



Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Obsoletes: 7049 (if approved)                                 P. Hoffman
Intended status: Standards Track                                   ICANN
Expires: 3 April 2021                                  30 September 2020


              Concise Binary Object Representation (CBOR)
                       draft-ietf-cbor-7049bis-16

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 is a revised edition of RFC 7049, with editorial
   improvements, added detail, and fixed errata.  This revision formally
   obsoletes RFC 7049, while keeping full compatibility of the
   interchange format from RFC 7049.  It does not create a new version
   of the format.

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 3 April 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  CBOR Data Models  . . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Extended Generic Data Models  . . . . . . . . . . . . . .   9
     2.2.  Specific Data Models  . . . . . . . . . . . . . . . . . .   9
   3.  Specification of the CBOR Encoding  . . . . . . . . . . . . .  10
     3.1.  Major Types . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Indefinite Lengths for Some Major Types . . . . . . . . .  14
       3.2.1.  The "break" Stop Code . . . . . . . . . . . . . . . .  14
       3.2.2.  Indefinite-Length Arrays and Maps . . . . . . . . . .  14
       3.2.3.  Indefinite-Length Byte Strings and Text Strings . . .  16
       3.2.4.  Summary of indefinite-length use of major types . . .  17
     3.3.  Floating-Point Numbers and Values with No Content . . . .  18
     3.4.  Tagging of Items  . . . . . . . . . . . . . . . . . . . .  20
       3.4.1.  Standard Date/Time String . . . . . . . . . . . . . .  23
       3.4.2.  Epoch-based Date/Time . . . . . . . . . . . . . . . .  23
       3.4.3.  Bignums . . . . . . . . . . . . . . . . . . . . . . .  24
       3.4.4.  Decimal Fractions and Bigfloats . . . . . . . . . . .  25
       3.4.5.  Content Hints . . . . . . . . . . . . . . . . . . . .  26
         3.4.5.1.  Encoded CBOR Data Item  . . . . . . . . . . . . .  27
         3.4.5.2.  Expected Later Encoding for CBOR-to-JSON
                 Converters  . . . . . . . . . . . . . . . . . . . .  27
         3.4.5.3.  Encoded Text  . . . . . . . . . . . . . . . . . .  28
       3.4.6.  Self-Described CBOR . . . . . . . . . . . . . . . . .  29
   4.  Serialization Considerations  . . . . . . . . . . . . . . . .  29
     4.1.  Preferred Serialization . . . . . . . . . . . . . . . . .  29
     4.2.  Deterministically Encoded CBOR  . . . . . . . . . . . . .  31
       4.2.1.  Core Deterministic Encoding Requirements  . . . . . .  31
       4.2.2.  Additional Deterministic Encoding Considerations  . .  32
       4.2.3.  Length-first Map Key Ordering . . . . . . . . . . . .  34
   5.  Creating CBOR-Based Protocols . . . . . . . . . . . . . . . .  35
     5.1.  CBOR in Streaming Applications  . . . . . . . . . . . . .  35
     5.2.  Generic Encoders and Decoders . . . . . . . . . . . . . .  36
     5.3.  Validity of Items . . . . . . . . . . . . . . . . . . . .  37
       5.3.1.  Basic validity  . . . . . . . . . . . . . . . . . . .  37
       5.3.2.  Tag validity  . . . . . . . . . . . . . . . . . . . .  37



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     5.4.  Validity and Evolution  . . . . . . . . . . . . . . . . .  38
     5.5.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  39
     5.6.  Specifying Keys for Maps  . . . . . . . . . . . . . . . .  40
       5.6.1.  Equivalence of Keys . . . . . . . . . . . . . . . . .  42
     5.7.  Undefined Values  . . . . . . . . . . . . . . . . . . . .  43
   6.  Converting Data between CBOR and JSON . . . . . . . . . . . .  43
     6.1.  Converting from CBOR to JSON  . . . . . . . . . . . . . .  43
     6.2.  Converting from JSON to CBOR  . . . . . . . . . . . . . .  44
   7.  Future Evolution of CBOR  . . . . . . . . . . . . . . . . . .  46
     7.1.  Extension Points  . . . . . . . . . . . . . . . . . . . .  46
     7.2.  Curating the Additional Information Space . . . . . . . .  47
   8.  Diagnostic Notation . . . . . . . . . . . . . . . . . . . . .  47
     8.1.  Encoding Indicators . . . . . . . . . . . . . . . . . . .  49
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
     9.1.  Simple Values Registry  . . . . . . . . . . . . . . . . .  50
     9.2.  Tags Registry . . . . . . . . . . . . . . . . . . . . . .  50
     9.3.  Media Type ("MIME Type")  . . . . . . . . . . . . . . . .  51
     9.4.  CoAP Content-Format . . . . . . . . . . . . . . . . . . .  51
     9.5.  The +cbor Structured Syntax Suffix Registration . . . . .  52
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  53
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  55
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  55
     11.2.  Informative References . . . . . . . . . . . . . . . . .  57
   Appendix A.  Examples of Encoded CBOR Data Items  . . . . . . . .  59
   Appendix B.  Jump Table for Initial Byte  . . . . . . . . . . . .  63
   Appendix C.  Pseudocode . . . . . . . . . . . . . . . . . . . . .  66
   Appendix D.  Half-Precision . . . . . . . . . . . . . . . . . . .  69
   Appendix E.  Comparison of Other Binary Formats to CBOR's Design
           Objectives  . . . . . . . . . . . . . . . . . . . . . . .  70
     E.1.  ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . .  71
     E.2.  MessagePack . . . . . . . . . . . . . . . . . . . . . . .  71
     E.3.  BSON  . . . . . . . . . . . . . . . . . . . . . . . . . .  72
     E.4.  MSDTP: RFC 713  . . . . . . . . . . . . . . . . . . . . .  72
     E.5.  Conciseness on the Wire . . . . . . . . . . . . . . . . .  72
   Appendix F.  Well-formedness errors and examples  . . . . . . . .  73
     F.1.  Examples for CBOR data items that are not well-formed . .  74
   Appendix G.  Changes from RFC 7049  . . . . . . . . . . . . . . .  76
     G.1.  Errata processing, clerical changes . . . . . . . . . . .  76
     G.2.  Changes in IANA considerations  . . . . . . . . . . . . .  77
     G.3.  Changes in suggestions and other informational
           components  . . . . . . . . . . . . . . . . . . . . . . .  77
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  79
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  79








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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
   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 is a revised edition of [RFC7049], with editorial
   improvements, added detail, and fixed errata.  This revision formally
   obsoletes RFC 7049, while keeping full compatibility of the
   interchange format from RFC 7049.  It does not create a new version
   of the format.

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.






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   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).

   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 the implementation complexity limiting how much
          effort can go into achieving that compactness.  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.




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       *  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
   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; the
      former can be addressed specifically by using "encoded data item".

   Decoder:  A process that decodes a well-formed encoded 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 (well-formed) 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 (see [RFC8742] for
      one application).  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.




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   Valid:  A data item that is well-formed and also follows the semantic
      restrictions that apply to CBOR data items (Section 5.3).

   Expected:  Besides its normal English meaning, the term "expected" is
      used to describe requirements beyond CBOR validity that an
      application has on its input data.  Well-formed (processable at
      all), valid (checked by a validity-checking generic decoder), and
      expected (checked by the application) form a hierarchy of layers
      of acceptability.

   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.

   Terms and concepts for floating-point values such as Infinity, NaN
   (not a number), negative zero, and subnormal are defined in
   [IEEE754].

   Where bit arithmetic or data types are explained, this document uses
   the notation familiar from the programming language C [C], except
   that "**" denotes exponentiation and ".." denotes a range that
   includes both ends given.  Examples and pseudocode assume that signed
   integers use two's complement representation and that right shifts of
   signed integers perform sign extension; these assumptions are also
   specified in Sections 6.8.2 and 7.6.7 of the 2020 version of C++,
   successor of [Cplusplus17].

   Similar to the "0x" notation for hexadecimal numbers, numbers in
   binary notation are prefixed with "0b".  Underscores can be added to
   a number solely for readability, so 0b00100001 (0x21) might be
   written 0b001_00001 to 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.

   Words may be _italicized_ for emphasis; in the plain text form of
   this specification this is indicated by surrounding words with
   underscore characters.  Verbatim text (e.g., names from a programming
   language) may be set in "monospace" type; in plain text this is
   approximated somewhat ambiguously by surrounding the text in double
   quotes (which also retain their usual meaning).








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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 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 defined in Section 3, a
   data item is one of:

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

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

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

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

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

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

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

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

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








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   Also note that serialization variants are not visible at the generic
   data model level, including the number of bytes of the encoded
   floating-point 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 number, (collectively "the
   argument", see Section 3) can be encoded.

2.1.  Extended Generic Data Models

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

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

   *  integer and floating-point values with a larger range and
      precision than the above (tag numbers 2 to 5)

   *  application data types such as a point in time or an RFC 3339
      date/time string (tag numbers 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 simple values or generic
   tags.

   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 tag numbers, 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
   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")



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   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,
   even while encoded as different major types, 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 7 in Appendix B,
   indexed by the initial byte.  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.

   31:  No argument value is derived.  If the major type is 0, 1, or 6,




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      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 constitute a data item at all but terminates an indefinite
      length item; all are described in Section 3.2.

   The initial byte and any additional bytes consumed to construct the
   argument are collectively referred to as the "head" of the data item.

   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 7).  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 unsigned 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



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      equal to the argument.  For example, a byte string whose length is
      5 would have an initial byte of 0b010_00101 (major type 2,
      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 (Section 1.2).  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 (that of Unicode) and encoding
      (UTF-8).  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")
      nor as 0x5c7530303061 (the characters "\", "u", "0", "0", "0", and
      "a").

   Major type 4:  an array of data items.  In other formats, arrays are
      also called lists, sequences, or tuples (a "CBOR sequence" is
      something slightly different, though [RFC8742]).  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 4, additional information 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 5, additional information 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.  Because items in a map come in pairs,
      their total number is always even: A map that contains an odd
      number of items (no value data present after the last key data
      item) is not well-formed.  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 ("tag") whose tag number, an



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      integer in the range 0..2**64-1 inclusive, is the argument and
      whose enclosed data item ("tag content") is the single encoded
      data item that follows the head.  See Section 3.4.

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

   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 7).

   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.

   Table 1 summarizes the major types defined by CBOR, ignoring the next
   section for now.  The number N in this table stands for the argument,
   mt for the major type.

     +====+=======================+=================================+
     | mt | Meaning               | Content                         |
     +====+=======================+=================================+
     | 0  | unsigned integer N    | -                               |
     +----+-----------------------+---------------------------------+
     | 1  | negative integer -1-N | -                               |
     +----+-----------------------+---------------------------------+
     | 2  | byte string           | N bytes                         |
     +----+-----------------------+---------------------------------+
     | 3  | text string           | N bytes (UTF-8 text)            |
     +----+-----------------------+---------------------------------+
     | 4  | array                 | N data items (elements)         |
     +----+-----------------------+---------------------------------+
     | 5  | map                   | 2N data items (key/value pairs) |
     +----+-----------------------+---------------------------------+
     | 6  | tag of number N       | 1 data item                     |
     +----+-----------------------+---------------------------------+
     | 7  | simple/float          | -                               |
     +----+-----------------------+---------------------------------+

       Table 1: Overview over the definite-length use of CBOR major
                  types (mt = major type, N = argument)










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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 ability to start sending a data item
   before all of it is known is often referred to as "streaming" within
   that data item.)

   Indefinite-length arrays and maps are dealt with differently than
   indefinite-length strings (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 zero or more 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.

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





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   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"

   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"





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   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 for byte string or text string with an additional
   information value of 31, followed by a series of zero or more strings
   of the specified type ("chunks") that have definite lengths, and
   finished 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 no chunks are present, the data item is an empty
   string of the specified type.  Zero-length chunks, while not
   particularly useful, are permitted.

   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.

   The design does not allow nesting indefinite-length strings as chunks
   into indefinite-length strings.  If it were allowed, it would require
   decoder implementations to keep a stack, or at least a count, of
   nesting levels.  It is unnecessary on the encoder side because the
   inner indefinite-length string would consist of chunks, and these
   could instead be put directly into the outer indefinite-length
   string.



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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 UTF-8 bytes of a single Unicode code
   point (scalar value) cannot be spread between chunks: a new chunk of
   a text string can only be started at a code point boundary.

   For example, assume an encoded data item consisting of the bytes:

   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.

3.2.4.  Summary of indefinite-length use of major types

   Table 2 summarizes the major types defined by CBOR as used for
   indefinite length encoding (with additional information set to 31).
   mt stands for the major type.


























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       +====+===================+==================================+
       | mt | Meaning           | enclosed up to "break" stop code |
       +====+===================+==================================+
       | 0  | (not well-formed) | -                                |
       +----+-------------------+----------------------------------+
       | 1  | (not well-formed) | -                                |
       +----+-------------------+----------------------------------+
       | 2  | byte string       | definite-length byte strings     |
       +----+-------------------+----------------------------------+
       | 3  | text string       | definite-length text strings     |
       +----+-------------------+----------------------------------+
       | 4  | array             | data items (elements)            |
       +----+-------------------+----------------------------------+
       | 5  | map               | data items (key/value pairs)     |
       +----+-------------------+----------------------------------+
       | 6  | (not well-formed) | -                                |
       +----+-------------------+----------------------------------+
       | 7  | "break" stop code | -                                |
       +----+-------------------+----------------------------------+

          Table 2: Overview over the indefinite-length use of CBOR
           major types (mt = major type, additional information =
                                    31)

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 3.  Like the major types for integers,
   items of this major type do not carry content data; all the
   information is in the initial bytes (the head).



















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    +=============+===================================================+
    | 5-Bit Value | Semantics                                         |
    +=============+===================================================+
    | 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       | Reserved, not well-formed in the present document |
    +-------------+---------------------------------------------------+
    | 31          | "break" stop code for indefinite-length items     |
    |             | (Section 3.2.1)                                   |
    +-------------+---------------------------------------------------+

         Table 3: 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 4 lists the numeric
   values assigned and available for simple values.

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

                          Table 4: Simple Values



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   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, and that only the one-byte variants
   of these are well-formed; more generally speaking, each simple value
   only has a single representation variant).

   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 numbers.)

3.4.  Tagging of Items

   In CBOR, a data item can be enclosed by a tag to give it some
   additional semantics, as uniquely identified by a "tag number".  The
   tag is major type 6, its argument (Section 3) indicates the tag
   number, and it contains a single enclosed data item, the "tag
   content".  (If a tag requires further structure to its content, this
   structure is provided by the enclosed data item.)  We use the term
   "tag" for the entire data item consisting of both a tag number and
   the tag content: the tag content is the data item that is being
   tagged.

   For example, assume that a byte string of length 12 is marked with a
   tag of number 2 to indicate it is a positive "bignum"
   (Section 3.4.3).  The encoded data item would start with a byte
   0b110_00010 (major type 6, additional information 2 for the tag
   number) followed by the encoded tag content: 0b010_01100 (major type
   2, additional information of 12 for the length) followed by the 12
   bytes of the bignum.

   The definition of a tag number describes the additional semantics
   conveyed for tags with this tag number in the extended generic data
   model.  These semantics may include equivalence of some tagged data
   items with other data items, including some that can already be
   represented in the basic generic data model.  For instance, 0xc24101,
   a bignum the tag content of which is the byte string with the single
   byte 0x01, is equivalent to an integer 1, which could also be encoded
   for instance as 0x01, 0x1801, or 0x190001.  The tag definition may
   include the definition of a preferred serialization (Section 4.1)
   that is recommended for generic encoders; this may prefer basic
   generic data model representations over ones that employ a tag.






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   The tag definition usually restricts what kinds of nested data item
   or items are valid for such tags.  Tag definitions may restrict their
   content to a very specific syntactic structure, as the tags defined
   in this document do, or they may aim at a more semantically defined
   definition of their content, as for instance tags 40 and 1040 do
   [RFC8746]: These accept a number of different ways of representing
   arrays.

   As a matter of convention, many tags do not accept null or undefined
   values as tag content; instead, the expectation is that a null or
   undefined value can be used in place of the entire tag; Section 3.4.2
   provides some further considerations for one specific tag about the
   handling of this convention in application protocols and in mapping
   to platform types.

   Decoders do not need to understand tags of every tag number, and 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 provide conversion
   hints when it is foreseen that the CBOR data item needs to be
   translated into a different format, requiring hints about the content
   of items.  Understanding the semantics of tags is optional for a
   decoder; it can simply present both the tag number and the tag
   content to the application, without interpreting the additional
   semantics of the tag.

   A tag applies semantics to the data item it encloses.  Tags can nest:
   If tag A encloses tag B, which encloses data item C, tag A applies to
   the result of applying tag B on data item C.

   IANA maintains a registry of tag numbers as described in Section 9.2.
   Table 5 provides a list of tag numbers that were defined in
   [RFC7049], with definitions in the rest of this section.  (Tag number
   35 was also defined in [RFC7049]; a discussion of this tag number
   follows in Section 3.4.5.3.)  Note that many other tag numbers have
   been defined since the publication of [RFC7049]; see the registry
   described at Section 9.2 for the complete list.












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      +============+=============+==================================+
      | Tag Number | Data Item   | Tag Content Semantics            |
      +============+=============+==================================+
      | 0          | text string | Standard date/time string; see   |
      |            |             | Section 3.4.1                    |
      +------------+-------------+----------------------------------+
      | 1          | integer or  | Epoch-based date/time; see       |
      |            | float       | Section 3.4.2                    |
      +------------+-------------+----------------------------------+
      | 2          | byte string | Positive bignum; see             |
      |            |             | Section 3.4.3                    |
      +------------+-------------+----------------------------------+
      | 3          | byte string | Negative bignum; see             |
      |            |             | Section 3.4.3                    |
      +------------+-------------+----------------------------------+
      | 4          | array       | Decimal fraction; see            |
      |            |             | Section 3.4.4                    |
      +------------+-------------+----------------------------------+
      | 5          | array       | Bigfloat; see Section 3.4.4      |
      +------------+-------------+----------------------------------+
      | 21         | (any)       | Expected conversion to base64url |
      |            |             | encoding; see Section 3.4.5.2    |
      +------------+-------------+----------------------------------+
      | 22         | (any)       | Expected conversion to base64    |
      |            |             | encoding; see Section 3.4.5.2    |
      +------------+-------------+----------------------------------+
      | 23         | (any)       | Expected conversion to base16    |
      |            |             | encoding; see Section 3.4.5.2    |
      +------------+-------------+----------------------------------+
      | 24         | byte string | Encoded CBOR data item; see      |
      |            |             | Section 3.4.5.1                  |
      +------------+-------------+----------------------------------+
      | 32         | text string | URI; see Section 3.4.5.3         |
      +------------+-------------+----------------------------------+
      | 33         | text string | base64url; see Section 3.4.5.3   |
      +------------+-------------+----------------------------------+
      | 34         | text string | base64; see Section 3.4.5.3      |
      +------------+-------------+----------------------------------+
      | 36         | text string | MIME message; see                |
      |            |             | Section 3.4.5.3                  |
      +------------+-------------+----------------------------------+
      | 55799      | (any)       | Self-described CBOR; see         |
      |            |             | Section 3.4.6                    |
      +------------+-------------+----------------------------------+

                  Table 5: Tag numbers defined in RFC 7049





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   Conceptually, tags are interpreted in the generic data model, not at
   (de-)serialization time.  A small number of tags (at this time, tag
   number 25 and tag number 29 [IANA.cbor-tags]) have been registered
   with semantics that may require processing at (de-)serialization
   time: The decoder needs to be aware and the encoder needs to be in
   control of the exact sequence in which data items are encoded into
   the CBOR data item.  This means these tags cannot be implemented on
   top of an arbitrary generic CBOR encoder/decoder (which might not
   reflect the serialization order for entries in a map at the data
   model level and vice versa); their implementation therefore typically
   needs to be integrated into the generic encoder/decoder.  The
   definition of new tags with this property is NOT RECOMMENDED.

   IANA allocated tag numbers 65535, 4294967295, and
   18446744073709551615 (binary all-ones in 16-bit, 32-bit, and 64-bit).
   These can be used as a convenience for implementers that want a
   single integer data structure to indicate either that a specific tag
   is present, or the absence of a tag.  That allocation is described in
   Section 10 of [I-D.bormann-cbor-notable-tags].  These tags are not
   intended to occur in actual CBOR data items; implementations MAY flag
   such an occurrence as an error.

   Protocols using tag numbers 0 and 1 extend the generic data model
   (Section 2) with data items representing points in time; tag numbers
   2 and 3, with arbitrarily sized integers; and tag numbers 4 and 5,
   with floating-point values of arbitrary size and precision.

3.4.1.  Standard Date/Time String

   Tag number 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 a text string that doesn't match the
   [RFC4287] format is invalid.

3.4.2.  Epoch-based Date/Time

   Tag number 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.

   The tag content 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



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   known as "UNIX Epoch time".)  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 tag content.

   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 number 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 number 1 support may
   restrict the tag content to be an integer (or a floating-point value)
   only.

   Note that platform types for date/time may include null or undefined
   values, which may also be desirable at an application protocol level.
   While emitting tag number 1 values with non-finite tag content values
   (e.g., with NaN for undefined date/time values or with Infinite for
   an expiry date that is not set) may seem an obvious way to handle
   this, using untagged null or undefined avoids the use of non-finites
   and results in a shorter encoding.  Application protocol designers
   are encouraged to consider these cases and include clear guidelines
   for handling them.

3.4.3.  Bignums

   Protocols using tag numbers 2 and 3 extend the generic data model
   (Section 2) with "bignums" representing arbitrarily sized integers.
   In the basic generic data model, bignum values are not equal to
   integers from the same model, but the extended generic data model
   created by this tag definition defines equivalence based on numeric
   value, and preferred serialization (Section 4.1) 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 number 2, the value of the bignum
   is n.  For tag number 3, the value of the bignum is -1 - n.  The
   preferred serialization of the byte string is to leave out any
   leading zeroes (note that this means the preferred serialization for
   n = 0 is the empty byte string, but see below).  Decoders that



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   understand these tags MUST be able to decode bignums that do have
   leading zeroes.  The preferred serialization 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 serialization).  Note that
   this means the non-preferred choice 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 number 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

3.4.4.  Decimal Fractions and Bigfloats

   Protocols using tag number 4 extend the generic data model with data
   items representing arbitrary-length decimal fractions of the form
   m*(10**e).  Protocols using tag number 5 extend the generic data
   model with data items representing arbitrary-length binary fractions
   of the form 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 representations.

   "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.










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   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 number 4) use base-10 exponents;
   the value of a decimal fraction data item is m*(10**e).  Bigfloats
   (tag number 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 can also be a bignum
   (Section 3.4.3).  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 6 for tag, additional
   information 4 for the tag number), followed by 0b100_00010 (major
   type 4 for the array, additional information 2 for the length of the
   array), followed by 0b001_00001 (major type 1 for the first integer,
   additional information 1 for the value of -2), followed by
   0b000_11001 (major type 0 for the second integer, additional
   information 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 6 for tag, additional information 5 for
   the tag number), followed by 0b100_00010 (major type 4 for the array,
   additional information 2 for the length of the array), followed by
   0b001_00000 (major type 1 for the first integer, additional
   information 0 for the value of -1), followed by 0b000_00011 (major
   type 0 for the second integer, additional information 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.

3.4.5.  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.



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3.4.5.1.  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 number 24 (CBOR data item) can be used to
   tag the embedded byte string as a single data item encoded in CBOR
   format.  Contained items that aren't byte strings are invalid.  A
   contained byte string is valid if it encodes a well-formed CBOR data
   item; validity checking of the decoded CBOR item is not required for
   tag validity (but could be offered by a generic decoder as a special
   option).

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

   Tag numbers 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 numbers suggest conversions to three of the base data
   encodings defined in [RFC4648].  Tag number 21 suggests conversion to
   base64url encoding (Section 5 of RFC 4648), where padding is not used
   (see Section 3.2 of RFC 4648); that is, all trailing equals signs
   ("=") are removed from the encoded string.  Tag number 22 suggests
   conversion to classical base64 encoding (Section 4 of RFC 4648), with
   padding as defined in RFC 4648.  For both base64url and base64,
   padding bits are set to zero (see Section 3.5 of RFC 4648), and the
   conversion to alternate encoding is performed on the contents of the
   byte string (that is, without adding any line breaks, whitespace, or
   other additional characters).  Tag number 23 suggests conversion to
   base16 (hex) encoding, with uppercase alphabetics (see Section 8 of
   RFC 4648).  Note that, for all three tag numbers, the encoding of the
   empty byte string is the empty text string.






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3.4.5.3.  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.

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

   *  Tag numbers 33 and 34 are for base64url- and base64-encoded text
      strings, respectively, as defined in [RFC4648].  If any of:

      -  the encoded text string contains non-alphabet characters or
         only 1 alphabet character in the last block of 4 (where
         alphabet is defined by Section 5 of [RFC4648] for tag number 33
         and Section 4 of [RFC4648] for tag number 34), 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.

   *  Tag number 36 is for MIME messages (including all headers), as
      defined in [RFC2045].  A text string that isn't a valid MIME
      message is invalid.  (For this tag, validity checking may be
      particularly onerous for a generic decoder and might therefore not
      be offered.  Note that many MIME messages are general binary data
      and can therefore not be represented in a text string;
      [IANA.cbor-tags] lists a registration for tag number 257 that is
      similar to tag number 36 but uses a byte string as its tag
      content.)

   Note that tag numbers 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.

   [RFC7049] also defined a tag number 35, for regular expressions that
   are in Perl Compatible Regular Expressions (PCRE/PCRE2) form [PCRE]
   or in JavaScript regular expression syntax [ECMA262].  The state of
   the art in these regular expression specifications has since advanced
   and is continually advancing, so the present specification does not
   attempt to update the references to a snapshot that is current at the



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   time of writing.  Instead, this tag remains available (as registered
   in [RFC7049]) for applications that specify the particular regular
   expression variant they use out-of-band (possibly by limiting the
   usage to a defined common subset of both PCRE and ECMA262).  As the
   present specification clarifies tag validity beyond [RFC7049], we
   note that due to the open way the tag was defined in [RFC7049], any
   contained string value needs to be valid at the CBOR tag level (but
   may then not be "expected" at the application level).

3.4.6.  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 number 55799 is defined for this purpose, specifically for use at
   the start of a stored encoded CBOR data item as specified by an
   application.  It does not impart any special semantics on the data
   item that it encloses; that is, the semantics of the tag content
   enclosed in tag number 55799 is exactly identical to the semantics of
   the tag content itself.

   The serialization of this tag's head 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 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 number 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 (preferred
   encoding); however, the present specification does not put the burden
   of enforcing this preference on either encoder or decoder.



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   Some constrained decoders may be limited in their ability to decode
   non-preferred serializations: For example, if only integers below
   1_000_000_000 (one billion) 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 therefore 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.

   The preferred serialization 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.
   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).

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











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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:

   *  Preferred serialization MUST be used.  In particular, this means
      that arguments (see Section 3) for integers, lengths in major
      types 2 through 5, and tags MUST be as short as possible, for
      instance:

      -  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.

      Floating-point values also MUST use the shortest form that
      preserves the value, e.g. 1.5 is encoded as 0xf93e00 (binary16)
      and 1000000.5 as 0xfa49742408 (binary32).  (One implementation of
      this is 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 form and repeat the process with a test
      conversion to a 16-bit float.  This also works to select 16-bit
      float for positive and negative Infinity as well.)

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

   *  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:




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      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.

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

      8.  false, encoded as 0xf4.

      (Implementation note: the self-delimiting nature of the CBOR
      encoding means that there are no two well-formed CBOR encoded data
      items where one is a prefix of the other.  The bytewise
      lexicographic comparison of deterministic encodings of different
      map keys therefore always ends in a position where the byte
      differs between the keys, before the end of a key is reached.)

4.2.2.  Additional Deterministic Encoding Considerations

   CBOR tags present additional considerations for deterministic
   encoding.  If a CBOR-based protocol were to provide the same
   semantics for the presence and absence of a specific tag (e.g., by
   allowing both tag 1 data items and raw numbers in a date/time
   position, treating the latter as if they were tagged), the
   deterministic format would not allow the presence of the tag, based
   on the "shortest form" principle.  For example, a protocol might give
   encoders the choice of representing a URL as either a text string or,
   using Section 3.4.5.3, tag number 32 containing a text string.  This
   protocol's deterministic encoding needs to either require that the
   tag is present or require that it is absent, not allow either one.

   In a protocol that does require tags in certain places to obtain
   specific semantics, the tag needs to appear in the deterministic
   format as well.  Deterministic encoding considerations also apply to
   the content of tags.

   If a protocol includes a field that can express integers with an
   absolute value of 2^64 or larger using tag numbers 2 or 3
   (Section 3.4.3), the protocol's deterministic encoding needs to
   specify whether smaller integers are also expressed using these tags
   or using major types 0 and 1.  Preferred serialization uses the
   latter choice, which is therefore recommended.



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   Protocols that include floating-point values, whether represented
   using basic floating-point values (Section 3.3) or using tags (or
   both), may need to define extra requirements on their deterministic
   encodings, such as:

   *  Although IEEE floating-point values can represent both positive
      and negative zero as distinct values, the application might not
      distinguish these and might decide to represent all zero values
      with a positive sign, disallowing negative zero.  (The application
      may also want to restrict the precision of floating-point values
      in such a way that there is never a need to represent 64-bit -- or
      even 32-bit -- floating-point values.)

   *  If a protocol includes a field that can express floating-point
      values, with a specific data model that declares integer and
      floating-point values to be interchangeable, the protocol's
      deterministic encoding needs to specify whether (for example) the
      integer 1.0 is encoded as 0x01 (unsigned integer), 0xf93c00
      (binary16), 0xfa3f800000 (binary32), or 0xfb3ff0000000000000
      (binary64).  Example 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 preferred
          (smallest of 16-, 32-, or 64-bit) floating-point
          representation that accurately represents the value,

      2.  Encode all values as the preferred floating-point
          representation that accurately represents the value, even for
          integral values, or

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

      Rule 1 straddles the boundaries between integers and floating-
      point values, and Rule 3 does not use preferred serialization, so
      Rule 2 may be a good choice in many cases.

   *  If NaN is an allowed value and there is no intent to support NaN
      payloads or signaling NaNs, the protocol needs to pick a single
      representation, typically 0xf97e00.  If that simple choice is not
      possible, specific attention will be needed for NaN handling.

   *  Subnormal numbers (nonzero numbers with the lowest possible
      exponent of a given IEEE 754 number format) may be flushed to zero
      outputs or be treated as zero inputs in some floating-point
      implementations.  A protocol's deterministic encoding may want to
      specifically accommodate such implementations while creating an
      onus on other implementations, by excluding subnormal numbers from
      interchange, interchanging zero instead.



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   *  The same number can be represented by different decimal fractions,
      by different bigfloats, and by different forms under other tags
      that may be defined to express numeric values.  Depending on the
      implementation, it may not always be practical to determine
      whether any of these forms (or forms in the basic generic data
      model) are equivalent.  An application protocol that presents
      choices of this kind for the representation format of numbers
      needs to be explicit in how the formats are to be chosen for
      deterministic encoding.

4.2.3.  Length-first Map Key Ordering

   The core deterministic encoding requirements (Section 4.2.1) 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.

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

   7.  "aa", encoded as 0x626161.

   8.  [100], encoded as 0x811864.



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   (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
   interchangeable, 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 a 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.

   The rest of 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.

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.



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   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.  Such an application also MUST have a
   matching streaming security mechanism, where the desired protection
   is available for incremental data presented to the application.

   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 encoded CBOR data
   items and present the data items to an application.  See Appendix C.
   (The diagnostic notation, Section 8, may be used to present well-
   formed CBOR values to humans.)

   Generic CBOR encoders provide an application interface that allows
   the application to specify any well-formed value to be encoded as a
   CBOR data item, including simple values and tags unknown to the
   encoder.

   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 (because [RFC3629] requires
   always using the shortest form) and so is not a valid CBOR item.
   Also, specific tags may make semantic constraints that may be
   violated, for instance by a bignum tag enclosing another tag, or by
   an instance of tag number 0 containing a byte string, or containing 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).








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5.3.  Validity of Items

   A well-formed but invalid CBOR data item (Section 1.2) 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 any validity
   errors 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 occur at the basic validity level of CBOR or in
   the context of tags (tag validity).

5.3.1.  Basic validity

   Two kinds of validity errors can occur in the basic generic data
   model:

   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.  See Section 5.6 for more discussion of keys in maps.

   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.

5.3.2.  Tag validity

   Two additional kinds of validity errors are introduced by adding tags
   to the basic generic data model:

   Inadmissible type for tag content:  Tag numbers (Section 3.4) specify




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      what type of data item is supposed to be used as their tag
      content; for example, the tag numbers 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.

   Inadmissible value for tag content:  The type of data item may be
      admissible for a tag's content, but the specific value may not be;
      e.g., a value of "yesterday" is not acceptable for the content of
      tag 0, even though it properly is a text string.  A decoder that
      normally ingests such tags into equivalent platform types might
      present this tag to the application in a similar way to how it
      would present a tag with an unknown tag number (Section 5.4).

5.4.  Validity and Evolution

   A decoder with validity checking will expend the effort to reliably
   detect data items with validity errors.  For example, such a 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 validity errors listed
   in the previous subsection.

   The set of tags defined in the tag registry (Section 9.2), as well as
   the set of simple values defined in the simple values registry
   (Section 9.1), can grow at any time beyond the set understood by a
   generic decoder.  A validity-checking decoder can do one of two
   things when it encounters such a case that it does not recognize:

   *  It can report an error (and not return data).  Note that treating
      this case as an error can cause ossification, and is thus not
      encouraged.  This error is not a validity error per se.  This kind
      of error is more likely to be raised by a decoder that would be
      performing validity checking if this were a known case.

   *  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
      number or simple value.










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   The latter approach, which is also appropriate for decoders that do
   not support validity checking, provides 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 the processing needed for validity checking may have an
   appreciable cost (in particular with duplicate detection for maps),
   support of validity checking 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 valid CBOR results from the encoder.  A generic
   encoder may also want to provide a validity-checking mode where it
   reliably limits its output to valid CBOR, independent of whether or
   not its application is indeed providing API-conformant data.

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 basic JavaScript
   number system treats all numbers as floating-point values, which may
   result in silent loss of precision in decoding integers with more
   than 53 significant bits.  Another example is that, since CBOR keeps
   the sign bit for its integer representation in the major type, it has
   one bit more for signed numbers of a certain length (e.g.,
   -2**64..2**64-1 for 1+8-byte integers) than the typical platform
   signed integer representation of the same length (-2**63..2**63-1 for
   8-byte int64_t).  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 that does not require deterministic encoding
   should accept values that use a longer-than-needed encoding (such as



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   encoding "0" as 0b000_11001 followed by two bytes of 0x00) as long as
   the application can decode an integer of the given size.  Similar
   considerations apply to floating-point values; decoding both
   preferred serializations and longer-than-needed ones is recommended.

   CBOR-based protocols for constrained applications that provide 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), might express a quality-of-implementation expectation
   that the integer representation is used directly.

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, conversion is simplified by
   limiting keys to text strings only; otherwise, there has to be a
   specified mapping from the other CBOR types to text strings, 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.  When processing maps that exhibit entries with duplicate
   keys, a generic decoder might do one of the following:





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   *  Not accept maps with duplicate keys (that is, enforce validity for
      maps, see also Section 5.4).  These generic decoders are
      universally useful.  An application may still need to do perform
      its own duplicate checking based on application rules (for
      instance if the application equates integers and floating-point
      values in map key positions for specific maps).

   *  Pass all map entries to the application, including ones with
      duplicate keys.  This requires the application to handle (check
      against) duplicate keys, even if the application rules are
      identical to the generic data model rules.

   *  Lose some entries with duplicate keys, e.g. by only delivering the
      final (or first) entry out of the entries with the same key.  With
      such a generic decoder, applications may get different results for
      a specific key on different runs and with different generic
      decoders as which value is returned is based on generic decoder
      implementation and the actual order of keys in the map.  In
      particular, applications cannot validate key uniqueness on their
      own as they do not necessarily see all entries; they may not be
      able to use such a generic decoder if they do need to validate key
      uniqueness.  These generic decoders can only be used in situations
      where the data source and transfer can be relied upon to always
      provide valid maps; this is not possible if the data source and
      transfer can be attacked.

   Generic decoders need to document which of these three approaches
   they implement.

   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
   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 their own for a protocol to
   require a deterministic encoding format.)

   Applications for constrained devices that have maps where a small
   number of frequently used keys can be identified should consider
   using small integers as keys; for instance, a set of 24 or fewer
   frequent keys can be encoded in a single byte as unsigned integers,
   up to 48 if negative integers are also used.  Less frequently
   occurring keys can then use integers with longer encodings.





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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 number.

   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 number and the tag content
   are equal.  (Note that a generic decoder that provides processing for
   a specific tag may not be able to distinguish some semantically
   equivalent values, e.g. if leading zeroes occur in the content of tag
   2/3 (Section 3.4.3).)  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,
   the decoder may provide a programming interface to perform this
   service for the application).  Specific data models are not able to
   distinguish values for map keys that are equal for this purpose at
   the generic data model level.









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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.

6.  Converting Data between CBOR and JSON

   This section gives non-normative advice about converting between CBOR
   and JSON.  Implementations of converters MAY 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.

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

   *  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.

   *  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.

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

   *  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.




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

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

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

   *  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.

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

   *  A bignum (major type 6, tag number 2 or 3) is represented by
      encoding its byte string in base64url without padding and becomes
      a JSON string.  For tag number 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.)

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

   *  For all other tags (major type 6, any other tag number), the tag
      content is represented as a JSON value; the tag number is ignored.

   *  Indefinite-length items are made definite before conversion.

   A CBOR-to-JSON converter may want to keep to the JSON profile I-JSON
   [RFC7493], to maximize interoperability and increase confidence that
   the JSON output can be processed with predictable results.  For
   example, this has implications on the range of integers that can be
   represented reliably, as well as on the top-level items that may be
   supported by older JSON implementations.

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:




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   *  JSON numbers without fractional parts (integer numbers) are
      represented as integers (major types 0 and 1, possibly major type
      6 tag number 2 and 3), choosing the shortest form; integers longer
      than an implementation-defined threshold may instead be
      represented as floating-point values.  The default range that is
      represented as integer is -2**53+1..2**53-1 (fully exploiting the
      range for exact integers in the binary64 representation often used
      for decoding JSON [RFC7493]).  A CBOR-based protocol, or a generic
      converter implementation, may choose -2**32..2**32-1 or
      -2**64..2**64-1 (fully using the integer ranges available in CBOR
      with uint32_t or uint64_t, respectively) or even -2**31..2**31-1
      or -2**63..2**63-1 (using popular ranges for two's complement
      signed integers).  (If the JSON was generated from a JavaScript
      implementation, its precision is already limited to 53 bits
      maximum.)

   *  Numbers with fractional parts are represented as floating-point
      values, performing the decimal-to-binary conversion based on the
      precision provided by IEEE 754 binary64.  The mathematical value
      of the JSON number is converted to binary64 using the
      roundTiesToEven procedure in Section 4.3.1 of [IEEE754].  Then,
      when encoding in CBOR, the preferred serialization uses the
      shortest floating-point representation exactly representing this
      conversion result; 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).  Instead of using
      the default binary64 precision, there may be an implementation-
      defined limit to the precision of the conversion that will affect
      the precision of the represented values.  Decimal representation
      should only be used on the CBOR side 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.  Protocol designs should attempt 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:

   *  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 easily 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.






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   *  the "tag" space (values in major type 6).  The total codepoint
      space is abundant; only a tiny part of it has been allocated.
      However, not all of these codepoints are equally efficient: the
      first 24 only consume a single ("1+0") byte, and half of them have
      already been allocated.  The next 232 values only consume two
      ("1+1") bytes, with nearly a quarter already allocated.  These
      subspaces need some curation to last for a few more decades.
      Implementations receiving an unknown tag number can choose to
      process just the enclosed tag content or, preferably, to process
      the tag as an unknown tag number wrapping the tag content.  The
      IANA registry in Section 9.2 is the appropriate way to address the
      extensibility of this codepoint space.

   *  the "additional information" space.  An implementation receiving
      an unknown additional information value has no way to continue
      decoding, so allocating codepoints in this space is a major step
      beyond just exercising an extension point.  There are also very
      few codepoints left.  See also Section 7.2.

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 the present specification.

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.





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   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.

   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 tag is written as an integer number for the tag
   number, followed by the tag content 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.

   A number of useful extensions to the diagnostic notation defined here
   are provided in Appendix G of [RFC8610], "Extended Diagnostic
   Notation" (EDN).  Similarly, an extension of this notation could be
   provided in a separate document to provide for the documentation of
   NaN payloads, which are not covered in the present document.








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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.  For example, "_" or "_3".
   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].

   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.)

   The detailed chunk structure of byte and text strings of indefinite
   length can be notated in the form (_ h'0123', h'4567') and (_ "foo",
   "bar").  However, for an indefinite length string with no chunks
   inside, (_ ) would be ambiguous whether a byte string (0x5fff) or a
   text string (0x7fff) is meant and is therefore not used.  The basic
   forms ''_ and ""_ can be used instead and are reserved for the case
   with no chunks only -- not as short forms for the (permitted, but not
   really useful) encodings with only empty chunks, which to preserve
   the chunk structure need to be notated as (_ ''), (_ ""), etc.

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.






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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 4.

   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, and other tags have
   already been defined since then.

   New entries in the range 0 to 23 ("1+0") are assigned by Standards
   Action.  New entries in the ranges 24 to 255 ("1+1") and 256 to 32767
   (lower half of "1+2") are assigned by Specification Required.  New
   entries in the range 32768 to 18446744073709551615 (upper half of
   "1+2", "1+4", and "1+8") are assigned by First Come First Served.
   The template for registration requests is:

   *  Data item

   *  Semantics (short form)

   In addition, First Come First Served requests should include:

   *  Point of contact

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

   Applicants exercising the First Come First Served range and making a
   suggestion for a tag number that is not representable in 32 bits
   (i.e., larger than 4294967295) should be aware that this could reduce
   interoperability with implementations that do not support 64-bit
   numbers.






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9.3.  Media Type ("MIME Type")

   The Internet media type [RFC6838] for a single encoded CBOR data item
   is application/cbor, as defined in [IANA.media-types]:

   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:  Many

   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:  IETF CBOR
      Working Group cbor@ietf.org (mailto:cbor@ietf.org) or IETF
      Applications and Real-Time Area art@ietf.org (mailto:art@ietf.org)

   Intended usage: COMMON

   Restrictions on usage: none

   Author:  IETF CBOR Working Group cbor@ietf.org (mailto:cbor@ietf.org)

   Change controller:  The IESG iesg@ietf.org (mailto:iesg@ietf.org)

9.4.  CoAP Content-Format

   The CoAP Content-Format for CBOR is registered in
   [IANA.core-parameters]:

   Media Type: application/cbor



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   Encoding: -

   Id: 60

   Reference: [RFCthis]

9.5.  The +cbor Structured Syntax Suffix Registration

   The Structured Syntax Suffix [RFC6838] for media types based on a
   single encoded CBOR data item is +cbor, as defined in
   [IANA.media-type-structured-suffix]:

   Name: Concise Binary Object Representation (CBOR)

   +suffix: +cbor

   References: [RFCthis]

   Encoding Considerations: CBOR is a binary format.

   Interoperability Considerations: n/a

   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:  IETF CBOR Working Group cbor@ietf.org
      (mailto:cbor@ietf.org) or IETF Applications and Real-Time Area
      art@ietf.org (mailto:art@ietf.org)




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   Author/Change Controller:  The IESG iesg@ietf.org
      (mailto:iesg@ietf.org)

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 secure channel such as TLS, is
   encrypted or signed, or has come from some other source that is
   presumed trusted.

   Section 4.1 gives examples of limitations in interoperability when
   using a constrained CBOR decoder with input from a CBOR encoder that
   uses a non-preferred serialization.  When a single data item is
   consumed both by such a constrained decoder and a full decoder, it
   can lead to security issues that can be exploited by an attacker who
   can inject or manipulate content.

   As discussed throughout this document, there are many values that can
   be considered "equivalent" in some circumstances and "not equivalent"
   in others.  As just one example, the numeric value for the number
   "one" might be expressed as an integer or a bignum.  A system
   interpreting CBOR input might accept either form for the number
   "one", or might reject one (or both) forms.  Such acceptance or
   rejection can have security implications in the program that is using
   the interpreted input.

   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 even arbitrary precision numbers) 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.)



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   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.

   The input check itself may consume resources.  This is usually linear
   in the size of the input, which means that an attacker has to spend
   resources that are commensurate to the resources spent by the
   defender on input validation.  However, an attacker might be able to
   craft inputs that will take longer for a target decoder to process
   than for the attacker to produce.  Processing for arbitrary-precision
   numbers may exceed linear effort.  Also, some hash-table
   implementations that are used by decoders to build in-memory
   representations of maps can be attacked to spend quadratic effort,
   unless a secret key (see Section 7 of [SIPHASH_LNCS], also
   [SIPHASH_OPEN]) or some other mitigation is employed.  Such
   superlinear efforts can be exploited by an attacker to exhaust
   resources at or before the input validator; they therefore need to be
   avoided in a CBOR decoder implementation.  Note that tag number
   definitions and their implementations can add security considerations
   of this kind; this should then be discussed in the security
   considerations of the tag number definition.

   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 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



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   interpretation than the one that will be used by a second
   application.  To facilitate consistent interpretation, encoder and
   decoder implementations should provide a validity checking mode of
   operation (Section 5.4).  Note, however, that a generic decoder
   cannot know about all requirements that an application poses on its
   input data; it is therefore not relieving the application from
   performing its own input checking.  Also, since the set of defined
   tag numbers evolves, the application may employ a tag number that is
   not yet supported for validity checking by the generic decoder it
   uses.  Generic decoders therefore need to provide documentation which
   tag numbers they support and what validity checking they can provide
   for each of them as well as for basic CBOR validity (UTF-8 checking,
   duplicate map key checking).

   Section 3.4.3 notes that using the non-preferred choice of a bignum
   representation instead of a basic integer for encoding a number is
   not intended to have application semantics, but it can have such
   semantics if an application receiving CBOR data is using a decoder in
   the basic generic data model.  This disparity causes a security issue
   if the two sets of semantics differ.  Thus, applications using CBOR
   need to specify the data model that they are using for each use of
   CBOR data.

   It is common to convert CBOR data to other formats.  In many cases,
   CBOR has more expressive types than other formats; this is
   particularly true for the common conversion to JSON.  The loss of
   type information can cause security issues for the systems that are
   processing the less-expressive data.

   Section 6.2 describes a possibly-common usage scenario of converting
   between CBOR and JSON that could allow an attack if the attcker knows
   that the application is performing the conversion.

   Security considerations for the use of base16 and base64 from
   [RFC4648], and the use of UTF-8 from [RFC3629], are relevant to CBOR
   as well.

11.  References

11.1.  Normative References

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







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   [Cplusplus17]
              International Organization for Standardization,
              "Programming languages — C++", ISO/IEC 14882:2017, Fifth
              Edition, December 2017.

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
              Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
              <https://ieeexplore.ieee.org/document/8766229>.

   [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>.

   [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>.



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   [TIME_T]   The Open Group Base Specifications, "Open Group Standard:
              Vol. 1: Base Definitions, Issue 7", Section 4.16 'Seconds
              Since the Epoch', IEEE Std 1003.1, 2018 Edition, 2018,
              <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
              V1_chap04.html#tag_04_16>.

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/>.

   [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>.

   [I-D.bormann-cbor-notable-tags]
              Bormann, C., "Notable CBOR Tags", Work in Progress,
              Internet-Draft, draft-bormann-cbor-notable-tags-02, 25
              June 2020, <http://www.ietf.org/internet-drafts/draft-
              bormann-cbor-notable-tags-02.txt>.

   [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>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <http://www.iana.org/assignments/core-parameters>.

   [IANA.media-type-structured-suffix]
              IANA, "Structured Syntax Suffix Registry",
              <http://www.iana.org/assignments/media-type-structured-
              suffix>.




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   [IANA.media-types]
              IANA, "Media Types",
              <http://www.iana.org/assignments/media-types>.

   [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>.

   [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>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [RFC7991]  Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
              RFC 7991, DOI 10.17487/RFC7991, December 2016,
              <https://www.rfc-editor.org/info/rfc7991>.

   [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>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.





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   [RFC8618]  Dickinson, J., Hague, J., Dickinson, S., Manderson, T.,
              and J. Bond, "Compacted-DNS (C-DNS): A Format for DNS
              Packet Capture", RFC 8618, DOI 10.17487/RFC8618, September
              2019, <https://www.rfc-editor.org/info/rfc8618>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/info/rfc8742>.

   [RFC8746]  Bormann, C., Ed., "Concise Binary Object Representation
              (CBOR) Tags for Typed Arrays", RFC 8746,
              DOI 10.17487/RFC8746, February 2020,
              <https://www.rfc-editor.org/info/rfc8746>.

   [SIPHASH_LNCS]
              Aumasson, J. and D. Bernstein, "SipHash: A Fast Short-
              Input PRF", Lecture Notes in Computer Science pp. 489-508,
              DOI 10.1007/978-3-642-34931-7_28, 2012,
              <https://doi.org/10.1007/978-3-642-34931-7_28>.

   [SIPHASH_OPEN]
              Aumasson, J. and D.J. Bernstein, "SipHash: a fast short-
              input PRF", <https://131002.net/siphash/siphash.pdf>.

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

Appendix A.  Examples of Encoded CBOR Data Items

   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.)  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')).






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   +==============================+====================================+
   |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               |
   +------------------------------+------------------------------------+
   |-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                           |



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   +------------------------------+------------------------------------+
   |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                       |
   +------------------------------+------------------------------------+
   |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 |



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   |                              | 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                         |
   +------------------------------+------------------------------------+
   |"\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               |
   +------------------------------+------------------------------------+



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   |["a", {"b": "c"}]             | 0x826161a161626163                 |
   +------------------------------+------------------------------------+
   |{"a": "A", "b": "B", "c": "C",| 0xa5616161416162614261636143616461 |
   |"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, 9, | 0x9f0102030405060708090a0b0c0d0e0f |
   |10, 11, 12, 13, 14, 15, 16,   | 101112131415161718181819ff         |
   |17, 18, 19, 20, 21, 22, 23,   |                                    |
   |24, 25]                       |                                    |
   +------------------------------+------------------------------------+
   |{_ "a": 1, "b": [_ 2, 3]}     | 0xbf61610161629f0203ffff           |
   +------------------------------+------------------------------------+
   |["a", {_ "b": "c"}]           | 0x826161bf61626163ff               |
   +------------------------------+------------------------------------+
   |{_ "Fun": true, "Amt": -2}    | 0xbf6346756ef563416d7421ff         |
   +------------------------------+------------------------------------+

                Table 6: Examples of Encoded CBOR Data Items

Appendix B.  Jump Table for Initial Byte

   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 | Unsigned integer 0x00..0x17 (0..23)            |
      +------------+------------------------------------------------+
      | 0x18       | Unsigned integer (one-byte uint8_t follows)    |
      +------------+------------------------------------------------+
      | 0x19       | Unsigned integer (two-byte uint16_t follows)   |



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      +------------+------------------------------------------------+
      | 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)                           |
      +------------+------------------------------------------------+
      | 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   |



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      |            | 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)                          |
      +------------+------------------------------------------------+
      | 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 3.4.1)                                 |
      +------------+------------------------------------------------+
      | 0xc1       | Epoch-based date/time (data item follows; see  |
      |            | Section 3.4.2)                                 |
      +------------+------------------------------------------------+
      | 0xc2       | Positive bignum (data item "byte string"       |
      |            | follows)                                       |
      +------------+------------------------------------------------+



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      | 0xc3       | Negative bignum (data item "byte string"       |
      |            | follows)                                       |
      +------------+------------------------------------------------+
      | 0xc4       | Decimal Fraction (data item "array" follows;   |
      |            | see Section 3.4.4)                             |
      +------------+------------------------------------------------+
      | 0xc5       | Bigfloat (data item "array" follows; see       |
      |            | Section 3.4.4)                                 |
      +------------+------------------------------------------------+
      | 0xc6..0xd4 | (tag)                                          |
      +------------+------------------------------------------------+
      | 0xd5..0xd7 | Expected Conversion (data item follows; see    |
      |            | Section 3.4.5.2)                               |
      +------------+------------------------------------------------+
      | 0xd8..0xdb | (more tags; 1/2/4/8 bytes of tag number 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)    |
      +------------+------------------------------------------------+
      | 0xfb       | Double-Precision Float (eight-byte IEEE 754)   |
      +------------+------------------------------------------------+
      | 0xff       | "break" stop code                              |
      +------------+------------------------------------------------+

                    Table 7: 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:

   *  the pseudocode does not "fail";





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

   The pseudocode has the following prerequisites:

   *  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.

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

   *  Arithmetic works as in C.

   *  All variables are unsigned integers of sufficient range.

   Note that "well_formed" returns the major type for well-formed
   definite length items, but 99 for an indefinite length item (or -1
   for a "break" stop code, only if "breakable" is set).  This is used
   in "well_formed_indefinite" to ascertain that indefinite length
   strings only contain definite length strings as chunks.































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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;                    // definite-length data item
   }

   well_formed_indefinite(mt, breakable) {
     switch (mt) {
       case 2: case 3:
         while ((it = well_formed(true)) != -1)
           if (it != mt)           // need definite-length chunk
             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 99;                    // indefinite-length data item
   }

               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
     unsigned mt = ui & 0x20; // extract (shifted) 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

   See Section 1.2 for some specific assumptions about the profile of
   the C language used in these pieces of code.

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) {
     unsigned half = (halfp[0] << 8) + halfp[1];
     unsigned exp = (half >> 10) & 0x1f;
     unsigned 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.






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   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
   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

   A discussion of CBOR and other formats with respect to a different
   set of design objectives is provided in Section 5 and Appendix C of
   [RFC8618].

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 item,
   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.






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   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
   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, which
   prevents 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 8 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 01 | 30 80 02 01 01 |
       |             | 02 02 01 03                | 30 06 02 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 8: Examples for Different Levels of Conciseness

Appendix F.  Well-formedness errors and examples

   There are three basic kinds of well-formedness errors that can occur
   in decoding a CBOR data item:

   *  Too much data: There are input bytes left that were not consumed.
      This is only an error if the application assumed that the input
      bytes would span exactly one data item.  Where the application
      uses the self-delimiting nature of CBOR encoding to permit
      additional data after the data item, as is for example done in
      CBOR sequences [RFC8742], the CBOR decoder can simply indicate
      what part of the input has not been consumed.

   *  Too little data: The input data available would need additional
      bytes added at their end for a complete CBOR data item.  This may
      indicate the input is truncated; it is also a common error when
      trying to decode random data as CBOR.  For some applications,
      however, this may not actually be an error, as the application may
      not be certain it has all the data yet and can obtain or wait for
      additional input bytes.  Some of these applications may have an
      upper limit for how much additional data can show up; here the
      decoder may be able to indicate that the encoded CBOR data item
      cannot be completed within this limit.






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   *  Syntax error: The input data are not consistent with the
      requirements of the CBOR encoding, and this cannot be remedied by
      adding (or removing) data at the end.

   In Appendix C, errors of the first kind are addressed in the first
   paragraph/bullet list (requiring "no bytes are left"), and errors of
   the second kind are addressed in the second paragraph/bullet list
   (failing "if n bytes are no longer available").  Errors of the third
   kind are identified in the pseudocode by specific instances of
   calling fail(), in order:

   *  a reserved value is used for additional information (28, 29, 30)

   *  major type 7, additional information 24, value < 32 (incorrect)

   *  incorrect substructure of indefinite length byte/text string (may
      only contain definite length strings of the same major type)

   *  "break" stop code (mt=7, ai=31) occurs in a value position of a
      map or except at a position directly in an indefinite length item
      where also another enclosed data item could occur

   *  additional information 31 used with major type 0, 1, or 6

F.1.  Examples for CBOR data items that are not well-formed

   This subsection shows a few examples for CBOR data items that are not
   well-formed.  Each example is a sequence of bytes each shown in
   hexadecimal; multiple examples in a list are separated by commas.

   Examples for well-formedness error kind 1 (too much data) can easily
   be formed by adding data to a well-formed encoded CBOR data item.

   Similarly, examples for well-formedness error kind 2 (too little
   data) can be formed by truncating a well-formed encoded CBOR data
   item.  In test suites, it may be beneficial to specifically test with
   incomplete data items that would require large amounts of addition to
   be completed (for instance by starting the encoding of a string of a
   very large size).

   A premature end of the input can occur in a head or within the
   enclosed data, which may be bare strings or enclosed data items that
   are either counted or should have been ended by a "break" stop code.

   *  End of input in a head: 18, 19, 1a, 1b, 19 01, 1a 01 02, 1b 01 02
      03 04 05 06 07, 38, 58, 78, 98, 9a 01 ff 00, b8, d8, f8, f9 00, fa
      00 00, fb 00 00 00




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   *  Definite length strings with short data: 41, 61, 5a ff ff ff ff
      00, 5b ff ff ff ff ff ff ff ff 01 02 03, 7a ff ff ff ff 00, 7b 7f
      ff ff ff ff ff ff ff 01 02 03

   *  Definite length maps and arrays not closed with enough items: 81,
      81 81 81 81 81 81 81 81 81, 82 00, a1, a2 01 02, a1 00, a2 00 00
      00

   *  Tag number not followed by tag content: c0

   *  Indefinite length strings not closed by a "break" stop code: 5f 41
      00, 7f 61 00

   *  Indefinite length maps and arrays not closed by a "break" stop
      code: 9f, 9f 01 02, bf, bf 01 02 01 02, 81 9f, 9f 80 00, 9f 9f 9f
      9f 9f ff ff ff ff, 9f 81 9f 81 9f 9f ff ff ff

   A few examples for the five subkinds of well-formedness error kind 3
   (syntax error) are shown below.

   Subkind 1:

   *  Reserved additional information values: 1c, 1d, 1e, 3c, 3d, 3e,
      5c, 5d, 5e, 7c, 7d, 7e, 9c, 9d, 9e, bc, bd, be, dc, dd, de, fc,
      fd, fe,

   Subkind 2:

   *  Reserved two-byte encodings of simple values: f8 00, f8 01, f8 18,
      f8 1f

   Subkind 3:

   *  Indefinite length string chunks not of the correct type: 5f 00 ff,
      5f 21 ff, 5f 61 00 ff, 5f 80 ff, 5f a0 ff, 5f c0 00 ff, 5f e0 ff,
      7f 41 00 ff

   *  Indefinite length string chunks not definite length: 5f 5f 41 00
      ff ff, 7f 7f 61 00 ff ff

   Subkind 4:

   *  Break occurring on its own outside of an indefinite length item:
      ff

   *  Break occurring in a definite length array or map or a tag: 81 ff,
      82 00 ff, a1 ff, a1 ff 00, a1 00 ff, a2 00 00 ff, 9f 81 ff, 9f 82
      9f 81 9f 9f ff ff ff ff



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   *  Break in indefinite length map would lead to odd number of items
      (break in a value position): bf 00 ff, bf 00 00 00 ff

   Subkind 5:

   *  Major type 0, 1, 6 with additional information 31: 1f, 3f, df

Appendix G.  Changes from RFC 7049

   As discussed in the introduction, this document is a revised edition
   of RFC 7049, with editorial improvements, added detail, and fixed
   errata.  This document formally obsoletes RFC 7049, while keeping
   full compatibility of the interchange format from RFC 7049.  This
   document does not create a new version of the format.

G.1.  Errata processing, clerical changes

   The two verified errata on RFC 7049, EID 3764 and EID 3770, concerned
   two encoding examples in the text that have been corrected
   (Section 3.4.3: "29" -> "49", Section 5.5: "0b000_11101" ->
   "0b000_11001").  Also, RFC 7049 contained an example using the
   numeric value 24 for a simple value (EID 5917), which is not well-
   formed; this example has been removed.  Errata report 5763 pointed to
   an accident in the wording of the definition of tags; this was
   resolved during a re-write of Section 3.4.  Errata report 5434
   pointed out that the UBJSON example in Appendix E no longer complied
   with the version of UBJSON current at the time of submitting the
   report.  It turned out that the UBJSON specification had completely
   changed since 2013; this example therefore also was removed.  Further
   errata reports (4409, 4963, 4964) complained that the map key sorting
   rules for canonical encoding were onerous; these led to a
   reconsideration of the canonical encoding suggestions and replacement
   by the deterministic encoding suggestions (described below).  An
   editorial suggestion in errata report 4294 was also implemented
   (improved symmetry by adding "Second value" to a comment to the last
   example in Section 3.2.2).

   Other more clerical changes include:

   *  use of new RFCXML functionality [RFC7991];

   *  explain some more of the notation used;

   *  updated references, e.g. for RFC4627 to [RFC8259] in many places,
      for CNN-TERMS to [RFC7228]; added missing reference to [IEEE754]
      (importing required definitions) and updated to [ECMA262]; added a
      reference to [RFC8618] that further illustrates the discussion in
      Appendix E;



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   *  the discussion of diagnostic notation mentions the "Extended
      Diagnostic Notation" (EDN) defined in [RFC8610] as well as the gap
      diagnostic notation has in representing NaN payloads; an
      explanation was added on how to represent indefinite length
      strings with no chunks;

   *  the addition of this appendix.

G.2.  Changes in IANA considerations

   The IANA considerations were generally updated (clerical changes,
   e.g., now pointing to the CBOR working group as the author of the
   specification).  References to the respective IANA registries have
   been added to the informative references.

   Tags in the space from 256 to 32767 (lower half of "1+2") are no
   longer assigned by First Come First Served; this range is now
   Specification Required.

G.3.  Changes in suggestions and other informational components

   In revising the document, beyond processing errata reports, the WG
   could use nearly seven years of experience with the use of CBOR in a
   diverse set of applications.  This led to a number of editorial
   changes, including adding tables for illustration, but also to
   emphasizing some aspects and de-emphasizing others.

   A significant addition in this revision is Section 2, which discusses
   the CBOR data model and its small variations involved in the
   processing of CBOR.  Introducing terms for those (basic generic,
   extended generic, specific) enables more concise language in other
   places of the document, but also helps in clarifying expectations on
   implementations and on the extensibility features of the format.

   RFC 7049, as a format derived from the JSON ecosystem, was influenced
   by the JSON number system that was in turn inherited from JavaScript
   at the time.  JSON does not provide distinct integers and floating-
   point values (and the latter are decimal in the format).  CBOR
   provides binary representations of numbers, which do differ between
   integers and floating-point values.  Experience from implementation
   and use now suggested that the separation between these two number
   domains should be more clearly drawn in the document; language that
   suggested an integer could seamlessly stand in for a floating-point
   value was removed.  Also, a suggestion (based on I-JSON [RFC7493])
   was added for handling these types when converting JSON to CBOR, and
   the use of a specific rounding mechanism has been recommended.





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   For a single value in the data model, CBOR often provides multiple
   encoding options.  The revision adds a new section Section 4, which
   first introduces the term "preferred serialization" (Section 4.1) and
   defines it for various kinds of data items.  On the basis of this
   terminology, the section goes on to discuss how a CBOR-based protocol
   can define "deterministic encoding" (Section 4.2), which now avoids
   the RFC 7049 terms "canonical" and "canonicalization".  The
   suggestion of "Core Deterministic Encoding Requirements"
   Section 4.2.1 enables generic support for such protocol-defined
   encoding requirements.  The present revision further eases the
   implementation of deterministic encoding by simplifying the map
   ordering suggested in RFC 7049 to simple lexicographic ordering of
   encoded keys.  A description of the older suggestion is kept as an
   alternative, now termed "length-first map key ordering"
   (Section 4.2.3).

   The terminology for well-formed and valid data was sharpened and more
   stringently used, avoiding less well-defined alternative terms such
   as "syntax error", "decoding error" and "strict mode" outside
   examples.  Also, a third level of requirements beyond CBOR-level
   validity that an application has on its input data is now explicitly
   called out.  Well-formed (processable at all), valid (checked by a
   validity-checking generic decoder), and expected input (as checked by
   the application) are treated as a hierarchy of layers of
   acceptability.

   The handling of non-well-formed simple values was clarified in text
   and pseudocode.  Appendix F was added to discuss well-formedness
   errors and provide examples for them.  The pseudocode was updated to
   be more portable and some portability considerations were added.

   The discussion of validity has been sharpened in two areas.  Map
   validity (handling of duplicate keys) was clarified and the domain of
   applicability of certain implementation choices explained.  Also,
   while streamlining the terminology for tags, tag numbers, and tag
   content, discussion was added on tag validity, and the restrictions
   were clarified on tag content, in general and specifically for tag 1.

   An implementation note (and note for future tag definitions) was
   added to Section 3.4 about defining tags with semantics that depend
   on serialization order.

   Tag 35 is no longer defined in this updated document; the
   registration based on the definition in RFC 7049 remains in place.

   Terminology was introduced in Section 3 for "argument" and "head",
   simplifying further discussion.




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   The security considerations were mostly rewritten and significantly
   expanded; in multiple other places, the document is now more explicit
   that a decoder cannot simply condone well-formedness errors.

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.

   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 Yasskin, Joe Hildebrand,
   Keith Moore, Laurence Lundblade, Matthew Lepinski, Michael
   Richardson, Nico Williams, Peter Occil, Phillip Hallam-Baker, Ray
   Polk, Stuart Cheshire, Tim Bray, Tony Finch, Tony Hansen, and Yaron
   Sheffer.  Benjamin Kaduk provided an extensive review during IESG
   processing. Éric Vyncke, Erik Kline, Robert Wilton, and Roman Danyliw
   provided further IESG comments, which included an IoT directorate
   review by Eve Schooler.

Authors' Addresses

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

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







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   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org















































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