST be expressed in the same byte as the
         major type;

      *  24 to 255 and -25 to -256 MUST be expressed only with an
         additional uint8_t;

      *  256 to 65535 and -257 to -65536 MUST be expressed only with an
         additional uint16_t;

      *  65536 to 4294967295 and -65537 to -4294967296 MUST be expressed
         only with an additional uint32_t.

   o  The keys in every map MUST be sorted in the bytewise lexicographic
      order of their deterministic encodings.  For example, the
      following keys are sorted correctly:

      1.  10, encoded as 0x0a.

      2.  100, encoded as 0x1864.

      3.  -1, encoded as 0x20.

      4.  "z", encoded as 0x617a.

      5.  "aa", encoded as 0x626161.

      6.  [100], encoded as 0x811864.

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      7.  [-1], encoded as 0x8120.

      8.  false, encoded as 0xf4.

   o  Indefinite-length items MUST NOT appear.  They can be encoded as
      definite-length items instead.

4.2.2.  Additional Deterministic Encoding Considerations

   If a protocol allows for IEEE floats, then additional deterministic
   encoding rules might need to be added.  One example rule might be to
   have all floats start as a 64-bit float, then do a test conversion to
   a 32-bit float; if the result is the same numeric value, use the
   shorter value and repeat the process with a test conversion to a
   16-bit float.  (This rule selects 16-bit float for positive and
   negative Infinity as well.)  Also, there are many representations for
   NaN.  If NaN is an allowed value, it must always be represented as
   0xf97e00.

   CBOR tags present additional considerations for deterministic
   encoding.  The absence or presence of tags in a deterministic format
   is determined by the optionality of the tags in the protocol.  In a
   CBOR-based protocol that allows optional tagging anywhere, the
   deterministic format must not allow them.  In a protocol that
   requires tags in certain places, the tag needs to appear in the
   deterministic format.  A CBOR-based protocol that uses deterministic
   encoding might instead say that all tags that appear in a message
   must be retained regardless of whether they are optional.

   Protocols that include floating, big integer, or other complex values
   need to define extra requirements on their deterministic encodings.
   For example:

   o  If a protocol includes a field that can express floating values
      (Section 3.3), the protocol's deterministic encoding needs to
      specify whether the integer 1.0 is encoded as 0x01, 0xf93c00,
      0xfa3f800000, or 0xfb3ff0000000000000.  Three sensible rules for
      this are:

      1.  Encode integral values that fit in 64 bits as values from
          major types 0 and 1, and other values as the smallest of 16-,
          32-, or 64-bit floating point that accurately represents the
          value,

      2.  Encode all values as the smallest of 16-, 32-, or 64-bit
          floating point that accurately represents the value, even for
          integral values, or

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      3.  Encode all values as 64-bit floating point.

      If NaN is an allowed value, the protocol needs to pick a single
      representation, for example 0xf97e00.

   o  If a protocol includes a field that can express integers larger
      than 2^64 using tag 2 (Section 3.4.4), the protocol's
      deterministic encoding needs to specify whether small integers are
      expressed using the tag or major types 0 and 1.

   o  A protocol might give encoders the choice of representing a URL as
      either a text string or, using Section 3.4.6.3, tag 32 containing
      a text string.  This protocol's deterministic encoding needs to
      either require that the tag is present or require that it's
      absent, not allow either one.

4.2.3.  Length-first map key ordering

   The core deterministic encoding requirements sort map keys in a
   different order from the one suggested by Section 3.9 of [RFC7049]
   (called "Canonical CBOR" there).  Protocols that need to be
   compatible with [RFC7049]'s order can instead be specified in terms
   of this specification's "length-first core deterministic encoding
   requirements":

   A CBOR encoding satisfies the "length-first core deterministic
   encoding requirements" if it satisfies the core deterministic
   encoding requirements except that the keys in every map MUST be
   sorted such that:

   1.  If two keys have different lengths, the shorter one sorts
       earlier;

   2.  If two keys have the same length, the one with the lower value in
       (byte-wise) lexical order sorts earlier.

   For example, under the length-first core deterministic encoding
   requirements, the following keys are sorted correctly:

   1.  10, encoded as 0x0a.

   2.  -1, encoded as 0x20.

   3.  false, encoded as 0xf4.

   4.  100, encoded as 0x1864.

   5.  "z", encoded as 0x617a.

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   6.  [-1], encoded as 0x8120.

   7.  "aa", encoded as 0x626161.

   8.  [100], encoded as 0x811864.

   (Although [RFC7049] used the term "Canonical CBOR" for its form of
   requirements on deterministic encoding, this document avoids this
   term because "canonicalization" is often associated with specific
   uses of deterministic encoding only.  The terms are essentially
   exchangeable, however, and the set of core requirements in this
   document could also be called "Canonical CBOR", while the length-
   first-ordered version of that could be called "Old Canonical CBOR".)

5.  Creating CBOR-Based Protocols

   Data formats such as CBOR are often used in environments where there
   is no format negotiation.  A specific design goal of CBOR is to not
   need any included or assumed schema: a decoder can take a CBOR item
   and decode it with no other knowledge.

   Of course, in real-world implementations, the encoder and the decoder
   will have a shared view of what should be in a CBOR data item.  For
   example, an agreed-to format might be "the item is an array whose
   first value is a UTF-8 string, second value is an integer, and
   subsequent values are zero or more floating-point numbers" or "the
   item is a map that has byte strings for keys and contains at least
   one pair whose key is 0xab01".

   CBOR-based protocols MUST specify how their decoders handle invalid
   and other unexpected data.  CBOR-based protocols MAY specify that
   they treat arbitrary valid data as unexpected.  Encoders for CBOR-
   based protocols MUST produce only valid items, that is, the protocol
   cannot be designed to make use of invalid items.  An encoder can be
   capable of encoding as many or as few types of values as is required
   by the protocol in which it is used; a decoder can be capable of
   understanding as many or as few types of values as is required by the
   protocols in which it is used.  This lack of restrictions allows CBOR
   to be used in extremely constrained environments.

   This section discusses some considerations in creating CBOR-based
   protocols.  With few exceptions, it is advisory only and explicitly
   excludes any language from BCP 14 other than words that could be
   interpreted as "MAY" in the sense of BCP 14.  The exceptions aim at
   facilitating interoperability of CBOR-based protocols while making
   use of a wide variety of both generic and application-specific
   encoders and decoders.

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5.1.  CBOR in Streaming Applications

   In a streaming application, a data stream may be composed of a
   sequence of CBOR data items concatenated back-to-back.  In such an
   environment, the decoder immediately begins decoding a new data item
   if data is found after the end of a previous data item.

   Not all of the bytes making up a data item may be immediately
   available to the decoder; some decoders will buffer additional data
   until a complete data item can be presented to the application.
   Other decoders can present partial information about a top-level data
   item to an application, such as the nested data items that could
   already be decoded, or even parts of a byte string that hasn't
   completely arrived yet.

   Note that some applications and protocols will not want to use
   indefinite-length encoding.  Using indefinite-length encoding allows
   an encoder to not need to marshal all the data for counting, but it
   requires a decoder to allocate increasing amounts of memory while
   waiting for the end of the item.  This might be fine for some
   applications but not others.

5.2.  Generic Encoders and Decoders

   A generic CBOR decoder can decode all well-formed CBOR data and
   present them to an application.  See Appendix C.

   Even though CBOR attempts to minimize these cases, not all well-
   formed CBOR data is valid: for example, the encoded text string
   "0x62c0ae" does not contain valid UTF-8 and so is not a valid CBOR
   item.  Also, specific tags may make semantic constraints that may be
   violated, such as a bignum tag containing another tag, or an instance
   of tag 0 containing a byte string or a text string with contents that
   do not match [RFC3339]'s "date-time" production.  There is no
   requirement that generic encoders and decoders make unnatural choices
   for their application interface to enable the processing of invalid
   data.  Generic encoders and decoders are expected to forward simple
   values and tags even if their specific codepoints are not registered
   at the time the encoder/decoder is written (Section 5.4).

   Generic decoders provide ways to present well-formed CBOR values,
   both valid and invalid, to an application.  The diagnostic notation
   (Section 8) may be used to present well-formed CBOR values to humans.

   Generic encoders provide an application interface that allows the
   application to specify any well-formed value, including simple values
   and tags unknown to the encoder.

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5.3.  Invalid Items

   A well-formed but invalid CBOR data item presents a problem with
   interpreting the data encoded in it in the CBOR data model.  A CBOR-
   based protocol could be specified in several layers, in which the
   lower layers don't process the semantics of some of the CBOR data
   they forward.  These layers can't notice the invalidity in data they
   don't process and MUST forward that data as-is.  The first layer that
   does process the semantics of an invalid CBOR item MUST take one of
   two choices:

   1.  Replace the problematic item with an error marker and continue
       with the next item, or

   2.  Issue an error and stop processing altogether.

   A CBOR-based protocol MUST specify which of these options its
   decoders take, for each kind of invalid item they might encounter.

   Such problems might include:

   Duplicate keys in a map:  Generic decoders (Section 5.2) make data
      available to applications using the native CBOR data model.  That
      data model includes maps (key-value mappings with unique keys),
      not multimaps (key-value mappings where multiple entries can have
      the same key).  Thus, a generic decoder that gets a CBOR map item
      that has duplicate keys will decode to a map with only one
      instance of that key, or it might stop processing altogether.  On
      the other hand, a "streaming decoder" may not even be able to
      notice (Section 5.6).

   Inadmissible type on the value following a tag:  Tags (Section 3.4)
      specify what type of data item is supposed to follow the tag; for
      example, the tags for positive or negative bignums are supposed to
      be put on byte strings.  A decoder that decodes the tagged data
      item into a native representation (a native big integer in this
      example) is expected to check the type of the data item being
      tagged.  Even decoders that don't have such native representations
      available in their environment may perform the check on those tags
      known to them and react appropriately.

   Invalid UTF-8 string:  A decoder might or might not want to verify
      that the sequence of bytes in a UTF-8 string (major type 3) is
      actually valid UTF-8 and react appropriately.

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5.4.  Handling Unknown Simple Values and Tags

   A decoder that comes across a simple value (Section 3.3) that it does
   not recognize, such as a value that was added to the IANA registry
   after the decoder was deployed or a value that the decoder chose not
   to implement, might issue a warning, might stop processing
   altogether, might handle the error by making the unknown value
   available to the application as such (as is expected of generic
   decoders), or take some other type of action.

   A decoder that comes across a tag (Section 3.4) that it does not
   recognize, such as a tag that was added to the IANA registry after
   the decoder was deployed or a tag that the decoder chose not to
   implement, might issue a warning, might stop processing altogether,
   might handle the error and present the unknown tag value together
   with the contained data item to the application (as is expected of
   generic decoders), might ignore the tag and simply present the
   contained data item only to the application, or take some other type
   of action.

5.5.  Numbers

   CBOR-based protocols should take into account that different language
   environments pose different restrictions on the range and precision
   of numbers that are representable.  For example, the JavaScript
   number system treats all numbers as floating point, which may result
   in silent loss of precision in decoding integers with more than 53
   significant bits.  A protocol that uses numbers should define its
   expectations on the handling of non-trivial numbers in decoders and
   receiving applications.

   A CBOR-based protocol that includes floating-point numbers can
   restrict which of the three formats (half-precision, single-
   precision, and double-precision) are to be supported.  For an
   integer-only application, a protocol may want to completely exclude
   the use of floating-point values.

   A CBOR-based protocol designed for compactness may want to exclude
   specific integer encodings that are longer than necessary for the
   application, such as to save the need to implement 64-bit integers.
   There is an expectation that encoders will use the most compact
   integer representation that can represent a given value.  However, a
   compact application should accept values that use a longer-than-
   needed encoding (such as encoding "0" as 0b000_11001 followed by two
   bytes of 0x00) as long as the application can decode an integer of
   the given size.

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   The preferred encoding for a floating point value is the shortest
   floating point encoding that preserves its value, e.g., 0xf94580 for
   the number 5.5, and 0xfa45ad9c00 for the number 5555.5, unless the
   CBOR-based protocol specifically excludes the use of the shorter
   floating point encodings.  For NaN values, a shorter encoding is
   preferred if zero-padding the shorter significand towards the right
   reconstitutes the original NaN value (for many applications, the
   single NaN encoding 0xf97e00 will suffice).

5.6.  Specifying Keys for Maps

   The encoding and decoding applications need to agree on what types of
   keys are going to be used in maps.  In applications that need to
   interwork with JSON-based applications, keys probably should be
   limited to UTF-8 strings only; otherwise, there has to be a specified
   mapping from the other CBOR types to Unicode characters, and this
   often leads to implementation errors.  In applications where keys are
   numeric in nature and numeric ordering of keys is important to the
   application, directly using the numbers for the keys is useful.

   If multiple types of keys are to be used, consideration should be
   given to how these types would be represented in the specific
   programming environments that are to be used.  For example, in
   JavaScript Maps [ECMA262], a key of integer 1 cannot be distinguished
   from a key of floating point 1.0.  This means that, if integer keys
   are used, the protocol needs to avoid use of floating-point keys the
   values of which happen to be integer numbers in the same map.

   Decoders that deliver data items nested within a CBOR data item
   immediately on decoding them ("streaming decoders") often do not keep
   the state that is necessary to ascertain uniqueness of a key in a
   map.  Similarly, an encoder that can start encoding data items before
   the enclosing data item is completely available ("streaming encoder")
   may want to reduce its overhead significantly by relying on its data
   source to maintain uniqueness.

   A CBOR-based protocol MUST define what to do when a receiving
   application does see multiple identical keys in a map.  The resulting
   rule in the protocol MUST respect the CBOR data model: it cannot
   prescribe a specific handling of the entries with the identical keys,
   except that it might have a rule that having identical keys in a map
   indicates a malformed map and that the decoder has to stop with an
   error.  Duplicate keys are also prohibited by CBOR decoders that are
   using strict mode (Section 5.8).

   The CBOR data model for maps does not allow ascribing semantics to
   the order of the key/value pairs in the map representation.  Thus, a
   CBOR-based protocol MUST NOT specify that changing the key/value pair

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   order in a map would change the semantics, except to specify that
   some, orders are disallowed, for example where they would not meet
   the requirements of a deterministic encoding (Section 4.2.  (Any
   secondary effects of map ordering such as on timing, cache usage, and
   other potential side channels are not considered part of the
   semantics but may be enough reason on its own for a protocol to
   require a deterministic encoding format.)

   Applications for constrained devices that have maps with 24 or fewer
   frequently used keys should consider using small integers (and those
   with up to 48 frequently used keys should consider also using small
   negative integers) because the keys can then be encoded in a single
   byte.

5.6.1.  Equivalence of Keys

   The specific data model applying to a CBOR data item is used to
   determine whether keys occurring in maps are duplicates or distinct.

   At the generic data model level, numerically equivalent integer and
   floating point values are distinct from each other, as they are from
   the various big numbers (Tags 2 to 5).  Similarly, text strings are
   distinct from byte strings, even if composed of the same bytes.  A
   tagged value is distinct from an untagged value or from a value
   tagged with a different tag.

   Within each of these groups, numeric values are distinct unless they
   are numerically equal (specifically, -0.0 is equal to 0.0); for the
   purpose of map key equivalence, NaN (not a number) values are
   equivalent if they have the same significand after zero-extending
   both significands at the right to 64 bits.

   (Byte and text) strings are compared byte by byte, arrays element by
   element, and are equal if they have the same number of bytes/elements
   and the same values at the same positions.  Two maps are equal if
   they have the same set of pairs regardless of their order; pairs are
   equal if both the key and value are equal.

   Tagged values are equal if both the tag and the value are equal.
   Simple values are equal if they simply have the same value.  Nothing
   else is equal in the generic data model, a simple value 2 is not
   equivalent to an integer 2 and an array is never equivalent to a map.

   As discussed in Section 2.2, specific data models can make values
   equivalent for the purpose of comparing map keys that are distinct in
   the generic data model.  Note that this implies that a generic
   decoder may deliver a decoded map to an application that needs to be
   checked for duplicate map keys by that application (alternatively,

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   the decoder may provide a programming interface to perform this
   service for the application).  Specific data models cannot
   distinguish values for map keys that are equal for this purpose at
   the generic data model level.

5.7.  Undefined Values

   In some CBOR-based protocols, the simple value (Section 3.3) of
   Undefined might be used by an encoder as a substitute for a data item
   with an encoding problem, in order to allow the rest of the enclosing
   data items to be encoded without harm.

5.8.  Strict Decoding Mode

   Some areas of application of CBOR do not require deterministic
   encoding (Section 4.2) but may require that different decoders reach
   the same (semantically equivalent) results, even in the presence of
   potentially malicious data.  This can be required if one application
   (such as a firewall or other protecting entity) makes a decision
   based on the data that another application, which independently
   decodes the data, relies on.

   Normally, it is the responsibility of the sender to avoid ambiguously
   decodable data.  However, the sender might be an attacker specially
   making up CBOR data such that it will be interpreted differently by
   different decoders in an attempt to exploit that as a vulnerability.
   Generic decoders used in applications where this might be a problem
   need to support a strict mode in which it is also the responsibility
   of the receiver to reject ambiguously decodable data.  It is expected
   that firewalls and other security systems that decode CBOR will only
   decode in strict mode.

   A decoder in strict mode will reliably reject any data that could be
   interpreted by other decoders in different ways.  It will expend the
   effort to reliably detect invalid data items (Section 5.3).  For
   example, a strict decoder needs to have an API that reports an error
   (and does not return data) for a CBOR data item that contains any of
   the following:

   o  a map (major type 5) that has more than one entry with the same
      key

   o  a tag that is used on a data item of the incorrect type

   o  a data item that is incorrectly formatted for the type given to
      it, such as invalid UTF-8 or data that cannot be interpreted with
      the specific tag that it has been tagged with

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   A decoder in strict mode can do one of two things when it encounters
   a tag or simple value that it does not recognize:

   o  It can report an error (and not return data).

   o  It can emit the unknown item (type, value, and, for tags, the
      decoded tagged data item) to the application calling the decoder
      with an indication that the decoder did not recognize that tag or
      simple value.

   The latter approach, which is also appropriate for non-strict
   decoders, supports forward compatibility with newly registered tags
   and simple values without the requirement to update the encoder at
   the same time as the calling application.  (For this, the API for the
   decoder needs to have a way to mark unknown items so that the calling
   application can handle them in a manner appropriate for the program.)

   Since some of this processing may have an appreciable cost (in
   particular with duplicate detection for maps), support of strict mode
   is not a requirement placed on all CBOR decoders.

   Some encoders will rely on their applications to provide input data
   in such a way that unambiguously decodable CBOR results.  A generic
   encoder also may want to provide a strict mode where it reliably
   limits its output to unambiguously decodable CBOR, independent of
   whether or not its application is providing API-conformant data.

6.  Converting Data between CBOR and JSON

   This section gives non-normative advice about converting between CBOR
   and JSON.  Implementations of converters are free to use whichever
   advice here they want.

   It is worth noting that a JSON text is a sequence of characters, not
   an encoded sequence of bytes, while a CBOR data item consists of
   bytes, not characters.

6.1.  Converting from CBOR to JSON

   Most of the types in CBOR have direct analogs in JSON.  However, some
   do not, and someone implementing a CBOR-to-JSON converter has to
   consider what to do in those cases.  The following non-normative
   advice deals with these by converting them to a single substitute
   value, such as a JSON null.

   o  An integer (major type 0 or 1) becomes a JSON number.

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   o  A byte string (major type 2) that is not embedded in a tag that
      specifies a proposed encoding is encoded in base64url without
      padding and becomes a JSON string.

   o  A UTF-8 string (major type 3) becomes a JSON string.  Note that
      JSON requires escaping certain characters ([RFC8259], Section 7):
      quotation mark (U+0022), reverse solidus (U+005C), and the "C0
      control characters" (U+0000 through U+001F).  All other characters
      are copied unchanged into the JSON UTF-8 string.

   o  An array (major type 4) becomes a JSON array.

   o  A map (major type 5) becomes a JSON object.  This is possible
      directly only if all keys are UTF-8 strings.  A converter might
      also convert other keys into UTF-8 strings (such as by converting
      integers into strings containing their decimal representation);
      however, doing so introduces a danger of key collision.  Note also
      that, if tags on UTF-8 strings are ignored as proposed below, this
      will cause a key collision if the tags are different but the
      strings are the same.

   o  False (major type 7, additional information 20) becomes a JSON
      false.

   o  True (major type 7, additional information 21) becomes a JSON
      true.

   o  Null (major type 7, additional information 22) becomes a JSON
      null.

   o  A floating-point value (major type 7, additional information 25
      through 27) becomes a JSON number if it is finite (that is, it can
      be represented in a JSON number); if the value is non-finite (NaN,
      or positive or negative Infinity), it is represented by the
      substitute value.

   o  Any other simple value (major type 7, any additional information
      value not yet discussed) is represented by the substitute value.

   o  A bignum (major type 6, tag value 2 or 3) is represented by
      encoding its byte string in base64url without padding and becomes
      a JSON string.  For tag value 3 (negative bignum), a "~" (ASCII
      tilde) is inserted before the base-encoded value.  (The conversion
      to a binary blob instead of a number is to prevent a likely
      numeric overflow for the JSON decoder.)

   o  A byte string with an encoding hint (major type 6, tag value 21
      through 23) is encoded as described and becomes a JSON string.

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   o  For all other tags (major type 6, any other tag value), the
      embedded CBOR item is represented as a JSON value; the tag value
      is ignored.

   o  Indefinite-length items are made definite before conversion.

6.2.  Converting from JSON to CBOR

   All JSON values, once decoded, directly map into one or more CBOR
   values.  As with any kind of CBOR generation, decisions have to be
   made with respect to number representation.  In a suggested
   conversion:

   o  JSON numbers without fractional parts (integer numbers) are
      represented as integers (major types 0 and 1, possibly major type
      6 tag value 2 and 3), choosing the shortest form; integers longer
      than an implementation-defined threshold (which is usually either
      32 or 64 bits) may instead be represented as floating-point
      values.  (If the JSON was generated from a JavaScript
      implementation, its precision is already limited to 53 bits
      maximum.)

   o  Numbers with fractional parts are represented as floating-point
      values.  Preferably, the shortest exact floating-point
      representation is used; for instance, 1.5 is represented in a
      16-bit floating-point value (not all implementations will be
      capable of efficiently finding the minimum form, though).  There
      may be an implementation-defined limit to the precision that will
      affect the precision of the represented values.  Decimal
      representation should only be used if that is specified in a
      protocol.

   CBOR has been designed to generally provide a more compact encoding
   than JSON.  One implementation strategy that might come to mind is to
   perform a JSON-to-CBOR encoding in place in a single buffer.  This
   strategy would need to carefully consider a number of pathological
   cases, such as that some strings represented with no or very few
   escapes and longer (or much longer) than 255 bytes may expand when
   encoded as UTF-8 strings in CBOR.  Similarly, a few of the binary
   floating-point representations might cause expansion from some short
   decimal representations (1.1, 1e9) in JSON.  This may be hard to get
   right, and any ensuing vulnerabilities may be exploited by an
   attacker.

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7.  Future Evolution of CBOR

   Successful protocols evolve over time.  New ideas appear,
   implementation platforms improve, related protocols are developed and
   evolve, and new requirements from applications and protocols are
   added.  Facilitating protocol evolution is therefore an important
   design consideration for any protocol development.

   For protocols that will use CBOR, CBOR provides some useful
   mechanisms to facilitate their evolution.  Best practices for this
   are well known, particularly from JSON format development of JSON-
   based protocols.  Therefore, such best practices are outside the
   scope of this specification.

   However, facilitating the evolution of CBOR itself is very well
   within its scope.  CBOR is designed to both provide a stable basis
   for development of CBOR-based protocols and to be able to evolve.
   Since a successful protocol may live for decades, CBOR needs to be
   designed for decades of use and evolution.  This section provides
   some guidance for the evolution of CBOR.  It is necessarily more
   subjective than other parts of this document.  It is also necessarily
   incomplete, lest it turn into a textbook on protocol development.

7.1.  Extension Points

   In a protocol design, opportunities for evolution are often included
   in the form of extension points.  For example, there may be a
   codepoint space that is not fully allocated from the outset, and the
   protocol is designed to tolerate and embrace implementations that
   start using more codepoints than initially allocated.

   Sizing the codepoint space may be difficult because the range
   required may be hard to predict.  An attempt should be made to make
   the codepoint space large enough so that it can slowly be filled over
   the intended lifetime of the protocol.

   CBOR has three major extension points:

   o  the "simple" space (values in major type 7).  Of the 24 efficient
      (and 224 slightly less efficient) values, only a small number have
      been allocated.  Implementations receiving an unknown simple data
      item may be able to process it as such, given that the structure
      of the value is indeed simple.  The IANA registry in Section 9.1
      is the appropriate way to address the extensibility of this
      codepoint space.

   o  the "tag" space (values in major type 6).  Again, only a small
      part of the codepoint space has been allocated, and the space is

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      abundant (although the early numbers are more efficient than the
      later ones).  Implementations receiving an unknown tag can choose
      to simply ignore it or to process it as an unknown tag wrapping
      the following data item.  The IANA registry in Section 9.2 is the
      appropriate way to address the extensibility of this codepoint
      space.

   o  the "additional information" space.  An implementation receiving
      an unknown additional information value has no way to continue
      decoding, so allocating codepoints to this space is a major step.
      There are also very few codepoints left.

7.2.  Curating the Additional Information Space

   The human mind is sometimes drawn to filling in little perceived gaps
   to make something neat.  We expect the remaining gaps in the
   codepoint space for the additional information values to be an
   attractor for new ideas, just because they are there.

   The present specification does not manage the additional information
   codepoint space by an IANA registry.  Instead, allocations out of
   this space can only be done by updating this specification.

   For an additional information value of n >= 24, the size of the
   additional data typically is 2**(n-24) bytes.  Therefore, additional
   information values 28 and 29 should be viewed as candidates for
   128-bit and 256-bit quantities, in case a need arises to add them to
   the protocol.  Additional information value 30 is then the only
   additional information value available for general allocation, and
   there should be a very good reason for allocating it before assigning
   it through an update of this protocol.

8.  Diagnostic Notation

   CBOR is a binary interchange format.  To facilitate documentation and
   debugging, and in particular to facilitate communication between
   entities cooperating in debugging, this section defines a simple
   human-readable diagnostic notation.  All actual interchange always
   happens in the binary format.

   Note that this truly is a diagnostic format; it is not meant to be
   parsed.  Therefore, no formal definition (as in ABNF) is given in
   this document.  (Implementers looking for a text-based format for
   representing CBOR data items in configuration files may also want to
   consider YAML [YAML].)

   The diagnostic notation is loosely based on JSON as it is defined in
   RFC 8259, extending it where needed.

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   The notation borrows the JSON syntax for numbers (integer and
   floating point), True (>true<), False (>false<), Null (>null<), UTF-8
   strings, arrays, and maps (maps are called objects in JSON; the
   diagnostic notation extends JSON here by allowing any data item in
   the key position).  Undefined is written >undefined< as in
   JavaScript.  The non-finite floating-point numbers Infinity,
   -Infinity, and NaN are written exactly as in this sentence (this is
   also a way they can be written in JavaScript, although JSON does not
   allow them).  A tagged item is written as an integer number for the
   tag followed by the item in parentheses; for instance, an RFC 3339
   (ISO 8601) date could be notated as:

      0("2013-03-21T20:04:00Z")

   or the equivalent relative time as

      1(1363896240)

   Byte strings are notated in one of the base encodings, without
   padding, enclosed in single quotes, prefixed by >h< for base16, >b32<
   for base32, >h32< for base32hex, >b64< for base64 or base64url (the
   actual encodings do not overlap, so the string remains unambiguous).
   For example, the byte string 0x12345678 could be written h'12345678',
   b32'CI2FM6A', or b64'EjRWeA'.

   Unassigned simple values are given as "simple()" with the appropriate
   integer in the parentheses.  For example, "simple(42)" indicates
   major type 7, value 42.

8.1.  Encoding Indicators

   Sometimes it is useful to indicate in the diagnostic notation which
   of several alternative representations were actually used; for
   example, a data item written >1.5< by a diagnostic decoder might have
   been encoded as a half-, single-, or double-precision float.

   The convention for encoding indicators is that anything starting with
   an underscore and all following characters that are alphanumeric or
   underscore, is an encoding indicator, and can be ignored by anyone
   not interested in this information.  Encoding indicators are always
   optional.

   A single underscore can be written after the opening brace of a map
   or the opening bracket of an array to indicate that the data item was
   represented in indefinite-length format.  For example, [_ 1, 2]
   contains an indicator that an indefinite-length representation was
   used to represent the data item [1, 2].

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   An underscore followed by a decimal digit n indicates that the
   preceding item (or, for arrays and maps, the item starting with the
   preceding bracket or brace) was encoded with an additional
   information value of 24+n.  For example, 1.5_1 is a half-precision
   floating-point number, while 1.5_3 is encoded as double precision.
   This encoding indicator is not shown in Appendix A.  (Note that the
   encoding indicator "_" is thus an abbreviation of the full form "_7",
   which is not used.)

   As a special case, byte and text strings of indefinite length can be
   notated in the form (_ h'0123', h'4567') and (_ "foo", "bar").

9.  IANA Considerations

   IANA has created two registries for new CBOR values.  The registries
   are separate, that is, not under an umbrella registry, and follow the
   rules in [RFC8126].  IANA has also assigned a new MIME media type and
   an associated Constrained Application Protocol (CoAP) Content-Format
   entry.

9.1.  Simple Values Registry

   IANA has created the "Concise Binary Object Representation (CBOR)
   Simple Values" registry at [IANA.cbor-simple-values].  The initial
   values are shown in Table 2.

   New entries in the range 0 to 19 are assigned by Standards Action.
   It is suggested that these Standards Actions allocate values starting
   with the number 16 in order to reserve the lower numbers for
   contiguous blocks (if any).

   New entries in the range 32 to 255 are assigned by Specification
   Required.

9.2.  Tags Registry

   IANA has created the "Concise Binary Object Representation (CBOR)
   Tags" registry at [IANA.cbor-tags].  The tags that were defined in
   [RFC7049] are described in detail in Section 3.4, but other tags have
   already been defined.

   New entries in the range 0 to 23 are assigned by Standards Action.
   New entries in the range 24 to 255 are assigned by Specification
   Required.  New entries in the range 256 to 18446744073709551615 are
   assigned by First Come First Served.  The template for registration
   requests is:

   o  Data item

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   o  Semantics (short form)

   In addition, First Come First Served requests should include:

   o  Point of contact

   o  Description of semantics (URL) - This description is optional; the
      URL can point to something like an Internet-Draft or a web page.

9.3.  Media Type ("MIME Type")

   The Internet media type [RFC6838] for a single encoded CBOR data item
   is application/cbor.

   Type name: application

   Subtype name: cbor

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations:  binary

   Security considerations:  See Section 10 of this document

   Interoperability considerations: n/a

   Published specification: This document

   Applications that use this media type:  None yet, but it is expected
      that this format will be deployed in protocols and applications.

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   Additional information:
     Magic number(s): n/a
     File extension(s): .cbor
     Macintosh file type code(s): n/a

   Person & email address to contact for further information:
     Carsten Bormann
     cabo@tzi.org

   Intended usage: COMMON

   Restrictions on usage: none

   Author:
     Carsten Bormann <cabo@tzi.org>

   Change controller:
     The IESG <iesg@ietf.org>

9.4.  CoAP Content-Format

   Media Type: application/cbor

   Encoding: -

   Id: 60

   Reference: [RFCthis]

9.5.  The +cbor Structured Syntax Suffix Registration

   Name: Concise Binary Object Representation (CBOR)

   +suffix: +cbor

   References: [RFCthis]

   Encoding Considerations: CBOR is a binary format.

   Interoperability Considerations: n/a

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   Fragment Identifier Considerations:
     The syntax and semantics of fragment identifiers specified for
     +cbor SHOULD be as specified for "application/cbor".  (At
     publication of this document, there is no fragment identification
     syntax defined for "application/cbor".)

     The syntax and semantics for fragment identifiers for a specific
     "xxx/yyy+cbor" SHOULD be processed as follows:

     For cases defined in +cbor, where the fragment identifier resolves
     per the +cbor rules, then process as specified in +cbor.

     For cases defined in +cbor, where the fragment identifier does
     not resolve per the +cbor rules, then process as specified in
     "xxx/yyy+cbor".

     For cases not defined in +cbor, then process as specified in
     "xxx/yyy+cbor".

   Security Considerations:  See Section 10 of this document

   Contact:
     Apps Area Working Group (apps-discuss@ietf.org)

   Author/Change Controller:
     The Apps Area Working Group.
     The IESG has change control over this registration.

10.  Security Considerations

   A network-facing application can exhibit vulnerabilities in its
   processing logic for incoming data.  Complex parsers are well known
   as a likely source of such vulnerabilities, such as the ability to
   remotely crash a node, or even remotely execute arbitrary code on it.
   CBOR attempts to narrow the opportunities for introducing such
   vulnerabilities by reducing parser complexity, by giving the entire
   range of encodable values a meaning where possible.

   Because CBOR decoders are often used as a first step in processing
   unvalidated input, they need to be fully prepared for all types of
   hostile input that may be designed to corrupt, overrun, or achieve
   control of the system decoding the CBOR data item.  A CBOR decoder
   needs to assume that all input may be hostile even if it has been
   checked by a firewall, has come over a TLS-secured channel, is
   encrypted or signed, or has come from some other source that is
   presumed trusted.

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   Hostile input may be constructed to overrun buffers, overflow or
   underflow integer arithmetic, or cause other decoding disruption.
   CBOR data items might have lengths or sizes that are intentionally
   extremely large or too short.  Resource exhaustion attacks might
   attempt to lure a decoder into allocating very big data items
   (strings, arrays, maps) or exhaust the stack depth by setting up
   deeply nested items.  Decoders need to have appropriate resource
   management to mitigate these attacks.  (Items for which very large
   sizes are given can also attempt to exploit integer overflow
   vulnerabilities.)

   A CBOR decoder, by definition, only accepts well-formed CBOR; this is
   the first step to its robustness.  Input that is not well-formed CBOR
   causes no further processing from the point where the lack of well-
   formedness was detected.  If possible, any data decoded up to this
   point should have no impact on the application using the CBOR
   decoder.

   In addition to ascertaining well-formedness, a CBOR decoder might
   also perform validity checks on the CBOR data.  Alternatively, it can
   leave those checks to the application using the decoder.  This choice
   needs to be clearly documented in the decoder.  Beyond the validity
   at the CBOR level, an application also needs to ascertain that the
   input is in alignment with the application protocol that is
   serialized in CBOR.

   CBOR encoders do not receive input directly from the network and are
   thus not directly attackable in the same way as CBOR decoders.
   However, CBOR encoders often have an API that takes input from
   another level in the implementation and can be attacked through that
   API.  The design and implementation of that API should assume the
   behavior of its caller may be based on hostile input or on coding
   mistakes.  It should check inputs for buffer overruns, overflow and
   underflow of integer arithmetic, and other such errors that are aimed
   to disrupt the encoder.

   Protocols that are used in a security context should be defined in
   such a way that potential multiple interpretations are reliably
   reduced to a single interpretation.  For example, an attacker could
   make use of invalid input such as duplicate keys in maps, or exploit
   different precision in processing numbers to make one application
   base its decisions on a different interpretation than the one that
   will be used by a second application.  To facilitate consistent
   interpretation, encoder and decoder implementations used in such
   contexts should provide at least one strict mode of operation
   (Section 5.8).

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11.  References

11.1.  Normative References

   [ECMA262]  Ecma International, "ECMAScript 2018 Language
              Specification", ECMA Standard ECMA-262, 9th Edition, June
              2018, <https://www.ecma-
              international.org/publications/files/ECMA-ST/
              Ecma-262.pdf>.

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

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [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>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <https://www.rfc-editor.org/info/rfc4287>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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   [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>.

   [TIME_T]   The Open Group Base Specifications, "Vol. 1: Base
              Definitions, Issue 7", Section 4.15 'Seconds Since the
              Epoch', IEEE Std 1003.1, 2013 Edition, 2013,
              <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
              V1_chap04.html#tag_04_15>.

11.2.  Informative References

   [ASN.1]    International Telecommunication Union, "Information
              Technology -- ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, 1994.

   [BSON]     Various, "BSON - Binary JSON", 2013,
              <http://bsonspec.org/>.

   [IANA.cbor-simple-values]
              IANA, "Concise Binary Object Representation (CBOR) Simple
              Values",
              <http://www.iana.org/assignments/cbor-simple-values>.

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <http://www.iana.org/assignments/cbor-tags>.

   [MessagePack]
              Furuhashi, S., "MessagePack", 2013, <http://msgpack.org/>.

   [PCRE]     Ho, A., "PCRE - Perl Compatible Regular Expressions",
              2018, <http://www.pcre.org/>.

   [RFC0713]  Haverty, J., "MSDTP-Message Services Data Transmission
              Protocol", RFC 713, DOI 10.17487/RFC0713, April 1976,
              <https://www.rfc-editor.org/info/rfc713>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [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>.

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   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [YAML]     Ben-Kiki, O., Evans, C., and I. Net, "YAML Ain't Markup
              Language (YAML[TM]) Version 1.2", 3rd Edition, October
              2009, <http://www.yaml.org/spec/1.2/spec.html>.

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Appendix A.  Examples

   The following table provides some CBOR-encoded values in hexadecimal
   (right column), together with diagnostic notation for these values
   (left column).  Note that the string "\u00fc" is one form of
   diagnostic notation for a UTF-8 string containing the single Unicode
   character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (u umlaut).
   Similarly, "\u6c34" is a UTF-8 string in diagnostic notation with a
   single character U+6C34 (CJK UNIFIED IDEOGRAPH-6C34, often
   representing "water"), and "\ud800\udd51" is a UTF-8 string in
   diagnostic notation with a single character U+10151 (GREEK ACROPHONIC
   ATTIC FIFTY STATERS).  (Note that all these single-character strings
   could also be represented in native UTF-8 in diagnostic notation,
   just not in an ASCII-only specification like the present one.)  In
   the diagnostic notation provided for bignums, their intended numeric
   value is shown as a decimal number (such as 18446744073709551616)
   instead of showing a tagged byte string (such as
   2(h'010000000000000000')).

   +------------------------------+------------------------------------+
   | Diagnostic                   | Encoded                            |
   +------------------------------+------------------------------------+
   | 0                            | 0x00                               |
   |                              |                                    |
   | 1                            | 0x01                               |
   |                              |                                    |
   | 10                           | 0x0a                               |
   |                              |                                    |
   | 23                           | 0x17                               |
   |                              |                                    |
   | 24                           | 0x1818                             |
   |                              |                                    |
   | 25                           | 0x1819                             |
   |                              |                                    |
   | 100                          | 0x1864                             |
   |                              |                                    |
   | 1000                         | 0x1903e8                           |
   |                              |                                    |
   | 1000000                      | 0x1a000f4240                       |
   |                              |                                    |
   | 1000000000000                | 0x1b000000e8d4a51000               |
   |                              |                                    |
   | 18446744073709551615         | 0x1bffffffffffffffff               |
   |                              |                                    |
   | 18446744073709551616         | 0xc249010000000000000000           |
   |                              |                                    |
   | -18446744073709551616        | 0x3bffffffffffffffff               |
   |                              |                                    |

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   | -18446744073709551617        | 0xc349010000000000000000           |
   |                              |                                    |
   | -1                           | 0x20                               |
   |                              |                                    |
   | -10                          | 0x29                               |
   |                              |                                    |
   | -100                         | 0x3863                             |
   |                              |                                    |
   | -1000                        | 0x3903e7                           |
   |                              |                                    |
   | 0.0                          | 0xf90000                           |
   |                              |                                    |
   | -0.0                         | 0xf98000                           |
   |                              |                                    |
   | 1.0                          | 0xf93c00                           |
   |                              |                                    |
   | 1.1                          | 0xfb3ff199999999999a               |
   |                              |                                    |
   | 1.5                          | 0xf93e00                           |
   |                              |                                    |
   | 65504.0                      | 0xf97bff                           |
   |                              |                                    |
   | 100000.0                     | 0xfa47c35000                       |
   |                              |                                    |
   | 3.4028234663852886e+38       | 0xfa7f7fffff                       |
   |                              |                                    |
   | 1.0e+300                     | 0xfb7e37e43c8800759c               |
   |                              |                                    |
   | 5.960464477539063e-8         | 0xf90001                           |
   |                              |                                    |
   | 0.00006103515625             | 0xf90400                           |
   |                              |                                    |
   | -4.0                         | 0xf9c400                           |
   |                              |                                    |
   | -4.1                         | 0xfbc010666666666666               |
   |                              |                                    |
   | Infinity                     | 0xf97c00                           |
   |                              |                                    |
   | NaN                          | 0xf97e00                           |
   |                              |                                    |
   | -Infinity                    | 0xf9fc00                           |
   |                              |                                    |
   | Infinity                     | 0xfa7f800000                       |
   |                              |                                    |
   | NaN                          | 0xfa7fc00000                       |
   |                              |                                    |
   | -Infinity                    | 0xfaff800000                       |
   |                              |                                    |

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   | Infinity                     | 0xfb7ff0000000000000               |
   |                              |                                    |
   | NaN                          | 0xfb7ff8000000000000               |
   |                              |                                    |
   | -Infinity                    | 0xfbfff0000000000000               |
   |                              |                                    |
   | false                        | 0xf4                               |
   |                              |                                    |
   | true                         | 0xf5                               |
   |                              |                                    |
   | null                         | 0xf6                               |
   |                              |                                    |
   | undefined                    | 0xf7                               |
   |                              |                                    |
   | simple(16)                   | 0xf0                               |
   |                              |                                    |
   | simple(255)                  | 0xf8ff                             |
   |                              |                                    |
   | 0("2013-03-21T20:04:00Z")    | 0xc074323031332d30332d32315432303a |
   |                              | 30343a30305a                       |
   |                              |                                    |
   | 1(1363896240)                | 0xc11a514b67b0                     |
   |                              |                                    |
   | 1(1363896240.5)              | 0xc1fb41d452d9ec200000             |
   |                              |                                    |
   | 23(h'01020304')              | 0xd74401020304                     |
   |                              |                                    |
   | 24(h'6449455446')            | 0xd818456449455446                 |
   |                              |                                    |
   | 32("http://www.example.com") | 0xd82076687474703a2f2f7777772e6578 |
   |                              | 616d706c652e636f6d                 |
   |                              |                                    |
   | h''                          | 0x40                               |
   |                              |                                    |
   | h'01020304'                  | 0x4401020304                       |
   |                              |                                    |
   | ""                           | 0x60                               |
   |                              |                                    |
   | "a"                          | 0x6161                             |
   |                              |                                    |
   | "IETF"                       | 0x6449455446                       |
   |                              |                                    |
   | "\"\\"                       | 0x62225c                           |
   |                              |                                    |
   | "\u00fc"                     | 0x62c3bc                           |
   |                              |                                    |
   | "\u6c34"                     | 0x63e6b0b4                         |
   |                              |                                    |

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   | "\ud800\udd51"               | 0x64f0908591                       |
   |                              |                                    |
   | []                           | 0x80                               |
   |                              |                                    |
   | [1, 2, 3]                    | 0x83010203                         |
   |                              |                                    |
   | [1, [2, 3], [4, 5]]          | 0x8301820203820405                 |
   |                              |                                    |
   | [1, 2, 3, 4, 5, 6, 7, 8, 9,  | 0x98190102030405060708090a0b0c0d0e |
   | 10, 11, 12, 13, 14, 15, 16,  | 0f101112131415161718181819         |
   | 17, 18, 19, 20, 21, 22, 23,  |                                    |
   | 24, 25]                      |                                    |
   |                              |                                    |
   | {}                           | 0xa0                               |
   |                              |                                    |
   | {1: 2, 3: 4}                 | 0xa201020304                       |
   |                              |                                    |
   | {"a": 1, "b": [2, 3]}        | 0xa26161016162820203               |
   |                              |                                    |
   | ["a", {"b": "c"}]            | 0x826161a161626163                 |
   |                              |                                    |
   | {"a": "A", "b": "B", "c":    | 0xa5616161416162614261636143616461 |
   | "C", "d": "D", "e": "E"}     | 4461656145                         |
   |                              |                                    |
   | (_ h'0102', h'030405')       | 0x5f42010243030405ff               |
   |                              |                                    |
   | (_ "strea", "ming")          | 0x7f657374726561646d696e67ff       |
   |                              |                                    |
   | [_ ]                         | 0x9fff                             |
   |                              |                                    |
   | [_ 1, [2, 3], [_ 4, 5]]      | 0x9f018202039f0405ffff             |
   |                              |                                    |
   | [_ 1, [2, 3], [4, 5]]        | 0x9f01820203820405ff               |
   |                              |                                    |
   | [1, [2, 3], [_ 4, 5]]        | 0x83018202039f0405ff               |
   |                              |                                    |
   | [1, [_ 2, 3], [4, 5]]        | 0x83019f0203ff820405               |
   |                              |                                    |
   | [_ 1, 2, 3, 4, 5, 6, 7, 8,   | 0x9f0102030405060708090a0b0c0d0e0f |
   | 9, 10, 11, 12, 13, 14, 15,   | 101112131415161718181819ff         |
   | 16, 17, 18, 19, 20, 21, 22,  |                                    |
   | 23, 24, 25]                  |                                    |
   |                              |                                    |
   | {_ "a": 1, "b": [_ 2, 3]}    | 0xbf61610161629f0203ffff           |
   |                              |                                    |
   | ["a", {_ "b": "c"}]          | 0x826161bf61626163ff               |
   |                              |                                    |
   | {_ "Fun": true, "Amt": -2}   | 0xbf6346756ef563416d7421ff         |

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   +------------------------------+------------------------------------+

               Table 4: Examples of Encoded CBOR Data Items

Appendix B.  Jump Table

   For brevity, this jump table does not show initial bytes that are
   reserved for future extension.  It also only shows a selection of the
   initial bytes that can be used for optional features.  (All unsigned
   integers are in network byte order.)

   +------------+------------------------------------------------------+
   | Byte       | Structure/Semantics                                  |
   +------------+------------------------------------------------------+
   | 0x00..0x17 | Integer 0x00..0x17 (0..23)                           |
   |            |                                                      |
   | 0x18       | Unsigned integer (one-byte uint8_t follows)          |
   |            |                                                      |
   | 0x19       | Unsigned integer (two-byte uint16_t follows)         |
   |            |                                                      |
   | 0x1a       | Unsigned integer (four-byte uint32_t follows)        |
   |            |                                                      |
   | 0x1b       | Unsigned integer (eight-byte uint64_t follows)       |
   |            |                                                      |
   | 0x20..0x37 | Negative integer -1-0x00..-1-0x17 (-1..-24)          |
   |            |                                                      |
   | 0x38       | Negative integer -1-n (one-byte uint8_t for n        |
   |            | follows)                                             |
   |            |                                                      |
   | 0x39       | Negative integer -1-n (two-byte uint16_t for n       |
   |            | follows)                                             |
   |            |                                                      |
   | 0x3a       | Negative integer -1-n (four-byte uint32_t for n      |
   |            | follows)                                             |
   |            |                                                      |
   | 0x3b       | Negative integer -1-n (eight-byte uint64_t for n     |
   |            | follows)                                             |
   |            |                                                      |
   | 0x40..0x57 | byte string (0x00..0x17 bytes follow)                |
   |            |                                                      |
   | 0x58       | byte string (one-byte uint8_t for n, and then n      |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x59       | byte string (two-byte uint16_t for n, and then n     |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x5a       | byte string (four-byte uint32_t for n, and then n    |
   |            | bytes follow)                                        |

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   |            |                                                      |
   | 0x5b       | byte string (eight-byte uint64_t for n, and then n   |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x5f       | byte string, byte strings follow, terminated by      |
   |            | "break"                                              |
   |            |                                                      |
   | 0x60..0x77 | UTF-8 string (0x00..0x17 bytes follow)               |
   |            |                                                      |
   | 0x78       | UTF-8 string (one-byte uint8_t for n, and then n     |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x79       | UTF-8 string (two-byte uint16_t for n, and then n    |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x7a       | UTF-8 string (four-byte uint32_t for n, and then n   |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x7b       | UTF-8 string (eight-byte uint64_t for n, and then n  |
   |            | bytes follow)                                        |
   |            |                                                      |
   | 0x7f       | UTF-8 string, UTF-8 strings follow, terminated by    |
   |            | "break"                                              |
   |            |                                                      |
   | 0x80..0x97 | array (0x00..0x17 data items follow)                 |
   |            |                                                      |
   | 0x98       | array (one-byte uint8_t for n, and then n data items |
   |            | follow)                                              |
   |            |                                                      |
   | 0x99       | array (two-byte uint16_t for n, and then n data      |
   |            | items follow)                                        |
   |            |                                                      |
   | 0x9a       | array (four-byte uint32_t for n, and then n data     |
   |            | items follow)                                        |
   |            |                                                      |
   | 0x9b       | array (eight-byte uint64_t for n, and then n data    |
   |            | items follow)                                        |
   |            |                                                      |
   | 0x9f       | array, data items follow, terminated by "break"      |
   |            |                                                      |
   | 0xa0..0xb7 | map (0x00..0x17 pairs of data items follow)          |
   |            |                                                      |
   | 0xb8       | map (one-byte uint8_t for n, and then n pairs of     |
   |            | data items follow)                                   |
   |            |                                                      |
   | 0xb9       | map (two-byte uint16_t for n, and then n pairs of    |
   |            | data items follow)                                   |
   |            |                                                      |

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   | 0xba       | map (four-byte uint32_t for n, and then n pairs of   |
   |            | data items follow)                                   |
   |            |                                                      |
   | 0xbb       | map (eight-byte uint64_t for n, and then n pairs of  |
   |            | data items follow)                                   |
   |            |                                                      |
   | 0xbf       | map, pairs of data items follow, terminated by       |
   |            | "break"                                              |
   |            |                                                      |
   | 0xc0       | Text-based date/time (data item follows; see         |
   |            | Section 3.4.2)                                       |
   |            |                                                      |
   | 0xc1       | Epoch-based date/time (data item follows; see        |
   |            | Section 3.4.3)                                       |
   |            |                                                      |
   | 0xc2       | Positive bignum (data item "byte string" follows)    |
   |            |                                                      |
   | 0xc3       | Negative bignum (data item "byte string" follows)    |
   |            |                                                      |
   | 0xc4       | Decimal Fraction (data item "array" follows; see     |
   |            | Section 3.4.5)                                       |
   |            |                                                      |
   | 0xc5       | Bigfloat (data item "array" follows; see             |
   |            | Section 3.4.5)                                       |
   |            |                                                      |
   | 0xc6..0xd4 | (tagged item)                                        |
   |            |                                                      |
   | 0xd5..0xd7 | Expected Conversion (data item follows; see          |
   |            | Section 3.4.6.2)                                     |
   |            |                                                      |
   | 0xd8..0xdb | (more tagged items, 1/2/4/8 bytes and then a data    |
   |            | item follow)                                         |
   |            |                                                      |
   | 0xe0..0xf3 | (simple value)                                       |
   |            |                                                      |
   | 0xf4       | False                                                |
   |            |                                                      |
   | 0xf5       | True                                                 |
   |            |                                                      |
   | 0xf6       | Null                                                 |
   |            |                                                      |
   | 0xf7       | Undefined                                            |
   |            |                                                      |
   | 0xf8       | (simple value, one byte follows)                     |
   |            |                                                      |
   | 0xf9       | Half-Precision Float (two-byte IEEE 754)             |
   |            |                                                      |
   | 0xfa       | Single-Precision Float (four-byte IEEE 754)          |

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   |            |                                                      |
   | 0xfb       | Double-Precision Float (eight-byte IEEE 754)         |
   |            |                                                      |
   | 0xff       | "break" stop code                                    |
   +------------+------------------------------------------------------+

                   Table 5: Jump Table for Initial Byte

Appendix C.  Pseudocode

   The well-formedness of a CBOR item can be checked by the pseudocode
   in Figure 1.  The data is well-formed if and only if:

   o  the pseudocode does not "fail";

   o  after execution of the pseudocode, no bytes are left in the input
      (except in streaming applications)

   The pseudocode has the following prerequisites:

   o  take(n) reads n bytes from the input data and returns them as a
      byte string.  If n bytes are no longer available, take(n) fails.

   o  uint() converts a byte string into an unsigned integer by
      interpreting the byte string in network byte order.

   o  Arithmetic works as in C.

   o  All variables are unsigned integers of sufficient range.

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   well_formed (breakable = false) {
     // process initial bytes
     ib = uint(take(1));
     mt = ib >> 5;
     val = ai = ib & 0x1f;
     switch (ai) {
       case 24: val = uint(take(1)); break;
       case 25: val = uint(take(2)); break;
       case 26: val = uint(take(4)); break;
       case 27: val = uint(take(8)); break;
       case 28: case 29: case 30: fail();
       case 31:
         return well_formed_indefinite(mt, breakable);
     }
     // process content
     switch (mt) {
       // case 0, 1, 7 do not have content; just use val
       case 2: case 3: take(val); break; // bytes/UTF-8
       case 4: for (i = 0; i < val; i++) well_formed(); break;
       case 5: for (i = 0; i < val*2; i++) well_formed(); break;
       case 6: well_formed(); break;     // 1 embedded data item
       case 7: if (ai == 24 && val < 32) fail(); // bad simple
     }
     return mt;                    // finite data item
   }

   well_formed_indefinite(mt, breakable) {
     switch (mt) {
       case 2: case 3:
         while ((it = well_formed(true)) != -1)
           if (it != mt)           // need finite embedded
             fail();               //    of same type
         break;
       case 4: while (well_formed(true) != -1); break;
       case 5: while (well_formed(true) != -1) well_formed(); break;
       case 7:
         if (breakable)
           return -1;              // signal break out
         else fail();              // no enclosing indefinite
       default: fail();            // wrong mt
     }
     return 0;                     // no break out
   }

              Figure 1: Pseudocode for Well-Formedness Check

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   Note that the remaining complexity of a complete CBOR decoder is
   about presenting data that has been decoded to the application in an
   appropriate form.

   Major types 0 and 1 are designed in such a way that they can be
   encoded in C from a signed integer without actually doing an if-then-
   else for positive/negative (Figure 2).  This uses the fact that
   (-1-n), the transformation for major type 1, is the same as ~n
   (bitwise complement) in C unsigned arithmetic; ~n can then be
   expressed as (-1)^n for the negative case, while 0^n leaves n
   unchanged for non-negative.  The sign of a number can be converted to
   -1 for negative and 0 for non-negative (0 or positive) by arithmetic-
   shifting the number by one bit less than the bit length of the number
   (for example, by 63 for 64-bit numbers).

   void encode_sint(int64_t n) {
     uint64t ui = n >> 63;    // extend sign to whole length
     mt = ui & 0x20;          // extract major type
     ui ^= n;                 // complement negatives
     if (ui < 24)
       *p++ = mt + ui;
     else if (ui < 256) {
       *p++ = mt + 24;
       *p++ = ui;
     } else
          ...

            Figure 2: Pseudocode for Encoding a Signed Integer

Appendix D.  Half-Precision

   As half-precision floating-point numbers were only added to IEEE 754
   in 2008 [IEEE754], today's programming platforms often still only
   have limited support for them.  It is very easy to include at least
   decoding support for them even without such support.  An example of a
   small decoder for half-precision floating-point numbers in the C
   language is shown in Figure 3.  A similar program for Python is in
   Figure 4; this code assumes that the 2-byte value has already been
   decoded as an (unsigned short) integer in network byte order (as
   would be done by the pseudocode in Appendix C).

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   #include <math.h>

   double decode_half(unsigned char *halfp) {
     int half = (halfp[0] << 8) + halfp[1];
     int exp = (half >> 10) & 0x1f;
     int mant = half & 0x3ff;
     double val;
     if (exp == 0) val = ldexp(mant, -24);
     else if (exp != 31) val = ldexp(mant + 1024, exp - 25);
     else val = mant == 0 ? INFINITY : NAN;
     return half & 0x8000 ? -val : val;
   }

               Figure 3: C Code for a Half-Precision Decoder

   import struct
   from math import ldexp

   def decode_single(single):
       return struct.unpack("!f", struct.pack("!I", single))[0]

   def decode_half(half):
       valu = (half & 0x7fff) << 13 | (half & 0x8000) << 16
       if ((half & 0x7c00) != 0x7c00):
           return ldexp(decode_single(valu), 112)
       return decode_single(valu | 0x7f800000)

            Figure 4: Python Code for a Half-Precision Decoder

Appendix E.  Comparison of Other Binary Formats to CBOR's Design
             Objectives

   The proposal for CBOR follows a history of binary formats that is as
   long as the history of computers themselves.  Different formats have
   had different objectives.  In most cases, the objectives of the
   format were never stated, although they can sometimes be implied by
   the context where the format was first used.  Some formats were meant
   to be universally usable, although history has proven that no binary
   format meets the needs of all protocols and applications.

   CBOR differs from many of these formats due to it starting with a set
   of objectives and attempting to meet just those.  This section
   compares a few of the dozens of formats with CBOR's objectives in
   order to help the reader decide if they want to use CBOR or a
   different format for a particular protocol or application.

   Note that the discussion here is not meant to be a criticism of any
   format: to the best of our knowledge, no format before CBOR was meant

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   to cover CBOR's objectives in the priority we have assigned them.  A
   brief recap of the objectives from Section 1.1 is:

   1.  unambiguous encoding of most common data formats from Internet
       standards

   2.  code compactness for encoder or decoder

   3.  no schema description needed

   4.  reasonably compact serialization

   5.  applicability to constrained and unconstrained applications

   6.  good JSON conversion

   7.  extensibility

E.1.  ASN.1 DER, BER, and PER

   [ASN.1] has many serializations.  In the IETF, DER and BER are the
   most common.  The serialized output is not particularly compact for
   many items, and the code needed to decode numeric items can be
   complex on a constrained device.

   Few (if any) IETF protocols have adopted one of the several variants
   of Packed Encoding Rules (PER).  There could be many reasons for
   this, but one that is commonly stated is that PER makes use of the
   schema even for parsing the surface structure of the data stream,
   requiring significant tool support.  There are different versions of
   the ASN.1 schema language in use, which has also hampered adoption.

E.2.  MessagePack

   [MessagePack] is a concise, widely implemented counted binary
   serialization format, similar in many properties to CBOR, although
   somewhat less regular.  While the data model can be used to represent
   JSON data, MessagePack has also been used in many remote procedure
   call (RPC) applications and for long-term storage of data.

   MessagePack has been essentially stable since it was first published
   around 2011; it has not yet had a transition.  The evolution of
   MessagePack is impeded by an imperative to maintain complete
   backwards compatibility with existing stored data, while only few
   bytecodes are still available for extension.  Repeated requests over
   the years from the MessagePack user community to separate out binary
   and text strings in the encoding recently have led to an extension
   proposal that would leave MessagePack's "raw" data ambiguous between

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   its usages for binary and text data.  The extension mechanism for
   MessagePack remains unclear.

E.3.  BSON

   [BSON] is a data format that was developed for the storage of JSON-
   like maps (JSON objects) in the MongoDB database.  Its major
   distinguishing feature is the capability for in-place update,
   foregoing a compact representation.  BSON uses a counted
   representation except for map keys, which are null-byte terminated.
   While BSON can be used for the representation of JSON-like objects on
   the wire, its specification is dominated by the requirements of the
   database application and has become somewhat baroque.  The status of
   how BSON extensions will be implemented remains unclear.

E.4.  MSDTP: RFC 713

   Message Services Data Transmission (MSDTP) is a very early example of
   a compact message format; it is described in [RFC0713], written in
   1976.  It is included here for its historical value, not because it
   was ever widely used.

E.5.  Conciseness on the Wire

   While CBOR's design objective of code compactness for encoders and
   decoders is a higher priority than its objective of conciseness on
   the wire, many people focus on the wire size.  Table 6 shows some
   encoding examples for the simple nested array [1, [2, 3]]; where some
   form of indefinite-length encoding is supported by the encoding,
   [_ 1, [2, 3]] (indefinite length on the outer array) is also shown.

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   +-------------+--------------------------+--------------------------+
   | Format      | [1, [2, 3]]              | [_ 1, [2, 3]]            |
   +-------------+--------------------------+--------------------------+
   | RFC 713     | c2 05 81 c2 02 82 83     |                          |
   |             |                          |                          |
   | ASN.1 BER   | 30 0b 02 01 01 30 06 02  | 30 80 02 01 01 30 06 02  |
   |             | 01 02 02 01 03           | 01 02 02 01 03 00 00     |
   |             |                          |                          |
   | MessagePack | 92 01 92 02 03           |                          |
   |             |                          |                          |
   | BSON        | 22 00 00 00 10 30 00 01  |                          |
   |             | 00 00 00 04 31 00 13 00  |                          |
   |             | 00 00 10 30 00 02 00 00  |                          |
   |             | 00 10 31 00 03 00 00 00  |                          |
   |             | 00 00                    |                          |
   |             |                          |                          |
   | CBOR        | 82 01 82 02 03           | 9f 01 82 02 03 ff        |
   +-------------+--------------------------+--------------------------+

           Table 6: Examples for Different Levels of Conciseness

Appendix F.  Changes from RFC 7049

   The following is a list of known changes from RFC 7049.  This list is
   non-authoritative.  It is meant to help reviewers see the significant
   differences.

   o  Updated reference for [RFC4627] to [RFC8259] in many places

   o  Updated reference for [CNN-TERMS] to [RFC7228]

   o  Added a comment to the last example in Section 2.2.1 (added
      "Second value")

   o  Fixed a bug in the example in Section 2.4.2 ("29" -> "49")

   o  Fixed a bug in the last paragraph of Section 3.6 ("0b000_11101" ->
      "0b000_11001")

Acknowledgements

   CBOR was inspired by MessagePack.  MessagePack was developed and
   promoted by Sadayuki Furuhashi ("frsyuki").  This reference to
   MessagePack is solely for attribution; CBOR is not intended as a
   version of or replacement for MessagePack, as it has different design
   goals and requirements.

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   The need for functionality beyond the original MessagePack
   Specification became obvious to many people at about the same time
   around the year 2012.  BinaryPack is a minor derivation of
   MessagePack that was developed by Eric Zhang for the binaryjs
   project.  A similar, but different, extension was made by Tim Caswell
   for his msgpack-js and msgpack-js-browser projects.  Many people have
   contributed to the discussion about extending MessagePack to separate
   text string representation from byte string representation.

   The encoding of the additional information in CBOR was inspired by
   the encoding of length information designed by Klaus Hartke for CoAP.

   This document also incorporates suggestions made by many people,
   notably Dan Frost, James Manger, Jeffrey Yaskin, Joe Hildebrand,
   Keith Moore, Laurence Lundblade, Matthew Lepinski, Michael
   Richardson, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray,
   Tony Finch, Tony Hansen, and Yaron Sheffer.

Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

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

   Paul Hoffman
   ICANN

   EMail: paul.hoffman@icann.org

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